RCSPORT FLYER

My Feedback: (11)

Does anybody know how much more power the Honda 35 has compared to the 31? I'm wondering if it would be worth trying. I have the 31 turning about 6600rpm's with an 18/8 prop.

tim220225

My Feedback: (4)

My 31 runs about the same. The 35 will turn a bigger prop and weigh a little more. I am still looking to work over a 35 but have not found one.

subarubrat

My Feedback: (1)

I fly both and I would describe the 35 as more of a workhorse and the 31 as a bit more responsive. The 35 will spin a 20x8 prop, more than the 31 which is happier with a wide blade 18x8 but it revs easier. What plane are you building?

RCSPORT FLYER

My Feedback: (11)

I'm building an SE5a from FunAero plans, blown up to 90 inch wingspan.Weight will probably be around 20 Lbs. more or less.I was hoping to use a 20 inch prop,but I haven't tried one on the 31 yet. I know I'll lose some rpm's, but should gain some thrust, hopefully.

subarubrat

My Feedback: (1)

The 31 isn't happy with a 20 inch prop, your better off with the 35.

av8tor1977

My Feedback: (6)

RCSPORT FLYER did you do any mods to your 31? What carb are you running?

Thanks,
AV8TOR

RCSPORT FLYER

My Feedback: (11)

I changed the carb to aWYK-38-1,enlarged the intake and exhaust ports and polished them.I haven't gained one rpm by doing this. Kind of a puzzle.I'm using the APC 18/8W prop. Around 6600 RPM in cool weather. Hot weather seems to really slow it down.

captinjohn

If that Honda is a 4 cycle....you may have to build or buy a high lift cam! Capt,n

Ralphbf

My Feedback: (27)

Just a quick note.

We use to remove material off the back of the cam lobe in our B&S go cart engine.
Dad was a Ford mechanic and he sanded the the non lobe side of the cam and re polished it with copus cloth.

He said there was very little pressure on that part of the cam and it would last as long as the engine would.

In retrospec it simply gave more lift and quicker cam timing.

You should be able to find out how much lift your engine could take before the valve spring bound up
and go from there.
Remember about the rocker arm ratio.

av8tor1977

My Feedback: (6)

Now why didn't I think of that.... of course, reduce the base circle diameter of the cam, and you will gain lift. Neat idea....

Thanks,
AV8TOR

davewallace

My Feedback: (29)

Flyer,
Here is a thought. If the GX31 piston is taller above the wrist pin then the GX35, the rod must be longer then the GX35. Due to the makers use of standard needle bearings, the longer rod would have the same bearings. So, put a GX35 rod into a GX 31 and get higher compression. If this is true, you may have to notch the piston slightly for valve clearence. Who would know?

Dave

tim220225

My Feedback: (4)

Guys here is what I know about the pistons etc. For starters here are the bores and strokes for these engines:

GX25 33mm bore 26mm stroke
GX31 39mm bore 26mm stroke
GX35 39mm bore 30mm stroke

The 31 piston needs to be fly cut on top to fit the GX35. The area above the top ring is taller or thicker than on the 31 piston. What you will do is increase the compression ratio. Stock is somewhere in the 8's as I recall. There is a formula I have somewhere but suffice it to say that you can get 10.5:1 safely with this mod and the engine will run fine. The issue is a coating of some sort on the pistons. You can break through it with a fly cutter, however no one has been able to say how if at all eliminating this coating will affect piston longevity. I talked to and got this information from a guy on the East coast that builds these engines for boats. He claims to get in excess of 10,000 RPM from these engines. He also produces cams for them. When I talked to him last I decided that for my use the GX31 I have would not gain much by swapping cams. Maybe with the piston mod on a GX35 and adding a cam you might be on to something. Now if I could only find a GX35. They have them on Ebay now for $225.00. By the way when you do these engines remove the compression release crap from the cam. I ran mine with a CH ignition and it would idle so slow the compression realease caused issues. Removed it and it runs great.

CricketGlider

Tim,
Who is it that builds these engines? I regularly get my gx 35 upto 11,000 rpms attached to my belt drive bicycle kit on the downhill side of a bridge that i ride every day. If I hold the trottle wide open with the rear wheel in the air I can get my tachometer/ hour meter to show over 10,000. The engine is stock. I removed the decompression system a while back due to starting issues and runs much better now. I also tapped the exhasust threads to a use 6 mm bolts instead of the the 5mm studs.

av8tor1977

My Feedback: (6)

Yeah, there is some combination of GX 31 and GX 35 parts that yields higher compression and more power, but I don't know what it is.... There was a guy doing cams for them too, but last I heard he wasn't doing them anymore. Check the boat forums. Those guys are real into hopping engines up and are very creative. They don't mind blowing a few engines up trying to find a good combo that works.

But if you really want to "rock" with a four stroke, try the Stihl Four Mix engines. There are several sizes in the 32 to 36cc range, and a brute of an engine; their Four Mix 65cc. Believe it or not, it makes about the power of a Zenoah G-62 engine, but with the sound and fuel economy of a four stroke. They are also a good bit lighter than the Honda engines, because they use a fuel/oil mix like a two stroke, and do not have a heavy crankcase containing oil and an oiling system.


AV8TOR

Mono

I've been using a modified GX35 for 3 or 4 years, it turns 20x8 master at around 7100RPM in a 104" glider tug/lolly bomber and has plenty of power.
I used a GX31 piston and machined the top down to give it about 10.5-1 compression, filled and reshaped the inlet port and unshrouded the valves, removed the compression release in the cam, gutted a GX25 muffler, installed a RCXL ignition and fitted a walbro carb from Dave's Discount motors that has the next size up bore, 10.5 mm I think.
A great little project, but not cheap!

ktan73

Very cool project now you have a great Engine that purrs nicely & with good milage too .
Did u make a prop flange for it or is there a shop that sells them ?
I heard that the stock card doesnot like inverted flights . What u did with yours ? point me to where i can buy a new carb so i will fly high above 2000m without caughing alot .

Ken

captinjohn

Mono, do you have any video fo that GX35 so we could see & hear how it runs? Is that a rotary valve carb? I like that primer bulb too. Thanks Capt,n

Mono

Hi captinjohn & ktan 73,
I don't have a video but I'll try and post one this weekend.
The carb came from Dave's Discount Motors in the USA, it's the same configuration as the original but has a slightly large bore and 2 mixture needles, it is a rotary valve type, Walbro WYK-194.
The priming bulb is great, just keep pressing it until there are no bubbles in the return line, plug the bell mouth and flick until it fires, one more flick and it just starts and idles.
I made all the brackets and adapters for the prop hub and RcExl ignition.
It's performance is very deceptive, it never sounds like is straining and with the 20 x 8 master will climb out at 45deg, the aircraft has 104" span and weighs about 10Kg.
Don't ever let anyone tell you that these are quiet with an open pipe, with the compression raised and ports cleaned up this thing has a very loud crackle to the exhaust and does not perform as well as it does with the muffler.