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Message started by DrunkenDwarf on 08/01/09 at 09:29:42

Title: main jet testing
Post by DrunkenDwarf on 08/01/09 at 09:29:42
I understand that jetting is tied to throttle position not rpm, but there has to be enough vacuum on the diaphragm to open up the needle/piston all the way.

So how fast (hard to gauge rpms with no tach) do you need to be going to actually isolate the main jet?

The reason I asked is because I'm screwing around and reduced my main jet to 145. I noticed that above 3/4 throttle the response was basically flat, but I couldn't tell if that was because the slide wasn't opening any further or if I had actually maxed out the main jet.

-D. Dwarf

Title: Re: main jet testing
Post by Serowbot on 08/01/09 at 10:34:35
Not sure exactly where vacuum drops to nothin',... Probably 3/4 throttle or so,... it might come up a little once your speed matches the throttle position.  But I'm mostly just guessin'...

Here's a way to test the main....
To test the main, accelerate from 30 or 40 mph, in 4th or 5th gear at full throttle for several seconds, then reduce throttle by about 1/8,...if power increases for a second, you are lean on the main jet.  Go up one jet size and test again.

PS,.. here's another jet diameter link...
http://suzuki.theatvchannel.com/invision/index.php?showtopic=72965&st=0&gopid=614995&

Here's an interesting throttle position chart I found... notice the overlap...  I believe it's for a Keihin, but the principle is the same...
http://i600.photobucket.com/albums/tt82/serowbot/calchart.gif?t=1249149340

Title: Re: main jet testing
Post by voldigicam on 08/01/09 at 11:28:57
http://www.motorcyclecruiser.com/tech/motorcycle_jet_kits_carburetion/index.html

http://www.motorcyclecruiser.com/tech/find_carburetion_problems/index.html

I put a throttle position indicator on mine.  Didn't even need to test drive - obvious that the hunting and weakness ramped up from 1/8 to nearly 1/4 throttle, then went away completely.  Bumped the pilot jet up and it's a whole new machine, putters along strong at 1/8 throttle 2800 rpm cruise.  Nice.  Doesn't have that off-idle dull spot followed by surge into decent power.  Just a nice even pull up to 5500 rpm.  Really, now I don't dare open the throttle too fast in second.  

Have fun.

Title: Re: main jet testing
Post by DrunkenDwarf on 08/01/09 at 13:41:58

4E584F524A5F52493D0 wrote:
Here's a way to test the main....
To test the main, accelerate from 30 or 40 mph, in 4th or 5th gear at full throttle for several seconds, then reduce throttle by about 1/8,...if power increases for a second, you are lean on the main jet.  Go up one jet size and test again.

I've tried that with 145, 152.5, and 155. Never noticed a change in power.

So far I've ridden with (Dyna exhaust):
55 pilot (1 turn out) / full spacer / 155 main
55 pilot (1 turn out) / no spacer / 152.5 main
55 pilot (1 turn out) / half spacer / 152.5 main
55 pilot (7/8 turn out) / half spacer / 145 main

Full spacer left me a weird surging condition 1/8 to 1/4 throttle.
No spacer fixed that, but killed my mpg.
Half spacer leaves me with a weird surging condition 1/8 to 1/4 throttle, but only when the engine is cold.

I just finished installing a 150, so I'll let you know how that goes. :)

Update:
Well, I guess I never hit "Post". It's now a few hours later and I just got back from a test ride with:
55 pilot (1 turn out) / half spacer / 150 main

There's a difference between 3/4 throttle and WOT now. Not a huge one, but I can feel it.

I guess the next step is to retest 152.5?

-D. Dwarf

Title: Re: main jet testing
Post by DrunkenDwarf on 08/01/09 at 13:52:33

3920232B2628262C2E224F0 wrote:
http://www.motorcyclecruiser.com/tech/motorcycle_jet_kits_carburetion/index.html

http://www.motorcyclecruiser.com/tech/find_carburetion_problems/index.html

I put a throttle position indicator on mine.  Didn't even need to test drive - obvious that the hunting and weakness ramped up from 1/8 to nearly 1/4 throttle, then went away completely.  Bumped the pilot jet up and it's a whole new machine, putters along strong at 1/8 throttle 2800 rpm cruise.  Nice.  Doesn't have that off-idle dull spot followed by surge into decent power.  Just a nice even pull up to 5500 rpm.  Really, now I don't dare open the throttle too fast in second.  

Have fun.

Thanks for the links. I've already got a throttle position indicator. Blue painters tape. Very classy.

The first article has some good tips. Thanks for the link.

-D. Dwarf

Title: Re: main jet testing
Post by DrunkenDwarf on 08/01/09 at 17:42:40

3B32373E243337560 wrote:
Full spacer left me a weird surging condition 1/8 to 1/4 throttle.
No spacer fixed that, but killed my mpg.
Half spacer leaves me with a weird surging condition 1/8 to 1/4 throttle, but only when the engine is cold.

Now that I'm thinking more about it. I wonder if my pilot is a tad lean? I am getting backfires on throttle close...

-D. Dwarf

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