Thorp Air Command - T18.net :: View topic - New thorp owner, would like to learn more!

The proper carb for a 125hp 290 is a 10-3346-1. For a 135hp 290d2 it is model 10-3565-1. The only difference between the two carbs is the nozzle. The carbs metering tube is called a nozzle instead of a jet like is normal terminology on autos and motorcycles. There is only one nozzle in the carb, not multiple ones. The nozzle number is 47-719 for the 3346 and 47-728 for the 3565.

If the engine is running rich, I would first check the float level with the clear tube method. If that is correct, you probably have too large of nozzle. Unlike an auto or motorcycle, your carb is equipped with an infinitely adjustable nozzle. It is adjustable with your right hand on the mixture control. I would fly full rich, wide open at low altitude and note egt (don't worry about it being too high as the actual number is irrelevant). Then lean to peak egt and compute the difference. This will tell you how rich of peak you are with the enrichening circuit active. You should see 100 to 150 degree difference. If more than that, you have too much fuel flow at full rich mixture. You could just live with it and pull the mixture to get the proper egt for whatever flight regime you are in, or spend some time to figure out why too much egt spread.

If you think the airbox is causing problems, remove it and do a runup without it to see if problem goes away.

Then check that the right carb is installed. If the model number matches the engine, that is still no guarantee that it has the proper nozzle installed or that it has not been drilled to a larger size. If the correct carb model, I would check that the correct nozzle is installed. If the correct nozzle number, I would check the nozzle id with a numbered drill bit. The correct id for the 47-728 nozzle is #42 drill bit (.093). I don't have the size for the 47-719 nozzle (would obviously be something smaller than .093).

I wouldn't chuck the carb if it is not worn out (excessive throttle shaft clearance, leaking accelerator pump, etc). This should be a fixable issue.

Author:	Binder [ Sat Oct 29, 2016 5:36 am ]
Post subject:	Re: New thorp owner, would like to learn more!
thorpdrvr wrote: The proper carb for a 125hp 290 is a 10-3346-1. For a 135hp 290d2 it is model 10-3565-1. The only difference between the two carbs is the nozzle. The carbs metering tube is called a nozzle instead of a jet like is normal terminology on autos and motorcycles. There is only one nozzle in the carb, not multiple ones. The nozzle number is 47-719 for the 3346 and 47-728 for the 3565. If the engine is running rich, I would first check the float level with the clear tube method. If that is correct, you probably have too large of nozzle. Unlike an auto or motorcycle, your carb is equipped with an infinitely adjustable nozzle. It is adjustable with your right hand on the mixture control. I would fly full rich, wide open at low altitude and note egt (don't worry about it being too high as the actual number is irrelevant). Then lean to peak egt and compute the difference. This will tell you how rich of peak you are with the enrichening circuit active. You should see 100 to 150 degree difference. If more than that, you have too much fuel flow at full rich mixture. You could just live with it and pull the mixture to get the proper egt for whatever flight regime you are in, or spend some time to figure out why too much egt spread. If you think the airbox is causing problems, remove it and do a runup without it to see if problem goes away. Then check that the right carb is installed. If the model number matches the engine, that is still no guarantee that it has the proper nozzle installed or that it has not been drilled to a larger size. If the correct carb model, I would check that the correct nozzle is installed. If the correct nozzle number, I would check the nozzle id with a numbered drill bit. The correct id for the 47-728 nozzle is #42 drill bit (.093). I don't have the size for the 47-719 nozzle (would obviously be something smaller than .093). I wouldn't chuck the carb if it is not worn out (excessive throttle shaft clearance, leaking accelerator pump, etc). This should be a fixable issue. This is excellent information! I have a lot to learn with respect to aircraft engines. Mechanical is mechanical but there are quite a few differences that I'm still learning about. The carb model is correct for my plane but I suspect just as you mentioned something has been changed on the inside. I need to get an NPT nipple and clear tubing to check that float height. I was trying to get the carb back on to do some more flying with an instructor before the winter sets in. This winter I'll tear back down and go through the nozzles to see if it's incorrect or has been altered. Knowing the proper nozzle part number is helpful as I can just order that and try it out. When testing the float height does fuel pressure make a difference? IE: Does it matter if I test it on the plane with it fueled up verses on the bench with a fuel bag? I know sometimes that fuel pressure can make a difference on carbs. Upon inspection there was no shaft play on any of the surfaces and everything looked in great condition. Based on logs there is little time on this carb since overhaul although it has been a number of years ago.

Author:	Bill Williams [ Sat Oct 29, 2016 5:48 am ]
Post subject:	Re: New thorp owner, would like to learn more!
Here is another thought. Check the induction tubes and flanges for leaks. This is a service problem with Lycoming engines after a period of service

Author:	Binder [ Sat Oct 29, 2016 6:03 am ]
Post subject:	Re: New thorp owner, would like to learn more!
Bill Williams wrote: Here is another thought. Check the induction tubes and flanges for leaks. This is a service problem with Lycoming engines after a period of service That was the first thing the local AP had me check. There was no obvious leak but when I put the plane away for the winter I'm going to replace the gaskets (or seals, whichever they are) on the intake tubes just in case. He said there could be some small leaks that might not be as obvious although he said it most likely wasn't a leak at this time.

Author:	fytrplt [ Sat Oct 29, 2016 6:34 am ]
Post subject:	Re: New thorp owner, would like to learn more!
Leaks at the cylinder to intake tube gasket make for a lean condition. Leaks here are common as the tube rather "floats" in the collar and wears on the paper gasket.

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