mercruiser 5.0 thunderbolt v mercarb backfire under load

[bolo78910]

helping a good friend on this one. 1999 merc 5.0 with thunderbolt ignition , from what he told me the boat was running great all summer, the lower unit went out and from the rookie mechanic install a new drive , it pure misfire backfire. i went to have a look, when they could not get it started.i dont know what the hell they was doing messing with the wire harnest, they have a ground wire going to the negative side off the coil . i have the grey wire from the ignition box going to the negative side off the coil now it started . all the spark plugs was black i meant real black . i install new one , took it for a ride and still backfire under load, i get the carb rebuilt still backfire under load . could it be the coil ?? or the ignition box, i also check the firing order also new cap, rotor and distributor sensor. it start right up and sound great just backfire under load. what next guys. thanks much for any info.

 

[bolo78910]

these are the parts we throw at it. distributor cap , rotor, distributor sensor. sparkplugs, rebuild carb, water seperator filter. carb filter,

only thing left is sparkplug wires. ignition module, coil, electris fuel pump, fuel pump oil switch. thanks

 

[achris]

Check the ignition module is not in 'base mode'. Check the timing while it IS in base mode.

Chris....

 

[Fun Times]

I have the grey wire from the ignition box going to the negative side off the coil now it started.

Did you mean of or off the - side of the coil??

There should be 2 grey wires at the - side of the coil...One for the tachometer and one for the ignition module. You need the ICM wire connected..... If the tachometer is shorted to ground internally or externally, the engine typically won't run or it may even run improperly.

Below are just some other thoughts I had texted out before rereading the quote above:
Make sure you have a V8 rotor and not a V6 rotor.

Since they had a ground to the - side of the coil, the coil might be internally compromised....Being the age of the engine and the things found wrong thus far, a new coil could be worth a try at this point.

Make sure the high tension coil wire rubber boot isn't leaking spark to the nearest ground.

^ Also while testing/checking the timing as mentioned by Chris, connect the timing light inductive pickup up to the high tension "coil wire" and ensure you're seeing a continuous flow of spark/flashing light especially while the issue is occurring. The light should basically stay on when running at higher RPM.

Another thought would be to ask the owner, did anyone touch the shift cable, or cable adjustments?
Did anyone forget to put the shifter into forward gear before stern drive removal?
If so, maybe the Mercruiser shift cable interrupter switch located on the engine shift cable bracket is now out of adjustment causing intermittent weird running/starting issues.

 

[bolo78910]

Thanks for the responds guys,. Yes he did have a rookie install a new shift cable, also the boat was running fine all season long, antil the lower unit gears got shot. That's when he got this person to install the the new drive ,plus change the shift cable also. He told my friend he has a short on the wire harness ECT. They cut and splice up some off the harness wires omg.

 

[bolo78910]

Admiral, Cris I red your reply s and all make great sense,. On the negative on the coil I now have two grew wires, one goes back to the clock, one goes into the ignition module box like Cris said it might be in base mode how will I fix that? Now the three wires from the distributor sensor one black ground wire. The other two goes into the module , ain't there sopose to be something going back to the shift interrupter switch?? I notice some loose wires there I will check.

 

[bolo78910]

Just a thought on the mercarb , there is a thick rubber spacer in between the carb they claim it will keep heat out off the carb. Now on the installation they have the regular thing gasket that sit first on the intake,then the spacer sits on top, then the carb ,shouldn't there be another thin gasket sits on the spacer then the carb?? In other words two regular thin gasket sanwich the spacer. ?

 

[Fun Times]

For the carb gaskets, itemnumbers 27, 28, 29 http://www.mercruiserparts.com/bam/s.../31837/2697/80

For going into base timing, https://www.perfprotech.com/blog/art...-engine-timing & this guy talks about (shows) what wire. https://www.youtube.com/watch?v=ZHNCICd4ZQ0

^ Make sure the other guy didn't leave a jumper wire to ground in place when he thought he was done.

Yes 2 wires into the ICM from the distributor sensor.. Then a White/Green wire out of the ICM to the shift interrupter switch being an Alpha stern drive. http://ww2.justanswer.com/uploads/Ha...9_scan0117.jpg

Sounds like you're being nice just calling him a rookie. Cutting wires and leaving others dangle. OMG is right.

I'd be worried he didn't set the new cable adjustments correctly too which could throw off the shift interrupter switch position.

Good luck.

 

[bolo78910]

Admiral thank for the links real helpful. So far I got the wiring corrected,. I put on a new coil it start right up with a lot of black smoke,. I let it warm up to temp sound great no miss or vibration,. But it still backfire under load. I will get the two flat gasket tomorrow for the carb , it still missing the top one. All this problem come along since the guys install the drive, could it be he did not install the drive in forward gear and the interrupter is acting up.? Sigh

 

[bolo78910]

Admiral , the way the carb is installed currently it has gasket #29. At the bottom then the spacer, gasket # 28 is missing, could that be a part off the problem. ?

 

[Fun Times]

But it still backfire under load. I will get the two flat gasket tomorrow for the carb , it still missing the top one. All this problem come along since the guys install the drive, could it be he did not install the drive in forward gear and the interrupter is acting up.? Sigh

Can you get a complete list from the owner of what the guy did to the boat? Sounds more than just installing another drive. If the shifting seems to be working okay then it's probably a non issue.....But knowing he/someone was in there changing things around, a good visual inspection would be a good idea.

Since the engine is running rough with heavy black smoke and backfire type sounds, then that could either be an ignition type issue, fuel pressure problem, Carb problems to start.

Admiral , the way the carb is installed currently it has gasket #29. At the bottom then the spacer, gasket # 28 is missing, could that be a part off the problem. ?

They had a few ways they installed these types of carbs as far as gasket placement goes. Lets see if the engine serial number, engine parts guide and a service manual will help show the correct way to install this carb.

Can you get the serial number?

 

[bolo78910]

Gm, thanks I will get the serial # after work tonight. And yes I spoke to marine mechanic out here in Brooklyn and he said the same thing about the shifting , say it not the problem if it shift ok. I keep you posted thanks

 

[bolo78910]

Engine serial. # 0L612762. 1999 year also when I took off the carb last night raw gas is sitting in the intake.

 

[bolo78910]

Update here guys, wow, ok went back to the boat today and put the correct carb gaskets, took it for a test run and man its still backfiring, it wouldnt even gets on plane just popping and bucking , pop pop pop pop pop rapid , i head back to the dock and start checking wires. this is what i came up with with the battery switch off i turn the keys and the boat want to crank wow, i notice there was no circruit breaker that use to be behind the carb. they run a new wire from the battery cut and splice it to one og the wire that goes to the starter, splice another and connect it to the slave solenoid, so it seem the slave was not working and they just bridge up some stupid wirine, lucky the boat did not catch fire, i disconnect the wire they rig up from the battery, i now turn the battery switch on turn the keys on and nothing click nothing , i put my tester on the two large nuts on the slave solenoid and there is no juice there, i was so **** i just pack up and call it a night, i will go back sunday to try put things back, , should i wire back a circruit breaker?? could this electrical mess is what causing the boat to back fire. thanks guys let me kno , i download the wiring dirgram just now.

 

[bolo78910]

update, went ack to the boat today, ground the timing purple wire to a good ground. hook up the timing light , let the engine warm up, point the timing light but the mark on the balancer was no where insight, i retard it i advance it but the mark on the balancer was no where close, it seems to runs a little better when i advance the timing, after that with the timing light connect to the num one plug wire the light was not flashing anymore , thats when the starter went out dead, i pull all the plugs turn the engine by the crank by hand stick my finger in the num one plug hole to mk sure it was on tdc, it seems it was its just that they wire the cap with number one at the back , i rewire the whole thing , now am waiting for a new starter to arrive, why did the timing light stop flashing, , thank in advance for any inputs.

 

[achris]

Silly question, you're sure you were on #1 (stranger things have happened. )? Port side front. If that's not working, try #6, that's the same place but on the 'opposite' side of the crank. The only other thing is if the rubber on the balancer has slipped.

Why did the light stop? Lots of possible reasons. If it's a powered timing light (most are these day) then a lead could have slipped off the battery terminal, the pickup may have been in just the wrong place, If it's got a switch (like mine) you may have bumped it to the OFF position.

Chris.......

 

[bolo78910]

Thanks Chris, good ??? With what they did to this engine who knows, I rewire the plug wires pull the distributor and out it back ,just want to start fresh, when the starter went out. Now I see why they runs a extra wire from the battery , it's because the starter fuse was blown so the bridge instead off replace it. I order a circuit breaker also , Bec to get to that starter in this Bayliner 2452 is no joke Chris I will explain it to him to make sure to disconnect the battery cable when working on it in the future. I will keep y'all posted., The starter is out without removing the manifold I only remove the riser. Also the starter was loose and only have one mounting bolt. Ahhg oh boy.