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redneckrodder 01-02-2005 08:45 PM

distributor timing problem? help
i just finished building a 350 engine. The motor sounds great, but when i go down the road it back fires under acceleration(i beleive). I think the distributor might be off a tooth, because when i start the truck it sounds like it's having a hard time starting because it is out of time. I really hope it is not a problem with the valves because the motor has not even been driven more that 2 miles, and sounds great while idiling and even when i goose it a little at stand still, but backfires when driving, and the valves were adjusted the old fashioned way...while the truck was runngin. Any fast help or info would be very much appreciated.

Max Keith 01-02-2005 09:05 PM

Did you use a timing light when you set your ignition?
Use a timing light to see what your initial advance and total advance are. if your engine barely turns over when you try to start it, that is very indicitive of having the initial advance set too far forward.

Try resetting your distributor at about 10 degrees of initial advance and see if you still have that problem. If you do, as far as the backfiring, it sounds like carburetor or possibly valve set too tight.

A quick way to check it out is to just loosen the distributor a bit and slowly retard it while it runs. If you are way too far in advance, you will notice a slight pickup in rpm, as you retard it.
But, to properly set it, you need a timing light.

Old School Nut 01-02-2005 09:53 PM

when you say backfire, do you mean out the carb or the exhaust????

troy-curt 01-02-2005 10:01 PM

I doupt that it would start and run if it was one tooth off. Sounds like the timeing needs adj.


redneckrodder 01-03-2005 08:07 PM

the backfire is out of the exhaust

underdog305 01-03-2005 09:13 PM

Just so you guy's know the correct terminology is Afterfire if it's out the exhaust, Backfire if it's coming out of the carb. :mwink: :D If it's coming out the exhaust, I think it could be an exhaust valve or it's way to rich with poor timing. Consider this: the valve would have to be at least slightly open to ignite the air/fuel and blow out the tail. The timing could be way too retarded (firing way too late). Easy fix!

hosep 01-03-2005 09:58 PM

Ok , here we go I had this same problem for years working on first gen SBC. I came across this somewhere and it worked. the more radical the cam the less margin you have to work with. I have done these SBC for 41 years. here it is. follow it to the letter and it will work. I did not write this only copied it.

Setting Valve lash
Centered on the base circle EO/IC
Exhaust opening/intake closing
When the exhaust valve just starts to open, stop cranking and adjust the lash on that cylinders intake valve.Crank the engine again watching the intake valve of the same cylinder. After it is fully open and the rocker tip is at the bottom of its travel crank slowly until the valve is 2/3 closed. Stop and adjust the exhaust valve on that cylinder. repeat on each cylinder.

With hydralic lifters I give them 1/3 to 1/2 turn from "0". not the factory spec of 1 full turn for lash.

This may be time consuming but it works. I remove the spark plugs when doing it to stop crank back from compression. I also use a remote cranking switch, ignition off .I then set the distributor to #1 at TDC checking the timing marks on the balancer. Then fire it up. Set the timing with the light, vac advance plugged. Check the weights in the disrributor for binding first. Sorry forgot that one. time it 6 to 8 degrees. Warm it up and run it at slow speed to purge any air from the lifters.
Now after all that put all the tools away but the wrench for the distributor. drive the car. hammer it, slow take off , do it all. and listen and use the seat of your pants to fine tune the timing. when you get it just right you will know. Just do very small moves on the distributor.
I think you have an exhaust valve hanging open. It is easy to do.
I use to do this for the guys with the door shut. You know one of those little secrets to piss them off:spank:
By the way any fire is a back fire. I never heard that other one. I go with miss fire, but the other sounds like politically correct bull.
That should fire some one up:D

underdog305 01-03-2005 10:15 PM


By the way any fire is a back fire. I never heard that other one. I go with miss fire, but the other sounds like politically correct bull.
That was the term I was taught when I went to school for Aircraft Maintenence 15 years ago. I'm about as far from politically correct as they come. It's a way of being specific about the direction of the fire. Direction is absolutely critical when troubleshooting a problem like this. Terminology is an important tool for diagnosis, otherwise nobodys on the same page.

hosep 01-03-2005 10:45 PM

I knew I would wind some one up!!!:D

underdog305 01-03-2005 10:51 PM

yeah yeah.. Does make sence though, doesn't it? laughing...

hosep 01-03-2005 10:52 PM

just having a little fun. no offence:D

redneckrodder 01-04-2005 06:29 PM

hey...i beleive it is the carb, it was backfiring(or afterfire lol) from the carb and exhaust, the timing is very good, had 3 other techs, including myself double check it. the carb was sitting for a good 5-6 months and was also used prior , it was just sitting on a shop floor...what do you guys think? im going to swap it with my buddies holley and see if it goes away then i'll now if thats the problem, but i was just wondering if that seems likely?

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