1982 18hp mercury WOT issue

[Spk101]

Hello I have a 18hp mercury two cylinder single carb engine. When I'm at WOT the engine loses power after running for about one minute. it drops to aproximatly 70% power and stay there consistently. When I back off the throttle it will regain full power for that given throttle position. this problem is very consistent.
Things i've tried:
1. fuel is high octane and new
2. the fuel pump is good and tank properly vented (hose ball stays hard at WOT and pumping it does nothing).
3. carb is clean and adjusted perfectly for idle.
4. adjusted timing advance rod forward and back but the original position ran best
5. fixed all fuel and air leaks on the engine and carb
6. spark plugs are new and look mid-dark brown (possibly slightly rich)
7. cylinder heads have some carbon buildup but nothing excessive (possibly running rich).
8. exact 50:1 fuel mix in tank

 

[flyingscott]

What is the compression? How did you see the cylinder head?

 

[Spk101]

I don't have a compression tester but i don't think that's the problem it has lots of power other than WOT. I can get the engine to give me most of my power if I back the throttle off to aprox 85-90% and I verified throttle position by looking at the butterfly in the carb. 100% (perfect open butterfly) is where the engine bogs down. I looked inside the cylinders from the spark plug opening with flexible camera. There's no scoring in the cylinder and everything looks great other than some mild carbon buildup in both cylinders. both cylinders have the same amount of buildup.

 

[flyingscott]

Rebuild the carb with the proper kit and give it a good soaking.

 

[Spk101]

Rebuilding and cleaning the carb didn't fix the problem. And yes the float is in spec height. Could it possibly be an ignition coil not working properly at WOT?

 

[Spk101]

I got a compression tester and cylinder 1 is 140psi cylinder 2 145psi.

 

[flyingscott]

Do you have spark on both cylinders? Is the throttle moving everything correctly?

 

[Spk101]

Yes the throttle is moving everything correctly and all the linkage is adjusted properly for carburetor and timing advance. I have normal power until full throttle, how would i check to see if there's no spark at full throttle only? Is it possible for a coil to stop working at high rpm only?

 

[Spk101]

New development. Engine is getting worse which is helping me troubleshoot. I'm losing lots of power now and I've cylinder 2 spark plug is very wet and black buildup. Tried switching spark plugs from cylinder 1 to cylinder 2 and it's always cylinder 2 with the problem. Swapped the ignition coils and it's still Cylinder 2 with the problem. Compression is still 145 in both cylinders so now I'm guessing it the stator or the switch. All wiring seems good, how do i find out which of the two is bad? the problem is sill somewhat intermittent.

 

[flyingscott]

www.cdielectronics.com has a troubleshooting guide for your motor. I would start with the switch box on your motor.

 

[Spk101]

Thanks,

 

[Sea Rider]

Is that a center console installation or still a tiller steering OB ? That's a 35 year oldie, is it salt or fresh water used ?. Are the grounds of each electrical component in mint shiny clean cond ? If not, file them gently with fine grit sandpaper.
Happy Boating

 

[Spk101]

It's a tiller in fresh water. engine and all components are very clean and all connections have been verified. I ordered both the switch box and trigger since I didn't have a marine electrical tester. When I took the flywheel off I was happy to see there was a new stator installed from the previous owner, probably trying to fix this same problem without success. The new parts should arrive today or tomorrow, I'll keep you posted if that fixed the problem

 

[Spk101]

If I change the trigger do I need to check/adjust the timing? or will it stay the same?

 

[Spk101]

I found the answer to my last question and "No" I do not have to adjust my timing since my flywheel has a key holding it to one position only. I only need to keep the same adjustments on my timing advance rod.

 

[Spk101]

With the switch box and trigger replaced the motor still has the surging problem at WOT. Need help

 

[racerone]

Checked crankcase compression ?----Removed powerhead to check for exhaust blockage ? ---Test run with another tank and hose ?----Replaced impeller ?

 

[nipperdog]

You've got a tough one here. I would positively suspect fuel delivery to be inadequate under full load except for the wet #2 spark plug. The fact that the problem is getting worse would make me think a fuel pump diaphragm has a tear which is getting worse causing fuel starvation at full load. Just to be straight here; plug #1 looks normal, while plug #2 is wet? Just for the record, is there any kind of anti-siphon device in your fuel system?
Edit: I like racerone's ideas too.

 

[Spk101]

It's not the tank or hose. New development now..... with all the new parts the spark plug #2 is now dry and perfect. two days ago I went out with the wife and kid and the motor was doing the same thing, (intermittent power) but it always drops by the same amount 20-30%. Today I went out alone and with little cargo in the boat....the motor acted very different WOT was amazing with great power and top speed, no motor surging at full speed no matter how long I stayed there it was running a much higher RPM alone which is probably the reason. mid and low range was having lots of power issues and would even stall frequently after slowing down from full throttle. After all this I think I'm back to a regular carburetor problem?? If I stay at low speed there is no problem with the motor, only after coming down from WOT. How do I check crankcase compression

 

[Spk101]

I forgot to mention that when the motor stalls it makes a sniff sound not a backfire. I'm thinking it may also be getting too much fuel if that's possible