Fuel pump routing - Open Tri-Z build

[starman11]

Building an Open Class Tri-Z motor, and have searched this forum all weekend. I cannot find the in/out labels on the stock fuel pump. I don't have one (tossed it) and I'm using a Mikuni dual-feed pump.

There should be arrows or markings on the stock pump indicating what goes where, if anyone has them I would be most grateful and will post a clear diagram so aftermarket pumps can be used if/when the stock ones become unavailable.

I've set up many fuel pumps, but the dual feed VM32 carb is killing me. I have vacuum pulse off the reed cage (Boysen), need to know how the fuel feed for the secondary powerjet is setup.

I think it's a pumparound back to the tank but I'm not sure. All the pics I've seen have double T fittings going off to places that don't exist on this motor.

I have thought about doing a common rail system to both feeds, but I have concerns about flooding through the powerjet.

The next step will be a methanol conversion and I'll post that too once I get the jetting right. If I can find the nitro conversion numbers I'll post them too, but 2 strokes & nitro are not really friends.

Thanks in Advance,
Tom

[250sx]

let me know if you still need this info and I will go take a pic of mine.

[starman11]

It would help a lot. The primary fuel feed I have working well with a pumparound system. I believe it was originally designed as a gravity feed.

My befuddlement revolves around the second fuel feed. It looks like it was designed as a midrange/ top end boost to prevent the engine from going lean at top end. It exits right at the mainjet, bypassing the float bowl needle completely.

What I don't know is where does the line go on the stock setup. I suspect there is a vacuum that holds it closed until intake velocity exceeds X, and then it provides fuel to that circuit. That could require some major engineering. :(

If that's the case I may have to got to another carb like the Mikuni SBN diaphragm off a jet ski that has a tunable midrange circuit and has a built in pump.

Right now it's breaking up on the high end, and I've swapped the coil and rechecked all the wiring with no net change. It's also running hot - around 160+ deg. water temp. Plug looks OK, brownish.

Tomorrows project is to verify I have no air leaks, but since it idles very civilized and runs up with the wheels in the air, I doubt that.

It only acts up under load, any gear. All I can say is :wtf: at this point.

Thanks for your help.

[250sx]

i'm a little confused now. do you need to know about the fuel line setup for your carb or the stock tri-z fuel pump? I can help with the pump but have no clue about the carb.

[starman11]

If you could tell me where the fuel lines run to & from - there are 2 on the 85 - especially the one towards the rear of the carb - I think that'll do it. We are very close now, rejetted the low side a tic and it's stronger than ever.

BTW, what redline on these motors?:2finger:

[84 200x man]

are you a honda man

[starman11]

No, I'm a 4 stroke Open guy who wants to play with 2 strokes now.

I abandoned all hope of figuring out what Yamaha was thinking when they went to that design. The boost box doesn't work as advertised and the rest makes no sense, so I plugged up the secondary feed, jumped the needle 2 notches and keep going up on the main until it comes in. Almost there, at a 400 main now. Fuel pump is a Walbro single feed now, about 20 LPH, the 36 LPH Mikuni was way too much for the needle & seat.

Just came back from running it for an hour in100+ degree heat and all seems OK, just a tic hot for my taste. Bigger radiator will go on tonight, see if that's where I'm lacking.

The motor is a beast compared to the CR & YZ 250's I've played with. Much better built, better port layout and the gearbox is really stout.:thumbsup:

[250sx]

:worthless: of your trike. :D

[Senator]

I abandoned all hope of figuring out what Yamaha was thinking when they went to that design.

I know how you feel dude :doh: