GS750 big bore kit

[KiwiGreg]

I’ve purchased a GS750C that has a 850cc big bore kit installed. Apparently it’s a Yoshimura kit.
I’ve never heard of a Yoshimura big bore kit, is there such a thing?

[zed1015]

II've never heard of a Yoshimura big bore kit, is there such a thing?

Yes! It will likely be an older modification or NOS kit if fitted recently as Yoshi piston kits for the GS750 don't appear to be listed on their current site.

PISTON | Yoshimura Japan

[Nessism]

Yoshimura was the driving force behind Suzuki's racing program in the USA, during the GS era. They sold piston kits, exhaust systems, cams, etc.

Some of their piston kits were hard core with high compression. Hopefully yours will get along with modern pump gas...

[KiwiGreg]

Many thanks gents - I was getting a little nervous.
I think I’ll go back to the seller and see if he has any further info he can share. He purchased the bike in 2021 and put the Yoshimura kit in in April 22.
As far as I can tell it’s done 97km in the last 8 years and about 10km in the last 3 years.
Any running in tips please ?

[Nessism]

Just drive it in a normal fashion. No racing around, or constant speed droning for long periods of time for a couple hundred miles.

[KiwiGreg]

Thank you for the advice

[KiwiGreg]

Hi all - back with a couple of questions on this project I bought to the forum a few weeks back.
Points and timing adjustments have improved things, but still not right.
A compression test (average quality testing gear) has shown:
Pot 1 - 100psi
Pot 2 - 100psi
Pot 3 - 110psi
Pot 4 - 90psi
And that’s with (I’m told) a new 850 Yoshimura kit in it.
so well below specs.
Plugs 1+2+3 are a little wet
Plug 4 is sooty
The Cam chain also seems noisy to my old ears.
The Yoshi kit is 2 years old with no miles on the road so I’m wondering if the build isn’t as good as I thought it was.
Any thoughts and advice greatly appreciated.
Cheers

[Nessism]

I would ride it normal for 500 or 1000 miles and then put some hard miles on it. I wouldn't give up on it yet. Valves are set correctly?

Check valves, then ride it HARD. Seat those rings! Check compression again in 1000 miles. Realize, that compression needs to be checked with a hot engine, throttle held full open.

[KiwiGreg]

Thanks gents - I’ll keep you posted

[Don R]

I've seen too many guys take a good motor apart for a low compression reading, often a regular automotive gauge will read low due to the volume of the hose. A fresh engine needs to break in as well as a parked engine, even if it is an older build.