Backfire in the Exhaust System

TSMITHEY51

MEMBER
1970 CHEVROLET EL CAMINO
62,000 MILES

Backfire in the Exhaust System. 350 300 hp 4bbl stock Engine has less than 250 miles on rebuild. Restored vehicle sits for a while between runs. Runs well when its running but backfire in the exhaust is a problem I continually fight with after its set for a while. I'm putting Sta-bil in the fuel at the recommended dosage.

Starts, Runs up, then tries to cut off followed by explosion in the exhaust system. Think maybe its the fuel pump? Somewhere else to look? This has been a problem since I bought the car and finished the restoration I've installed a Crane Fireball XP-7000 Ign System, New Wires, New Cap, Cleaned the fuel system, replaced the Carb (with a stock rebuild). At wits end. Condensation in the fuel system?

I've had this issue before and usually after you run a bit and the condensate is pulled through with a good fuel addative like Techron Concentrate Plus and a little bit of rough running it will recover and be OK but not this one.

No probelm with the Corviar Spyder that sits next to it in the garage and gets run at about the same frequency.

6 Replies

HMAC300

MECHANIC
48,601 POSTS

Backfire usually means it 's out of timel or the firing order is screwed up. On a 350 chevy/gmc the #1 is at 11:30 or so and runs clockwise. Firing order is 1-8-4-3-6-5-7-2 trace your wires carefully, maybe 5 & 7 are messed up. Also it could be condensation in dist cap causing this. Due to infrequent driving and area of u.S. Normally a good dist cap will take care of this or see if dist screen is blocked and not venting. NAPA and standard are two quality parts for caps. This is about I can think of at the moment, unless your exaust is collapsed in front of the muffler as GM cars did that due to double wall construction of pipes.

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Saturday, October 27th, 2012 AT 6:26 PM

TSMITHEY51

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3 POSTS

Rule out the exhaust - its all new dual pipes professionally installed ( stainless steel) from the manifold to the end. Thinking I'll replace the fuel pump. Only thing that hasn't been replaced since the restoration and maybe the cap (again) with a new NAPA (My favorite parts source). Dont think its the wires ( new -well a couple of years ago) and and in the spring when I cleaned the fuel system and replaced the fuel it ran really good. Ran OK in July but now in October its back to loading up the exhaust system and baskfiring. Oh by the way also a new fuel filter in the spring along withnthe fuel system cleaning (pumped out all the fuel and dropped the tank and dried it out).

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Saturday, October 27th, 2012 AT 9:14 PM

HMAC300

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48,601 POSTS

I didn't say the wiresere bad I said crossed like 5& 7 maybe or it can be the exaust vavle isn't opening enough or intake not opening. Fuel pum p is not going to cure it especially if it is loading up. If it's loading up it morel ike a fuel problem of way to rich or choke hanging up. Recheck yoru timing with vacuum disconnected and blocked. And what cam shaft is in this? Is it way to big wher ethe car will only idle at 15-1700 if it is then you ahve the wrong one in the car.

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Monday, October 29th, 2012 AT 1:28 PM

TSMITHEY51

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3 POSTS

Thanks hmac. You have confirmed likely a fuel problem (again) and I may swap the carb for a new one I have ont the shelf and by the way I'll check the wires for crossed up especially 5 & 7 and I'm going to replace the dist. Cap. I know the one on there is fairly new but its "new old stock" so it could be breaking down as well. I'll check the cam too for which one was installed at the rebuild just to confirm but the idle at 800 - 900 works OK. A little lope but not much. I still have all the paperwork. Engine rebuild was done when I bought the car but had 0 miles on it.

I'lll retrace through the simple things first again and see what goes on.

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Monday, October 29th, 2012 AT 6:57 PM

HMAC300

MECHANIC
48,601 POSTS

One other thing what kind of carb is on teh car now an dwhat will you be putting on it? If the car will idle around 800/900 don't worry abou twhat I said with camshaft. However if you cannot find this then pull the valve covers off and see if the rockers all look to be working ok.