Diagnosing proper main jet size?
Posted October 06, 2006 - 10:07 AM
Regardless TT rocks!!!
Ok, so I'm trying to dial in my bike at my new eleavation 0-2000 and from looking at Indy's chart, I know I should be using a 48 pilot and either a 168 or 170 main. My question is what symptoms will I encounter to tell me which would be better. I currently have a 165 and my bike feels pretty good especially since I just moved down from 5200' and it can breathe better. If they are close, I'd prefer to only go up to a 168 because I know they'll be times when I'll want to take it back up to 5200' and I'd prefer to not have to rejet just for a weekend trip. Any advice?
Also, will my Zipty FMS give me enough adjustment to keep me from having to switch back to a 45 for a ship trip up to 5200?
Posted October 06, 2006 - 11:51 AM
Posted October 06, 2006 - 05:50 PM
i don't think you'll be able to get away from changing back to the 45 pilot jet when you go back to higher elevations (5000 ft. +), there just isn't enough adjustment with the fuel screw to cover a full jet size.
Posted October 06, 2006 - 06:05 PM
It is easier to tell if slightly lean than slightly rich.
Lean it will take a bit to wind up, breathe heavy.
If rich, it will stumble, like ignition cutting in and out.
Perfect, it will take off like a cat shot up the ass! You will know when you got it right.
Test in 3rd or 4th gear, limits the wheel spin. Just hold it wide open.
Revs don't determine jetting so you don't have to be going max rpm.
Posted October 06, 2006 - 06:50 PM
Posted October 06, 2006 - 07:53 PM
Posted October 11, 2006 - 05:24 AM
Once again its very dependent on bike setup (pipe and airbox then temp and alt.)
The important thing is get the MJ set FIRST.
All the above previous advice is good but follow above and fiddle a bit and then you will soon know what works for YOUR bike.
Posted October 12, 2006 - 10:34 PM
Posted October 13, 2006 - 07:07 AM
What causes the popping (backfire?) when I let off on the throttle? Also, my main problem is that it bogs for a second when I first snap the throttle. I thought that the pilot jet controlled 0-1/4 throttle so I don't understan why I should first change the main? I'll probably do that and drop to 4 on the red needle all at the same time but I'm thinking my main problem is the pilot jet unless I'm confused.
1. Popping on decel is lean condition on decel (normal) but ACV on WR adds fuel to compensate.
2. Bog on snap is EITHER too much fuel or too little from AP circuit. Very difficult to tune out totally because of the dynamics of opening the throttle full from idle. Please search on this, many discussions...
3. MJ for WOT because you want the bike to perform and produce MAX power first, then when that is right you tune the pilot for low throttle openings and then needle for 1/4 to 3/4 throttle.
Posted October 13, 2006 - 09:02 AM
Posted October 16, 2006 - 12:51 PM
Posted October 18, 2006 - 05:25 AM
Thanks for the comments. Mine is an '04 so I don't have AIS. The popping started (or got more noticeable) when I went from 5200 to 1200 feet. What's ACV? Also, what's AP circuit? Is that the pilot jet? I ordered a 48 pilot jet which should be here any day so I'm thinking that might help with the bog because it never had it before I came to lower elevation where I'm certainly running leaner.
1.5200 down to 1200 > no jetting change then lean (pop) assuming the bike was jetted correctly for 5200. Check Indys / Clark 4131 charts, probably UP a size on all jets.
2. ACV = Air Cut Valve
Best got to www.thumperfaq.com (for 250 but same principle) The WR carb has one (not sure about 04) the YZ not. Basically, on closed throttle the vacuum opens a valve that allows more fuel to the intake = richer so less pop.
3. AP = Accelerator pump (linked to throttle opening) >squirts fuel into venturi to compensate for extreme vacuum and lack of fuel on initial opening. See ARINs guide and tutorial on FCR carbs (search) - you need to understand the carb on a thumper until FI does away with it...
Put the 48 in, try a 168MJ and #40 AP jet to start with. Needle on #4 should be close.
One at a time with the above or you will lose track of cause and effect.
1. MJ first
2. Pilot second
3. needle third
4. AP jet next
Firts two are easy, the others a little tougher to do..
PS. read up and educaate yourself then you can diagnose jetting problems and fine tune. Then set and forget for known conditions.