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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4724
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a m40c with some issues

Customer Question

Hey Jeff I have a m40c with some issues
Submitted: 1 year ago.
Category: Boat
Customer: replied 1 year ago.
Motor has no power and can't get enough rpms to plane 14' Carolina skiff. Actually runs nearly fine if you can get on plane. Runs great on the hose. Has good spark and has 125psi in both cylinders and was recently worked on by mechanic who supposedly tuned carb and adjusted timing. However boat does run ok without a full load
Expert:  Jeff G. replied 1 year ago.

So the biggest problem you have is getting it on plane? If the boat is lightly loaded how does it do? Does it idle well? Is this something that has just started?

If the timing is OK then the ignition should be fine, that compression is fine. That really leaves fuel or set up.

Post back with your answers to these questions.

Thanks

Customer: replied 1 year ago.
the biggest problem is getting on plane. When the boat is lightly loaded it does ok but still takes far too long to plane. Idles just fine. Yes it just happened one day it has previously ran well and no real changes were made it is on a 20" transom and I believe it is a short shaft but it ran well previously. I'm in the process of replacing all of the fuel lines and ball
Expert:  Jeff G. replied 1 year ago.

With the engine down check where the cavitation plate of the lower unit is in relationship to the bottom of the boat. How much higher? It is probably too high on the transom.

Did you replace the propeller?

Do you have any rpm figures?

Customer: replied 1 year ago.
The cavitation plate sits about two inches above the bottom of the boat yes I did replace the prop to a 9 pitch from a Yamaha 40 but it previously ran well the way that it is
Expert:  Jeff G. replied 1 year ago.

Being that high will make it a bit harder to get on plane.

Let me look up the timing specs on that engine. If the timing is off it will cause lack of power problems. I'll get back to you in the morning from the shop.

Expert:  Jeff G. replied 1 year ago.

On the starboard side magneto plate are some marks for the timing. With the throttle at full throttle, forward gear, engine off, check that the 25* mark on the plate is lined up with the split line between the crankcase halves. Adjust the throttle stop as necessary.

If the timing is off by more than a few degrees then it will make it harder to get on plane.

If the engine wants to bog down when getting on plane try to enrichen the idle mixture screw 1/8 of a turn.

Let me know what you have.

Customer: replied 1 year ago.
I cannot make out any timing marks on the Stbd side there is a groove there that lines up with the case split for the entire throttle range (starts at one end at no throttle and is at the end of the groove at WOT I'm in the process of replacing fuel lines and ball and filter my gas didn't have any water but was a little on the green side I won't be able to put I. The water for a couple days because of this hurricane
Expert:  Jeff G. replied 1 year ago.

There should be timing marks on the plate under the flywheel.

Customer: replied 1 year ago.
On the port side there are some vertical lines but no numbers on WOT in gear the split lines up to the far left of these lines
Expert:  Jeff G. replied 1 year ago.

Let me get you a pdf in the morning.

Expert:  Jeff G. replied 1 year ago.

here is a pdf of the timing.

Customer: replied 1 year ago.
Sorry I am not able to view the pdf
Expert:  Jeff G. replied 1 year ago.

I tried it on another computer and it comes through for me. Do you have a pdf viewer on your computer? If not it is a free download from Adobe Acrobat.

Customer: replied 1 year ago.
It seems the be correct the first photo is WOT the next is no throttle no gear
Expert:  Jeff G. replied 1 year ago.

You ran the Yamaha prop with no problems correct?

Run the engine, have a friend at the throttle. Remove the air silencer from in front of the carb. Squirt a little fuel into the carb while trying to accelerate, does the rpm go up?

Customer: replied 1 year ago.
I have been running a Yamaha prop with no previous issues. Ok I will try that migh be a couple days because of this weather
Expert:  Jeff G. replied 1 year ago.

Let me know, curious problem. Check the sync between the carb and ignition. The timing should be at wide open or nearly so before the carb opens. If the carb opens too soon it will bog down.

Customer: replied 1 year ago.
Just took a look at it and it looks like the carb opens far before the timing is wide open really it's about on the fourth mark or so
Expert:  Jeff G. replied 1 year ago.

On the same page as the timing I sent is the throttle cam sync. As you can see from the design of the cam it causes the timing to advance before the carb really opens much. Is your throttle adjusted this way?

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