402 backfire problem

[soala70]

Sat up for about 3 months now it backfires through edelbrock carb. put new plugs , plug wires points and condensor, and checked timing. still does it just as bad. help if any suggestions.

[bryanw1968]

Sounds like you might have an exhaust valve that is stuck. If the intake valve is working and getting fire it will come back out the carb if there is no where else for it to go. If this is the case throw some Marvels Mystery Oil at it and let it set for a day or two.

[corn]

I always thought popping through the carb was an intake valve out. Guess one is just as bad as the other huh.

[bryanw1968]

You're right I typed that wrong. Doh!

[rlm316]

First thing that I would do is check that the firing order is correct on the cap. A misplaced plug wire will also give you this. Was the distributor ever out while the motor was down? Possibly 180* out, 1 TDC on compression stroke? Also, how did the motor act before storage? Good luck!

[soala70]

well I got the carb rebuilt by a friend cleaned out tank put new filter in still wont run it will try but now its gone from backfiring out the carb to backfiring out the exhaust . If I crank it hold the gas to it it will run a little bit but backfire about every other fire. thought has to be spark plug wires nope checked it again right on and distributor hasnt been out .HELPPPPPPPPPPPPPPPPP.

[cdowns]

absolute best way to check plug wires is to remove all of them and start from square one or in this case cylinder 1// you can stare at a miswired dizzy all day long tellin yourself its right but its not, i've done it so many times that the 20minutes it takes to strip them put #1 on tdc compresson and rewire correctly is time well spent

you have taken off the dizzy cap to make sure the rotor is pointing at what you think is the #1 cyl position on the cap right??

[Steve Vos]

Been there, done that. Not necessarily comforting words. I have a '69 GMC 402 that set for some time while doing the body rebuild. When it did run it was only at an idle. I went through the HEI distributor, wires, timing, new lifters, all to no avail. I checked the firing of the number 1 wire and had erratic results so I thought I had a bad distributor or bad wires. I finally, under the advice of a local mechanic, changed to one heat range hotter plug. It seems the plugs were fouled from all that idling and that can give you false firing. That did it for me. Mine has the Edelbrock performer cam and intake with a speed demon carb. It runs great now, but finding the problem was rough. I am only a shade tree mechanic so diagnosis doesn't always come easy. It also helps to run it more often, even now if it sets for more than two weeks it is hard to start. I wish you well and hope this helps.

[68gmsee]

Originally Posted by soala70

well I got the carb rebuilt by a friend cleaned out tank put new filter in still wont run it will try but now its gone from backfiring out the carb to backfiring out the exhaust . If I crank it hold the gas to it it will run a little bit but backfire about every other fire. thought has to be spark plug wires nope checked it again right on and distributor hasnt been out .HELPPPPPPPPPPPPPPPPP.

Now that you've eliminated the wires, gas, carb, you can check:

Fuel/air mixture. Make sure you have no vacuum leaks and play with the carb settings to see if it changes the problem.

Worn valves. (check with compression tester)

Timing/timing chain.

Worn cam. (flattened lobes-common in older high mileage 350's)

Make absolutely sure the plugs are good. Looks can be deceiving with plugs and they're cheap and easy to replace.

Keep us posted.

[soala70]

spark plugs new ac delco Im going to look at timing tomorrow then off comes the valve covers

[Longhorn Man]

it kinda sounds to me like the dist hold down bolt is loose or something.

[Blazer1970]

Bad distributor cap? You are not using one of those caps that re-position the wires using a circuit board inside the cap are you?

[OJOE]

check a few possible problems, start by getting timming mark on TDC then pull dis cap, rotor should be on #1 while you have it off check for cracked cap or a bad rotor. another problem often missed is fuel flow, make sure you are getting fuel pressure to carb. good luck

[Huck]

Does this happen at idle or when reving? If your vac advance diaphram has a hole in it and doesn't advance, you will go nuts with throwing parts at it. Put a Vacuum gauge on the motor---and also do a quick compression check. Between the two, you should zero in on the problem. Huck