82 18 merc high speed miss

I have an 1982 18 Merc short shaft manual start, it's black ,with blue lettering not xd .
I bought it used a couple of years ago and it never has run right . It was hard to start, stalled, wouldnt restart,hard on fuel, and had a high speed miss when I did get it going. I rebuilt the carb replaced the bulb and fuel line and over the last couple of seasons replaced evey electrical component on the motor yes every thing.
The only problem now is I still have a high speed miss, a good one. It's like it is starved for gas for second . I took the carb apart apart and noticed that the float needle was stuck in the seat . It came out easily but I cleaned things up in there and the needle and seat looked great . I still got a high speed miss. the main jet is .046 NOT .052 as the manual says.
My compression is 105 psi even both cylinders. My question are is the low compression causing this, and if yes, is it feasable to rebuild this motor.
I have a pile of money into the electrical already, and have lots of time on my hands.
Thanks in advance,
hockeyteeth

Re: 82 18 merc high speed miss

Hi
Welcome to the board.

Have you looked at the fuel pump? If you are only starved at high speed,it could be your fuel pump is not suppling enough fuel to the motor. I would do a complete fuel system check. If you can find .046 jet then I would replace it.
It will help you on fuel savings. Clean or replace any filters
Compression should be fine on the motor any thing north of 100 psi. Right now I don't see a cause to rebuild your motor, but it can be overhauled.
Hard to start? Have you done a link and synch ? Hard starting is often from timing being off. This also can cause a miss also not enough advance or to much Worth a check anyway.

If you don't find anything in the fuel system, use a timing light to see if your dropping a cylinder at high speed under load.
post back what you find

Re: 82 18 merc high speed miss

Thanks for the reply!
It is no longer hard to start just has high speed miss or hesitation at wide open.
The fuel pump from what I gather is the diaphram on the side of the carb [walbro round bowl] I replaced that the first season when I did the carb cleaning and rebuild.

The other thing I was wondering about would be float height ????. If the float is set too low would this impact high speed running and cause the miss??

One other thing on further inspection of the main jet it is a .046 . The manual calls for .052 at my elevation. I did not think the miss would be the .046 initialy. If the jet was too small, would I not have a lean condition, right across the whole fuel range . Plugs black/brown ,,bit of oil fouling
any and all input greatly appreciated.
Thanks, H.T.

Re: 82 18 merc high speed miss

I have narrowed the high speed miss or hesitation in the fuel delivery system to the float needle seat .
The carb in question is a WMC 10. It looks like it was taken from another motor as this [WMC 10] is the carb for the 18 E start. Mine is manual.

Anyway, the seat, after careful inspection with a magnifying glass, is worn to the shape of the tip of the needle. That is ,instead of being square on the shoulders of the seat, they are cone shaped. This cone shape holds onto the needle and starves carb bowl and then the motor for gas.

This is what I think anyway.????
Now the problem is they sell a needle but no seat . The seat looks like it will come out for replacement, but why bother, if I can't get another seat .

Any thoughts??
Thanks in advance,
Ken

Re: 82 18 merc high speed miss

Hi,
Needle and seat replacement would be where I would start. Check your float to make sure it's not gas logged. Set the float height by the book. Try to get the right jet for your motor, you do not want to run lean at anytime. When you get the mix right open it up 1/8 turn to the rich side.Running rich is better than running lean.

Re: 82 18 merc high speed miss

The WMC 10 was use in the 18, 20 and 25 HP engine depending on the year of manufacture. You right the seat is not removable some are not quite round but usually a new needle will take care of the problem.

Re: 82 18 merc high speed miss

Thanks guys, for the input .
The needle was new a few seasons ago when I overhauled the carb.
I may consder a new carb .
Thanks again everyone
Ken