Wiseco Pistons and Jetting

[IT400C]

Hi All,

The Background Story:
When I first got the IT400C it was running a reasonably fresh 0.25 Yamaha and rings.  The bike seemed to be running well apart from the motor sounding a bit mechanically noisy.

After pulling it apart, I discovered that the barrel was within tolerances for 1st oversize.  At the upper end of tolerance, but still within tolerance.  The rings were sealing properly, with no blow past, and correct end gap, but the Yamaha piston that was marked 0.25 was actually undersized and out of round?? 

As CD6 was fast approaching, and I had no time to source a new Yamaha piston kit, (and not wanting to ruin what was still a good 0.25 barrel), I took my spare worn out barrel down to have re-bore to a new 1.00 Wiseco piston.

The Question/Problem:
With the original barrel/piston combo, the jetting seemed spot on, with no hesitancy anywhere in the rev range.  It pulled well, and revved well.

With the new barrel/re-bore/1.00 Wiseco piston, it pulls well down low, but doesn't want to rev out quite as well as before.  And it seems to have a hesitancy in the upper mid-range?  It doesn't feel right at 1/2 to 3/4 throttle.  It's not so bad when you start from low down and pull all the way through the rev range in one gear.  But it is especially noticeable when you go down one gear more than usual into a corner.  When it's running a constant 1/2 throttle it feels very hesitant. 

I can, and will eventually get round to fiddling with the jetting, but my question is this.  Has anyone come across this sort of thing before?  What causes it? I suppose I'm trying to understand why a seemingly well jetted bike now seems poorly jetted...

Thanks,

Tony

[oldyzman]

Mate i am no expert but i think it is possible that the new wiseco may have altered the compression and you may be getting a bit of detination. or is it possible that the squish if it is applicable on the combustion chamber design, was by fluke better on the old piston, I will be interested to hear what the other more knowledgable doods say... Is the petrol fresh in the bike....
OLDYZMAN

[David Lahey]

How well the rings seal on the bore can also affect the mixture ie there may be a bit of blowby with the Wiseco.
Another thing that can affect the mixture is that the displacement is a bit bigger now
How about the crown height - exhaust and transfer port timing?

[Nathan S]

You're talking about 35 year old parts, that weren't made all that carefully in the first place.

I'll bet that if you had both barrels on the bench, and a pair of verniers, you'd find a lot more than just the bore is different. Production updates, production tolerances, previous "port jobs", previous machining work are all likely and can all result in the sorts of issues you're talking about.

[Lozza]

At the upper end of tolerance, but still within tolerance.  The rings were sealing properly, with no blow past, and correct end gap, but the Yamaha piston that was marked 0.25 was actually undersized and out of round?? 

 I can, and will eventually get round to fiddling with the jetting, but my question is this.  Has anyone come across this sort of thing before?  What causes it? I suppose I'm trying to understand why a seemingly well jetted bike now seems poorly jetted...

Thanks,

Tony

Pistons are not round they are oval to begin with and they are tapered from bottom to top. Your measurements should be taked 10mm from the base of the skirt.
Yes on numerous occasions. The bigger bore creates higher velocties in the carb drawing more fuel through. Had one went 2 jets sizes down for a 2mm overbore. It only seems to happen on well carburated bikes. Just do plug chops like you did before no big drama.

[caps 999]

is the porting differnt on the other barrell by chance

[IT400C]

Thanks Guys for your input..

Yes, it was nicely jetted with the previous barrel.  I wasn't aware that pistons were supposed to be out of round, but I suppose it makes sense - differential expansion, hot exhaust gases going out through the front/top, cold fresh charge coming in through the rear/bottom.

It was the manager of the company doing the re-bore that was surprised by the figures, so maybe it wasn't that it was out of round, but more out of round than he expected?  Don't know...

I did wonder about the shape of the Wiseco piston.  I didn't pay too much attention to the exact shape of the Wiseco when I installed it, but if it's was a different shape at the piston crown, I suppose that would male a difference to the combustion as well.  Anyway, all of the things mentioned above may make a difference..

Because I hardly ride the bike at the moment I just adjusted my riding style to suit the different power characteristics. (CD6 was the first ride after the re-bore, and a short ride last weekend was the second..  maybe 40 minutes riding in 8 months.).  Guess I'll have to spend the time jetting...   

[TT5 Matt]

maybe it wont rev out is the fact that the motor is super tight eg low hours and give it a chance to loosen up,did the rebore shop give the right piston to bore clearence?forged piston like wiseco's like alot more clearence than jap cast piston's,was the barrel honed enough to remove the fine groves left by the boring bar?if not you cut out rings fast and it robbes power

[john d]

 It would be worth a tear down to see if the ports were chamfered. If they were not you get blow by on the rings and ring failure in short order.
Let you compare the new piston to the old also to see any differences.
john d.