308 Carburetor popping issue

This has been a persistent problem for some time.

It seems to happen with the majority of the cylinders, but the worst is with cylinders #7 & 8.

There is popping up through the intake trumpets at somewhat consistent intervals.

I have a YouTube video of the phenomenon that is unlisted (private) so can be only viewed with this link:



You can see the needle on the air gauge fluctuate with the pops. This is not a cold engine, it is at operating temperature and when I shot this video, the outside air temps were quite warm (like 80*F. May have been even warmer than that. Put it this way, I was pretty sweaty when working on this).

I rebuilt all of the carbs with new gaskets and regreased the throttle shaft bearings - no change. I swapped out the positions of the carbs to see if the popping would travel with the carb in the new location on the engine - no change. I checked valve clearances at #7 and the other cylinders as well - #7 is spot-on (Int = 0.008", Exh = 0.013") and the other cylinders are within range.

I've ordered new caps and wires and will install those once they arrive (I had new rotors already). I've replaced the extenders some time ago. #7 spark plug is oily though. Not dripping wet, but not dry and properly-colored like the other plugs.

I haven't checked fuel pressure yet, so I don't know if that could be a culprit. I did replace the fuel pump approx. 4-5 years ago with a NOS, OEM BCD-Corona pump, which I bench-tested before installing and it was putting out the correct pressure and volume. It would have very little mileage and time on it.

I had also installed next half-size up idle jets (so 57 instead of the stock 55) and it improved my readings on my exhaust gas analyzer (I don't have the results as I had been pressing the wrong button on the machine and it didn't get saved). It improved the readings from the last time I measured with the 55 idle jets installed and the car does drive better with the 57s. In fact, this popping issue seems to be only happening at idle and once warm, is hard to keep adjusted to maintain a constant speed. Prolonged stops at traffic lights and it's almost like it's going to stall.

Anything else I should look into that may be the cause of this? My good friend Rob Garven mentioned the carbon canister lines and this system is active on my car and is tied into this bank as per original spec.

Thanks in advance for any suggestions...

 

Are the cylinders balanced? This usually calms more and more as it gets 'dialed-in'. Have you pulled out idle jets to look for blockage. Right after carb work, I've seen particles block these. Comes from disturbing hidden passages. I'm assuming carb o-haul didn't include removal of all lead plugs to clean passages. I've had one ridiculous carb problem which finally was resolved by pulling all of the plugs to clean passages. One seemingly non-critical passage had a lot of carbon in it, and wouldn't let that carb set correctly. (How we love them so!)

Also - float height set perfectly? Since you mentioned having a gas analyzer, have you looked at the offending cylinders? Can you look at lambda? I find easier to deal with than just CO. Is the mixture relatively close between cylinders?

And of course, ignition is carefully set on both banks? If one bank is pushing or dragging due to timing discrepancies, this makes the process that much more fun.

Most importantly ~~~~ Remain patient. You'll find it.

PS: Motor looks great!

 

Have you tried a small turn rich on that cyl?

 

Popping back through the carbs is often due to a lean condition. Agree with the advice given by absostone.

 

When I had that problem with my 308 GTS, it turned out to be a burned exhaust valve.

 

check compression first before you try to do other things.
if ok then otherwise I think to lean mixture

 

I’m no pro.

Problem didn’t follow the carb when swapped, stayed at same cylinder.

Oily plug of the involved cylinder sounds like an issue that can’t be ignored. Agree with compression and possible leak down test.




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I didn't read that well this morning

 

Yes, the air flows were all dialed in to the 5kg mark (keeps the engine around 800 to 900 rpm idle speed).

I submerged each carb in my ultrasonic cleaner using gun wash, but no, I did not drill out the lead plugs. It was quite effective though as I had to slop the bath out with each carb (the solvent was that dirty). I blew out each passage with compressed air.

I’m running the Pertronix triggers in one dist. body (the MR 193 unit I believe. It houses both triggers so the advance curve is the same for both banks). Yes, timing is set at 7*btdc. Floats set with my vernier dial caliper.

My analyzer uses a probe that goes into the tail pipe, but I have extra fittings that I was planning to set up a hose so I can test the individual cylinders.

 

Yes, I thought this was that lean spit condition, so yes, I gave the mix screw a good turn out (plus the bigger jet).

 

Ugghhh...

 

Yes, that was next on my list of checks - compression/leak down

 

Peter- Will keep my fingers crossed it is not a burned valve.

 

Me too, you think valve was tight and hung open?

 

But why would that cause the hicupp at idle?

 

Beats me, but that is what she did, spitting at idle. Could not hear it at all at speed.

 

Had a friend with a Ford 351 with 4 Webers on it. Just couldn't get it to run well. Had the same issue with popping thru one of the carbs. He finally gave up and decided to switch to an Edelbrock manifold with a Holley carb. As he was taking the old manifold off he could see where the intake manifold gasket must have shifted a little when the manifold was installed. The out of place gasket caused a small intake leak. He reassembled with a new gasket and it ran fine after that. I'd check for a leak.

 

Ha, I'm keeping every finger on my hand crossed that it isn't a burnt valve!

 

That's why I pulled the valve cover off to check the clearances... I was thinking along the same lines that if there was carbon build-up, or a bent valve, it would drop the valve down and there'd be a huge gap. Not so, every intake clearance was at 0.008".

 

Makes sense. When I did my SBC 305 motor out of my truck last year, I found it tricky trying to get those things to stay put while dropping the intake manifold in place.

A multi-carbureted engine like this is so much more sensitive than a single carb/TB feeding all cylinders. That's why Rob mentioned the carbon canister purge line and tubing at the carb bases as maybe a source of a leak (which, #7 and 8 are first in line in that hose's run). FWIW, I replaced the three-way valve a few years ago, when a batch of NOS OEMs were available. Considering it was NOS, it would still be a part that would be a couple of decades old...

 

Before I cleaned the carbs, I noticed certain throats had heavy soot below the throttle plates. You'll notice the cylinder numbers I marked on the base of the carbs.

As I mentioned, #7 popped the most, but there were traces of popping from nearly all carbs (hard to tell as they were faint and sporadic, so harder to pin-point which ones they were).

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The carbs on the US cars were set up lean to get through emissions, which leads to the popping.
Clean, re-jet, and sync the carbs as per "Birdmans" instructions here. It cured my popping. I also converted my points to Pertronix MR-183 electronic ignition and changed the plug wires.
My 308 now runs great.

http://www.birdman308.com/service/floats/float_height.htm
http://www.birdman308.com/service/sync/carb_synch_tutorial.htm

 

Gotta ask just to rule out~~ Was car running properly before recent work? This would help rule out the 'non-adjust' issues. ie dist caps / wires / basic engine hardware. Now, to make it run right. First off (IMHO) I wouldn't be concerned as to the flow number. More important is that all pots are breathing the same amount. I concur that lean will cause this, but you checked with emission analyzer. If too lean, HC would be very high. Too rich and both CO & HC would be high. Does your machine read AFR or Lambda. I really like this function because targeting a CO level is very cookbook and works well after everything else is dialed. Also, having the ports available to read on each cylinder is a great diagnostic tool.

Some things to watch for that tripped me up: Don't get too sensitized by carbon soot by butterfly's. The vac line for the canister was a concern, so I cleaned thoroughly the internal passage. No help. I plugged it off for testing - no help. Misfiring and popping on multiple cylinders can simply mean cylinders pushed or dragged. These motors are quite tough, and while big events can happen, are on the rare side. Inspection tubes can crack. If this occurs, CO leaks out immediately. This scared me that I had dead cylinders. Weirdest carb problem by a large margin was a scrupulously clean carb still had debris under a lead plug, and only after removing plug and getting lots of carbon soot from passage, did idle jet respond and allow tuning. (BTW - in this case, I was running v-large idle jets, like 68 or 70 to tune but was not comfortable with this anomaly.) After the soot-ectomy, the 55 was right on. My car has some miles on it, so this might possibly explain the collection of soot? I diagnosed this problem using a gas analyzer, and going 1 cylinder at a time after I was confident in synch.

When you set the balance bank-to-bank, did you have the linkage disconnected?

Are you getting upstream of cat emission data?

Hang in there!

 

I once cured the popping by changing the plug wires. Three were bad. Good luck,Mike

 

Yes, I've got the Pertronix set-up in my car.

I've always set the float height to 50mm.

I had experiences many, many years ago that if the float height was any higher than that, the car would die stopped facing uphill. I live just out side of Vancouver that is relatively flat, but the city itself is very hilly in some parts (think to a lesser degree than Seattle, or San Francisco). Whenever I'd stop at a light on a steep road, the car would start to burrble, sputter and felt like it was going to die. Only heel-and-toeing the brake and throttle would keep it alive. Totally embarrassing. I bench tested the carbs and yes, if you tilt them far up enough, fuel will spill out of the float bowl vent and pour down the main jet wells (and then trickle out of the discharge venturis) causing flooding. Tilt it back down to a more normal, flat level and no flooding.

By keeping the float height at 50mm, that problem disappeared. Only on a killer steep hill would it start to do that again, but I haven't come across that around these parts...