SWAPPING JETS between DIFFERENT MODELS of BS38

[ANLAF]

I am about to swap jets and need some guidance.

I have had some great help from the guys, especially 5twins on the carburettors. I thought I would start this thread so others can see what is going on.

background - I want to swap the main and pilot jets from my 76/77 BS38s with the main and pilot jets of my 78/79 BS38s. The reason is I have pods and straight-through pipes so 5twins suggests if I want to use the 76/77 carbs (which I do) they will benefit from larger mains and pilots (and lowering the float needle one position) - 5twins says the larger 27.5 pilots and 135 mains will take me in the right direction to compensate for the custom set-up (pods and straight-through pipes), and lowering the needle position will make the mixture leaner to balance the bigger jets which deliver more fuel.

QUESTIONS
1. is there a guide to swapping jets - this is my first attempt, and I would like to do the reading first before I get in there; and,

2. re lowering the needle position, that is putting it to a lower number (from three to two) or is it moving it down the needle itself (apologies for the simplistic question, but I am not sure).

Carburettors are a mystery to some of us.

Anlaf

 

[5twins]

I'm not sure what you did with the needle (as per the comments in your other thread) but to lower or lean it, you want to move the e-clip up one slot from the #3 to the #2 position. This allows the needle to sit lower in the slide by 1mm. This means that at any given throttle (slide) opening, the needle will be 1mm lower in the needle jet and allow less fuel flow.

I don't know if you've swapped the jets yet but be sure to read the numbers on them. There's what came stock in the carb and then there's God only knows what a P.O. could have swapped in, lol. And being that most people don't have a clue about carbs and jetting, they usually tend to go big, way big, when they attempt something like this.

 

[ANLAF]

Thanks, 5twins. I am just reading through the amckayltd carb guide to locate the pilot and main (in the bowl I see).

Yes, I will check the numbers and see if they correspond with those you gave me (as stock items expected in there).

Looking at the mixture screw (number of turns) I had mine way out in the 3 1/2 range - hence richness and coughing carbon dust.

Anlaf

 

[5twins]

Yep, that would do it, lol. That's over twice the spec. And make sure you have good fitting screwdrivers for the slots in the jets. They're soft brass and easily buggered up. Grind some down if need be to make them fit right. I've taken to grinding those 1/4" hex screwdriver bits down to get perfect fits. They're cheap so you're not out much if you screw up the grind job, lol.

 

[ANLAF]

I have swapped the jets with the care those little brass pieces deserve. I could not read any numbers (not even with my good eye) but the donor jets had a larger bore. I am presuming, at this stage, they are the stock (for the 78/79) 27.5 pilots and 135 mains now snugly in my 76/77 carbs.

The jet needle is now at position #2 - that is the slot next to the top (please let that be right).

The mixture screw (for 27.5 pilots and 135 mains) is 2 1/4 turns out - but can I clarify, 5twins, or are you suggesting 1 1/2 + or - 1/2 as in the original 76/77 carbs set-up (25 pilots and 122.5 mains)?

Anlaf

 

[jd750ace]

Yep, that would do it, lol. That's over twice the spec. And make sure you have good fitting screwdrivers for the slots in the jets. They're soft brass and easily buggered up. Grind some down if need be to make them fit right. I've taken to grinding those 1/4" hex screwdriver bits down to get perfect fits. They're cheap so you're not out much if you screw up the grind job, lol.

I do the same thing if some of my gunsmith screwdrivers don't fit. For the deep recessed ones ya gotta get the extension bits!

 

[5twins]

Yes, stick with the original mix screw setting for the '76-'77 carb set as your baseline (1.5 turns out). If you were doing all this to the '78-'79 carb set, you would use it's baseline setting (2 1/4 turns out). These baseline settings have more to do with the carb set years and how the mix screws and the passageways they meter are designed than on the jet sizes used. Sometimes when larger pilots are installed, the mix screws like slightly less turns out than the baseline. A general rule of thumb is 1/4 turn less for each size up on the pilots - but that's not etched in stone. Yours may still want 1.5 to 2 turns out. The bike will tell you what it likes best by how well it runs. You set the screws where it runs best no matter where that may be.

I suppose I should expand a little here on the theory behind mix screw settings. The manufacturers will usually spec between 1 and 3 turns out on the screws. There are a couple of reasons for this. First, the screws are spring-loaded so they hold the setting you give them. Get much more than 3 turns out and you lose that spring tension. The screw can move off it's setting, even vibrate right out. Second, the screws usually have a tapered or stepped end that fits into a similar shape at the bottom of the screw hole. For these two surfaces to interact together and flow correctly, you don't want the screw almost closed or too far open. If you have to go above or below these points to get the bike to run right, it's recommended that you change the pilot jet size again. That will bring your "best running" setting back within the desired screw range (1 to 3 turns out).

 

[ANLAF]

That's great, 5twins, and now all swapped and set.

Just to confirm, position #2 for the jet needle is the second slot down from the top (blunt end) of the needle (five being nearest the pointed end) isn't it?

Anlaf

 

[5twins]

Yes, that is correct. Some of us even shim the needles with 1/2mm thick washers to achieve half step settings, but we won't go there now ..... or yet anyway, lol. That's for fine, fine tuning.

 

[naessdrawos]

Since the knowledge being thrown around is great. Does anyone know any good books that talk about carburetion and understanding it better.

 

[Johne]

I am looking for some guidance or confirmation. I have a 79 with pods and through pipes, and a set of very worn 79 BS38's which run OK but the POs look they have tried to get them to work with a hammer and the float bowl threads are all stripped. They are not nice. I also have a set of 76 BS38s with a few bits missing, but fortunately those bits are OK on my 79s. I was going to rebuild these 76s with new diaphragms, jets etc to get a good refurbished set. (they are currently being ultrasonically cleaned) So the question is can I use the 79 slides with appropriate 79 needles and jets etc in the 76 bodies, or do I need to stick with the 76 slides and use the jets etc appropriate for these? The set up works with my 79's and so when tuning I should have something close if I use the 79 internals. I have read the guide which is where I was alerted to the slide change over and the above thread seems to imply that it can be done. Thanks for any response.

 

[5twins]

Yes, it should work. Along with the '79 slide, you'll need to use the '79 5O2 needle and the '79 Z-2 needle jet. The '76 4M1 needle is too short to work in the newer design '79 slide. That was the big difference in the slides. The newer one mounts the needle higher so a longer needle is required. It also spring-loads the needle. This gives more precise metering. In the older slide, the needle was rather loose and could jiggle around quite a bit, in particular, up and down about 1/2 mm. That's half a needle clip position. The spring-loaded needle is more firmly held in place. This gives smoother running, mostly at lower RPMs.

 

[Johne]

Brilliant - that was what I was hoping for. And the reasons make sense. I have all the parts I need between the two variants. Really appreciate such a rapid and informed response. Thank you.

 

[5twins]

I should mention, the only spec you'll want to stick with for the '76 carb set is their initial mix screw setting (1.5 turns out). Start at and fine tune the screws from there. Don't start with the '79 initial setting spec of 2.25 turns. These recommendations are based mostly on the screw style. The '76 screw is the old fashioned type with a long gradual taper to a point. The '79 screw is a pollution mandated type with steps instead of a smooth taper. They generally get set more open.

 

[peanut]

Johne I had the same problem with the float bowl threads on my 79 BS38s with a few stripped threads .
I solved the issue by fitting some new hex head stainless screws and also adding a few thin M3 nuts where necessary.

 

[Johne]

Thank you both. The initial mix screw stuff was interesting - will check that out. Thanks Peanut - the carbs I inherited have that done and they do not leak. But they have generally been abused and one float bowl has chunks out of the lip where it the gasket goes. And impact marks. And other stuff. The bodies on the 76 carbs are just dirty - so I want to create a good set and doing it this way means I can still ride on the warmer (10C+ ) days whilst I get all the stuff together. I am looking forward to it all being done now.

 

[peanut]

Thank you both. The initial mix screw stuff was interesting - will check that out. Thanks Peanut - the carbs I inherited have that done and they do not leak. But they have generally been abused and one float bowl has chunks out of the lip where it the gasket goes. And impact marks. And other stuff. The bodies on the 76 carbs are just dirty - so I want to create a good set and doing it this way means I can still ride on the warmer (10C+ ) days whilst I get all the stuff together. I am looking forward to it all being done now.

understand completely. I have been looking out for a used pair of late BS38 carbs for 3+ years and not found a single set .
My set does work fairly well but I have never got them perfect.
Last month I bought 2x pairs of GPZ500 carbs and there is an excellent thread about them on here somewhere