Carb Help

[sophistical]

Hello! I hope this finds everyone en route to Oshkosh instead of facing a week of work like I am. (I do hope to take off on Friday to get up there).

I have a '57 PA22/20-150 with a narrow deck O320. It's been running a little rough around 1500 RPM meaning it will stumble a bit when passing through that RPM range. (Steve, if you read this you will recall this when you flew my plane in February; then I think you assessed that it might be the carburetor jetting.) I asked my A&P to go ahead and have the carburetor rebuilt; he agreed and sent out for an overhauled carb. Apparently I had a -11 carburetor on there (should we hang on to that; I guess they are long gone?). He got a nice new -12 and the same problem exists; in fact it's worse; there is a pronounced stumble between 1500 and about 1900 RPM. It's no fun if you need to goose the throttle when coming in for a landing. He asked around and did some digging and came up with SB258 (link) but he didn't have time to remove everything to see if my sump has the taper as described in that article. He wants to try the -32 as prescribed by that bulletin. I do not want to go that route as I understand the -12 is more efficient than the -32 from reading on this forum.

Anyone else have this problem? I'm hoping the answer is that I need the insert and then the -12 will work fine but would like some assurance. Again, we don't know if it has the taper at this point and we have to tear the plane apart again to figure that out. Also; is the -11 carb worth anything; I'd never heard of it before.

[Gilbert Pierce]

I've had a -12 for 15 years. Never had a problem like that. Sounds like float level wrong to me.

[Steve Pierce]

I would eck the sump, no nd for the -32 from my exrience. The carburetor overhauuld be able to rectify this probm.

[Lownslow]

That rpm range is the transition from the idle jet to the main jet. Just curious... what fuel are you using? If auto fuel, is it fresh fuel? Have you had your timing checked recently? Have you verified there are NO intake leaks?

Lou

[sophistical]

I'm using auto fuel but it's been going on for months no matter what fuel I use (I purchase it from different places and burn 100LL as well.) My A&P checked and adjusted the timing slightly at Annual (when this carb was installed). Good call on intake leaks .. I'll remind my mechanic to check for that. I'd hope the float is good on a new carb but since the problem has existed on 2 carbs I'm thinking it's something else. So intake leak or possibly the tapered sump. I'm just trying to narrow it down without spending too much time and effort; took a 5 weeks to get the carb and annual done; don't want it down for another several weeks; I prefer to spend the summer flying!

I did find at least one post on this site where someone found that fixing the taper improved the smoothness of the engine. I'm hoping that's the problem but we won't know until we tear into and then I'll have to have the piece manufactured or source it somewhere.

[Steve Pierce]

My recent experience with intake leaks is that the run rough except at full throttle.

[Pacerfgoe]

Have you made any adjustments to the accelerator pump throw? There are 3 different setting you can use, maybe your not getting a big enough shot of fuel. It's easy to check and adjust.

[Lownslow]

It seems there is something other than the carb that is causing your problem. I think you are on the right track, just focus on the simple things first, then work to the more expensive. You may even check your primer possibly allowing air into the intake system. I am curious to what you will find. Best of luck.

Lou

[mmoyle]

To bad the automotive trick to check for vacuum leaks....isn't a good idea. Used carb cleaner, sprayed at the base of the carb, manifold....if that didn't change the idle RPM....start spraying the vacuum lines. Maybe your oil pan/induction needs new paint? Good excuse to change out the gaskets, clamps and hoses....


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[richas]

To bad the automotive trick to check for vacuum leaks....isn't a good idea. Used carb cleaner, sprayed at the base of the carb, manifold....if that didn't change the idle RPM....start spraying the vacuum lines. Maybe your oil pan/induction needs new paint? Good excuse to change out the gaskets, clamps and hoses....


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It is better to use Propane. You can use a small torch with the burner removed and a piece of rubber hose attached. Be advised you are releasing a flammable gas... Was my technique of choice when I was a BMW tech in a previous life.

Rich