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Discussion Starter · #1 ·
Hey everyone I used to be a member here many years ago under a different user name. I've since lost the email address I used to register so I have a new account.

I'm having a backfire issue I can't seem to solve on my brand new SP350/357 crate engine. I'm getting the normal exhaust backlash I'd expect from short pipes but my driver side has a loud backfire out of the exhaust when I let the throttle snap shut to shift. If I keep the throttle from completely closing all the way it doesn't do it.

Engine is the GM 350/357 crate with 100 miles on it. Intake is a 3x2 setup with 2G's. Center carb is the only one with an idle circuit. Exhaust is a set of ceramic coated Speedway lakester headers with their baffles and turnouts. Trans is a GM NV3500. Ignition is brand new Pertronix distributor with their cap, rotor, wires, and external coil. Base timing is 12*as per GM but I've tried as high as 16* with no change. No vacuum advance as per GM and total timing is 32. I've tried hooking up the vacuum advance just to see and it didn't help.

I've tried almost everything I can think of at this point. Compression on all cylinders is good....180-190. Leakdown consistent on all cylinders as well so I don't think it's a ring or valve issue. I also relashed the valves on the driver side to be sure but they all ended up within about 1/4 turn of how GM had them. The engine idles at about 750 with 14 hg vacuum that varies +-1 hg...which seems to be common for this engine. I set the idle screws for max vacuum. Truck idles fairly well and runs good...pulls fine and cruises fine. Just the loud backfire on the one side.

About 2 weeks ago I found the driver side header slightly warped. I installed a set of RemFlex gaskets and thought it was sealed but still had the backfire. After doubling back again today I found that header still leaking at the flange by pushing air into the pipe with my shop vac and spraying soapy water around everything. All the welds were good but had the flange leak as well as a leak at the gasket for the turnout. I was able to carefully pull the header and bend it with my press today to within .010 of flat. I reinstalled the same Remflex gasket since it was basically new still and got no more bubbles around the header flange from the same test. Thought for sure it was sucking air and causing my backfire but it's still doing it! The leak at the turnout is so close to the exit I can't imagine that's causing it. I got frustrated and pulled the turnouts and baffles now and a lot of the cheap Speedway packing is already gone....although the side without the backfire is actually in way worse condition. I'm going to pick up new packing from a motorcycle shop tomorrow and swap which sides the baffles are in.

Besides that the plugs look good...a bit rich but I just went down from 62 jets in the center carb to 60's. I haven't really done any wide open throttle driving yet to draw from the end carbs. Rear end carb seals great and the front draws very little air when it's closed as well. I even put blockoff plates under them last week and ran it and still had the backfire. Cylinders 1 and 7 on that side have the lowest pipe temps of the entire engine at idle. From looking inside the intake those 2 ports sit furthest and highest from the center carb so I'm assuming they're getting the least fuel at idle. They pick right up when RPMs come up. Not sure if they go so lean when the throttle snaps shut that the fuel enters the pipes and ignites there? Could it be another carb jetting or idle adjustment issue?

I've been trying to solve this for 2 or 3 weeks now with no luck so any help is greatly appreciated. Sorry for the long post but wanted to get all the info in here for what I've tried. And pic of the truck just for attention. Thanks in advance! Wheel Tire Sky Car Vehicle
 

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Discussion Starter · #5 ·
Open the idle mix screws another 1/8 and tell us what happens
I tried that today as well....drove it at highest vacuum and got the backfire, opened them 1/2 turn more, then 1 full turn more, then all the way back to 1/2 turn further closed than highest vacuum.....backfire was the same at all of those settings.

I've also gone around with starting fluid and checked for vacuum leaks with nothing notable.
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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Even near the end of the system a leak is a leak.

I haven't seen one in a while, but some carburetors used a "dashpot" as part of the metering system. It would prevent the throttle from closing as quickly to prevent a rich condition from increasing emissions or even stalling the engine. It sounds like it may be worthwhile to consider, if you have no luck otherwise.
 

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Discussion Starter · #9 ·
Thanks for the suggestions.....the plug wires are routed properly....been checked several times. I'm confident the flange is sealed at this point so I don't need to cut it.
I was thinking about a dashpot yesterday....not sure if there is room on tri-power setups but I may ask the carb builder about it.
After I repack the baffles today I'll use new gaskets for the turnouts and add some copper sealer to see if it helps with those leaks at the end.
 

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1979 Chevrolet Malibu 496-TH400-9" (cruiser). 1992 Chevrolet S10 355-700r4-7.625" (daily driver).
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In the event that an exhaust leak or vacuum leak isn't the cause, I'm curious to see what the solution would be on a tri-power set up. On a 4 barrel a couple of ways around situations where more air is needed with the primaries in a good place on the transition slots is to slightly open the secondary or some folks drilled holes in the secondary butterfly. Then tune the idle circuits and so on from there. I have no experience with the tri-power to know how or if that would translate to that set up.

When you do meet with success on this, I hope you follow up. Welcome back!
 

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I had a header with a crack on a tube weld at the collector that caused a back fire noise on decel.. Header was the short design , not long tube. The back fire noise was under the hood.
 

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Discussion Starter · #12 ·
I had a header with a crack on a tube weld at the collector that caused a back fire noise on decel.. Header was the short design , not long tube. The back fire noise was under the hood.
You had exhaust after that right? All my welds look ok and like I said the turnout and hookup gaskets are right at the exit from the pipe but you never know. I'll silicone them in reinstall and see.
 

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There are two things that cause exhaust backfires and that's retarded timing and air leaks.
When you have unburned A/F in the exhaust, once it gets pushed out into the header and meets the oxygen in there it'll self ignite in the tubes and go bang.
Either make it so it will completely burn in the cylinder OR close off the exhaust so it can't burn in the tube.
Check your ignition closely.
 

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Discussion Starter · #14 ·
There are two things that cause exhaust backfires and that's retarded timing and air leaks.
When you have unburned A/F in the exhaust, once it gets pushed out into the header and meets the oxygen in there it'll self ignite in the tubes and go bang.
Either make it so it will completely burn in the cylinder OR close off the exhaust so it can't burn in the tube.
Check your ignition closely.
I'll try pressurizing the exhaust again tomorrow after I repack the baffles and silicone the 2 end gaskets. Would it be possible to have a leak between the 3 and 5 cylinders on the header? I didn't see signs of it on my gasket but maybe they're cross contaminating each other? Aside from a bad cap I'm not sure what else would be wrong g with the ignition system. I've tried different plugs. I could swap the wires from side to side I suppose but they're brand new and a timing light on each shows they're receiving spark.

I'm still hoping it's an air leak I haven't sealed yet but like I said after having the header on a press yesterday and reinstalling the flange showed no signs of leaks. The 2 lower gaskets are right before the exit of the system but I'm hoping for a miracle on one of those as the other side has minimal leakage at those spots. Beyond an air leak what about the possible theory of the 1 and 7 cylinders idling much leaner? That would cause more of a misfire at idle than causing the pipes to load up when the throttle snaps shut right?
 

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There are two things that cause exhaust backfires and that's retarded timing and air leaks.
I agree with what you said.

If there is not an air leak, I think the mechanism for the exhaust backfire is a situation that presents like retarded timing would.

The same way mechanical advance curves are "all in" by 2500 or 3000 and don't require the need for additional advance as a consequence of the velocity and better mixture in the combustion chamber allowing for a faster burn. Essentially the faster burn "creates" the scenario that additional timing would if the burn speed remained constant.

In the case of the abrupt (and maybe too complete) throttle closure, I think the column of atomized fuel (that was doing fine at higher velocity) loses it's ability to mix as well in the chamber. Leading to a low velocity over rich condition that can't burn fast enough. It presents the same way as retarded timing would with the incomplete burn in the chamber and the pop in the exhaust.

The open headers give that mixture quick access to fresh air. It may not happen or be noticeable in a full exhaust system.

For the same reason the dashpot helped with emissions, I figured smoothing out the transition from fast to slow moving mixture could help prevent or lessen the deceleration backfire. Maybe even buy some time for the now higher vacuum signal to compensate the timing a bit better?

The old adage that lean pops through the carburetor and rich pops in the exhaust might be a fitting explanation. I'm not arguing here. I guess I'm agreeing in a different way? :)
 

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Discussion Starter · #20 ·
I repacked the baffles today and swapped which side they are on. I also cleaned the surfaces for the turnout and bottom hookup on the driver side and reinstalled the gaskets with copper RTV on both sides as well. I want to let it set up good tonight. Tomorrow I'll leak check the driver side again with the shop vac and soapy water then take it for a ride.
 
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