750 kit?? Your opinion?

[gggGary]

So I got a no title bike last winter. Got it from a guy, who got it from a guy..........who traded for work done on his Harley
So the story that came along with was: guy said it has an 850 kit in it. Well we know how true THAT was but... Still, maybe a 750 kit? So anyway today I got to thinking, someone mentioned an indicator can be compression. I just tested a regular 650 cold not run for a while kicking it over and got 135, 142. So what the heck, I tested the "850" bike. pulled 162, 165 carbs off by kicking it. This bike has not run for at least a couple of years. Hmm can it be? Maybe it does have a kit. Whaddaya think? It is not a big fin mikesxs kit for sure.

 

[OakBehringer]

Hmmmm... All I can think of is this:

You could put the bike at bottom dead center, remove a spark plug, take a long straw and put it in the spark plug hole. Get the end of the straw that's in your bike into the opposite corner of the jug, where the piston meets the cylinder wall. Make a mark on the straw where it's coming out of the spark plug hole. Do the same thing on your stock 650 and you will find that it's not going in as deep as the 750 kit. If it goes in the same distance, no kit.

 

[xjwmx]

Or you could measure out 750 ml of oil and see how much will go in the spark plug hole. Remember to get it out before putting the plug back

 

[yamaman]

Maybe just measure 1 cylinder & only pour in 375ml lol!

Why don't you believe it's 850 Gary?

You know the only way to find out for sure don't you!

You could just have a peep through the plug hole. The Yamaha & Wyseco pistons look fairly similar, but the Ross 840cc pistons are their own style. I dont know what breed of pistons Halco used?

 

[xjwmx]

Maybe just measure 1 cylinder & only pour in 375ml lol!

Umm...Errrrr....yeah that would work too

 

[OakBehringer]

Put on stock exhaust and air filters, if it's a big bore kit your jetting will be way off!

 

[dwyatt]

Could always tear down the motor and check it all out. It's not as big a job as you'd think.

 

[jay760]

Halco used sp370 pistons, but they only fit the 533 engine. Without measuring it some how, if you could get some light into the barrel some how, maybe a small torch in the inlet track ( when the valves opening of course ) if you can see the edge of the piston there will be a distinct line where the edge of the piston is recessed by a few mm so it misses the head, unlike standard pistons which are domed to the outside edge

 

[grizld1]

Gary, you can kill two birds with one stone here. You're going to need to know whether that thing has streetable CR (<10.5:1) anyway, so here's a quick inspection.

First off remove the camshaft and cam chain so the valves don't open and either secure the head with spacers on the studs or reinstall the head cover; light torque (8 ftlbs. or so) on the large fasteners will do it, no need to reinstall the small bolts. Position the motor so that a spark plug hole is vertical. Take a plastic screw-on top from a bottle or sealant tube with a flat surface on one end and a nozzle on the other, cut the nozzle so the length is the same as the length of your spark plugs from end of thread to gasket, and set it in the plug hole, using it as a small funnel. Locate TDC and use a graduated cylinder to add 30 cc. of oil. After that, use a dosage measuring vial from the drug store to bring the oil level up to the bottom of your little funnel. Gently rock the engine a bit to dislodge any air (there shouldn't be any, but check) and top up again if necessary. Record the number: that's your combustion chamber volume (HV).

Rotate the motor to BDC and fill again as above; the amount you add is your cylinder volume (CV), which is your approximate displacement. Add CV to HV and divide that total by HV to find approximate static CR. If you want to be AR you can correct for plug nose volume (usually <0.5 cc.), but this is a quick procedure for close-enough numbers and that correction won't make much difference.

 

[XSLeo]

On my 75 when I did the top end, second over pistons, carbs off I got 165 lbs compression each side. After break in I got about 150 carbs on.
An "850" should make a bit more. I might expect 185 or more cold. The 750 kits are often around 200 lbs.
A compression check is just one way to get a ball park guess at the engine size. About the only way to know for sure is to pull the head and measure.

 

[Apexdc]

This is all good info. I am going through the same question on "Prom Queen", my old flat tracker. Thought it was a 650, but one previous owner thinks it is a 750. For one thing, I know it has a TON of compression.

Thanks to all!

 

[XSLeo]

I rebuilt my 75 into a 750 with one of Mike's kits. With it on the stand, turning it over with the E start, no carbs I got 195 PSI compression.
I haven't checked in since.
Leo