Fuel(?) Problem at high RPM

Hey all- I've got an '02 70hp two-stroke Tohatsu. It's in great shape all around other than the fact that last summer, after running more or less fine the first time out, it subsequently would not rev under load above roughly 4000 RPMs. After another day or two, it would hardly even run, and eventually stopped. At idle, it would rev fine as I recall, but couldn't, like I said, in gear. At the time, I thought it was due to the fact that we had a fiberglass tank, and hadn't used the boat in a month or so, and that perhaps the ethanol in the fuel finally ate away some of the resin, and it gummed up the fuel injectors or pump. The boat yard looked at it in the fall and fixed it (I assumed), and junked the fg tank in favor of two eyesore red plastic ones.

Fast forward to the present, first run out a couple weekends ago, wouldn't rev above 4,000, and it would stall at idle, which it had never done before. Next day, basically the same, except it did run higher for maybe half an hour. This past weekend, worked perfectly fine and ran up as high as I desired (I don't think I quite got up to the RPM limit), except for the idle problem. Then yesterday, other than the idle problem, worked fine. UNTIL, that is, I got into a little bit of chop and the engine had to fight against the waves, at which point it reverted back to the <4,000 RPM game, and even then it would kind of halt at 3,500 and slowly work it's way up to 4,000, at which point it hit a ceiling.

I lean toward thinking this is still a fuel problem, but we've got plastic tanks, a new external fuel filter, new external fuel lines, new plugs, and good gas. Supposedly, the problem was fixed. Any thoughts on what this could be or a remedy if you know? I have not tried priming the bulb while the motor struggles at 4,000 because I'm usually alone and can't reach it from the wheel (and I think I did try that last summer to no avail). Also, if it matters- we used to winterize it ourselves by emptying the fuel tank, running the engine dry, then spraying fogging oil into the cylinders and air intake and turning it over a few times; we would let the yard winterize it and get bottom oil changed, etc etc every couple of years.

First, as the boat yard what was done, aside from new tanks. Then, you will need someone with the proper expertise and a Factory service manual to do a proper diagnosis. Could be a fuel problem, but also the ignition needs to be checked for a proper diagnosis.

Ok, thanks. I guess, as always, I was being optimistic it might be easier.

It probably is easier... Diagnostics could include something as easy as squeezing the primer to determine whether extra fuel pressure addresses the problem. Knowing what the yard did, aside from a new gas tank, would help. See if you can find out exactly what was done, and... if you or a friend has 1- a Factory service manual, 2- some decent tools and equipment, and 3- average-to-good mechanical skills and abilities, you will be on your way to getting a decent diagnosis. After that, the rest will be routine.

Paul (or anyone else), I'd like to pick your brain once more.

Had a local yard clean out the carbs (twice) after having them chide me for not using 93 octane gas with K100G additive, which they swear by. So, I began using both of those for every tank I run through, and it's been running better. This weekend, however, after going through roughly 24 gallons of 93/K100 since the last service, the same problem struck again, lasted about an hour and a half, and then went away (no more stalling at idle, though). If I'm using the best combo of gas possible, can it be that there is carburetor damage or something else wrong with the fuel system?

I double checked what the yard did last fall- new tanks, external fuel lines, added external fuel filter, changed internal fuel filter. The service from this summer consisted solely of cleaning gunk out of the carb, twice.

I don't have a service manual, nor do I possess a variety of tools, not to mention my relatively basic mechanical knowledge. Just looking to get your opinion compared to whatever they may tell me at the yard.

The 93 octane is fine, but typically not necessary. For your 70C, 87 r+m/2 (91 research) is enough octane; higher does not hurt. That said, the premium fuels sometimes have more detergents and better stabilization additives -- so not a bad choice. I have a customer who runs 100 octane Low Lead (non-ethanol airplane fuel) with success, proving that higher octane is not a bad thing. We have been recommending the K100 products (for gas outboards, we sell the MG version) for a few years, because many boats have the fuel sit for weeks before using it all up. If the fuel is fresh, and always will get used up in a week or 2, the K100 isn't critical. It's more important to run the carbs dry at the end of the day to prevent varnishing of the passages. This is a bigger problem in the newer, epa-rated, carbed 4-strokes (because they have smaller passages that get restricted more easily), but it's always a good habit.

Depending on what the yard did to clean the carbs, you may still be experiencing the remnants of dissolved polyester resins from the old FG tank. The only way to properly clean the carbs is by completely disassembling them, submerging them in carb dip for several hours, and then washing them out (either with clean water, if the carb dip is compatible, or spray-type carb cleaner), then re-adjust the floats, reinstall the carbs, and finally verify link and sync.

We have run into similar problems with carbed V-8 inboard motors and FG tanks, thanks to E-10 gas. Even after replacing the tanks and flushing the fuel system up to the carbs, there is sometimes some residual gunk in the carb passages that can be difficult or impossible to clean out with a spray-only approach to carb cleaning. However, over 90% of the time, a long dunk in dip has been successful. In the other 10%, the carbs had to be replaced. ;-(

Long story short... If you are certain that you now have good fuel in good tanks and lines... and are certain that the ignition has been properly verified by a decent mechanic... you are probably still experiencing carb issues.