What cause one cylinder to run very lean?

[rogeroadster]

I have a SBC in my 29 roadster that runs very lean only on the #5 cylinder.  It has a new Holley 650 on an Edelbrock manifold.  Any ideas??

[enjenjo]

Bad lobe on the cam.

[UGLY OLDS]

Possible vacuum leak in only that cylinder  :?:  :?:   I know it's a long shot ...But .... :?


Bob......

[wayne petty]

carbon build up on the back of the intake valve.. usually do to a loose guide, bad valve stem seal... or one that has slipped off the guide and is riding up and down on the valve stem....


the thick carbon found on the back of some intake and a hand full of exhaust valves can choke off one cylinder..

the carbon absorbs the fuel when cold... but can block the air flow also..


also... are there any vacuum hoses that are hooked to the back of the intake???

like the power  brake booster?   could that be leaking?? does the engine idle speed change when you pinch off the power booster hose or the PCV hose?

[rogeroadster]

carbon build up on the back of the intake valve.. usually do to a loose guide, bad valve stem seal... or one that has slipped off the guide and is riding up and down on the valve stem....


the thick carbon found on the back of some intake and a hand full of exhaust valves can choke off one cylinder..

the carbon absorbs the fuel when cold... but can block the air flow also..


also... are there any vacuum hoses that are hooked to the back of the intake???

like the power  brake booster?   could that be leaking?? does the engine idle speed change when you pinch off the power booster hose or the PCV hose?

These are all good answers.  I hope it is not the cam. It is a flat tappet cam.  I will check for vacuum leak but it idles OK.  The only vacuum system is the vacuum modulator and it is tapped into the runner for the #8 cylinder.  The engine only has 1800 miles on it.  I'll check the lift on the cam with a dial indicator.

Thanks for the ideas.

[wayne petty]

do you have the old rocker cover that has the cut away and the gasket glued on....


so you can start the motor without puking oil all over the exhaust???


fire it up.. let it idle... look at the push rods...   are any spinning significantly faster than the others.. or twisting back and forth...

either of these may indicate problems...

please.... if you have a cam that has gone flat...  and are going to install new cam and lifters...  while you have the timing cover off...   shove some strong rare earth magnets into the pan...  make sure they stick to the pan.. not up to the rotating assembly...

you can also get them smaller at radio shack...  shove them through the drain plug opening...

there is going to be metal filings scattered through the oil system... remember.. the oil filter bypasses some of the oil when cold...

you might stop right now.. swap the oil filter.. cut the used one apart... examine it closely .. use a magnet... a clean magnet... on a white flower sack dish cloth.... got metal... not got metal?


valve action still need to be examined... but you will know if there is metal in the filter..

[38HAULR]

Considered a leak around the intake gasket at #5?  
Frank.

[rogeroadster]

do you have the old rocker cover that has the cut away and the gasket glued on....


so you can start the motor without puking oil all over the exhaust???


fire it up.. let it idle... look at the push rods...   are any spinning significantly faster than the others.. or twisting back and forth...

either of these may indicate problems...

please.... if you have a cam that has gone flat...  and are going to install new cam and lifters...  while you have the timing cover off...   shove some strong rare earth magnets into the pan...  make sure they stick to the pan.. not up to the rotating assembly...

you can also get them smaller at radio shack...  shove them through the drain plug opening...

there is going to be metal filings scattered through the oil system... remember.. the oil filter bypasses some of the oil when cold...

you might stop right now.. swap the oil filter.. cut the used one apart... examine it closely .. use a magnet... a clean magnet... on a white flower sack dish cloth.... got metal... not got metal?


valve action still need to be examined... but you will know if there is metal in the filter..

I'll try running it as you suggest.  I cut the filter every time I change the oil which has been 3 times on this car in 1800 miles.  I've seen no indication of metal particles in the oil yet.  I do run a zinc additive in the oil.  If the cam turns out to be flat I'll go entirely through the engine not just replace the cam and lifters.

Thanks for the ideas.

[rogeroadster]

Considered a leak around the intake gasket at #5?  
Frank.

I'll check that today with a spray can of starter fluid.

[tomslik]

Considered a leak around the intake gasket at #5?  
Frank.

I'll check that today with a spray can of starter fluid.

fire extinguisher handy?


i'd use something not quite as potent....

[Bruce Dorsi]

Considered a leak around the intake gasket at #5?  
Frank.

I'll check that today with a spray can of starter fluid.

It's also possible that the intake gasket is broken at the underside of the manifold at the #5 port.

[Learpilot]

On my son's Chevy truck with a 250 6. It would tick like a stuck lifter. I though that a cam was bad ,but it turned out to be a rocker nut backing out as it ran. The no cost fix was to take the nut off and hit it with my "BFH" Big Fine Hammer on the flats. I put the nut back on , no more problem.
Rick

[Hooley]

Just a little more on the subject. Our Henry J drag car sets on the trailer most of the time. A few weeks back we were at the track when we tried to start it, it hydrolicked . Pulled plugs and let water out of #6 and #8. Water not anifreeze.  I started to cover the scoop with water proof stuff[trash bag]. Forward to today. it had set for a couple weeks and has rained a lot.
I found the problem today. The intake gasket was pushing above the head intake and rain water was running in the motor at that place. The car had still ran the numbers at the track.  We used some rear-end oil on the parting and could see it sucked it the hole.  
It could be something strange like this.

[exsedan]

Wonder what the final outcome to this problem turned out to be??..............jb EXSEDAN

[Hooley]

I been wondering too.