Last night i swapped my 86 dodge van from the crappy worn out Holley 1945 one barrel to a freshly rebuilt (by me) slant six specific Carter BBD. The engine runs good and idles very smooth, but when i rev the motor at idle I hear a popping noise going through the exhaust and if I do it enough it will actually have flames pop out the tailpipe! I am guessing that this isn't very good for my catalytic converter.

Is this just a symptom of the carb running too rich? Should I lower the metering rods? I set everything to the specs in the rebuild kit, so I am rather surprised to see this happen.

Any suggestions will be appreciated.

Kewl!

Something's a little out of whack, though... How long is your tailpipe, by the way? That's unusual for flames to make it back that far!

D/W

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If it ain't broke, fix it!

It is the stock exhaust line, so I am guessing roughly 15-20 feet long. The flames have to make it through the catalytic converter AND the muffler (a 2.5 foot long round truck type muffler).

I wish I had that feature.
It'd make tailgaters think twice.

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The Old Goat '83 D150 225-2

AKA Ice Titan

Sounds like a combination of excess fuel (rich) and a leak in the exhaust system, probably further down the line (behind the cat).

Are you sure the cat still has its 'guts'?

-S/6

My neighbor used to have A Case SC tractor that would shoot a flame out the exhaust. You could only see it at night. It would suck in air through the "weep hole" that most Cases have in the exhaust manifold. Keeps your engine from filling up with water when it rains down the exhaust pipe

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

I can't vouch fopr the condition of any of the exhaust system. I knwo that a previous owner deleted the first of the two catalytics the van came with, but I suspect the second is original. The muffler is not original though.

I will try lowering the metering rods and seeing if that fixes it.

If I understand you correctly, these exhaust system flames are happening when you rev up the engine and then let the throttle snap shut: vrrrooOOOOMmmmPopPopPop!!

What's going on here is that a whackload of raw fuel is ejected from the engine when the throttle snaps shut, because there's insufficient air to burn it within the engine. The fuel hits the hot exhaust system and explodes when it reaches an area where there's enough oxygen to burn (catalytic converter!).

Why does this happen and how do you fix it?

"Wet manifold flashoff": When you rev the engine, you create relatively high manifold pressure. The pressure cap on your radiator teaches you that raising the pressure raises the temperature at which a liquid boils. It works the same way in an intake manifold: Raise the manifold pressure by revving the engine, and fuel that had been in vapour phase condenses and coats the manifold walls. Then you snap the throttle shut, the manifold pressure drops into deep vacuum (high engine RPM vacuum "sealed off" by the closed throttle plates) and all that condensed fuel evaporates ("flashes off") and stifles the fire in the cylinders 'cause it's way too rich to burn. Out the exhaust valve it goes!

The factory had a few ways of dealing with this: Make the throttle close slower, increase the ignition timing advance during deceleration, and modify the secondary air injection system.

A slow-closing throttle dashpot is not optional, but mandatory, on some installations. Sounds as if yours is one of them. It acts like those building door regulators that make the door close slowly instead of slamming: It causes the throttle to close gradually instead of suddenly. This allows air to enter the intake manifold during deceleration to dilute the fuel that's flashed off the manifold walls. The mixture keeps burning in the cylinders 'cause it's no longer too rich to do so, and the amount of raw fuel reaching the exhaust system is significantly reduced. It will not be hard for you to find a suitable bracket and the dashpots are, I believe, still available. The factory changed from dashpots to electric idle-stop solenoids in the mid-'70s, but rigging up a control system for the latter type of unit will be much more difficult for you than simply installing an appropriate dashpot.

A vacuum advance control valve as used in some '63-'69 versions of the Clean Air Package is a very, very handy device. It increases spark advance only under conditions of extremely low manifold pressure (heavy vacuum, i.e., deceleration) to give the rich, slow-burning deceleration mixture time to burn in the cylinder so it won't be ejected partially-burnt into the exhaust system. These valves are around; there's a guy on eBay who has them new, but you probably won't want to pay his price. A less costly way to experiment with this to see if there are gains to be made in your case: Connect the vacuum advance hose to manifold vacuum rather than ported.

Make sure your basic ignition timing and idle speed are set correctly and that your air injection system, if so equipped, is fully intact and working correctly. If the previous owner hacked the catalyst(s), then it's likely he also hacked the air injection system. Your '86 would have been originally equipped with an air-switching valve that would shunt the air injection to the exhaust ports under certain conditions including rapid engine deceleration. That way, most of the unburned fuel in the exhaust stream burns well upstream of the catalytic converter and muffler, greatly reducing exhaust system popping and flames out the tailpipe.

Was your '86 originally equipped with an oxygen sensor and Holley 6145 feedback carburetor? If so, an appropriate choice for a 2bbl carburetor is the feedback BBD off an '82-'84 Diplomat/Gran Fury/New Yorker Fifth Avenue with 318, and keep the \6's feedback system in place. Nonfeedback catalyst systems are the pits!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Dan- good info! I will hve to read it several times to digest it.

THe van originally was a California emissions package- two catalysts, a spark control computer, a lean burn computer controlled carb, an air injection system with a port upstream and downstream of the catalytic converter. The Holley 6145 carb was completely worn out (vacuum and fuel leaks) and the computer was no longer working properly, so I swapped the ignition control computer out for an HEI ignition system and old style electronic ignition distributor, and I swapped the carb for the Carter BBD from a Super SIx slant. The AIR injection system is still hooked up and functional. It is controlled by vacuum so I hooked it into a constant feed which means it injects all the air straight into the catalytic converter (a previous owner removed the forward most catalyst).

I have a feedback Holley 2280, but I dumped the computer.

The pops occur both on accelleration and decelleration, no matter how fast or slow I work the throttle. The van puffs out black smoke when I rev it and also is very rich when I start it up. I have confirmed that the choke does pull off fully in the first few minutes of operation and my curb idle mixture screws are set at 2 1/4 turns out.

I guess I need to get the O2 sensor hooked back and start playing around with mixture settings. Is there any way to bypass the ignition control features of the computer and just use it to control mixture? I really hate to put the computer back on the van, but I don't want to have my exhaust line blow up.

Thanks for the help!

They pretty much came that way from the factory.
Oof...restore the air-switching system so that it works correctly.
That's more than a dashpot and some timing changes will fix. Sounds like a much more serious problem, at least a very faulty carburetor up to possibly even one or more burnt exhaust valve(s). Could also be vacuum leaks at work.
Won't help you until you get much, much closer to proper mixtures.
Certainly. Just keep running your HEI system. As long as the distributor/coil aren't connected to the computer, there's nothing the computer can do about the ignition.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

...very good 'splanation Dan. I agree, his problems are greater than just slowing down the throttle shutting. A little pop pop on decel never bothers me, my '88 (amongst others) does just that (no mods except "lost" cataclismic converter and duals) - I alwez suspected the air pump pumping into the exhaust contributed to that, don't know if the switching curcuit works or no. Of course I'm one of those stinkin' redneck Americans who loves fouling up the ozone layer with my nasty hydrocarbons, chlorofluorocarbons and Budweiser flatulence.

D/W

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If it ain't broke, fix it!

Okay, I will give it a shot. I just wasn't sure if the air/fuel calculations made by the computer required a tach feed from the ignition circuit. If the computer can actually separate out the air/fuel ratio calculation from the timing calculations I will give it a try.

I am pulling 18 inches of vacuum at idle so I don't think there are any vacuum leaks, and I re-read my shop manual last night and discovered that i hooked the AIR pump up so it is always ionjecting at the cylinder head, NOT in the catalytic. I will dig the computer and hoses and stuffd back out and start reinstalling them. THe bad part is that some of the plastic hoses broke during removal. I will have to fabricate something to replace them.

...there's 90% of your problem!

D/W

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If it ain't broke, fix it!