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Rough Running

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Alistair
Posts: 113
Estimable Member
Topic starter
 

Sid has sat most of the last half of 2018 due to circumstances beyond my control, but finally I have him out for engine runs. First thing to note is the last 20 hours of flying were on 100LL, no TCP. Engine would not run at all, rough and coughs and bangs - found a stuck exhaust valve which with judicious employment of the rope trick resumed normal operation.

Ran it up again (sounds normal after he cleared out the cobwebs) still on 100LL. 2000 rpm, left mag, runs rough and quits, right mag, runs rough and quits. Handful of carb heat and it runs just fine, 100 rpm drop both sides on mag check. Drained all the fuel and refilled with 91 octane mogas, exact same situation, it's 50+ degrees and sunny, the carb temp gauge reads just on the top side of the yellow arc so there could be carb icing...

So, I'm thinking the carb heat is really just richening the mixture, in which case I either have the wrong size jet in the carb, or an induction leak that's leaning out the mixture. The carb is an overhauled unit from Marvel Schebler, the one I sent was a 'P' model (for Bellancas with a fuel pump) and they sent me back the non-'P' model (same part number.

So... suggestions...? I'm not comfortable flying until I fix this, so I'm hoping it's simple...

Al

Al Corden
N97555 - Sid
1946 108

 
Posted : 24/03/2019 6:51 pm
Brad Schmidt
Posts: 5
Active Member
 

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Posted : 27/03/2019 10:35 pm
Brad Schmidt
Posts: 5
Active Member
 

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The picture above is what I used to work through my rough engine. Hope this helps. I had induction leaks.

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Posted : 27/03/2019 10:37 pm
Alistair
Posts: 113
Estimable Member
Topic starter
 

Thanks for the info - seems the primer and primer lines are a big item here... I'm working on it...

Al Corden
N97555 - Sid
1946 108

 
Posted : 28/03/2019 2:15 am
Alistair
Posts: 113
Estimable Member
Topic starter
 

Brad - where were your leaks..? Book says to pressurize the induction system, but I haven't worked out how to close all the inlet valves at the same time...

Al

Al Corden
N97555 - Sid
1946 108

 
Posted : 28/03/2019 6:42 pm
resto108
Posts: 278
Reputable Member
 

You don't have to get all the intakes closed, just plug the tailpipes.

 
Posted : 29/03/2019 12:47 pm
Brad Schmidt
Posts: 5
Active Member
 

Start by check the whole intake system the bolts just backed out. Mine was loose. We just ended up replacing all the intake and exhaust gaskets.

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Posted : 29/03/2019 12:52 pm
Alistair
Posts: 113
Estimable Member
Topic starter
 

Did not think of plugging the tailpipes... Spoke to Marvel Schebler today and Jeff suggested I tape a shop vac into the intake and spray soapy water onto the joints..

Onwards and upwards...

Al

Al Corden
N97555 - Sid
1946 108

 
Posted : 29/03/2019 10:12 pm
lawheelock
Posts: 774
Prominent Member
 

Al,
As you know, letting an engine sit for months is not good!
I do not recall if your engine had the older stamped rocker arms or the newer forged rocker arms. If you actually found a stuck exhaust valve, did you carefully inspect the rocker on that valve to see if it might have cracked? If it is the stamped variety it may well have. You might also need to pull the pushrod and inspect to see if it is bent especially if your rockers are forged.
Matt's rough 150 was from a cracked (broken) rocker after the exhaust valve had stuck due to 100LL. I journeyed to where it was and with his help, fixed it for him.
Most intake leaks, unless massive, will only affect the idle. Also, one of the things that the chart about carburetors miss is the fact that stumbling can also be due to a stuck ball in the accelerator pump circuit in the carb. Once at full power , it will not affect the running of the engine...only the acceleration.
Bottom line remains: F150 engines simply will not tolerate 100LL! Unless the intake system has been taken apart since I put it together, It should not have any intake leaks unless the manifold to cylinder bolts have backed out, which sitting should not have backed them out. As I recall, your engine had the later hex head intake bolts instead of the original F150 allen head intake bolts which were safety wired. But yours did get all new lockwashers.
And, if you have not done it already, I would suggest draining the carburetor bowl to make sure it does not have water in it due to condensation with the engine sitting for so long.

Larry Wheelock, A&P/IAStinson 108 N584LW 180 LycTexas in Winter; Indiana in Summer

 
Posted : 30/03/2019 5:33 am
Alistair
Posts: 113
Estimable Member
Topic starter
 

Larry

Unfortunately I had to use 100LL since mogas just isn't available anywhere except a few places, and Swift fuel is equally problematical. Do you remember if the guides were bronze or not..?

Things have taken a turn for the worse when I ran it again, sounded like the valve had stuck again, but closer examination reveals the valve seat on #5 intake has come out is is loose in the cylinder, don't think it is broken. Until I pull the cylinder I won't know the extent of the problem. I take it the removal is straightforward..?

This also begs a question... How common an occurrence is a loose valve seat..?, It certainly puts a doubt in my head for when I'm over the PA ridge lines - not many options out there... So - pull all six and send them out for overhaul..? It has me spooked.

Al

Forgot to say... I drained the fuel bowl, drained all the 100LL out and filled with 91 mogas. The rockers are forged, and the bolts are hex. The intake is secure, the only leak I think would come from the primer lines - that investigation is on the back burner for now.

Al Corden
N97555 - Sid
1946 108

 
Posted : 30/03/2019 6:45 pm
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