BS38 carbs pod filters effect on mixture

Just a thought.... how about removing the throttle levers from the carbs and winding the throttle return spring (around the shaft seal) one more revolution tighter? It might hold them closed better. :shrug: :twocents:
 
If it does require 3/4 of a turn out to idle right then maybe the P.O. already changed pilots, and to a size that's too big. But we'll never know since you won't check the sizes. Good luck getting it sorted.
 
Peanut, hang in there...

Your description of the idle change after pressing on the throttle shaft and observing your videos leads me to believe that you may have worn throttle shaft bores in the throttle body, an issue I've not seen discussed here in excruciating detail yet.

Wish I was there to draw on a chalkboard, complete with appropriate arm-waving, but we'll give this a try.
The scenario goes kinda like this:

When new, the steel throttle shaft fits closely in the aluminum throttle shaft bore in the aluminum throttle body. The throttle valve disc is precisely fitted into the throttle shaft slot so as to completely close-off the throttle opening when fully (minus stop screw) closed. The throttle disc edges are slightly angle-bevelled so that the throttle disc fits snugly to the bore walls. Also, when in this fully-closed position, the transfer slots/ports are covered, not exposed to engine-side vacuum. The idle port is the only exposed port in this position.

This is the proper restore configuration after full carb disassembly, or after replacing throttle valve seals.

Here's where the problem comes in. The BS38 throttle valve area is about 2 square inches. It must resist the pressure difference between ambient atmospheric (14.7 psi) and manifold vacuum (as low as 1 - 1/2 psi). This means that as much as 10 - 25 lbs of force is trying to press the throttle shaft towards the engine port. Also, manipulating the throttle with the throttle cable induces rubbing wear and a load in the direction of the pulling cable (in this case, up). Combine those two forces and the longtime wear, you'll end-up with a throttle shaft bore that's wallowed up and to the left (10 o'clock, as viewed straight on at the throttle arm from the left side of the bike). You can confirm this by wiggling the throttle arm/shaft (engine off) up/down and left/right. Anything more than a couple thousandts shows a worn throttle shaft bore.

If the carb is properly reassembled, and the idle stop screw set a fraction of a turn off fully stopped, the bike should start and idle fine, until the throttle is 'blipped' while the engine is running. Then the combination of vacuum and cable force will pull the throttle shaft off-center (up and to the left), and the throttle valve disc will now prematurely contact the bore, or get very close to it, causing mechanical sticking and transfer slot/port starvation, AND admitting excess air at the opposite edge of the disc, creating a lean condition, and a high idle.

Good hi-dollar carbs are fitted with replaceable bushings (like Holley), but most are not. I've been able to successfully line-bore and bush worn throttle bodies, but the setup time takes forever. It's worth it though, if if you're trying to restore a $2000 Marvel-Schwebler!

I've noticed several posts here where some folks have restored functionality by replacing throttle shaft seals, but that is short-term.

An effective band-aid can be done by recentering the throttle valve disc with the throttle shaft displaced toward the worn zone. Tricky, can be done, that's the way my wore-out BS38's are setup now. Also, setting the idle ignition timing at its most retarded position helps, as this forces you to nudge the idle stop a little higher, avoiding the disc/bore interference and eccentricity near full close. Of course, the idle mix screws have to be set to accomodate this situation.

Boy, I sure hope all this makes sense. Oh, well, food for thought. Wish I were there...

My carbs will eventually need to be line-bored and bushed. Don't know if this service is already available for wore-out BS38 carbs. And, refitting throttle shaft seals is not the same thing. I've thought about making a rebore/bush jig for BS38's, but that's another project awaiting. Has anybody else line-bored and bushed carb bodies?
 
much has been made about pod filters jetting. I could never get mine good until I bought the foam type. it is all about erratic turbulence at the mouth of the carbs at the air ports. that's what I call them anyway. foam works. I stepped the idle and mains up just a couple sizes and couldn't be happier. I don't bother with those k&n types anymore.
 
TwoMany, that's a fine description of the problem; new seals won't fix worn bushings. The BS series carbs do in fact use brass bushings. They can be extracted but they're splined and stepped, so there's no off-the-shelf stock that can be used to replace them and they'd be a real bear to fabricate. A fix might be to line bore and bush the old bushings. If you set up for that job you'd be the only guy doing it, as far as I know.
 
I was hoping to get some advice from someone that knows what they are talking about :laugh:

you don't use a manometer to correct idle or correct backfiring. :doh:

You have received a lot of good advise:wink2: Out of sync carbs will cause both of them issues. I have fixed a bunch of peoples "bench synced" bikes with a simple set of mercury stix. Good luck with your bike maybe you just need a little more RTV goop around the boots :thumbsup:
 
Your description of the idle change after pressing on the throttle shaft and observing your videos leads me to believe that you may have worn throttle shaft bores in the throttle body, an issue I've not seen discussed here in excruciating detail yet.

an excellent write up twomany albeit a little wordy :wink2:
.
yes there is a little wear to the shaft/buses but without being able to compare with a new set I would say that the wear was well within tolerance. This bike has only done 18k miles which is pretty modest mileage to expect significant wear. Its easy enough to temporarily apply some paste to the shaft ends to do a test to see if the wear is of any significance.

What I have discovered however is some huge gaps around the butterfly vanes where they seat on the venturi surface .:yikes:

At this point 30+ years ago I would probably have launched into a detailed explanation of Bernouilli's Theorum lol :laugh: .Theres a name to strike terror into the heart of an Engineering student :laugh:

The carbs were definitely stripped , cleaned and reassembled by presumably the PO before I got them but due to time restraints I only checked the float levels and gave them a cursory checkover and fitted them . :(:doh: big mistake !

I have never seen such ill fitting vanes so I am not sure whether they are as originally fitted ? or if someone has stripped them and rebuilt them without properly aligning them or switched them between venturis perhaps ? either way the carbs are going to need to be completely stripped and refurbished now over the winter.

I might even consult a local precision Engineer about the possibility of rebushing the throttle shaft buses and replacing the shafts.:wink2:
I certainly need to do something about a stripped thread on one of the bowl fixings ...yes the only blind one of the four !:doh:

I have done a significant amount of reading around the net on the BS38 carbs and when I've refurbed mine I intend to draw up a full troubleshooting flowchart but whether I get around to it remains to be seen.

:wink2:

thanks for your advice and encouragement .. its much appreciated. I know I come across as a grumpy old git sometimes but in my defence I would say that it is because I am a grumpy old git !
 
Just a thought.... how about removing the throttle levers from the carbs and winding the throttle return spring (around the shaft seal) one more revolution tighter? It might hold them closed better. :shrug: :twocents:

I like your thinking :thumbsup:

I think I realised the problem straight away. the butterflys were not synced properly . The RH idle adjustment screw was set slightly too high preventing the LH butterfly from returning fully . It took only a moment to correct .
 
You have received a lot of good advise:wink2: Out of sync carbs will cause both of them issues. I have fixed a bunch of peoples "bench synced" bikes with a simple set of mercury stix. Good luck with your bike maybe you just need a little more RTV goop around the boots :thumbsup:

chris I appreciate your imput and suggestions thank you but I will probably not be rushing out and buying a set of manometers just yet.

In my experience It is neither necessary or essential to use manometers to set the carbs in sync for initial tuning. :wink2:
You'd have to be pretty ham-fisted ,or blind ,to set your butterflys that far out of sync for it to effect your idle circuit that much.
 
Peanut, no offense meant, but when 5twins and I wrote the XS-650 Garage Carb Guide, he did a very nice job in the chapter on tuning for modifications, and you won't get more information by asking him to write it all again just for you. And for cryin' out loud don't smear gumpucky on your carb spigots (they aren't stubs unless somebody decided to "build" them with a Sawzall).

no offense taken. like me, you have no tact, diplomacy or social interactive skills so you say it how you see it and to hell with what people think of you . :laugh:

I have read the carb guide several times and it is very very good although it does contain some fundamental errors and ommisions .What it didn't tell me was the likely cause of a high idle.

lets not get hung up on what to call the carb holders, spigots, stubs, etc etc they get called different things around the World but we all know what we are referring to don't we !:doh:

There is nothing wrong with applying some hi temperature rubber silicon around the groove on the carbs to ensure an airtight fit to the rubber (insert your preference here xxxx) .many of these fittings are now hard and inflexible and probably need replacing which is all very well if you can afford the price or it is within your budget .
You wouldn't fit your windscreen rubber seal without applying some silicon would you

I was not looking for an explanation of how the carbs work or how to install and set them up. i specifically asked for some urgent suggestions as to what could be causing the idle to rise to 3000rpm because I had only a few hours to correct it . I have had some good suggestions and there are at least 2x other causes of a high idle that have not been mentioned here that I have discovered for myself.

I now have what I need and without the pressure of an imminent MOT test the following day, I have the time to strip and refurbish the carbs at my leisure.
 
an excellent write up twomany albeit a little wordy :wink2:

Thanx, peanut! Too bad you missed out on all the arm-waving...

What I have discovered however is some huge gaps around the butterfly vanes where they seat on the venturi surface .:yikes:

Bingo!

At this point 30+ years ago I would probably have launched into a detailed explanation of Bernouilli's Theorum lol :laugh: .Theres a name to strike terror into the heart of an Engineering student :laugh:

I'd love to hear about Coanda's theorums!

I have never seen such ill fitting vanes so I am not sure whether they are as originally fitted ? or if someone has stripped them and rebuilt them without properly aligning them or switched them between venturis perhaps ? either way the carbs are going to need to be completely stripped and refurbished now over the winter.

Do you need for us to find the throttle valve threads? I believe 5Twins and xsleo posted some very clear pics here somewhere.

I might even consult a local precision Engineer about the possibility of rebushing the throttle shaft buses and replacing the shafts.:wink2:
I certainly need to do something about a stripped thread on one of the bowl fixings ...yes the only blind one of the four !:doh:

If you look into the rebushing possibilities, I'd like to know what you find. For a universal BS38 rebush jig, I'd like to know what surfaces can be relied upon as dependable base reference points.

There's a thread here somewhere about bowl screw repairs.

I have done a significant amount of reading around the net on the BS38 carbs and when I've refurbed mine I intend to draw up a full troubleshooting flowchart but whether I get around to it remains to be seen.

Wow, good luck. Takes much longer to document this stuff than to do the actual fix.

thanks for your advice and encouragement .. its much appreciated. I know I come across as a grumpy old git sometimes but in my defence I would say that it is because I am a grumpy old git !

There's a strong correlation between grumpy and pain. My chemotherapy nurses knew to avoid me if I wasn't cracking jokes.

Is your back acting up?
 
I like your thinking :thumbsup:

I think I realised the problem straight away. the butterflys were not synced properly . The RH idle adjustment screw was set slightly too high preventing the LH butterfly from returning fully . It took only a moment to correct .

Wait a minute, did you just say they were out of sync? Was that after that exact possibility was suggested by at least 2 people? Nah can't be, as such a professional as yourself must have seen that before us amateurs even had the remote possibility of thinking such a thing. Good luck and happy grumping. It truly is contagious.
 
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chris I appreciate your imput and suggestions thank you but I will probably not be rushing out and buying a set of manometers just yet.

In my experience It is neither necessary or essential to use manometers to set the carbs in sync for initial tuning. :wink2:
You'd have to be pretty ham-fisted ,or blind ,to set your butterflys that far out of sync for it to effect your idle circuit that much.

No need to go out and "buy a set of manometers" you can make then for probably a little over a pound. About 5 meters of poly tubing and a meter stick and a few zip ties. If you had even bothered to look at the thread that I had mentioned rather than just arbitrarily dismissing every option that was presented to you without even considering them viable. But hey at least you can (but probably won't) come back later and let us know how bad off your "perfectly synced" carbs were. :thumbsup:
 
There's a strong correlation between grumpy and pain. My chemotherapy nurses knew to avoid me if I wasn't cracking jokes.

Is your back acting up?

:wink2: yes ...continual pain can put your patience on the limit can't it. I'd be interested to hear of your experience some time .

I don't recognise myself some days . The other day I wrote in my notepad in large letters 'Why am I always so frigging angry'?

Lugging the bike in and out of the house this week has made me realise that this bike is going to be too heavy for my back which is gutting.
I've been looking forward to this moment for over 15 years.

I'm hoping that my back will recover by Spring so I'll hold off making a decision about the bike until then.

I cannot even get the bike on the stand by myself.:( .

I'm sorry to have missed your graphic demonstration lol. Thankfully Coanda was still at school when I was Uni so I was spared his theorums
 
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Friend of mine from S. Africa had excruciating back pain. Found temporary relief using 'gravity boots' (hanging upside-down). Later transferred to Houston, where he claims has the world's best back specialists. They found a disc blowout, pressing on his spinal cord. Fixed it, he's fine now.

Mild back pain relief can be found with Bromelain, a pineapple extract.
 
Rub a little on the sides of your carbs too because they're obviously suffering as well, lol.
 
The last vestiges of my L5/S1 disc disintegrated in 2005 and I tore the supportive ligaments to the base of my spine.
Every now and again, if I am not careful how I move, a nerve gets pinched and it can take weeks or months to recover.
Could be a lot worse , at least I can still walk most of the time.
 
I have been criticised for not posting my solution to this thread but in all fairness the bike was put away in the garage for months over Winter and I completely forgot about it.

it took a lot of work and testing and carb cleaning resetting etc but in the end I the problem of the hanging idle was discovered to be due to several issues . Firstly the throttle shafts had too much sideways float and consequently the butterfly valves were not centering in the venturis . Secondly the throttle shaft return springs had weakened over the previous 35 years and adding an additional return spring brought the throttles back to rest . I also found the choke body gaskets were introducing excess air
I'm sure that in the process of stripping and cleaning and resetting the carbs over several months also had some hidden benefits .

The over riding lesson that I learnt was the need to be absolutely meticulous in everything if you want consistant and reliable performance.
lastly I would belatedly like to thank all those members that offered advice even if I didn't accept It ;) it was appreciated .

It is interesting to note how unhelpful skulls 'contributions' were even back then in 2013....nothing changed there then:laugh2:
 
I would belatedly like to thank all those members that offered advice even if I didn't accept It ;) it was appreciated .

It is interesting to note how unhelpful skulls 'contributions' were even back then in 2013....nothing changed there then:laugh2:

I'll bite.............seeing as how you have bought me into this, claiming how my contributions don't help now as they didn't then 5 years ago, is a slur on me and my integrity and contributions as a member

RG gave some advice............you dumped on him and then called his advice as amateurish

Ippity gave some advice........you dumped on him

XSchris gave some advice .........you dumped on him

This was before i posted so if the advice i gave was not to your liking.........."over revving a cold engine will tear the sump filter"......

Claimed you had synced you carbs after some advice was given to sync the carbs.........later on you stated you hadn't synced the carbs..........That's just straight out lying............
 
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