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.

I want to say it’s the coil. Does that engine have coil in cap? Swap cap and rotors from port to starboard.

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.

Not trying to state the obvious but, you double checked the firing order and all the wires are on tight?

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

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.

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.

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.

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

I’ve 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?

I second the choke check.

He said he swapped the carbs and it did the same thing so not choke or acc pump. Shot in the dark but the ign module is grounded to the block in the rear with main ground. Take nut off and clean all corrosion off the stud and washers and re install.

Chris

Choke heater wire getting 12 volts.

Was it timed in base timing mode? I believe you have to do that with the TB-V.

Two odd things I ran into sepetately.

Years ago had to grab a Thunderbolt IV cap from Napa on a Sunday The posts where numbered wrong. Uggh.

Also, have had 2 engines (454 and 502) with broken valve springs that still held the valves so compression okay and idle speed okay, but start to give throttle and pop, pop.

I haven’t run into, but could swear I have read over the past 100 years on OSO and others, the bolt pick up assembly or module causing weird issues like this for a few people.

The choke is mechanical off the choke stove spring. I verified the choke was open fully. This distributor does not have a base timing feature. From what I have read, because there is no knock sensor to provide feedback it is just a electronic advance. All 4 caps (new and old) are numbered the same. I put the old cap and wires from the port engine on the motor in question and had the same backfire.

any ticking sound at idle, collapsed lifter, intake valve way out of adjustment, time to pull a valve cover

Choke +12 volt would not follow carb, he is in MI and it most likely is cold up there. Backfire through carb usually indication lean carb or ignition.

Which timing module you have? Is it the 1" brick older style or the triangle, if the triangle it will have a number on it.
I also have heard that the timing pickup block inside the distributor will cause some weird problems.

Strong compression was mentioned earlier. I’m thinking something is up with the distributor.

Also, the manual states that the module has to be in base timing mode to set. It made no mention of a difference between having the knock module or not.

0F124358

Backfire through carb above 1200 rpm

454 MAG. BRAVO (GEN. V) GM 454 V-8 1992-1994 - 0D805069 THRU 0F304999

The serial number range can be either TB4 or TB5. In another thread OP said there is only 4 wires coming from the ICM which makes it a TB4

Timing should be 8* BTDC

Compression 130 to 145 psi

Swap carbs with no change

Edit: no knock sensor or module

I'm thinking Dist module 87-892150Q02
Even more so if the module is the old 2 wire style

No ticking at all. engine stars great and idles smooth. Engine is completely stock and has 450 hours on it.

Valve train or hung valve, doesn’t take much. Pop the valve covers.

spark has to be super screwed up to act that bad without load

I’m still banking on the TB-V f’ing up but only because I’ve had similar happen. Hoping it’s not mechanical.

Was it your module or the pickup in the distributor?

Friend’s boat... He was told the module was bad and unobtainable.

I ordered a Voyager conversion for it and ran the hell out of it.

I guess its time to switch distr. and module and see if its not a mechanical problem.

NEWEST UPDATE---- 5/25/20. swapped over a known good coil, still backfiring. Tried retarding the timing to 0 deg BTDC and even ATDC and still backfiring.
timed engine to 0 deg BTDC, marked the alignment of the #1 tower with the coil tower, spun engine over to 0 deg BTDC on the balancer, pulled the cap off and noticed that the rotor was not pointing towards the #1 tower but was actually about 20 degrees BTDC on #1. Swapped distributors and the engine ran fine and no backfiring. So now 1 of three possible solutions. A bad module, a bad pickup coil, or the rotor was not phased correctly. The module for this distributor is $400 and is obsolete and the pickup coil is another $50. At this point, I dont have ready access to the boat. and have told the guy that we can upgrade both distributors for a little more than the cost of replacing parts on just one distributor. I'm looking either at the DUI distributors (HEI) type distributor or the A-Team Performance (marine HEI ) distributor. I want to replace the whole distributor. Does anyone have personal preference of these two brands or another brand/system that is better?

The A-Team is complete low cost Chyna right ? Compared to medium cost questionable Chy-na things.

GM Delco Voyager EST kit for the win. It’ll come with the coil and wiring as well as spark plug wires for about $450 per side.

I think ebasicpower.com has them for $430.

Agree Delco EST

i would check the fuel system from the carb back to the tank,it is easier for the fuel pump to suck air than fuel.through the years i have seen loose fittings on both sides of the screw on fuel filter,cracked fuel hoses,cracked fittings at the tank&fuel selector valves.to me it sounds like a lean condition backfire.add in,i just worked on a 39 sea ray that had your same problem,the seal in the fuel selector valve dried up and allowed air to be sucked past it but it never dripped a drop of fuel.

Valve sticking or rocker arm loose? Pop the valve covers?

only 1 tank that both motors draw off. the port runs fine. Swapped carbs and still have the problem. Swapped distributors and the problem follows the distributor

looks like you found the problem.

Probably timing sensor if a thunderbolt ignition