70 hp rod question

[TownsendsFJR1300]

I was not trying to say that the jet sizes were his problem, That thought just came to me for some reason and I posted it

No problem..

Good question thou. If running pre-mix(now), should jet sizes be slightly larger? I wouldn't think so, but...

[boscoe99]

being you pre mix and having a scored #2 piston/cylinder and a bearing go bad, I would think the carb was not dumping enough fuel and oil into that center cylinder and it overheated(running lean) along with lack of oil got bearing and cylinder /piston.

makes me wonder if the jets in a pre mix motor are a different size than an oil injected motor due to the specific gravity of the fuel changes when you mix oil into the gas.

I started wondering the same after reading your post. Here is an instance where the same carburetor is used on both a premix and oil injected model.

63D-14301-00-00 YAMAHA CARBURETOR ASY 1

Another instance:

6H3-14301-0A-00 YAMAHA CARBURETOR ASY 1

[sasko1980]

After using premix, the motor run fine.
The carbs were clean and the pilot screws were as in manual 1 1/4 turn out.
There was no jet size changes.
As taking a head apart , all cyl and pistons and bearings were lubed, oily from premix.
In my opinion the destroyed pin from rod bearing came in contact with the piston and left a cyl wall scratch.
When you look a piston top side it has a small pinched out holes, like it was done by a small screwdriver and a hammer.
Could this be from lean condition or too rich condition?
Again the carbs were taken down several times for cleaning because I found especially on this forum that it all about carbs. So in basic i cleaned the clean carbs.
Again if it was the fuel pump, then a lower cyl would make a blast from a too rich condition ; right ??

The only thing that was done before this problem I returned the red lever from middle carb to its original position (normal position). When i bought the motor the red lever was in open position, but even in open positon ran fine.

Hit me!

[TownsendsFJR1300]

After using premix, the motor run fine.
The carbs were clean and the pilot screws were as in manual 1 1/4 turn out.
There was no jet size changes.
As taking a head apart , all cyl and pistons and bearings were lubed, oily from premix.
In my opinion the destroyed pin from rod bearing came in contact with the piston and left a cyl wall scratch.
When you look a piston top side it has a small pinched out holes, like it was done by a small screwdriver and a hammer.

!

In BOLD ^^, are we talking about the wrist pin in the PISTON, or ? What bearings in the top end are you talking about or is the engine already tore down?

A broken piston ring could also cause that damage. We've seen pic's on this forum of the piston ring RETAINER PIN, loosening (bent upwards) and then the ring(s) rotated, caught a port and tore up a cylinder.

Can you post some pic's?

If torn down already, specifically of the piston ring retainer pin and the bore.

.

[99yam40]

If the motor ran good with red lever in the open position, either the cards had a problem ( running lean) and needed the extra fuel from the open position or the prime start diaphragm and/or passages had a problem and was not pumping the extra fuel it should have been while in the open position

[sasko1980]

In BOLD ^^, are we talking about the wrist pin in the PISTON, or ? What bearings in the top end are you talking about or is the engine already tore down?

I suppose that the destroyed rod bearing pin on the crankshaft did the mess.
I will try to post some pictures.

In the meantime I'm looking in another way.
What yamaha 70 BETOL 1995 have in common with a 60HP motor?
I found a used motor but on the stamp is 6h2-81, year1996, 44.1 kW, Mass 106kg.
there is no letters except 6h2-81.
Can this motor be identified ??
Some 60 have a same crankshaft as 70hp. I just need to be shure that is the same ,and for that I need a code/model number from that 60hp.

[TownsendsFJR1300]

Are the cases split yet and crank/rods/pistons yanked yet?

[panasonic]

Another continent, europe, adriatic sea, croatia.

I thought to buy a new crankshaft from yamaha in my country.

Then im left with a scorred cyl. Also im thinking of a new sleeve to be preesed in, or to rebore to fit a 0.040 piston inside. Then i have a upper and lower on 2.854 and middle 2.874.
Potential problem or not ???

Yes a potential problem in my opinion. The larger piston in the bored out cylinder will weight more than the other two and may cause balance problems. The Pistons should be all the same size.

So bore them all out OR resleeve the damaged cylinder.

[sasko1980]

Somewhere i read that yamaha oversize pistons are the same weight as a standard 72mm piston.Maybe even on thia forum. Right or not ??

Im also for inserting new sleeve.

What about 6h2-81 ddo you have any data about this motor in previous post??

Again thnx for helping me (all you forum guys)

[sasko1980]

I do machine work all the time and been in shops that do this, very precise work. The more cylinders on the crank, the more difficult it is to repair.

Panasonic - Do you have some extra spec (dimensions)about pressing a 70hp crankshaft, and how its done, what to take care of in process.
In service manual there are some, but must be more then that.
If you can please send, it will much help if i decide to press it up again in a shop.

TNX

[rodbolt17]

while I have never done it I used to watch the guys at the Kawasaki shop do it.
they had some impressive jigs and press fixtures.
I send mine to Carolina machine and tool.
formally Ackermann marine.
it is in Wilmington NC.
one oversize piston wont hurt anything.
I don't do it but have seen it done.
I have also watched Ron at Ron's marine in Portsmouth VA sleeve the cylinders.

[panasonic]

Panasonic - Do you have some extra spec (dimensions)about pressing a 70hp crankshaft, and how its done, what to take care of in process.
In service manual there are some, but must be more then that.
If you can please send, it will much help if i decide to press it up again in a shop.

TNX

No I am sorry I don't have any specific specs for rebuilding crankshafts. Like rodbolt I have only seen it done, mostly skidoo and motorcycle cranks. A lot of special tools, jigs and fixtures to do this kind of work. And most importantly a very good machinist to do the work.

I do know that in the UK they are big into old two stroke motorcycles and there are machine shops there that specialize in rebuilding cranks. Google can help you find a shop somewhere close I am sure.

[panasonic]

These guys might be able to point you in the right direction.

Motorcycle & Motorsport parts » Athena Aftermarket Division

[sasko1980]

one oversize piston wont hurt anything.
I don't do it but have seen it done.
I have also watched Ron at Ron's marine in Portsmouth VA sleeve the cylinders.

All 3 cyl are with 72.5mm (2.854) pistons.
The middle has to be rebored to piston 72.75mm (2.864).
Will it hurt anything??, I'm looking for stress on crankshaft ??
Yamaha has a 2 piston oversizes.(Std,1st,2nd) Others have more.
Can you point out a good brand for buying that one oversize piston(3rd or 4th) ??

tnx

[TownsendsFJR1300]

Wiseco has been been making pistons for decades. Might do a search there...