I recently fitted a new cam and lifters but notice that there is a slight misfire on idle. I pulled the plugs and two of them were black the rest were fine, swapped them around and the same two cylinders keep fouling the plugs, so swapped out the plug wires, still the same, checked the timing still the same. I sprayed the inlet manifold and the carb base with wd40 but there was no change the idle.

Today I had it idling for a while and using an infrared heat gun I got a shock, cylinders 5 & 8 are down on heat by a long way, the other six cylinders were over 500 degrees but cylinder 5 was around 300 degrees and cylinder 8 was down to 200 degrees.

The engine is basically a stock L48 except the cam which is nothing mega just a very mild uplift on the standard cam made by Herbert, the inlet is an Edelbrock torquer dual plane item that has a slight spread bore inlet, the carb is the original quadrajet which I did work on last year to stop fuel draining out the plugs in the base and it has a set of tubular headers and a twin exhaust system without an x pipe. The ignition is the standard Hei set up.

I removed the carb today and noticed that the two offending cylinders are fed by the upper plane of the manifold, really close to each other and this is now steering me towards a carb issue once again.

Has anyone had similar issues and if so what was your repair, I’m not going to be driving the vehicle till this is resolved as too much fuel in the cylinders will be washing the lubrication of the cylinder walls and eventually will do more damage.

 

A carb problem won't cause a misfire on 2 cylinders, even on the same plane. Not possible. You either have a cam with 2 flat lobes or an ignition problem. You don't have a carb problem.
Lars

 

Any time two cylinders have an issue, you have to look for a correlation. In your case #5 & # 8.
Are they next to each other in the block? Nope. No correlation there.
Do they share an exhaust system? Nope. No correlation there either.
Are they next to each other on the distributor cap? No again.
You maybe on to something about sharing a common fuel induction.

You have 8 cylinders and four barrels. That's a barrel for two cyl. However, because its a dual plane, I 'm think that half the carb feeds half the cylinders. So logically, there should be two more cylinders with issues.

Your cyl temps definitely show that #5 & #8 are not firing. If those two cylinders were getting too much fuel they should still fire, raising the temps of those cylinders.
I'm thinking this is an Ignition issue. You can always check the OHMs on the plug wires. I have had plugs with 300 miles on them stop firing. Only way I knew was with the thermo gun like you did.

 

Thanks for your quick responses. My reasoning for the a carb issue was that the plug wires have been replaced and after that I then swapped them about from cylinder to cylinder and the same two cylinders still had the fault. I did the same with the spark plugs, all new then swapped them about and still the same cylinders. It was only when I looked in the inlet manifold that I saw the two runners for the “dead” cylinders were next to each other.

I will check the lift on the cam lobes, however, I will be surprised if they are different as it has done no mileage since been installed and the break in procedure was done as per the manufacturers recommendation, not to say it hasn’t done any damage but will check later and report back.

I have already checked the rotor arm and the distributor cap for any tracking or obvious signs of failure and they both looked fine but I will get new ones just to be sure.

 

Have you done a compression test? Black plugs. No heat, do you have proper compression?

 

Have you put a vacuum gauge on this engine to see what's going on?

 

Pull the valve covers to see if the engine has a broken valve spring or a stud pulled out X 2. How did you set the valves up? Jerry

 

Valve adjustment too tight on those two cylinders?
How did you adjust the valves?

 

the rockers and studs are a good thought.....have you checked the cap, rotor, points

 

Since you recently installed the cam, I would suspect flat lobes, lifter issues or an adjustment problem.
What type of cam did you install?
Before you run it anymore I would pull the valve covers and start checking everything listed above.
It wouldn’t hurt to open up the oil filter to see if there is any debris inside.

 

Hate to write it but it does sound like valve trane issue/s

 

stilltrying

#5 and #8 share the same intake runner. Note image.

That may be your connection.

Mapman

 

See post #2.

 

so you think theres a rag or something sucked in there blocking it?

 

And 2 and 3 also but they aren't having an issue. See post 2 in this thread.
The question about valve adjustment hasn't been responded to.

 

Don't arbitrarily disregard troubleshooting information.

Problems, with only a single cylinder, eliminates some potential problem areas and makes others more likely.

Problems with two cylinders begs questions of connectivity.
Related by firing order?
Related by physical connectivity? (i.e. side-by-side)
Related by intake runner? (our situation)
Unrelated by ignition/fuel/air connections? etc.

Problems with all four cylinders served by a carburetor side, points toward a common problem.

Then again, everything may also just be happenstance.

Point is, examining potential connections helps focus troubleshooting efforts.

Mapman

 

The only thing they have in common is the plenum...unless something is blocking it they have no other connection

Fuel, spark ,air......the comonalites to look at are the cam, rocker studs failing if they are pressed in, distributor, rotor and cap.....the intake isnt the issue unless the back end of it isnt bolted down securely and its sucking so much air it isnt getting a charge....doubt it

 

Thanks to all for your responses.

As stated the compression on all cylinders are within 10% of each other at around 150 psi.

I removed the valve covers and the offending cylinders are lifting the same as the rest of the engine, so no flattened lobes on the new Herbert’s cam or failed lifters (unless when running they are not pumped up)

The valve springs all look fine. The rockers are on push in studs but nothing looks wrong compared to each of them.

When I installed the new cam and lifters I set them to zero lash then three quarters of a turn on every one of them, when I realised something was not right I did them again with the engine running, this time undid them till heard a ticking noise, wound them back in till the noise disappeared then a further three quarters of a turn.

I haven’t fitted a vacuum gauge, other than to set up the quadrajet a while ago.

MapMan, that is exactly the reason I was thinking carb as they are the only connecting thing I could see between the two cylinders, however, I have just ordered a new distributor cap, rotor arm and coil to either cure or eliminate them from the equation. It will be a couple of weeks before they arrive and can be fitted so will not be able to report for a few weeks.

once again thanks to all for your input.

 

the carb has no clue where the fuel air mixture is going. It can not be a carb issue...As you say you have eliminated everything except for a bad lifter but it sounds as though you you investigated those by resetting those when the motor was running. If the offending cylinders studs protrude through the nuts the exact same as all the other studs then thats probably not the issue which leaves the electrical side. Where you getting a strong spark at those 2 plugs?

 

looking at your cap 8 and 5 are directly across from each other.....that could be an issue. Did you pull the cap off the distributor and look at the inside of it for abnormal wear. I had a retainer fail and the shaft would bounce up and down causing the rotor to eat away at the inside of the cap.....similar issue to what you are having. As it got worse the issue spread to different cylinders