The Stovebolt Site Forums The Shop Area The HiPo Shop Need help with diagnosis

My '58 GMC has a 350 with a Turbo 350 trans in it. This is the engine that was in the vehicle when I bought it.

It has always run rough. Not real rough but kind of like it wasn't firing on one cylinder. It is especially noticeable when I drop it in gear. I'm getting a little bit of smoke (not black, maybe gray) out of both sides of exhaust. The plugs are black and sooty. No unusual noises knocks, or clatter. No bubbles in radiator or milky oil. When starting, it fires right up no problem.

Here are the specs:

The engine is early 70's SBC 350 ('72 if I recall correctly)

Mileage is unknown but I would imagine it's got quite a few on it.

Intake is an Edelbrock performer.

Carb is Holley R84010 w/ manual choke, part of the 4010 series, I guess.

Here is what I have done so far:

Replaced the points distributor with an aftermarket HEI - no change

Replaced the plugs and ignition wires - no change - plugs are sooty.

Check firing order - it is correct.

Timing is OK.

Removed and reinstalled intake with fresh gaskets. - no change

Sprayed carb cleaner around base of carb while running - no change.

Run compression check as follows:
#1- 150
#2- 130
#3- 142
#4- 150
#5- 150
#6- 130
#7- 150
#8- 150

Engine has a PCV valve on one valve cover. Other valve cover has a straight line running to a fitting on the air cleaner. Vacuum advance line running to distributor is OK. There are no open vacuum fittings.

Any ideas? I'm out of 'em.

Advance the timing until it pings slightly on the road under load. Recheck # 5 and # 7 plug wires for being in the right order(easy mistake to make).Other than that,if you can get a scope (like a Mac # ET-985 Cylinder Balance Analyzer) that will kill each cylinder to see which one is the problem child.

Thanks gtow69,

I have had the timing set all over the place and the firing order is correct - same thing. Like I stated in my first post, I've had two different ignition systems on it with the same result. I'm beginning to think it may be a vacuum leak somewhere or the carb itself. Would this cause my plugs to be sooty and black?

Mike,

Dry sooty plugs are a typical sign of over fueling. With the HEI you should be able to fire through just about any rich mixture.

If you look at the list of things you have addressed, it covers all the suspects. I think this is why gtow69 recommended the scope check...you have already done the things most mechanics would have done and still have the problem.

The carburetor sure seems like a prime suspect to me. It is one of the few items that hasn't been swapped and the plugs are pointing to it.

If you have the ability, I would check the float level first. I would also dig around some more with the carb cleaner focusing on the intake to head area.

When you installed the HEI did you dump any resistor in the old ignition circuit? Is there a vacuum hose to the tranny modulator...is it in good shape?

You said it 'runs rough' but I wonder if you don't mean it runs fine but 'idles rough'. My bet is it runs great but just idles funky. You are getting close, don't give up!

Stuart

Small block Chevy V-8's are notorious for wearing out camshaft lobes, usually the exhaust lobe on #4 or #6 cylinder. It was most common on mid-1980's 350's and 305's. The engine gets progressively rougher-running, yet it will pass a compression test, and until the cam goes totally flat, a cylinder balance test is inconclusive. The exhaust gas that remains in the bad cylinder due to the partial valve operation dilutes the intake mixture when the intake valve on the bad cylinder opens, resulting in low manifold vacuum, intermittent bad mixture in other cylinders caused by exhaust gas dilution, and enough effect on the carburetor power valve to result in very rich mixtures.

Pull the valve covers, and monitor the valve lift, particularly on the passenger's side exhaust valves.
Jerry

wow jerry. thats probably the most interesting bit of information i've seen in a long time.

Thanks gentlemen,
Stuart, yes I bypassed the igntion resistor when I installed the HEI so that wouldn't be the problem. It idles rough but the "miss" is also apparent at higher RPM just not as noticeable. I will double check the hose to the tranny modulator. It would sure be nice if that were the problem. I like a simple fix! I'll keep you posted.

Jerry, I will take your advice and pull a valve cover and observe the valve lift. I really hope that's not the issue.

I did notice something I've never seen before. When I pulled the vacuum advance line I could feel plenty of vacuum but the inside of the line was wet with fuel. that shouldn't be I don't think.

If it ends up pointing to the carb as the culprit (it may be going that direction) I have a new edelbrock carb and manifold still in the boxes that I could try.

Thanks again for putting on your thinking caps. I sure don't want to pull this motor and replace/rebuild when maybe I could get it to run decent for a while. I hope it's something minor.

Might want to replace your plug wires & boots if you haven't already done so. They can melt & short out to the headers.

After a lot of semi-educated guessing, GM figured out the problem causing the flat cams was the EGR valve. The EGR passage ran through the cyilnder head right between #4 and #6, and the heat of the exhaust gas would cause the valve guides to tighten up. That overstressed the cam lobes. It would also "coke" the oil in that area to the point I've had to drive pushrods out of solid chunks of carbon lodged in the cylinder head.

In the mid-80's, GM would not warranty an engine if it ran 10W-40 oil, because the oil-coking problem seemed to be worse with that weight range than any other. The refused to specify a brand name that was causing problems- - - - - coughcoughquakerstatecoughcough! They just told dealers to refuse warranty coverage for all engines running 10W-40.

The change to electronically-switched EGR's in remote locations from the cylinder head/intake manifold area pretty much cured the problem.
Jerry

Thanks Britcon, I did replace the plug wires and boots with no change.

Hotrod Lincoln, I believe this motor is pre-EGR. I may swap out the carb and manifold to see if that makes a difference. I would really like to solve the problem but if I can't, I have a good running 283 that can go in with a little work. I just need to drill the intake manifold to take the vacuum fitting for the auto trans and get an adapter to hook that 283 to the trans so that I have a starter mount. The 283 that I have uses the bell housing starter mounting.

I still hope I can get this one to run decent though.

Mike,

I would check the cam with a dial indicator on all lobes, or replace it with a cam and lifter kit (because that what I would have done anyway). Then I would check the carb jets. If you have straight dual exhaust and are getting smoke out of both pipes, you are running rich on both banks. My guess is primary jets and/or power valve issue. Does your carb have the check ball to protect the power valve in event of a back-fire (Holley 125-500)?

This is standard on new carbs, not sure when it was introduced. A blown power valve will richen up the mix at all engine speeds

you may want to tune your power valve anyway (take idle vacuum and divide by two).


Found this bit of information on checking power valves here A blown power valve will have the greatest effect at idle and low RPM. You can check the carb by unbolting it from the manifold, putting the nuts back on the studs, and plopping the carb onto the studs. If you have an electric pump, turn it on. For a mechanical pump, completely remove the coil wire, and crank for about 15-30 seconds. The manifold and carb will be separated by a 1/2 inch or so, enough to see if any gas drips into the plenum when the engine is off (Don't try to start it!). If gas is present, check where it is coming from. There is a hole near the throttle plate that leads to the PV chamber. A blown PV will cause gas to drip from this hole(s). If it is dripping from the bores, the float level may be too high, or your carb has a crack.

Scott

I would pull the valve covers & make sure the valves are all opining & closing like HRL says. Also look for any valves standing taller then the others. That can indicate a recessed valve seat. If not that it,s looking like the carb. Loop the vacuum hose up high between the carb & dist. That way any gas that gets in will flow back to the carb if the vacuum chamber is not leaking. Same goes for the power brake hose.

I don't even have my motor running yet but am fascinated by all this information. You guys could write a book on this stuff. We should form an association and publish some kind of book on Chevy engines or AD trucks or something. Think about the notoriety the prestige the power... We could be on all the morning TV shows. Ah, but I'm just a dreamer...

This is interesting. Jerry I knew Chevy had cam problems, but had never read anything about why. My EGR equipped 350 is running rough and now I'm wondering if that may explain it. I was already going to spend time troubleshooting it today.

No one has mentioned the compression differences. Cylinders 2, 3 and 6 are a bit lower than the others. I thought a 10% difference highest to lowest was considered enough to cause concern. The 20 psi difference 150 to 130 sure exceeds that.

Could the rough idle be in part the result of the low compression in three cylinders?

Yes, compression differences can contribute to rough running. One way I learned to check on such problems was the original cylinder balance test Sun Electric Co. reccomended. The engine was run on only two cylinders, like #1 and #6 on a small block Chevy, for instance. Those cylinders are exactly 360 degrees apart in the firing order. The manifold vacuum at 1,500 RPM was recorded for that cylinder pair, then the next set of companion cylinders, 8 & 5 would be tested, then 4 & 7, then 3 & 2. A noticeably lower vacuum reading would indicate trouble in one of the two cylinders being tested. That procedure worked better than shorting out one cylinder at a time and recording RPM and vacuum drop. Yes, an engine WILL run on 2 cylinders!

Another test is the cylinder leakdown test, where a pressure regulator is used to set 100 PSI against a dead-ended air line, then the air is fed through a nipple adapted to a spark plug shell into a cylinder at top dead center with both valves closed, and the crankshaft locked. A pressure reading is taken. Let's say the cylinder retains 92 PSI. The leakdown is 8% for that cylinder. By listening at the intake, exhaust, and oil filler, and looking for bubbles in the radiator, the leak can be isolated very accurately.

All these tests can be done in a relatively short time, with little or no disassembly of the engine. Sun's slogan used to be "We Test, Not Guess!"
Jerry

When I asked for help with the diagnosis I didn't expect to get so many responses. Thanks to all of you. You have certainly given me a lot to consider.

I was wondering about the compression differences and whether that could actually be a cause for it to run rough. I guess I still have some diagnostic work to do.

The motor is not EGR equipped but of course that doesn't mean there can't be an issue with flat spots on the cam.

BTW, I verified that the engine is a 1972 model.

As far as I can determine the problem can be in one (or more) of three areas. Ignition, fuel system, or mechanical. With the tools and resources I have available I have decided to approach this methodically and try to eliminate one thing at a time.

Since I have replaced the points distributor with an HEI and still see the same problem it tends to make me think that it's probably (I do say "probably") not the ignition.

That leaves me with a carb/fuel system/vacuum related problem or a mechanical problem. I think I will install the new carb and manifold that I have in the shop and see if that improves anything. If it does, great! If not, then I move onto the mechanical possibilities.

Hmmm...mechanically probabilities probably point to valve train issues. Could be: flat cam, the timing chain had jumped a tooth or two (I have never had that happen but I guess it could be theoretically possible), bent pushrod, or a problem with one or more valves, rocker arms or valve seats. I'm not getting any funny valve train noises that I can detect.

It I can eliminate any possibilities other than mechanical (I guess I should say "probabilities") then I have to determine whether I want to tear into this motor or not. I don't want to get into a total rebuild which is where this could lead and I don't want to throw money at an already tired motor by patching it up. You know...like going through the heads and doing a valve job without doing the bottom end. Frankly I don't want to spend that kind of time or money right now. If it's an easy and inexpensive fix then OK but if not then I have a good running 283 waiting in the wings. I'd just like to have this one running right if at all possible.

I guess my questions now are: Does my thinking seem logical? Is there something I'm not considering?

I should have that new carb and manifold on in a few days but before that I will pull the valve covers and do a visual inspection of the valve train. I'll keep you all posted.

Thanks again guys!

If your timing chain is sloppy, pull the distributor cap, and rotate the engine backwards. If the engine moves a fair amount prior to moving the distributor, you could have a really sloppy T-chain. In fact, since you have the intake off, I would spend the $50 for a new T-chain anyway, and since you are in there, pull the lifters and look at the lobes down the holes

Scott

If there is any vacuum line comeing out of the opening in the intake runner behind the carb. plug it off & see if that effects the engine. A vacuum leak in this area can act like a miss on 2 cyls. because it draws on the one runner. Get some plastic pliers & pull one plug wire at a time & see if you can isolate it to one cyl. It even helps to lock the emerg. brake good & put it in drive. This is a poor mans Sun machine. A rich carb can put soot on the plugs & cause missing & rough idle.

Come on lets go back to basics.He claims it sounds like it's missing.With the engine running and no special gadgets remove one spark plug at a time .Listen for the engine rpm change when you remove and reinstall the wire.You find one with no rpm drop you have your cylinder and diagnosis from there.If they are all the same then you look else where.No one mentioned a running compression test ,should be 50% of the cranking compression.

Had a firebird with a transplanted 350,It alway's had a miss that I couldn't track down.I went through all the above mentioned stuff and gave up!I gave the car to my son who took out the motor and tore it down looking for anything that could be a problem.Found one piston (don't rember wich one) the top ring land was gone.even though the compression was equal all the way around?Sometimes thing's are hard to find without a full teardown.My 2 cent's. Tom

My first clue was the black sooty plugs. Some one already addressed over rich fuel. Check the carburetion first. I don't know what those carb numbers given were, but if the cfm is too large, you could be combatting that.

Also, it is assumed that the engine has a fair amount of miles on it. One could also assume that the valves and guides are loose and the valve seals are no longer performing their job. This would allow excessive oil to contaminate the cylinder and create a "sooty" problem. That excessive oil could also give an inaccurate compression test. Like HotRodLincoln suggested, a leak down test would help to determine some problems quickly.

OLD GOlD,
Feel free to drop me a PM I can help you through the process of elimination.

61 Suburban

I would do a leak down test as well before replacing anything. Start on the cyl. with the most loss during that compression test. It's a quick and easy test. They sell leak down testers (cylinder differential test gauges) from the major tool suppliers for this test or make your own.

your statement
Replaced the plugs and ignition wires - no change - plugs are sooty.

after replacement of the plugs how long does it take for them to get sooty?

when you are at idle.... and you rev up the engine, can you notice the engine cut out and run rough at idle too?

this may sound crazy and it should not account for the sooty plugs but sometimes if you have a bad motor mount it can cause similar condition about running rough when you put it in gear, usually it will clear up after you get moving.. so can a bad valve.......

does it back fire at all?

I replaced the carb and manifold with the new Edelbrock pieces I had and NO MORE SMOKE or sooty plugs. That old Holley looks like it may have been the culprit as far as that goes.

It runs better but still not perfect. a little rough at idle still. I am getting a little backfire on occasion, nothing huge but it tells me something isn't quite right. I think the old motor is tired and may have some valve issues.

Joker, the motor mounts seem to be OK.

I haven't performed a leakdown test yet but I'll have to do that soon so I can learn a little more about what may be happening.

I also changed the oil and noticed that it seemed a little thin and had a slight gas odor.