Bs38 mystery hanging idle with 35 pilot

[zoomoto]

hey guys.

Thanks for all the help previously.

Well...as it goes. You fix one problem only to discover more. Here’s my current set up.
•Pamco with e-advance
•PMA/Capacitor
•uni pods
•pandemonium ya mommas (planing on modifying them for mufflers)
•78-79 bs38s with matching internals
•jbm diaphragms
•145 main, 35 pilot, dropped needle 1 setting, fuel screw 3 out, float level 25
•shimmed throttle shaft bushings to eliminate side to side slop.

Bike idles well but is surging and hanging up. Loves to stay at 3k rpm.

Not sure how I’m lean? Those are big pilots.

Thoughts are:
• timing issue without proper tdc mark (advances properly)
•out of sync (did bench sync with bits and waiting on manometer from a friend)
•recently replaced most o rings on carbs. I’ve been through them a lot.

Any input would be awesome.

I picked up a pair of sudco vm34s I wasn’t planning on installing until this winter along with rebuild.

 

[gggGary]

•shimmed throttle shaft bushings to eliminate side to side slop.

That'd be my guess; Can you push the throttle shaft closed on the LH carb to stop the hanging idle?

 

[5twins]

Yes, the butterfly plates are a pretty snug fit in the bores, or should be. The shafts need a little side to side "float" or endplay or the plates can bind against the side of the bore when trying to close fully. If you have the plates out for shaft removal and shaft seal replacement, an important step when reassembling is to center them or they'll bind. To do that, you must install the return springs first. They pull the shafts towards them. If you install and tighten down the butterfly plates first, the springs will pull them into the bore wall and they'll bind.

 

[peanut]

Its not really clear at which throttle position the lean mixture is occuring.
Each throttle position uses a different combination of components and jets/needles etc.
Don't give up on these carbs yet ...a replacement set simply could add other issues.

I had the exact same issues with my first set of BS38s but eventually I managed to completely cure the rising and hanging idle issues.

Could we have a little more info .
What do you currently have your idle speed set to when the engine is just started cold and the choke off. ?
As Gary asked.... when your idle is hanging at 2-3000rpm what happens if you force the throttle shaft stop lever down with your thumb ? does the idle speed come down at all ?

 

[zoomoto]

Thanks for the responses.
Sorry for the late reply.

Bike is really happy at 900. Getting it up to 1100 feels like it takes more revolutions of the screw than it should.

Butterflies are not hanging up, unfortunately. Wish it was that easy. Full return without hang up even with idle speed screw backed all the way out.

Thanks for the encouragement, guys!

Does anyone know average header temp? Bought a thermo gun and I feel I’m getting oddly high readings around 700.

Also. Just randomly lost spark. Was idling the bike and it turned off. Thought it was run out of gas cause tank was super low. Dropped the carbs. Confirmed butterflies and raised needle to middle setting, re assembled and now no spark. So that’s not cool.

 

[5twins]

The 1100 to 1200 RPM idle speed spec is the hot idle speed. It may very well idle at 900 to 1000 when 1st started and cold. That's one of the tricks with these - getting the idle speed set just right so it doesn't stall when cold but doesn't idle too high once warm. When I 1st got my 650, I thought the 1100-1200 idle speed was fast, the motor just sounded too "busy". I turned it down to about 1000 but then noticed the bike would randomly stall at stops. Also, you get no charging output that low, heck, you barely get any at 1100-1200, lol. I use the 1100-1200 spec now and have grown used to it. It sounds normal and right to me now for this particular bike.

 

[peanut]

What do you mean by 'bike is really happy at 900 '?
I take it that you mean the engine will idle at 900rpm yes ?

What do you mean by 'butterflys are not hanging up '? in your initial post you said that your idle was hanging .?........you also said you had a rising idle ... has that been solved now ?

edit I see that 5Ts post has coincided with mine so I'll leave you with 5T as he has forgotten more than I know about carbs

 

[zoomoto]

congratulations ! you've once again managed to avoid answering any of anybodies questions

What do you mean by 'bike is really happy at 900 '?
I take it that you mean the engine will idle at 900rpm yes ?

What do you mean by 'butterflys are not hanging up '? in your initial post you said that your idle was hanging .?........you also said you had a rising idle ... has that been solved now ?

edit I see that 5Ts post has coincided with mine so I'll leave you with 5T as he has forgotten more than I know about carbs

Sorry, reading too many threads. I’ll stop multi tasking.

My cold idle just off choke is at 900. That’s where it settles and idles consistently after first started. Warms up to 1000-1100

5twins mentioned butterfly binding, mine do not bind they snap shut fully and are centered using the loose screw procedure, snapping them close while loose before tightening down.

Went through everything on the carbs again to double check for mistakes, climbing idle is gone. Still slow return to idle and running lean around 1/2 throttle when on test ride. Made progress there. Brought back to garage let it idle for a minute then it died now no spark. So I’ll hunt that down now

 

[zoomoto]

Once I get spark back I’ll Force the butterflies closed and see what that does.

Thanks again guys. I’m in the process of opening a small business and can’t put the time into this that i want for the time being. I’ll keep updating

 

[5twins]

You might want to re-visit your float level settings. The spec for the '78-'79 carb set is 24mm. That would put slightly more fuel in the bowls than 25mm and may help with the problem. Yes, your 35 pilots are pretty big, so is your 145 main, but your open pipes may account for that. In my '78 carbs, I run 30 pilots and 140 mains but I have mufflers.

 

[zoomoto]

Yeah I contacted Pandemonium and they tell everyone with my carb set up to run 35, 145, and needle clip on 4.

Which sounds nuts. I asked to confirm and he did.

I’ll let you know how it goes.

 

[zoomoto]

Made progress!

Floats to 24 mm and set needle to stock position.

Still a little slow to return to idle.

 

[zoomoto]

I pressed the butterfly’s shut and there was still the same slow return to idle.

Would that maybe mean that there is an air leak somewhere?

 

[peanut]

I pressed the butterfly’s shut and there was still the same slow return to idle.

Would that maybe mean that there is an air leak somewhere?

possibly ....... it can be a useful diagnostic test . ..not as some wrongly assumed ,as some kind of a 'kludge' ? permanent solution ........but as a very simple instant diagnostic test.

If with an extra spring attached your idle then returns smartly ... you have pin-pointed the problem in seconds. If it doesn't then you can probably eliminate the return springs or cable binding as the issue and look elsewhere.

Pressing the throttles shut with your thumb is another simple instant diagnostic test . Provided that you have the throttles synced if pressing the throttles shut doesn't reduce your idle then you can pretty much eliminate friction or binding of the throttle cables or your butterfly disks being the issue



Heres another simple diagnostic that I have frequently used to trace elusive minute air leaks . I smear a little vasoline (petroleum jelly) around suspect parts of the inlet stubs and carb bodies to see if that will 'pinpoint' tiny air leaks. Its simple , quick ,extremely effective and simple to wash off once a problem has been found. You could have used it on your throttle seals for example to 'prove' that there was a vacuum leak issue .if I had suggested it I would probably have drawn all the PC brigade down on your thread

..I hadn't realised that you had cut-offs. She sounds good ...not too far off now I shouldn't have thought

 

[zoomoto]

Went for a 45 minute ride! Went well. Didn’t leave me stranded. Still more tuning to do.

Here’s what the plugs looks like

 

[zoomoto]

Feels a little sluggish off the line. A little break up in mid throttle.

If I drop the needle one more setting I will be right where pandemonium suggests for their exhaust. Going to try that.

 

[zoomoto]

I meant raise the needle

 

[zoomoto]

Raised the needle a half step after lean plug reading and lack of power in midrange.

Bike comes off the line much better but now there’s breakup in the midrange. Return to idle is snappy and there is no creeping.

Thinking about raising floats to 23mm??