Technical Revisiting exhaust backfire on decel issue

If any of you remember last fall I went crazy trying to track down an exhaust backfire/afterfire on decel only when the throttle is completely shut. I thought I finally found my issue last week as I noticed fuel leaking from every gasket and screw hole on my 3 2G carbs. After talking to Mike's Carburetors to see if I had a gasket issue with the cap fuel here in NJ he told me it wasn't the gaskets but actually too much fuel pressure. I was up around 6psi so I got an adjustable regulator last week and dialed it down to 3.5psi. I was hoping this decrease would lean me out a bit and help the raw fuel that's obviously getting into the exhaust but no such luck.

Here's a refresh on my setup, symptoms, checks. Engine is a GM crate 350/357 with less than 1,000 miles on it running Rochester 2G carbs with progressive linkage. The exhaust is a set of ceramic coated lake headers from Speedway with their baffles and turnouts. The truck gets some backlash out of both sides on decel but also gives a strong backfire out of the exhaust when I let the throttle snap shut to shift or slow down.....almost always from the driver side....occasionally from the passenger side.

Carbs have been gone through twice professionally and the truck seems to run well...idles fairly steady and pulls well. Compression and leakdown tests were good last year. I also did a fresh leakdown on just the driver side today while the engine was warm.....numbers still look good...less than 5% on the worst cylinder. Exhaust has been leak tested with a shop vac blowing air in as well as smoke tested...no leaks at the flange or in the welds. Ignition timing has been checked several times. I believe at last adjustment I bumped up to around 13 or 14 base and 34 total plus vac advance. I also swapped the Pertronix distributor i had in it initially for an MSD unit with no change. This engine does not make steady vacuum but from the few posts I've seen online that seems to be how this combo is. After dialing down fuel pressure today and readjusting my idle screws for max vacuum it fluctuates between about 15 and 17hg. It's not a constant flutter ..it'll be fairly steady at one end for a few seconds then float down a bit for a few seconds and eventually back up again. With good compression and leakdown tests plus no spark plug that looks wildly different than the rest I have to believe I don't have a sticking valve right?

So at this point I have to believe that the lake header is just allowing either too much fresh air into it or not enough of the spend gasses out past the baffle and it's lighting the unburnt fuel inside the header right? I was going to build a full exhaust under the truck this winter and got side tracked with other projects. Any thoughts?

Below are the specs on the engine as well as a pic showing the intake and exhaust setup. Thanks for any advice. I do want to eventually build a full exhaust underneath still but if I could find a way to stop the backfire for now I'd be thrilled.

 

Also last season I blocked off both end carbs and still had the issue so it's not fuel or air getting in from the end carbs

 

Well,,,,according to the vacuum gauge chart,,,it is either a valve sticking,,or leaking ,,,,,or ,,a carburetor issue.
Which one do you think it might be ?

You have done compression and leak down,,,,they pass .
Have you considered replacing the intake with a regular 4 barrel for test purposes ?
Those 3 deuce setups are notorious for creating havoc .

Tommy

 

Turn the pressure down some more just to eliminate over fueling, or lower the floats as a test.

 

Someone was discussing something similar happening to a drag car and his was response that it was likely oil being sucked down a valve guide by the closed throttle high vacuum and firing off.

 

Brand new or reconditioned heads?
I have an old 390 truck where I bought a set of heads from a machinist, cause they had been redone with exhaust seats. He milled them down and told me to check pushrod length, that was 25 years ago when I was stupider. I installed them without checking length. Ran great but back fired during deceleration.
My cure was to throw it in neutral

 

So I haven't tried a 4bbl intake and carb....mostly out of stubbornness. I don't want to dirty it up cleaning gasket materials, coolant, etc plus I'd have to find someone with a good setup that fits Vortec heads.

Heads are brand new as this is a new GM crate engine. As for oil being sucked down....anything is possible but wouldn't I see oil on the plug? Tomorrow I'll try to turn the fuel pressure down even lower and see what it does.

I took a video just now of the vacuum gauge at idle but not sure how I can upload it. I'll see if I can maybe post it somewhere else and link it to here.

 

No one is paying attention to the vacuum reading,,,it is leading you to the solution .
Vacuum should never fluctuate !

Tommy

 

Vacuum readings that fluctuate constantly (bouncing) is a problem with valve sealing. What you describe with a steady needle that changes between 15 -17 inches, but steady, is a different issue. Fuel is most likely the cause. Although a PCV system problem could do it.

 

Many moons ago I had that same "backfiring" issue with long tube, Hooker Headers into straight-through Cherrybomb-type mufflers, into 45* dogleg pipes side exiting before the rear tires.
I got a fix-it-ticket and simply changed mufflers to some claimed to be free-flowing, chambered, "Turbo Mufflers" and the pops were gone for keeps.
I'm not a believer in the "some back-pressure is needed" school by any means. But that was all I did and I have to say it worked in that instance.

 

I've thought about my exhaust being too short and needing more back pressure. But I also haven't heard of anyone else having this backfire issue with lake headers. Could be from the cam in this engine plus the short exhaust?

What about the power valve in the 2G? Could I be having an issue with that opening at idle and making it too rich?

 

And is anyone able to open those video links?

 

No luck.

 

Have you tried to run with your vacuum advance disconnected just to see if your problem goes away? The other thing I would try is feadjusting your valves, particularly the exhaust side. You may have one or two valves a little to tight.

 

I'm no expert...... but it's the "perfect storm" of something. You (we) just have to figure it out.

 

A few comments.
The adjustable fuel regulator will have no affect on the engine's air/fuel ratio (unless you set it much lower than 3.5 pounds). It is just keeping fuel pressure from holding the needle valve open, when the carb bowl is already full.

The backfire is NOT the problem, it is a symptom, (unless it is caused by the very short exhaust system). You still need to identify what is CAUSING the backfire. I'd start with one carb and back to the 62 or smaller jets to see if you can get it to idle smoothly and run around town at light throttle smoothly.

Until you can getting it running nicely on the center carb, the end carbs are just complicating your effort to identify the performance/mpg problem. Also, is the vacuum advance being used? It should be.

Have you reread this thread lately?
https://www.jalopyjournal.com/forum/threads/my-turn-to-start-a-rochester-2g-thread-i-guess.1268138/

Keep at it, you will get it figured out.

 

I'm sticking with what I said in the last thread... Put some real muffler's on it and the problem will more than likely disappear.

 

I've run it with the vacuum advance on and plugged off both. Also I've played with base and total timing as well as advance curve with no changes for the better .

Pprather.....once it warms up the truck idles well with just a little lope. Acceleration is good....seems better with the lower fuel pressure now but I haven't driven it in months. It may actually need larger jets in the end carbs on the top end but haven't gotten that far yet. I had gone all the way down to 58s I believe in the center carb and it ran like crap. The current jets seem to offer the best part throttle performance.

 

Truck was off to upholstery for a bit this winter....I had planned on putting full exhaust on it. I probably still will as soon as I finish the old 280Z I have in my shop right now.

Interior looks insane though.....even if it backfires lol

 

Look's pretty Bad Ass from here! NICE!!!

 

Is it possible high vacuum at throttle shutdown, is sucking one/both of the other carbs open slightly, feeding fuel into the exhaust?

 

It did it with the end carbs blocked off too.

 

Doh...I remember reading that...never mind

 

Ok let's see if these links to my vac gauge videos work.....these aren't private. It was smoother earlier when the engine was fully warm but this is the general idea.

https://hosting.photobucket.com/images/i/Resto_Paul/20230320_170211.mp4

https://hosting.photobucket.com/images/i/Resto_Paul/20230320_170122.mp4

 

With the vacuum reading going up and down but not fluttering I'm wondering if you have a problem with valve adjustment or perhaps a slightly bent valve stem causing it to not seal properly. Could also be a weak valve spring letting a valve float under high vacuum. Also, if it only happens when you snap the throttle shut the exhaust will be full of excess fuel for a second. I think moving the mufflers farther back would keep them a little cooler, maybe cool enough that the exhaust gasses would not self ignite. If the mufflers, being that close to the engine, have baffles in them they could easily become red hot. That and a little excess fuel would make a wonderful boom. The principal behind any exhaust system is to cool the gasses before they exit. That's what determines how much noise they make. One simple thing you could try is to put dashpots on the carbs. They slow the closing of the throttle to idle a bit . Commonly referred to as an anti stall device.

 

Yeah the baffles are right at the end of the header now so not much of a run to cool the gasses.

If I let the throttle close slow with my foot it alleviates the problem.

 

There's your problem. Easy fix. A lot of cars with automatic transmissions used dashpots on Rochester 2G series carbs. Tons of other makers, too. Do some research or talk to a professional carburetor person about what GM cars used them. There are surely any number of them that will mount to your carbs. Usually mount to a couple of the screws on the top.

 

I'll have to call my carb builder tomorrow. Also gonna be seeing one of the old time hot rod mechanics at work tomorrow so I'll pick his brain more and show him my vacuum videos too. I just feel like if it was a valve I'd see other symptoms...but that could be wishful thinking I suppose.

 

My straight 6 ford did that. Short pipe with a cherry bomb.
Loved coasting down hill.


Then I rebuilt the carb and it stopped
Coincidence?
I don’t know

 

Wow! This is timely I was thinking about this thread just today as I was out in the '51Coupe.
HardcoreZ28, if you want to eliminate backfire on deceleration do this.
Do not let the throttle snap shut. When you back off keep a little light pressure on the accelerator. Easy Peasy....backfires or pipe pops are gone.
I really think it's a non-issue that can be corrected with simple driving habits unless you just want to sound off.