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oversize valves affect on jetting?

  • Thread starter Thread starter paco13
  • Start date Start date
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paco13

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If all other factors remained the same and you had larger stainless valves installed in the head, what in general would it require as far as jetting changes? I assume more fuel would be needed, correct?
This would be on a previously modded engine, that was jetted fine for those mods, then larger valves installed later.
Thanx.
 
paco13 said:
If all other factors remained the same and you had larger stainless valves installed in the head, what in general would it require as far as jetting changes? I assume more fuel would be needed, correct?
This would be on a previously modded engine, that was jetted fine for those mods, then larger valves installed later.
Thanx.
Click to expand...

Probably no changes needed. The bigger dia valves will allow more fuel/air into the engine at higher rpms, but the existing cams, port dimensions and carb size will still be your limiting factor.

If your present valve size is the main restriction, you may need to increase the size of the main jet after fitting bigger ones.
 
What engine Paco? If you are referring to the 1100, bigger valves will make a difference in power! Let me know if you want more info or call me at 714-356-7845. Ray.
 
More info

More info

Hello guy's thanks for the replys.

The engine is 0verbored, Wiseco 1166 10.25-1 comp.

Web cams grind # 190 370 lift 242 dur.@.050, Timed 106* IN., 108* EX

street ported head. about 15 thousandths off the deck.

Stainless 28.5 IN Valves 24.5 EX.

Stock 34 CV'S With KN pods. I Know I Know, can't afford RS' carbs right now, all the good deals come up when I'm broke it seems and now its heat season here.

V/H meg Exhaust with comp baffle core.

Current jetting mikuni 160 main, 50 pilot, Factory pro needle 4 from top.
 
That's a pretty BIG main jet but with CVs, it is possible the motor wants that much fuel. Just know that when you can afford to do it, a set of 36 or 38 RSs should have that motor around 150-155 at the rear wheel! I have a set of 36 RSs when you are ready. Ray.
 
I kinda thought that was BIG for the main too, thats why I posted the question. But it actually feels like it needs more fuel this way. Plus the plugs have almost no color.
It is around 40-45 degrees around here though, and I'm running pump gas with ethanol ( all we have anymore).
Not getting any detonation that I can tell.

Heres what it does, lets say your cruising top gear 50mph and want to accelerate around a car W/O downshifting. If I roll the throttle to wide open it pulls smoothly, but builds slowly. If I roll it only to half throttle it actually pulls much stronger and quicker.
Now if I'm in second or third gear I can whack the throttle WO from 2000rpm Puttin' along and it will either spin the tire or loft the wheel AND send you to redline like IN A HEART BEAT.
 
With what you described & the ethanol in it I would guess it needs more main jet. You MIGHT reach a point with the CVs that they just won't flow enough air or fuel for your engine. Minimum, you may have to go to 36mm CVs but your best bet would be a set of RSs. Ray.
 
My mate is running a very similar motor i built 17 years ago including the 1mm oversize valves thats now using 36mm gsxr semi flatside carbs & pods with 160 mains but i cant remember what the pilots were, runs well on pump p*** (97 ron) 152 rwbhp

I couldn never get it right on the 34,s so i agree you need 36's :)
 
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