Motor has to catch up

My M70B Tohatsu has a weird p

My M70B Tohatsu has a weird problem. When I push the throttle down to WOT is goes up to about 3000 rpms for about 10 seconds then it surges for a couple of seconds and then goes up to about 5200 rpms (WOT) what could this be? Also if I am running for a while and then shut down. If I try to start up after about 1-2 hours it is hard to start. Thanks in advance. Joe

" Joe,

As for it bein

" Joe,

As for it being hard to start after 1-2 hours of sitting, the engine is cold at that point, it's going to start about the way it will when cold, and probably requires priming, throttle and choke. If a (after 1-2 hours an engine is probably not hot) hot engine is hard to start it can be a phenomenon called "vapor lock", heat from engine messes with fuel, keeps carb dry... I've never actually seen it, but people talk about it happening. Another possibility is the carb is a little clogged, making it run lean and that will also make starting hard, but if it runs well normally, it's probably not that.

As for the surge.. you are saying it goes right up to 3000, then through the roof (say 6000), then down to 5200 RPM's ? This probably is caused by cavitation - the prop looses "traction" for some reason at that point in planing out the boat. You may want to give some hydro-foils a shot (cheap fix, under $30) they will aid in bringing the boat up to plane and getting the prop to grip. It also may be corrected by just changing the physics a bit, trim (tilt) the engine a little up or down (using power trim or whatever adjustment you've got) or move a little weight in the boat the the stern or bow. Could just be the boat too.

There's an off chance your prop is damaged and slipping, but it doesn't sound like the case here, as the RPM's drop when the boat hits top speeds and they also hold at 3000 for a while. And lastly, 10 seconds is a long time to plane out, is it really 10 seconds or more like 5? Also, is this a new thing or has it allways done this on the boat it's being run on currently?

Jon "

" Joe,

I just looked ba

" Joe,

I just looked back up at your post.. If you mean surges, as in the engine pulses (up and down a few times around that RPM) for a few @ 3000 RPM before coming up to speed or hesitates around that RPM, you are most likely dealing with a clogged low speed jet and nothing to do with cavitation.

Jon "

The best way I can describe i

The best way I can describe it is that I put the throttle all of the way down and it goes to about 3000 rpms it will start going along and the when the boat starts gaining some speed it will start to act like the high speed circuit is trying to kick in. after a couple of little surges it will go to wot. It seems that it could be electrical of some sort I am not sure. It is not a cavitation etc. or the carbs. They have been bathed and cleaned 3 times in the last month and a half.

" Joe,

Yah, that'

" Joe,

Yah, that's hesitation, I misread you on "surges", that's actually what it's called but I was thinking of something else. I don't get how it could be electrical as it seems to be running correctly at all other points other than that transitional period. It really sounds like fuel.

Just for kicks, try giving it a good decarb, use whatever brand of spray-in-carb stuff they sell for marine in your area. I have found that with my merc, every so often (depends how much low speed running I do, but about 20 hours usually) it will start hesitating and runs perfect after a decarb. Rarely do magic fixes like this actually work, but it's cheap and easy enough. After that, check the compression and be sure you are dealing with a good engine before wasting time diagnosing.

Checking spark is easy enough; be sure all cylinders are firing with strong spark.. doubt it's it, but proove to yourself that good spark is there. Just for kicks as well, pull the fuel line off where it feeds the carbs, pull the MOB switch to prevent ignition and pull off the spark plug leads to be safe; squeeze the primer bulb and observe smoothe easy flow through the line. Turn over the engine (hopefully this engine lets you do that with the MOB switch yanked); observe that the fuel pump is sending some sort of steady pulse to the carbs for a few seconds.

I'd look into fuel issues. Assuming you are SURE the carb's are clean; If that engine has an adjustable low speed jet, try opening (loosen screw, counter-clockwise) it up a little, say 1/8 of a turn, and if that doesn't work try another 1/8 etc.. If the engine runs like hell, with no increase in performance, put them back. Next thing I would suspect is something to do with syncronization; like the throttle is not correctly line up with the approximate RPM range and ignition. Pretty much perform the tune up procedure, Timing and Syncronization. I can't help you with that, but a repair manual will have it. Also, take a look at the plugs, that may give you a clue of what's happening. If it's running lean at that RPM, chances are it's running funny at others at well, regardless of performance. Hope this helps,

Jon "

" I don't know anything ab

" I don't know anything about how Tohatsu motors are set up, but have you considered the timing advance might be hanging up a bit? I have no idea what rpm the advance goes to max advance (perhaps well below this point?), but I'd look at this to eliminate the possibility anyway. "