'86 Tri-z carb swap

[Stephon_xr500]

I picked up a '86 Tri-z 250 over the weekend that was carbless. A quick phone had my dad dusting off a box with a complete motor carb and radiator from a '82 yz250 rebuilt about 10 years ago but never made it on to the bike again..... back to my tri-z.

The motor seems to be in GREAT shape, I would like to use the carb from the yz. It's bigger than the tri-z carb because it won't fit with out some stretching but seems possible. It looks like the vacuum lines for the fuel pump are all intact, does the fuel pump get its vacuum from the crank case to the left of the jug?



My ultimate goal is to get it ready to motocross race. I've been racing a little bit here and there on my '85 350x but I think the tri-z might be set up a little better for motocross and at $125 I couldn't pass it up.

The Tri-z needs wheels too but my buddy is loaning me a pair of spare blaster wheels.

[Rider414]

I have heard that a carb set up for a fuel pump is different than a carb set up for gravity feed (your YZ carb).

[YTZ drew]

You will need to change the float needle and seat to one meant for a fuel pump, usually gravity feed carbs have a 3.3mm inlet and the ones for fuel pumps have a 1.5mm inlet. I'm not sure what the stock Tri-Z size is, but it might show on a parts microfiche.

[El Camexican]

It looks like the vacuum lines for the fuel pump are all intact, does the fuel pump get its vacuum from the crank case to the left of the jug?

Right side of the JUG on the Tri-Z cylinder is where the pulse that powers the fuel pump is sourced from an elbow tube. If the YZ cylinder doesn't have one you can add it. The nipple on the LEFT side of the cases is for crankcase ventilation and vents into the atmosphere.

[Stephon_xr500]

I was under the impression that the fuel pump ventilation came from the carb itself. Sounds like the fuel pump should be intact on my tri-z!! I traced the system around with my fingers the only line missing is the main fuel line from the pump to the carb.

So now I just need to figure out how to limit fuel flow?

The yz carb is definitely bigger I think it might be a 36mm.

[El Camexican]

Try starting with a Tee fitting from the 3WW store with the large return line fitting. That alone might take care of everything.

[Stephon_xr500]

I fit the carb in place today after cleaning it out. The main get is a 320 I hope that's not to big. I had a chance to check the motor over has great compression and plenty of spark! I drained all th fluids out and should be able to get it all filled up tomorrow. I figure once I get it running I can worry about getting it dialed in. What are the warning signs I should be looking for as far as to much fuel? Over flowing while running? High idle?

[YTZ drew]

If the fuel pump is overpowering the float and needle, the carb will overflow. You'll know it for sure if its too much. As far as the jet goes, it will probably be in the ballpark, if not a little small. A smaller engine breathing thru a bigger carb pulls less of a vacuum, and usually needs bigger jets.

[Stephon_xr500]

The carb came off a 82 yz 250 it's the same size motor. I'll have to order some jets... I've been wanting to play around with the jetting on my 85 350x too!

[Stephon_xr500]

here are sointpics of the mounted yz carb and a few other pics of the tri-z getting cleaned up










has any one else cracked that "T" connector in the fuel line?

[badass350x]

must be a mikuni carb correct?

[badass350x]

here are sointpics of the mounted yz carb and a few other pics of the tri-z getting cleaned up








has any one else cracked that "T" connector in the fuel line?

As said above replace it with the aluminum one from Sblt500's replacement T fitting!! I would pass on the JB weld mess...

[Stephon_xr500]

Yes its a mikuni, I'll be ordering the t fitting when I get a chance. I was just trying to see if I could get it running first.

[Stephon_xr500]

I picked up material to recover the seat today, I couldn't find any red so I just got black. I'll try and work on the seat this evening

[Stephon_xr500]

I got the seat recovered today... I should have waited to find red material I think.