Outboard Motor Question

Hope someone can offer up some suggestions...

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, but this is just too fast to troll.

In going through things here is what I have done as part of preventitive maintenance as I didn't know how old some of the fuel system was when I purchased the boat.

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 made new gasket for reinstallation
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 to 1.75 to 2 turns out 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.

After going through all the suspected fuel related issues I started looking at the ignition system. I found a scan copy of a factory maintenance manual online and checked the OHM readings per the manual on the coils and everything was within spec.. The timing advance is all mechanical via linkage as a direct relationship to throttle position and upon inspection the timing was off. It is supposed to be 6ATDC at idle and I was getting about 8-10BTDC at idle. So I did the proper adjustments and checked with timing light. Idled much better, but what it did was move the clunkiness described earlier to a higher point in the RPM range now it happens at the 5-6mph range on GPS.

Manual says that it should be at 25deg at WOT (5,000RPM) and when I was fishing Sunday night I checked it with digital timing light at WOT and I was at 15-16deg and 5,200RPM.

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

I need another set of thoughts here guys as I am probably missing something very simple on diagnosis to get this motor running smooth at any throttle position.

THANKS in advance

I built a 35' pontoon "party barge" / ski staging area this spring. It has a POS Force 85. It was doing the same thing until I put 1 litter of Lucas throughout it. It worked, not sure if it will help you though.