Jetted bad enough to stall in five minutes??

Torchard

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My 76-77 carb set is jetted incorrectly - me and the previous owner had it jetted for the later set: currently 27.5 pilots and 140 mains. I have other jets on order to align w my pods and pipes - two into one w baffles. Is this jetting far enough off to explain why the bike is essentially unridable after it warms up? I’m lucky to get 5kms from home. It stutters, it stalls, it sputters, it runs rich - although the plugs aren’t totally blackened. I have a strong charging system, the ignition system all seems to be operating properly. I’ve checked coils over and over. I’ve swapped other coils in, check floats, adjusted mix screw blah blah blah… sigh. If the consensus is that this has everything to do with very poor jetting then I’ll just relax and do nothing until they arrive. But otherwise this is driving me insane I’ve done all that I can do to figure out what’s going on. Thanks. Therapy over.
 

5twins

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I suppose that could happen. 140 mains are way big for a '76-'77 carb set. The most I've been able to use in one of them is 132.5's. 135's caused break-up in the upper midrange under heavy throttle. I should probably also mention that 2-1 exhausts can be difficult to tune for. They're not designed with performance as their #1 objective but rather just to fit on the bike in question. That means headpipe lengths, collector diameter, and total length may not be ideal.
 

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Well 5T, I was hoping someone with your experience would come along and say, “Absolutely! that’s your problem. No chance that it’s anything else. Just relax - go have a beer. Find something new to obsess over. When the proper jets arrive that bike will run like a lovely little kitty.” ;). But still, thanks.
 

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My bike (77, 750 BBK, pipes, pods, came with 137.5s. I am at 127.5s (stock 122,5s) where the plug center is now tannish brown. I am persuing getting my pilots correct at this point. My bike ran pretty good on throttle with the 137.5s but burbled an aweful lot at part throttle.
 

Torchard

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Thanks, this is why I suspect that even with the correct range of workable jets the problem will not be resolved. It’s killing me.

My bike (77, 750 BBK, pipes, pods, came with 137.5s. I am at 127.5s (stock 122,5s) where the plug center is now tannish brown. I am persuing getting my pilots correct at this point. My bike ran pretty good on throttle with the 137.5s but burbled an aweful lot at part throttle.
 

Jim

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Thanks, this is why I suspect that even with the correct range of workable jets the problem will not be resolved. It’s killing me.
Yeah, I suspect you're right. You're only one up on the pilot. That should be close enough for you to at least keep it running somewhat smoothly at the 5 min mark. Unless of course you're crankin' it up into the mains when it's getting rough 5min in.
So I guess my first question is have you tried the full range on the carbs when it gets bad? Pilots, needles... mains.... they all act the same?
You said... "It stutters, it stalls, it sputters, it runs rich - although the plugs aren’t totally blackened." So, that at all throttle settings?
Reason I ask is your signature says PMA and caps. That piques my spidey sense.
 

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They act similar - breaking up and stuttering. Problem is worst at off idle/pilot. It kind of stumbles and will die at a stop. Sometimes even when holding the idle a bit high at a stop it’ll miss and quit. And then is hard to start as if fouled.

By all measures the PMA, and now battery, with capacitor, are operating fine. 13+ volts at battery. Coils test fine and even when swapped with another coil, the problem persists.
 

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I’m going to investigate the ground on the e-ignition which calls for a sound frame connection. Upon review of the schematics and installation instructions, perhaps I don’t have the best ground. Currently, all grounds run to a common block, to cap, to motor, motor and frame are grounded. Thoughts?
 

Torchard

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I suppose that could happen. 140 mains are way big for a '76-'77 carb set. The most I've been able to use in one of them is 132.5's. 135's caused break-up in the upper midrange under heavy throttle. I should probably also mention that 2-1 exhausts can be difficult to tune for. They're not designed with performance as their #1 objective but rather just to fit on the bike in question. That means headpipe lengths, collector diameter, and total length may not be ideal.
Thanks. I should add that I’ve since learned that two-into-one set ups add more complexity and likelihood of never quite getting things aligned. I didn’t know this at the time. I still don’t know much. The lengths are equal-ish and the collector diameter aligns with the pipe size (not easy to find). Frankly, I’m happy to land on an 80/20 range of performance at this stage and given my intended use. I picked up a set of stock pipes recently in the event that I return to something a bit more in keeping with what the engineers had in mind. ;).
 

5twins

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When BMW was endurance racing back in the mid '70s, they found their twin ran best with a 2 into 2 featuring long, gently tapered megaphones. This 650 likes a 2 into 2 exhaust as well. Stock headpipes appear to have an OK diameter but it's a case of "looks are deceiving". They are a pipe in pipe design so the actual I.D. is quite a bit smaller than the O.D., only like 1.25". They are very restrictive. The aftermarket 1.5" O.D. headpipes are much better, about maybe 1 3/8" inside. Combine them with the original design Commando mufflers from YamahaXS650.com and you'll have a very nice system. I have them combined with 1.5" MAC headpipes (N.L.A.) on my '78. They look good, sound great, and perform very nicely .....

Mammys2014.jpg
 

Jim

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I’m going to investigate the ground on the e-ignition which calls for a sound frame connection. Upon review of the schematics and installation instructions, perhaps I don’t have the best ground. Currently, all grounds run to a common block, to cap, to motor, motor and frame are grounded. Thoughts?
It never hurts to recheck and clean the grounds. Also no such thing as too many grounds.
 

Torchard

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It never hurts to recheck and clean the grounds. Also no such thing as too many grounds.
I have added a couple of dedicated grounds recently - frame to motor. And for one ride my problem seemed solved. Apparently not. So, good time to recheck w meter and confirm the ignition specifically is well earthed. I’m getting a beaten down by this I’ll admit.
 

5twins

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For the typical "pods and pipes" mod many of these bikes get, the usual re-jetting scenario is one or two up on the pilots, one to three up on the mains, and the needles leaned a step to compensate for the larger mains. But the '76-'77 set is a little different than the other BS38 carb sets due to the very small mains it had stock (122.5). It may need a little more on the mains, like 4 or 5 sizes up. If you study this jetting chart I drew up, you can see how main jet size is related to needle jet size. Larger needle jets get smaller mains and smaller needle jets can use larger mains. The Z-8 needle jets used in the '76-'77 set are the largest ones put in any 650 carb set, and they get paired with the smallest mains ever used .....


Carb Specs Reduced Size.jpg


Now, you may have read jetting recommendations on the web (MikesXS, Minton Mods) calling for mains in the 140's. That may work on some of the other carb sets that had smaller needle jets and bigger mains to begin with, but you'll never be able to go that big on a '76-'77 set due to that large Z-8 needle jet it has. As I mentioned earlier, 132.5's were the biggest I could fit, but that's 4 sizes above stock. Keep in mind, when re-jetting it's not so much the size of the jet that matters but rather how many sizes above stock it is. By comparison, on my '78 with the '78-'79 carb set, I run 140 mains. This does align with the common recommendations but it's only possible to go that big because of the small Z-2 needle jets, and it's only 2 sizes above the stock 135's.

When you're close on your jetting, one size can make a big difference, so it's best not to skip around or skip sizes. Try every size in succession. Yes, when it's all said and done, you're going to end up with extra jets but it's just the cost of doing business, lol. $40 in jets is still way cheaper than taking it to a shop. They would charge you several hundred dollars I'm sure.

Now let me talk about pilots a little bit. I'm one size up on my '78. I'm playing around with an XS500 carb set adapted for 650 use on my '83 at the moment. It's jetted based on a '76-'77 650 set (Z-8 needle jets, 4M1 needles). I had tried another '76-'77 set on my '78 several years ago so based my jetting on that, in particular the one size larger pilot. Well, on the '83, this wasn't enough, I had to go up another size. I had lots of popping on decel I couldn't tune out with the mix screws. Then I remembered - years ago when I was first tuning the carbs on my '78, I was up 2 sizes on the pilots. Then I rebuilt the topend and ported it. This porting changed the flow and I was able to reduce the pilots a size to just one over stock. The '83 isn't ported so I guess that explains why it needs 2 up on the pilots.
 

Torchard

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For the typical "pods and pipes" mod many of these bikes get, the usual re-jetting scenario is one or two up on the pilots, one to three up on the mains, and the needles leaned a step to compensate for the larger mains. But the '76-'77 set is a little different than the other BS38 carb sets due to the very small mains it had stock (122.5). It may need a little more on the mains, like 4 or 5 sizes up. If you study this jetting chart I drew up, you can see how main jet size is related to needle jet size. Larger needle jets get smaller mains and smaller needle jets can use larger mains. The Z-8 needle jets used in the '76-'77 set are the largest ones put in any 650 carb set, and they get paired with the smallest mains ever used .....


View attachment 216164

Now, you may have read jetting recommendations on the web (MikesXS, Minton Mods) calling for mains in the 140's. That may work on some of the other carb sets that had smaller needle jets and bigger mains to begin with, but you'll never be able to go that big on a '76-'77 set due to that large Z-8 needle jet it has. As I mentioned earlier, 132.5's were the biggest I could fit, but that's 4 sizes above stock. Keep in mind, when re-jetting it's not so much the size of the jet that matters but rather how many sizes above stock it is. By comparison, on my '78 with the '78-'79 carb set, I run 140 mains. This does align with the common recommendations but it's only possible to go that big because of the small Z-2 needle jets, and it's only 2 sizes above the stock 135's.

When you're close on your jetting, one size can make a big difference, so it's best not to skip around or skip sizes. Try every size in succession. Yes, when it's all said and done, you're going to end up with extra jets but it's just the cost of doing business, lol. $40 in jets is still way cheaper than taking it to a shop. They would charge you several hundred dollars I'm sure.

Now let me talk about pilots a little bit. I'm one size up on my '78. I'm playing around with an XS500 carb set adapted for 650 use on my '83 at the moment. It's jetted based on a '76-'77 650 set (Z-8 needle jets, 4M1 needles). I had tried another '76-'77 set on my '78 several years ago so based my jetting on that, in particular the one size larger pilot. Well, on the '83, this wasn't enough, I had to go up another size. I had lots of popping on decel I couldn't tune out with the mix screws. Then I remembered - years ago when I was first tuning the carbs on my '78, I was up 2 sizes on the pilots. Then I rebuilt the topend and ported it. This porting changed the flow and I was able to reduce the pilots a size to just one over stock. The '83 isn't ported so I guess that explains why it needs 2 up on the pilots.
Thanks for the detail here. I use your guide often. I’ve got mains and pilots on order - stock and two sizes up on each (I think). My choice of 140 was due to confusion over the carb set I have. Now that that is clarified I’ll get back to what’s in the zone for the 76–77 series w mods. It did rip up the highway the other day w the 140s but whatever chronic issue I have caught up w it so I can’t really use that as a reference. T.
 

5twins

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Yes, these CV carbs will run with jets that are too big because they work on engine demand, only feeding in the amount of fuel the motor can use. When you whack the throttle wide open that doesn't necessarily mean the slides are lifting all the way. Those too large mains aren't flowing 100%.

I base my main jet size selection on when that size I've chosen starts causing break-up and stumbles through the upper midrange (4K-5K) under heavy or full throttle application. Now normally you don't drive the bike like this, using full throttle and winding it up to 5K or 5500, but for jet size testing, you have to. Drive it easy and the glitches won't show. So, once I get the stumbling, I'll lean the needle a step to fix it. Then you can go up another size on the main but that usually brings the stumbling right back. So, once I get stumbling with the needles leaned a step, I drop back down one size on the mains and call that good. You could lean the needles another step but I'm not a fan of running them at their extremes (#1 or #5 slots). I like to keep them in one of the 3 middle slots.
 

Torchard

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Yes, these CV carbs will run with jets that are too big because they work on engine demand, only feeding in the amount of fuel the motor can use. When you whack the throttle wide open that doesn't necessarily mean the slides are lifting all the way. Those too large mains aren't flowing 100%.

I base my main jet size selection on when that size I've chosen starts causing break-up and stumbles through the upper midrange (4K-5K) under heavy or full throttle application. Now normally you don't drive the bike like this, using full throttle and winding it up to 5K or 5500, but for jet size testing, you have to. Drive it easy and the glitches won't show. So, once I get the stumbling, I'll lean the needle a step to fix it. Then you can go up another size on the main but that usually brings the stumbling right back. So, once I get stumbling with the needles leaned a step, I drop back down one size on the mains and call that good. You could lean the needles another step but I'm not a fan of running them at their extremes (#1 or #5 slots). I like to keep them in one of the 3 middle slots.
I considered leaning the needle on the more extreme end, but based on what I’ve read - and what you’re saying here - not advisable. Masking an issue at best. Thnx.
 

Torchard

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So, problem resolved! If you go way back to the start of this thread it was about stuttering and stumbling after warming. Short story: the XS-Charge ignition is faulty. I swapped in an earlier generation I happened to have (which I thought was also damaged) and the bike ran better instantly. After warm up it continued to run very reliably, good idle, good response on throttle. A bit of blurps and pops - so still tuning needed. But after much too much effort I’m so relieved. T.
 
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