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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 · #3 ·
Like I said I fixed the below pictured leak today. The only other one is at the ends of the lakester header....basically at the end of the pipe. Automotive tire Motor vehicle Bumper Water Hood
 

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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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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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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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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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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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Discussion Starter · #21 ·
Checked the driver side exhaust this afternoon with the shop vac blowing air into it and soapy water again....no leaks. Took the truck for a long cruise and it still backfires out of just the 1 side. Other than the backfire it seems to idle and run well.

I didn't have time to let it cool down and pull the plugs again before work tonight but I'll inspect them tomorrow. I cleaned the carbon off all 4 driver side plugs before I left today and took a 40 mile ride of primarily highway and some stop and go local. The plugs looked a little rich before my drive but this is the first highway cruising it's seen. If the plugs still look rich upon inspection tomorrow I'll probably try stepping down from a 60 jet to 59s or 58s. Maybe when the throttle snaps all the way shut that last instant rich mixture doesn't have enough air to ignite in the cylinder and when it hits the header there's enough scavenged oxygen from the lake pipes being so short to ignite it? If I really ease out of the throttle on my shifts or feather it so it barely stays open I get no backfire.
 

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Discussion Starter · #22 ·
Just pulled my plugs....pics below. I'm not fouling anything but seems a bit rich still right? This was after about 40 miles of mostly highway driving. I have 60s in my center carb right now....I think I have 57s and 59s on hand. Worth leaning it out a bit and seeing if less fuel helps the backfire and overall combustion I suppose?
Wood Grey Finger Thumb Nail
Finger Thumb Wood Auto part Nail
Fluid Gas Audio equipment Auto part Automotive tire
Azure Wood Grey Flooring Pattern
Wood Grey Finger Thumb Nail

Fluid Gas Audio equipment Auto part Automotive tire

Finger Thumb Wood Auto part Nail

Azure Wood Grey Flooring Pattern
 

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Discussion Starter · #24 ·
So I went down 2 sizes but didn't change idle mixture or timing....backfire seems to be worse instead of better....I'm stumped. I'm going to try to get another highway run in later or tomorrow and see if the plugs at least look like they leaned out with less black deposits. Then maybe bump the timing to see if it helps?
 

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Discussion Starter · #26 ·
Does your exhaust have a cross over? H or whatever. My LS1 backfired "popped" when I let off the acelerator and only on one side. Connected the exhaust with an H and no problems since.
No....it only has the lake headers that dump infront of the doors....no under vehicle exhaust
 

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Discussion Starter · #27 ·
Just went for another cruise....backlash with the occasional loud backfire out of the driver side is still there. Other than that it feels like it's running good. I tried bumping timing up during the ride by several degrees with no change. Pulled 2 plugs when I got back and they were slightly cleaner than before. Ground straps have no soot....faces have some still but not as much as before the jet change. Wondering if I still need to go down another size or two....although I don't think it'll effect the backfire. At this point I'm about ready to give up and live with it as being the nature of the combo
 

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Discussion Starter · #30 ·
Are these carburetors from a dedicated tri-power set up or did 3 of the same carburetors go on a tri-power intake?

From what I can tell, the 2 outer carburetors on the factory set up didn't have idle circuits and had special butterflies to permit better closure of the outer carburetors at idle. Do the outer carburetors have idle circuits?
These were built to be end carbs...no idle circuit and heavy butterflies. I've had the ends completely blocked off with plates I made and it didn't stop the backfire

I'm going to completely swap the wires between both sides today and see if the backfire follows. Going to call Pertronix and see if there's a good stock application cap and rotor I can replace with locally as well.
 

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Discussion Starter · #31 ·
Swapped wires with no change...still backfiring on driver side. I also feel a hesitation now when I punch it at part throttle so stepping down 2 jet sizes yesterday may have been too much.

On the issue of timing advance....like I said bumping initial didn't seem to help. The Pertronix distributor comes with a few sets of springs.....I'm running the silvers since they seemed to be fairly non aggressive as a base point. Wondering if stepping up to the next set would help or hurt? The advance chart is below...
Blue Font Line Parallel Rectangle
Blue Font Line Parallel Rectangle
 

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Discussion Starter · #32 ·
I went up to the next advance curve setting ....once I bumped the jets back up to the 60s the truck feels snappier with more power but still backfires. May just live with it at this point after I do a few more plug chops for the air fuel ratio. Then I may put an undercarriage exhaust on this winter because honestly it's a bit too loud for me....I must be getting old
 
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