RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

petvetdoc

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Picked up a 1985 Yamaha 70 HP ETLK. It has been sitting, I think, for 19 years! took carbs apart, cleaned carbs, float bowls and needle valves, blew out with compressed air. Cleaned fuel filter. Replaced water pump. Fuel line, for testing now, is hooked directly to barb on fuel filter, bypassing quick connect fitting. Backed boat in lake on trailer. In forward gear, am only running 2100 RPM at WOT (still on trailer, boat is rotten). If I advance the warm-up lever, can get about 2800 RPM.
I'm thinking one or more partially clogged main jets? How to narrow down which carbs? I didn't remove main jets when I had carbs apart, just shot carb cleaner up them followed by blowing out with compressed air.
Should I remove carbs, take main jets out and re-clean everything? I'm leery of running a cylinder lean while trying to let it clean itself with carb cleaner/ Seafoam /etc in fuel.
 

99yam40

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

Yes take apart and clean properly along with pump,lines, and tank. and then adjust like the service manual calls for
Old fuel residue will be a problem in all of them.
If you want to run the motor like that you need a Yamaha test prop for doing it.
without the boat moving it is overloading the motor
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

without the boat moving it is overloading the motor[/QUOTE]

Is it possible that it is just the overloaded condition with the boat stationary that is keeping RPM down at 2100? It runs smooth as a top, and accelerates nearly perfectly, (occasional slight hesitation with rapid throttle opening). I cleaned the carbs thoroughly, with the exception of removing the main jets. I shot cleaner and air through them multiple times, just didn't take them out. As you know, the more times you take something apart, the greater the chances of tearing seals, breaking something, etc.
If it were running normally, should advancing the rapid warm up lever increase RPM in a warmed up engine? Does it advance timing or is it just throttle/choke?

Not trying to argue the point. I'll take them back apart if I should, but that is the big question if running on the trailer like that is enough to limit WOT RPM.
 

emoney

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

My vote wouldn't be for that, but I guess anything is possible. I've yet to see one that wouldn't go WOT on the trailer, even though it's not recommended, of course. Are you sure you don't have a throttle cable issue? If warm up lever is getting beyond WOT, sounds like it. Have you tried to see what happens if you skip the controls and advance the throttle at the motor? The warm-up lever should just be a mechanical limiter on throttle, as opposed to a choke...that's separate.
 

Capt Ken

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

Both levers work the same throttle cable and the warm up lever should not be able to be lifted with throttle handle moved out of neutral. So something is wrong with the box. Go ahead and buy three carb kits and rebuild the carbs. That thing is 28 years old and could probably use it.
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

I've yet to see one that wouldn't go WOT on the trailer, even though it's not recommended, of course.

That is the real issue, then. I'll tear the carbs down again. I was opening the throttle at the engine not at the control box as the cables are shot.

Thanks for the replies,I'm trying to make sure I have the engine right before I install it on another boat. That way, if there is a problem after the install, I'll know where to look for my problem.
 

99yam40

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

If you are moving throttle at the motor make sure you are moving all of the linkage.
Not sure about your motor but myC40 has a wiper on CDI that has to move with throttle plates to advance the timing.
if it does not advance timing it will not have any power
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

I redid the carbs and tested the motor on the trailer. It runs perfectly smooth, but maxes out at 3100 RPM. I think it is the load on the motor limiting RPM in gear on a stationary boat. I'm going to assume it is OK, switch it over to a new boat with new control cables and go for a sea trial! If I am wrong, I did a lot of rigging for a motor I don't have the bugs out of yet!
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

for anyone who faces this question in the future. being strapped to the trailer is what was keeping RPM down. Once out on the water WOT is 5400 RPM
 

99yam40

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

Yes take apart and clean properly along with pump,lines, and tank. and then adjust like the service manual calls for
Old fuel residue will be a problem in all of them.
If you want to run the motor like that you need a Yamaha test prop for doing it.
without the boat moving it is overloading the motor

So you did not believe what I said before.

The test prop for your motor would have showed the motor was OK while strapped to trailer
 

Silvertip

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

You cannot lock the brakes on an automatic transmission car, put the car in gear and floor the gas pedal and expect the engine to go to red line. It's because the engine has reached the "stall" rpm of the torque converter (or in other words, you've loaded the engine)' The same thing occurs with a boat. You cannot expect it to reach WOT when tied to the dock or sitting on the trailer. If it did, you would have a very ineffective prop (or no prop at all).
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

So you did not believe what I said before.

The test prop for your motor would have showed the motor was OK while strapped to trailer

No, I did believe you before. You were spot on! I was too cheap to buy a test prop! I wanted to post the final result so anyone in the future, reading this thread, won't be misled by lower RPM's while boat is hooked to a trailer and will know it is due to the overload condition.
 

99yam40

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

Usually you do not buy the test prop, you just take it to a Yamaha shop and have them test the motor with the test prop or see if they will loan it to you. Depend on how well they know you and how much you have spent there in the past.
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

Ok, gotcha. You taught me something there. Didn't think of a loaner plan. I plan to buy a stainless prop anyway from someone. Next time I'll know.
 

99yam40

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

The prop i was talking about is a diagnostic tool that the dealers have to make sure motor can produce the proper Hp to produce the correct rpm while boat/motor is in a tank or just on a trailer
 

petvetdoc

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Re: RPM @ WOT on 1985 Yamaha 70 HP on trailer in gear? Carb issue?

Yes, I have seen them. I hadn't thought of the loaner plan idea.
 
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