Superchicken wont hit max RPM

[ClumpDigger]

Ive been tearing my hair out over this superchicken for almost a year now I can not for the life of me get this thing to "open up". its a 1992 SC650 i swapped a big pin 750 out of a 97 STS rebuilt motor new top end and crank seals. Full STS pipe with a XP waterbox pro k flame arrestors and rad dudes 2" exhaust exit 10/16 hooker as recommended by impros pump stuffer and a PJS scoop grate.

When i originally put it together i put a single SBN44 on it, drilled out a kawi 750 single carb intake and put an adapter on for the SBN. I bought jets from watcon he suggested: 120 l 140 H 2.3 n/s and 95gr spring. it would start right up and idle great but ran lazily, wouldnt open up at all ran like a gurgly honda generator. when i would switch the gas to off i would get a boost of speed so i started off by going lower and lower on my main jet. got down to like 120 main jet and then it wasnt really improving anything so i took off the SBN and put the stock intake on with the dual keinin CDKii's.

swapping the CDKs back on helped considerably (stock jets for STS) but its still not there. messing with the high screws the max rpm i see on my tiny tach is 5700 so its a good amount of RPMs missing. im starting to think its some sort of pump issue, if i hit a wave and my prop comes out of the water then you can hear it open up like it should and you get a good boost of speed until the pump fully gets back in the water then it goes back to the 5600rpm. The impeller is basicaly brand new, no cavitation, everything looks fine in there. sounds great revving it out of the water. Is it possible its overpropped? i dont know anything about props or prop tuning i just listen to what impros or jet maniac has to say.

Any ideas on where i should start testing? here is a pic of the POS for your time

 

[smoofers]

Dumb question, did you rehab the fuel pickups in the tank at all? Is your tank check valve operating properly (should let air into the tank, but not out)?

 

[ClumpDigger]

Yes ive had the pick ups off a couple times, the "on" is good and has the stock filter thing. the reserve rotted off at the end so i cut it and put some fuel line on it and a little screen dongle thing on the end. both look good. the tank vent is new, arrow pointing toward tank iirc. reeds are stock and look great. motor was pressure tested before i put it in. a real head scratcher.
just read this thread :

Built for Speed, Tuned Like a Tractor: The Ugly Truth About Over-Propping​

sounds like my issue but i thought a 10/16 would be underpropped?

 

[fastgtfairlane]

Have you tried drying out the exhaust at all? Big pin, heavy sit down hull and a 10/16 probably not the best combo. The big pin isn't known for good bottom end power because it has a higher exhaust port timing. Might want to try loosening the pump load a little with either boring the nozzle or adding shims behind the impeller. Other option might be a stubby pump cone. A ski with a pump load that is too much will absolutely be lazy and hard to tune.

 

[ClumpDigger]

Have you tried drying out the exhaust at all? Big pin, heavy sit down hull and a 10/16 probably not the best combo. The big pin isn't known for good bottom end power because it has a higher exhaust port timing. Might want to try loosening the pump load a little with either boring the nozzle or adding shims behind the impeller. Other option might be a stubby pump cone. A ski with a pump load that is too much will absolutely be lazy and hard to tune.

how do i go about drying out the STS pipe? have not tried that. impros told me that would be a great combo and the prop was like $360 or something crazy lol :'( ive seen others say its a good combo too. i dont know anything about pump tuning but it made sense that if i made the outlet bigger it should unload it, i have a nozzle from a 750ssxi that i think bolts up to the 650 pump my next step was going to be changing that and go from there it certainly looks bigger. pitch wise would going to a 9/15 help my cause and going up like a 11/17 make it worse in theory? i know the numbers are the pitch but not really sure what the pitch represents.

 

[fastgtfairlane]

Yes the numerically lower number means less pitch. Will allow more rpm but a less top speed. Similar to changing rear end gear in a car. Id have to look at the sts pipe to determine how to dry it out. Basically just reducing water flow will allow more rpms. You can also put a larger reduction nozzle on that will hsve the same affect as a lower pitch impeller.

 

[kevbo]

Prop open the engine cowl and see if anything changes. What’s the top speed? If needed the pump load can be lightened by bending the trailing edges up .100”. I never measured an SC nozzle. All plastic 650 units are over 82mm and metal 750 units are 76 or 80mm. Larger diameters do make more thrust.

 

[ClumpDigger]

Prop open the engine cowl and see if anything changes. What’s the top speed? If needed the pump load can be lightened by bending the trailing edges up .100”. I never measured an SC nozzle. All plastic 650 units are over 82mm and metal 750 units are 76 or 80mm. Larger diameters do make more thrust.

i propped up the seat when i had the mikuni on and there was no change, i think i did it with the keinins on but will try again tonight just to be sure. not sure of the top speed id guess mid 30's. i just measured my ssxi nozzle its 80mm not sure of the SC size but i saw it may be 82mm so i might be working backwards there but i have an X2 at the lake so i might swap its nozzle this weekend instead of the ssxi and see where that gets me. ordered a stubby pump cone too. thanks for the tips guys.

 

[fastgtfairlane]

The stubby cone will increase the volume while reducing pressure at the back of the pump. This will loosen the pump a little as well. I have also seen them hurt performance if the impeller is low enough already. It is similar to boring the nozzle.

 

[Storbeck]

Not 100% sure, but I believe the chicken uses an odball nozzle that is aluminum and 82mm, but the location of the bolt holes for the steering nozzle is different than all of the other common kawasaki nozzles so it's not easy to swap around. Slightly longer.

What you're describing sounds to me like you are probably over propped, but to be honest I would have thought that motor should be able to pull that pitch.

 

[ClumpDigger]

Have you tried drying out the exhaust at all? Big pin, heavy sit down hull and a 10/16 probably not the best combo. The big pin isn't known for good bottom end power because it has a higher exhaust port timing. Might want to try loosening the pump load a little with either boring the nozzle or adding shims behind the impeller. Other option might be a stubby pump cone. A ski with a pump load that is too much will absolutely be lazy and hard to tune.

I noticed the water fitting on the cylinder head of my 750 was swapped from the 3/8ish size tapped to a 1/2 for some reason and thats the water that feeds into the headpipe. could there be too much water in the pipe from making this fitting bigger for whatever reason? i know the basics of pipe tuning from messing with b pipes and that 1/2 turns on the screws and where you put the water is cruicial for making power. could i just put one of those fuel line clamps on that line to restrict water and see if thats my problem? i think i have another 750 head somewhere but its a small pin head i could swap that on too i guess.

i thought it was odd that impros recommended a 10/16 prop for my sp750 swapped X2 and my bp750 swapped chicken being one is quite a bit smaller than the other so i would assume different prop pitches would be required for both but lots of other chicken ppl run a 10/16 without issue i guess and i have faith in the guys at impros and my X2 rips pretty good

 

[Quinc]

Are you running the correct manifold to headpipe gasket? There are two different ones and you want the one that does NOT let water through.

 

[WFO Speedracer]

Is the fitting on the pipe swapped too, if it's still 3/8' it won't make any difference .