"freshly" built engine backfires, will not start?

There are two different firing orders for this engine depending on the camshaft you have. I would also check the compression to make sure the engine is in good shape.

https://www.2carpros.com/articles/how-to-test-engine-compression

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

Hello TOTALMC,

It definitely sounds like a timing issue to me. But to have it backfiring through intake and exhaust is strange. Is it doing it at the same time?

Was a bigger cam added or longer pushrods?

Was rocker arm ratio changed?

Is the timing pointer on there?

Do you have a build sheet for it?

I would check to see if the harmonic balancer is in the right position. We used to call it dead timing. Take out the plug on cylinder 1. Put a long screwdriver in the hole and turn the engine by hand while watching the screwdriver. Watch it move up and as soon as starts to move back down, turn the motor back. Then I would rotate the engine back 365 degrees and have an assistant put their thumb over the spark plug hole and see if there is pressure and make your mark with a white paint pen on the balancer. Then you will know true TDC.

Then you can adjust your distributor from there.

I would also check the firing order. I read that newer aftermarket cams use the 302HO and 351W firing order.

For yours it should be 1-5-4-2-6-3-7-8. If it has a newer aftermarket cam it could have the 302HO firing order which is 1-3-7-2-6-5-4-8
Also, if you are using a carburetor with electronic choke, unplug it while trying to time it.

You can see if the valves are open by performing a leak down test. OTC makes a pretty nice tester. It can be found on amazon. That way you can see if it is coming from the intake or exhaust more than the other. Do this while turning the motor and see what changes. You will need a compressor for this test as well.

You can also use just compressed air into cylinder on when at TDC at combustion cycle and see if you get any leakage like air through the intake and turn the motor by hand until there is no more leaking air.

I would try these few things. At the very least it will give you more information on where the valves are located in relation to piston movement.

I hope this helps. Let me know if I can help in any way. Try these few things and let me know what you come up with.

Thank you,
Brendon

I have it wired
1-5-4-2-6-3-7-8

Thought the other firing order was for the newer style 302, and customer says this is the original , fresh back from the machine shop about 5 years ago, but I'll still try that other firing order before I remove the valve covers and check their operation.

Tried the other firing order, and it popped and spit even worse, so going back to 1-5-4-2-6-3-7-8. which just happens to be embossed on the top rear of the intake manifold.

Removed spark plugs (which were new) and it was evident there were two cylinders on each side backfiring.

Pulled valve covers and spun engine by hand. rocker arms, springs and pushrods all seem to have normal operation.

tried setting distributor in at TDC, and both 10° advance and 10° retard. still nothing but spitting and fire, through the carburetor and out the header and I didn't even put fuel to it to try to eliminate if it's a carburetor problem or not, just trying to start it on a one second shot of starter fluid.

I plan on renting a leak down tester, customer has a compressor, so I can try that.

Everything under the valve covers looked brand new (it is supposed to be a fresh 5-year-old build)even the assembly lube is visible and still viable.

I'm at a loss why it won't start and keeps with these multiple backfires.

Any other ideas would be greatly appreciated.

Good morning, Totalmc,

What are you running for a carb and distributor?

So, whether your base time is 10-degree advance or 10- degree retard you have the same result? It backfires through the carb and exhaust?

What about vacuum leaks? Everything capped off that isn't being used?

What was the reason for them having the motor built and not using it for 5 years? Were they having issues with it too maybe?

I would think whether it is timing or a carb issue it would backfire on all cylinders not just a couple. To me that signals more a valvetrain issue. Just a thought.

If it was built and sat for 5 years, it's possible the valves need to be adjusted. Also, with it backfiring through the carb and the exhaust both valves would have to be open so I would go through and adjust the valves.

Thank you,
Brendon

Brendon,

Running a stock 2-barrel carb and points distributor.

Backfires in basically any position.

No visible vacuum leaks, all ports capped off.

Engine was rebuilt and installed in car, then just left on back burner for 5 years, awaiting the finish of the rest of the build.

I'll attempt the valve adjustment and let you know.

Thanks again

Hello Totalmc,

Okay. Sounds good. I will continue to look at some information from my end and see what I can come up with.

Let me know how you make out on your end with the valvetrain, and we can go from there.

Thank you,
Brendon

Thanks Brendon, will do.

Good Morning, TOTALMC,

We always ripped out the points type distributors and just replaced them as well as the carb.

But it is obvious this customer wants this to be restored and have original parts.

I thought maybe they tried starting it and the carb is gummed up with old fuel. Since timing adjustments don't seem to make any difference, I am wondering if it is something with the carb. Maybe adjusting the Idle Air Screw will help. When we installed Edelbrock or Holley carbs I always turned the Idle Air Screw all the way in and came out 2 1/2 turns and see how it went. I don't know if yours was rebuilt or not. It might need to be gone through and cleaned.

As for the distributor it seems to be getting spark so that seems fine. I would take the hose off for the vacuum advance and plug it with a bolt in case that is messing with something. I would think the vacuum advance wouldn't kick in until it is actually running but I would give it a shot and see if anything changes. I always removed it even when just setting the base timing.

I believe it is MSD that makes a retro fit kit so you can use the original distributor and just replace the points and condenser.

Unfortunately, it's hard to tell what the last person did for work or what issues they may have had so checking everything is your best bet like you have been doing.

Hope this helps. Let me know how the valvetrain looked and if there were any issues found.

Thank you,
Brendon

Brendon,

I tried to eliminate carb issues from the equation last time. disabled fuel to the carb, was only trying to start it on a one second shot of starting fluid, and still backfired like no tomorrow.

I figured that way, with no fuel going to the carb, I could eliminate that as part of the issue.

Next time I will try to cap the vacuum advance to see if that helps any.

Thanks again for all your input.

Hello TOTALMC,

You are very welcome. You are right, I remember you saying that. Sorry just trying to think of any possibility I can.

If I can help any further, please don't hesitate to let me know. I will work with you and help the best I can as long as you need it.

Thank you,
Brendon