Rejetting and lowering the jet needle

[ejdirin]

I fired up my project (1979 XS650 SF) the other day using a temporary fuel tank (ie, gatorade bottle) and I could see the fuel just pouring in. I burned 1/4 of the bottle in probably 8 minutes of idling. I installed POD filters and basically straight pipes with minimal baffles.

I took the carbs apart (1979 Mikuni). Everything looked clean, but to be safe, I took everything apart and soaked all the small parts (jets, starter plunger, float needle/seat, etc) in carb cleaner and blew compressed air through everything.

The main jets are #135. I have some #145's on the way. First I'm going to try to reassemble the carbs as close to stock as possible (float at 24mm, #135 MJ, #27.5 PJ, etc) and see how that goes, because most of what I've read says that the bike should run ok if the carbs are stock, regardless of the new filters/pipes. I'm also using a non-vacuum petcock.

My questions are: Should I use the #145 MJs? And, is there any reason to lower the jet needle?

Here is a comment from a doc I had on file:

"Bub pipes require jetting changes to the stock Mikuni carbs. We found that K&N R-135/2 air filters (they replace the airbox) are quieter and cheaper than the OEM-type K&N replacement filters. After fitting the Bub pipes and K&N filters, the main jets
should be replaced with No. 145 jets, the needle should be dropped to its lowest
position (remember, lower the needle not the clip), and the plastic caps removed
from the idle mixture screws. Adjust the mixture screws for fastest idle, keeping the
idle speed down to less than 1200 with the idle-speed screw. The idle-mixture screws
should be set so that turning them a quarter-turn in either direction will cause the
idle speed to fall slightly."

Will this work for me? I know a lot of this will be trial and error, but those jet needles are a pain to adjust. Thanks in advance, I'm a carb rookie

 

[Tomterrific]

Even though sawn off pipes are loud they may have excessive back pressure at certain rpm. Many will be surprised to know that Brit bikes need the main jet reduced in size when the mufflers are removed (running straight pipes). Putting a restrictor inside a pipe is garanteed to choke off the exhaust. Those BUb pipes were straight through and long. In your case I'd start with stock jetting and work out all the other problems before trying to fiddle with jets changes.

You may need to poke a thin wire through the idle jets. Difficult to get an idle jet clean with only solvent and air.

Tom Graham

 

[ejdirin]

Thanks Tom. I removed the MJ and PJ without any trouble. There wasn't any varnish holding them in tightly. I stuck a wire through all of them, visually inspected them and soaked them. Was there anything I missed?

I'll try to run them stock tomorrow. I'm just confused as to why the fuel was just pouring in, but the bike seemed to be running well. If its any help, I had to pull the idle mixture screw out about 4 turns....

 

[bloodthirstysystem]

i pretty much run the same mods you did with pilot jet: 30, main: 140. needles at 3rd slot, mix screws 2.5 turns and it runs good. stock jets will definitely be lean..

changed my needle height a few times too. you'll be able to do it really fast after a couple times. you do not need to remove the carbs to change the needles!

 

[ejdirin]

So you changed them a few times but ended back up in the stock position (3rd slot)? I'll have to order a set of pilot jets then. Did you ever try #145 MJs?

 

[bloodthirstysystem]

yeah i went back to the stock position. i got a set of 145's but never tried them since the 140's worked. (i have 78 BS38 carbs) i'm pretty sure too rich OR too lean will both ruin fuel economy

 

[Sundie]

i'm terribly unclear why someone would suggest bigger jets (richer) then say lower the needle (leaner). Why not, you know, just raise the needle?

These carbs are *really* easy to run rich. I would stick with stock jetting, to be completely honest. *maybe* a step up on the pilot, and *maybe* two on the main, for totally open intake/pipes. But i'd have a hard time justifying that for most people who run open pipes, since they rarely sit at 6k+.

 

[bloodthirstysystem]

not sure if just raising the needles will do it. it seems like it would work! i read so many times to go up on the pilot and main..

ejdirin - just spend an hour reading this: http://www.amckayltd.com/carbguide.pdf

 

[Sundie]

raising the needle will give you more fuel during the 3/4 -> WOT throttle. Otherwise - it's actually not bad to run a little lean at "even" throttle - "even" being "holding it at a cruise". Most modern closed loop FI systems will run you slightly lean in those conditions.

 

[XSLeo]

The needle's range is about 1/4 to 3/4 throttle. WOT is controled by main jet size.
Rasing the needle will have a slight effect on WOT and at just off idle.
I might reccomend leaving the needle in the middle position, get the pilot jet right. Get the main right and adjusting the needle to get the mid-range right.

 

[marvintheminnow]

The needle's range is about 1/4 to 3/4 throttle. WOT is controled by main jet size.
Rasing the needle will have a slight effect on WOT and at just off idle.
I might reccomend leaving the needle in the middle position, get the pilot jet right. Get the main right and adjusting the needle to get the mid-range right.

how do you know when the pilot is right and then after that how do you know the main is right? ive tried everything and would like to start at stock and approach it this way one jet at a time.

 

[inxs]

- pilots affect idling...idling is optimised by adjusting the idle and air mix screws to get a balance between smooth idling and low rpm...the choice of pilot jet is made once the air mix screw setting is determined, screw set close to min-needs larger jet, screw set too far out-needs smaller pilot jet...then reset air-mix and idle screws
- mains effect open throttle ranges...sharp throttle response indicates leanness, increase main jet size until the engine bogs at wide open throttle then decrease jet 1 or 2 sizes

 

[grizld1]

Guys, take a close look at inxs' diagram and think about it--it's one of the few component-and-range charts I've seen that emphasizes the strength of secondary and tertiary effects both upstream and downstream.

Some carburetors isolate effects of component changes better than others; older Mikuni designs (VM and BS series) do it pretty poorly. Sundie, you asked why stepping up MJ size would force a leaner needle setting. Answer: because the bigger MJ fattens the upstream mix. Here's more: if you have to drop the needle to compensate for the fat MJ, you'll more often than not, particularly in VM's, have to fatten the PJ to compensate for upstream effects of the leaner needle setting.

Bottom line: if you try to do jetting as a straight-line process, you won't get anywhere with it. Think in terms of a tightening spiral of change and compensation. If you're working smart with the carbies you'll often nail problems by ambushing them upstream and downstream from the trouble spot.

 

[marvintheminnow]

- pilots affect idling...idling is optimised by adjusting the idle and air mix screws to get a balance between smooth idling and low rpm...the choice of pilot jet is made once the air mix screw setting is determined, screw set close to min-needs larger jet, screw set too far out-needs smaller pilot jet...then reset air-mix and idle screws
- mains effect open throttle ranges...sharp throttle response indicates leanness, increase main jet size until the engine bogs at wide open throttle then decrease jet 1 or 2 sizes

ive got the bike idling good with 1 size over on the pilot with the stock mixture screw adjustment however the bike is hard starting and once it starts it will idle good but onc the throttle is applied it dies.

 

[grizld1]

Once more, don't read component-and-range charts too literally. In BS-series carbs, slide lift occurs late--usually after 4K rpm. That means you're on the pilot circuit and main circuit bypass for a larger part of the throttle range than with many other designs. You can set the fuel screws for a decent idle and still have PJ's in place that are too small for off-idle operation, and be sure pilot and main circuit bypass nozzles in the spigot are clear. BTW, there are very strong interactions between needle position and the pilot circuit, even at initial throttle opening and even with modern carbs with better component isolation. I just finished a round of tuning on the TM36/68 radial smoothbore pumpers on my XS650D. Midrange was just a tad lean, so I installed a pair of richer 9EBY1-50 needles; but they weren't richer in the initial taper, and with the clips at #3, engine idling fine, and mixture screws well within range, the bike barely ran off-idle. With the clips at #4, off-idle transition and low-throttle operation cleared right up.