More jetting questions....

[tomale]

Ed, it is good to see you on this site.
I am sure that many of you know Ed chesnut, but many of you do not.. He is a long time vintage racer and is very active here in the Northwest... He works tirelessly to promote vintage racing and especially crosscountry and trials

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)

[skiracer]

Thanks for the input Larry.  If I still had an air leak, would the bike run well?  Wouldn't there be some revving issues?  The bike ran very well.  I had it out trail riding for two days, for a total of 4.5 hours, and a portion of the RR.  I did a pressure test on the motor, with no leaks around the head and base gaskets prior to riding the bike.  The crank seal is brand new.  The plug had been a very light brown without any deposits on it.  Thanks, James

quote:Originally posted by Larry Perkins

James,

I think you still have some air issues.  On that bike you should not have to go beyond a 320 main jet, a 55 pilot jet, and a 285 needle jet.  40 to 1 is a good ratio but you should not seize at 50 to 1 even.  Too many times people keep messing with jetting and there is a mild air leak at the base gasket, intake, or crank seal.  I can't imagine you should have seized even at 290 on that bike if all is right in the air department.  My 2 cents worth.  Also needle position won't be the difference between seizing or not.  It will just help you fine tune what is already close.

Larry P

[skiracer]

Hi John, thanks for your thoughts on this.  I am running a Mikuni 36mm, brand new.  Very interesting bit from Golden Specro.  Ring gap is per specs.  Thanks, James

quote:Originally posted by john durrill

James,
The difference in your oil to fuel ratio is small ( unless your oiling bell curve is very sharp like Ron hinted at ). Take a look at this FAQ from Golden Spectro's web site
 FAQS

Question:

This may be a ridiculous question, but I would like to know the correlation between air/fuel ratio and oil/fuel ratio rich/lean mixture. Is it acceptable to lean out the oil/fuel ratio from that of the manufactures recommendation to compensate for rich running conditions. I use Golden Spectro 2stroke oil in my '99 KDX 200. Manufacturer recommends 32:1,I run the Golden Spectro at 40:1 and sometimes leaner and I find this to be acceptable by reading my spark plug. Please, could you inform me if I'm off track?

Answer:

Its not a ridiculous question and you are right on track. Golden Spectro is ideal at 50:1 in the KDX 200 and the affect on fuel mixture is as follows:

A change from 32:1 to 50:1 oil fuel mix = 1% in percentage of oil present. 50:1 = 2% oil and 32:1= 3% oil

So, at 50:1 you are 1% richer on FUELfollow that? 1% less oil = 1% more fuel

Mikuni jets are metered in percentage increments. So, a 170 main jet is 10% leaner than a 180 main jet. In order to compensate exactly for the oil ratio change, not accounting for air density, you would need to go leaner 1%. If you have a 180 main you would have to scrounge around for a 179 main jet. Good luck! They don't make them.

More important though is what happens in the low and midrange because we can feel the bike running a little richer there when we go leaner on oil. Well, some of us can. So, the best course of action is to turn out the air screw a 1/4 or 1/2 turn to clean up the bottom to compensate for changing from 32:1 to 50:1. THAT'S ALL!



Eds advice to starting rich and working lean is sound advise.
 Larrys trying to keep you with a full head of hair with the advise on checking for air leaks chuckle chuckle!!!!!!
  I think you missed something IMPORTANT! in Ron C's note. Check the ring end gap. If it's too big ( even with a new bore and little time ) blow by from the rings could damage the oil film on the cylinder walls and cause a seizure.
 I have seen more that one engine seize like you described. when backing off full throttle to 3/4 or so and the needle needle jet combo could be lean at that throttle setting.
 Are you running a Mikuni or a Bing for a carb?
john d. the curious

[Larry Perkins]

James,

I am baffled.  The pressure check should have found a leak.  Was this done after the seizure or before.  If before perhaps your cylinder/head bolts need retorqueing.  They sometimes can loosen after a period of rideing.

Larry P

[skiracer]

LOL!!!  The pressure test was done before the seizure.  That was how I found the original leak.  Yea, I know what you are going to say.....you need to check in now, and you are right.  Yes, as the shop manual also tell you about retorqueing too.  Thanks again for your input.  James

quote:Originally posted by Larry Perkins

James,

I am baffled.  The pressure check should have found a leak.  Was this done after the seizure or before.  If before perhaps your cylinder/head bolts need retorqueing.  They sometimes can loosen after a period of rideing.

Larry P

[Paul Danik]

I am sure you folks have a handle on the technical end of this problem, but in my mind there could possibly be another factor.  When I first started running events that required any road riding I was instructed to not hold the throttle in one position, unless wide open, for any length of time. If you had to run wide open on a road section to make up time in an extreme situation I was advised to pull the handlebar mounted choke lever down occasionally to give the engine a bit of extra lubrication.

I always have run some pretty tall gearing and typically would not be topped out at maximum RPM for any length of time on a road. A 140 main jet was the norm for a 125 Penton with a Bing carb, we found the correct drill size to make the 140 into a 145 to get a bit more fuel for those situations.

Hope this helps.
Paul