2001 two stroke 9.8 - engine races / won't slow after a rebuild

I had to quickly bug out of Mexico when COVID hit, unable to flush/fog and properly store this clean Tohatsu dinghy motor. Upon returning to the mother ship six months later the hot, salty 2020 summer had seized it up. Finally got this motor home and tore it all down for a crankcase de-rust / lubrication. Dodged a bullet (so far, anyway) and got it all back together. Started it up today, but it's only running at WIDE open. I've probably done something wrong to the advance, perhaps? It all seemed to go back as I found it. Tried disconnecting the carburetor from the twist tiller, so I could turn the advance with grip twist, and work the carb separately, to no better result. What makes a very simple two stroke rev up super high like this? (and here I was so proud of my first ever engine tear-down / reassemble...)

Thanks in advance for your ideas!
Greg

The first half or 2/3 of throttle advance is just ignition. As you near the end, the carb is also opened.
Does it run full tilt with the carb at idle?

Yes, it's going full tilt regardless of the carb (unhooked the carb from the tiller). I've screwed up the advance somehow, not realizing there are alignment marks, perhaps, when you reinstall the flywheel? Is it possible to put the stator/exciter stuff in backwards?

I really like these Nissan/Tohatsu 8/9.8s and need to find a source for a real repair manual! (no dealers nearby)

You likely have the spark way advanced.

Hello Paul. I've studied up on how to set the timing advance. By any chance can you provide the idle and high speed limit values (compared to TDC) for the Nissan/Tohatsu two stroke 9.8? And how do I reach your shop to buy a proper factory service manual? I need to get this motor running in the next couple of days, before it hits the road from my California workshop back to the boat in Mexico. The repair manual can come a bit later.
Many thanks in advance, for the numbers of advance!

Update - Just found the numbers right on the Tohatsu website. And learned they still sell this motor in some countries.
idle is 1 degree after TDC, fast is 17 degrees before TDC. It doesn't give the connecting rod lengths, but I'm pretty sure i didn't mess with those. OK, here goes tonight in the workshop...

Oh boy, there is no apparent user adjustability on this motor's spark advance. The metal advancement plate has a slot to slide over the plastic limit tab, and that thing is screwed into a set position. No moving any of that. There is no TDC mark on the flywheel, nor any degrees-off marks on the stator/exciter/electrics housing. There is a bit of adjustability on the small linkage arm going between the vertical throttle positioning / cable connections stem and the spark advancement plate, but its already set for no play, and the plate hits the limits properly based on the twist grip throttle min/max positions.
So, either i'm not understanding another method to advance / retard spark timing on this motor, or something else is making it high-rev uncontrollably. I don't think it would be the CDI, as the motor was running well before the crank rust-seized.
Any way to adjust spark timing on these? Or any other possible causes to investigate?
Thank you!
-Greg

Just found the relevant pages of the Tohatsu repair manual. Reading this procedure (copied in below), it sounds like I probably have something like the cables backward or other simple problem. Will be following the official steps now.


The 8 and 9.8 engines are different electronically from other one and two cylinder engines. The 8 and 9.8 engines incorporate an electronic switch (Thyristor SCR (Silicon
Controlled Rectifier)) which is actuated by signal voltage to provide stable, high performance secondary sparking.
The magneto is a simultaneous, multi-cylinder ignition type and would, if allowed, cause ignition in both cylinders at the same time. Ignition however occurs only on the compression stroke side (TDC side). Ignition is caused in only one cylinder at each spark timing. The other cylinder is not ignited, although sparking is caused in both cylinders at the same time.
The timing is adjusted only after the powerhead has been disassembled and reassembled. Timing is controlled by the position of the advancer link rod which is adjusted so that the coil plate turns counterclockwise to contact with the stopper when the throttle valve is closed fully when the advancer lever is moved.
8/9.8 Adjustment Procedure
1. Move the advancer lever with a finger to fully open the carburetor throttle valve. At this time, adjust the length of pole joint cap of the advancer link rod so that the coil plate stopper hits the projection of the set plate lower as the coil plate rotates to the left.
2. Assemble the throttle cable into the throttle drum and attach the nut to the throttle cable holder of the motor cover lower. At this time, be careful that the cable is not attached to a wrong location.
3. Adjust the nut of the throttle cable that was attached to the throttle cable holder so that the coil plate of the magneto rotates to the left at the position where the handle grip stops when it is turned to the left and hits the coil plate stopper, and then the coil plate of the magneto rotates to the right at the position where the handle grip stops when it is turned to the right and hits the coil plate stopper.

Hello again folks. The motor is assembled properly per the Tohatsu factory manual. It will start right away, but immediately revs up to max rpm. Spark advance is not adjustable on the 8/9.8 two stroke. So where to go next... basic voltage values from the CDI are within specs, but maybe there's more complex testing to do? I am seriously stumped as to what makes this two stroke run up like this. Any other thoughts out there?
Thank you.
-Greg

I am sure spark is not an issue. Verify timing.

Hi Paul. Well it looks like this is likely a carburetor problem. I was able to briefly get the motor down close to idle but still wanting to race back up. Removed the carb and upon closer inspection found the main throttle flap not closing down completely, and some other corrosion issues. And one of the jet adjusting screws broke off. So I'm willing to buy a new carburetor for this motor. Thinking of trying the $45 China knock off ones. Or are they a total waste of money?

It's interesting and perplexing this model motor does not have any obvious timing marks. Understandable since the advance is not adjustable but seems rather unhelpful to their customers doing troubleshooting?
Thanks again.

Once the timing is set during assembly, you don't need to readjust it.
The Factory carb is about $200. There are 2 versions, depending on whether you have remote or tiller. YMMV if you try an aftermarket. Frankly, they will sell anything, even if it doesn't fit the inlet or linkage.

Follow up:
Yes, this was a carburetor issue. The $36 chinese carb looks on the surface to be a good clone of the original, and the engine is running fine in the test tank with this replacement carb. It's not clear how I either damaged the carb during the rebuild or if the corrosion issues were to blame. Also the new clone is supposed to be the 9.8 hp jetted model, but I won't get a good gauge of that until I can return this motor to its Takacat dinghy back in Mexico next month.
Thank you Paul for your commentary in this thread, and the many, many more you provide guidance on.
Greg in Novato, CA