Tohatsu M40C popping/surging rhythmically

[henryst]

I recently purchased a non-running Tohatsu M40C. I got the motor running this morning, and it seems to run smoothly, but every second or so the engine pops and surges in RPMs a bit, repeatedly and rhythmically until I shut it off!

To get the motor running initially, I diagnosed that it had no spark, and traced the problem to a bad ignition coil. I replaced the coil with the one from an identical parts motor, which made the motor start, but now I’m having this popping/surging problem. Interestingly, the coil from the parts motor is built into the CDI unit, while the coil on the other one was separate (even though motors are identical).

I’d really appreciate any help identifying the problem here! I’ve eliminated the carburetor, by switching it out with the one on the parts motor (the problem didn’t change). Thank you for your help.

 

[henryst]

Here is a short video of the problem I am having. Using spark testers, it seems the 'popping' sound is happening on every ignition. Thank you for any help.

 

[pvanv]

You can Verify timing per the Factory service manual, and revisit Properly cleaning the carburetor.

 

[Sea Rider]

Seems you have connected two spark plug tester in between each plug cable and spark plug side ? If so, remove both and test without them. Which spark plug brand and model are currently installed, were the plugs gaped to 1.0 mm tight ?

Happy Boating

 

[henryst]

Thanks for your help guys. I’ve been using NGK BR7HS-10, as per the service manual. I put in a different set of spark plugs this morning and removed the testers- no luck, still having the same problem.

I did notice that when I removed the plugs, one was very clean and the other had a dark oily substance on the base. I know the owner before me put in a new set of reed valves himself. Could this perhaps be the problem, ie. they were installed incorrectly? I have a parts motor, so easy to switch things over/try things out.

Thanks again for all your help.

 

[henryst]

Also, timing has been verified as correct (lining up dash below flywheel with mid section of crankcase) and carburetor has been eliminated by switching with parts motor carb (same issue).

 

[henryst]

Another thing I’m thinking of... is it possible that this new, combined CDI/Ignition Coil I’ve installed is not compatible with the original magneto system on this engine. In other words, the magneto system is only set up to work with the original, 2-part CDI and ignition coil? As a reminder, however, I did get the replacement CDI/coil of a slightly older, identical, running parts motor.

 

[Sea Rider]

Would you say that there's a very bright spark produced inside the plug tester at each cylinder when the motor is cranked with the cord. If hard to tell put the motor in a dark place, garage and check that out. How many DVA volts is the Pulser and Exciter Coil producing ? If out of range the CDI won't function properly...

A M40C is a work horse. Reed Valves usually don't go bad unless fingered. Why did the previous owner replaced the original reed valves factory installed ?

At full throttle grip the timing advance must stop at 25º, right at middle of the thin line formed by both matting surfaces while the carb's butterfly is fully open at 180º. Check if well synched..

Happy Boating

 

[henryst]

Thanks so much for your help. Unfortunately I don’t have a garage but I can try and go out tonight and test the brightness of the spark. I’d say the testers were as bright as I’d seen them on other motors. However, when I pulled a plug and held it near the block, the spark that jumped was not as blue/bright as I would have expected. More orange colored.

The exciter coil was at 110-120V on the DVA. The pulser was at about 5V. I’ve run both these test multiple times.

The previous owner said he replaced the valves because the motor wasn’t running, and he could see through the valves when he held them up to the light. It didn’t seem to me like he really knew what he was doing, and I believe the problem was an ignition problem (which I addressed by changing out the CDI/ignition coil).

Re-checked the timing and it appears to be good, assuming 25 degrees is the dash on the far left, which from another forum I believe it is. At this point I’m wondering if maybe the replacement, combined CDI/coil is also faulty. Unfortunately I don’t know how to test this type of combined unit with a DVA/multimeter.

I really appreciate your help. Thanks again.

 

[Sea Rider]

You're welcome,

Is it possible to test both spark plugs outputs with a variable distance gap tester ? The issue is that if one spark plug if firing low when cranking, imagine at load pushing a boat. Check at nigh if both spark plugs produces a good visible spark inside each tester, test both plugs independently, not at same time with a dual tester.

To understand, that motor uses an independent siamese ignition coil and a CD box, you have installed a dual IG and CD unit, right ?. Check that both spark plug's boots have shinny internal contacts, all electrical components grounds and where they bolt to should be immaculate clean and shinny.

Had an electrical issue which drove the owner near insane, the spark plug's cable end portion that screws to the plug caps had both blackened cable portion ends which didn't deliver full IG power to both plugs, cut 2 mm off each side, screwed back both caps and problem solved, that was a real tricky one...

This is the M40C Time advance setting with full butterfly opened at 180º

Happy Boating