n5lp

n5lp

fugio ergo sum
My airplane has a carburated O-360. Once in a while, in the last few months, there is an issue after the first start of the day. The fuel pressure goes high, around 7 or 8 PSI, and the engine runs very rough at idle. I think it would die, but I can keep it going by leaning to near idle-cut-off.

During and after the run-up things are much better and the engine always runs great at higher power settings. For the rest of the day there is no further problem.

I'm thinking this is a mechanical fuel pump problem but I don't know how the pump regulates pressure. This weekend, at a fly-in breakfast, I asked two different mechanics about the issue and they didn't seem to have any idea.

Do I just need to replace the pump or is there something else I can do? I have checked that the vent line is clear.
 
Larry,

Make sure when you test run your engine as you work through this problem to keep an eye on your CHT's so you don't glaze your cylinders.

Your carb should have been flow (bench) tested and the mixture should be set correctly. If this is not the case, check with the company you got the carb from.

It is possible that you need to have the thing adjusted. Worse case, it is around $600 to have it overhauled.
 
N941WR said:
Larry,

Make sure when you test run your engine as you work through this problem to keep an eye on your CHT's so you don't glaze your cylinders.

Your carb should have been flow (bench) tested and the mixture should be set correctly. If this is not the case, check with the company you got the carb from.

It is possible that you need to have the thing adjusted. Worse case, it is around $600 to have it overhauled.
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I should have mentioned that this engine has about 1,000 hours on it, all in this installation. CHT is not an issue as this only happens after a cold start and only while at idle or near idle. This is an occasional problem only, which makes me think it has nothing to do with adjustments; wear, maybe.
 
If 8 psi is flooding the carb then it's time to rebuild it, needle/seat/float assy replacement should do it. Check the new SB out on these so you get the latest and greatest parts for it. The fuel pump sounds ok to me, 8 psi is within limits.
 
Walt said:
If 8 psi is flooding the carb then it's time to rebuild it, needle/seat/float assy replacement should do it. Check the new SB out on these so you get the latest and greatest parts for it. The fuel pump sounds ok to me, 8 psi is within limits.
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I just checked my notes. The OH for my MA3 was $700 in December of 2005 and it was done by AVstar.

The thing was bolted on and worked as expected. 200+ hours now and no issues.
 
Only just...

Walt said:
If 8 psi is flooding the carb then it's time to rebuild it, needle/seat/float assy replacement should do it. Check the new SB out on these so you get the latest and greatest parts for it. The fuel pump sounds ok to me, 8 psi is within limits.
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...the usual specification is 0.5 psi to 8 psi at the carb. inlet. So the 8 psi is exactly at the upper limit.

If the guages are not very well calibrated, this could easily be above the limits....

However, at 1000 hrs, a new seat/valve assy would certainly do no harm...:)
 
Thank you all

It seems the consensus is that I need to do some carburetor work. Now I have to decide whether to learn about it and do it myself or send it off to a good shop.

Apparently the mechanical pump doesn't really have a regulator but by design should only be able to put out so much pressure?
 
n5lp said:
It seems the consensus is that I need to do some carburetor work. Now I have to decide whether to learn about it and do it myself or send it off to a good shop.
Click to expand...

Larry, unless you have a flow bench to test the carb you should send it in.
 
I don't think there's anything wrong with your carb. If there's too much fuel pressure certainly fuel will leak past the float needle. It wouldn't hurt to remove the bowl and check the float needle though. There could be some junk on it. But before I would do that I would suspect the pump. 8 psi is high for a pump with a thousand hours on it. There are two things that can affect the mechanical pump pressure. 1. The spring, and normally over time it looses its stiffness and thus pressure, and 2. the venting of the pump to the crankcase. Plug the vent and the pressure goes up. Or, plug the breather and the fuel pressure will go up.

I would pull the pump, take it apart, and clean it out, and maybe give it to an A&P who has done this before to do because there's a trick to disengaging the plunger arm to take it apart. Or I'd borrow a pump and see if your problem goes away.

1-3 PSI is normally what I typically see with diaphragm pumps. 8 psi is definitely on the high side of the median, and again, especially for one with a bit of time on it.
 
valve problem?

First flight of the day, cold engine, makes a valve problem come to mind. Not sure why leaning would help or aggravate problem. Perhaps 80psi air into each cylinder, 1/time, cold, and listen, might be a place to start?
 
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