Technical Part-throttle popping through exhaust under load?

Ever since I got this 56 Olds a few weeks ago I've been working on dealing with some mechanical gremlins. For those who didn't see my thread, it's a 56 Olds Holiday 98. Basic running gear consisting of the factory 324 Olds with a Jetaway.

Under part-throttle acceleration, the engine will pop/crackle out of the exhaust. It has a harsh, choppy idle. Under WOT, it is far less prone to the popping, but seems down on power, and seems to struggle to pull through the RPM band, though it gets there eventually. On the highway, it sails down the road at speed, but when the road grade increases or if I want p***, when I lean into the throttle (not floor it) it starts popping back through the exhaust. Around town, under low rpm under load situations, it will pop out of the exhaust. Under both scenarios it may also either backfire in the intake or detonate, which causes a stumble, and a noticeable odor in the car. The latter is less frequent than the popping in the exhaust.

When I got the car, it ran but poorly, as it had been sitting for a number of years. In the past 2 weeks I have performed the following;
-Replaced the Rochester 4jet with a newish Holley 600 cfm, electric choke, vacuum secondaries. Carb has approximately 5000 miles on it, and was known to me as a flawless runner until I removed the engine from the car it was in about 3-4 months ago. Floats were checked and seem to be properly adjusted.
-Replaced the fuel tank
-Removed the factory mechanical fuel pump, installed a block-off plate, and added a new Carter rotary vane style electric pump. I also plumbed ALL new lines from the tank all the way up the carb, including a new sender, and filters before the filter and carb. Quite literally the entire fuel system is new.
-Swapped the factory '56 distributor for a '61 distributor (swapped the gear) with a Pertronix ignitor. I also swapped in a new vacuum advance and rotor while I was in there.
-New plug wires
-New plugs gapped to .035 (Pertronix calls for .005 additional gap over factory spec)
-Timing appears to be correct. I rotated the engine over by hand, with valve cover off, watching intake shut, #1 plug pulled, and my hand on a screw driver feeling the piston rise and stop. This correlated to the factory timing mark on the balancer. I set it at 2 degree BTDC, which is what my Motor Manual calls for. I also tried advancing the timing to about 7-8 degrees BTDC.
-Nothing seems amiss under the left valve cover. I haven't pulled the right one yet.

At first I thought it felt like it was a fuel delivery issue, like the car would run itself out of gas through an obstruction.

Then I thought maybe it was an advance/timing issue, or even a misfire from worn components.

With the changes I've made, the car runs much better, and is drivable. The Pertronix especially has made starting the car worlds easier, a night and day difference. But the issue I've described has persisted. It has done basically the same thing with both carbs, both fuel systems, both points and Pertronix, with and without the new vacuum advance, and at both initial timings. That's telling me it's an issue independent of that. I'm running out of stuff to change.

The idea of a bad cam has crossed my mind, but I feel like that would cause symptoms at all RPMs and driving conditions, not just some. The other thing that has crossed my mind is a bad intake gasket or bad vacuum leak, which could be causing a lean condition a**** other issues.

Anybody have any other thoughts or suggestions?

 

Sounds like a cam lobe is going away to me.maybe ?

 

I'm not sure, I've never had a cam go bad on me. I have had a rocker loosen up, and when that happened the engine sounded like it was going to come apart and sounded like death at all RPMs. If the symptoms of a flattened cam lobe are similar to a loose rocker, and net result is a loss of valve lift, then the symptoms I'm getting now are nowhere near that extreme. But again, I've never had a cam go bad on me, so I really can't say.

 

Measure, don't guess.! Use an old MoToRs Repair Manual and perform a full Tune-Up. Compression test, and a vacuum test will tell you quite a lot about the internal condition. Some Marvel's oil or your favorite potion in the gas and oil will help free up sticky piston rings, valves, and lifters from sitting so long.

Don't overlook silly stupid stuff, either. Make sure for example, all the spark plug wires are in the proper firing order. This happens more often than you think, and it might not even necessarily run particularly poorly, though it will under load.

 

When the distributor was changed, which cap was used?

Ben

 

The one off of the '61. Like the carb, that distributor was working just fine on a 394 4 months ago. I plan to replace the cap as well shortly.

 

I am with Truck64, do a compression test and use a vacuum gauge. That will give you quite a bit more info on the condition of the "innards".

 

Could have a weak exhaust valve spring causing it not to seat well, or maybe some carbon not playing nice. I'd check the ignition at night and see if any sparks are jumping around and do the compression check that was suggested. I'd try checking the springs and maybe the old water in the carb trick...........

 

If you don't already own a vacuum guage, here's a good excuse to go out and getcha one . . .

http://www.gregsengine.com/using-a-vacuum-gauge.html

 

If the car ran well before being idle for several years you probably have some sticky valves and rings. A vacuum gauge connected to manifold vacuum will tell the story. If the gauge reading fluctuates at idle, you have valves that are not sealing. Dump some Marval Mystery Oil in the gas tank and the crankcase and drive the hell out of it for awhile. You'll be amazed at how much better it runs.

 

My dad's 56 Chevy did the exact same thing, but probably worse, would die once in a while. Started easy but just had no balls, popped through the exhaust, would not accelerate.

It had a 235 that had a Pertronix kit swapped into it. We dug into everything, rebuilt a stock carb to replace the new replacement that had already been swapped onto it, dropped and cleaned the tank and lines, lots of tuneup parts, ect. It was perfectly timed when the Pertronix got swapped in.

After a ton of frustrating measures, we finally figured out that the Pertronix kit does not "clock" itself to the same spot as factory points. It essentially moved the timing by 15 degrees without actually moving the distributor. 235 distributors don't rotate in the hole like most V8 distributors do, you have to loosen up a collar on the distributor and rotate the head of the distributor while the base stays stationary. Once it was re-timed, it ran perfectly.

I'm not sold on the Pertronix kits in distributors. I've known of a few that had the magnets fly apart inside the distributor at speed, crippling the car, and trying to figure out the 235 was extremely frustrating.

What made you decide to swap out the distributor in the first place? My mechanical experience is very limited compared to others here, so I could very easily be wrong, but what you describe sounds like a timing issue to me.

 

There were a couple reasons as to why I wanted to swap the distributor. First, the lack of availability of quality points, or frankly any points at all. The reality is that if I'm out on the road somewhere and my points burn up, I stand virtually no chance of finding points over the counter at a Pep Boys or Advanced. But every major auto parts supplier carries a Pertronix ignitor in stock. Secondly, there's less to go wrong with the elimination of the ballast resistor and condenser. And finally and perhaps most importantly, putting a full 12v to the coil instead of the ~6-9v to the points makes for a much hotter spark. I can say here that after swapping the distributor, it was a night and day difference in how easily the car started and how it needed virtually no time to warm up. I was impressed.

If the timing was off though, wouldn't that be indicated at the timing mark with the light? The engine was manually set to TDC, which was independently correlated to the timing mark. If the distributor was off, wouldn't that indicate with the timing light that's based off the fire to #1?

I hear you in regards to this feeling like a timing issue. Any time I've had a car pop through the carb it's been a timing issue. But this is primary an issue with the pop out of the exhaust, not the carb, and only under certain conditions, so it's a confounding issue.

I'll grab a vacuum gauge and see what it tell me.

 

Vacuum guage and compression check.

 

I agree with a compression check and running the engine with a vacuum gauge if the compression is okay. However, is there a chance that you have a plug wire out of order?

 

I highly doubt it. This has been a consistent issue since I loaded the car on the trailer bringing it home. I’m certainly not above making a mistake, but I’d have had to made it 3 separate times and with the same result as when I got the car.


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But, what if it was wrong from the start?????? If you swap 5 and 7 on a SBC, it runs, is down on power, and pops out the exhaust.

 

The Pertronix is GOOD. Bet you a steak dinner.
I was thinking the distributor cap. You changed that, though.
Probably when you find the problem, you will kick yourself because you did not twig right away.

Ben

 

Mechanical advance free? I put in a Pertronix once and the slip on reluctor held up the advance.

 

Ok, I put a vacuum gauge on it and **** is definitely not right.

At idle the pointer bounces back and forth between 5-15 inches, which coincides with the idle chop. If I listen closely under the hood, I can hear a light tap that goes with it.

Blip the throttle and it will immediately drop to 2 inches, then up to 20 coming off.

At a constant 3000 rpm, at first it bounces a bit around 15 inches, but then smooths right out at 20.

I’ll do a compression test tomorrow. I need to keep the car mobile for a couple days since it’s behind my daily driver in the shop.

Thanks again for all of the replies, this is a helpful lesson for me in engine diagnostics.


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When I first got my 98 coupe 20 years ago, it hadn't been running for a number of years. Finally got it to go but would only run on 6 cylinders. Found out the cam had 2 perfectly round lobes, apparently a known problem with the rocket motors, through lack of oil getting to the top end. Found a replacement cam and all was well for a short period of time, until a skirt came off a piston.

 

This should help you figure out what the vacuum gauge is trying to tell you.

https://dannysengineportal.com/vacuum-gauge-test-tells-you-a-lot-about-the-condition-of-your-engine/

 

Great link. It would seems a sticky valve(s) could be the culprit. Which would make some sense given the pop out of the exhaust, and occasional pop in the intake despite seemingly correct ignition timing.

I painted a couple rooms in the house today, but tomorrow I'll do a compression test and see which cylinder or cylinders are acting up. If a valve is hung up, I'd imagine compression will read low in that cylinder, and the rocker will probably show excessive lash. Maybe some Marvel Mystery Oil or Rislone will free that up.

 

It could be something as simple as rust on the valve seats from the engine setting . Try getting the motor good and warm in take a bottle of water with it idling fairly fast slowly feed the water down the carb and it will turn to steam and clean up the carbon.
I have used Sea Foam many times to remove deposits and carbon build up in cylinders.
Saved a family's vacation once when I worked at the Fred Harvey garage at the Grand Canyon. They came in with the Ford 6 cylinder really knocking and they were on their first day of a 2 week vacation.
the old Granada had been used to run around town and it built up carbon on top of the pistons. They got it out on the highway and it was hammering.
A half a Solo cup of water later and their vacation was saved. Got me a 50 dollar tip and pissed off my boss because he told them it had to be rebuilt.

 

I would be looking at your electrical system. With the engine idling tug on your wiring harness,watch for engine stumble .look for wires Frayed,burnt,pinched or on exhaust .Take a close look at that distributor.

 

There's no subs***ute for a compression test in a full Tune-Up but the vacuum test with a mechanic's gauge comes pretty close and is fast. Testing compression is kind of a pain in the *** at best and abuses the battery and starter a little bit. When a stock engine pulls a steady 19" to 21" the compression is more than likely very acceptable, goose throttle and the needle should drop to about 5" and then quickly rebound to 25".

If the rings or cylinder bores are worn excessively the compression will be low, but oftentimes low compression is caused by gum, carbon, and varnish. A solvent can really help with this. Basically a kind of glue has rendered everything stuck, especially after sitting a long time.

 

Alright, we're getting somewhere. Got out in the shop and continued the diagnostic journey.

First, before pulling the plugs for a compression test, I wanted to make sure visually everything was moving. I pulled both valve covers and fired it up. Visually, everything moves. Rockers go up and down on all cylinders, pushrods spin freely as the engine runs. There doesn't appear to be any excessive tapping from the rockers and the tone doesn't change when I take my hand and push on the moving rocker.

Next onto the compression test. My Motor Manual calls for 150 psi at cranking speed. Here were my values:

1 - 125 psi
2 - 130 psi
3 - 140 psi
4 - 137 psi
5 - 145 psi
6 - 150 psi
7 - 0 psi
8 - 145 psi

Well, well, well... a dead cylinder. Since this is an old, unrebuilt engine, I didn't expect factory compression, and aside from #7 reading zero, none of the values really give me cause for alarm.

Next I ran the compression test on cylinder #5 again and let it sit there. My thinking being that if the head gasket was blown it would be bleeding my compression into the neighboring cylinder. But after 45 mins, the needle hadn't moved, so I'm thinking the head gasket is probably OK.



I put an inspection camera down the spark plug hole in cylinder #7 and observed a wet piston, and when I dipped a screwdriver into it and pulled it out, it seems to be a combination of unburned gas and oil, making me think the valve guides are toast.

My impression now is that somehow the valve or valves in #7 aren't seating or hanging up. Again, the rockers are moving, so there is at least some lift, but there's n compression. If the exhaust valve is hung partially open, it would explain the pop. But I wanted to get a better idea of what's happening before I start dumping **** into the engine to try to free the valve up.

Any ideas on how to free that up?

 

Me, I would just bite the bullet and pull the head.

Ben

 

Hook up compressed air and see if it comes out the intake, exhaust or cc.

 

Great idea. Might get the trifecta there haha

 

I had a similar experience with my 55 tbird with a 351 Cleveland. Same symptoms you have. Changed distributor, carb, timing ,etc. It was a exhaust lobe going flat. The more I drove it, the worse it got, was a new mellings cam and lifters. Good luck