[NSRCA-discussion] Fuel Regulation in 2C Engines

[DaveL322 at comcast.net]

Jerry is right on target.

More power generally means more fuel is being burned (or being burned more efficiently) - a tuned pipe on a 2C can do both (add power and/or add efficiency).  So when the pipe is added (and tuned) to increase power, the main needle is richened up - and when the engine is "off the pipe" in the midrange, the midrange is slightly richer due to the richer main needle.  A common "trap" with the 2Cs is to lean the idle to alleviate the richness in the midrange and this is really only a partial cure.

As Jerry noted, lengthening the pipe generally leans the midrange, and adding nitro generally leans the midrange.

Good pipe design (and tuning) and concurrent design of the pipe and carb reduce pipe jump and mixture irregularities across the throttle curve.

Way back, OS changed (leaned) the fuel metering on the 140RX carb to alleviate a rich spot in the midrange that was popuarly blamed on the carb - this change appeased the majority - but I actually like the original carb better using a longer tune length (compared to the majority at the time).

Regards,

Dave Lockhart
DaveL322 at comcast.net




> > -----Original Message----- 
> > From: nsrca-discussion-bounces at lists.nsrca.org 
> > [mailto:nsrca-discussion-bounces at lists.nsrca.org]On Behalf Of Jerry Budd 
> > Sent: Tuesday, March 07, 2006 12:28 AM 
> > To: NSRCA Mailing List 
> > Subject: Re: [NSRCA-discussion] Fuel Regulation in 2C Engines 
> > 
> > 
> > >5. It is clear that all 2-cycle engines will have this behavior. 
> > 
> > I guess I have to offer a different opinion on this. If you have the 
> > low end set lean (and most running the OS140Rx are running the bottom 
> > lean to get the mid-range somewhat reasonable) you will not be able 
> &g! t; to get the motor to settle into a low idle quickly. And that 
&g t; > presents itself as poor down line breaking, as well as the occasional 
> > flameout. When I ran the OS140's I bought four motors and then 
> > picked the two with the best carbs (aka midrange) so I could set the 
> > low end where it belonged. I also used higher nitro fuel (Magnum #1) 
> > to help lean the midrange, which also allowed me to run the pipe 
> > about an inch longer (which helped soften the pipe "bump"). 
> > 
> > I had good luck (post '02 Nats) and good power (8400 rpm, APC 17x12) 
> > with the OS's, but I found that the throttle response wasn't the best 
> > if you used a lot of throttle to control speed. When I switched to 
> > the Webra 160 I was able to set the top end rich and still have more 
> > power than I knew what to do with (because of the enormous 
> > torque/very conservative exhaust timing), and also set the idle 
> > sligh! tly rich so the motor would quickly settle into a low stable 
> > idle. Of course this was a lot easier to do with the MC carb since I 
> > didn't have to compromise on the mixture settings. 
> > 
> > But, anyway, to bring this back to point. The most prevalent reason 
> > for a 2c to have an erratic idle with poor down line braking is a too 
> > lean idle and a hot motor (like Evil Eric said). Adding a fuel pump 
> > or a pressure regulator or whatever won't fix that. 
> > 
> > Just my $0.02 worth, 
> > 
> > Jerry 
> > -- 
> > ___________ 
> > Jerry Budd 
> > Budd Engineering 
> > (661) 722-5669 Voice/Fax 
> > (661) 435-0358 Cell Phone 
> > mailto:jerry at buddengineering.com 
> > http://www.buddengineering.com 
> > _______________________________________________ 
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