Some guys cut the hook off the return spring to loosen it, its still needed to hold the cam down on the pin.Or use a high torque servo that has standard size case.Futaba 3305 has plenty of torque to over come the spring.
Nothing runs as good as the WY type of carbs on the four strokes, the WA and WT will run, but not as good.Throttle response will not be anywhere near as good as the WY with the WA or WT.
The butterfly type of carbs also require new manifold to supply pulse for pump and the large throttle bore needs to be transitioned into the port diameter over a distance long enough to not have the taper angle in the manifold bore too steep or fuel seperation will occur.I've seen WA 167s dribbling fuel out of carb at idle because of this.

If you already have the WYK, I would cut the spring as mentioned elsewhere. It is not a super fun job but it can be done. I've done @ a half dozen of them.

J

I removed the bent over end at the top which made very little difference so I then removed the other bend being careful to keep the ends smooth to help them slide. The spring still loads up and gives resistance so I found no benefit.

Briggs has very poor customer relations. I've had a very bad experience with them. Do yourself a favor and avoid them. Today's B&S is not the fine company they once were.

something is wrong, unhooking it lets off a lot of tension.

The cut off ends are probably catching. I put a slight bend inwards at the ends. I also cut off a loop or two, there is still plenty of spring.

I experimented with the original carb and naffed it up by cutting off a coil, for some reason it jammed it up! Stil gotta get a prop adapter made, I have a mate with a lathe but I have no materials other than a 16" length of 3 3/4" ali, a right sod to cut and a lot of material to remove to!. I don't like the idea of a basic shaft with a hole tapped the size of the crankshaft thread and attaching with thread lock. I would rather machine out the centre where the shaft nut goes to a circle, machine off the face and 2 original tapped holes which the clutch was attached to then fit a Zenoah type prop adapter not using the crank thread at all. The machined central hole with keep things central and the 2 8mm tapped holes will hold the adapter on if you see what I mean.
I'm sure I can overcome the throttle issue with a fiddle, just been attending to other things for the last few days and I also have a little 22cc 2 stroke I'm converting to

I have found that on the WYK, that it is best to leave the spring complete. Just lift it out of its notch and unload it to almost closed. Than carefully cut a new notch. Try not to disturb the area where the seal is.

motorhd

TBH I still don't understand why a normal Walbro cannot be sued, I have spoken to my local garden machinery place and they say they all work basically the same. I have also picked up a couple of small 2 strokes that use the WYK type carbs so if they will work on a 2 stroke surely a normal Walbro will work on a 4 stroke? Much easier to get hold of a Normal carb which has H & L jet adjustment.

I plan on trying some various Walbro carbs on my 34 once I get the conversion finished. A friend of mine has already converted his (yet to fly though) and it hand starts beautifully. Only downside is the low revs. I'm certain a revised carb and intake will make things more compact and add a few RPM.

Can anyone advise as to a safe upper rpm limit for these engines?

I'm sure I read somewhere that used in boats they are revving to about 10,000 but don't quote me I have tried a couple of Walbros but could not get them to run right but I suspect I have an issue with the carb I tried which was a 10.5mm bore and has the H & L needles, I could not get it to tick over, perhaps a velocity stack may help the throttling but not sure about tick over.

The Briggs engine...

I work for a briggs dealer and have had too many failures. The plastic cam drive gear on the crank is very weak, had them fail on the 1st tank, the best part is the parts!! GOOD LUCK...

Sorry I hijacked the thread, I was talking about the Honda GX31 conversion.

The boat guys are running Hondas with a full support crank, Honda has the engine rated for 10,000 from the factory. With the cantilever crank the B&S shouldn't run over 8500.check B&S recommendations.

The honda gx 25 series, now thats a better choice, havent seen them blow except from abuse...

Have a SPAD bipe in the works that will run the 35..

So what wingspan and weight/type aircraft are you guys using your BS, Honda etc 4 strokers in? Ther has been debate on another forum that says they don't have enough hp to be of any use, I have been defending the Honda GT31 as it appears to be the best Honda weight for hp IMO. Any of you guys got pictures of your 4 strokers in your airframes?

Is there a replacement alloy drive gear or, does anyone have the know-how to produce better gear drives? If a better part can be used I'd rather put it into the engine before it flies.

If anyone is still looking at this thread, here is a couple of pics of my recently completed plane with Honda GX 25...
I had the same issues with the carb return spring, every time I tried cutting the hook off, I ended up with a spring that wouldnt coil tight enough to fit in the space above the barrel. Did any of you guys remove the barrel to cut the spring, if so, how did you do this ?
You may see the added plate ofer the carb, this was to hold a spring on top of the barrel.

Certainly am watching and with much interest. Nice to see the Honda in an airframe, how does it fly? What's the noise like with just a header pipe? I have changed my carb to a normal type Walbro and it's transformed it, I just had to drill the manifold to fit a pipe to the carb for the pump and it works a treat. I forgot to mention mine is the GX31.

I is pretty low on power but flies the 14lb model ok.
noise is low and sounds great, idles at @ 1300rpm with a "put put" sound !!
Which carb did you use ?, the carb on it is a walbro but the return spring issue is a real pain in the a**.

That sounds promising, I intend to put my 31 in a 1/4 scale SE5a. I cant tell you which carb I used, it came from my local garden machinery shop off a scrap strimmer, it's at my mates right now having the prop driver made so I cant just nip out and get numbers for you. I'm hoping to throw a 18 x 6 wood prop at about 7,000rpm with any luck. I will report as soon as I have run it with the newly made driver.

My 25 pulls a 16 x 8 at @ 7k, tried an 18 x 6 but it goes down to about 6.6k and doesnt appear to pull as well.
I heard the 35cc will pull an 18 x 8 to 7k with no problems........

If you look far enough back you will see that I have a couple of the 34cc B&S engines and one 40cc B&S engine. I used a 34cc on my Fokker DVII (for about two years) and it flew fine...a little underpowered but if you have plenty of space to climb out it is fine...great power once it is run out in the air for loops, etc. The DVII is 22lbs (1/4 scale). I bumped that plane up to 40cc B&S (Thanks Jtisch!) and it has excellent power and flight now, pulles right off the ground and up into the air. The 40cc is no heavier than is the 34cc. I use the 34cc on a Fokker DVIII (BalsaUSA) and another on my 1/4 scale Bud Nosen Aeronca champ. The 34cc flies both planes excellent. The best thing is, you can mount them inverted! A buddy has the Honda 31cc on a BalsaUSA Fokker DR1 and it is an amazing flier. I would post flight videos but the host that I was using disappeared (Putfile.com)

Putfile did itself in,they went offline a couple months ago. Why not reload your vids to You Tube?

I would...if I knew where I saved them!!!
Saved them so good I can't find them now, lol