home made velocity stacks and cheap pistons 73 buffalo

[two-stroke-brit]

while at the gap i had a chance to ride a H1 and a H2 and was pleasently surprised at the low rev rideabilty of both bikes which were fitted with pods and chambers.
the owner said that the bikes were much improved buy his addition of home made velo stacks .
these were made out of plastic plumbing elbows.
so i decided to make some and see if i could get the same improvement.




all went to plan but was disapointed in the result,
marginal at improvement best.
i think because the kwakas are in higher state of tune the differance is more noticable.
which got me thinking about skiming the head to up the comp ratio a bit.
as the comp when i rebuilt the motor was only about 115/120.
which brings me to the real subject of this thread.
when i rebuilt the motor i used "cruzin image" pistons .
no probs sofar with them but i read on the kwaka forum that some of there members found that the position of the wrist pins relative to the crown and skirts was about 1 mm out resulting in low comp and affecting port timming.
has anyone on this forum measured them in respect to the oem pistons.
it never occured to me to measure the against the oems before fitting.
cheers mark

[two-stroke-brit]

i thought i would try the solder wire thro the sparkplug ploy to check the squish size.
not sure if i am mistaken in thinking it should be about 0.040 thousands .
turned the motor over and it didnt deform/squash the solder at all.
leading my to think the wrist pin/pistons are not high enough and inturn giving the low comp numbers.
i guess simple fix would be to get a replacement head and skim to suit.
how much can be safley skimed of a fresh head.
presumably all the heads from different years are the same.
or maybee skinier gasket from cometrics or both.
any help in direction would be great.
thanks mark

[tz375]

GT750 does not have a squish band. The design is known as a quiescent chamber and by definition is more open that a squish chamber but lacks the high velocity squish effect and the quenching effect on a large mass of head close to the block.

You can raise the compression by skimming the block and/or head but even when they are down to say 40 thou at the outer edge, there is still no squish band to measure. The Phat Trakka project has metal removed from the block and head and uses a special Cometic gasket and of course it still doesn't have a squish band so we'll test on the dyno and see when/if detonation starts and will adjust timing or fueling to avoid it.

The stock design is fine for a low compression cooking motor, but has serious issues as power rises.

Interestingly, early GT head gaskets had a bore size of around 72mm (the gasket not the bore of the cylinder). later models and most aftermarket gaskets are closer to 76mm and that creates a small area of gas/air mix between the head and block that tends to detonate and causes blown gaskets etc.

To achieve a squish effect, you have to either weld up the chambers and start again or use a head with squish type inserts. I have a welded head waiting to be machined but prefer an insert if the head an inserts are designed to have sufficient rigidity and water flow. The ones I have seen that blew gaskets, had oversized gaskets and were race inserts being used with street gas even though they may have been sold for the street.

The intakes are a different story. Why do they work on an H2? Probably because it generates stronger pulses through teh intake tract which in turn are reflected by the rear face of filters. A spacer changes the resonant frequency to one that's so low that it doesn't much matter, and the curve in the pipe makes it impossible to reflect a wave back down the intake.

With a GT, I suspect that intake pulses are weaker and that the intake path is already much longer than on an H2. Which of those is most important, I know not, but I suspect the results would be different on a tuned motor running higher RPMs.

[two-stroke-brit]

i thought i had read in another thread that the squish band had to be either machined or inserted somehow.
so how much do you think is safe to take off a replacement head?,
and i think the cometric gaskets are a closer fit to the bore size?.
cheers
mark

[Suzukidave]

If you think the wrist pin height is off that could also affect the timing as well as the compression. That may be a good thing to check .. the compression .

[two-stroke-brit]

does any one know what the standard measurements from the wristpin to crown,
and from the pin to the skirt base are.
thanks mark

[Suzukidave]

My dig mic reads 27.50mm from the top of the wrist pin hole to the top center of the crown and 34.22mm from the bottom of the wrist pin hole to the skirt .

[two-stroke-brit]

Thank you for that when I open it up I will check the CI pistons
Mark

[tz375]

There is no way to machine a squish band into the head. The CR would be up around 18:1 or something ridiculous. You could weld up the chamber and machine it with a squish band, but it has to be deeper than stock to avoid excessive CR.

Stock Suzuki pistons have a deck height of -.05 - -1.0mm ie they are below the top of the barrels by 1/2 - 1 mm. When you lift the head you will be able to see if they are lower than that at TDC.

Cometic will make you a gasket at whatever bore size you want.

Before you tear into it in, try another compression test and if the numbers are still low, borrow a different gauge to verify the results.

[two-stroke-brit]

thanks tezzer will do.
cheers mark

[two-stroke-brit]

Before you tear into it in, try another compression test and if the numbers are still low, borrow a different gauge to verify the results.

i checked the comp with a buddys tester and it shows 140 on all three ,
so if it aint broke dont fix it
so i think i will leave well enough alone.
cheers mark