7.5 hp mercury won't idle

I have two 7.5 mercurys, 1974 & 1980. Neither of them will idle down. both have had new plugs and carbs professionally cleaned with ultrasonic cleaners, carbs are like new. They still won't idle down. I was told that it is likely the stator...on both motors? Is this a common problem with these motors? They run great wot but can be hard to start (weak spark at low rpm?). Any help is greatly appreciated...thanks.

What do you mean when you say they "won't idle down" - they die out if you back off the throttle OR they just won't slow down when you back them down?

Stator, trigger and switch box problems are extremely rare on the Thunderbolt 4 ignitions used on these models - because of that I would think it's extremely unlikely that both are experiencing the same issue, that being a fault in the low speed winding's.

If you are getting good spark at each plug below 1500'ish rpms, then your low speed winding's are probably fine. You can remove the spark plugs, and ground them to the block. Pull over the rope start and observe the spark. If you get a good blue lightning bolt with a "snap", you have tons of power.

The first thing I would investigate, if you are confident in the service to the carbs, would be to see if they were properly adjusted.

I would fully seat the idle adjustment screw then back it out 1 1/4 turns as a starting point and then give that a try.

The 1974 won't start with the gap less plugs mercury recommends. I have to use standard plugs to get it started. 1980 uses the very common ql77j4c and starts hard as well. I have turned the slow idle needle every which way but lose with no change. They won't idle below 1100rpm (appox). Both have spark in both cylinders. I don't have a spark gap tester. I just grounded them to the power head.

Kimcr,

did you read the previous posts?
your advice is generic and not specific to what has been posted.

I bought a working 7.5 merc to help me figure out the problem. It idles perfect and tops out perfect. I have begun swapping parts from a non working 7.5 to the working 7.5 to find out the problem. Swapped carbs, working motor ran better! Swapped stators, still ran great. Swapped ignition coils and plug wires (one at a time) still ran great. Gonna do power pack and ignition trigger this week. Any suggestion from anyone would be great...thanks.

i see issues with the lower crankshaft seals on these wonderful motors.--------------Usually install new lower seal when motor is apart

Would the lower seal disturb the idle but not be an issue at wot?

I have two of these motors with the exact same problem, won't idle down. I think the problem will be simpler than that. I feel it's ignition related. Gonna eliminate that first. If the power heads need to be torn apart they are both parts motors.

..."..carbs professionally cleaned with ultrasonic cleaners"

Uh-huh.

Jeff

Jeff,

why by would the carbs work perfectly in the working motor and not in the non working motors?

"uh huh"

This is the second 9.8 I've heard of that won't run on gapless plugs. One would assume low energy output is responsible, and that usually means a weak stator.

Jeff

Jeff,

please read the previous posts regarding the stator...thanks.

cdbrummond said:

Jeff,

why by would the carbs work perfectly in the working motor and not in the non working motors?

"uh huh"

Click to expand...

cdbrummond said:

Jeff,

why by would the carbs work perfectly in the working motor and not in the non working motors?

"uh huh"

Click to expand...

It may be because the minimum timing advance screw is wound in too tight, thereby not letting the mecanical advance plate not move counterclockwise enough. On mine circa 1973 there is no idle adjustment on the carb, the carb does not work off the handle throttlegrip (Per-say),, all the way at the lowest handle position, my carb's throttle plate is fully shut, when you twist the throttle on the tiller handle it advances the timing "first" just a little, that increases the rpm, (which is why some people tinkering with these things will just assume that screw is a idle setting screw, well it aim't. but the carb does no tcome into effect yet, it is secondary to the advance moving first , kicked in by a drag finger cam that will contact the throttle plate bar.
Summary you may not have your min TIMING" stop backed off enough, some may consider this screw setting as part of the carb linkage. when it is not actually part of the carb linkage it controlls advance. to further answer your question of why would carb work on the good motor,, because you are not transferring the mis advance stop with the carb cause it is not connected.
Disclaimer , I am just a rookie on these motors, seems things are going around in circles here so figured I would take a chance with this and feasibly it could be part of your problem.
good luck,

I got the 1974 close to running perfect. One of the issues was ignition coil #1 was weak. It is idling down now but it is not running very smooth. I have turned the slow idle needle from .5-2.5 turns and it only dies out if you go too far either way. Starts hard too. I talked to a local merc tech and he told me how to check the crank shaft seal so I'm gonna do that next. I still haven't figured out the 1981 but will work on it eventually and post the results.

I've had to replace the seals in several old Mercs like that, to make them idle and start properly. The top seal is easy to change--you can pick it out with a punch--but the lower requires a power head removal and cases splitting.

Jeff

Thanks for the advice. If the seal was pressed in I would have to part the motor out. The merc mechanic I talked to said I could spray a good amount of carb cleaner under the flywheel and see if the motor smooths out. Gonna try it tomorrow.