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Terrible backfiring through carb

19K views 42 replies 27 participants last post by  Todd Buttrick 
#1 ·
I have a new (rebuilt) 302 with a holly carb that barely runs. The engine backfires terribly through carb and even catches Fire in the carb. Pretty sure the timing is at least close and firing order is correct. Even verified the firing order by removing the valve covers. Any help would be appreciated. When it does run, it shakes pretty good.
Thoughts?
Thanks Mike
 
#4 ·
hmmmm. If you've confirmed firing order, I'd suspect ignition. What are you running for ignition bits Mike? You sure it's an HO 302 and not marine?
 
#5 · (Edited)
Newly built 302. Could it be timing gear is off or even 180 deg. off on distributor. What kind of fuel pressure do you have? Are you using correct firing order? Mike
 
#6 ·
Backfiring through carb is definitely timing or firing order. You could always try the other firing order to see if its better or worse. Get a timing light on it if you can get it to run long enough and you may be way off even though it runs. Let us know the outcome once you figure it out.

Mike
 
#9 · (Edited)
I could never do the finger-over-plug-hole to determine TDC but here is something I came up with that works well if you have a compression checker like this:



Remove the gauge and just using the hose, screw the hose adapter into #1. Take a small square of electrical tape and put it over the opposite end and press it firmly on the small OD of the adapter. Prop it up vertically and you can use both hands to turn the engine and watch for a little bubble in the tape to confirm TDC when timing mark is at or near zero. Works great.
 
#12 ·
Firing order:

302 HO/351W firing order is 1-3-7-2-6-5-4-8
Small Block Ford 260, 289, 302 order is 1-5-4-2-6-3-7-8

Verify the firing order shown above is correct for your engine and that the dizzy is not 180* out.
 
#13 ·
I'll Admit To It...

On my first Cobr@ replica, I had a 390 that I did the rebuild on, and had excessive back firing on my initial start-up. I found that I had guessed the rotation of the rotor wrong and had the wires in "reverse" order. It took a few minutes to figure it out but when I did it ran fine from then on.

Regards, Rick.
 
#14 ·
My firing order is 1-3-7-2-6-5-4-8 counter clockwise. Confirmed this by watching the valves open and close too. I checked the valve tightness on the passenger side. I can rotate the pushrods by hand. I will check the driver side today. I am using an all in one distributor from MSD. It was what came with the engine. Standard 5.0 coil. It takes forever to start. Once it does it runs ok but still back fires out the carb. Eventually it just dies and will not restart. I have spark the whole time as checked with a timing light.
Mike
 
#17 ·
My firing order is 1-3-7-2-6-5-4-8 counter clockwise. Confirmed this by watching the valves open and close too. I checked the valve tightness on the passenger side. I can rotate the pushrods by hand. I will check the driver side today. I am using an all in one distributor from MSD. It was what came with the engine. Standard 5.0 coil. It takes forever to start. Once it does it runs ok but still back fires out the carb. Eventually it just dies and will not restart. I have spark the whole time as checked with a timing light.
Mike
Being able to rotate the pushrods doesn't tell you much, assuming hydraulic lifters. The fact that it dies and won't restart also supports the idea that the rockers are too tight. Are they adjustable or pedestal? If you're confident in the firing order and timing, I'd start by re-checking the valve adjustment. :beer:
 
#16 ·
I had a similar issue. Check your valve cover / rocker arm clearance.

The valve covers I was using didn't provide enough clearance...which I initially didn't identify, so i adjusted the timing (way out)...complicating the issue. Switch to different valve covers, reset the timing...now running great.

Good Luck!
 
#18 ·
Hi Mike

Is it a roller cam or flat/hydraulic................. maybe a cam going flat?

Even if the break-in is done correct it has happened where a cam can still can still go flat.

Just a thought.

If not that I am in the Ignition/Coil/Timing/Firing Order camp.
 
#19 ·
I bet million dollars you are 180 degrees out. At your TDC keep turning the engine and see if the exhaust valve starts to open. Ive done this a bunch of times.
 
#21 ·
Do more ...

With so many unknown's in the motor ( assuming it has been worked on & you didn't do it ) follow the timing pattern further out and don't trust the balancer mark .... make your own from # 1, think it is every 90 deg .... this will eliminate allot of the questions and allow you to recheck the valves at the same time ..... after that it is likely the dizzy .....
 
#22 ·
This happened to me and it was timing. I tried everything until a friend of mine that has a mustang shop came to help. He knew immediately. What threw me off was then I would advance or retard it, it would get worse. It wouldn't run enough to put my light on it. Since it got worse, I just knew timing was right and it was something else. I even tried changing firing order.

That's when my buddy told me the HO firing order actually has a spot that if its far enough out, it gets worse before it gets better. He pulled it, advanced a tooth, dropped it back in and the damn thing fired up and ran perfectly. I was ecstatic and embarrassed at once.

Also, the TFI module being not fully seated can cause it, but that's on efi cars.
 
#23 ·
This happened to me and it was timing.
He pulled it, advanced a tooth, dropped it back in and the damn thing fired up and ran perfectly. I was ecstatic and embarrassed at once.

=
Exact same thing - had TDC & wires ran correct. it was a tooth off. So frustrating.

Hope that's all it is. Good luck Mike.
 
#24 ·
Wire in the whole

When you feel pressure on your finger and you think your close to top dead center, stick a piece of wire into the cylinder and rotate engine to make sure you are at top dead by holding wire in your hand. Also, if your timing marks are in the right place on the harmonic balancer in relation to the timing pointer you should be able to use that to get the distributor really close. If the distributor is set right, valves are adjusted, and spark is good, it's time to pull the front cover and make sure the timing gears aren't off by a tooth! Not a fun job, but it can cause the problem. Hope it is something simple to fix. Oh and number 1 piston is the front right piston, but I'm sure you know that.
 
#25 ·
I've had two experiences like you describe. First was because I had the original 302 SBF firing order on the dizzy with an aftermarket HO cam. Rookie mistake. It made a lot of nasty noises and backfires when trying to start and run. For my most recent build, the first start also included backfires, flames back through the carb, etc. It was pure timing. My guess at the initial dizzy position was at least an inch off. Sounds like you've verified the firing order. So timing is my guess. But it could be deeper as others have suggested with the cam timing itself being off.
 
#26 ·
#28 ·
Todd MSD 8352
I really don't think the distributor cares where number one is as long as it is at top dead center.

Redfish might be on to something. I noticed when I took the valve covers off that one of them was damaged underneath. Looked like someone took a hammer to the splash plate that covers the oil fill. Maybe its actually hitting the rockers and not allowing the valves to close. Will change them. Its a roller motor so there should be no issues with the cam.
Mike
 
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