Low rpm wot 70hp

[jaredluke]

Hey guys my 70hp yamaha 2 stroke with 220 hours is only revving to approximately 3900/4000 rpm. I say approximately because the tachometer doesnt work and top speed at wot is 22 knots which would be around that rev range. The motor is mounted on a 15 foot centre console so should be seeing figures of around 33/34 knots. The motor starts fine idles fine and runs smooth. Pulled the plugs off one at a time while the motor was running and from what i could hear all cylinders are firing.

[TownsendsFJR1300]

Year, model #?

[jaredluke]

Year model is 1999

[99yam40]

sounds like carbs need to be cleaned.
what did the plugs look like?
check the fuel pump also

[TownsendsFJR1300]

Has the engine been sitting awhile (with old fuel in it) or used regularly?

Have you checked the fuel filter for crap/water.

Perhaps drain the carbs (drain screw) again looking for crap (catch what comes out)?

[jaredluke]

Hey scottvnot sure mate just purchased the boat. Ive gone through a full tank of fuel but will check what you have listed

@99yam just lut new plugs in. Didnt take much notice of the old plugs but will pull the new ones again soon and have a look. Ill check out the fuel pump

[TownsendsFJR1300]

Yep, prior "history" helps a bunch.

As 99Yam posted, carbs clog up pretty quick with old, un-stabilized fuel. Very possible their clogged.

The fuel pump diaphragm especially if original, does wear out, leak, etc.


The main jets specifically from what your describing, part #21:

http://www.boats.net/parts/search/Ya...TOR/parts.html

Should you clean the carbs, clean all the system, innerds of the carb, fuel filter, etc.

PULL all the jets, just a "spray of carb cleaner" don't count. If you don't see light thru them, they ain't clean...


And you really don't want to run it that way, possible lean, and burn up the engine...

A really bad case of old fuel and varnish:


.

[zenoahphobic]

Interesting that fuel residue has gone past the seal, must have leaked a lot of fuel well before this dismantle, So a problem was evident for some time.

[TownsendsFJR1300]

Interesting that fuel residue has gone past the seal, must have leaked a lot of fuel well before this dismantle, So a problem was evident for some time.

The varnish stayed inside the float bowl (carb), none visible on the outside.
The bolt (now next to the float) IS the MAIN JET and easily clogs with old ethanol.

The generator (10HP Tecumseh as I re-call) it came off of sat for an unknown, LONG time (customers machine).

After several "soaks" in the Sonic cleaner:

*Un-fortunately, after cleaning, the needle seat was found to be bad and was irreplaceable...






.

[rodbolt17]

a tip for cleaning that seat????
use fine valve lapping compound, a q-tip cut in half and a drill.
chuck 1/2 a Q-tip,dip in the compound and give her ell.
you would be amazed how many I have saved.

[TownsendsFJR1300]

a tip for cleaning that seat????
use fine valve lapping compound, a q-tip cut in half and a drill.
chuck 1/2 a Q-tip,dip in the compound and give her ell.
you would be amazed how many I have saved.

Thanks.

I remember that tip you posted awhile ago and have used it once (worked great, especially cleaning the part/sides where the needle resides in).

This carb was really old and the seat wasn't metal but plastic or something.

It literally disintegrated and didn't look anything like a seat...

[99yam40]

Thanks.

I remember that tip you posted awhile ago and have used it once (worked great, especially cleaning the part/sides where the needle resides in).

This carb was really old and the seat wasn't metal but plastic or something.

It literally disintegrated and didn't look anything like a seat...

I have seen some carbs with hard rubber(composite of some kind maybe) seats that you could replace.
trick was finding the carb kit that came with it

[TownsendsFJR1300]

I have seen some carbs with hard rubber(composite of some kind maybe) seats that you could replace.
trick was finding the carb kit that came with it

It was a customers "one of many", OLD, "garbage picked generators". He was very thrifty on the repairs allowed.

I did get most of his machines running, (2 generators already fixed and gone) the red one shown was the problem child:



Sorry for the small pic, uploaded from CL... My lap top with the original pic died...

[99yam40]

I was just letting you know about the ability to replace those seats if you can find them

[TownsendsFJR1300]

I was just letting you know about the ability to replace those seats if you can find them

Yep, I know, and thanks. Finding the correct one is the key. And even if I find one, replace it, I don't know if the engines still going to run or not (as their may very well be other issues) and it's been sitting for at least a decade in the weather..

10HP Briggs on another of his junk picked generators. Cleaned the carb, got it running but ran crappy, wouldn't go over 3600 RPM's and wouldn't start cold. Bad misfire too. Not starting cold leads to the carb (it was spotless), but not running at higher RPM's and NOT starting once hot and the misfire. Just not worth more time into it with a very limited budget.


I just bought a "junk" 10 HP Briggs, 5500, 8500 watt generator. Water in the tank and carb (with associated corrosion). I did a leak down test BEFORE anything, and it was down to 86% @90PSI (borderline tear down-rings leaking -into the crankcase).

Cleaned the carb (nasty), "sea-foamed" the combustion chamber (while doing the carb). Eventually got it running great. Re-checked the leak down, it was now up to 97/98%.
It's already sold and getting picked up 12-29-17 (by my lawn guy).


If it was mine (my time is free to me), I would have dug deeper but he wouldn't spend the money with no guarantee..