86 tri-z power valve questions

[Stephon_xr500]

I've been building an 86 tri-z with the hopes to MX race it here in AZ. I put a carb from a 82 yz250 on it, because it was carb-less when I got it. I had it running on Monday and was going to do a lap around the neighborhood and plug chop on the way back home. Well needless to say I didn't make it back home with a running trike, I had to push it home. I had pulled over to adjust the clutch cable and it started reving really high with out twisting the throttle, it wouldn't shut off with the switch or when I pulled of the plug wire, Then it backfired and shut off. Fuel mix was 50:1

so at this point I think I need to pull the top and apart to examine and see if I did internal damage. I have a 1982 YZ 250 motor that was rebuilt and never made its way back into the bike. It has been sitting for about ten years, so the seals may need to be gone through. This motor has the power valve set up on the jug. I feel if I'm going to have to do any major motor work to the tri-z motor I should upgrade it if I've got the parts available. What are the differences if any between these motors?

197654

197655

[C.J]

It's a direct bolt on swap. Just gotta drill out the power valve actuator plug on the Z clutch cover and then put the bearings and seal in

[christph]

Funny you should post this topic. I was just reading about the 82 YZ 250 the other day as I was entertaining the possibility of a power valve motor. After reading the write up on the motor (see link below) I don't know if you will be happy with the results. They say the motor has a good bottom end but flattens out on top, although I'm sure it could be made better. Besides, the stock Tri-Z setup can be improved considerably. I've got a Sprock cool head (with a race dome), V-Force reeds, reed spacer, 38mm carb, K&N air filter, a DG pipe (I want to get a Jemco), and an additional boost port (see the Yamaha Wrench Report). I'm really happy with mine, although I'm a gear head so I'm always entertaining other possibilities, i.e., the power valve.

http://www.pulpmx.com/stories/look-back-old-moto-mags/gps-classic-steel/gp%E2%80%99s-classic-steel-12-1982-yamaha-yz250

[C.J]

I'm in (was in?) the process of building a PV Z with an 87 YZ top end. Same thing just a bigger intake that's angled. I figure after getting the "shifter kart" (POS) porting cleaned up, 38mm Mikuni, Cobra or Phase-5 pipe (I have both), and getting the gearing and powervalve set just right, it should be a bomb trail rig. But that's pushed way back now after the fire so y'all lemme know if/when you get your set ups done

[86T3]

I dont know anything about trizs but i was paying attention last weekend to a conversation between Ed Bergemen and Jason Hall. They said that you cant put a gravity fed carb onto a bike with a fuel pump because blahblahblah is diferent and the carb wont work. To find out what blahblahblah is you'll have to get ahold of one of these guys. Sorry i cant help any more.

[Stephon_xr500]

Should I give up on the yz carb? I have a stock carb now but thought since I had the bigger carb available I should use it.

[MTS]

You need to change the return line "T" to a brass one with a larger hole to run the gravity fed carb, the backfire was hopefully the mag key shearing.

[DohcBikes]

I dont know anything about trizs but i was paying attention last weekend to a conversation between Ed Bergemen and Jason Hall. They said that you cant put a gravity fed carb onto a bike with a fuel pump because blahblahblah.Who can't?...yes blahhhhhblah.

[El Camexican]

My issue was resolved when I found the main jet someone had jammed into the return line:rolleyes: but I picked this up anyway http://shop.3wheelerworld.com/productdetails.php?productid=108 I assume it can be drilled out if need be. Lots of people on here are running gravity feed carbs and there are a few threads on here about this topic as well as alternative carbs that are better suited to a pressurized system.

[Stephon_xr500]

I've already changed the "T" fitting to one that allows more flow, it was kind of by accident. When I first got the trike I broke the "T" fitting while cleaning out the lines, the local auto parts store had a universal vacuum line "T" I just had to cut down the length but the hole the fuel flows through are bigger.

[C.J]

You can change out the needle/seat assembly to run the GF carb on a FP system. That's (just about) the only difference

[christph]

I put a Keihin 38 mm PWK on my Tri-Z with the fuel pump with no problem, and I'm pretty sure it was designed with a gravity fed setup in mind.

[El Camexican]

You can change out the needle/seat assembly to run the GF carb on a FP system. That's (just about) the only difference

That would be on a Mikuni right? That option is not available on a gravity fed Keihin, the orifice is cast, or drilled into the carb. Some guys (ZMpolaristech) use carbs off sleds to get the pressurized option. I called Sudco a couple years back about this and they said that those carbs were not available at the time, but that with a 200 unit MOQ they would make them for like $180 bucks each if I recall correctly.

[Stephon_xr500]

I must have knocked the vacuum line off the cylinder when I adjusted the clutch cable.



This line was off when I started looking at it today. It runs again but still feels very flat in the top end.

[adamszwheeler]

Doesnt the wrench report say to use a 38mm off an early yz? Doesnt mention anything about modding the carb/ fuel set up other than what jets to use. The back fire may have been from rich/ fouled plug. Was having back fire issues tuning my 36mm vm on my stage 1, always super rich when it happened.

[El Camexican]

I must have knocked the vacuum line off the cylinder when I adjusted the clutch cable.



This line was off when I started looking at it today. It runs again but still feels very flat in the top end.

DUDE! That hose will come off again as soon as the temperature drops a few degrees. Get a new hose and leave some slack in it.

[Stephon_xr500]

I think the backfire may have been from the carb "running out of gas" since the pump had no vacuum.

[Stephon_xr500]

New hose is on the list of parts I need for it, along with a clutch cable

[Stephon_xr500]

So I swapped the 320 main jet for a 420 main jet. It feels alittle better but still feels like it Boggs down after 1/4 throttle twist I'm gonna have to go bigger still

[Stephon_xr500]

I managed to get the clutch cable to work for a lap around my neighborhood (I think I'm missing a clip that holds it on to the "U" fitting on the motor case)

So I plug chopped; I got it nice and warmed up, got it into 6th wide open Throttle, clutch killswitch coast to a stop.


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