Jetting Going from 36Mik's to 40's?

[santito]

One thing that I must add to this is , I would highly recommend that a set of EGT gauges be used when playing around with swapping carbs or getting into rejetting carbs. This is a good way to monitor your engines exhaust temps to help in preventing the unexpected burn down. This can happen even as low as 1/2 throttle and running on the jet needle. I run these on all my sleds when setting up the carbs and to monitor while running.  CHEAP INSURANCE !!!!!!!!!!
 Vic

What do you look for as a "SAFE" temperature range?

[Vintagesleds.com]

[HOOTS]

 That differs too much with placement of probes . You MUST determine your baseline temp by wash & plug read, then use that temp reading for future tuning.

[santito]

Ok.. so I got me a guage and got a good baseline for EGT and now looking to make some adjustments. For the full story... its a SA2 340C with a vm36. Manual says that its supposed to be a vm36 and it is very rich on the low end... smokes like a freight train and loads up something fierce. Running it in the upper mid to WOT it's all good.. just the low end. With that being said and the main is a 320... should just the main be changed? or should I also swap the pilot jest as well? Was looking at moving to a 310 main and checking that.

[HOOTS]

 If your wfo chops look good & temps go way down or its loading up at idle & mid, try dropping your needles a groove. If just at idle & big rich bog when cracking throttle, adjust air screws, if you are more than 3/4 turn from 11/4 turnes out on them, (to get bog out) then change pilots.

[santito]

If your wfo chops look good & temps go way down or its loading up at idle & mid, try dropping your needles a groove. If just at idle & big rich bog when cracking throttle, adjust air screws, if you are more than 3/4 turn from 11/4 turnes out on them, (to get bog out) then change pilots.

So far changing the clip on the needle does not seem to make any effective change for the low end richness. Also have adjusted to airscrew to no effective change as well... mid range to wide open is good.. plugs are a nice toasty brown... but low end you'll gag on the smell of raw fuel and yep.. if you go to goose it you get nothing but a bog and have to feather the juice to get it up enough to clear out.

[Docjim]

I like Ahoods comment.    If velocity is low in theory you would need more jet.       You could get rich quick if you could develop a gauge that would tell you if you where rich or lean at any given throttle position.    So you inventors there ya go!!    This is a gauge that would be used in all forms of motor sports.

[Cameron670]

air velocity and hence pressure across the jets will go down.  pressure difference is what moves the fuel.  so you may not be "moving" as much air as you think. just that you decreased the air velocity.

I like this post!   When I put a 670 in a 97 SD 440 zx (I was 21 remember) I started off using the 34's original to the sled.
I was certain that i would need bigger jets. A week later I was right back to the original 440 jet sizes. Its like jetting is
making the hole in the jet the correct proportion to the big hole thru the carb. The 670 drawed on the small carbs harder,
but fuel flow was still in correct proportion. I feel this contributes to the posted topic, hopefully no one see's it as a hi-jack!!

KJC

[AHood]

that is exactly right cam. same reason you see most 2 stroke modern moto X motors (whats left) both the 125' and 250' use almost the same carb and similar main jet sizes.  these posts help people understand the theory of carburetor tuning i think instead of just saying go up a few sizes.  teach a man to fish... ya know.      so by inference if anyone has a motor that "requires" a huge main jet you could probably afford to use a smaller carb and gain better overall performance.   

[340RVFA]

the gauge Doc talked about is already out there. its an 02 sensor and used in fi cars since the 80s. stand alone sensors with a gauge is available