Carb jetting

The new jets are wrong. The new ones are the Vm22/210 type. The ones that I pulled out are proper 90/96. I guess I'll try 45's in the proper jet and see how that helps me.

I'm still a bit confused, because my plugs show signs of richness, but I believe it was you, who said that the cracking and popping from my exhaust is a sign of leanness. Could this simply be a matter of tuning and still the wrong pilot jet?
 
Your carbs have 3 circuits, one for low speed, one for midrange, and the main for high RPM operation. It's possible to be lean in one area but too rich in another. In your case, your midrange or main may be running too rich, blackening your plugs, while your low speed circuit (which contains the pilot jet) may be too lean, causing the popping on decel.

Carb_Circuits.jpg


When you decelerate with the throttle closed, using the engine as a brake, your carbs are basically running on the low speed or pilot circuit. The slide is all the way down and the needle is stuck all the way into the needle jet. This lets very little fuel/air mix flow from the needle jet. By design, the low speed circuit feeds your motor the fuel it requires at this time.

Something else you should probably verify is that the needle is the proper one, in your case a 4N8 with the clip in the #4 slot from the top. Many of the rebuild kits for these carbs come with a generic Y-22 needle. It is too short (and therefore, too rich) for any of the '74 and newer carb sets. It will only possibly work in the '70-'73 sets. If the P.O. put those Y-22 needles in, that could be part of (if not most of) the problem.
 
The 75 used two throttle cables, one for each carb.
To properly adjust the idle air mix screws you need to use the dead cylinder method. This requires you "kill" one cylinder while adjusting the other. With the stock points it is easy, just unplug one plug wire. If you have an aftermarket ignition, you need to build some adapters.
One easy way is to find a nail that is the same size as the tip of the spark plug and fits snuggly in the plug cap. Hook a jumper wire from the nail to the plug. Now with the engine running hook another jumper from ground to the nail. Don't try to hook this second jumper to the nail first, it will go to ground through you. Not fun.
Once you 'Kill' a cylinder back off the idle on the running cylinder to as low as you can and still run. Now adjust the idle mix screw for best idle. Repeat on the other cylinder.
Once you get the mix screws set you use the almost the same procedure to sync the carbs. "kill" one cylinder, adjust idle screw on running carb out till the engine quits. Now "Kill" the other cylinder and back out the other idle speed till the engine quits. Now both carbs are synced. Start the engine, it will idle too high. Now back off both idle screws the same amount to get proper idle speed.
It's all in the book.
Leo
 
I have my dad's old '77 Standard that hasn't run in 25 or 30 years and was in sad shape. I had the nastified carbs vapor blasted (does a beautiful job) and will reassemble soon. Bike has lost its rotted mouse nest (intake box) and aftermarket pipes, and presently looks like I will have pods and Emgo shorties, at least until they annoy me too much. I live at 7200 feet. Anyone want to take a flyer as to what jets to install to start with?

Thanks!
 
Hey guys, I have a 78 xs650 I just finished and now having trouble tuning my carbs. I am running with mikes xs performance air pod filters and I have strait through drag pipes. timing is good. idles ok and runs fine from idle to about 3000-3500rpm then starts breaking up pretty bad, like it is starving for fuel. im assuming ill need to re jet but not sure where to start or what size jets to use, as well as what notch I should have my slide needle set at. if anyone can give me a base setting to start with, with the mods I have would be greatly appreciated!
 
nooch said:
Hey guys, I have a 78 xs650 I just finished and now having trouble tuning my carbs. I am running with mikes xs performance air pod filters and I have strait through drag pipes. timing is good. idles ok and runs fine from idle to about 3000-3500rpm then starts breaking up pretty bad, like it is starving for fuel. im assuming ill need to re jet but not sure where to start or what size jets to use, as well as what notch I should have my slide needle set at. if anyone can give me a base setting to start with, with the mods I have would be greatly appreciated!
Click to expand...


I'll offer my input, I'm no expert but I'll tell what most are gonna say. If 5twins see's this he's gonna tell you need to read the carb guide, it can be found on here. He wouldn't be wrong either, the guide is helpful. As far as the mikes pods, not your best choice. Get some Uni's. I can't see you signature on my phone, but you should specify what carbs your running. I can only guess bs38's?? Having said that, yes you will need to rejet. Not sure what size jets you have but likely need to go up at least one to two sizes on the mains and pilots, maybe more. That's my two cents.
 
Should've also mentioned checking your float height, floats for holes, diaphragms, and if you haven't already cleaning and rebuilding.
 
Carbs are Bs38s, Floats are set at 22mm slightly heavy, no holes in floats, diaphragms are in good shape no crack tears or leaks... I read a bit if the carb guide and I understand what it is telling me to do, I'm not a very good carb guy so I was basically looking for a preferred starting point from someone who has a similar set up... So I'm not trying to tune something with the wrong size jets And to see if anyone has had to play with the slide needle at all. Another silly question... What are the next sizes of primary and main jets up from stock, I'll Oder some in as well as some uni filters too
 
According to the card guide your float level should be 24+/-. Your main jet is a 135, pilot jet is 42.5.
 
Although your float level isn't far off, it can cause big problems. Trust me I know. Adjust your floats, might see a difference.
 
There were several variations of the BS38 carbs so you will need to determine what year bike the carbs are off of before changing any jetting (there is plenty of info on here about how to identify what year carbs you have). Once you have the type (which you already have said are BS38's) and year of the carb you can determine what the stock jetting was (the stock jetting changed with each year set, every couple of years or so) then you can find out from the carb guide what a good base line jetting is and work from there. For instance lets say that you indeed do have 78-79 BS38's the stock jetting was 135 main & 27.5 pilot, for pods and open exhaust a good base line wuld be around a 140 or 142.5 main and 30 or 32.5 pilots. A good idea is to get a couple extra sizes of each type of jet and follow the carb guide for tuning.
 
awesome thanks! the year of my motor is 1978 se model and it has the stock carbs on it. I did a bit of research and you are right with the 135 and 27.5 jets. I appreciate your info. I think ill order the next 3 sizes up from stock and play with those. Do you think I will have to start adjusting my slide needle also with the bigger jets?
 
From my experience one up on the pilots is good two or three up on the mains should get you there.
Leo
 
Yes, you will most likely need to lean the needles a step with the bigger mains and pilots. If you don't, you will probably experience break-up in the upper midrange, say between 4 and 5K, at heavy throttle openings.
 
figure8 said:
Although your float level isn't far off, it can cause big problems. Trust me I know. Adjust your floats, might see a difference.
Click to expand...

I had my floats set starting at 25mm and worked my way down to a heavy 22mm. the reason I have them set there is it was running lean up top and having them at 22 seems to have solved the problem...somewhat, mind you I am running "apparently" shitty air filter haha and my jets are too small now. so im going to order a few bigger jet sizes and uni filters, ill reset my floats to 24mm and start again :doh:
 
Yes, your current 22mm float setting is just a "band-aid" fix. I experimented with the whole float range (23-25mm) on my '78 carbs. With proper jets, they were happiest at the given spec, 24mm.
 
thanks 5 twins, so I should probably go with the 24mm setting after I up my jet sizes, correct? and should I just try that to first before leaning my needle a notch?
 

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