#8 cylinder is doing something or should I say nothing.

I have a 1970 350 rebuilt to L46 specs. Engine was balanced, port and polished and cycled at the professional engine shop before I picked it up. I watched the guy compression test it to 160PSI on each cylinder before I took delivery. It has been Tuned by Lars and absolutely screams above 1000RPM. Since then I have installed another carb (also rebuilt by Lars).

Heres the problem though. In the last couple of weeks the idle has been rough and it sounds like engine is missing. The symptoms were pretty similar with either carb on the car (both QJETS). I have put it on a SUN engine analyser and it shows that during balance test #8 is virtually dead. If you put a temp gun on the manifold (rams horns) then the temp out of number 8 is about 150 degrees cooler then the other cylinders. It is also showing that the fuel air ratio coming out the right hand side exhaust is 2-5 percent higher.

It gives all symptoms that number 8 is not burning fuel at idle.

I have compression tested #8 and it still reads 160PSI. (as do #4,#6)
We did a rudimentary leakdown test last night and it was the same on #8 as on #6. The valves and rockers seem to move as they should. Did smoke test of manifold and no leaks vsible.

I have swapped carbs, adjusted idle mixture screws.

I have changed the rotor, distributor cap (both new AC DELCO) have new Taylor wires and Autolite plugs (gapped at 0.035"). I even swapped several spark plugs just in car. We have checked the engine in the dark and there are no visible shorts. Strong spark going to number 8 when pulling of plug lead.

Timing is 34 degrees at 2500+ and around 18 degrees at 1000 RPM. (without vac advance connected).

ANY IDEAS?

 

I would swap ignition wires and see if that makes a diiference. Take a wire from one of the other cylinders. I would find it hard to believe it's a carburator since it feeds 4 cylinders on a dual plane intake. You mentioned it has the same compression as other cylinders so chances are that the rings and valves are OK. Have you at least eye balled to see if the lift of the cam is the same on this cylinder as compared to the others? It wouldn't be the first time some had a lobe worn off the cam in a short period of time.

 

I'll try another lead on #8 tonight. I haven't measured the lobe but will do.

THANKS!

 

You said you saw the rockers moving, so the lobes are probably ok. Usually when they wipe the rockers just kind of twitch around.

I too would suspect a wire simply because that's the only thing you didn't check. But seriously I've had wires that arced inside the boot. Sometimes the clip will work its way off the wire or it can get smashed or shifted and not engage the plug correctly. check!

 

Might have an intake leak sucking air into the cyl.

 

Fevre,

I did a smoke test on the intake last night. No leaks.

Good advice though thanks.

(now i have to amend the top post )

Cheers,

Pete

 

I will get some new leads and also put a dial indiator on the rockers. If I just compare the lift to the adjacent cylinders is that OK. They should all have the same lift (I hope).

 

I didn't notice you stated you looked at the rockers and valves. If a lobe was off it should have been noticeable, a few thousands difference will not be drastic.

 

For what it's worth, I had a 350 that rounded the lobe off the cam in a short time, pulled the valve cover off and there it was you could see it while the engine was running. The rocker arm on the intake side was quivering moving ever so slightly.

Gook Luck in your hunt

Tom

 

[QUOTE=stingry]#8 cylinder is doing something or should I say nothing.

I have a 1970 350 rebuilt to L46 specs. Engine was balanced, port and polished and cycled at the professional engine shop before I picked it up. I watched the guy compression test it to 160PSI on each cylinder before I took delivery. It has been Tuned by Lars and absolutely screams above 1000RPM. Since then I have installed another carb (also rebuilt by Lars).

Is Lars the person that writes the technical papers? How do I find out mor info on Lars carb rebuilds?

 

Tom,

It certainly a hunt! I hope its not a lobe. I guess I will find out tonight.

Pete

 

[QUOTE=IMEZRU]He has papers under www.corrvettefaq.com

Email Lars. His email is: v8fastcars@msn.com

Most knowledgeable guy on Vettes/Engines I have ever met. Might have to make him an honorary Aussie just because he appreciates beer so much!

 

Where do your headlights get vacuum? Stock manifolds have the fitting in #8 intake runner. I have seen this before. Plug this fitting and see if the condition (miss) goes away.

 

A fairly simple way of checking for spark at #8 is to hook an old plug to the #8 wire and lay it agains the head. You should see a spark when you turn it over. If nothing, then you at least have isolated the problem.

 

I had the same probelm once turned out to be a bad spark plug, and it was brand new I just changed them

 

Good plan! I am testing that now also.

Thanks!

Pete

 

Will do.

Thanks,

Pete

 

Well he said that the cylinder was running cooler and was he seeing more fuel-air coming out of the right bank. Both of which would not indicate a lean condition as would be caused by an intake/vacuum leak. Cold cylinder + gas out the exhaust says ignition to me. *shrugs*

 

I haven't tested the engine yet but I had lunch with another member of our club. How does this theory sound????

1. Vaccumn leak through headlight relay then into manifold at intake runner for #8.
2. Mixture going into #8 is VERY LEAN and #8 does not always fire.
3. Mixture in #8 gets pumped out without firing hence manifold is colder and fuel-air on RH exhaust is higher then LH as the fuel is still getting to #8 but just gets pumped out the exhaust.

So even though the cylinder is lean, it is SO LEAN that it actually stops ignition and the cylinder runs cooler.

Is it possible to be TOO LEAN and not combust. Remember I am an electrical engineer so keep the words SMALL!

 

Yes, and that mixture is referred to as "air". But seriously, it'd be damn hard for that to happen. The spark (assuming there is one) is going to ignite whatever fuel vapors are in the cylinder. The combustion of which requires oxygen (duh) which is pulled from the air also in the cylinder. Too much air, and you're running lean. Meaning all the gas is burned, but there's still air in the cylinder. This in turn causes oxygen in the exhaust and increased cylinder temperatures. On the other hand running rich means you have more fuel in the cylinder than you have oxygen to burn it with. This will typically lead to lower cylinder temperatures and fuel out the exhaust.

And of course if you don't even ignite what's in the cylinder, you'll get fuel/air out your pipes.