Motor will not accelerate under load.

I have a 1978 Mercury 500 50 hp thunderbolt. The motor will start up and idle fine. With ears hooked to the motor using the fast idle or by puting it in gear, it will rev up just fine. With the boat in the water i can put the motor in drive and it will run at idle speeds but as soon as I try to throttle up, the motor will stop running. It has a new trigger and stator. It is missing the mercury switch.

Thanks

Re: Motor will not accelerate under load.

Any ideas maybe electrical or fuel? Any ideas appreciated.

Re: Motor will not accelerate under load.

Tennman12 said:

Any ideas maybe electrical or fuel? Any ideas appreciated.

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i been having the same problem with my 150 xr4, i have gone thru this thing with a fine tooth comb, new trigger, new stator, timing check, cleaned carbs, FINALLY after thinking the new stator was bad i ordered another new stator, same result, so i went and bought a simple spark tester and determined that it was only running on 3 cylinders, bad switch box! so start with the simple things first and belive me if you don't have one you need a factory manual to go by, not clymer or solec!

Re: Motor will not accelerate under load.

Well I purchased a new fuel tank, line, and hookups today. Did the timing check and the carbs have just been rebuilt. So yeah im going to try the cheaper options first, probably to no resolution. I definitely need to invest in a factory manual. I am using a Clymer right now and it has been somewhat helpfull, but the detail just isn't there. I had a local friend mention that it might be the ignition coils. I know all of the parts and what not. The issue is I dont know how to determine what does what and how to diagnose the problem by testing this or that. Hopefully the factory manual will give me better advice and in more detail. Thanks for the advice.

Re: Motor will not accelerate under load.

Test spark to ensure a bright 7/16 spark onall cyls.

Test compression to ensure all cyls are healthy.

Has the carb recently been cleaned? A dirty idle circuit can idle fine and rev like crazy, but bog and die as soon as the throttle is opened while under load.

Re: Motor will not accelerate under load.

My 150 mercury was doing the same thing and it was the switch box not spark on 3 cyclinders, so for sure check the spark. Also to learn from my mistake make sure you dissconnect the battery before you replace the switch box, I fried the first one putting it in.

Re: Motor will not accelerate under load.

Thanks guys for the great info!