Another Re-Jet Job

[noears]

I know there are a lot of threads and all sorts of articles on re jetting, all have been usefull, but some are contradicting.

I have a 78 special, I am running K&N pods and straight pipes with internal baffles.

My standard jetting when I got the bike was;

Pilot = 27.5
Main = 135

The bike would run and ride but would struggle at mid range.

I have now kept with standard pilots and moved to a 140 main and moved the needle to the second slot but it runs worse, if I take my pods off and watch the carbs as I rev the engine the carbs seem to have excessive fuel and the engine cuts, my low end is now the problem,

The jets I have are

Mains,135, 140, 142.5, 145, 147.5, 150, 155, 157.5,

Pilots,#27.5 #30.0 #35.0

Can someone please give me some direction of where to go, I have done so much work on this Motorcycle and the carbs are letting me down big time,

Thanks!

 

[5twins]

You're missing a couple important jet sizes you may need - 137.5 mains and 32.5 pilots. On second thought, 137.5 mains would probably be too small for your pipes. When you lean the needle, you need to bump the pilots up. Put the 30s in there. Mains above 145 usually aren't called for. Using K&Ns, you may not be able to even run mains in the 140s. I suggest changing to UNI pods, it will make the whole tuning process easier.

 

[noears]

Thanks,

Im now on a #30 Pilot and a #140 main,needle is in slot #2, with k&n filters on.

It idles well but from idle to 1/4 throttle it stutters and wants to die,

 

[5twins]

Have you re-set your mix screws? They need to be re-set any time you change pilots. Spec for your carb set is 2 1/4 turns out. The larger pilots may require a bit less, like 2 maybe. That's not etched in stone though. Your bike wants what it wants and will tell you so by the way it runs. If mix screw tuning doesn't help, try the 35 pilots. They are usually a bit rich for this carb set but see if they help. Like I said, you're missing the one you may really need, the 32.5. It's just what's needed sometimes for the '78-'79 carb set.

 

[noears]

Yeah I change them each time I try I different set up, wind them all the way in and work my way out.

 

[noears]

How about my float level? Leave them in the second slot?

 

[5twins]

If you mean the needles, yes, leave them in the #2 slot. That's #2 from the top now. Make sure you're counting the clip slots from the right direction .....

 

[noears]

Sorry I did mean the Needles, They are in the 2nd slot down from the top,

My local bike shop have #32.5 pilots and 137.5 mains, so I'll order those

 

[5twins]

You may not need the 137.5 mains with the pipes you're running. Something in the 140 to 145 range will probably be better suited to your needs. However, as I mentioned, the K&Ns may not allow you to run mains that big. The pleated filter element design disrupts the air flow. It manifests itself mostly in the upper midrange, causing break-up and stumbling with larger mains. You may need to go bigger on your mains but the K&Ns aren't going to let you.

 

[gentlemanjim]

I'm tinking that its not a jet issue. bit bad slide diaphrams. I've seen ones with out pin holes or tears not work properly. When you open the throttle do both slides rise even slightly?

 

[noears]

Ok, so I appreciate everyones words of wisdom but I am still having trouble with the jetting and I have a bike show next weekend I need to go to.

I am now running the bs38's with 32.5 pilots and 140 mains.

It idles nice and steady, low and high range are ok but mid range it stutters and runs like crap.

I run Uni filters and internally baffled straight pipes. My needle clip is in the 3rd slot and it doesn't make much difference if its 2nd.

Very frustrated.

 

[gentlemanjim]

If the diaphragm's are good then put the needle in its lowest postion. If that doesn't solve the problem then you need to shim the needle with small wahers to raise it up out of the main jet. I had the same problem with Kawasaki 650 and now it runs clean at all throttle positions.

 

[scabber]

What is the pipe size. 1 1/2 or 1 3/4 it would make a difference. In selecting the proper jets for your system. What do the sparkplugs look like ?

 

[noears]

Gentleman jim by the lowest position do you mean #5 slot?

The pipes are 1 1/2 inch Scabber

 

[noears]

Sorry scabber, Spark plugs are generally black, not sooty and not oily.

 

[5twins]

Your plugs are probably black because you're running larger than stock jets but haven't leaned the needle. Needle position will have a big effect on plug color. Have you checked your float levels? The spec for your carb set is 24mm +/- 1mm. They like 24 (not 23 or 25). The jetting you have should be working for you. There may be something else amiss with your carbs like the float levels I mentioned or maybe the o-rings on the needle jets are shot. There's also the possibility that the needle jets are worn. If so, you would run rich and needle position changes may not have much effect.

 

[scabber]

Black plugs means it is running rich. The float levels like 5twins said is really important. If set wrong it can cause the midrange problem. The 137,5 would be my choice. If you can not get it to run right on the 140.

 

[5twins]

The chokes could be another problem area. If the rubber tips on the plungers have gone bad, they may not be sealing in the "off" position as well as they should. It would be like running on partial choke all the time. Also check that the housing screws are tight.

 

[noears]

Ok, so I am now on the 137.5 mains and it is running better than it has yet.

My needle clip's are in the #2 slot but I'm wondering if I should try the #1 slot, she still has a slight stutter and seems rich

 

[gentlemanjim]

Ok, so I am now on the 137.5 mains and it is running better than it has yet.

My needle clip's are in the #2 slot but I'm wondering if I should try the #1 slot, she still has a slight stutter and seems rich

Increasing the main jet has a similiar effect to raising the needle. My advice is go maybe one size larger on the main jet and raise the needle by putting the clip in the lowest postion (closest to the tip) As I said before it sounds like your jetting was good as the engine idled well and had high speed as well. If a stumble is off idle or mid range it is the needle postion and needs to be raised. Forget about the pipe diameter everything has an effect on running just don't over think it. And be certain that the vales are adjusted, carbs are synced, plugs and wires and good, point/condenser if equipped are all in spec.