Home Forum Members Pictures Contact Events

Go Back   Bay Area Two Stroke > Technical > Aprilia Tech
Register FAQ Members List Calendar Mark Forums Read

Reply
Thread Tools Display Modes
  #1  
Unread 05-18-2020, 08:02 PM
crunchbucket's Avatar
crunchbucket crunchbucket is offline
Junior BAT
 
Join Date: Aug 2018
Location: Bay Area, CA
Posts: 9
Default RS50 (85cc Stroker kit) TM24 Carb

Wondering if anyone can offer some advice here.
I bought what I believe to be dankersons RS50 build he had for sale last October. Pretty much 100% sure.

The bike runs great and there are some impressive areas where the power is just spot on.

It had a TM24 with what I think was a 210 main and a 15 pilot. Stock needle, I didn't write it down. I have gone to a 220 main and a 25 pilot jet. I have a wideband 02 sensor on it and I am trying to tune out a lean range from about 6K to about 8K rpm. This lean condition is more pronounced when I flick the throttle open very quickly in this RPM range. It goes to like 18:1 or even 20:1 until it hits 8k to 9K then back to a nice healthy 12.5 or 13 to 1.

Based on the fact that this is not partial throttle leanness based on RPM, but WOT leanness in this same range, it doesn't seem like the needle. If I go richer on the main, it gets too rich once it gets over 9K.

If I could get the bike to stay around 12.5:1 the entire power band I'd be super happy.

Any ideas on how to tune a fast twist WOT lean spot in the mid range?

Once I solve this, I'll move to the second pronounced trouble spot which is some misfiring around 11K and up. :-)

BTW, the bike had a 12 tooth front sprocket on it. That seemed a bit small to me so I put on a 15 just to see how far it would go. This thing make such good power that I was hitting 65MPH today and still pulling with a good amount to go. It has no problem pulling my 180LB load up my steep driveway in 1st gear either. Impressive.
Reply With Quote
  #2  
Unread 05-19-2020, 06:25 AM
jkv357's Avatar
jkv357 jkv357 is offline
Junior BAT
 
Join Date: Nov 2017
Location: Middleton, WI, USA
Posts: 61
Default

What does it do when held at 1/2 throttle/midrange? Are you able to isolate different throttle positions (1/8, 1/4, 1/2, etc) and read the mixture?

I tend to think in terms of throttle position only, and not RPMs when carb tuning.

Any time you whack the throttle open quickly I would expect a momentary lean mixture due to the difference in flow between air and fuel.
Reply With Quote
  #3  
Unread 05-19-2020, 06:48 PM
crunchbucket's Avatar
crunchbucket crunchbucket is offline
Junior BAT
 
Join Date: Aug 2018
Location: Bay Area, CA
Posts: 9
Default

Yeah. I hear you on the throttle position.

But this issue is basically WOT down low and it is fine.
WOT above say 8.5K it is fine.
WOT in between 6K and 8.5K goes super lean until it hits 9K.

If I slowly, and I mean very slowly, twist the throttle open as it raises through the RPMS it is less of a pronounced lean, like 15:1.

It is like it takes a certain air velocity flowing in at the higher RPMS to suck enough fuel in.

I don't have a lot of experience with float level or atomizer changes. I wonder if tweaking on those settings could help this midrange RPM WOT lean situation.

Thanks for your thoughts.

I can get some data on the various throttle positions and the AFR in general through whatever RPM ranges those positions will operate through.
Reply With Quote
  #4  
Unread 05-20-2020, 11:34 AM
jkv357's Avatar
jkv357 jkv357 is offline
Junior BAT
 
Join Date: Nov 2017
Location: Middleton, WI, USA
Posts: 61
Default

Quote:
Originally Posted by crunchbucket View Post
Yeah. I hear you on the throttle position.

But this issue is basically WOT down low and it is fine.
WOT above say 8.5K it is fine.
WOT in between 6K and 8.5K goes super lean until it hits 9K.


If I slowly, and I mean very slowly, twist the throttle open as it raises through the RPMS it is less of a pronounced lean, like 15:1.

It is like it takes a certain air velocity flowing in at the higher RPMS to suck enough fuel in.

I don't have a lot of experience with float level or atomizer changes. I wonder if tweaking on those settings could help this midrange RPM WOT lean situation.

Thanks for your thoughts.

I can get some data on the various throttle positions and the AFR in general through whatever RPM ranges those positions will operate through.
That would make sense with a big carb on a small engine.

There just isn't enough air velocity at low RPMs to draw the fuel up efficiently - is what I get out of the situation.

Not sure if port size and timing would have any effect, but since they don't make powder that well in the lower RPM range (compared to closer to peak power RPMs) it's possible that flow is just not good at that point.

Does it run poorly/stumble or do you just know the mixture is off?

My Derbi EBE-075 came with a 24mm Flatside and it wouldn't even run in the mid-range because it was so rich. I ended up with the leanest Jet Needle and Needle Jet I could get and it runs well now, but not much happens if I open the throttle quickly below about 7500.

It pulls better with a slow roll-on of the throttle than it does if you whack it open. Probably the same thing is going on - not enough air velocity to draw the fuel up with the large abrupt throttle opening compared to a slower/smaller opening and more consistent air velocity.

I'd be curious to know if you find a solution.

Pretty sure I'm geared 15/42. Up 1T on the countershaft from how i got it. Gives decent cruise RPMs and ok acceleration. Best I can tell (speedo is in kph) it tops-out about 75 or more. I was following it one day and was indicating 75 mph - and it was pulling away.

Last edited by jkv357 : 05-20-2020 at 03:54 PM.
Reply With Quote
  #5  
Unread 05-20-2020, 07:14 PM
crunchbucket's Avatar
crunchbucket crunchbucket is offline
Junior BAT
 
Join Date: Aug 2018
Location: Bay Area, CA
Posts: 9
Default

Nice!

I get you on the velocity thing. I'm just trying a few more things.

The carb came with a 5N14 Needle. Hard to find info on this. From what I read it is supposed to come with a 5N13. Again, not on any charts from Mikuni. All in all, I would think the needle isn't doing much on a WOT whack.

Today, for giggles I swapped out the pilot jet from a 20 to a 40. I was reading some Mikuni literature that states the pilot has to cover for some of this whacking before the main will flow enough to cover the whole thing. I imagine that to be 1-2 seconds, not the 6+ seconds that the AFR is well over 14.6:1. I have fried pistons before, so not interested. Looking for 12.5:1 as much as I can get it into the whole RPM range. My thoughts are if I can just prove that I can get this area under control, I can tune the other areas around it. It didn't promise anything. I think I still saw 17 and 18:1 range numbers in this range.

Not sure what to try next.
Reply With Quote
  #6  
Unread 05-24-2020, 12:01 PM
twinturba's Avatar
twinturba twinturba is offline
Moderator - $ Donating Member
 
Join Date: Oct 2005
Location: San Francisco
Posts: 1,022
Default

Never experimented with wideband O2 on 2 stroke bike. Not sure it's the best way to measure your fuel mixture as most top 2 stroke tuners use EGT or Det counter. My guess is that the function of the expansion chamber with mixture may cause different O2 readings particularly in the power band. Another factor affecting O2 reading may be the oil mixture. Again, I am just guessing as I have no O2 experience on 2 stroke machines.
__________________
Jenius Biker Blog

Current FTR, Diavel, KilLeR, Metro, Grom, GTS300, RS250³,Newd, TT, Max, Nastro, Bangkok, 85 ADV

Past: SC1000s, CeRbi, SPR, 959 Corse, CB300R, SRMotard, Versys, Typhoon, NCXD, RC390², Madass, PW50, KTM BR 65, Aprilia BR50, Aprilia RS125², DRD, WR250X, Derbi SM⁷, Aprilia RS50¹⁰, Derbi GPR³, Aprilia RS250², Mito², NSR50², NS50, HRC RS125, HRC RS80, YSR63 (1st 2stroke!), FZ600, KLR250(1st Bike!)
Reply With Quote
  #7  
Unread 06-17-2020, 06:46 PM
dankerson's Avatar
dankerson dankerson is offline
SUPER BAT - $ Donating Member
 
Join Date: Oct 2005
Location: cali
Posts: 2,677
Default

Main jet change smell is in the air. I think I kept it fat for the break in.
__________________
2000 Aprilia RS 50/TP85 cc stroker kit
2003 Derbi Super Motard with a CR85R race engine
2001 Honda RS 250R that beat my fastest times already
2006 Aprilia RS125 Street legal
4 2004 go-peds all with race engines
2017 Ford F2fity to haul all my bike with
Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -7. The time now is 06:31 PM.


Powered by vBulletin® Version 3.6.8
Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.

Ad Management by RedTyger