How to fix a engine with a lean cylinder

[mason1968]

Hi,

I have a small block Chevy with one cylinder (#6) that seems to be running lean.

The engine is a 350 with a B&M 144 supercharger and is carbureted. I recently installed a set of headers with ceramic coating. The coating on the primary pipe for cylinder #6 has burned away.

What can cause one cylinder only to be lean?

I have a new MSD ignition system installed. I think it might be a leak in manifold to cylinder head gasket or I might have a problem with my values in that one cylinder.

Any info would be helpful.

Thanks
Matt

 

[gmjunkie]

Hi,

I have a small block Chevy with one cylinder (#6) that seems to be running lean.

The engine is a 350 with a B&M 144 supercharger and is carbureted. I recently installed a set of headers with ceramic coating. The coating on the primary pipe for cylinder #6 has burned away.

What can cause one cylinder only to be lean?

I have a new MSD ignition system installed. I think it might be a leak in manifold to cylinder head gasket or I might have a problem with my values in that one cylinder.

Any info would be helpful.

Thanks
Matt

May not be lean at all!! May have a bad valve seat!! junk!!

 

[mason1968]

Is it possible to test the engine?

I have the abilty to perform a compression test and a cylinder leak down test. As for the leak down test what owuld I be llooking for..

 

[gmjunkie]

Yes!! If that cyl is low on Compression!! Could even a valve starting to burn!! In Pre 72 engine's the valve seat's were softer than thay are now!! Thay had Lead in Fuel to cushin then!! Now all unleaded fuel and the seat's have to be harder So the valves don't beat them out!! Now on older engine's when rebuilding I have the Machine Shop put all NEW seat's in!! junk!!

 

[L48]

A compression test is where I would start. If you are burning the coating off the pipe it sounds like the fuel is getting burned up in the exhaust.

I doubt it will pass that test. If it does you are on to something more obscure.

Has this problem been going on since you put the engine together or did it just come up?

I could guess there is some variation in the cam grind on number 6 or maybe there is something wrong with the plug wire - off by one, losing some of the spark.

In any case red hot exhaust probably means the fuel is getting there - but it is burning up in the header rather than port, for whatever reason (and forgive me if I am stating the obvious to you).

 

[mason1968]

The engine is late model GN 350 create motor. So the issue of vale seats and leaded fuel should not be a concern.

L48 -

I just installed the headers about 2 weeks ago and noticed it. I talked with the manufacture of the headers and they are going to honor the warrenty and recoat the headers for free. By the way the headers are DOUG's I like the quailty and the fact hey will re-coat them.

I do not know if this a new problem or not. I originaly had the stock cast manifolds and never noticed anything..

Last night after driving the car a little I used my multimeter which had a themocoupler (measure temp) attachment. At idle I checked the temp on the outside of the header near the heads. This is the results: Cyl #1 ~ 350-375 F, cyl #2 & 4 ~ 350-375, Cyl #6 +415 and climbing.

I tend to beleive that the the flame front is passing by the exahsut valve.

Would a burnt valve do this how about a miss-adjsuted rocker arm that is not allowing the valve to fully close????

Thanks for the help.

 

[curtis]

The leakdown test should tell you if you have a leak at a valve. Listen very carefully for the sound of excaping air into the intake (bad intake valve or seat) or the sound coming thru your exhaust pipe. And yes it is possible the lash is too tight on a particular valve, thus holding it open. could be a bent valve. If heads were off and standing on end on the floor and it got knocked over, it could have hit a valve top and bent it. In that case it would probably not close fully again after it initially opened up. Compression test should indicate if a cylinder is not sealing. Could also be a broken ring or broken ring land on a piston from the high cylinder pressures you make with a blower. the temperatures you show definately indicate something is different on that cylinder. I saw a race engine at the track once that had a big chunk of gasket wedged in the intake port right in front of the valve. That cylindr also ran hotter than the rest.

 

[mason1968]

Ok I have perfromed a compression test on the passenger side bank
CYL PRESSURE
#2 160 PSI
#4 160 PSI
#6 150 PSI
#8 155 PSI

The compression testing looks good.

I also performed a leak down test, but I am not sure if I am doing it correctly. For cylinder #6 and #8 I apperently have a leak down of 15% (had set gage to 100 psi, connect and read 85 psi) I also hear a leaking sound but not sure from where. I think the sound is coming from the lift valley. I have the valve cover off and I hear the leak but not from the valves them selves. I think I can feel air leaking past the valve stems but again not sure.

I have also removed and examined all of the spark plugs. My first impression is that the engine itself is lean, but I only have about 50 miles on the plugs. The porcelin looks very white, I do not see the combustion ring.

Any ideas?

 

[norvalwilhelm]

I too have a blower and when reading plugs I run 2 different heat ranges. I would run one step colder plug in that cylinder.
Big blocks are bad for this. Some cylinders just seem to run leaner. I put a colder plug in those cylinders.