850 big bore tuning

ekrohn · 07-16-2009, 09:07 PM

Hello Harry,

A few months back, you did the 850 big bore upgrade for my SXR. I finally found time to have the parts installed. The shop owner and former racer tells me that he was expecting more snap out of the hole. We are still in the tuning stag, so what should we also look for?

I am running pump gas, 170 lbs, CDI-9R ignition module, v-force reeds, dual Novi carbs, wet pipe, 17/18 cut back Skat track prop, pump cone, grate, bored nozzel, light weight charging flywheel.

I have not ridden the ski yet, but shop owner is very knowledgeable and thinks it should snap harder out of the hole. Any suggestions as to what we should look at next sure would be appreciated.

thank you,
Eric Krohn
Austin, TX

Group K · 07-17-2009, 11:27 AM

By far the most common tuning issue we se on the big bores is that guys have too steep a prop (or too small a nozzle). We usually use a Skat C-75 swirl 9/17 with 80mm-81mm nozzle .... your pitch "sounds" a bit tall.

The best way to confirm that you have ideal pitch is to get the peak rpms on smooth-water with a digital tach (7300+ is ideal). It's "very" important to keep in mind that during this testing you should have the top and middle adjusters on your FPP head pipe closed (usually 1/2 to 3/4 open on the bottom only is best). After the ideal peak numbers have been reached, then you can re-open the top screw for more bottom end.

Also note that the smooth water digital tach test is also the best way to find the best high speed screw setting (the best setting will always yeild the best "sustained" peak rpm numbers).

I have little faith in indicated guage readings...but yours sounds a bit low ... did you prep your own head, or did we??

Respectfully, Harry

Quote:

Originally Posted by ekrohn

Hello Harry,

A few months back, you did the 850 big bore upgrade for my SXR. I finally found time to have the parts installed. The shop owner and former racer tells me that he was expecting more snap out of the hole. We are still in the tuning stag, so what should we also look for?

I am running pump gas, 170 lbs, CDI-9R ignition module, v-force reeds, dual Novi carbs, wet pipe, 17/18 cut back Skat track prop, pump cone, grate, bored nozzel, light weight charging flywheel.

I have not ridden the ski yet, but shop owner is very knowledgeable and thinks it should snap harder out of the hole. Any suggestions as to what we should look at next sure would be appreciated.

thank you,
Eric Krohn
Austin, TX

ekrohn · 09-12-2009, 12:25 AM

Hello Harry,

My prop pitch is a 13/21 Dynafly.
Looking for fastest 0 - 40. Is this the correct prop?

Thank you,
Eric

Group K · 09-14-2009, 01:17 PM

Eric,
I'm sure the Solas prop can work well, so long as it is pitched to offer the correct "smooth-water" peak-rpm for the engine and nozzle setup you have. The ONLY way to know if the pitching is right is to do a digital tach run on smooth water and confirm that you are reaching the correct target peak.

If you are running the FPP wet pipe, the smooth-water peak should be 7300-7350. THERE IS "NO WAY" TO TELL IF THE PITCH IS CORRECT WITHOUT DOING THIS TACHOMETER TEST. Please note that when you do the tach test, the middle and top water adjusters on the headpipe MUST be closed ... and 1/2 -3/4 turn opening on the bottom adjuster.

Respectfully, Harry Klemm

Quote:

Originally Posted by ekrohn

Hello Harry,

My prop pitch is a 13/21 Dynafly.
Looking for fastest 0 - 40. Is this the correct prop?

Thank you,
Eric

ekrohn · 09-15-2009, 02:22 AM

Hello Harry,

Sounds like the tachometer test is critical. I will follow up once this is completed.
Sure appreciate all your time.

Eric