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YPVS question
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Author:  graham [ Sat Jun 18, 2011 1:14 am ]
Post subject:  YPVS question
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
Author:  Ja-Moo [ Sat Jun 18, 2011 11:42 am ]
Post subject:  Re: YPVS question
big o flywheel??????
Author:  graham [ Mon Jun 20, 2011 10:02 am ]
Post subject:  Re: YPVS question
some thoughts....... :problem:

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
Author:  evansje1 [ Mon Jun 20, 2011 4:25 pm ]
Post subject:  Re: YPVS question
What the heck is a powerjet carb? I don't think my RZ has them.
Author:  Evans Ward [ Mon Jun 20, 2011 4:28 pm ]
Post subject:  Re: YPVS question
evansje1 wrote:
What the heck is a powerjet carb? I don't think my RZ has them.


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!
Author:  evansje1 [ Mon Jun 20, 2011 4:46 pm ]
Post subject:  Re: YPVS question
Evans Ward wrote:
evansje1 wrote:
What the heck is a powerjet carb? I don't think my RZ has them.


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!


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.
Author:  Evans Ward [ Mon Jun 20, 2011 4:58 pm ]
Post subject:  Re: YPVS question
evansje1 wrote:
Evans Ward wrote:
evansje1 wrote:
What the heck is a powerjet carb? I don't think my RZ has them.


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!


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.


That one, I do not know...... We'll both get an educumacation on this one!!! :geek:
Author:  evansje1 [ Mon Jun 20, 2011 5:10 pm ]
Post subject:  Re: YPVS question
Evans Ward wrote:
evansje1 wrote:
Evans Ward wrote:
evansje1 wrote:
What the heck is a powerjet carb? I don't think my RZ has them.


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!


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.


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


Copy that have a good one.
Author:  graham [ Mon Jun 20, 2011 6:21 pm ]
Post subject:  Re: YPVS question
This is a powerjet RZ carb
Image

And this is where the jet itself is located
Image

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
Author:  evansje1 [ Mon Jun 20, 2011 6:56 pm ]
Post subject:  Re: YPVS question
Thank you graham, very good description of the power jet circuit, even I understand your explanation. :clap:
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