Optimum carbs by CC
04-27-2010 | 11:49 PM
#1
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From: Salinas,
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Optimum carbs by CC
I am having a great time with the converting hobby. I appreciate everyones help.
One of the mysteries, to me, is selecting carbs. Mostly I grab the box of carbs and try them until something works. I have found that the WT542 (13.49mm) with and accelerator pump is the most versitile. It works on every engine I have done from 20cc up to 65cc. The 65 turns up more RPM with and HDA 137 (16.61mm) but does not idle as reliablly or transition as well as the WT542. All my inbetweenie engines like the WT542 real well.
I have been playing with some little Stihls 20cc-ish. The running one likes a WA226 (7.37mm) for smooth but the WT542 for fast. I am looking to EPAY some carbs, possibly in the 10mm range. I think they should have accelerator pumps? I see the Zenoah G23 uses the WT643 (12.7mm). They must have some some testing? Seems big though. I bought three of the big HDA's only to find out they are too big. I'd like to get this purchase right the first time
The point of this rambling is to find out what are favorite carbs for various size engines. What has works well for you all? Maybe break it down by displacement?
15cc
20cc
30cc
40cc
50cc
60cc
70cc
larger??
Is there a rule of thumb? Maybe 10mm for 20cc, 12mm for 30cc, 14mm for 40-60cc, Over 60cc 16.6mm? I am envisioning a plot on a graph of cc vs. mm of carb. Is that too simplistic?
One of the mysteries, to me, is selecting carbs. Mostly I grab the box of carbs and try them until something works. I have found that the WT542 (13.49mm) with and accelerator pump is the most versitile. It works on every engine I have done from 20cc up to 65cc. The 65 turns up more RPM with and HDA 137 (16.61mm) but does not idle as reliablly or transition as well as the WT542. All my inbetweenie engines like the WT542 real well.
I have been playing with some little Stihls 20cc-ish. The running one likes a WA226 (7.37mm) for smooth but the WT542 for fast. I am looking to EPAY some carbs, possibly in the 10mm range. I think they should have accelerator pumps? I see the Zenoah G23 uses the WT643 (12.7mm). They must have some some testing? Seems big though. I bought three of the big HDA's only to find out they are too big. I'd like to get this purchase right the first time
The point of this rambling is to find out what are favorite carbs for various size engines. What has works well for you all? Maybe break it down by displacement?
15cc
20cc
30cc
40cc
50cc
60cc
70cc
larger??
Is there a rule of thumb? Maybe 10mm for 20cc, 12mm for 30cc, 14mm for 40-60cc, Over 60cc 16.6mm? I am envisioning a plot on a graph of cc vs. mm of carb. Is that too simplistic?
04-28-2010 | 09:19 AM
#2
Senior Member
My Feedback: (19)
RE: Optimum carbs by CC
Displacement is part of the equation. RPM is another. The last part is how much peak power you're willing to sacrifice for midrange running and transition. If you calculate volume flow and size the carb for 60-80m/s velocity through the venturi it will be close.
Pe Reivers has a nice calc in his spreadsheet.
http://mvvs.nl/utilities.htm
Pe Reivers has a nice calc in his spreadsheet.
http://mvvs.nl/utilities.htm
04-29-2010 | 05:33 PM
#3
RE: Optimum carbs by CC
Like Greg said above, many factors come into play such as Displacement, RPM's, Port Size and Shape, Port Timing, Compression ratio, Intake Valve type (Reed, Rotary, Piston, etc.), Exhaust Type and Size among others.
