I just started digging into a friend's engine and am looking for some hints on which direction to head. I have a plan in mind, but want to get other ideas so we can get this thing running right as easily as possible.

71 Olds 350, rebuilt within the past 1,000 miles, had various running problems since assembled. Low compression pistons, stock heads worked over, Performer intake, correct # qjet that I've gone through. New Accel HEI, wired properly. Memory is fuzzy on the cam - it's a Comp, either XE262H or 260H. In either case, relatively mild. The assembler put in stock rockers and bridges. Looks like the bridges are new, maybe the rockers too.

The problem: #5, #7 and #4 are, for all intents and purposes, dead. She shakes and stumbles at idle, vacuum is around 10" @ 700 rpm, backfires if you try to run it hard. Can limp around town OK.

I know 5,7,4 are dead because I pulled the plug wire and got NO difference in the running of the engine. Pull any other and the engine dramatically falls on its face.

1) Ignition
Timing was re-checked. Sitting around 12deg base, although the mechanical advance comes in really early.
Wiring is new and done correctly. Full IGN to the HEI.
All 8 plugs had gaps around 0.020". Regapped to 0.045", no change.
All plugs look practically new, although with a slight red tint. No tan, white or black deposits, NO OIL. 5, 7 and 4 were NOT WET.
I tried swapping #3 and #5 plugs. No change. Problem stayed with #5.
Tried a known-good spark plug wire on #5. No change.
Checked with noid light (spark tester) on #5, Noid lit correctly.
Holding the boot close to the head results in a visible spark that jumps out of the boot to the head. The jump and intensity is approximately the same between #3 and #5.

2) Fuel
I put on a known-good carb just for kicks. No change. Mechanical fuel pump working fine. It's a dual plane intake and we've got both planes involved here.

3) Compression
The starter is a stock style, but new. It's very tired and can barely turn over the engine even with a booster battery and a 150amp jolt from a charger. Hence I didn't get good compression numbers.
Regardless, both #3 and #5 came back with ~ 145psi. I'm certain that's low because the engine was turning so slowly. What I was looking for was consistency. With them pumping up the same, that tells me there's nothing catastrophically wrong with #5.


4) Cam
I was not precise, but it appears the stock bridges induce about 3/4 of a turn preload on the lifters. That's the stock rocker bolts which are course thread, whereas the standard "1/2 turn preload" is for fine thread studs, right? I did not run with the valve cover off in the interest of not making a mess. I do have half-cut V/Cs for thus purpose but it's still messy. Anyone make clear plastic valve covers yet?


Plan right now:
Install a ministarter and run a better compression test.


Any ideas where to head from here?


a) Ignition: I considered swapping a known good dizzy, but it's a points style with Pertronix, so it'd take a little while to redo the wiring and whatnot. Plus I'd expect Accel to be good out of the box. Given that I am seeing spark and we're not getting backfires, I don't think this is a high priority.

b) Cam/rockers: I have a set of roller tip rockers and SB pushrods sitting around here that I can swap on. I could envision the stock bridges causing valves to hang open and cause something like this, but why only three cylinders? Is this worth chasing?

c) Cam failure of some sort: Maybe?

 

Here's what jumps out at me; your statement of 3/4 turn preload on lifters. You say you are using stock rockers, thus there is no adjustable preload, the rocker hold down bolts are run down all the way tight, there is no adjustments on the factory set up. Snug all the rocker assemblies down tight and report back, just might be your problem......

 

When I say 3/4 turn preload, what I mean is when tightening the bridge hold-down bolts, in tandem, with that cylinder having both lifters on the base circle, the bolt requires 3/4 of a turn from the point where the pushrod begins to be loaded (i.e., zero lash) until the bolt is seated and torqued. All the bolts are tight. I was trying to determine how much pre-load the stock rocker setup ended up forcing on the aftermarket cam and lifters.
Obviously not running stock rocker bridges loose. That would be a quick test.

 

Ok, it wasn't very clear, understood.

 

i don't know how many times i've seen #5 and #7 plug wire get crossed but that doesn't explain #4. don't overlook wiped cam lobes.seems to be common,especially with a recent rebuild.

 

x2

ck for spark at the plug ends of the dysfunctional cyls' wires.

 

If the rocker hold down bolt is 5/16x16 then each turn is a 1/16 inch. 1/16 inch equals roughly .060 (.0625 to be exact). 3/4 of that would be .045 inch so I'd say you're in the ballpark.

 

Check for vacuum leaks and vacuum hose routing. That cam is on the large side for 8 to 1 compression and 10" is low for vacuum. The motor will like a lot of vacuum advance and would run better with even more initial timing. 145 psi is about right for compression with your specs. I would also degree the cam in that motor.

 

3) Compression
The starter is a stock style, but new. It's very tired
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new, but tired? Smells fishy.


and can barely turn over the engine even with a booster battery and a 150amp jolt from a charger.
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Remove the plugs and turn the engine by hand. Loosen all the belts. See if it feels right. Both directions.

If it feels too tight, then slacken or remove all the rockers to remove valve action and see how just the crank and pistons moving feels. If the balancer bolt comes out rather than the motor turning CCW from the front, that's bad. I have seen this. 1-1/8" socket required.


Hence I didn't get good compression numbers.
Regardless, both #3 and #5 came back with ~ 145psi. I'm certain that's low because the engine was turning so slowly. What I was looking for was consistency. With them pumping up the same, that tells me there's nothing catastrophically wrong with #5.
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I dunno, could still be wiped lobes on the cam, it happens frequently. Your dead cylinders are all close on the cam.


4) Cam
I was not precise, but it appears the stock bridges induce about 3/4 of a turn preload on the lifters. That's the stock rocker bolts which are course thread, whereas the standard "1/2 turn preload" is for fine thread studs, right?
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That should not be an issue. If your rockers ALL have a correct 1/2-1.5 turns of the pedestal screw for preload, that indicates not a wiped lobe issue.


Plan right now:
Install a ministarter and run a better compression test.
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OK sure
I still say remove plugs and feel the machine by hand.
You can also spin it with the new starter with no plugs, verify sparks, then add stuff back on, like the plugs.



a) Ignition: I considered swapping a known good dizzy, but it's a points style with Pertronix, so it'd take a little while to redo the wiring and whatnot. Plus I'd expect Accel to be good out of the box. Given that I am seeing spark and we're not getting backfires, I don't think this is a high priority.
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Spark at every [dead cylinder] plug wire? Trust but verify.



b) Cam/rockers: I have a set of roller tip rockers and SB pushrods sitting around here that I can swap on. I could envision the stock bridges causing valves to hang open and cause something like this, but why only three cylinders? Is this worth chasing?
============
I would measure lift at each problem cylinder lifter first.


c) Cam failure of some sort: Maybe?
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entirely possible- but should leave rockers loose if it's any more wear than your preload figure.

 

12 degrees BTDC is waaaay too low for an HEI. That would cause the low vacuum. HEIs should be at 20.

Vacuum leak is the first thing that jumped out at me, especially given the dead cylinders. Set the timing at 20, get it to idle, then put a towel slowly over the top of the carb and see if rpm increases. Also, Does the vacuum gauge fluctuate wildly at idle?

 

Well I'd say if it was 5-7-2 not firing the distributor cap is probably not on all the way seated. That #4 is something strange though. At any rate I believe you have an electrical problem. Use a known good HEI spark tester not a noid light The spark gap is a lot larger then a standard ignition. You may be getting spark to the wire but not the plugs. MAW test Ohms on the all the wires while off.

 

Except that it is NOT cylinders 5,7, and 2, it is cylinders 5, 7, and 4.

 

Thanks for repeating that back to me

 

Intake leaks along the intake<->head surface, thankfully on the top side. It's always the stuff you assume doesn't need to be checked.

Edelbrock Performer, stock 7 heads, Felpro head gaskets, no surfacing done anywhere. It leaked really bad last year with a turkey tray gasket (installed by the assembler) so I re-did it with an SCE fiber gasket. Looks like the exhaust crossover heat has been hard on the gasket - it's folding and cracking around the crossover ports. Doesn't explain #7, though. Leaks are on #5 and #7 significantly, with a smaller leak on #4. Others seem to seal up well.
I checked this intake with a good straight edge before installing and it appeared to be in good shape.

 

What type plugs are you running? It makes a great deal of difference. I bought an old Chebby pickup once and it was misfiring. Did a complete tune-up on it. A/C Plugs, cap, rotor, points, condenser, and wires. Still ran rough! Cleaned the carb, ran rough. Did a compression test and one side was a bit low, nothing serious, but lower than the other side. Bought a newly rebuilt set of heads and put them on, changed out the cam while I was at it. Still ran rough! Put in an HEI distributor and Accel coil with larger wires, still ran rough! My loving wife suggested new plugs and I told her that was one of the first things I did. She says, "They're cheap, aren't they?" I put in some A/C Rapid Fire plugs and that thing ran like a turpentined cat! Always, Always, Always use the best spark plugs you can get!

 

Good old R45S. Even gapped to 0.040" for the HEI.

It's air leaks. Finally got it in my head to check with some carb cleaner. Engine falls on its face when spraying at the top of 5, 7 and 4 intake runners at the head. Not subtle at all.

 

I agree always use AC spark plugs.

What is puzzling is that the plugs in the "dead" cylinders are not fuel soaked. Is there no fuel getting there? Or are they really firing?

I seem to remember an issue with a Performer intake and a qjet carb possibly not mating correctly. I could be wrong on that but remember something.

I also have had two "new" cams fail. I have had new lifters fail. I have had bad distributor caps that are new. I have had bad plug wires that are new. In fact I remember once having three bad spark plugs.

You stated with slow turn over that the compression was at 145. That really is not that bad. All depends on the cam and how much turn over it has and how does that compare to the other cylinders? Did you do the compression check on the engine warm and with all the plugs out and the choke flap held open?

I have had bad plug wires and could not tell until we ran it with the hood open in the dark and it looked like a light show with the plug wires sparking all over the place. Sometimes just moving plug wires will cause them to fail.

Just some more input. Hope you figure it out soon.

Larry

 

Yes. He has massive leaks between the intake and the heads on those cylinders.
There is no fuel getting there.


He figured it out:
- Eric

 

You have two choices, try a different brand of fiber gaskets or get the intake shaved slightly. Those seem to be the thickest gaskets too.

 

Yeah, it's only an issue at idle (anything over idle moves enough air to overcome the leak. Yes, those cylinders will still be comparatively lean, but this car is for crusing around town), so the owner is going to enjoy it for the summer and we'll take another crack at it in the fall. At this point I believe quite strongly in surfacing any aluminum intake.

I've used both types of SCE, the Cometic, Flatout and Mr Gasket composite, as well as metal. Still like the plain SCE the best.

The frustrating bit is when I re-installed this intake last year it sealed up great. The owner got back on the road. Then when he pulled it out of the garage when the weather warmed back up the intake gasket got all f'd up. And yes, we waited a few days for the silicone to set, didn't use end rail gaskets, retorqued a couple of times after several heat cycles.

 

If anything I have only got the composite intake gaskets to fit one time due to milled heads and crooked intakes.