Poor Idle Question

[MAN - GA]

Motor is a 1980 Spirit 40hp (2 cylinder/2-stroke). Same motor as a Suzuki DT40 it was just labeled Spirit on the cowl b/c it was not till a few years later that Suzuki actuall marketed their motors in the US with Suzuki on the cowl (this is based on what I gathered from internet info) - all the part references/diagrams/serial numbers check to Suzuki DT40 parts. Here is info on how it runs and then I will explain the symptoms as I am at a loss on what else to check.

Starts cold within 2-3 seconds of cranking - generally doesn't want any choke to start. There is no stumbling/hesistation on acceleration and winds out just fine and will run at WOT all day without issue. Impeller is new with good pee stream and exhaust water it spits will be warm to touch even at WOT. Overall the motor is great until I come off part to WOT conditions and try to go to low idle say just in gear. Just in gear pushes boat about 2.5mph on GPS and I am quoting this to help give relevance to the problem. Generally after this WOT condition back to just in gear it will want some throttle to help it stabilize sometimes taking several minutes to stabilize back to a clean idle condition. It will act like a miss, sometimes sputter & die, but if I give enought throttle to go to 4.5 - 5mph on GPS it will idle perfect all day and this is just too fast to troll.

I am not a parts thrower and try to figure out the problems first, but in going through things here is what I have done as part of preventitive maintenance anyway as I didn't know how old some of the fuel system was.

1. Replaced tank (had another newer one on hand), fuel lines, fuel filter, and tank/motor line connections
2. Replaced fuel line from motor fitting to fuel pump and fuel pump to carb
3. Removed fuel pump and inspected for cracks and cleaned block port and reinstalled with new gasket I cut myself
4. Removed carb, cleaned and made new gasket
5. Checked all internals of carb and nothing appeared to be bad and float appeared OK (actually no visible sediment in fuel bowl
6. Adjusted low speed idle screw for best idle and have turned it all the way in and it will kill motor

This sucker is a great motor outside of the low speed idle symptoms and I have not invested in a new fuel pump yet because it runs fine at part to WOT conditions. When I reset the float in carb it does the same idle issues as before. Sometimes it gets hard to idle especially if I shut off for a few minutes and then restart - this is generally about the amount of time it takes to park at dock to get trailer backed back into water for loading.

I need another set of thoughts here guys as I am probably missing something very simple on diagnosis.

THANKS in advance

 

[bluegillfisher]

I'm new at this too, so my two cents--be sure you really cleaned the idle screw hole. I cleaned my carb three times before it would idle right. Last time I soaked carb and blew out jet holes with compressed air. runs good now.

 

[JMichael]

Other than stripping the carb down and giving it another good cleaning the only thing I'd suggest trying would be to tweak the mixture very slightly. You could try going both ways (richer/leaner) but just a very slight amount.

 

[Pappy]

What you may have going on is actually more normal than you think.
If you have tuned this engine for optimum idle quality using your idle mixture screws then you are probably close to the lean limit - nothing at all wrong with that!
Have you noticed that your idle RPM is always just a little higher right after you come off plane then settles back down the longer you idle?
Now follow me...when the engine is first started and it is running correctly the cylinder and crankcase is fairly wet with fuel since the mass air flow is low. This is normal for a carbureted engine.
When you plane off and run for a while the crankcase becomes well scavenged of fuel, well enough so that when you come off plane the engine will be a little too lean until the cylinder and crankcase wets back down and the temp changes to the conditions you tuned it at.
Do yourself a quick check to see if this is correct and pop the choke a little when you come off plane and see if that helps. If it does then you need to make a decision to either live with it or richen the idle mixture a little and sacrifice a little idle quality.
Couple of other things you can do to help some.....
If you are truly well tuned then the transom angle of the engine will also make a difference in idle quality and overall idle RPM. Try it! Trimmed down will lower the idle RPM and trimming up will raise it. Along with the changes in fuel mixture (fuel droplets don't travel uphill to the cyls. from the carb as well as downhill) the back pressure is changing as well. If you have tilt and trim then drop the trim back down when you come off plane and the engine will stabilize faster as well.
BTW - when you raise your RPM up a little to keep it running when you come off plane you are activating an off-idle circuit in your carbs that is compensating a little.

 

[MAN - GA]

Here is an update - I started tinkering with the motor a little more and decided to ohm out the ignition system and check timing to be sure it wasn't something spark related that was causing the issues. I discovered that the timing was off pretty bad. The manual says 6* ATDC at idle and it was about 10* BTDC. Made the appropriate adjustments on the mechanical linkage and now it is purring like a kitten - haven't lake tested to fine tune idle in gear or do a few WOT pulls to see how far it revs out, but definately night and day on the idle quality.

As a side note I did a compression check and both cylinders fell exactly in the middle of 115 and 120psi readings on the guage on warm motor - checked each cylinder 3x with each reading being the same - thought this was pretty good considering the motor is 32yrs old