Narrowing down air leak 1978 BS38s

Pete.....................you might have a Craftsman brand that was sold by Sears here in Canada. I have had my Craftsman hand impact tool for 43 years, and it's a high quality tool. As soon as I bought my first Japanese motorcycle, a motorcycle buddy of mine told me to buy one, and he was so right!
 
Update. Went for an awesome ride yesterday! I greased the advance rod and I clipped the springs on my advance unit and that seems to have fixed the issue of the idle hanging, although I guess the carbs could have just been way out of sync. I did have to turn the sync screw out around 2.5 turns and then turned the mixture screw on the left carb out around 2 full turns. Seems like that was pretty far off and it felt like I was still compensating for some kind of air leak still. The left side has pretty much always been the issue.

Anyway it runs pretty good, I only get pops when I'm going along at a good speed and I let off the throttle.. between 3k and 4k rpms. Going to do some more messing around today.
 
Last edited:
The factory recommended setting for the mix screws on your '78 carb set is 2.25 turns out. But these factory recommendations are just a starting point, a "ball park" setting that should allow the bike to run and idle. Best setting should be close to that but may not be exactly that. Since you've added pods, probably have different mufflers as well, and didn't change the stock pilot jet size, your best setting will probably be more in the 2.5 to 3 turns out range.

The usual jetting routine for the '78-'79 carb set and mods like yours is about 2 up on the mains, needles leaned a step, and 1 or 2 up on the pilots. Stock pilots should work OK as long as you haven't changed the needle setting, but once you do that, a larger pilot will probably be needed. The thing is, the needles on your carb set pretty much always need to be leaned a step with any main jet size increase, even just 1 size.
 
@5twins thanks again. Your replies all over this site have been super helpful to me as a major newbie. I re-read the car guide again and I think I'm slowly starting to understand jetting some, although I think I just need to wait for the additional jets I ordered to come in so I can start playing around more and get a feel for it. Right now I'm only one up on the main, 137.5.

I guess what I was trying to say earlier is that I'm still suspecting an air leak or something out of the ordinary on the left side. I ended up getting carb sync gauges to double check my sync work, I was pretty close but I adjusted a bit more and they appeared to be in sync. I started with both idle mix screws 2.25 turns out, then the left side still sounds like it's bogging, kind of spitting, not nearly as rhythmic as the right side. So, on the left side I turned the idle mix screw OUT another 2 full turns. This seems to help and the bike will ride and run ok. From what I understand though, that is richening the carb quite a bit and in the carb guide I read that a turn or so away from the recommended setting usually indicates some issues. Do you think there could still be some sort of leak (throttle shaft seals or elsewhere) or could this be because I have switched to pod filters?

The left side on this bike has always seemed to be the issue, even when I first fired it up with the stock airboxes (even though they were falling apart).

One other issue that I'm not sure is related or not is that in first gear if I get up to 3500 - 4000 RPMS the bike totally loses power and I have to shift up to second to continue riding, this is when the bike is plenty warmed up. It doesn't ever recover until I shift up to second (lean or rich? probably lean) Another issue is that after riding for awhile (bike is warm) if I come to a complete stop (sometimes at a stoplight or just pulling over) the engine will shut off and will have a hard time starting again. I feel a bit at a loss in terms of diagnosing some of these, can you point me in the right direction?

Cheers
 
It does sound like you still have some issues with the left carb. Yes, opening the mix screw makes the idle circuit richer, and you shouldn't need to have it 2 turns more than the spec even with pods. If both carbs required that, I might think jetting, but only one points to a problem with that carb. You said you cleaned them but I'm thinking maybe not well enough. I think you may have a partially plugged idle circuit on the left carb still. There's more to cleaning the pilot circuit than just the jet. You have to clear the passageways that lead from the jet into the carb's main bore, the delivery path for the fuel/air mix. As a possible "quick fix", you might try this reverse flush method .....

http://www.xs650.com/threads/revers...bs38-carburetor-pilot-circuit-idle-jet.42002/

But the best method is blowing cleaner through the passageways with compressed air. I've outlined the procedure many times before. A little searching should turn one of those threads up.
 
@5twins by looking at those posts I think you might be right, I may have missed cleaning parts of the idle circuit. I'll get the carbs off and report back. Thanks
 
@MaxPete @5twins It seems as though the throttle shaft seals may have been the culprit all along. I had ordered 2 seals from boats.net awhile ago but I had figured it was the carb holders and was afraid of stripping the butterfly screws so they still haven't been replaced (I also now realize I should have ordered 4, so I ordered two more). I replaced the boots on both carbs and it was making a pretty loud chirping sound still. I thought maybe it was still the boots but it was tightened down pretty good and theres no way the new boots were leaking with those gasket between the intake manifolds. I used a tube to hear the leak (chirping sound) and then sprayed carb cleaner between the carbs near the inner shaft seals and I'm getting an idle change and a change in sound.

So now I have the JIS screwdriver set, ordered more throttle shaft seals and will attempt to replace when they come in. Is there anything specific I should be aware of when replacing or is it pretty straightforward?
 
H Moebius: it sounds as though you have the problem on the run but 5Twins is really the carb man here - I am a total amateur, so I will defer to his advice.

Cheers,

Pete
 
So I was changing my throttle shaft seals and it looks like I have a leftover part... a really thin washer. I'm guessing maybe it goes on the throttle shaft next to the seal, anyone know?
 

Attachments

  • IMG_3481.JPG
    IMG_3481.JPG
    167.1 KB · Views: 217
TwoManyXS1Bs said:
That looks like the thin springy washer that goes behind the choke lever, or the bowl drain bolt washer...
Click to expand...

Hmm, it's not the bowl drain bolt washer those are sort of reddish in color on mine and I'm pretty certain I put those back on, I took the choke pieces off the body but I didn't completely disassemble the bolt and choke plunger... is it not a part of the throttle shaft assembly??
 
I think 2M nailed it, the shim washer on the choke lever assembly. It fits on the stepped spacer, between it and the choke lever .....

Qge0dBM.jpg


I measured one and it's the same size as what you have there. Also, I looked at some spare linked sets of 38s I have and don't see any thin steel shim washers on the butterfly shafts, just the plastic spacers.
 
@TwoManyXS1Bs ohhhh i see what you meant now! I think I was thinking of the choke plunger assembly. thanks @5twins. I put that back on and the throttle shaft seals have been replaced. Carbs back on the bike and it runs, doesn't seem like there is an air leak anymore at first glance but I still need to sync the carbs and adjust the idle.
 

Similar threads

7
'77 XS360 flooding on left side at wide open
Replies
5
Views
130
74Torino
7
ForestHasAnXS650Too
New-Old XS650 Special journey - My initial problems
2
Replies
22
Views
559
jetmechmarty
jetmechmarty
O
Yeah I'm an Old Rookie Running without carb manifold gaskets for 2 years
Replies
9
Views
832
Kevin Werner
Kevin Werner
johne67
Exhaust Sucking In Air
Replies
13
Views
1K
Slippery
Slippery
fenbrancis
One Cylinder Just a Touch Too Rich
2 3
Replies
40
Views
2K
dps650rider
D
Back
Top