I recently blew a head gasket on a '91 GMC Safari with a 4.3 liter V6 TBI (as some of you who frequent the off-topic section already know). I had the heads reworked and got it all back together. I started it up and it was backfiring through the throttle body real bad. I took the distributor out and rotated it a little and it improved, but still backfired. So I tried to rotate it a little more but it was too much.

I don't have a timing light so this is a little difficult. No matter what I do I cannot keep it from backfiring. What do you guys think? Valves need more adjustment? I really can't think of anything else. I have checked all the wires, grounds and cannon plugs. I do not think I have a vacuum leak. Please help!

Joey, the engine is out of time that is why it is backfiring through the intake. Try getting ahold of stan browning he is very good with chevrolets and has helped me many a time. You should be able to take the distributor cap off and roll the engine over till the no.1 piston is top dead center. then remove the distributor and restab it with the rotor pointing to the number 1 position where it is marked on the distributor cap.

wes

Thanks for the response. I just got off the phone with my dad, he has my copy of the manual. Unfortunately he is iver 200 miles away The book said put the motor at #1 compression stroke, adjust some of the valves (I've got them written down, you don't need to know those specifics) then rotate it 360 degrees to #4 compression, do the other valves.

The prescribed method for valve adjustment is as follows: tighten the adj. nut until you can no longer freeley turn pushrod with fingers, tighten until there is no las, then tighten 1 full turn. This is done cold. does this sound right? It sounds like the valve would be cracked open at this point.

Ok, update:

Went by the book on adjusting the valves. Don't do this. The book is wrong here, or there was further adjustment needed. I don't have it here in front of me so I cannot say for sure. But in either case, the valves are where they are, after having adjusted them twice, no thrice.

The distributor: I was about 180 degrees out, and the firing order was wrong even though I had the wires marked. Got everything back together and first attempt at the start yeilded more backfiring. I decided to try and advance the distributor one tooth (suspected when installed it may have been a little retarded). Tried to start again and the battery started going dead. So I've got it charging and I'll start again tomorrow. For now I am going to eat supper.

Any further thoughts will be greatly appreciated.

Joey
usually if the thing is back firing through the intake it is out of time,if after you get it in time it still backfires it is going to be a plugwire crossed.Quickest way to set timing on this vehicle is to set the timing mark at the balancer to 0 and stab the distributor to point to no 6 plugwire on cap.let me know if you need further assistance,my cell 214-538-9071 just call me,also on valve adjustment tighten the valves on all cylinders until you can count 3-4 threads at the top of the studs when done,this will get you the closest without second guessing.

Thanks Stan. Sounds good to me.

I agree that the timing is off.. Over tighting/adjusting of the valves will also cause a backfire. On used lifters as yours, I would recommend 1/2 - 3/4 max...

The best way to determine TDC is by compression or removal of the valve cover and observe the opening of the intake valve while bumping the engine over. Once the intake valve starts to close, check the timing mark on the Balancer and set it on 0. Check the position of the distributor rotor in relation to #1 on the Distributor cap. If all other adjustments to the valves and correct firing order on the wires, your engine should start and only need the timing light to top it off... Sounds easy, huh... Good luck.. Feel free to call.. (254)394-0540 John

Thanks fool. That's my plan of action. And that's what I did to set the distributor but unfortunately I tried messing with it again yesterday. I re-adjusted the valves since talking to my dad because that full turn on the nut was way too much. Maybe I'll get a little noise but at this point I'm sick of missing work.
Anyway valve covers are back on so I'm going to take the #1 plug out and determine TDC that way, and reset the distributor once and for all.

I hope I don't have to call you guys but thanks for the support and advice.

Edit: I'll be finding the compression stroke on #1 with the plug out, then using the balancer to set at #1 TDC.

IT'S ALIVE!!!

Alright, Hit the Road!!

100 mile round trip from Canyon Lake to San Antonio and back with no ill-effects! Thanks guys.

HEY STAN YOU WORK AT A DEALERSHIP SOUNDS LIKE YOU KNOW WHAT YOU ARE TALKING ABOUT