Tuning Help

[wacko]

Hey everyone,

I need some advice. I recently did the PMA swap on my bobber project since my charging system was shot. Now the bike is charging great. Starts first kick, idles good. But when I give it heavy throttle when accelerating it seems to hesitate and recover over and over. I don't think it was doing this prior to the PMA swap but didn't have it on the road too much due to the charging problem.

My first thought was the carbs, so I had them apart today and cleaned out the jets which seemed to be in pretty good shape. I didn't have snap ring pliers so I was not able to completely take them apart but cleaned them out as best I could. Put them back together and no difference

So just looking for some other things that may be causing the hesitation??? I am no mechanic but have a manual and am willing to put in the time.

Thanks in advance

 

[5twins]

If you've changed mains and/or pilots, you may need to adjust your needle setting as well.

 

[wacko]

I haven't changed the jets but it looks like the main jets have been changed. The mains are currently 145 and the pilot is still stock at 27.5. I need to pick up some snap ring pliers to get at the needle setting.

Could the hesitation be caused by anything else? Timing, Valves, Cam Chain? Or is it probably the carb just needing to be dialed in?

 

[5twins]

I'm leaning towards the carbs. With a main jet that big, you definitely need to lean the needles, especially on a '78-'79 carb set. This particular carb set came with a 135 main stock, the biggest main ever put in a 650 from the factory. This large main puts the midrange on the verge of being too rich. It doesn't take much of a main jet increase to push it over the edge and you're up 4 sizes.

 

[wacko]

Where would you recommend the needle setting be?

 

[5twins]

Stock setting is the #3 (from the top) or middle slot. Raise the clip to the #2 slot. This will lower and lean the needle setting. If you find it already moved to that slot then you could try the lowest setting or it could be that those 145 mains are just too big. Getting the midrange to run clean is one of the big limiting factors for main jet size selection on these carbs. Even if the bike will cleanly pull a large main like a 145 in the high RPMs, if you can't correct the midrange running through needle adjustment, your only option is to reduce the main jet size.

 

[wacko]

Cool, I will give that a try.

Thanks again 5twins.

 

[5twins]

It appears your carbs have been messed with a bit in the past so when you get inside that slide, make sure the needle and it's spacer washers are assembled correctly. Put the washers in the wrong spots and that will throw the needle height all off, regardless of clip position .....

Properly assembled, your needle should look like this .....