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Mercruiser 43 v6 backfiring

burren2004

New member
"I have installed a replacemen

"I have installed a replacement 4.3 V6 with a new vortec marine engine, the problem is, when I get to 3000 revs (2000 under load ) it backfires and looses power, I have checked and replaced all tunderbolt ignition components but no change, I notice now that if I pull the plug wires from the left hand side of the engine, it makes no (or very little) difference, whereas if I pull a wire from the right it cuts out.
can anyone tell me what the problem might be"
 
Bad set of wires crossfiring.

Bad set of wires crossfiring. Did you save the old ignition parts for spares? Get some heavy duty marine grade wires.
 
compression test it - I wonder

compression test it - I wonder if the rocker arms might be cranked down too tight on that side or something.....
 
"I changed and swapped wires a

"I changed and swapped wires around and still the same, so I think I have ruled out electrical problems ( if you ever can )
I didnt do a compression test or any mechanical tests because I figured -new engine, no need!! but I will have to at this stage so it will take a few days as the boat is 40 miles away and a weekend project.
thanks for the advice, guys"
 
This is a "dirty trick&#34

This is a "dirty trick" check for ingintion wires checking and not reccomended expcet for when in desperstion!!!!!!!!!

Get a cheap 12 volt test light from a auto parts store. Connect the alligator clip to a groung. Stab the sharp pointed end ( if needed sharpen the end as thin and sharp as possible so you do not distort the wire with a large hole after stabbing) into the wire comming off the distributor cap you want to check. This will ground out the wire you are testing and avoid pulling it off and getting shocked or worse sparking it in the motor area. Try not to make a hole when doing this. Stab it in just enough to short the wire to ground.

Makes short work on testing spark to each cyclinder.
 
I should have typed/spelled it

I should have typed/spelled it this way. Not enough coffee yet...........LOL

This is a "dirty trick" check for ingintion wires and not reccomended except for when in desperation!!!!!!!!!
 
this was a complete new drop i

this was a complete new drop in motor from Mercruiser? did it come with manifolds and Y pipe?
 
I would check carburetor. If y

I would check carburetor. If you are getting no change in idle when removing wires from one side the you may not be getting fuel to that side.

The way a dual plane intake manifold works is it splits the fuel to the left bank or the right bank.

Seen this before!!! You may indeed be getting spark but no fuel and that would also explain the backfire due to lean condition and the low rpm.

Check fuel delivery first..........

COFFEE IS FINIALLY KICKING IN!!!!!!
 
"I checked for a spark with a

"I checked for a spark with a loose plug grounded onto the engine and there is a spark in the front cylinder on the left (#1), I didnt check the other two
The engine was a short block (no risers or manifolds)I assembled it on the bench and dropped it in and it seemed to run fine first but deteriorated as the summer went on last year.
This summer has been a disaster here (Ireland) so I havent been out that much to test it,
It runs reasonable in the lower revs and only starts backfiring at 2000 rpm under load and higher without load, so it can be used for fishing
That is interesting about the carb, I adjusted the distributor to advance the timing the last time i started it and it flooded the engine, so I removed the plugs to dry them and the plugs on the left side were wet, would this happen if the carb was the problem ?
I have to get down this weekend to work on it and will do a compression test, if that is ok - how can I check to know if it is the carb ?"
 
"Which carb is it? Mercarb 2

"Which carb is it? Mercarb 2 BBL w/single mixture screw? If it is a Mercarb, does the idle mixture set easy? If not, then the base gasket should be 1/4" thick. Rebuild the carb and make sure all the idle circuits are clean."
 
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