Kawi2strokes.com Forum

Learned Gentlemen...... I have been working on yamaha twins since the late 60's and rebuild cranks to give me some toy money in my semi-retirement see: http://crankshaftrebuildingservices.com.au/
I have rebuilt 30 or so RZ 250/350 cranks over the last 20 years, and have just picked up 5 more cranks from a wrecker to rebuild and use as core exchange stock.

In all cases, except when caused by piston failure..... the left hand big end is the faulty one.

Why is this so?

I have a couple of thoughts, but what do you think?

On the air cooled models, and even on the LC (pre YVPS) this is not as obvious, although I would say that it is still slightly in favour of the L/H being faulty.

Graham

big o flywheel??????

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Twist the throttle, tilt the horizon, and have a great time. What triples are all about...........

some thoughts.......

With regard to the L/H cylinder……. Well I think is has to do with the crank seal behind the Ignition hardening and letting a small air leak happen….. But this is basically identical to the LC’s so why not the same with them?

Well the only other difference (besides the YPVS) is the powerjet carbs….. I thought for a while that cruising just below the Power jet functioning may have been giving a lean running condition and the additional air leak from the seal causing the l/h to run hotter/leaner etc and this may be so, but then when asking a couple of customers what their riding was like just before it happened…..it wasn’t cruising!!!

So I had him bring his carbs in, just out of curiosity and checked out the power jet circuit. Now on the retro fitted powerjet carbs that I had worked on in the past the Power jet is at the end of the fuel supply tube, high up on the back of the carb….. But on the RZ it is located at the bottom of the fuel supply tube, in the bottom (side) of the float bowl…..and it was restricted by gum and muck in the bottom of the float bowl.

So possibly a partially working power jet, combined with a slight air leak is causing this problem…….Maybe? But check those power jets; they are just waiting to be blocked.

Graham

What the heck is a powerjet carb? I don't think my RZ has them.

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1973 RD350, 1979 RD400F, 1984 RZ350, 2014 Suzuki Vstrom 1000
CHRIS AND JEFF ARE THE MEN

Canadian RZ carbs JE. They also got individual carbs with regular pulls unlike the Mickey Mouse contraption we got in the US with the carb bank, linkages, and push/ pull cables!

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1972 Kawasaki H2 Mach IV (Candytone Blue), 1976 Suzuki GT750 (Maui Blue)
1984 Yamaha RZ350 (KRR- Yellow/Black).

“Jeff and Chris are THE MEN!”
RIP John E. Evans

OK EW, then what is the power jet he is talking about. I've seen these mentioned before but was too shy to ask. Now that I'm a RZ expert I've got the nerve to ask.

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1973 RD350, 1979 RD400F, 1984 RZ350, 2014 Suzuki Vstrom 1000
CHRIS AND JEFF ARE THE MEN

That one, I do not know...... We'll both get an educumacation on this one!!!

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1972 Kawasaki H2 Mach IV (Candytone Blue), 1976 Suzuki GT750 (Maui Blue)
1984 Yamaha RZ350 (KRR- Yellow/Black).

“Jeff and Chris are THE MEN!”
RIP John E. Evans

Copy that have a good one.

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1973 RD350, 1979 RD400F, 1984 RZ350, 2014 Suzuki Vstrom 1000
CHRIS AND JEFF ARE THE MEN

This is a powerjet RZ carb


And this is where the jet itself is located


The basic principle of operation is:

The remainder of the carb is conventional, and the pilot, cutaway, needle & needle jet and main jet function as normal.
There is a tube from the float bowl to an area where vacuum will exist when the throttle is wide open. In this tube there is a jet, about the size of the pilot jet in the carb, that meters an extra amount of fuel in addition to the main jet at WOT.
This allows you to use a slightly leaner than normal main jet so that the transition (when excellerating) from needle to main mixture is not overly rich, but the correct amount of fuel is delivered for long periods of WOT.

When these first were developed circa 1972? I was racing a TD3 powered superkart and fitted these as an aftermarket kit that was available at the time (they are still available about $30 a carb).

We found that by adjusting the length/diameter of the hose we could delay the time it took for the power jet to become operational (by a second or two) this gave us an advantage (in a kart) at places like entry to a long sweeper, where the slightly leaner mixture would give a crisper response and better excelleration ....enough to pull alongside a compeditor for the run through the sweeper.....and just before detonation would set in, the powerjet richened it up......

Ahhhhh memories....... I won't tell about the times I got it wrong though.....

Graham

Thank you graham, very good description of the power jet circuit, even I understand your explanation.

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1973 RD350, 1979 RD400F, 1984 RZ350, 2014 Suzuki Vstrom 1000
CHRIS AND JEFF ARE THE MEN