Thread: Jetting a GP1200 with Mods..
08-14-2009, 02:51 PM #1
Jetting a GP1200 with Mods..
Hey there Fellow "Old Schooler's"...
Having myself a little bit of difficulty figuring out the jetting on my newly resurrected GP1200..
Here's what it has in terms of modifications..
Group-K ported cylinders.. (.25mm over)
Ported crankcases,. done them myself.?(very similar to the ones in this thread.. http://www.greenhulk.net/forums/showthread.php?t=17008)
VF1 Reeds, Riva reed spacer plate.
Riva CDi box (rev b)
Stock head for now..(has compression of #1.. 129,\ #2.. 130,\ #3.. 130, this is at five hours on the motor after the rebuild.)
R&D power plenum F/A.
Lightly ported intake manifold with filled crossover channels..
The motor was built up with all new Yamaha gaskets and seals..
Pressure tested at 10psi forever.. (well overnight anyway.?)
Currently the hull and pump setup are:.
Protec intake grate with wing.
Beach house sponsons.
NuJet 6.5 prop.. (best rpm so far is 7020)
Trued hull.. (never again..)
Ok then moving along to the issue in hand..
The carbs came to me jetted at
#1, 117.5 low, 135 hi, 1.2 n/s. shinny silver spring..??
#2, 115 low, 135 hi, 1.2 n/s. shinny silver spring..??
#3, 120 low, 135 hi, 1.2 n/s. shinny silver spring..?? (its possible i've got mixed up on the #1 and #3 low jet positioning.. )
The carbs are stock 44 sbn's with the chokes removed and a primer installed..
After fitting the carbs to the motor and firing it up.. they dribbled like crazy.. (think more poured than dribled..??)
I pulled them back out and fitted some gold springs.. (dribbling was vitually eliminated.. if the throttle was blipped a couple time's they'd dribbled a little while idling then stop..????? )
While out riding this ski..
The hole shot is not there it bogs for about two or three seconds, then clears out and takes off..??
If i ride along at about 1/8th to 1/4 throttle for 20-30 seconds then punch it it bogs for a second or less then it take's off..??
Everywhere else in the throttle range it just goes..!!
If i feather the throttle out of the hole it'll take off pretty nice..
The ski will literally go vertical after it clears out.. (this thing can jump..!!)
I was initially thinking its way rich on the lows..??
So i dropped them to 112.5's across the board.. (i turn out.. #3, 1 and 1/8th out)
It feels like it holeshots a little better but still bog's for a second or two instead of 2-3..??
Turned the lows in to 5/8th's out with #3 at 3/4
This might have improved it marginally..?? (was on a different day unfortunately..)
After reading this thread from the past.
I'm now beginning to think it's actually a lean bog i'm experiencing and not rich at all..
I've ordered up a set of 105 lows i was going to use in conjunction with 142.5 hi's..
My thinking was this would give the same overall jetting as the 112.5's and 135 above..
Since anything over quarter throttle the ski pulls great..
My plan is either pull the head or stick a bore scope down the plugs holes to check the wash..??
The intention for this ski is serious wave jumping and drag/bouy racing..
So i'll likely try to source a shred plate or shim the shit out of the R&D..
I've also got in hand a couple of skat swirl 14/19 props and an XO..
Think i'll bend one of the skats to a 14/17.. (opinions on what this would do to the holeshot..??)
OK then after all the rambling on above..
What i'm after is known jetting that works for my spec's.. (will be adding a 150 comp head, when i source one..??)
I'm begining to think i might be needing somthing like 125 lows and 125 hi's with a 1.5 n/s..
What i really really don't want to do is melt down this motor..?!!
I'd much rather spend the time and $$$'s on jets and n/s than pistons and rebore's
Anything all you old school guys can offer up i'll be happy to hear it..
08-14-2009, 11:14 PM #2
Gerry, I think it is rich not lean, just from how you described it. There is an easy test to find out, pump the primer while you are hitting it off the hole if it gets worse "rich" if it improves "lean"
As for a recommendation on jetting, I would go down to 105 lows, leave the high where it is. Change to 1.5 N&S Gold spring and double up on the little check valves under the metering block. If you have not done it already, drill the returns out and add a jet to the return line. Adjust it for for 1-2 psi idle and 4-5 psi WOT. This will balance the pressure in all three carbs for consistent metering and give your fuel pump check valves a little more life.
08-15-2009, 01:31 PM #3
i would do EXACTLY what Bill said. To the letter. You could epoxy your cross channels in the intake as well..i would (groupk thing on the triple pipe 1200 write up).
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