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Backfire thru carb problem on starbord engine------HELP

Backfire thru carb problem on starbord engine------HELP

Old 05-20-2020, 10:13 PM
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Default Backfire thru carb problem on starbord engine------HELP

Here is the basic info on this problem I am having. The engine is a bone stock 1994 Mercruiser 454 mag engine with he Thunderbolt V ignition and a Weber carb (looks a lot like the old Carter AFB's) The engine is a gen V block and has 250 hours on it. The engines are in a 1995 Sea Ray 31 SS Bravo drives. I replaced plugs, cap, rotor and wires on both engines today. The insides of the distributors were clean and not rusty or corroded. Checked the timing and factory is 8 deg BTDC, After replacing these parts, the starboard engine will backfire thru the carb and runs with a noticeable vibration above 1200 rpm. I The engine starts strong and easily and idles smooth at 700-730 rpm. If I rap the throttle starting from idle speed, the engine backfires thru the carb. If I bring the rpms up to 1200 and rap the throttle, it backfires thru the carb. I check the timing several times at idle and tried 8 deg, 10 deg, and 12 deg and get the same backfire result. I check the timing advance at 1500, the timing is advancing to 18 degrees. I tried adjusting the idle mixture screws and the same backfire. I swapped the carb from the port engine and get the same result (the port engine runs great and does not have ANY backfiring condition). I tried doing a cylinder drop test and am seeing the rpm"s drop about 20-30 rpm with every spark plug I ground out. I though a lean condition or bad accelerator pump but swapping carbs between port and starboard proved that wrong. These engines ran fine last season, when I pulled the spark plugs, all the plugs were sooty black and fouled out Im out of ideas and open to all suggestions. Ill recheck the firing order tomorrow, but even if that was wrong, wouldn't it start backfiring as soon as the engine started and get worse as the engine is slowly accelerated??? This engine starts good, idles smooth and will rev up slowly without backfiring, but like I said if I rap the throttle quickly, it will backfire.
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Old 05-20-2020, 11:09 PM
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I want to say itís the coil. Does that engine have coil in cap? Swap cap and rotors from port to starboard.
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Old 05-20-2020, 11:26 PM
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Valve train issue, loosing a cam lobe, lifter, hanging or damaged valve. Chased a similar issue last year, was pulling my hair out, turned out to be rocker trunnions with damaged needle bearings, getting hung up and hanging valves.

Last edited by sailtexas186548; 05-21-2020 at 06:46 AM.
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Old 05-21-2020, 05:07 AM
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Not trying to state the obvious but, you double checked the firing order and all the wires are on tight?
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Old 05-21-2020, 07:13 AM
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was it doing this before you swapped the cap, rotor, and wires? If so I would agree you have a valvetrain issue. I had the same problem from a broken valve spring. Pull the valve covers, take a look, then start it up and take a look to make sure everything is moving correctly. I would also try a compression test
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Old 05-21-2020, 12:26 PM
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I will say that I've seen similar issues with the Thunderbolt V. On a small block we moved to a Voyager system and the engine ran like a top. If that's not the issue, then I think you have something holding an intake valve open. It doesn't take much.
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Old 05-21-2020, 01:43 PM
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Had similar issue on my 454 many years ago. Idled fine, ran fine until you put it under load, then backfired thru carb. Checked the usual spark, fuel, etc. problem showed itself with compression test - #3 was about 40 lbs low. Pulled head and had small hole in intake valve. Which came from a chunk of piston that had gotten hung up in it. Hopefully your issue is less severe/expensive, but my next step would be the compression and/or leakdown test.
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Old 05-21-2020, 02:08 PM
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Assuming it wasn't doing it before the work, I would first take a break, have a beer, go back and double check plug wires/firing order, then start re-installing the old parts until the issue went away starting with dist cap.

Also, should look around for a vacuum leak. Maybe a vac hose that got pulled loose or if old, may have cracked when disturbed by maint activity.

If none of that helps, attach a vac gauge to intake vac and look for low and/or bouncing readings and compared to good eng.
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Old 05-21-2020, 08:08 PM
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UPDATE--- checked the firing order and that was correct. Pulled al the plugs and no broken porcelain. They all came out black though. Did a compression test and got 130-135 psi on all cylinders and compression built fast on the 1st revolution. This engine has (450 hours on it). Pulled the new cap and wires and inspected it for any breaks in the cap ( nothing) and nothing burnt or wrong with the new wires. Installed brand new plugs, and the original cap and wires. The engine stared quickly and idled smooth, adjusted the idle mixture screws to 2 tuns and 800 rpm. And it still has the same problem. It still has a wierd vibration at 1200 Very slight, and when I rap the throttle from 800 rpm it still backfires. When I slowly rev it up to 1to 1500 it is fine. If I bring the engine speed up to 1200 and rap the throttle it will still backfire. I was told that last year the engine was very hard to start. had to crank it a lot and pump the throttle a lot to get it to kick and hold the rpms up until it warmed up. At this point I'm thinking it may be the ign module not advancing enough. Im going to taker the easy way out and replace that. The boat is not mine, I am just doing a tune up on it for for extra cash. If the module doesn't fix it, I'm going to have to tell the owner that Its beyond my time commitment and I dont have the space to keep it out in my street to dig further into it. for anyone interested the serial number on the engine is 0F124358 and i believe it is a thunderbolt IV ign. It has the module mounted to the distributor body. The engine is a1994 454 mag with bravo drive

Last edited by phughes69; 05-21-2020 at 08:17 PM.
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Old 05-22-2020, 06:43 AM
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Iíve never seen an ignition module fail in that way. Many different symptoms but not that one. Is the choke opening fully? Accelerator pump working properly?
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