spring height

[dirtymike]

I have two sets of BS38 mikuni from 1976-1977 era. I have managed to make a good set from the two. I ran across a difference in the diaphragm spring height. The one i have been running is the 4 5/8. The other two measure 4 3/4. This doesn't appear in my SM. I think the 4 5/8 was a standard for 1976-1977. Opinions, facts. Thanks Dirty

 

[gggGary]

An 1/8" difference? Just use a matched pair and no worries, would be my thought.

 

[5twins]

Yes, I'd say the difference is due to one pair being a little sacked is all. I have encountered different spring lengths on other carbs for other bikes but the difference is much more pronounced, like an inch or better (750 and 920 Viragos come to mind).

 

[dirtymike]

Thanks for the input, picked the best parts and got a nice working set. Didn't figure that little would make a difference. thanks Dirty

 

[DogBunny]

I hate to differ with gggGary, and I really hate to differ with 5twins, especially when it comes to carbs, but 78-79 uses the "new style" slide, and 70-77 uses the "old style" slide, and the springs are different. At rest, the diaphragm spring for 70-77 is approximately 4-5/8' long, while the 78-79 diaphragm spring is approximately 4-1/8" long at rest.
The parts fiches show 256-14933-00-00 for 70-75, 584-14933-00-00 for 76-77, and 2F0-14933-00-00 for 78-79.
EDIT: Okay I just read the whole thread.
Perhaps there is a third length, the 4-3/4" that dirtymike measured, which would be for either 70-75 or 76-77. Anyways, in this case, 5twins was right, as usual, gggGary too, just use a matched pair.
Sorry for the huge time waste, but I have to admit that I've always been a bit obsessed with the different spring lengths...

 

[dirtymike]

Anybody know why there is different spring height. Pro's or con'. Thanks Dirty

 

[5twins]

The slide was redesigned for the '78-'79 carb set. It's bottom extends higher up into the slide which probably explains the need for a slightly shorter spring. I think spring length (and strength) is chosen as soft enough to give good throttle response and not hinder slide lift but strong enough to insure the slides close back up and don't hang open.

On the Viragos I mentioned, the 920 carbs had longer and (no doubt) stronger springs. Other than that and larger jetting, they were pretty much identical to the 750 carbs. I suppose the greater vacuum "pull" through the 920 carbs generated by the larger motor warranted that.

 

[dirtymike]

So the spring is the mechanical means of closing the slide. Stronger the spring the quicker the slide closes, weaker the spring the quicker it opens. Correct. Dirty

 

[5twins]

Pretty much so, yes. I've never tried the shorter '78-'79 springs in a '77 or older carb set but it might make for an interesting experiment. Maybe it would improve throttle response and quicken slide opening a bit. As long as the springs were still strong enough to close the slides properly, it might make for a neat little mod.

 

[dirtymike]

Both set of springs came out of two sets of 76-77 BS38's. The air hole config on the induction side (12,4,8)O'clock. The springs that i have in reserve are 4 3/4. that's a 1/8" longer than SM for 1976-77. Maybe the PO placed them in there for some reason. Thanks Dirty

 

[5twins]

Part numbers can be deceiving. Sometimes the number changes but the part does not. Sometimes the part changes but the number doesn't. In this case, only the first 3 digits change and many times they are just a model designator. But it could indicate the part is different as well. There's no way to really tell just from the number. You would have to compare the actual parts. If there were differences in the rest of the number string then there is a better chance the parts are all different. With just the first 3 numbers differing though, who can say.