1990 Merc 40hp tiller idle issues

[robert1115]

Hello all....looking for any and all help regarding an idle issue. The engine is a 1990 mercury 4 cyl tiller. The problem is that I cannot get this thing to idle for me. I have soaked the carbs, I have ultra-sonic dipped the carbs, rebuilt them, replaced gas lines, rebuilt fuel pump, yet still cant get a smooth running engine. All the "basics" have been covered (tank, supply line, bulb and so on) I have muffs hooked up, and not using a tank. I understand that the "final" adjustments will have to be done in a tank or pond in order to dial it in, but I should at least get a running engine out of the water at a low RPM. The engine will fire off, and run if I give it throttle, but once the low speed jets kick in the motor falls flat on its face. I have the WMA 7 carbs on this setup, and Ive adjusted the idle screws to the 1 1/4 turn mark, and have been tinkering with them, trying to get better results by turning in or out. Turning out in small increments and giving things time to adjust has given the best results, but still not anywhere near where they should be. Thus far, I have been adjusting both carbs the same time. Is there something I'm missing here with this old war pig? Compression is not perfect, but I'm only off by about 15 psi in the bottom cylinder. That shouldn't make a difference with this issue, but throwing it in there. Anyway, thanks for reading this and any help or advice other then "your motor is junk" or "or adjust the timing" would be greatly appreciated!!!!

 

[racerone]

Your opinion will vary.-----------Maybe it is not a carburetor / fuel issue !--------Perhaps remove bypass covers to inspect bottom piston / rings.----Post your compression numbers and does spark jump a gap of 3/8" on all leads ?

 

[robert1115]

Compression from top to bottom cylinder ~~115 110 115 95~~ Spark will jump 3/8 on all leads. Wasn't stated in the original post, but I replaced the stator as well because of weak spark. I would have to agree that it very well may not be a carb fuel issue. But like stated, I can start and run her, but when the throttle is backed down, it'll smooth out for a spell, then fall flat on its face. This may be a stupid question, but is one carb more responsible for idle then the other? Meaning am I countering any adjustments by doing both at the same time?

 

[racerone]

No, The air that goes into the top carburetor feeds the top 2 cylinders.-----Air that goes in the bottom carburetor only goes to the bottom 2 cylinders.----In actuality your motor can be said to be 4 seperate motors with a common block and crankshaft.------Your opinion may be different but I suggest you look into the compression issue on # 4 cylinder !

 

[robert1115]

Gotcha!! Thanks!!! Although the compression is lower in the bottom cylinder, would it really effect idle to a great degree? I'm by no means a technician, but I guess its the only thing that I haven't really taken a strong look at. I know the fuel pump runs off the cylinder above this one. So, at this point, how would I discern if this motor is in actuality salvageable or junk? Is there something other than worn rings, scored cylinder wall, or bad head gasket that cause the low compression? Uncertain how to proceed from here. Thanks again!!!!

 

[racerone]

There is no head gasket to leak compression on that motor !----I already mentioned the bypass cover inspection to you !

 

[robert1115]

Looks like I've been chasing the wrong gremlin(s). On a positive note, the fuel system has been taken care of!!!

 

[robert1115]

***UPDATE***
​After flow testing the carbs, it was found that one of them had a blockage in the low speed circuit and subsequently given an acid bath overnight. Additionally, the floats on one of the carbs was out of adjustment, thus flooding the cylinder out. However, after re-installation, the problem persisted! After consulting with several techs, they all concluded that the problem was electrical related. Having already gone through the system, I double checked the trigger and the stator witch both fell within factory specs. With a timing light attached to #1 cylinder wire, It could be seen that the cylinder was getting spark right up until the motor stalled at idle. This process was repeated for all four cylinders, and all cylinders were getting spark. So, I decided to test the switch box, which also checked out fine. However, it was found with this particular setup (and maybe others) that there really isn't a "reliable" test. So from here it was decided to bring it to one of the techs that I had spoke to earlier. After lengthy diagnostics, he concluded that, in his humble opinion, that the switchbox was indeed bad. By now I'm really questioning it, and begrudgingly accepted an offer to purchase a new switch box, brought the boat home, and went to work. The motor now purrs like a kitten............

​I have learned a lot from this process, and these forums!! Thanks for all the help!!!!