Engine Mods.....what works and what doesn't

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geezagillard
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Engine Mods.....what works and what doesn't

Post by geezagillard »

This is what I mostly look for when trolling the site so thought it might as well be an area of discussion so we can summarise for newbies or people like me who may have missed something over the years. I have rebuilt and raced almost all the 250s (except a TZR) at some point in the last 30 yrs and realised every engine has responded to different mods and you couldn't approach a standard motor with the same ideas.

What works for the KR1s?

Compression.....seems to respond very well to more octane/ higher compression
Flowing the bottom end .....5 hp available with a bit of careful work
Open airbox retaining trumpets?
Bigger carbs (35mm).....2hp everywhere?

What the KR1s doesn't necessarily like?

A higher/wider exhaust port height....more than a mill seems to be a backward step?
Aftermarket chambers.....nobody seems to be able to add much to the standard design except weight loss?
Reed blocks.....the largest of any 250 from memory....perhaps vforce adds something?
Ignition....worth doing?
Jetting......too rich will obviously reduce power but once in the ballpark not worth leaning any further? This has been my experience but happy to be contradicted.

I know trying to put it all in a nutshell like this is problematic because of the variations in tune but it does seem as though there are some consistencies.

My trackday bike has single ring Wisecos (YZ125) with reprofiled head and squish (race fuel only) with 1mm base gaskets, no filter or lid to the airbox and carbon end cans. Not had it on the dyno but seems to have the measure of other 250's I have ridden or encountered on the track and I weigh 96kg before leathers....perhaps a real 62-65hp....hard to say of course.
Last edited by geezagillard on Sat Nov 21, 2015 10:20 pm, edited 1 time in total.
DougB
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Re: Engine Mods.....what works and what doesn't

Post by DougB »

Good idea for a topic. Not sure if my tuppence is worth much as I've mainly followed other peoples' advice but it's worked nicely so here's what I did to a completely standard set up.

Flowed the cases with advice from Ben (Luders), but without adding any filler to the cases (didn't trust myself that it wouldn't come adrift in the future).
Matched/flowed the barrels to cases, some work up the exhaust port to match up the valves and port to headers but nothing on any ports in the barrel
Pipercross air filter.
Skimmed head, tighter squish (got it a bit too tight!)
Zeeltronic ignition - altered the valve open/close times and tweaked the ignition curve a bit
Totally stock carbs/airbox & exhausts

Don't know what power it does but compared to before the work it was much crisper & pulled harder, top speed was up a few mph, full rev range was better.

Sadly the pistons let me down so had to strip & rebuild it with old pistons & a different cycl head. It's still better that before any of the work was done but it's not quite what it was. Hopefully going to rebuild to top end this winter to get it back to where it was.
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Re: Engine Mods.....what works and what doesn't

Post by Luders »

I have had excellent results out of my expansion chambers, but your statement is true of off the shelf pipes
geezagillard
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Re: Engine Mods.....what works and what doesn't

Post by geezagillard »

Nice work bozbridge .....great to have real world experience to draw on. Have you tried no filter/no lid? I also remember tightening up the squish too far back in the day and getting some contact with the head. We reprofiled the shape with the wiseco pistons anyway so managed more compression without going to close to the piston....from memory .8mm.

Luders are you happy to elaborate on your chamber experience? At some point I guess a new chamber design becomes necessary when shifting up the rev range with porting and ignition mods all working together or have you managed to retain the relatively low RPM operation of the standard pipe?
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Re: Engine Mods.....what works and what doesn't

Post by Luders »

No my pipes were specifically designed for my engine tune. I'm not sure how they perform on less trick motor.
Will probably find out next year though, just out of interest.
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Re: Engine Mods.....what works and what doesn't

Post by DougB »

geezagillard wrote:Nice work bozbridge .....great to have real world experience to draw on. Have you tried no filter/no lid? I also remember tightening up the squish too far back in the day and getting some contact with the head. We reprofiled the shape with the wiseco pistons anyway so managed more compression without going to close to the piston....from memory .8mm.
I've not tried anything with the airbox at all, I did so much to the motor in one go I wanted to keep everything else stock so I could set it up. Biggest worry is having it run too lean. Carbs run air screw out 1/2 turn and needles on the 2nd groove down. The pistons had been just touching the head but I think that was because they wore out around the Gudgeon pins and allowed the piston to come too close (see my post about Mitaka piston trouble). Hoping when I get it rebuilt I'll use .7-.8 squish again.
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Re: Engine Mods.....what works and what doesn't

Post by Luders »

Housing the carbs in the airbox is the way forward if you can be bothered to make a completely new bespoke one.
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Re: Engine Mods.....what works and what doesn't

Post by ozzy1 »

I'm sure I recall reading increasing the thickness of the side cover that is to do with the kips valve volume also increases the low down power .
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Re: Engine Mods.....what works and what doesn't

Post by Luders »

Spacing out the resonator chambers to increase its volume will have that affect of low down, but it's not really worth doing. If you are riding it at low revs, I suggest buying a large V-twin 4-stroke.
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Re: Engine Mods.....what works and what doesn't

Post by ozzy1 »

I have :mrgreen:
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scooble
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Re: Engine Mods.....what works and what doesn't

Post by scooble »

the whole point of powervalves was to make 2 strokes more flexible.
Whats the point of disregarding a potential method of optimising the power available in order remain faithful to the peaky two strokery cause?
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Re: Engine Mods.....what works and what doesn't

Post by TwoStroke Institute »

An example recently got just shy of 61hp on our dyno (reads about the same as most dynapro's) from an Aprilia RS with tyga pipes and an ignition. That was in about half an hour on the dyno. So there is heaps of value in pipes and an ignition.I could only put 1 aftermarket part or make 1 modification it would be an ignition every time. I sort of proved this with a CR250, I managed to extend the over rev out and make it shoot off corners, still ran standard pipe, comp and squish. Compared to a "highly tuned stage 3 blah blah" CR 250 was night and day, much much easier to ride fast with 2000 rpm of over 40hp compared to 200 rpm.

KR-1 cylinder needs lowering and reprofiling to make the main exhaust port open first. Then fix the crank so it can rev harder and extend out the over rev. Resonators work over a very specific rpm range so spacing it out or making it smaller tends to make a dip in the power after it's closed.
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