No8 Wire Extraordinaire
I've only had one RV dirt ike. One issue was it loaded up pretty bad going down a long hill closed throttle so if you encountered something you wanted to blip to get the front wheel over it did nothing for about a second by which time you had already bounced over it.Originally Posted by Flettner
the other issue was it had about 4" of ground clearance. . . Because it was s GP100 with knobblies
Don't you look at my accountant.
He's the only one I've got.
Forum whore
don't let truth get in the way of a good story, yes they were a bit like that. How big a cut away was your RV?I've only had one RV dirt ike. One issue was it loaded up pretty bad going down a long hill closed throttle so if you encountered something you wanted to blip to get the front wheel over it did nothing for about a second by which time you had already bounced over it.
the other issue was it had about 4" of ground clearance. . . Because it was s GP100 with knobblies
CanAm (rotax), ran with RV for a long time and got away with it.
Obviously this TIP/sliding gibs will take care of those issues these days.
Moped rider
I'm in the process of a top end rebuild on the YZ250, I've decided to go 2.1mm larger on the bore since the cylinder needs replating anyway and the pistons are readily available.
At the moment the cylinder has a double base gasket, all the porting looks as cast.
However I've noticed a few oddities:
-The floor of the transfers is about 1.4mm below the piston edge at BDC.. Why would it be like this? Can I raise the port floor with something?
-There is a large lip (protrudes approx 0.5mm into the flow path) around all of the ports except the main exhaust, the overbore should cut through all of these, will that be a problem?
My plan is to:
-Measure the heights of all ports as they are
-Have the cylinder bored 2.1mm oversize
-Clean up the port edges, in case the lips aren't completely removed
-Recheck heights of the ports, hopefully they haven't changed too much.
-Radius all port edges
-Remove big casting flash but leave the ports standard.
-Smooth the transition from the aux exhaust ports into the main duct?
-Smooth/enlarge the Boyesen ports
-Have Nikasil applied
Does this all sound reasonable? I'm happy with the engine's power delivery, don't really want to mess it up.
Forum whore
nath88, are you going to run the TPI?
PM me and I'll help you with the 'mark two' setup if you like.
No8 Wire Extraordinaire
The story was really for the porpoise of entertainment. There can't have been a much less suited dirt bike. Although I've never ridden old brit attempts.
But it would be interesting to know how a modern and clever approach hopefully sidesteps a potential issue.
Looking forward to a ride account and YouTube of the bike in action. 360 should really rip in the dirt.
Don't you look at my accountant.
He's the only one I've got.
Hardcore Biker
normally i dont like the idea of using a larger piston unless theres a real good reason to do so. especially if the new piston is heavier. i cant think of why yamaha would want the port floors below the piston top but alot of trail riders like to machine a few mm off the bottom deck and drop the cylinder down. supposedly it gives better low rpm power. was the cylinder purchased second hand ? if the piston top rises above the cyl top deck then its likely someone has machined the bottom. you can thank eric gorr as i think he may have been the one who started the cyl lowering trend with dirtbike riders
Moped rider
I'm starting to think I should have stayed at stock bore, especially since the new piston has a 1mm wide flat step around the edge of the dome. Will make cutting the head a bit more difficult and probably a bit worse flow across the edge when the port is just opening.
The piston edge is 1.3mm below the deck with 2 base gaskets, the top would be above the deck for sure with the dome. May have been machined, I'll have a look for evidence.
Cylinder came with the bike, not sure of it's history. I'll measure all the ports tonight see where they're at.
No8 Wire Extraordinaire
Think yourelf lucky, the GasGas come with the Exhaust floor not full exposed by the piston, sort of a reverse of Jans port floor raising. Overseas forum members all seem obsessed with raising the barrel so it matches. You know, like for a squillionth of a second, and to hell with the timing.
Don't you look at my accountant.
He's the only one I've got.
Hardcore Biker
Yes, i´ll post the progress.
They did a fun test yesterday to see if it responded on a small carburetor.
27mm old round throttle keihin made 28.0hp to the wheel.
Today they will go back to 32-34mm carb again and test(29.2 was with 32mm carb), but as they haven´t got any racefuel(they want the engines 'footprint' before going to methanol) yet they will be very careful.
As i understood from phonecall, they had no corrections made on that 29.2 pull, it might have been around 29.5-29.8hp
And he also mentioned they had problems with slip on the dynoroller.
So i´m excited, as i suspect it actually burned up some power due to tireslip, pipe is tuned to 14000rpm and they got the power at 14300rpm as the peak might have rolled over.
Hardcore Biker
34.1hp at this point, but at very high rpm´s 16467rpm.
But, i figure it is because petrol in a pipe designed for methanol.
Hardcore Biker
Oops.. fast update now. =)
They tested E85 fuel, 35,3hp at 16000rpm
As suspected, the power will be at lower rpm on methanol in future pulls.
Forum whore
Having been thru the exercise of proving that a hot exhaust duct is no good for power I tested ways of getting better cooling in the TM kart engines
I work on.
The first was to figure out why the factory originally intended to have all the cooling water enter the cylinder via ports under the duct, from the case,and then deleted this
and added a hose to feed water in above the exhaust port - from the side..
After many long hours of thought and trial tests, here is the result.
With all the water flowing over the hot duct, it heats ALL the water,that then flows over the transfer tops,into the head and out.
Thus the transfers are actually being heated up,always a bad idea.
So I first drilled two 3mm holes up thru the plugs under the exhaust duct, and added a water fitting hole into the cylinder facing the boost port.
This allowed some cooling of the duct, but forced the coldest water volume over the transfers.
Result - 1 to 1.5 Hp everywhere.
But the tech minions decided this was illegal,as I had made an extra hole in the cylinder that wasn't shown in the homologation papers.
OK, so fuck them,the papers dont show any water fittings at all, so i made a long screw in manifold that also directs all the cold water, back, over the transfers.
Result, almost the same power, and 1st and 2nd at the National champs over Easter.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Moped rider
Measured the port timing on the YZ250, cylinder raised 0.4mm.
Exhaust PV Open 185° Blowdown 34°
Exhaust PV Closed 160° Blowdown 22°
Aux Exhaust 185°
Rear Transfer 120°
Main Transfers 117°
Exhaust seems right but transfers seem low.. The only reference I could find says stock is 121 degrees, can anyone back that up? Should the rear transfer port open first?
No8 Wire Extraordinaire
That's quite a stagger. Sorry did you bore this significantly? Transfer angled will be more affected.
Don't you look at my accountant.
He's the only one I've got.
Moped rider
Nope, stock cylinder... I'm measuring so I can see the difference once it's bored. Seems like the transfer ports are cast about 1mm too low. Would explain the piston/port mismatch at BDC.
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