[454me]

This summer I bought a 71 Chevelle with what is supposed to be a 350 lt1 motor. The numbers on the block and heads seem to support this. I drove it 150 miles home and drove it for several weeks. It had a clutch problem when I bought it. It was bending the frame bracket every time you pushed in the clutch. It was not a heavy pressure plate, it had some problem. Changing the bracket and clutch,pressure plate and throw out bearing fixed that problem. Then it started to miss. Just a little at first then got worse. Now it is dead on cylinders 1 and 5. I tried swapping plug wires with a known good one but it stays on 1 and 5. I next tried new plugs. Still there I had another Holly carb so I tried that since it seemed like they were rich. No difference. I then set the points (it has a mallory comp 9000 dual point dist). I wanted to get rid of that so I ordered a HEI dist. On trying to install I found that it wont clear the intake. It has a edelbrock c4b manifold. So a new manifold is in the works this winter. I set the timing and it seems to backfire in the intake sometimes with hesitation. I ran a compression check and all cylinders are between 169 and 175lbs. I can touch the headers and the 1 and 5 are just warm where as any other cylinder is hot. You can also tell the exhaust is hot out the right bank and barely warm out the left. I tried moving the plug wires in the dist one plug wire over and turning the dist to make up the difference just in case there was a bad dist cam. The plugs all spark real good. I finally timed it by ear. Turning the dist reving the motor to get the least hesitation. It pulls hard all the way to 6500rpm but still has 2 dead cylinders. The popping in the intake rarely happens now. I ordered a dial and magnetic base and my next step is to measure the cam. Thats what I think is wrong. 2 cam lobes worn off. I installed a anti power valve blowout kit in the carb and put in a new power valve to. The only other symptom is the vacuum reading. At idle it varies from 10 to 14 real fast. As the rpms come up it steadies and rises to 21 or higher. I think if the cam lobes are bad I will pull the heads and make sure they have the hardened exhaust seats for unleaded fuel. And have a valve job done. Then it will be new cam,lifters,springs,retainers,locks, and timing chain and gears. Any other thoughts would be welcomed.

Gary

 

[19_Chevelle_69]

Sounds like the cam I just replaced in my SBC in my truck. Mine had bald lobes on #1 Exhaust, and #2 Intake.

You can easily pull the valve covers off, and see if all the rockers are moving....



Don's Chevy Shop forums

 

[mike in nz]

pull the manifold off carefully & inspect the gaskets . you will find it has a faulty gasket . if you dont wont to do this just yet you can block off the PCV valve , start the engine & remove the oil filler cap , put the palm of your hand over the hole if it sucks your hand on to valve cover the inlet manifold gasket is leaking into the crankcase . you can also carefully spray engine start (etha ) around the manifold & if the engine rpm increases you will find the manifold gasket is leaking to atmosphere . hope this helps you & saves you from changing cams etc .

 

[DoubleVision]

I would think the cam is bad, I`ve never seen a intake gasket leak so bad that it missed at all RPM ranges, if the intake gaskets were leaking that bad around the base of the manifold it would using a lot of oil also. Since you know which cylinders it is, pull the valve covers and run the engine, if you see one of the rockers of that cylinder not moving very much you`ve found the problem, also inspect it for valve train damage, broken rocker arms, bent push rods, broken valve springs, pulled head studs etc.

 

[mike in nz]

try intake gaskets first .its hard to imagine that you've had mechanical damage on 2 cylinders on the same bank occur at the same time (possible but unlikley)

 

[RCastle]

If it is the intake gaskets causing this problem,you can take a can of carb cleaner and spray the suspected ares of the intake.If it is leaking that bad to kill two cylinders than when you lightly spray the carb cleaner on the intake/head mating surfaces,with the engine running,you should notice an idle increase.

 

[454me]

I did spray the intake and carb area to check for leaks. None found. I also ran the engine with the valve covers off feeling the rockers. They all seem to move the same. I spun the push rods while running to check for a bent one. I cant believe that the inake could be leaking bad enough to do this and not leak on the top also but it is possible. I also truned the engine over with a breaker bar and watched the valves they all seem to be working the same. Thats why this has me puzzled I have done every thing I can think of other than micing the cam which is the next thing when I get a day off. I did enjoy all the replies though.

gary

 

[docvette]

Vacuum

Doc here:imp:

Did you say you had a rapid fluctuation on the Vacuum at Idle?

and it cleaned up with a little RPM's?

That is indicative of worn Valve guides ..

My manual say's...

Quote:
"Gauge reading : Needle vibrates excessively at Idle, but steady's as engine speed increases...
Indication: Worn Valve Guides..

But with the missing and intake popping..I wouldn't put it past burned intakes,...at least on those two cylinders anyway...

Probably ran out of lead build up in there (the guides) so maybe it's a good time to do the valves...

Doc imp:

 

[firebird_red]

burned intake

Doc,

Is there a quick way to check for a burned valve w/o pulling the heads?

 

[poncho62]

Re: burned intake

Doc,

Is there a quick way to check for a burned valve w/o pulling the heads?

Compression test....................

 

[firebird_red]

He said compression looked okay at 169-175 on all cyl.

 

[poncho62]

Well then, the valves should be OK then................right?

 

[docvette]

Compression

Doc here,

Two ways I know of...

One is kinda shade-tree, but does give you an idea..

With it running,(and If you don't have a big *** nasty Cam) Place a business card over the offending tail pipe, if it doesn't bend out full time, but slaps against the pipe..(Wap, Wap, Wap) Think Valves...

The other is wet and dry compression tests..If it goes up on those cylinders WET that's a pretty good indication something is going wrong down there...

Doc imp:

 

[troy-curt]

Is it a dead miss?

With eng. running, pull the PCV and see if there is excessive crankcase pressure.

Troy

 

[454me]

docvette I agree that the vacuum says bad valve guides but I cant see why it would miss with good compression. If the guides were bad enough to make the miss I would think the compression would be low for those 2 cylinders. I was also wondering if the miss could be causing the vacuum reading to pulse.

 

[docvette]

Burns

Doc here:imp:

I'm thinking maybe burns or pits around the valve seats in those two cylinders along with leaky guides...

Try a compression/leak down test...

The miss would account for some of the fluctuation, but less rapid ...besides, then It's back to the same question, Why is it missing...on those two cylinders only.



Docimp:

 

[LilSmoke]

Run the valves ,then go get a moroso single gauge leak down tester and leak it.

 

[454me]

OK heres the dope. I ran the cam and it cant be the problem. Last night I decided to try something to clean out the cylinders. I have done this before and it works ok. I ran the engine and slowly poured a pint of water down the carb. Reved the motor to keep it from dieing. It actually seemed to run a little better after that. Then I took it for a ride. Still missing and you have to slip the clutch to get it moving since its only got 6 cylinders working. I then decided to check the timing chain. I put my dial on the dist rotor and turned the engine clockwise. Set the dial and turned it counter clockwise. It didnt move 1 degree on the crank before the dist started to move the dial, So the timing chain cant be sloppy. I then removed the plugs. 1 and 5(the 2 bad cylinders) were black and the rest were brownish white. So next I removed the valve cover on the drivers side. Right where the #1 exhaust valve was in the valve cover I see signs of the water. That show me that the guides are the problem. There is no way for water to get there except through the oil or guides and the oil shows no sign of water. Then I put 120 lbs of air on the cylinders. They all seem to sound the same. Air is leaking slowly but must be going around the rings since its quiet in the carb and exhaust. So I then ran the cam.

Intake Exhaust
#1 467 494
#3 465 476
#5 470 483
#7 476 477

So unless somebody has an idea I am going to pull the heads and have them done. I will decide on the rings when I see the buildup at the top of the bore. I dont really know how many miles are on this engine so it could be ok.

Any Ideas Gary

 

[troy-curt]

I don't understand your reasoning of water in the valve cover but not in the oil. That moisture wont cause a miss unless it it coming through a crack in the head normally around the valves. Then the plugs would not be black. Black plugs tells you that plug is not firing. Does the coolant look funny? The moisture could just be condensation, after the engine warms up it should go away.

Troy

 

[454me]

No there was no water there until after I ran it through the carb which tells me itscoming up through the guide. The antifreeze looks normal and the oil in the pan is normal. I dont think the moisture caused the miss. The miss was causing the water to go there during the overlap phase of the cam cycle. Its just my guess since every test I do says there is nothing wrong but its still a dead miss in 2 cylinders