Merc 305 Running rough when warm

[harleyman1975]

I would start with a rebuild (good thorough cleaning of carb)...if not confident you can do job, don't wing it...take to a pro. You want this done right.

 

[bronko69er]

I've rebuilt a few carbs before, watched a video to re-familiarize myself with the rochester and learn about problem areas to look for (epoxy wells etc.). Got it apart last night and soaking in cleaner. It'll go back together and on the muffs tonight. Lake testing tomorrow. Didn't see anything obviously suspicions internally,thing were pretty clean, although there WAS a little fuel puddled in the manifold. Another observation to note: When I unscrewed the fuel line, it was still at pressure from running 30min prior. Doesn't this indicate that the float and needle are sealing properly?

 

[alldodge]

When I unscrewed the fuel line, it was still at pressure from running 30min prior. Doesn't this indicate that the float and needle are sealing properly?

Yes

Also find some fine wire and run it thru the small channels to make sure their cleaned out

 

[bronko69er]

Another thing I just remembered , when pulling out the primary metering rods, the "power valve piston assembly" was quite sticky in its bore.

 

[QBhoy]

Sounds like a failing auto choke symptom. Disconnect it and turn it to permanently closed. sounds like a worth while try.

 

[G_Hipster]

Time to revive this. Still having problems. Things I have done this year include general maintenance tasks such as oil and filter change, outdrive oil change, replacing the water separator/fuel filter, resetting the base timing, adding a carb spacer, additional blower and ducting to reduce heat soak in the carb. After getting up to operating temperature (180F), and idling or sitting for a minute or more it struggles to get out of the hole, sputtering at mid through full throttle for anywhere from 5 to 30 seconds before it comes back to life instantaneously. After that point it runs strong and as is should, I can come off plane then re-plane with no issues, as long as I don't stop and idle for very long. During this behavior, I can definitively say that the carb is not hot. Its barely above ambient temps. The only thing left I can think of doing is rebuilding the carb (rochester/quadrajet 4bbl), but I really don't like the idea of throwing parts at problems as a means of diagnosis (even though I just did that with the carb spacer and blower).

I was experiencing similar issues with mine. Very sporadic random, stuttering and cutting out. Sometimes getting on plane sometimes after coming down. More pronounced when warm. Chased through all the normal stuff. Not enough to see in the oil or lock it up but it was sucking water on the right side exhaust.

 

[bronko69er]

I was experiencing similar issues with mine. Very sporadic random, stuttering and cutting out. Sometimes getting on plane sometimes after coming down. More pronounced when warm. Chased through all the normal stuff. Not enough to see in the oil or lock it up but it was sucking water on the right side exhaust.

Interesting that you mention that, when I got the boat it seemed underpowered so I did a compression check and found a dead cylinder, turned out to be a tuliped valve, which, as I understand can be cause by water reversion from the exhaust. Is that what you were experiencing?

 

[bronko69er]

Update after the carb rebuild: it runs even worse now, sputters almost non stop. I was only able to get it to run smooth a few time but it didn't last long before sputtering again and falling off plane. I had a buddy driving while I was fiddling with choke levers etc., nothing I did would improve it. I didn't change any adjustments on the carb just cleaned and resealed/gasketed. Interestingly though, when I tried to tune it after first startup, turning the idle mixture screw all the way in did not decrease the tips at all. Also backing the throttle stop off the lever entirely only got the idle down to 800. I did have to turn it up on the lake to keep if from stalling though.

 

[alldodge]

didn't change any adjustments

In my opinion your carb was flooding. It either had a float which was sinking from being gas soaked or was not adjusted correctly. You need to pull it back apart, weight float with gram scale or replace the float. Then recheck all adjustments and make sure the sheet your working off of is the correct one for your carb

 

[bronko69er]

I did check the float level (1/4" below top) which matched the chart on the instructions. I'll search on that level and see if its correct. I looked it over very carefully and did not see any signs of holes. It also felt very light and I could not hear or feel and sloshing when shook it. I would be very surprised if the float was flooded, but I suppose it's possible.

 

[alldodge]

When checking make sure the float arm is straight, then bend the tang to adjust level, not the arm

 

[bronko69er]

Not sure what you mean by tang, but it appears that there is only one location to bend the float structure, a V notch in both arms.

 

[bronko69er]

And reading another thread, looks like someone else didn't have success until they were closer to 11/32" for the float level.

 

[alldodge]

Correct no tank this is a Q-jet, 11/32 or 3/8
The ones below can soak and get heavier then spec

 

[bronko69er]

Yep, thats the one. I'll get in there and readjust. Probably order another one too.

 

[bronko69er]

Adjusted float down to 3/8". No change in behavior, still runs like crap. I'm out of ideas.

 

[alldodge]

If the carb is still flooding the motor then its still a carb issue. What are you seeing?

 

[bronko69er]

Some things to note from last night's test run: At cold idle (750ish rpm), thermal choke still mostly closed, the idle adjustment screws do not seem to do anything. I can turn them all the way in and not affect the idle. I have turned them out 2-3 turns without seeing any change in idle or performance characteristics. Warm idle after a quick cruise was up around 1200, then later when hotter it struggled to idle at all, stumbling and dying. Also, once warm and thermal choke mostly open, at idle, manually forcing the choke more open or more closed brings down the idle.

This last time I was inside the carb (a few nights ago), I double checked all the passages for clogs, didn't find anything. Also, after sitting for 2 weeks the carb was completely dry, does this speak to a fuel leak into the manifold?

Trying again tonight. going to fiddle with the choke under power to see if it gets better or worse, also brought some starting fluid to spirits into the carb during load to see if it's a lean situation.

 

[alldodge]

the idle adjustment screws do not seem to do anything. I can turn them all the way in and not affect the idle.

This says your idle jets are damaged. You should be able to screw them in and kill the motor which is shutting off the fuel. Your unable to shut off the fuel so their wide open.

 

[harleyman1975]

That carb is leaking internally. It's time to admit defeat and bring it to a pro or buy a reman unit.