Solving backfire problem on passenger side

[HSSS427]

Have a 427 side oiler, with Shelby aluminum heads and a Mighty Demon 750. I continue to get a back-fire on deceleration, and sometimes in between shifts (again a decel as I let off the gas). It only happens on the passenger side. I've tried to tighten down the header bolts at the head to make sure I'm not getting an air leak there. Using copper gaskets. The headers are individual pipes that connect into the collectors just before the pipes exit the body. The collector pipes slide into the header pipes and there are 2 bolts to hold it together. Was wondering if this could be the source of air, and if so, what can I use to seal this connection that will endure the heat and won't blow out.

Also, there's a flange that connects the collector to the side pipe with the idea that you could easily unbolt the side pipes to run straight headers. The flange connection is just about under the front edge of the doors. Is this too far down the pipe to cause a back-fire issue?

If it's a carb adjustment issue, what type adjustment should I be looking at, particularly given that it's only happening on one side of the engine. Float height? Jets - larger or smaller?

Finally, is it reasonable to expect this engine to run without back-fires or is that just nature of the beast?

Thanks!

[azjulian]

Hi - couple of questions on this

did you buy your demon new ? what I mean by that is do you know which primary and secondary jets you are running and are the symetrical right and left....

Have you checked that all your plugs are sparking properly / esp have you checked your dist cap is the engine running smooth ?

Have you checked your compression, do you have even compression on both sides ?

demon have 4 point adjustment of idle fuel mixture. after taking careful note of the settings of each of the screws ensure that they are all at the same setting. open them up on the pass side to richen and remove the backfire.

Also have you checked your float level settings on primary and secondary ? Too high would allow backfire, but would expect from both sides....

[Excaliber]

Usually back fire, especially on one side only, is caused by air leaking into the exhaust some where.

[Cobrabill]

Ditto Excaliber.Air is getting into the exhaust system.

[Tony Aprile]

I agree with a possible gasket leak at the headers to cylinder head and or header flange to sidepipe.

Another possibility is an intake manifold gasket leak right at the one of the intake runners (happened to me). Back fires on decelleration

Tony

[HSSS427]

So my leading theory is air leak on passenger side. The obvious spot would be primary tubes into collector since it's just a tolerance fit with the tubes sliding into one another. Any suggestions on some type of goop to use to cause that fit to seal up?

Will also check the carb settings, but it is a new Demon that I got from Don Gould at FourSecondsFlat - he adjusted it and tuned it for the engine specs I gave him, but that was 150 hp and 160 ft lbs of tourque ago prior to rebuild. #77s on primary and #80 on secondary.

Could the stage II port and polish and new cam mean I need to re-jet to larger size?

Engine is running smooth ie no missing. Will check compression.

Will try simple stuff first though ie the header pipes.
Thanks!
Scott

[CJ428CJ]

Scott,

I've had the same problem with my slip fit header to side pipe connection and I'm about 99% certain that is the cause of your backfire. I've tried just about every type of "goop" there is out there and none of it has lasted for more than a couple of months. I've used the hi temp silicone that some have recommended and it lasted no more than a couple of hours. The best stuff I found was some black tar like muffler sealer that I bought at a Napa auto supply store but it would only last a few months. I finally made some modifications to my side pipes to solve the problem. I just got the car back on the road yesterday after making the mods and so far so good. I'm confident I've solved the problem.

When I get home tonight, I'll take some photos and post them here so you can see what I did but I'll attempt to describe it too. I removed the side pipes and welded a 1 inch long piece of 1/2 stainless tube at the very end of each sidepipe opening (I welded it perpendicular to the sidepipe opening). I then used a die grinder with a 1/8 think blade to cut a notch through the stainless steel tube and the sidepipe (make sure you cut the notch to a depth that is less than the distance that the header slides into the sidepipe). I then inserted a 1/4 in bolt through the stainless tube. After reinstalling the sidepipes, I can then tighten the bolt which solidly clamps the sidepipe to the header.

Chris

[speed220mph]

HSS427: Air leak, as you've already addressed, is the first possibility. As for a leaky intake, if the manifold has a common plenum chamber, backfire should occur on both sides. Another scenario, and sadly one I encoutered, was misfire in a cylinder, causing excess fuel to enter the exhaust, which exploded when it joined with hot gasses from the other primaries. Problem was coolant leaking into cylinder on decel, cooling or leaning the charge and causing the misfire. A leaky head gasket could also cause the problem.

I suggest you pull the plugs on the passenger-side bank and check for differences. Rather than a compression test, do a leak-down test. Have the radiator cap off so you can see if there are air bubbles in the coolant when pressure is applied. If not, the problem could still be a leak, but air that leans the mixture, causing a missfire on decel in one cylinder, so listen for a hissing sound in the crankcase and carburetor. Good luck.

[HSSS427]

CJ428CJ - Thanks for sharing your experience. Interested to see the photos. Wonder if I could put some hose clamps - like what you'd use on a radiator hose - around the outside of the pipes and screw them down real tight to try and close the gap. Or perhpas get some that are wide and wrap header tape around the joints and screw them down? I ask as I don't have access to welding equipment - seems that's the next thing I need to learn how to do.

[sharpe 1]

Here is some sealer that may help with the problem. Walker (acousti/seal) part#35959. I got mine at napa auto store. Avoid welding the pipes unless you put in a flange. Good luck.

[azjulian]

Easy way to check if you have an exhaust leak (be careful its hot!) is to take a rag bunch it up and block the exhaust outlet on the side of the car - you should feel pressure. If you have a leak you'll hear air exiting from that point. If you have a helper at hand that person can track down where the leak is coming from.

In the past when I've found a leak and its gasket related, I've taken every thing apart, sanded down the flanges and used sealant - if everything is flat you don't need the bolts very tight to maintain a leak free system.

[speed220mph]

HSSS427: I hate to butt in again, but my car is also an ERA with slip-fit primaries. These may fit loosely when cold, but swell when hot to seal the joint. One problem that is created when a header leaks is the header and head errode around the exhaust ports under the gasket, ensuring that a new gasket will leak. I had to mill the exhaust-header mounting face on both the head and headers to fix the leak. You already know how The backfire on my engine was fixed by pinning the cracks in the old tunnel-port heads.

[azjulian]

just to answer your previous question on re-jet of the demon...

if you have ported your heads and changed cam, then you will have increased the airflow into your engine - its entirely possible that the ratio fuel to air is still correct to the setup that you were given - you just need to make sure that you are not running too lean.

[HSSS427]

Well I think I've fixed it. Bought and installed the Byson header gaskets mentioned here, and then took some of the Permatex hi-temp copper gasket sealer and smeared a good coat inside the collector ends - about 1" deep into the pipes. Slipped it back together and tightened up the connector bolts, then tightened up the headers to the heads. Took it for a test run and no back-fire! Question now is, how long will the gasket sealer hold up, and how much of the issue was at the collector vs the heads. Could see clear indication where I was getting blow by with the copper gaskets, but I haven't changed out the copper gasket or sealed the collector connections on the driver side and not getting any problem on that side.
Thanks for all the advice! Now I'll probably start mucking with tuning the Demon carb since I've got a lot higher flow on the heads and 150 more HP than I started with. However, no noticable issue in the way the car is running.
Thanks!