Technical Odd test result while diagnosing rough running engine

I'm hoping this group might be able to lend a little help. I'm in the process of trying to revive a SBC 350 in an old '59 GMC 1/2-ton. The motor is from a '72 A-body with a Quadrajet carb and an HEI distributor. It had been running reasonably well until about a year ago when the carb started flooding and pouring gas all over the intake manifold. I tore it apart for a quick clean and rebuild but thereafter its been a struggle to get it to start or idle at all. I've tried swapping carbs with a known working Q-jet unit with no change. I pulled the intake manifold figuring maybe something had fallen in and been sucked down one of the runners during the time I was rebuilding the carb and was restricting airflow to one or more cylinders, but it was clear. I've double-checked timing and it is at 16* at idle with the vacuum disconnected coming up to 32* by 2500 RPM. I've replaced spark plugs and wires, but not the cap, rotor, coil, or HEI module as all are relatively new and at least for the cap and rotor show virtually no signs or wear at all. I ran a compression test on all cylinders:

1 - 178
2 - 178
3 - 187
4 - 165
5 - 190
6 - 165
7 - 172
8 - 170

It seemed a little odd to me that 3 and 5 were so significantly higher than the rest, but not enough that I figured it would cause the engine to run this poorly. The thing that really left me scratching my head was the cylinder power balance test. As I was pulling spark plug wires and recording the RPM dips, when I got to cylinder 5, not only did the RPMs not go down, they shot up from around 620 to nearly 950 and stayed there idling smoother although not smooth. Once I reconnected the plug lead, the RPMs dropped back to 620 and really rough. I had a similar result with cylinder 4. All others recorded about a 50 RPM drop. I'm really not sure what to make of it. Any thoughts or suggestions would be welcome.

 

Plug wires routed correctly? Odd disconnecting one would increase RPMs, was it #5 or 7 by chance?

 

That's exactly what I thought. It happened on both 5 and 4. I've checked the wires at least 6 times, including disconnecting and running them all from scratch twice. I think I might start reciting 18436572 in my sleep...

 

And you got the cyl numbering correct? 1357driver 2468 pass

 

Lol. I had 5 and 7 swapped before, ran decently, but did lack some power. I never went through the unplug a wire a time drill though to see what would happen. My buddy had 5 and 7 arcing to each other, but I don’t recall his issues on how it ran. But you have new wires so should eliminate that I think.

But back to when this started happening with the carburetor, etc. my feeling is it has to be related. How? I don’t know.

 

Yup. And don't worry, I won't be offended if there are any seemingly obvious questions here. I'm hoping it will be something simple and stupid I have somehow managed to overlook rather than a serious problem that could require major surgery. I haven't double checked valve lash yet, but that might be next. Although I can't come up with any reason that lash and this could be related.

 

I'd be checking to see if the valves are all opening fully. If you are losing cam lobes, weird things like that can happen.'

 

If an exhaust valve isnt opening it would still give compression and could give you results like yours . Especially the high cyl readings .
Having a fuel dump could kill your oil ( thin it ) and kill your cam . You need zinc in all non roller older chevys . Many cam lobes rounded with out .

 

A vacuum gauge is a great diagnostic tool. Connect one to any manifold vacuum source and see what that tells you.

 

Id pull the valve covers an check lash , im guessing you rounded a few lobes and the high compression cyl are not letting the pressure out and continue to pump pressure via the intake valve working .

 

Everyone says you need 3 things to run BUT its not correct . You really can have compression and no exhaust valve = no fire . It drove me crazy about 50 yrs ago . Bent pushrod on the exhaust ,

 

This ^

 

So youre thinking , why would my rpms rise when I remove spark , well , think of what happens when no exhaust is allowed. You have an explosion pushing the piston down but trapped , now the intake opens and it contaminated the incoming charge , and other cylinders. Now remove the spark and no contaminated mix .
Intake , piston is going down sucking in mixture , compression piston going up compressing the mixture, combustion, boom piston going down with expanding exhaust gases NOW THE PROBLEM, piston going up but NO exhaust so the charge is still compressed and being recompressed , intake opens , pressure in the cylinder is more powerful than the vacuum sucking in soo it escapes and contaminates the charges .

 

Time to walk the dogs .
Tomorrows coming fast , good luck

 

That totally makes sense. It has been a long road of trying different things since the carb originally started flooding last year. It totally makes sense that the oil could be significantly contaminated and the lack of lubrication has caused an issue with the valve train. I will pull the valve covers tomorrow and check all the valve actuations to see what’s happening. Thanks so much for the tip. I hope I can get away with just a pushrod replacement, but I suspect I may be looking at valves/guides too...

 

If you've got one of those hand held infrared thermometer guns, run the engine a few minutes to get things warmed up. Point the thermometer at the exhaust manifolds or headers near each exhaust port. You'll probably see lower exhaust temps at the ports for #3 and #5.

You might even be able to spot a couple of "colder" exhaust ports with a squirt of water on the manifolds and watching how quickly it boils off at each exhaust port. Or doesn't.

And don't be too surprised if you find a couple of bad exhaust lobes.

 

Valves and guides should be okay , the cam and lifters are the high pressure areas that need good oil with zinc . You cant use regular mobil 1 and think its okay . You need to use a racing oil or any oil if you add zinc addative . Coffee time .

 

On the cylinders that picked up rpm were you holding the wire off the plug allowing the spark to jump the gap ? if so the 2 plugs are probably just dead and holding the wire off increases the energy of the spark firing the plug.

 

First thing I would do is put a single points distributor in with a resister and a stock coil. Those HEI units can cause all kinds of weird issues.. I remember in the late seventies, the mechanics at the tune up shop next door to me would be cussing up a storm when those HEIs first came out and not too many mechanics had any experience with them.. I got to laugh, today people love them..
I'm still into the single points distributor triggering a CD box ignition...

 

Q jets are finnicky. I believe they are the best street carburetors ever devised. Back when I was cutting my teeth on Chevys (late 1980s), I believed a Q-jet really had one good rebuild in it. After that one overhaul, it needs special attention by someone that really knows their stuff. Back then we would run one, build it and when it started to act up, we'd find another. We learned to avoid the later smog models. Keep in mind that they were 15 years old then, now they are over 50 years old. A "good one" now may be one we would have put on the shelf then.

Engine...
As Squirrel says I would look at the valves. They may be adjusted too tight. If the engine is running now, I would check it with a vacuum gauge. Look for that needle to jump.
I would try a "hot lash" adjust on it. An old valve cover with the top cut off is a useful tool. Less mess.
With a 49 year old engine it's very possible that the rocker studs could be lifting. Check those rocker studs and see if any are obviously taller. It's common for rocker studs to "pull" on SBCs.

 

vacuum gauge trouble shooting guide here might give you some clues...

Vacuum Gauge troubleshooting (earlycuda.org)

 

Usually when a exhaust lobe fails and exhaust valve stays shut, the exhaust has no where to go and when the intake valve opens it "pops" out the intake valve into the carb. Seen that alot back on the 80's SBC's.

 

Thanks everyone for the input. I should have some time later today to dig in a little further. It sounds like the quickest, easiest, and cheapest next step is to pull the rocker covers and check valve movement and lash. Logically, an exhaust valve that is stuck shut, or not opening enough, forcing combustion gases back into the intake makes a lot of sense to me. If that’s not the case I figure I can look into additional areas like the dizzy.

I hadn’t mentioned the vacuum gauge readings earlier because I found them to be too wildly erratic to be of any use. They were literally all over the dial and not rhythmic in any way that I could tell. Exhaust gases forcing their way back into the intake could definitely explain the wild needle. I will take a look at that vacuum gauge chart a little closer though and see if it can provide any additional ideas.

As for the spare carb, it was lent to me by a neighbor off his running vehicle strictly for ruling out my carb rebuild. His was purring like a kitten and mine sounds like a wounded wildebeest in its death throes. Popped it back on his ride and it’s perfect.

To answer TA DAD’s question, no, I pulled the plug wire away completely from the plug. Got my fair share of shocks in the process.

First thing for today is valve covers. Wish me luck.

 

Honestly, change the spark plugs. In a lot of cases that I have seen by having a fuel system go out of whack it can compromise the plugs and cause a weakened spark. While you’re compression readings are somewhat uneven, it’s not that alarming considering an engine with hydraulic lifters has to be ran 5-10 minutes and then tested with the throttle wide open to get an accurate reading. And also, plugs are cheap and a good first place to start

 

Yep, spark plugs and wires were changed after I did the compression test, given that I already had the old plugs out. They were pretty fouled but it ran exactly the same with the new plugs and wires, which led me to try the cylinder power balance test. That's when I called in the cavalry. Thanks again everyone. As soon as I can sneak out of work today, I'll pull those covers and give everyone an update.

 

When you replaced the carb/ manifold gasket(s) are you SURE you used the ones correct for that carb/ manifold combination ? Many used multiple gaskets with heat deflector plates that blocked heat riser ports in the manifold , engine would still run albeit with excess exhaust dilution .

 

I'm not good at posting things , this is the best I could do . Do some research on " SBC hot slot manifold"

 

Yeah, I got the 1/4" thick insulating gasket for the base due to the unrestricted heat riser ports for the divorced choke thermostat.

 

If it has exhaust there, you need the stainless shim and high temp gasket

Sent from my Trimline

 

For that vintage , the the thick base gasket may be the wrong choice