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520 rich/misfire?

F

fanofthesimplelife

Active member
just changed the oil and plugs in my motor because i noticed it running a bit rough and now out of the right cylinder bank at WOT it will smoke black not constant but puffs i was thinking maybe mag issue new plugs are ngk AB-6
 
black garbage probably only had 20hrs on them the back cylinder plug had a small rust spot by the tip it looked like a heat rust though not a water rust
 
With new plugs installed (no run time on them) start motor and immediately go to low-medium power to warm up. Once warm run it hard for a bit and shut down. Pull the plugs and see what you get. Looking to see if all cylinders are rich or just 1-2?

By black garbage, do you mean they are oil fouled or fluffy black from being rich?
 
It could be mags, but both would have to go weak at the same time for all plugs to show. Rich sounds like the correct diagnosis. All cylinders rich points to the carb. Did this just start or always done it? Recent change might indicate a stuck float. What carb? If Holley or similar a blown power valve is a good suspect. Otherwise it might just need jet tuning.

Did you recently add an air cleaner? That will richen the mixture some. I believe this is a new boat to you (??) so note any changes you made since it ran right.
 
Need to know what carb, if MA6 the aneroid could have vibrated out, there is an MA4-5 version that has the aneroid as well, unsure of the # on that one, a runup and a gradual move of the lean rich lever could show a jet issue, also a stuck or plugged economizer valve could cause a rich condition.
 
if and only if the carb is a MA-6AA helicopter carb and has not been converted or modified.....I had a very similar problem with an 0-540 and a 6AA carb last week and what had happened was the AMC unit had failed and was dumping unjetted fuel into the main nozzle
 
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