Finally swapped carbs.

[XSLeo]

I was having some issues with the BS38's I was running. Occasionally fuel leaking, off idle stumble. Low mileage.
I finally swapped them for a set of BS34's from the parts bike.
The jetting is all stock. 132.5 mains, 42.5 pilots. Air screws set 3 1/4 out. The same mods as always, 750 kit, cleaned up ports on head, Unifilters, Mike's 1.5 headers, Emgo shorty mufflers.
Runs very well. Needs very little choke, idles well. no off idle hesitation. It doesn't pull as strong as the BS38's, I think I need to go up one on the main and shim the needle. It has a very slight hesitation in the 7/8 to full throttle area. Milage about 57 mpg on the first tank full. I was running it kinda hard to test the jetting so it might be better on a regular ride, know better next run.
After I got the BS34's on I was looking over the BS38's. I think one of the diaphrams is bad. When you lift the slide, plug the hole, release the slide the left side stays up, the right slowly lowers. About 5 seconds. Between that and the bad floats or float valves could have been my running problems.
I know that the BS34's use the large round main jets, can the small round main jets be used just for testing? I have plenty of them. Once I find the size the bike likes I can buy the right ones.
One other thing, the 75 steel side covers won't fit on the bike with the BS34 carbs. Guess I'll have to mod a set.
Leo

 

[littlebill31]

I have not used the small jets for the main. The air jet is a small main jet and I think the thread is the same. The only thing I could think would be them being too small, diameter wise. Thus leaking around the main jet washer that is between the main jet and needle jet.
I think it would work though.

 

[XSLeo]

Thanks, I was trying to see if some one had tried the small jets before I pull the bowls and try myself. Hate to go that far and find out they won't.
Leo

 

[littlebill31]

I think they would though. I just went out and the thread is the same. The jet size is the same on the large diameter and small. i.e 135 on a large diameter is a 135 on the small. And the jet tightens nicely against the washer, sealing it against the needle jet.
The washer is thick enough to maintain a good seal.

 

[XSLeo]

Thanks, That helps alot. I have the small round from 127.5 to 137.5. I'll start experementing and see what happens.
Leo

 

[grizld1]

Mikuni small round mains meter identically with the large round mains and can be substituted with no issues.

 

[scabber]

Try the 137.5 and 45 piolet should get you close.

 

[littlebill31]

Thanks grizld1. I was waiting patiently for Leo to test it 'cuz I've been thinking about it since he posted this.

 

[5twins]

Starting in '78, one of the big improvements to the 650 carbs was the spring-loaded needle. This keeps the needle position consistent. Earlier carbs had a big plastic washer over the needle and there could be as much as a half MM up-down play between it and the needle clip. That means your needle height could bounce up/down and change by 1/2 MM or half a clip notch during operation. Not a very accurate set-up in my opinion.

 

[littlebill31]

The first time I took one of them apart, and saw the free-floating needle, I thought I lost a piece. It didn't look correct to me. You have to make sure the seals and gaskets are all working properly and not leaking, yet here is this needle just flopping around.

 

[bluebikerblan]

Leo....you mentioned some concerns about the diaphragms, one being bad etc. Since you're seeing signifigant difference in function, it may well be time to give the JBM replacement diaphragms a shot? If you do so decide to rehab the BS 38's, replace the old diaphragms with a set....not just one. I've been hearing a lot of good things as to high end performance. Stands to reason as well that such would be the case with CV's.
Best, Blue

 

[XSLeo]

When I work with the BS38's I planned on new diaphrams as well as new float valves, new needles and needle jets. Those carbs are 76-77 bodies with brass from a 74 carb set. It was confusing at first, but once I figured out what parts were in the carbs things went much better.
Thanks for all the advice.
Leo