i've converted a Ryobi 31cc to go on my 80" SPAD but have a small problem. The engine will start and run great but after it runs for a few minutes the crank will move into the case about 1/8" causing the prop adapter to bind against the front of the engine. This is a long shaft and I have put the crank in a different case and it did the same I only have one long shaft crank so was not able to swap it.
Any ideas or suggestions.

Specs:
Long shaft Ryobi excellant compression
Walbro 324 carb
CH EI prop adapter
RCexl ignition
JAG muffler
JAG mount
MAS 18x8 prop getting 8000rpm when running

Thanks
Rex

The prop adapter must seat tight against the taper and/or the inner race of the front bearing without touching the case itself

I've checked the prop adapter several times and it is seated. I have machined the back of it to give it extra clearance. With the crank seated tight against the inner bearing the adapter has better than 1/8" clearance. After it runns for a few minutes there will be a 1/8 or better space between the crank and the inner bearing.
Thanks
Rex

Sounds like the crank fit in the bearing is a little loose.   This is a bit strange, as usually on Ryobis they are a bit too tight.</p>

You will probably have to machine a groove in the crankshaft for a circlip to prevent the crank from "walking".  Be careful, because if it walks back far enough it is going to smack the reed valve stop and make a mess....</p>

AV8TOR</p>

I'm confused. It seems to me that if the hub is locked down on the inner race of the front bearing, then the hold bearing has to move aft.
This to me means that the outer surface of the front bearing is too loose in the block and it is shifting back, and the locked in crank with it.
NO ?

The ryobi has a seal out front with a bearing behind it ( unless I have identified my engine wrong ) if it had a front sealed bearing you could bulid the prop hub to set against the inner race.  I'm going to pull the engine back apart and clean everything then put it back with a little loctite on the inner race and reassemble it. If that does not fix it I will cut a groove for a circlip and solve the problem.
The engine runs to well when it don't have a problem to junk it out.
I was able to get 8000rpm with a 18x8 MAS which I think is reasonable.  It should pull my 14lb SPAD around better than the wore out  weedeater engine I have on it now, it has little compression left but runs.
Thanks for the help any comments,ideas or observation are greatly appreciated

Rex

I have one crankcase and crank assembly that seemed to do the same thing, especially whenI  used an electric starter, but at other times also. I used some rough emry on both the crank, and inside the bearings, and then on the housing for the bearing, then used indutrial strength sleeve seal (similiar to thread locker) and to date, the bearings and crank have stayed in place.  Worth a try. The sealer is available at most Auto parts stores.  Flathead

You could be on to something I have been using a electric starter.  Thanks for the tip on the sleeve seal

Rex

Use JB weld OR make a spacer that will fit between the inner bearing race and prop hub. That way the crank can't move back.

8000 rpm on a mas 18x 8 If this is a stock engine im sorry im not buying that. What have you done to this engine? any mods? be specific.

I'll have to check the RPM's again but that is what I read. I've been playing with these toys for 30 years so my Globee tach has been around a while it could have personal problems....

My Ryobi has a stock "20" carb with opened muffler and it does the 18-8 at just above 7000

Give us some details on how this engine is built up.

What? "20" carb ?

Ipicked up two 31 CC's at the City recycle center today. They havea Walbro WA Carb with a "20" clearly stampted on the carb. The Carb bore looks to small to me though.
I plan on messing with them this summer.

The numbers cast into the throat of the carb is the venturi size in 64's of an inch.</p>

20 = 20/64" = .32125" = 7.9mm venturi.  A very small carb.  The weedies come with small carbs and often low port timings so that they can be run at full throttle and even if the string breaks they won't rev high enough to hurt themselves.</p>

The Ryobi responds well to an 11mm venturi carb, which has a "28" casting number in the carb throat.  A "26" carb will work fairly well on a Ryobi too if you happen to have one of those.  I use 11mm.</p>

AV8TOR</p>

</p>

</p>

Now thats some good info to know. i had no idea what he was talking about. With that size carb the 7000 rpm sounds about right. If you get a 12.7 mm carb i think you can expect at least 300 more rpm and that number is kind of conservative.

But only if he opens up the exhaust as well....  A bigger carb seldom helps much if the muffler is restrictive.</p>

AV8TOR</p>

So far it looks like I have it fixed. I cleaned all surfaces with contact cleaner and put the bearings and crank back in with locktite was able to run it for about 15 minutes with no problems. I'll find out for sure when I get back home I had to come to Shreveport to work for two weeks so I can get more money for toys and pay a few bills.

Thanks
Rex