Railridr

I am in the process of converting 10 Homelite motors, and I was looking for 11mm carburetors for my conversions. I went through all of the carburetors on the Walbro and Zama website, looking for any carbs that have an 11mm venturi. I compiled a list of the carbs that I found. This list includes all 11mm carburetors that I found. Carburetors on this list may of may not be suitable for model airplane use as they may not have high or low speed needles, or they may have a strange throttle connection. In this list I have included information such as if the carb has a choke, a primer, and needles for mixture control. The list of Zama carbs also includes equipment manufacturers and the models of their equipment that may use the listed carb. The Zama 11mm carbs have a 16mm throttle bore, and the Walbro 11mm carbs have a 15.84mm throttle bore

The link for product lookup search on the Zama Carburetors web site

http://www.zamacarb.com/prod_lookup.cfm

The link for product lookup search on the Walbro's web site

http://wem.walbro.com/walbro/family2.asp

Zama Carburetors

C1U-K19, No choke, Primer, 2 needle, Echo, J/D 21C, 21S
C1U-K20, A-C, No Choke, Primer, 2 needle, Echo, SRM-2501/2501S, SRM-2510
C1U-K21, No Choke, Primer, 2 needle, Echo, SRM-3000, J/D 30S
C1U-K21A-B, No Choke, Primer, 2 Needle, Echo, SRM-3000, J/D 30S
C1U-K22,A, No Choke, Primer, 2 Needle, Echo, J/D 38B, SRM3800
C1U-K23, No Choke, Primer, 2 Needle
C1U-K23A-B, No Choke, Primer, 2 Needle, Lesco, LST24501/LHE2475
C1U-K34, No Choke, Primer, 2 Needle, Echo, SHR-2100
C1U-K34A, No Choke, Primer, 2 Needle, Echo, SHR-2100

C1Q-E3, No Choke, Primer, 2 Needle, Efco, Jet 300,400
C1Q-E4, No Choke, Primer, 2 Needle, Efco, Jet 300,400
C1Q-E6, No Choke, Primer, 2 Needle, Efco, EFCO 8742

C1Q-H17D, Choke, No Primer, 2 Needle, Homelite/JohnDeer, 200 Chain Saw
C1Q-H35, Choke, No Primer, 2 Needle, Homelite/JohnDeer, 33-38cc Chain Saw
C1Q-H42, Choke, No Primer, 2 Needle, Homelite/JohnDeer, 33-38cc Saw
C1Q-H43, Choke, No Primer, 2 Needle, Homelite/JohnDeer, 33-38cc Saw Non Em

C1Q-M27-C, Choke, No Primer, 2 Needle, McCulloch/Jenn Feng, 32cc Chain Saw
C1Q-M27D, Choke, No Primer, 2 Needle, McCulloch/Jenn Feng, 32cc Chain Saw
C1Q-M28-C, Choke, No Primer, 2 Needle, McCulloch/Jenn Feng, 35cc Chain Saw
C1Q-M28D, Choke, No Primer, 2 Needle, McCulloch/Jenn Feng, 35cc Chain Saw

C1Q-S44,A, No Choke, Primer, No Needles, Stihl, FC55 FS46 FS55 HL45
C1Q-S45, No Choke, Primer, No Needles, Stihl, FC85 FH75 FC75 HT75 HL75 S75/80/85 HT70/75 KW85
C1Q-S49,A, No Choke, Primer, No Needles, Stihl, HS45
C1Q-S49B, No Choke, Primer, No Needles, Stihl, HS45
C1Q-S50, No Choke, Primer, No Needles, Stihl, BG55/65/85 SH55/85
C1Q-S53,A, No Choke, Primer, No Needles, Stihl, FS46 FS55 HL45
C1Q-S54, No Choke, Primer, No Needles, Stihl, 4228
C1Q-S55, No Choke, Primer, No Needles, Stihl, BG55/65/85 SH55/85
C1Q-S56, No Choke, Primer, No Needles, Stihl, FH75 FC75 HT75 HL75 FS75/80/85 HT70/75 KW85
C1Q-S58, No Choke, Primer, No Needles, Stihl, FS55
C1Q-S60, No Choke, Primer, No Needles, Stihl, 25 CC ST / Brazil
C1Q-S62, No Choke, Primer, No Needles, Stihl, 4140 CA Version
C1Q-S63,A, No Choke, Primer, 2 Needles, Stihl, FS75/80/85 HT70/75
C1Q-S64, No Choke, Primer, 2 Needles, Stihl, BG45/46/55 SH55/85
C1Q-S66, No Choke, Primer, 2 Needles, Stihl, EMU ST FS45/46/55
C1Q-S67, No Choke, Primer, 2 Needles, Stihl, 4228 EMU HS45
C1Q-S68, No Choke, Primer, 2 Needles, Stihl, 4229 BG85 Blower
C1Q-S68A, No Choke, Primer, 2 Needles, Stihl, 4229 BG85 Blower
C1Q-S69A, No Choke, Primer, 2 Needles, Stihl, 4137 EMU Trimmer
C1Q-S70, No Choke, Primer, 2 Needles, Stihl, HS45 Hedgetrimmer
C1Q-S71, No Choke, Primer 2 Needles, Stihl, 4140 FS55 Trimmer
C1Q-S74,A, No Choke, Primer, 2 Needles, Stihl, 4314 BT45 Drill
C1Q-S79, No Choke, Primer, 2 Needles, Stihl, 4610 EMU Minni-Tiller
C1Q-S95, No Choke, Primer, 2 Needles, Stihl, FS38 Stringtrimmer

C1Q-SK8, No Choke, Primer, 2 Needles, Yanmar Brushcutter, Honda B.C./S.T., Shibaura M35D
C1Q-SK9, No Choke, Primer, 2 Needles, Lesco B.C./S.T., Shibaura M35D

C1Q-W14, Choke, No Primer, 2 Needles, Poulan/Weedeater, 36/42 LE CSI Non AV Saw
C1Q-W14A, Choke, No Primer, 2 Needles, Poulan/Weedeater, 36/42 LE CSI Non AV Saw
C1Q-W2A, No Choke, No Primer, 2 Needles, Poulan/Weedeater, Super 25 Chain Saw
C1Q-W8, Choke, No Primer, 2 Needles, Poulan/Weedeater, Wood Shark Chain Saw, Patriot 1900 Saw
C1Q-W9, Choke, No Primer, 2 Needles, Poulan/Weedeater, Sears 2.6 18" LS Saw, Poulan 2250 LE 18", Poulan Pro 221 16" LE CA, Poulan Pro 260 18" LE CA, Poulan Pro 2250 18" LE CA
C1Q-W9A, Choke, No Primer, 2 Needles, Poulan/Weedeater, Sears 2.6 18" LS Saw, Poulan 2250 LE 18", Poulan Pro 221 16" LE CA, Poulan Pro 260 18" LE CA, Poulan Pro 2250 18" LE CA

Railridr

Walbro Carburetors

11.11mm
WT-105-1, No Choke, No Primer, 1 Needle (no low speed)
WT-108-1, No Choke, No Primer, 2 Needles
WT-130-1, No Choke, No Primer, 2 Needles
WT-131-1, No Choke, No Primer, 2 Needles
WT-139-1, No Choke, Primer, 2 Needles
WT-144-1, No Choke, No Primer, 2 Needles
WT-145-1, No Choke, Primer, 2 Needles
WT-146-1, No Choke, No Primer, 2 Needles
WT-214-1, Choke, No Primer, 2 Needles
WT-239-1, Choke, No Primer, 2 Needles
WT-243-1, Primer, No Choke, 2 Needles
WT-249-1, Choke, No Primer, 2 Needles
WT-250-1, No Choke, No Primer, 2 Needles
WT-252-1, No Choke, No Primer, 1 Needle (no high speed)
WT-258-1, Choke, No Primer, 2 Needles
WT-261-1, No Choke, No Primer, 2 Needles
WT-265-1, No Choke, Primer, 2 Needles
WT-271-1, No Choke, No Primer, 2 Needles
WT-289-1, Choke, No Primer, 2 Needles
WT-29-1, No Choke, No Primer, 2 Needles
WT-297-1, No Choke, Primer, 2 Needles
WT-301-1, No Choke, No Primer, 2 Needles
WT-323-1, No Choke, No Primer, 1 Needle (no high speed)
WT-324-1, Choke, No Primer, 2 Needles
WT-337-1, No Choke, No Primer, 2 Needles
WT-347-1, Choke, No Primer, 2 Needles
WT-348-1, Choke, No Primer, 2 Needles
WT-350-1, Choke, No Primer, 2 Needles
WT-356-1, No Choke, No Primer, 2 Needles
WT-358-1, No Choke, No Primer, 2 Needles
WT-378-1, Choke, No Primer, 2 Needles
WT-382-1, No Choke, No Primer, 1 Needle (no high speed)
WT-391-1, Choke, No Primer, 2 Needles
WT-40-1, No Choke, Primer, 2 Needles
WT-404-1, No Choke, Primer, 2 Needles
WT-407-1, No Choke, No Primer, 2 Needles
WT-410-1, No Choke, No Primer, 2 Needles
WT-415-1, No Choke, Primer, 2 Needles
WT-42-1, No Choke, No Primer, 2 Needles
WT-433-1, Choke, No Primer, 2 Needles
WT-437-1, Choke, No Primer, 2 Needles
WT-438-1, Choke, No Primer, 2 Needles
WT-462-1, No Choke, Primer, 2 Needles
WT-464-1, No Choke, No Primer, 2 Needles
WT-47-1, No Choke, No Primer, 2 Needles
WT-478-1, No Choke, No Primer, 2 Needles
WT-499-1, No Choke, No Primer, 2 Needles
WT-504-1, Choke, No Primer, 2 Needles
WT-511-1, No Choke, No Primer, 2 Needles
WT-516-1, No Choke, No Primer, 2 Needles
WT-517-1, No Choke, Primer, 2 Needles
WT-518-1, Choke, No Primer, 2 Needles
WT-522-1, Choke, No Primer, 2 Needles
WT-524-1, No Choke, Primer, 2 Needles
WT-527-1, No Choke, No Primer, 2 Needles
WT-530-1, No Choke, No Primer, 2 Needles
WT-534-1, Choke, Primer, 2 Needles
WT-551-1, No Choke, Primer, 2 Needles
WT-553-1, No Choke, Primer, 2 Needles
WT-563-1, No Choke, No Primer, 1 Needle (no high speed)
WT-568-1, Choke, No Primer, 2 Needles
WT-569-1, Choke, No Primer, 2 Needles
WT-572-1, Choke, No Primer, 2 Needles
WT-576-1, Choke, No Primer 2 Needles
WT-588-1, Choke, No Primer, 2 Needles
WT-613-1, No Choke, Primer, 2 Needles
WT-624-1, Choke, No Primer, 2 Needles
WT-625-1, Choke, No Primer, 1 Needle (no high speed)
WT-626-1, Choke, Primer, No Needles
WT-627-1, Choke, No Primer, 1 Needle (no high speed)
WT-628-1, Choke, No Primer, No Needles
WT-629-1, No Choke, No Primer, 1 Needle (no high speed)
WT-630-1, Choke, Primer, 1 Needle (no high speed)
WT-631-1, No Choke, Primer, 1 Needle (no high speed)
WT-634-1, Choke, No Primer, 1 Needle (no high speed)
WT-642-1, Choke, No Primer, 2 Needles
WT-662-1, Choke, No Primer, 2 Needles
WT-669-1, Choke, No Primer, No Needles
WT-678-1, Choke, No Primer, 2 Needles
WT-684-1, No Choke, No Primer, 1 Needle (no high speed)
WT-689-1, Choke, No Primer, 2 Needles
WT-690-1, Choke, No Primer, 2 Needles
WT-695-1, Choke, No Primer, No Needles
WT-698-1, Choke, No Primer, No Needles
WT-700-1, Choke, No Primer, 2 Needles
WT-702-1, Choke, No Primer, 2 Needles
WT-707-1, Choke, No Primer, 2 Needles
WT-709-1, Choke, No Primer, 2 Needles
WT-718-1, Choke, No Primer, no needles
WT-72-1, No Choke, Primer, 2 Needles
WT-729-1, Choke, No Primer, 2 Needles
WT-74-1, No Choke, No Primer, 2 Needles
WT-77-1, No Choke, No Primer, 2 Needles
WT-86-1, Choke, No Primer, 2 Needles
WT-88-1, Choke, No Primer, 2 Needles
WT-94-1, No Choke, No Primer, 2 Needles

11.11mm
WTA-12-1, Choke, No Primer, 1 Needle (no high speed)
WTA-13-1, Choke, NO Primer, 1 Needle (no high speed)
WTA-14-1, No Choke, No Primer, 1 Needle (no high speed)
WTA-15-1, No Choke, No Primer, 1 Needle (no high speed)
WTA-2-1, Choke, No Primer, 1 Needle (no low speed)
WTA-3-1, Choke, No Primer, 1 Needle (no high speed)
WTA-4-1, Choke, no Primer, 1 Needle (no high speed)
WTA-5-1, Choke, no Primer, 1 Needle (no high speed)
WTA-6-1, No Choke, No Primer, 2 Needle
WTA-7-1, No Choke, no Primer, 2 Needle
WTA-9-1, Choke, no Primer, 1 Needle (no high speed)

11.11mm
WA-137-1, No Choke, No Primer, 2 Needles
WA-138-1, No Choke, No Primer, 2 Needles
WA-144-1, No Choke, No Primer, 2 Neesles
WA-150-1, No Choke, Primer, 2 Needles
WA-154-1, No Choke, Primer, 2 Needles
WA-162-1, No Choke, No Primer, 2 Needles
WA-166-1, No Choke, No Primer, 2 Needles
WA-167-1, No Choke, Primer, 2 Needles
WA-173-1, No Choke, No Primer, 2 Needles
WA-174-1, No Choke, No Primer, 2 Needles
WA-175-1, No Choke, No Primer, 2 Needles
WA-190-1, No Choke, No Primer, 2 Needles
WA-192-1, No Choke, Primer, 2 Needles
WA-193-1, No Choke, Primer, 2 Needles
WA-196-1, No Choke, No Primer, 2 Needles
WA-197-1, No Choke, No Primer, 2 Needles
WA-203-1, No Choke, No Primer, 2 Needles
WA-208-1, No Choke, No Primer, 2 Needles
WA-210-1, No Choke, No Primer, 2 Needles
WA-213-1, No Choke, Primer, 2 Needles
WA-222-1, No Choke, No primer, 2 Needles
WA-55-1, No Choke, No Primer, 2 Needles

11.11mm
HDA-113-1, No Choke, Primer, 1 Needle (no high speed)
HDA-131-1, No Choke, No Primer, 2 Needles
HDA-135-1, No Choke, Primer, 2 Needles
HDA-147-1, Choke, Primer, 2 Needles
HDA-149-1, No Choke, Primer, 2 Needles
HDA-155-1, Choke, No Primer, 2 Needles
HDA-168-1, Choke, No Primer, 2 Needles
HDA-169-1, Choke, No Primer, 2 Needles
HDA-201-1, Choke, No Primer, 1 Needle (no high speed)
HDA-206-1, Choke, No Primer, no Needles
HDA-210-1, Choke, No Primer, 2 Needles
HDA-220-1, Choke, No Primer, 2 Needles
HDA-59-1, Choke, No Primer, 2 Needles
HDA-60-1, Choke, No Primer, 2 Needles
HDA-64-1, Choke, No Primer, 2 Needles
HDA-70-1, No Choke, Primer, 2 Needles
HDA-90-1, Choke, Primer, 2 Needles

11.13mm
WT-15-1, No Choke, No Primer, 2 Needles
WT-21-1, No Choke, No Primer, 2 Needles
WT-570-1, Choke, No Primer, 2 Needles
WT-8-1, Choke, No Primer, 2 Needles
WT-83-1, No Choke, No Primer, 2 Needles
WT-9-1, Choke, No Primer, 2 Needles

11.13mm
WA-106-1, No Choke, No Primer, 2 Needles
WA-1-1, No Choke, No Primer, 2 Needles
WA-112-1, No Choke, No Primer, 2 Needles
WA-113-1, No Choke, No Primer, 2 Needles
WA-115-1, No Choke, No Primer, 2 Needles
WA-126-1, No Choke, No Primer, 2 Needles
WA-133-1, No Choke, No Primer, 2 Needles
WA-141-1, No Choke, No Primer, 2 Needles
WA-142-1, No Choke, No Primer, 2 Needles
WA-151-1, No Choke, No Primer, 2 Needles
WA-155-1, No Choke, No Primer, 2 Needles
WA-18-1, No Choke, No Primer, 1 Needle (no high speed)
WA-19-1, No Choke, No Primer, 2 Needles
WA-200-1, No Choke, No Primer 2 Needles
WA-25-1, No Choke, No Primer, 2 Needles
WA-32-1, No Choke, No Primer, 2 Needles
WA-33-1, No Choke, No Primer, 2 Needles
WA-34-1, No Choke, No Primer, 2 Needles
WA-39-1, No Choke, No Primer, 2 Needles
WA-44-1, No Choke, No Primer, 2 Needles
WA-53-1, No Choke, No Primer, 1 Needle (no high speed)
WA-54-1, No Choke, No Primer, 1 Needle (no high speed)
WA-58-1, No Choke, No Primer, 2 Needles
WA-64-1, No Choke, No Primer, 2 Needles
WA-65-1, No Choke, No Primer, 2 Needles
WA-73-1, No Choke, No Primer, 2 Needles
WA-75-1, No Choke, No Primer, 2 Needles
WA-78-1, No Choke, No Primer, 1 Needle (no high speed)
WA-8-1, No Choke, No Primer, 2 Needles
WA-85-1, No Choke, No Primer, 2 Needles
WA-86-1, No Choke, No Primer, 2 Needles
WA-92-1, No Choke, No Primer, 1 Needle (no low speed)
WA-96-1, No Choke, No Primer, 2 Needles
WA-99-1, No Choke, No Primer, 2 Needles

sides7

My Feedback: (9)

Just wanted to say THANKS for the work you did to post the info on the carburators. I printed it off as soon as I saw the post and have used it several times already. I'm sure there are a lot of others out there that did the same thing . THANKS!

Sides7

av8tor1977

My Feedback: (6)

Railridr, are you converting these motors as reasonably priced resale items, or as hot rods, and what size motors? The 11mm carbs cannot really be effectively fitted to the 25/30cc series without enlarging the manifold adaptor to the point where you get into the pulse hole. Then you have to rig an external pulse line to the carb. Also, for the 25cc motors, 11mm is just a touch big unless the motors are hopped up in other ways.

Just trying to help,
AV8TOR

Railridr

I am converting these more or less as part of an informal club project. There are a couple of other guys that are doing conversions as well. We are converting them all at the same time to save on shipping costs, and to get better parts prices. We plan on doing some modifications to the motors that we have, but will stop short of changing the intake, exhaust, transfer port timing, and machining material from the cylinder for extra compression. My list of planned modifications is as follows.

Modify Manifold to match carburetor venturi
Carburetor pulse pressure From crankcase
Bowman Rings/ pin rings in place
Machine Exhaust port bridge
Machine Intake port "nub"
Exhaust modifications, larger exhaust outlets on muffler
Stuff crankcase
Match and smooth transfer ports to crankcase
Contour transfer port area on piston bottom
Velocity Stack

we may also machine and balance the flywheels,
and remove the excess material from the crankcase.

I Live close enough to Peter Faith @ http://www.wackerengines.com that I can drive over and pickup prop adapters, Velocity stacks, and flywheels from him.

I know that there are better motors that are more powerful, and weigh the same, but we got these motors for a song and dance at one of our local repair shops. So far we are looking at about 70 dollars in parts per motor once we finish the conversions. The cost will be more if will machine the flywheels.


As for numbers, I cleaned up one of the long shaft motors and ran it with a 16 x 8 Master Airscrew classic prop, and got 6900 with the stock carb, and stock exhaust w/ baffles.

I installed a larger 11mm carburetor, a Walbro WT-21. It tached 7200 with a 16x8 Master Airscrew Classic. Everything else on the engine was unmodified. I was still using the stock exhaust w/ baffles. I also constructed a thrust stand to measure the engine thrust. I measured between 7.5 and 8 pounds of thrust with the 16x8 Master Airscrew Classic turning 7200 RPMS.

I sealed the exhaust "EGR" hole, and used some High temp silicone to seal the head gasket ( it was leaking), and saw 7400 RPMS with about 8.5-9lbs of thrust. I removed the exhaust baffles from the muffler, and I was able to obtain 7600 RPMS and about 9.5-10 lbs of thrust. I should also note that after removing the baffles, and tuning the carburetor a little, I was able to get a Nice 1400-1600 RPM idle. After this, I ran the Homie with an APC 16 X 8. This prop tached 7600 RPMS, and pulled 10-11 lbs of static thrust, and averaged 10.5 lbs of static thrust.

The engine I have been testing with has been used a lot. It started life as a blower in 1997, and has been run enough to wear out the piston ring. Once I got it cleaned up I could see that this motor has a lot of life left in it. The cylinder walls still look good and have no scratches. This motor will run really well when I finish my mods. All of the other motors that I have are in similar or better condition.

All of my testing so far was performed at 81 degrees air temperature, air pressure was 30.12" (1019.9 mb) and humidity was about 47 percent.

Have I mentioned that this is my first conversion experience. It has been a lot of fun so far. The only thing that is a drag is cleaning up these motors.

Attached are some pictures that I took of my test stand. I used scrap wood, some aluminum, a set of wheels/trucks from an old skateboard, and a digital fish scale. at 10 lbs the accuracy is +- about 6 ounces. I have since modified the stand by installing a piece of 1/4 wood down the middle between the wheels, to keep the carriage from shifting side to side. I have also lowered the aluminum rails to keep the carriage from bouncing up and down. Needless to say, the first test run without these mods were interesting.

w8ye

My Feedback: (16)

Just for the record, try a 18-8 on your test stand to see if the thrust will be greater? Also try the other brand of black prop.

Enjoy,

Jim

Railridr

I plan on testing a great number of props. The problem is I currently do not have $200 dollars to purchase all the different props I want to test with. At the moment I have to settle for the prop that I have on hand.

av8tor1977

My Feedback: (6)

Hi,

Nice work and testing procedures. If you are interested more in thrust than speed, an APC 18 x 6 would load the engine approximately the same as a 16 x 8, and would provide noticeably more thrust. A little trick that I use is to leave the base gasket for the cylinder out, and seal that area with Yamabond from the motorcycle store. That lowers the cylinder about .025" or so, and raises the compression a bit. You can also carefully take this amount off the piston top at the exhaust port area to re-gain the little bit of exhaust timing lost by lowering the cylinder. (That way you don't have to get into actually porting the cylinder.) On one of mine, with the basic mods you mentioned, this mod I just described, and a 10mm carb, I get 8200+ rpms on an APC 16 X 8 at 4500' altitude, 80 degrees, and 29.92 barometric pressure. (I should also note that I use CH electronic ignition and am thus able to optimize the timing.)

Good luck and have fun,
AV8TOR

SNIGGEMANN

My Feedback: (10)

I would agree with AV8TOR on the carb size for a 25cc. I had been trying to dial in a WT-324 (11.11mm) Walbro but was not having the best of luck getting good idle or transition. The high end worked great but it seemed like I just could'nt get it to run reliably. I did open up the carb adapter a little but probably not as much as it needs. I'd be interested in how Railrdr's come out after his mods. I changed carbs to a WT-318 (9.53 mm) and it idles, transistions and is good on the high end. I am running on a CH ignition and with a 16X8 APC I am able to get 8100 rpm. Keep us posted on how your progress is coming.

Railridr

Thanks for the tips guys. I am mainly using the thrust measurement to help document any gains with the modifications that I am making. We will see how well these motors run once I finish up the mods. My test engine runs and transitions well with the 11mm carb. That may change once I start modifying things. I hope that the 11mm carbs will work well, I purchased 20 of them at 10 dollars each off of the other auction site. If they do not, then I can use them when I start playing with some Ryobis and Ryans, that I found locally.

Railridr

Something else that comes to mind, Has anyone figured out what the optimum port timing should be for intake, exhaust, and transfer, that will provide maximum power, but without a tuned pipe? Most of the information that I have found is for motors that are running tuned pipes, and usually on boats.

av8tor1977

My Feedback: (6)

Hi,

120, 140, 160 degrees for Transfers, Intake, & Exhaust works pretty good for operation around 8000 and assuming you're not going to lug it down with too big a prop. For me, an APC 16 x 8 seems to be about optimal. For operation around 9000, you can add 5 degrees to each, but the motor will get a little more finicky. You have to remember though, that everything in a motor works together. If you don't mill out the squish band in the head and lower the cylinder to raise compression, you will definitely be hurting yourself. Of course, lowering the cylinder changes the port timing so do this first and then start checking and changing port timing. Also, use two outlets of 7/16" i.d. or larger in the muffler, with no baffles or other obstructions inside. Otherwise, it's like stuffing a big cam in a car engine without making other complimentary changes; it could run worse!! Optimizing the ignition timing is also important. I have read that some nice power increases have been found by advancing the timing on the magneto equipped Homeys. (I use electronic ignition exclusively, and it is easy to change the timing to get the most power.) With the magneto, you can do away with the crank key, and advance the flywheel. If you use a little Loctite on the taper, it won't go anywhere. Remember to carefully chamfer your port edges after porting to protect the rings and piston!!

As an afterthought, any work that the engine has to do other than turning the prop results in a net loss of thrust. I've never read any actual tests, but the little fan blades on the magneto have to be absorbing some power. If I were to run a magneto, I would take them completely off, though I know this is a hassle.


Good luck,
AV8TOR

captinjohn

AV8TOR Post #8 Can you explain the port timing thing. What about the intake port? Do you mill off the top of the piston there as well as near the exhaust port? If you mill of metal off piston top ...would that not lower compression? Do you have a photo of what this mill job would look like? Thanks Capt,n PS Keep up the good work

av8tor1977

My Feedback: (6)

Hi,

Ummm, yes and no.

Ok, let's think about this. The piston top uncovers (and therefore opens) the exhaust and the transfer ports. The piston top never goes below the intake port top, and has nothing to do with it's opening and closing. The piston skirt opens and closes the intake port. As the piston goes up, the piston skirt uncovers the intake port, and the fuel air mixture enters under the piston. As the piston goes back down, it closes the intake port, then compresses the fuel air mixture that entered the crankcase area below the piston, and when the transfer ports are opened by the descending piston, this mixture is forced up on top of the piston for later combustion. That's why they call them "transfer ports"; they literally transfer the mixture from the crankcase to the combustion area.

So, this means several things if you think about it carefully. If you lower the cylinder, you have of course also lowered the ports. So the exhaust and transfer ports, controlled by the top of the piston, will open later and close earlier. Conversely, the intake port, controlled by the piston skirt, will open earlier and close later. This is all because the piston is in the same position and going up and down as it used to, but the cylinder and ports are lower in relation, and this is how it changes the port timing.

Now what does this all mean? Well, since the intake port is now opening earlier and closing later, it has effectively increased timing. You may find that after you lower the cylinder, that the intake timing is very close to what you want with no modifications. Lucky huh?

The exhaust and transfer ports however, are a different matter. Since they now open later and close earlier, their effective timing has been reduced. You therefore have to work on them, (or the piston top) to recover the lost timing, or to increase it to whatever your intended specs are. It is definitely "cheating" a bit to chamfer the top of the piston at these ports to get your timing and I really only recommend it if you can't, or for some reason really don't want to, alter the port itself. You are right Capn John, it does lower the compression just a bit. Not a lot, but every bit tends to count in these little motors. Cutting edge racing tuners sometimes do this to fine tune their porting numbers, since it is a lot cheaper to replace a piston than a cylinder if you go too far with your port timing. Once they get their porting dialed in, they can then port their actual cylinder ports to those specs, and install an unmodified piston.

However, sometimes we cheat anyway. What I have done, is paint the piston top ring land at the exhaust port area with dykem. (A magic marker will work.) Then reach in through the exhaust port and gently mark the port sides on the piston top ring land. Now you know where you need to modify the piston. Then mark down from piston top the distance that you would like to "modify the port", and file or grind a chamfer down to this mark, the width of the exhaust port. I don't go back much farther than about 1/8" or so from the edge of the piston towards the center on these small motors. I really don't particularly recommend this procedure, as you have now lost some compression, and weakened the piston just a bit at a high temperature area. If done, I would only do it enough to restore the timing lost by lowering the cylinder a bit to gain compression; not for serious port timing changes. Now, having added those caveats, I will say that I have done this on a few motors large and small, and never had a problem.

Man I hope this all makes sense. It's a little difficult to describe without more words, and diagrams.

Good luck and hope I helped,
AV8TOR

captinjohn

AVATOR: Very well explained. Just thinking about the compression....is it realy that important to raise it? Is the gain that much? It would seem that it would be a lot less trouble to mill the head. Or replace piston with a high dome to get more compression?

captinjohn

Another thanks for all the Carb data...very usefull!!!

av8tor1977

My Feedback: (6)

Compression is very important, especially to complement the other changes. Raising compression, as long as you have the fuel to run high compression ratios, is one of the most effective modifications you can do to an engine. (Note that with these small bore engines the fuel octane rating is generally not a problem.) It is so important to efficient engine operation, that the automotive manufacturers went to great lengths (and expense), to design fuel injection and computerized anti-knock ignition systems to be able to get the compression back up in car engines to increase efficiency. They did it to be able to run high compression again with todays junk gasoline, after having to lower the compression to ridiculous levels when the gas went to ##$%&*@ in the early '70's.

High compression is what we call "all good". Most other engine modifications tend to be compromises. Increasing intake and exhaust event timing makes the engine lose low end torque. Same with larger carbs, in addition to "soggy" running characteristics at low rpms. But high compression increases power and efficiency at all rpms. Better fuel mileage, more power, and if you have good fuel, no negatives! When you modify an engines port (or cam) timing, as I said, you lose low end torque. You can get a lot, and sometimes all of that low end torque back with higher compression, as well as more power at the top end.

If/when I modify an engine, setting a good compression ratio is the first part of the plan. For a mild modify, I do exhaust, carb, and compression on these engines. Then, and only then, if I want more I start porting and changing port timing.

Hope this helps,
AV8TOR

The Raven

OK, I know this is an older thread but can anyone advise on what is the ideal carb choice for a Homelite 25cc?

I currently have a ZAMA carb and have removed the mixture restrictors (alas no high speed needle), bored the carb out slightly, blocked the throttle plate holes for lower idle, removed the choke, minor port cleaning etc.

In this configuration the engine runs fine but appears to lean out at full throttle (about 6800rpm), choking the carb helps [presumably increasing vacuum to the jets] giving 7500rpm. Ideally I want a carb with a high end mixture and primer bulb that will give me all the power the engine can reliably give.

Can anyone with 1st hand experience recommend a Walbro carb to suit? Note that I am in Australia and will probably need to source one from the US (ebay perhaps) and cannot afford to buy the wrong carb.

w8ye

My Feedback: (16)

Any of the 11mm venturi Walbro's in the list at the beginning of the thread would be great on a Homelite 25.

The Standard for several years was WA167-1 Walbro for a Homelite 25.

Enjoy,

Jim

ryan@ddm

The WA-167 is an excellent choice.

rollmyown

Hi Railridr,
I just want to add my thanks for your effort to post the carb specs, I also printed them.
John

The Raven

Thanks for the confirmation guys, I am specifically considering the WA-167. My main concern revolved around a few comments which hinted that 11.1mm sized carbs may be a little too large for the 25cc Homelite.

Now that it's confirmed the WA-167 is OK I will go ahead and place an order for one.

Thanks for the advice.

Moondog express

My Feedback: (18)

Can anyone advise me on what is the ideal carb choice for a RCS 180

I currently have a Walbro-225a carb and have had nothing but problems. I´ve had this engine for 2 year and have sent it back to have it checked, they say their isn´t a problem. I would like to replace the carb with a new one or another brand.


Can anyone with 1st hand experience recommend a Walbro or another brand carb to suit? Note that I am in Mexico and will probably need to source one from the US (ebay perhaps) and cannot afford to buy the wrong carb.

The Raven

Hi,
I have no idea what an RCS180 is beyond 30cc's. In which case a WA-167 is probably what you want. I've used the WA-167 on both a Homelite 25cc and a Ryobi 31cc (soon a 30cc Homelite). It makes all the difference! For a Ryobi, you will see at least a 800rpm improvement (with cooler running and better tuning) than the stock WT-275. For a Homelite 25cc the revs increase a full 1000rpm over the standard ZAMA carb (with cooler running and better tuning).

If you want to buy a WA-167 I will recommend an ebay seller [link=http://stores.ebay.com.au/Captain-Never-Sails-Engine-Service_W0QQssPageNameZstrkQ3amefsQ3amesstQQtZkm]Captain Never Sails[/link]. I have bought at least three carbs off him and he is a very reputable trader.

Crusty

Gotta gongratulate your ingenuity Railridr....... yet another use for a skateboard