merc 9.8 power loss after warm up

im having some probs with my 78 merc 9.8. it runs fine untill it gets warm and running at full throttle it will bog down to about idle speed. it keeps happening everytime i run full speed now kinda sputters in and out & sometimes picks back up to speed for a few seconds. if i run at half throttle i never have any problems. squeezing the primer bulb during doesnt help any. im thinking its the switch box. if anybody has any suggestions i would really appriciate it i cant get any help on iboats.com, also i just got my carb rebuilt so thats ok, thx john :confused:

You might have dirty carbuetor. Run it without front carb cover at full throtte put your hand over carb and see if it picks up. I did this to motor and foud out dirt got in it. Because it didt.n run gas out of carb

Dirty carburetor:confused:

Pepper

My 88 40hp Evinure was doing the same thing. Ran better a half throttle because the plate where close half way. When I choke the carb it ran better. The man said I had dirty carb he clean them and ran good. It may not be getting gas, because the carbs are dirty.

If the carb truly was rebuilt, then it should be clean. If the carb rebuild did nothing to improve, I would lean towards the coil. Sounds like the coil is getting hot and the impedence is dropping to the point that your spark is not what it should be when warm. You can take the coil off, measure the resistance of the coil cold, stick it in an oven, and then measure the resistance hot (200 deg.). If you see a big drop in the resistance, replace the coil. Good Luck!

It sounds like you are overheating. Check the water pump, or engine might be full of lime causing it to over heat after running for a while at full speed.

thanks for the tips, the carb is running great i had the same problem before rebuilding it & it runs much better other than my losing power once the engine gets warm, the coils checked out ok, also i have a new water pump & impeller. So im thinking it must be something else...this is a new motor to me & i would suggest having a pro look over any motor you are trying to buy before you go in the hole like im in!

I'm having the same prob. with my 9.9 86 merc. Talked with a couple buddies of mine & they suggest rebuild fuel pump, kit cost @ 10.00. I have the same prob @ WOT. One pull it starts right up again, runs WOT for about 2 min. & dies out. Havn't done this yet with the new boat on the water but it's a cheap first start. If you do rebuild pump let me know if that worked.

My guess is the carb is still dirty it takes very little to jack one up or it could be a reed valve thats not closing properly due to it being laquered up,we had a johnson that was doing the same thing and after cleaning the carbs several times we pulled the reed valves and they were covered in laquer and not closing after we fixed that it ran like a new one.

I had an 85 HP. merc that did about the same and found water in gas after much trouble shooting to no avail. I had built in tank which set out in rain and recessed cap on tank...just a thought..


chuck61938

It could also be a head gasket. Motor warms up thermostat opens up lets water in cylinder presto no power. I had a 15 h.p. do the same thing.

Good thought mean green. i will definately update once this is solved