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H2 flatspot issues
http://kawi2strokes.com/forum/viewtopic.php?f=16&t=9044		 Page 1 of 15
Author:  h2b780 [ Wed Aug 27, 2014 5:10 am ]
Post subject:  H2 flatspot issues
Hi! I have a bad flatspot @5-6000rpm. on my 780cc h2 with pretty agressive porting. Haven't degree it yet but exhaust is raised
4-4,5mm. and widend to about 70% of bore. Don't remember how much, but transfer's are opend up all the way and both raised and widend to.
Intake also opend up and milled to change the angle for a strait shot and take bigger carb's. Carb's are TM38 Mikuni (snowmobile rack carb's)
Pipes are also snowmobile stuff.. (yamaha srx 700) but now heavely modified. Tuning lenght from piston to start of stinger is right about 1100mm.

Besides the flatspot bike runs really great! Pretty strong lowend and great topend, pulls to 9500.. Have tryed several tuning lenght's and other combination's like long header short belly, long belly-long header, short header-short belly.. And only diff. is the flatspot and peak move's around :banghead So i really not want to change my current tuning lenght. Have to work now but will be back with more info and pics.
Author:  Zambia [ Wed Aug 27, 2014 6:36 am ]
Post subject:  Re: H2 flatspot issues
A cheap option might be to try some VM 38's - I believe they have more linear flow than TM's.
Author:  Walms [ Wed Aug 27, 2014 7:37 am ]
Post subject:  Re: H2 flatspot issues
I haven't seen the specs on your pipes yet but I could imagine a sled pipe has a fairly steep baffle cone to give max hp over a narrower range considering the use of a centrifugal clutch.
If you have played around with tuned lengths to no avail, maybe your next step is to try some more gradual baffle cones.
Author:  husson73 [ Wed Aug 27, 2014 3:37 pm ]
Post subject:  Re: H2 flatspot issues
Zambia wrote:
A cheap option might be to try some VM 38's - I believe they have more linear flow than TM's.
VM are more linear than TM , yes or exactly they are more smothy at low range compare to the TM but at 6500 usually no trouble and no real difference
Have you power jets on your TM :?:
Author:  h2b780 [ Wed Aug 27, 2014 5:49 pm ]
Post subject:  Re: H2 flatspot issues
husson73 wrote:
Zambia wrote:
A cheap option might be to try some VM 38's - I believe they have more linear flow than TM's.
VM are more linear than TM , yes or exactly they are more smothy at low range compare to the TM but at 6500 usually no trouble and no real difference
Have you power jets on your TM :?:
Thats right! Had to open up throttle cutaway alot with the TM's beqouse it was pigrich down low (1/8 throttle)
And that cleared up that area but i need to try smaller nozzles to, only their isn't any avalible.. They are Ski-doo oem. one size only:part number 327 O3
But 159's are the same design and are avalible from size N4 and up, but.. they are a little shorter :banghead But i will try them anyway..
Want to lean her out just to se if that will improve the flatspot? Would be nice to hear what other's use for jetting in their ported H2
Ohh and no i don't have power jets on the TM's (340mj) but i have VM 34's with power jets that i use to run before.

Thank's for your input guy's! :thumbup:
Author:  h2b780 [ Wed Aug 27, 2014 6:14 pm ]
Post subject:  Re: H2 flatspot issues
Walms wrote:
I haven't seen the specs on your pipes yet but I could imagine a sled pipe has a fairly steep baffle cone to give max hp over a narrower range considering the use of a centrifugal clutch.
If you have played around with tuned lengths to no avail, maybe your next step is to try some more gradual baffle cones.
I will sertenly try
that if i can't cure it with jetting and/or zelltronic. Curren't baffle is 115x28x300mm. =8,25 degrees. Don't know if that's to steep? but if it is i will be happy?
to grab the grinder again.. (my next bike will be a single.. :lol: ) But it's a pretty big job so i tought i would try the easyer one's first.. ;)
If i'm to try a different baffle, what angle do i shoot for? Don't wanna mess up my tuninglength but have about 75mm. bellylength to play with.
And if baffle is lengthen to 375mm. it's gonna be 6,6 degrees..
Author:  Neville Lush [ Wed Aug 27, 2014 6:50 pm ]
Post subject:  Re: H2 flatspot issues
What air filter do you have? Are you saying your stinger ID is 28mm?
Author:  Jim [ Wed Aug 27, 2014 7:17 pm ]
Post subject:  Re: H2 flatspot issues
h2b780 wrote:
Curren't baffle is 115x28x300mm. =8,25 degrees.


I get 8.5° for the 1/2-angle, but close enough. That's pretty close to "normal" and within a degree of the Denco design.
Author:  jyrgNorway [ Thu Aug 28, 2014 2:03 am ]
Post subject:  Re: H2 flatspot issues
You got pretty aggressive port times.

I also have 1 set cylinders, ported aggressive. I just found out, my conservative higgspeed worked better with those cylinders than both my Figarolies AND Dencoes regarding performance all over.The result really surprised me.
I get home from Barents Sea tonight, and will do some measurements on my higgspeed. In fact, they work surprisingly well with heavy ported cylinders!!!!

Does your motor ping at all at full throttle on pipes? If you hear nothing at all, with a square ex port and none squish band heads, you are probably too rich. This causes even worse flat spot gurgling as far as I have experienced. And raising transfers make the whole thing even more difficult. What is your transfer duration?

Is your flat spot just as bad in every throttle openings?

Jorgen
Author:  h2b780 [ Thu Aug 28, 2014 4:19 am ]
Post subject:  Re: H2 flatspot issues
Jim wrote:
h2b780 wrote:
Curren't baffle is 115x28x300mm. =8,25 degrees.


I get 8.5° for the 1/2-angle, but close enough. That's pretty close to "normal" and within a degree of the Denco design.
Yes i think so to, couse 10-12 degree's would be for roadracing so around 8 should be pretty "streetable"? But every engine is different.. so if a 6 degree would "do" it for me i will go for it, but i doubt it..
Neville: I run uni filters and 28mm. is od. bafflecone stinger is 24mm.
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