Mercruiser 250 inline 6 rpms intermittently dropping

[Cbastuan]

Yeah, as it was idling I screwed both screws in all the way until lightly seated, the motor stumbled and ran worse, backed them out until it smoothed out, but it still had the odd miss and stumble, the further I back them out the richer it runs correct?

Mind you this is all in the driveway, so I'm not sure if I will get better results on the water with the motor under load and in gear which I read is when you should be adjusting your idle and the mixture screws .

The idle mixture screws wouldn't affect or cause the Intermittent hesitation/surging at higher rpms though? Would the float level being off cause a problem like that?

 

[alldodge]

The mixer screws should effect the motor before they are all the way in, my guess is the mixer screw needle tips are damaged from someone screwing them in tight. The mixer screws will have no effect on hesitation or missing at high rpm, these happen because of you main fuel jets, float level and fuel pump pressure.

 

[Cbastuan]

Update,
Had the carb apart today, needle tips looked good, everything looked clean, blew all the passages out with compressed air again.
Only thing I noticed was maybe I forgot to tighten down the needle seat (kids must have distracted me, lol) maybe it was finger tight and slowly worked it's way loose after a few times out from the vibrations? not sure if that could be the root of all of my problems, have it all back together and will test tomorrow and report back.
Thanks for All the replies.

 

[Cbastuan]

Been a while so I thought I'd post an update.

Solved the running rich issue, must have been the needle seat not tightened down, motor runs really well at higher rpms now no hesitation or bogging, pulled the plugs after a little run out on the water nice brownish color to them, not black and fouled anymore.

The motor was getting harder and harder to start and continued to run really rough and missing while idling, While I was doing my troubleshooting I did notice the distributor shaft had some side to side play in it, so I went ahead and replaced the distributor as well.

Starts effortlessly now with a little pump of the throttle and idles much better as well, however while setting the timing after replacing the distributor, every once in a while it seemed as the it would fire the spark of a little too soon every now and then (you could hear it in motor and see the timing light go off too far advanced) motor would be idling at around 750. Only happens at idle, the timing light advances very smoothly as I give it throttle and doesn't miss a beat.

So, not sure if this is an issue or not, and not quite sure what else to troubleshoot or not look at what could be causing this seeing as I how after the new distributor I have replaced every component regarding the ignition.

I was thinking of getting a pertonix flamethrower coil to see if this would perhaps help.

Oh, and I checked the compression, pretty much 130psi across all cylinders except #4 was 135

Thanks for all the help so far

 

[alldodge]

Compression numbers look good, could be better, was the throttle at wide open?
I'm not a petronixs fan so I'll let someone else answer that one

 

[Cbastuan]

Compression numbers look good, could be better, was the throttle at wide open?
I'm not a petronixs fan so I'll let someone else answer that one

Yeah, throttle was wide open while checking compression