2 Stroke Carb Jet Cleaning

[TownsendsFJR1300]

The only empty holes I can think of on a crank would be where they drilled them to balance the crank

Exactly.

Back in the day, besides porting, etc, you stuffed the balance holes with a cork and epoxy. The idea was to lessen the volume in the crankcase and force that much more of the fuel mix back to the top end (instead of remaining in that small space).

This was in the late 70's, was very common. I don't remember how well, if it worked back then...

[99yam40]

hard to believe it would stay in there at high RPMs.
seems it would throw out the balance a little , the cork and epoxy had some weight

[TownsendsFJR1300]

hard to believe it would stay in there at high RPMs.
seems it would throw out the balance a little , the cork and epoxy had some weight

That was done on 1975-1977 Yamaha YZ 125's and 250cc's which are dirt racing bikes. Did that and rode them thru many AMA sanctioned moto-cross races. I don't know what the top RPM was of the engine (no gauges) but at a minimum 8,000 RPM's for 20 minutes. Roller needle bearings on the lower connecting rod to crank. Blew up a lower bearing in a race (major knocking and shut down before destroying anything) but the corks NEVER came out. There were 3 of us racing, all cranks "stuffed", never an issue.

Back then, they were coating the cylinders with a chrome like substance(forgot the name of it), but no steel sleeve, no boring.

[Jason2tpa]

Here's my prop info:

SS Michigan Wheel Apollo XHS
993146-13x21

What's your thoughts for my S115TLRW using this prop. Thks guys!

[TownsendsFJR1300]

Not me, I'd get the prop professional's insight as that's what he does....

[Jason2tpa]

Did you look at the Yamaha performance bulletins to find a rig like yours to see what prop they used?

Where can I find the Yam performance bulletins?

[TownsendsFJR1300]

Where can I find the Yam performance bulletins?

Outboard Performance, Performance Bulletins | Yamaha Outboards

[Jason2tpa]

Alright so after doing some research on the 21" prop pitch currently on the motor it looks like this pitch is the highest amount of pitch recommended. Low end would be 15. So yah, looks like this motor is overpropped.

How do y'all feel about using the standard pitch inch/rpm scale? For every 1" you decrease pitch you increase rpm 150-200.

So if I'm getting 4500 at 21" I could decrease prop pitch to either a 15" or 17". Even using the high end of 1"/200rpm and installing a 15" theoretically puts my rpms at 5500. Couple questions for ya:

--By decreasing prop pitch you increase hole shot BUT decrease top end speed. Let's assume installing a 15" gets my rpms to max of 5500. Can I expect the top end speed I'm currently experiencing to DECREASE?

--You guys spoke of a yamaha test wheel. I'm assuming this is a loaner prop a yamaha dealership offers? Can y'all give me a few details how to go about getting a test wheel?

Big thanks as always!

[Jason2tpa]

I second the test wheel test.
running overpropped can and will lead to wristpin failure.

use test wheel YB1611 for the 95-05 40 and the 98-02 c40.
should turn yb1611 to 4900-5100 rpm.
if it wont the engine still has issues.
if it will the rigging or prop is incorrect.

Rod-
Can you recommend the test wheel for my model, '98 S115TLRW?
Thks mate

[rodbolt17]

test wheel for the s115tlrw is yb1624, it should turn 4900-5100.

[99yam40]

Just to clarify, the 21" and 15", etc, is NOT the pitch, its the diameter of the prop.

Pitch would be the angle of the prop blades

the prop he has now is a 13 X 21
the diameter is 13" and the pitch is 21"
pitch is given in " not degrees

Pitch is defined as "the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood." For example, a 21-pitch propeller would move forward 21 inches in one revolution.

[TownsendsFJR1300]

Pitch is defined as "the distance a propeller would move in one revolution if it were moving through a soft solid, like a screw through wood." For example, a 21-pitch propeller would move forward 21 inches in one revolution.

Fixed, my boo

[99yam40]

OP did you find a boat and motor set up like yours in the Yamaha Performance bulletins?
what prop did they use and what RPMs & speed did they get at WOT?

[Jason2tpa]

test wheel for the s115tlrw is yb1624, it should turn 4900-5100.

Thanks bud!

[Jason2tpa]

OP did you find a boat and motor set up like yours in the Yamaha Performance bulletins?
what prop did they use and what RPMs & speed did they get at WOT?

I did. The recommended props range in pitch from 13-15 at max 5500 rpms. No speed is listed. Overall, the pitch range of props for my engine is 15-21". So again, theoretically, the max rpms of 4500 im currently producing with the 21" match up. Highest recommended pitch resulting in the low end of rpm range.