Do I need bigger carbs?

[snowprophet1]

I know cub cylinders make good power and right now I can't compete in a 500' run, but in short runs I do ok. My setup is stock cyls drag ported, 6mm stroke, long rods, cpi inframes, and 33mm flatslides. On the dyno my stock carbs make 72rwhp, and the 33mm carbs made a wider more usable powerband by a couple hundres rpms, and made 75rwhp. I make 200psi on the compression gauge. I didn't try lowering the compression yet, but I don't seem to have the top end I think I should have. If I went w/bigger carbs, would it make a definate difference or not really much? It is just under 400ccs. Just wondered if anybody was in my situation and had noticably better top end w/35s, 39s or whatever. Really pulls nice in short runs, but I need more top end. Thanks everyone!!

[shanYE west]

I probably would run 35's.. but.. 200psi compression is a bit high too.. If it were me.. I'd back that down to about 165-175ish and play with timing.

[sleeper06]

I run 39mm pwks on my stock cyl,my bike flies but I also make alot more hp than you,your motor might not need 39s who ported it

[J-Madd]

Please give tire/gearing info and what peak hp rpm was on the dyno.

[snowprophet1]

I can't upload my dyno chart I just created. It is almost 2,000 KB and it says I can only upload 50K? I'll sent it to you and see if you can shrink or post it or look at it.

Edited August 27, 2010 by snowprophet1

[snowprophet1]

Please give tire/gearing info and what peak hp rpm was on the dyno.

 

75RWHP @ 9,500rpm @ 4,000' above sea level. 15/41 w/21" tires for 500'. My guess is I'm just a touch higher than I should be. I wanted 180 psi on this setup, but ended up at 200 psi on first attempt, so that is where I tried it at. Possibly wouldn't drop off as fast w/a little less compression. I did the porting and my brother has a couple Haas cncs so we put the stroke at 6mm, figuring that stroke would work best for the transfer port duration. VP110 gas. No weight shaved, and swingarm is 3 1/2" longer.

[df350nos]

I had pretty much the same motor setup on my little bike for along time and I ran 35 mikunis and they worked awesome. Very tuneable and pulled hard.

[snowprophet1]

Thanks. I'll try tweeking it on the dyno one thing at a time once I get my street bike and other atv back together.

[chrisp250r]

i never had anything dyno's and i may be wrong but im not sure... isnt 75 kinda low output for a 6 mil? i was told my stock stroke with port magic drag port should be makin 67-70 range, just askin

[sandtazam]

Does it feel like you run out of power or does it feel like you run out of gear?

[RadarRacing]

if you have a ricky stator that is always the first thing to check. I had a 95hp motor make 77hp with a ricky stator. they just sort of make the same hp for about 3000 pms. If you can post what your port durations are that may help. If you have lower transfer port durations than you should that can hurt topend. 75HP isnt really that bad for a 350 but you are bigger than that. If you have 32 or 34 degrees of blowdown and 196 to 198 exh and at least 126 degrees if trans duration but not over 130 you might look elsewhere. Did you cut the skirt of the sleeve down, widen the exh port to 70% of the bore size or so?? Bigger carbs probably wont add much to that setup right now.

[snowprophet1]

I can't find my whole book of notes when I did this, but I remember when I ported it I left a little on the table so i could fine tune slightly by changing base gasket thicknes or my other set of pistons. I can't remember the exact specs I ended up with, but exh was 198*-200*, and transfers were 128*-130*, and it was between 70%- 73% on the bore, and the rings didn't fall in yet. The next time I work on it, I'll verify what I have. I'm thinking 200/128, and if I can find a combo that can keep it at 200 or under I'll get the tr up to 130 if they aren't already, along w/lowering the compression while I'm at it. Then I'll dyno it and if it doesn't work, I'll try a slightly bigger carb.

As for cutting the skirt of the sleeve down, I changed it on the transfer port sides a little, but not the skirt itself. With the bigger stroke on the stock cylinders, I'd probably want to keep all that support at bdc since the piston is going lower than a stock stroke now. I did all I could w/the intake and added the extra intake ports to the transfers down low.

Just curious how your ricky stator hurt power so bad?

Right now I am running a stock ignition setup, and modified my timing plate for up to 7* advance. For sure when I shift into 5th, it doesn't pull as strong as the gear before (goes lean getting off and on the throttle, so maybe I just can't get a hard clean rev after the throttle blip)?. I might put a momentary kill for the 5th shift, and not have to let off the throttle, and see if my times change drasticly.

Edited September 12, 2010 by snowprophet1

[RagunCajun]

Let off the throttle? does that mean you do NOT have an override?