Mercruiser 43 backfiring under load

"94 mercruiser 4.3 thunderbolt

"94 mercruiser 4.3 thunderbolt ign 4bbl can get to about 40 mph starts backfiring thru carb till i back it down to 20 or less. rebuilt carb, replaced wires, cap, rotor, fuel filter, plugs, reset timing, checked firing order, isolated wires. Will run @ 4000+ rpm in neut. all day no backfire. Any suggestions????"

Timing Module??

How many ho

Timing Module??

How many hours on motor?

Check compression?

What spark plugs?

What is initial timing set to?

Check timing at 2500-3000 rpms and see what full advance is? Best to use a timing advance light to keep mark on or near TDC. you should see at least a total of 32-34 degrees.
Is the back fire from the carb or the exhaust?

Some one here has the test proceedure to test ing system maybe they can post it.

If all ingnition tests good and compression is good then the next place to look is fuel.

If all looks good than maybe a bad cam lobe as a last resort. Does motor run smooth or does it have a slight miss once in a while?

"Tests for Thunderbolt Ignitio

"Tests for Thunderbolt Ignition:
W/ignition key on AND BILGE WELL VENTILATED OF GAS FUMES!!!

#1 - Connect your voltmeter neg. lead to the engine ground and pos. lead to the white/red wire terminal at the dist. It should read 12 volts.

#2 - If 12 volts is present, remove the coil spark wire from the distributor and connect it to a spark gap tester to ground. Remove the white/green lead from the dist. terminal. Turn the ign. key to on and strike the white/green lead to ground. If there is spark, replace the ignition sensor in the dist.

#3 - If there is no spark, substitute a new coil and repeat test #2. Now if you get spark, install a new coil. NO SPARK, replace ign. amp.

#4 - In #2, if there is No voltage present, disconnect the white/red wire and check it again for 12 volts. If 12 volts is present, replace the ignition sensor inside the dist. cap. If no 12 volts present w/it disconnected--ignition amplifier is shot and must be replaced."

"will test ignition, and advan

"will test ignition, and advance but does not make sense to me that this only happens under load. if bad module or timing or carb would it happen @ hi rpm in N ??"

"backfires thru carb. new plu

"backfires thru carb. new plugs from dealer gapped to spec. cant remember mfg. inital timing was @ 5B moved to spec 8B made no difference. not sure on hours or compression, but like i said seems to run perfect even @ hi rpm. Maybe thinking timing chain or dist. advance? but again runs great @ hi rpm in N tried to watch it under load on the water but very hard to tell fuel delivery wiyh it shooting flames!"

"KG et al:

Ignition tests g

"KG et al:

Ignition tests generally only work when the engine won't run.

Mike:

"Will run @ 4000+ rpm in neut. all day"

It will blow up if you try; keep RPMs at 1500 max. in neutral per Merc.

Under the rotor is the optic sensor that triggers the amplifier. If it is heavily rusted it can fail at high RPMs. The coil is another part that can fail from heat at high RPMs. Try another coil. If that doesn't work, zip tie two bags of frozen blue ice to the amplifier to keep it cooled. If it works w/the blue ice, you need an amplifier.

Instead of blue ice you can use a hair drier at idle and cook the amplifier a few minutes to see if the extra heat causes it to fail. Same heat test can be done to the coil.

Dealer price is $400-600 but there are lower priced OE units available. If you buy a used one the price will be near what a new one is available. Let us know."

"thanks, did not run all day,

"thanks, did not run all day, but did for a couple minutes after i just had the backfiring problem with no backfire. already replaced the coil same problem. would the amplifier heat up more on the water than off? optic sensor looks ok, clean. i guess i'll try that on the amplifier any other ideas??"

"would it happen @ hi rpm

"would it happen @ hi rpm in N ??"

Neutral doesn't load the engine.

"i'll try that on the amplifier any other ideas?"

Make sure the ground to the amplifier is squeaky clean and TIGHT.

"will try cooling module and c

"will try cooling module and compression and checking spark adv. boat is about 3 hrs away i only get there on weekends (if lucky) anything else i should try when i'm there next time please let me know so i can be prepared would like to get this worked out before the snow flies!! thanks, mike"

"A motor not under load will t

"A motor not under load will typically run PERFECT!
Just the nature of the beast.

When under load that is when timing advance needs to work.

Just so you know there is NO mechanical timing advance. It is all controlled by the timing advance module that guy referes to.

Guy, I thru that in because it does include a end result pointing to the ampliphier.
He has repalced everything else.

The more I read thru this and the fact that he has a back fire I am stsrting to suspect fuel issue.

One question??? I forget and am at work rite now, Does the 4.3 have the anti knock sensor?? It was the mid 90's when they introduced the next generation of thunderbolt ingition and added the anti knock sensor on the V8's but I am not sure about the V6's.

Also what about your fuel pump? Is it maitianing pressure, ie: oil pressure sender/switch is it mafuctioning?"