Tri-z PowerValve

[cr480r]

Years back I had a Suzuki LT250 that had similar symptoms. RM cylinder, RM single ring piston and light pin, head cut 215psi, 'early style reed cage(big) w/ medium carbon techs, PT rev pipe, 39 PWK, clamp-on, lightened flywheel..... Bike started and ran flawless at part throttle, but once on main circuit it ran flat and sounded like lean bog. Tried jets from 150-200 with little change.. I messed with floats, petcock mods, vent etc.. No change. I finally swapped the complete ignition system from a friends bike.. Instantly had a rich condition.. Started dropping jets till it cleaned up.. 168-170 I think.. Bike was now fast, had fantastic mid-top power that didn't end.... until it broke a rod a few rides later.. It had been losing spark when it came on the pipe, and it got worse as I added more powerful components .. Caused me fits. I isolated the ignition problem down to the stator, even though it still tested OK by the book. Every since then I always suspect ignition trouble when a bike is unresponsive to jet changes.. High compression will show you a weak ignition everytime. .018 plug gap can help. I know lower compression can give gains up top, but high compression bikes will rev just fine when everything is right.. And pull better "off pipe"

[NeverLift]

onformula1,
Tyler did a fantastic job great guy to deal with. I love the swingarm. For the head i wanted it to be pump gas 180 PSI but that didn't happen. I would like to have it turned in a lathe to lower the compression. What should the clearance be between the head and piston to bring me back to the 180psi range? I did run with the stock head and still had this problem. The only difference i noticed with the Sprock head is that it pulls much harder down low now. Carbon tech reeds are on the shopping list i knew before i posted that you were going to recommend them. Now with this Intake setup i'll have to get a top mount from John.

Last night I eliminated the long intake tube and came up with this. Hopefully it works better, it was well after dark when i finished. I tried to make the airbox work but with the different carb angle it's a no go at least with what i had on hand. Next time out i will adjust my timing in 1mm increments as you suggested.

nstyle73,
I am running 110 in her so i don't believe it's detonating. I have ran the stock head and sprock head with the same issue. I think i bought these triples from you.

cr480r,
Sounds like You had my exact problem. I am going to test these other suggestions this weekend. If nothing comes of it i will source some "new" used ignition components. Everything i have is within spec of my factory service manual.

Thanks for all the advise guys, I've got some work ahead of me this weekend. Hopefully the wife doesn't bombard me with honey do's.

[yaegerb]

onformula1,
For the head i wanted it to be pump gas 180 PSI but that didn't happen. I would like to have it turned in a lathe to lower the compression. What should the clearance be between the head and piston to bring me back to the 180psi range?

Need to figure out with the volume of your current head is. My guess is its 18CC or below...possibly even 15. I agree with Milner, those are some high numbers you are playing with. I see you are using 110....dunno if that's high enough for the comp's you are running. You need to perform a UCCR to gauge your octane rating. I can tell you my 250R was running like crap about 2 years ago and I performed the UCCR and found my uncorrected compression ratio to be 16:1 and should have been using 116 or higher octane...food for thought. My comp at the time was 218. My head ended up being 15cc's. That was a time bomb waiting to go off. BKM gave me an 89 TRX head to try and solved all my problems. The 89TRX head was stock at 23cc's.

To drop that comp back down you need to get the bowl re chambered, I am only guessing here, but 23-25 CC's should be sufficient.

[El Camexican]

Perhaps its as simple as adding a second base gasket? Raise the ports and lower the compression all at once, should equate to more top end RPM, no?

[yaegerb]

Perhaps its as simple as adding a second base gasket? Raise the ports and lower the compression all at once, should equate to more top end RPM, no?

That will definitely help...that's an oldschool fix. It will come on the pipe in the upper RPM's since you are raising the ports though. I like my power low-mid. All depends on preference.

[NeverLift]

Need to figure out with the volume of your current head is. My guess is its 18CC or below...possibly even 15. I agree with Milner, those are some high numbers you are playing with. I see you are using 110....dunno if that's high enough for the comp's you are running. You need to perform a UCCR to gauge your octane rating. I can tell you my 250R was running like crap about 2 years ago and I performed the UCCR and found my uncorrected compression ratio to be 16:1 and should have been using 116 or higher octane...food for thought. My comp at the time was 218. My head ended up being 15cc's. That was a time bomb waiting to go off. BKM gave me an 89 TRX head to try and solved all my problems. The 89TRX head was stock at 23cc's.

To drop that comp back down you need to get the bowl re chambered, I am only guessing here, but 23-25 CC's should be sufficient.

My dome was engraved with the CC on it but i can't remember what it says. i'll pull it off and take a look.

[NeverLift]

Perhaps its as simple as adding a second base gasket? Raise the ports and lower the compression all at once, should equate to more top end RPM, no?

I seem to remember that someone sent you a thick base gasket for your engine build. If you didn't use it maybe you'd be willing to part with it? I'm a little weary of using 2 base gaskets might be inviting a vacuum leak.

[yaegerb]

My dome was engraved with the CC on it but i can't remember what it says. i'll pull it off and take a look.

If you can give me the bore, stroke and CC numbers off that head I can do a quick and dirty comp ratio for you.

This will be "ball park" only because it won't take into consideration any gasket thickness or decking on the cylinder.

are you still running this crank and bore and is that a stock stroke crank? If so, then all I need are the CC's off the head.

85 YZ250 crank shaft Hot Rods connecting rod & bearings
Wiseco YZ250 piston 68.5mm

[onformula1]

Cometic gaskets will make you any thickness you want.

Sent from my SM-G386T1 using Tapatalk

[El Camexican]

I seem to remember that someone sent you a thick base gasket for your engine build. If you didn't use it maybe you'd be willing to part with it? I'm a little weary of using 2 base gaskets might be inviting a vacuum leak.

Yes, that was courtesy of the esteemed Mr. 3 Wheel Drive, but it measured out to be the same thickness as the ones Versa puts in their kits. I would not be scared at all to lightly coat two with Yamabond and see what happens. At the very least you'll be able to determine if it revs higher and then you can go back to one gasket and get the head modified.

Speaking of nice guys, would you believe I'm wearing a Barrett Jackson shirt today?

[NeverLift]

If you can give me the bore, stroke and CC numbers off that head I can do a quick and dirty comp ratio for you.

This will be "ball park" only because it won't take into consideration any gasket thickness or decking on the cylinder.

are you still running this crank and bore and is that a stock stroke crank? If so, then all I need are the CC's off the head.

85 YZ250 crank shaft Hot Rods connecting rod & bearings
Wiseco YZ250 piston 68.5mm

Bore is 68.5mm Stroke is 68mm it's an o-ring head so there is no gasket volume to calculate. I can hopefully pull the head tonight.

[yaegerb]

Bore is 68.5mm Stroke is 68mm it's an o-ring head so there is no gasket volume to calculate. I can hopefully pull the head tonight.

True, but I assume you are running a base gasket which effects squish, plus do you know if the base or head surface has been decked on the cylinder? That's another factor.

[NeverLift]

Yes, that was courtesy of the esteemed Mr. 3 Wheel Drive, but it measured out to be the same thickness as the ones Versa puts in their kits. I would not be scared at all to lightly coat two with Yamabond and see what happens. At the very least you'll be able to determine if it revs higher and then you can go back to one gasket and get the head modified.

Speaking of nice guys, would you believe I'm wearing a Barrett Jackson shirt today?

I do have a spare base gasket. I figured you'd look good in red.

[NeverLift]

True, but I assume you are running a base gasket which effects squish, plus do you know if the base or head surface has been decked on the cylinder? That's another factor.

I bought the complete YZ motor and it looked like i was the first one to break it open. No silicone covering the cases, no stripped bolts and stock bore.

[cr480r]

I did run with the stock head and still had this problem.

^^I think a few missed this.

110 octane is not an issue for 15-16:1 UCCR on a two stroke.. 100 usually works from my experience.. Powervalved bikes can also trap a higher PSI for a given UCCR.. My kx250 trapped 245 with a thin base & stock dome lightly skimmed for .040 clearance.. They are around 220 stock. Cranking pressure only tells half the story unless comparing engines with fixed ports of similar duration.

You need to perform a UCCR to gauge your octane ratio. I can tell you my 250R was running like crap about 2 years ago and I performed the UCCR and found my uncorrected compression ratio to be 16:1 and should have been using 116 or higher octane...food for thought. My comp at the time was 218. My head ended up being 15cc's. That was a time bomb waiting to go off. BKM gave me an 89 TRX head to try and solved all my problems. The 89TRX head was stock at 23cc's.

All my modded 250's have been over 15:1... I have never used a dome smaller than 21cc with a domed piston. Even with a flattop piston you would have likely been over 17:1 with that 15cc dome.. that engine might have trapped 250-260psi on my gauge.. If your gauge doesn't have two functioning valves, and an adapter with 3/4" thread length it's gonna read low on a small engine.