Issues at idle

[TomG]

I've been following positron's thread on his carb rebuild hoping to find some clues, but I have an issue I need help with.

Today I was finally able to finish a long-delayed project of installing new instrumentation. I finally have a tach, which as it truns out, is both a blessing and a curse. I've always thought the engine idled high, but because she ran so darn well, I never dwelled on it (no pun intended!) Today my suspicions were confirmed. At idle in neutral, she runs at 1,400 RPM (according to the gauge). In gear, she turns 1,900 in forward (with a dirty prop) and in reverse, she turns 2,400, so I think the tach is pretty close.

I attempted to set the idle set screw on the throttle to slow her down and she did not like it one bit. She started to bog a down and I couldn't get her to idle lower than 1,100. I tried to increase the mixture on the carb, but the idle mixture screw is already fully seated (?).

So if I'm running full rich on the idle mixture and she really doesn't like the idle set screw decreased, where should I start looking? Does this sound like an idle jet that is clogged? Or an air leak? Should I just rebuild the carb?

Engine is a rebuilt MMI A-4 in 2008. Late model carb. I added the Indigo PCV kit this past spring.

At the higher idle, she runs like a champ.

I appreciate any advice!

[Loki9]

Sounds like a clogged idle jet or passageway. I'd take the carb apart and clean it, poking small copper wires through all the jets.

That was the problem on my recent rebuild, exactly the same behavior as yours.

[smosher]

The idle jet is an air bled which means you adjust of the amount of air going into the idle jet, mine is set to 1 1/2 turns from seating. Fully seated means its fully rich, turn it to start 1 1/2 turns out. I usually adjust it so it doesn't bog down when I give it power, if it bogs its too rich, could be other thinggs also, but this is where I would start

As Loki9 said you could have a clogged jet, and I had the same, wasn't until I poked small wire through the jets it went away, this is after trying carb cleaner.



Steve

[Mo]

Tom,
On my old carb that came with the engine I had (what I think was) an air leak that caused the same issue. I believe it was leaking around the the gasket /seal that connected to the throttle lever itself. I'm not quiet sure because I just cleaned up another carb that came with the boat and put it on...then all was fine.

I did try cleaning that particular carb, blew out the jets etc...it should have worked fine...but alas...same issue continued. I just put on another.

Maybe someone has some experience with "what I thought was the problem"...keep in mind that that was 4 years ago.

[hanleyclifford]

1400 rpm is not "idle". I doubt if your idle system is functioning at all. At that rpm you are running the carb "off-idle". Also, your timing could be way off, probably retarded. The 1400 rpm idle would mask this by running up the centrifugal advance. Before going any further I would advance the timing until I could slow the engine down to around 700 rpm with the idle mixtute screw 1 or 1 1/2 turns off seat. Set the timing at TDC using an automotive strobe type light (at 700 rpm). Then race the engine to 1500 rpm and confirm that total advance is around 17 degrees BTDC. Lastly, set the idle mixture screw.

[Mo]

ah ha

Hanley,
Great approach....didn't think of that for this particular case.

[hanleyclifford]

Hanley,
Great approach....didn't think of that for this particular case.

Mo - I'm not certain timing is the issue but it does have to be correct before carb adjustments are made. It is possible the idle system is plugged in which case the engine would only run "off-idle". Another possibility is a vacuum leak.

[TomG]

Thanks one and all.

Hanley, how would electronic ignition figure into the timing issue?

I'm leaning (no pun intended) toward a clogged idle jet. The engine is an MMI rebuild and I seriously doubt the timing at the distributor has been changed since it was installed in 2008.

It's funny after writing down the symptoms and then re-reading my own post, it seems like a blocked idle port is the likely culprit. I'll let you all know what I find.

Thaks for letting me bounce this off you all.

[hanleyclifford]

It would seem that with it's greater dwell that electronic ignition would retard the timing if distributor position remained fixed but I am only speculating here - others may know more about this. In any case before proceeding in this case I would check timing with a light. Also before taking the carb apart I would blast every hole I could get to with carb cleaner or starting fluid - use the little red tube that comes with the can.

[Marian Claire]

And do not forget the safety glasses. Dan S/V Marian Claire

[ndutton]

When was the last time it idled properly and what if anything was done to the engine between then and the first high idle episode?

I really like Hanley's thought on timing at least as something to eliminate. Instead of checking with a timing light (can't see the front of my engine anyway so I think of other means) I'd have a look at the advance weights under the distributor plate to be sure they are free moving and functioning properly. It could be they are not and the timing was manually advanced some time ago to compensate. That could explain the poor running at lower RPM.

I'm not saying the carb isn't the problem, it might be as you suspect.

[hanleyclifford]

Could some one please post the picture of Tom Thatch's accessory drive timing pointer? A way to use a strobe light without access to the flywheel!

[ndutton]

Yep.

As I typed my post his pointer popped into my head.

[Dave Neptune]

Idle

Tom, unless you have moved the timing I doubt it is an issue with your engine, check to see if the weights are stuck and be done with the timing. I suspect an air leak or a clogged jet which would not affect the timing. You can spray a bit of "motor crack" around the carb flanges and the manifold to check for air leaks while idling (high). If the engine rev's up you have an air bleed also could be around the pcv valve. You could even plug it for the testing to eliminate it as a possibility.
If the above provides nothing back out the idle speed screw so the throttle can drop back a bit. Set the idle mix screw at 1 1/2 turns out and slowly throttle back until she begins to die and then APPLY some choke. If the choke helps you have a plugged idle passage or jet. The idle jet is tiny so poke something through it like a small wire. I do suggest that you dissassemble and blow air through the passages if you get this far. Some times you can rev the engine a bit and while opening the throttle close the choke and the additional sucking will sometimes break loose a plugged idle.
My engine will not even run with the idle at full rich (mix screw closed), when adjusting if I get to less than 3/4 turns out she starts to run so rich she stumbles and dies.

Dave Neptune

[jpian0923]

Neil, Hanley, That thatch picture is helpful if you know what it means. Care to explain? Will everyones pointer point to the same spot on the accessory drive pulley?