Carb Jetting

[09classic650]

getting ready to rebuild and rejet the carbs,

doing the pod intake

free flowing pipes (stockers hacked before the muffler with turnout tips)

bike is a 1980 with bs34's

where is a good start for the jetting, links for rebuild kit and jets are very helpful

 

[breahn riley]

http://www.xs650.com/forum/showthread.php?t=43
the most info ever, its kinda overwhelming how much you can do to tune your carbs

 

[09classic650]

yeah kinda hoping someone else here had the same setup and knew where a good starting point for the jets would be, i know how to tear them down and rebuild, just looking for a good start point, maybe only have to buy 3 or 4 sets of jets instead of 6 or 7

 

[breahn riley]

start two sizes above stock on the main and one bigger on the pilot. i think its 140 and 45?

 

[Gordon]

classic, the best place to start is with the stock set-up. Of course, you will have to remove the plug's on the top of the carbs for access to the mixture screws. If you DO encounter problems, then I would suggest that you send a PM to XS JOHN, for some of his expertly tuned diaphram needles.Not only do you get two FULLY adjustable needles, but a set of instructions that a fifth grader can decipher. He will suggest what jet's to use, and also tip's on trouble shooting IF problems occur. They are worth every dollar you pay for them. But, like I said, start with the stock set-up, just back out the mixture screws to about 2-2 1/2 turns.
I wish I asked that question BEFORE I bought a whole slew of jet's & needles. Well,you got one up on me. I hadn't worked on motercycle carbs for decades, and my memory wasn't as good as I hoped. Good luck, these are not bad carbs, to work on or tune. They were lean from the factory, but we here can help with any problems that might show.BTW, you can get stainless allen bolts for the top's & bowls from boltdepot.com

 

[09classic650]

i got lucky, my old man was a motorcycle mechanic in a Honda/ yamaha/ Snapper dealership, he had me at one point about 15 years ago where he could take three different types of carbs apart put em in a B-15 Chemtool dip/soak for a night and i could put them back together right, carbs im good with, electrical on the other hand i understand how it works but only when simplified, btw does anyone know of a parts list for a pamco setup and rectifier for an 80, headlight and tail light only, no turn signals

 

[Gordon]

Yea, thats cool. On the needles from John, he really knows these carbs well. They are so well made, that they actually reduce cylinder temps. He was a Yamaha tech forever.
As far as wiring, on the home page, there is a link for several electrical diagrams,from bone stock to bare bones electrical, like your looking for. Also, somebody posted a link for new diaphram rubber boots only. Alcohol safe, which all the fuel sold in the states has now. Pretty cheap, too. Think it was $35 a pair. Just info for you. I am going to order some for myself, just to have them. My diaphrams are a little soft.

The diagrams (electrical) have a set-up to run the stock TCI ignition & charging system. PamcoPete also posted a replacement rectifier/ regulator set up that you build yourself for cheap money.

 

[ironwill]

Thought I would throw this in here. I have pods, with straight pipes that I cut an angled slot in and slid a 1/2" washer into so exhaust can escape on both sides. Currently tuning it. Mains at 150, needle set with clip in bottom slot. Plastic washer that normally goes on top of the clip is under it, to raise it more. Pilot jet is a 35. Pulls great 1/4 throttle on, too rich from idle to 1/4. Going to put the 30 pilot back in to lean it down a bit, tried the mix screw, but still too rich (basically backfiring, and gets choked up on all the fuel) The 30 pilot caused a flat spot just off idle, though very small one. Reading the designers prefer the needle clip in the middle slot, so thinking I need a little bigger main, that will allow me to raise the clip, and get the plactic washer out of there. I don't think that will help the flat spot, but maybe it will. I have tried the mix screw in multiple positions with every set-up change. Am I on the right track with a bigger main? Any ideas? Thanks.
P.S. I have the BS38 carbs. ('79)

 

[Gordon]

I am going to presume you got John's needle's, so I went with what he recommended, 32.5 on the pilot,middle on the needle jet, & I think, not real sure, a 135 on the main.You could PM him, as he IS THE MAN on BS34 carbs. Start with his recommendation. I don't have an open exhaust like you, but got Mikes 2-1 header with the taper tip muffler. Float level is very important, too high will give a real rich problem. I got a metric grease fitting that fits the bottom plug on the carb bowls. Drill out the spring & remove the check ball inside the fitting, screw into the bowl plug using a small 'o' ring to seal it against the bowl. Finger tight is enough, then use a clear hose turned up to see where the float level is. Should be just below the rim of the bowl. I used a spare battery vent hose, but any clear hose that fits the grease fitting will do. That method can be used on the bench also.

 

[ironwill]

ooopps...sorry Gordon, I didn't mention I have the BS38's. Nor sure what Johns needle is. Mikes didn't offer a 32.5, so I got the 30 and 35, unless I missed it somewhere which is entirely possible.

 

[Gordon]

XS John, he's on this site. He makes needles for the BS34 carbs, not sure about 38'S. PM him, as he can & will help. I don't remember what the stock 38'S have for jet's, so,,, you are probably way too rich on the main for sure. All these CV carbs use the pilot jet all the way through the RPM range, so any mods there will affect the whole running operation.
Look in the carb guide for some place's to start at. The 34S were real lean for the EPA mandates at the time, because everything came through California, & had to meet their emisions. The 38'S were allot better, because the EPA wasn't there for them.
I did the same thing when I changed my exhaust, figuring it would be way lean if I didn't go up at least 2-3 on all the jet's, but it really wasn't. Close to stock is where your going to find the running best.
The other thing is make sure the rest is good, cam chain adj, valve adj., advance unit for the timing along with the point's. Wait, you said your's has the TCI, right?Kinda got a little of a mongral huh? All the same, the stuff still applies.

 

[ironwill]

Actually it is points. I've adjusted the timing chain, adj. valves, set point gap, w/new points, set timing to -F- at idle. With the stock jetting it would idle, and rev slightly, but would die in the midrange. Once I got the 145 main in it, and the 30 pilot, it would rev from idle to 1/4 throttle, and would rev at wide open, but die in the mid-range. I kept raising the needle, and it kept getting better. The higher I got the needle the better it would run. Now with 150 main, and needle all the way up (clip in bottom slot) plus the plastic washer it runs the best, other than a tiny flat spot just off idle, maybe 1/8-1/4 throttle. According to the carb guide the cure is slightly larger pilot, but that made the idle to 1/4 throttle too rich. Putting the 30 pilot back in today, and will try again.

 

[Sundie]

Something i've experienced, take it for what you will: With open pipes/intake, these bikes will run fine on the stock jetting. By "fine", i mean, they will start, you can boot down the road, everything will run okay, they'll just be lean on 3/4 -> full throttle. So if you put all stock jets in and it had problems, i might be looking elsewhere first.

Or that the carbs are cursed. I've seen that happen before too. Swap a totally stock set of carbs in, bike runs fine, set all the jets on the other carb to that setting, put them on, runs like ass. Only solution in that case is to bury them face down in consecrated earth.

 

[ironwill]

Ok, this is where I'm at, and its running sweet. '79 XS650, pods, straight pipes with a built in slight restriction about 3" in from the end of the pipe. Factory head pipe. Points. BS38 carbs. 150 main, 135 pilot, needle set in bottom position with plastic washer plus one additional metal washer under the clip. Mix screw turned to lean then out maybe 1/16 of a turn. Pulls hard from idle to full throttle, and sounds sweet. Sounds rich, I know, but thats where it runs sweet. Need to get the clip back to middle position, according to the guide, this is done by a bigger main. Is that possible?

 

[Gordon]

Yea, it is possible, even if they are not available. You get a micro drill set, available from eHobbyTools.com. I suggest you get an extra set of bit's also, as they are so small that they can & do break easliy. Most set's only go to #74, but are available down to #80. That is really small. Someone had posted a drill bit to jet size chart somewhere, either here or the rider site. Use that as a guide to what pin drill set you will need. If a #74 is smaller than what you need, then go with that set. They cost about $12,plus $7 for shipping. They were really quick,too, if I remember correctly. I don't usually forget slow ass shipper's. The trick is to get the new hole absolutely 90 degrees to the base, an angle will flow ok, just not as well as a 90.

 

[ironwill]

Yea, I just checked Mikes, he has them up to 160. Does anyone else that you know of run jets over a 150 main? Sounds rich, but I think thats the only way to get the needle clip back into the center slot.

 

[xsjohn]

I'm with Sundie on this one.....Are your floats set at 24MM ........and I don't see how it can run with the needle position and jets you are describing......and if it does I would be careful of ring and cylinder wash with all that gas.........

xsjohn

 

[Sundie]

Out of curiosity - what's your compression?

At the jets you're describing, a pair of 38's should be soaking/fouling plugs regularly. I think the only reason you're getting away with "it seems okay" is because you're running points and not wasted spark - if you had wasted spark in there, i bet it would be popping like crazy as the second spark on the exhaust stroke hit all the unburnt fuel you're leaving.

From your description, i'm not sure if you're following proper tuning method for these. Which is:
1 - start with everything lean enough to get the bike to idle.
2 - sync your carbs at idle
3 - WOT from 4k -> redline to get main dialed in. Start small, when you get to the size where it stumbles, come back two.
4 - WOT from 2500 -> 5k to get needle dialed in.
5 - 1/2 throttle from 0 throttle from 2500 -> 5k to get pilots dialed in.
6 - individual idle adjustment screws
7 - main idle stop adjustment

It's also important to do these while the engine is under load (like a 2nd gear pull), not just sitting on the center stand.

While it's a long process, it's a much longer and frustrating process to change things in the middle, then go back and change the mains/idle. And there's no other real method to do it with vacuum slide carbs.

 

[ironwill]

I appreciate the help, and will go back and start over. I have been using this as a guide http://www.amckayltd.com/carbguide.pdf

I haven't done a compression check, or checked the floats. I will have to pick up a compression tester, and perhaps put fresh oil in it just to be safe. I'll check the tech section, for instruction on float setting. Didn't think what I had sounded right.

Once again, I appreciate all the help.

 

[ironwill]

Ok, I put everything back to stock. 27.5 Pilot. 135 Main. Needle in middle position. Mix screw set rich minus 1/32 turn. It idles fine and revs until around 1/8-1/4 throttle then falls flat on its face. It will also rev fine at 3/4 to full throttle. Anything mid-range it starts popping, will run briefly, then like it runs out of fuel, and dies. This is sitting in the garage, as I don't think it will go down the road like this.