Backfire through carb, thunderbolt 5

[nextboat1]

Hello,
I recently purchased a new to me boat and am having some issues. The boat is a 1998 Rinker Captiva 232 BR with a Mercruiser 5.7, Alpha drive, and Thunderbolt 5 ignition. The engine serial number is 0l020702. All testing has been done while running on the muffs in the driveway. After a couple pumps of the throttle, the boat will fire right up and run at about 1000 rpm. It seems a bit rough, but smooths out a bit once warm and will then idle correctly at 650 rpm. However, when I try to rev the engine rapidly it will backfire through the carb and then increase rpms as intended. If I slowly increase rpms it will not backfire. I'm not sure where to begin my process as far as diagnosing the problem is concerned. Does this sound like more of an ignition problem or carb? I cannot say for sure, but the cap and plug wires look to have been changed recently. I also noticed that the tach occasionally goes "whacky" and will give a false reading. Eventually it will return to the correct rpm. Could this maybe have something to do with the problem? Thus far, I have verified that the accelerator pump is in fact squirting fuel, all plug wires are secure on both the cap and plugs, and the engine temp gauge was verified to be reading correctly at 160 once warm with an IR thermometer. Any help will be greatly appreciated. Thanks

 

[alldodge]

Hello,
I recently purchased a new to me boat and am having some issues. The boat is a 1998 Rinker Captiva 232 BR with a Mercruiser 5.7, Alpha drive, and Thunderbolt 5 ignition. The engine serial number is 0l020702. All testing has been done while running on the muffs in the driveway. After a couple pumps of the throttle, the boat will fire right up and run at about 1000 rpm. It seems a bit rough, but smooths out a bit once warm and will then idle correctly at 650 rpm. However, when I try to rev the engine rapidly it will backfire through the carb and then increase rpms as intended. If I slowly increase rpms it will not backfire. I'm not sure where to begin my process as far as diagnosing the problem is concerned. Does this sound like more of an ignition problem or carb? I cannot say for sure, but the cap and plug wires look to have been changed recently. I also noticed that the tach occasionally goes "whacky" and will give a false reading. Eventually it will return to the correct rpm. Could this maybe have something to do with the problem? Thus far, I have verified that the accelerator pump is in fact squirting fuel, all plug wires are secure on both the cap and plugs, and the engine temp gauge was verified to be reading correctly at 160 once warm with an IR thermometer. Any help will be greatly appreciated. Thanks

Back firing is not from a carb issue unless it happens at high rpm. Suggest checking your timing, it should be a 8 BTDC when put in base timing. Base timing is done by connecting the Purple/White wire (no 13) to ground. Connect a jumper to the wire an adjust timing

 

[ibrw1]

I just fixed my backfire through carburetor. I have a 1999 crownline Mercruiser 5.0L Thunderbolt 5
Ignition. I chased this problem for weeks! It's fixed now.

I'm no expert so anyway here you go.

The backfire into the intake manifold can only happen
If fire is introduced into the intake manifold. So to me, an intake valve has to be open when the ignition fires. The exhaust valves are in the exhaust manifold. So it's not them.

Suck, Squeeze, Bang, Blow.

My theory, the plug was / remained HOT, the intake valve opens to suck in a lung full of gas and air and a HOT plug pre ignited the fuel.

So, my spark plugs were the wrong type. They were cross referenced at local part store.

Anyway, I put the plugs in that the manufacture recommend.

It runs great.

My Rinker tach, same thing. Loose corroded wire.

 

[nextboat1]

Thanks guys. I will verify tomorrow that the plugs are correct and gapped correctly and that the timing is correct.

I know that the boat has seen very little use in the past four years (maybe 40hrs), and has sat for at least the past month if not more. Do you think these symptoms could be caused by something as simple as bad fuel? Just a thought.

 

[ibrw1]

Backfire through the Carburetor, tach issues, my opinion, nope. I think that the admirals suggestion is always a great idea. Mine is worth a look too. Also, consider looking at the cap for cracks and moisture. Check firing order from cap to plugs. Yadda yadda. Keep posting! This place is full of very useful information. Lots of interesting, insightful and intelligent People at this site. Have a great week!

 

[NHGuy]

Try it with the grey tach wire disconnected from the distributor. Disconnect and insulate the end so it does not contact metal. Just put a finger tip from a rubber glove over the wire end and secure with a zip tie or piece of twine .A bad tach signal can interrupt your ignition.

 

[NHGuy]

When my 1988 5.7 backfired through the carb it had some good looking, but as it turns out, bad .old cracked wires.

 

[Scott Danforth]

Bad cap will make it fire thru carb. So will a lean condition (lean sneeze)

 

[flipbro]

So will a bad cam lobe..

 

[nextboat1]

After some tinkering, its fixed! I checked the timing and firing order, both were ok. Pulled the fuel filter and drained, gas looked good with no water present. Decided to pull the plugs and all eight were fouled. I ended up replacing the plugs, cap, and rotor. Started ot up and she ran great, idled smooth, and no backfiring. I did notice a bit of black smoke out he exhuast. I readjusted the mixture screw on the carb. According to the previous owner, the carb was replaced two years ago. I dont think the carb was ever properly adjusted, therfore the engine was running rich, thus the fouled plugs. Thanks guys.

 

[alldodge]

An engine running rich will not cause a back fire all by itself. Idle mixers are only for idle, not for running above 1500 rpm. Glad its running good but I do think this will be coming back around for another look later

 

[nextboat1]

Alldodge,
What other issues do you think I could have going on. I hope the boat continues to run well, but if there could be a larger issue I would like to deal with it now rather than later.

 

[alldodge]

The cap could have been doing it if it was tracking inside, and maybe wires if they were cracking. But since you said it would do this while taking off and having black plugs I'm seeing a carb rebuild in your future.