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Backfire through carb

Old 05-07-2015, 12:33 AM
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Default Backfire through carb

I have been fighting this issue for a while now so this is going to be a lengthy post. I used the search function and have found some good info here but never a resolution to the problem.

The boat is a 25' Farallon with a Marine Power 454. I purchased the boat a little less than a year ago. Since I have had it there was always a bog when accelerating. Seemed to bog down when the secondaries started to open on the carb and then it would take off and get up on plane and run great. The "bog" progressively got worse and started to backfire through the carb occasionally. Now it is to the point where it will back fire through the carb every time and then totally lose power.

My initial thought was a lean condition. I replaced the fuel filters and the fuel pump. Had the Holley carb professionally rebuilt. Verified fuel lines and check valves were good. Fuel is new and fresh. Same issue.

I then thought maybe it was an ignition issue. I replaced The cap, rotor, coil, plugs, and wires. Same issue. I then said screw it and replaced the entire distributor and ignition module. Timing is set correctly and seems to be advancing properly. Still have the back fire through the carb?

The engine runs great in neutral. It starts right up and will rev all the way up to 4k RPM's with no issue? It only does it under a load? I am kind of at a loss at this point. I really don't know which direction to go next? I was thinking maybe there could be a valve issue or bad cam but it runs great in neutral? Any suggestions would be greatly appreciated.
Old 05-07-2015, 05:32 AM
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I know small block Chevy's had bad cams. The lobes are surface hardened and once it goes, the lobes wear right off. If it's easy to pull the valve covers, do that and check the lift on all the rocker arms.
Old 05-07-2015, 05:43 AM
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Backfire through carb is all timing related. Put a timing light on it and see where your at.
Old 05-07-2015, 05:52 AM
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How old is the engine and did it sit ? Maybe sticking intake valve.
Old 05-07-2015, 06:00 AM
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Sounds like timing to me too. Could your ECM be bad and causing it to retard the timing? Sounds like your base timing is good, but your off idle timing "controlled by the ECM" is off. That is, if it has an ECM.

Could be a bad ground too.
Old 05-07-2015, 06:17 AM
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I am pretty sure the timing is good. Base timing is set @ 12 degrees ATDC which is spec for this motor. When I run it up to 4K RPM's and put the light on it the timing advances. I am not sure how far but it looks to be around 30 degrees ATDC. The ignition system is the older delco HEI with external coil with centrifugal timing advance. The boat did sit for a year or two before I got it so it could be a sticking valve? I will pull the valve covers when I get a minute and see if I can find anything.
Old 05-07-2015, 06:20 AM
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Sounds like you have a Holley. Time to learn how to rebuild it. Kits are inexpensive
Old 05-07-2015, 06:25 AM
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Secondaries are opening too early..lean bog. Adjust the carb so secondaries don't open so early. Unless it's mechanical secondaries, which is really bad, it needs a stronger spring in the vacuum unit.

Also, may have wrong accelerator pump set-up. Not enough shot, or too short a period of time for shot.

Last edited by billinstuart; 05-07-2015 at 06:39 AM. Reason: addl. info
Old 05-07-2015, 06:29 AM
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A backfire through the carb happens when part or all for the combustion from a cylinder is released through the intake valve rather then the exhaust valve. This can happen as a result of incorrect timing or an intake valve that is partly stuck open. All you need to do is figure out which. A simple compression test will tell you if you have a valve even partly stuck open. That will at least narrow down the search..
Old 05-07-2015, 06:42 AM
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Put a pint of ATF in your fuel tank, it should clean everything up and might fix your problem if it is valve related.
Old 05-07-2015, 07:05 AM
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I'm hoping the timing your stating is before top dead center not after. If it is in fact 30 ATDC that would be your problem.
Old 05-07-2015, 07:25 AM
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Originally Posted by Evensteven View Post
I'm hoping the timing your stating is before top dead center not after. If it is in fact 30 ATDC that would be your problem.
wouldn't run at all.

30 degrees total is adequate.

While it could be a valve issue, lean bog creates the same problem. Secondaries should never open until 2500-3000 rpms. Because there's no accelerator pump on the secondaries (unless it's a double pumper) it's common for a lean stumble condition to occur.

Just because it's rebuilt doesn't mean it's adjusted correctly.
Old 05-07-2015, 08:42 AM
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exhaust gas restriction
Old 05-07-2015, 10:06 AM
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is it a steady backfire, as in every time a steady popping noise at higher rpm or once , twice the ok.
What is engine vacuum at idle and 3-4 k under load. Steady Vacuum or bouncing.
Could be lean timing or engine mechanical. Check fuel pressure and volume, float settings.
I would start with timing.
Old 05-07-2015, 10:44 AM
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Sounds lean to me.

Get someone to physically restrain secondary throttles as you power up, then gradually let them open.

Once over the hump and at steady planing cruise, does it run ok then? What about full power, smooth? What rpm do you get?
Old 05-07-2015, 12:02 PM
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Since it runs great when up on plane it's not a sticking valve. Or anything else internal to the engine.

It's probably the carb, either accelerator pump not functioning correctly or the secondary's opening too soon.
Old 05-07-2015, 03:26 PM
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Possibly ignition wire cross induction,make sure the wires are routed properly and just for fun,do a compression test to rule out a burnt or sticky valve.If it's backfiring through the carb,it's firing into the intake side somehow,as in a valve is open or firing on the wrong cylinder.
Old 05-07-2015, 05:33 PM
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If it is a Holley carb and the engine backfired, the power valve will be blown. Timing may have started the event, but Holleys do not like backfires. They do not like dirt, water or me
Old 05-07-2015, 05:38 PM
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Originally Posted by AtTheWheel View Post
Since it runs great when up on plane it's not a sticking valve. Or anything else internal to the engine.

It's probably the carb, either accelerator pump not functioning correctly or the secondary's opening too soon.
x2.........lean condition = backfire
Old 05-07-2015, 07:47 PM
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I would check the timing chain. Pull out the #1 cylinder sparkplug and rotate the crankshaft manually to bring the piston up to TDC and see if the rotor button is pointing to the #1 piston. Your timing chain can jump a couple of teeth and create timing problems.

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