BST38s hanging revs

[Upshotknothole]

So I think I first noticed this a few days ago, but yesterday when doing about a 3 hour ride I really started to notice it. When shifting especially, the revs are hanging and not dropping when rolling off the throttle slightly. Usually up around 5-6K when doing "normal" city and back roads riding. If I let go of the throttle and let it snap shut, it's fine. Adjusted the throttle cable in the garage earlier and pulled the throttle apart, everything there seems normal. It's not an air leak as I've dealt with those way too many times and it returns to normal idle if I snap the throttle closed. I'll be pulling the tank off tomorrow and looking into it.

Maybe a sticky throttle cable, though it doesn't feel bad. I can try lubricating it tomorrow when the tank is off. Anything else it might be? I'll check the throttle linkage too. They're CV carbs so not really sure what could cause the revs to hang.

Thanks, just looking for ideas before I tear into the bike tomorrow. The bike is a full fairing 750, and there's so much factory crap under the tank, I wouldn't be terribly surprised if something was just interfering with the throttle.

 

[george 1100]

Sticky throttle, vacuum leak, lean pilot a good place to start 

[Upshotknothole]

3 minutes ago, george 1100 said:

Sticky throttle, vacuum leak, lean pilot a good place to start 

Cool, I might see if I have a spare throttle cable, otherwise I'll give this one a good lube and give the carbs a quick going over.

[Captain Chaos]

I'm having the same problem now with my B12 engine and carbs. Roll off the throttle, revs drop slowly. Snap it shut, all ok. With warm engine only. Had been OK for years but a few weeks ago this problem appeared.

Tried new spark plugs, a new throttle cable, no difference. Tried without throttle cable, no difference. Ultrasonic cleaned the carbs and reassembled carefully, no difference.

Now going to adjust the valve clearances (weather permitting, it's too nice to work on the bike, better go for a ride on one of the other bikes ) and see what difference it makes.

 

 

[Upshotknothole]

Glad to know I’m not the only one, and I can still ride the bike but it’s fucking distracting shifting gears. Makes me actually use the clutch to shift. Gonna pull my carbs apart in the morning, if I have to do the valves, I’m gonna drop in an 1127 and rebuild the 750 later. 

[Captain Chaos]

Oh yes and on my bike the spark plugs 1&2 are good, 3&4 are black, like it's getting too much fuel in those cilinders. Carb clean and rebuild didn't help. That's why i thought it might be a compression/valve clearance issue. 
 

Been using the bike for 8 years with only oil changes so it's about time i do some maintenance. 

[Captain Chaos]

 

[imago]

I've had this issue in the past and done the things listed above with no joy. I also noticed that in the cooler months it doesn't happen unless the engine gets really hot, sitting in traffic for example.

In the end I just put it down to one or both of two things. Firstly when it's taking warm air into a warm engine it's enriching the mix slightly, which may be just enough if it's at the rich end of acceptable mixture on pilot/low rpm. The other possibility that occurred to me was a slight tightening of the slides and/or the butterfly pivots.

I think the first is the most likely so maybe try a 1/4 turn on the mixture screws as a first easy/cheap thing to try.

[Captain Chaos]

23 minutes ago, imago said:

maybe try a 1/4 turn on the mixture screws as a first easy/cheap thing to try.

Tried it, no improvement.

Gonna report back after the valve clearance check (could take a while though, hot and sunny weather here. And a free day )

[Swiss Toni]

It couldn’t be the enrichment plungers on 3&4 not coming fully off, could it?

[Captain Chaos]

5 minutes ago, Swiss Toni said:

It couldn’t be the enrichment plungers on 3&4 not coming fully off, could it?

Checked that as well. Working in the prescribed manner. 

[Captain Chaos]

First i had a Dame Blanche 

 

and then I checked the valve clearances. All within spec. Only one intake valve on cil. 3 was slightly tight, adjusted it closer to the upper limit (0,15). 

now going to reassemble the thing and test ride it. Weather and beer permitting of course. 

[TonyGee]

3 hours ago, Captain Chaos said:

I'm having the same problem now with my B12 engine and carbs. Roll off the throttle, revs drop slowly. Snap it shut, all ok. With warm engine only. Had been OK for years but a few weeks ago this problem appeared.

Tried new spark plugs, a new throttle cable, no difference. Tried without throttle cable, no difference. Ultrasonic cleaned the carbs and reassembled carefully, no difference.

Now going to adjust the valve clearances (weather permitting, it's too nice to work on the bike, better go for a ride on one of the other bikes ) and see what difference it makes.

 

 

it could be the float valve tips are slightly worn and weeping a wee bit, or maybe the O ring on the valve seat. ive seen the O rings go hard over time and start to crack. 

[Captain Chaos]

18 hours ago, TonyGee said:

it could be the float valve tips are slightly worn and weeping a wee bit, or maybe the O ring on the valve seat. ive seen the O rings go hard over time and start to crack. 

Can't think of anything else at the moment. After my holidays i'm going to order new valves and seats. Any recommendations? Original Suzuki cost €60. EACH. O-rings are almost new, replaced a few weeks ago.

This morning while it was still not too hot (at 8:00 it was already 23 deg. C) i went for a ride. The hanging revs have almost disappeared, but it's still there. And now plug #1 is black instead of 3 & 4. 

 

[Captain Chaos]

why is the fucking pic upside down?

plug #1 is on the right. 

[TonyGee]

4 hours ago, Captain Chaos said:

now plug #1 is black instead of 3 & 4. 

feck thats a strange one !!!!  is their any chance you have picked up some dodgy petrol and the valve tips have some crap on them ? 

[Captain Chaos]

Same petrol as always, from one of the larger servos in town. My other bikes and car have no problems with it.

I think the only solution is turbo and EFI.

[Captain Chaos]

For clarity, while I checked the valve clearances the carbs were in bits again and I reassembled them very carefully, checked and cleaned everything, adjusted float levels again, the whole lot. Left the fuel tap on PRIME for half an hour and no fuel leaks anywhere.

Then I went for a ride and now plug 1 is black.

I think anyway I will replace the float needles and seats. Any suggestions for which to buy or avoid?

[TonyGee]

2 hours ago, Captain Chaos said:

Any suggestions for which to buy or avoid?

we know OEM are probably the best if you can afford them   but their are some after market Japanese made parts that seem to be ok, but cant remember the name of them right now   

[TonyGee]

the name just come to me, "Tourmax"  their made in Japan.  Ive used them a few times with no problem so far !!!!!  

[Captain Chaos]

Ah yes Tourmax. Bought them once I remember now, they were good.

[Captain Chaos]

@Upshotknotholesorry for the thread hijack from hell.

[Upshotknothole]

No worries, we have the same problem, and definitely worse on hot days. 

[Upshotknothole]

Worn out throttle cable. As soon as I noticed that if I turned the bars right to full lock, the revs went up, knew what it was. Made the mistake of deciding to fuck with it last when I got home from the bar, and discovered that there’s only one adjuster nut on the end of the cable. 

 

Tried fitting another cable that was handy, but it was too long. Gonna screw around with it this afternoon and see if I can botch something together while waiting on a new cable…or I’m just gonna throw on a spare set of RS38s and see what happens.

By the way, I fucking hate working on stock bikes. So much extra crap, and I could have sworn I had a hose on the breather before…

Edited July 7, 2022 by Upshotknothole

[Upshotknothole]

Reassembled everything, same problem. The throttle cable is loose in the damn switch housing. Bolt and clamp that hold it in place are snug, I think the little hole that it sits in is worn out and indented. Gonna try swapping out the switch gear, can I use washers to keep it from moving around?