The Stovebolt Site Forums The Shop Area Engine Shop Carb Backfire

First, thanks for all the help. I got my 270 in....finally, and after figuring out that my distributor was 180 deg. out of whack, it fired right up.

I plumbed my coolant lines last night and tested the engine again to make sure that there were no major leaks, there werent.

While testing this out, I ran it as close to idle as I could and when I got too lean, I would get a nice pop out of the carb. When I run it with a little more throttle that goes away. I finally shut it down after about 2 minutes of running and it knocked (at least it did not immediately stop trying to fire) and gave one last big belch out the carb.

Is this just what happens prior to breaking it in and setting the valve seats? I have checked my lashing on the valves and they are set good. What is going on here? I would love any advice you might have. The weather here has been BEAUTIFUL and I am chomping at the bit to get my truck back on the road for the first time in 3 years!!!!!!

Thanks in advance.

Tom

Did you check the timing ?

I checked the timing the best I could. I am not sure how to better set the timing. I set it originally with 1 at UDC and place the distributor cap on the distributor. I have not really done much/any adjustment from that point forward. What exactly is the process that I should use? Also, would that allow unburnt fuel to pass back through the carb?

I would like to understand what is happening as well as how to fix it.

Thanks again,

Tom

Try setting the timing with a vacuum gauge. Rotate the distributor until you get the steadiest vacuum at the highest rpm..

Make sure you have the right firing order too. It will run but roughly if you have it wrong. It's 153624 going clockwise on the distributor.

I thought you said it was 536241?

That's a little too close to my social security number for comfort.

Just so I do this right the first time.....I put the vacuum gague on the intake manifold, run the engine while checking the gague, then loosen the bolt holding the distributor to the block and twist the distributor until I get the highest level of vacuum?

I have read that I need to do this at varying RPMs. Is there an ideal RPM range (weighted average) that I should tweek to?

Again, I appreciate the advice. I just really want to understand why as much as how.

Thanks,

Tom

Don't over analyze this, Tom. Let the engine idle, hook up the vacuum gauge, spin the distributor....After that it will hopefully be Happy Motoring for you and your new 270..

Everybody has their own method for tuning an engine. I prefer to use a timing light, and I first set my motors to about 10 degrees at TDC. Then fine-tune carb and timing from there. Any body ever tried setting an engine using a vacuum gauge, then seen what the timing is with the light?

Yhep.

thanks for all the information, it really helped. I got the vacuum gauge on and was running around 15 lbs in it current situation (backfiring through the carb). Then I advanced it 10 degrees and pulled 19 - 20 lbs at idle and no more backfire, just a nice clean idle.

Now if I can figure out my throttle linkage I will be in good shape.

Also, I am running around 150 degrees, basically dead center of my temp gauge and my oil pressure pegged 60 lbs at startup and backed off to about 50 after idling for 5 - 10 minutes (150 degrees).

Does all this sound about right? Is there anything else that I should be paying attention to? I see no leaks, and I hear nothing really abnormal. Unfortunately I have only heard this engine run with spun bearings prior to this week so I dont have a good comparison.

I appreciate all the help from you all!!!!!

Thanks,

Tom

It sounds like you are in good shape. That's a pretty healthy vacuum and oil pressure you have there. If the valves are a bit ticky that's normal, but if they clatter they probably need adjustment. Other than that it should be smooth and quiet.

Tom, Idling that engine is probably the worst thing you can do to it after a rebuild. Get your throttle linkage hooked up and operate that motor under a load to help seat the rings..