Weird vacuum test on carbs

[Lachie]

Hi ok there lots of this stuff around but I'm getting a weird test result when I'm checking for a suspect Vacuum leak.

'89 750 btw c/w - BST36's

Ok so Just rebuilt carbs new slide guides standard jetting one float not right ok set that.

Now take her for a ride and revs hanging on off throttle, not able to set idle properly misses on low speed accel, stalls.

Ok sounds like a vacuum leak can sort that had a bit of trouble lining up all the old boots........... could be.

Get propane bottle and hose run let the bike warm up and only once did it look like the revs rose but that was nearer the throttle linkage, couldn't get it to do it again however every time I stuck the hose near here the revs died



and if left there would stall the bike
tried and tried again same result revs die take away and back up to idle she went. so I'm a bit confused by this cause by all accounts a vacuum leak and propane the revs should go up .

Hmm tried it again and maybe more noticeable around the choke slide but nothing if I sprayed it with wd40,

If I put some pressure on the Choke plunger it also dies so wondering if the plunger can get worn to the point of causing this problem?????

I wouldn't think that was right either if it's drawing air in should rise in revs???????

Any body got some ideas before i strip it down before I tried the same with WD40 but couldn't get it to do anything

Before we go too far all I changed was the slide guides, needles emulsion tubes and Mains (up one from 102.5 to 105) I've added a 4-2-1 so trying out the jetting.

All pilots and other settings are exact like before and had no problems at low engine speeds before.

Actually had the opposite problem with the worn slides too rich and missing at WOT........

[steves]

It's very possible the boots that the carbs fit into on the motor side are worn out, or the orings on those boots are worn. I spent more time that I'd like to admit chasing a vac leak that was caused by the boots.

[Kreylyn]

You have two plastic caps right there that could be a vacuum leak. Take off the caps, verify the diaphragms are properly seated, and make sure the caps have a flat smooth surface.

Krey

[moto]

Get propane bottle and hose run let the bike warm up and only once did it look like the revs rose but that was nearer the throttle linkage, couldn't get it to do it again however every time I stuck the hose near here the revs died



and if left there would stall the bike
tried and tried again same result revs die take away and back up to idle she went. so I'm a bit confused by this cause by all accounts a vacuum leak and propane the revs should go up .

The rpm would only go up if the mixture was lean before the propane was wafted near the leak. If it was correct with the leak, then the propane might make the mixture go rich, causing the behaviour you describe.

If I put some pressure on the Choke plunger it also dies so wondering if the plunger can get worn to the point of causing this problem?????

Are you applying axial or radial pressure?

All pilots and other settings are exact like before and had no problems at low engine speeds before.

Does this mean that fuel screws and sync have not been adjusted?

Regards,

Derek

[Lachie]

Just for Masochistic fun I'm going to post this.

The problem:
BTW the bike has done 100,000 K (@60,000 Miles)

So the bike was not pulling properly mid to high rpm badly missing etc etc
So I pulled the carbs apart and find white stuff all through under the diaphragm at first I thought this might have been dried grease or such but searching through interweb and find it's due to the slide guides worn and on inspection the needles were badly scored etc.

OK new guides needles emulsion tubes Mains 105 up one from my original jets and all apparently matching the OME spec for this Carb

Pic of worn slide guide there's a post around here somewhere about this but just for comparison



The extra knobs on the top help stop the slides wearing through the bottom of the slide as circled.

Ok all new parts back in

#4 butterfly was open so quick sync on the butterflies all seem pretty good.
A/F screws 1,2 &3 a little leaner than "normal" at two turns #4 at 2.5 (Could it have been higher due to the butterfly ??? rhetorical question)

Since the low speed jetting had seemed pretty good and #1 cylinder seemed to show good colour on the plug and was also the least worn of all, set them all to 2 as a base line
(ok I can hear everyone saying 2.5)

Put it all back together
FFS (For F* sake)
Just tweaking up (seriously not a lot of pressure here) the accel cable and

Snap!



Hmm looks a bit rustic but works ok



So now we're up to were I started the thread.

(Derek)
If I pushed on the slide in the direction of how it moves then the pressure was axial on the plunger

Taking it all apart again and checking everything in reverse order, I'm going to start this all again and properly go through the low speed side

The enrichment slide was really not closing properly and I'm thinking at least #1 and 2 circuits are slightly open since I hadn't taken these out possible stuck with some crude???? or is the slide bent ?? will have a good look through this

(Steves)
All boots fitted and snug, check

Right I start taking these apart to go right back to whats suppose to be a standard setting and check through all the pilot jets and circuit again I did a superficial check before but will try it a bit better.

FFS !!!!





Ok I'm really not liking this now these carbs are so tender, is it just cause they're 30 years old!!!!!

(BTW this was under the choke cable attachment maybe an idea to put a washer under the wire bracket as maybe the pressure had caused it to split.)

With the carbs off now all the enrichment plungers snap back quite well so I'll give them a bit of lube and clean out and make sure they follow suit when all back together.

Now I understand the basic principles of carburetor devices but trying to understand how all the bits work together is a bit mind boggling.

Would the diaphragm cap been leaking? hmm don't know. I would have suspected when I sprayed the area including the enrichment plungers with WD40 then something would have happened. Even if too rich and dying as Derek has suggested ( which sounds plausible). That is if there is a leak there?!? the broken lug is on the otherside of were the test resulted in a change to idle.

So this is the big question really,,,,,,, one test (propane) says there's a leak but the specific test (WD40) can't find it??????

I guess there should be a vacuum above the diaphragm by air being drawn across the two holes adjacent to the needle at the bottom of the slide. Otherwise how else would the needle/slide be lifted, as there is no other connections above the diaphragm, but don't know if the broken lug would have caused this it was all basically in place until I undid the screw! The plastic cap really only just rests on top of the diaphragm outer ring with just the two screws holding it down, so it doesn't really seem to be a great secure seal??? Maybe - perhaps.

Interestingly air enters the underside of the diaphragm from the bellmouth and also there is a connection to the enrichment circuit on the otherside.



and it would be really interesting to actually know what these did. I guess (my theory) is that the air going through the enrichment plunger helps draw the fuel out of the bowl. At only just off a seated position the air inlet from the diaphragm to enrichment side isn't open though. But when open would draw air from underneath the diaphragm (actually lowering the air pressure under the diaphragm) perplexing indeed!!!

Found this and is quite interesting to explain CV carb 's

http://advrider.com/index.php?threads/the-bst-40-bible.347184/

So by this at idle the pressure would be equalized either side of the diaphragm (e.g. not lifted) so there shouldn't be a vacuum on the top side of the diaphragm at idle.

I like slide carbs better they do seem a lot less complicated

Anyway,

So the plan is now to go back through and recheck all jets and settings etc and set A/F to 2.5. Eventually I will then Vacuum Sync the carbs which was the original plan to start with but it should be running nearly right before then. Hopefully!

Here's a good game - drop the little white plastic washer that sits under the needle clip on your garage floor - then time yourself how long it takes to find it again LOL

[Lachie]

So I've been looking through all the online info about CV carbs to get a better understanding.


Factory pro suggests a lean idle mix can cause the same symptoms as I've had hmm! still does not go near my leak test though.


Interestingly no info about setting the idle adjuster screw,,, in setting the idle and it would appear the CV carb should idle with the butterflies closed?


and should run and idle on the pilot and A/F jet.


Any comments on that one???


If so, I should probably look at upsizing the pilot jet.

[92xjunker]

The throttle plates are not closed at idle. That's why a bench sync is done at the middle of the first hole. Your stalling issue is due to the propane cutting off the oxygen needed for combustion. A lean mixture will cause multiple issues from high idle to popping threw the exhaust and flat spots in excelleration. As far as changing the pilot jets... I tend to think " it was designed to run on this size jet so, it isn't the problem" a clogged passage or jet may be the issue.

[Lachie]

Yep your right I was just musing info from the net. Sort of like my friend self diagnosing she was dying LOL

Any road I had also thought I had 32.5 Pilots but in actuation I have 37.5's hmm shouldn't jump to conclusions on these things it's not the first time as there is no rhyme nor seems reason to the jetting in various countries spec'd carbs. i.e. I had gotten 105's (mains) thinking they were standard for my carb based on the needle number vs country code vs handbook but what were in there were 102.5's

I did plan on putting it all back together tonite I had gotten some carb cleaner to spray through all the little orifices and found something interesting.

So in my first post i wondered if the enrichment plungers were leaking so I did this test.



Now I'm not sure (unless the pressure from the spray can is lifting the plunger) but I don't think this should be happening??????

It did not do it on #4 so I would say not.

3# &#1 were the worse with #2 following not far behind. Tried a few times #4 no,.... others yes,.... but it did drop off (maybe because the spray bottle was getting low or maybe getting better?????

#1 plunger not much to see



Compared to #4 (on the left) still not a lot visible difference but under a magnifying glass does look like a deeper groove in #1



#4 Retainer is showing signs of old age also



From all that's happened I don't think I could trust that they don't leak so off to get some more parts hmmmm!

I hope nothing else breaks cause I'm already investing a lot of money into an old possibly just worn out set of carbs, for a couple of hundred extra I could have gotten a brand new set of RS smoothbore Mikuni's to bolt on!

[Lachie]

Ok then I guess thats not happening



I re did what I did before and looks like the plunger is lifting with the pressure so hopefully just replace all the old rubber bits and see!!!

[moto]

Any road I had also thought I had 32.5 Pilots but in actuation I have 37.5's hmm shouldn't jump to conclusions on these things it's not the first time as there is no rhyme nor seems reason to the jetting in various countries spec'd carbs. i.e. I had gotten 105's (mains) thinking they were standard for my carb based on the needle number vs country code vs handbook but what were in there were 102.5's

Austria, California, Singapore, Switzerland and USA came with 32.5 pilot jets. All others came with 37.5. '95-'97 Germany came with 107.5 main jets. '89-'94 Sweden came with 102.5 main jets. '89-'97 USA came with with 105 on cylinders 1, 2 & 4, and 110 on cylinder 3. All others came with 105 main jets.

Compared to #4 (on the left) still not a lot visible difference but under a magnifying glass does look like a deeper groove in #1

The cold start enrichment slides in these pictures look fine.

#4 Retainer is showing signs of old age also

I would replace the cold start enrichment slide shank seals.

I hope nothing else breaks cause I'm already investing a lot of money into an old possibly just worn out set of carbs, for a couple of hundred extra I could have gotten a brand new set of RS smoothbore Mikuni's to bolt on!

That may be, but Keep in mind that they will come with their set of exciting challenges.

Regards,

Derek

[Lachie]

Yep cheers
Thanks for the feedback, Just really didn't know what the plungers were suppose to look like but that gives me better confidence.


Sweden country code is E17 I believe but when going through the parts catalogue my country code setup E27 or maybe even E24 and is a different set up so don't know how I ended up with the 102.5's


Anyhow the main, emulsion tube, needle & pilot setup should be the same as the UK spec bike now and will try setting the A/F at 1 &3/4 to start


I'm going through everything that could be a possibility and replacing any thing dubious like the enrichment seals (and the little rubbers in the pilot jet towers, I did have a sneaky thought to fill in the transfer port ALA GSXR but that's probably not a good idea the air jet is probably sized accordingly and who knows what else they did to make this work)) so hopefully I won't have to end up dealing with "a new set of Challenges" LOL

[Lachie]

Last update

So I've spent the last couple of weekends and odd times annoying the neighbors tuning the bike.

So whats wrong with revving a bike at 11:00 at night some people just don't have a sense of humor........... Meeh!

Replaced the choke slide rubbers and the little rubbers in the bottom of the Pilot jet tower (to be honest even though the ones in there were not very snug and fall out by themselves It didn't seem to change anything)


So the carb setup I've finally got is 5EZ61-3 needle jet, 105 Mains,37.5 Pilot jet.

But it's still not 100%

It'll pull WOT great

Dropped the needle one notch to eliminate the stutter in the midrange great

Next was to try and sort the pilot started at 1 3/4 as per the manual for this jetting and slowly worked my way up cause that just wasn't working had massive low speed stutter.

I thought the floats were pretty right I had checked them before putting it back together but when I finally got to 4 Turns out on the pilot jet rechecked them again

Btw I resync'd the carbs every second time having them out with the Manometer

The setup - Start



Getting better nearly there can spend so long tweeking this (why is that middle screw at such a hard to get angle??!!!)



So rechecked the Float heights and I don't think I must have done it right the first time and ended up adjusting them all to around 14.5 mm which was about a mm lower then were they were set so this should make the bike a little richer. Put the pilot down to 3 1/4 and was much better but needed more.

Down to 2.5 and was way worse so ended up on 3 turns out and pulls really nice when warmed up.Great

I ended up syncing the carbs at 2250 revs I had started out just doing it at about 1500 revs as I had read you should just do it just above idle but decided 2250 was probably a better place to do it. I don't know but anyway.

The Factory Pro tuning says you should tune the low end for smooth idle and 4k 2nd gear operation which it does, in fact it goes great at any acceleration.

So now it is just at 5th or 6th gear cruising it has a little lag at 4,500 to 5,500 revs roll on and if I slightly drop the throttle a little feels like it's starving which is more just annoying then anything.

Having spent so much off time trying to sort the tuning I've just decided to get it back on the road proper and ride it for a while maybe if it gets to me too much or when i get bored I'll rip it apart again and try the float at 14mm i'm thinking it maybe still be a little lean in that area or maybe play with the needle again to see if raising it back up cures the lag in the 5-6 gear range midrange cruise state, but is accelerating so great in all other ways, I don't want to stuff it up .....

Postscript:

cause it was quick and didn't take a lot of disassembly I lifted the needle back to the original middle position and it fixed the leaned out midrange but somehow the lowend was bogging down. So i relented and took it all apart again. Went over the float heights spent a bit of time on this and made sure they were all at 14.5mm turned the PA screw down to 2 3/4 and vacuum sycn'd the carbs at 1750 this time.

Now it pulls absolutely great through all the rev range and there is just the slightness hesitation @ 5000 in 5th and top at cruising speed which i can live with ATM. Some thoughts on this is either the pipe has a little flat spot around this range which is why it's been difficult to tune this bit out or maybe I could go one size up in the main as it's just struggling a bit just on the needle.

Oh well that's maybe a challenge for when I really get bored LOL

[Lachie]

Yeah Nah! after daily riding it for a week I couldn't live with it.

But I did realise it wasn't a rev range that was the problem it was the throttle position.

At 1/8 to 1/4 throttle it's buzzing between the pilot the float and the needle.

So thinking maybe doing some more tuning around this area but where to start first or do I just suck it try a bigger main jet and start it all again.

Hmmm, Factory pro suggests you shouldn't redo the needle position after the float and pilot are set but... hmm it is the easiest thing to try.

I had previously set the needle on position two to get of the overly rich flat spot out but then gone back to the middle after resetting the floats and pilot.

Decided to try it on position 4 to get the needle further out of the jet at that throttle position and it worked. Take her for a blast now and it's sorted at all rev ranges and throttle positions
sensational finally ............. Well it's been fun learning the relationship between all the bits but glad it's finished... I hope fingers crossed......... I did take her for a good blast and lots of different riding situations and looking feeling great