Another jetting question : commando exhaust system

[D Stroy]

Hi guys,
Thanks in advance for any help you offer, reading through many of the posts has already been very informative. I am new to riding motorcycles and also a new xs650 owner. I have a '79 special that is all stock except for a new Commando Exhaust system from Mikesxs (bike came without exhaust). A friend did me a big favor and cleaned the carbs for me as well as got it to the point that I could ride it. The jets were stock sizes (27.5 pilot, 135 main) but in order to get the bike to pull through the full throttle range, the needle clip had to be moved to the richest setting. This has allowed me to ride the bike, pass license tests, etc. The more I ride it though, the more I suspect the original lean condition isn't fully remedied. I also recognize that it is not ideal to be n either the richest or leanest needle setting. I will start with what I think are symptoms, feel free to say otherwise.

- Idling - bike starts right up with choke and when I take choke off I have to give it a little throttle to keep it running until the bike gets hot. (Though I figured this is pretty normal) After the motor is hot, if I leave it idling, it does ok for about 30 seconds but then after that the rpms start to drop and it will stall unless I give it gas. If I adjust the idle speed screw/throttle stop to where it stays running, once hot, each time I pull the clutch or come to a stop the rpms stay high at about 3000 rpm unless I turn the idle speed way down. So I basically can't idle without either giving it just a little gas or having it float at high revs.

-Riding - when I first take off, there is a hesitation right off idle after which the bike jerks into motion, but then I can go through the rest of the gears no problem. Once in a while it can feel a little jerky at the ~1/4 throttle range. Also, when riding with a passenger this starvation/jerky behavior happens when just going along.

-I am assuming this all means lean running. Both in other posts I have read here as well as in the Commando Exhaust paperwork, it has been recommended to go 140-145 in the mains and potentially one size up in the pilot jet. (From 27.5 to 30 I assume) In other posts I have read here they say go up one size in the mains per mod, which would move my stock mains to a 137.5, not the 2-4 size jump that is recommended in the Commando literature. I am planning on ordering the 30 pilot jets. My question is about the discrepancy about which mains. If I go for 1-2 sizes up I would order 137.5 and 140 mains, but if I follow recommendations from Mikes, order 140 and 142.5. Does anyone here have experience with this exhaust system or have a recommendation on which range of main jets to order? I would prefer to only order 2 sizes that are most likely to work and narrow to 1. Please let me know what you think as well as if I am on the right track in thinking that my bike is running lean.
Thanks,
-Dustin

 

[D Stroy]

I forgot to mention there is crackling and popping upon deceleration.

 

[KeithB]

I have an '80 Special with the same exhaust but Uni foam filters and the Canadian needles and needle jets. Stock idle jets.
Tried main jets from 130 up to 145 and ended up at 132.5 (stock)
The 140's made the bike run like crap at full throttle. It seems to like the 132.5 to 135.
Been adjusting the needles and ended up with them at the second last low clip position richening the mixture in the midrange.
The main jets seem to fly in the face of what I have read, but I have tested the options repeatedly and this is what works for me.
I think these mufflers are more restrictive than I thought.

 

[Fleksta]

With mine ('79 Special), I just went up to a 32.5 pilot and 140 main. No needle or mixture adjustment seemed necessary.

Are you running the stock air filters or pods?

 

[jchrisk1]

Sounds like you are lean. Probably due to your carbs not being clean or air leaks. You really need to read the carb guide to understand these carbs.

http://www.amckayltd.com/carbguide.pdf

KeithB's carbs are bs34's and his jetting won't pertain to your carbs. Sounds like you have 78-79 bs38's.
With your needle being in the richest position, you have something wrong. It should be in the middle slot, give or take one. I have the same carbs as you and I run a 140 main, needle in the second position with 30 pilots. I could probably go up to 32.5 on my pilots, but I like it where it is. Every engine is different, so you just have work on it until it's right. I think you should read the carb guide and clean them yourself.

Have you checked for leaks and are you running fuel filters?

Fleksta, 37.5 pilot? That's rich!

 

[Fleksta]

Fleksta, 37.5 pilot? That's rich!

My bad, I meant 32.5

 

[jchrisk1]

Shameless plug.

I've got genuine Mikuni jets I'll sell you.
30,32.5,35 pilots
140 mains

$5 a pair to your door.

 

[D Stroy]

I am running the stock air filters. The carbs have been cleaned and I don't believe that there are air leaks. (I am assuming you are referring to the floating idle) No fuel filters. When I rejet, it'll be apart anyways and I can make sure there aren't air leaks. The float only happens if I have the throttle stop set for higher idle though. I have read the carb guide, many times.
-Flektsta, what mods put you up to 32.5 pilots and 140 mains?
-jchrisk1, I'll get back to you on the jets. What setup do you have that lead to your jet sizes?

-It sounds like from what you guys are saying that the mains shouldn't have to go too much higher to stock. Does that mean I need either the 137.5-140 range or 140-142.5? Also sounds like upping the pilots to 30 could help.

 

[jchrisk1]

I'm running Uni pod filters and open pipes with 4" baffles.
The 135 mains are rich to begin with. You shouldn't need to go any higher than 140. One or two up on your pilots should be plenty as well. I put 35's in mine and it was breaking up pretty bad. The 32.5's weren't too bad but were still a little rich so I went with 30's and set my mix screws to the rich side. I'm liking this setup so far. You'll just have to try a few different things and see what works.
Are your floats set to 24mm? Have you synced them?

 

[gggGary]

+1 on are the carbs synched? #1 suspect with floating idle and back firing.

 

[Fleksta]

-Flektsta, what mods put you up to 32.5 pilots and 140 mains?

The only mods with the current jetting are the Commando exhaust and K&N OEM style filters. This weekend I'll be swapping to a Pamco HO ignition and Uni pods; so I'll probably have to tinker with the carbs again.

 

[D Stroy]

Thanks for all the help guys. I think I am going to order the 30 pilots, 137.5 and 140 mains and see how that goes. I'll make sure they are synched as well. Is it worth ordering the float bowl gasket as well, do they really tear that easily?

 

[Fleksta]

My philosophy is that it never hurts to have spares. If you do pull them apart and have a gasket break; its much better to have a new one on hand then to wait a week until you can ride again.

 

[jchrisk1]

For what it's worth, I would put some fuel filters in line, make sure the pilot circuit is clean, put the needles back in the middle position, sync them, and start from there. You shouldn't be having such a problem with them for just having changed the exhaust. Good luck with it and let us know how it goes.

 

[Underdog]

yea it sound lean on the pilot circuit or sync is off i wouldn't re jet i would first pull all the jets out and clean them thoroughly then set the float and install inline filters once its running syn the carbs and set idle mix screws if you still have a problem once this all of this is done spray something nonflammable unless you like fire like me everywhere on and around the carbs do this with the air box on and avoid getting it in the air box to prevent confusing you if you get a rpm change you have a vacuum leak

 

[D Stroy]

I know I have left this thread dead for a while, figured I will update for those who helped:

So far I have put in 30 pilots, 137.5 mains, replaced the needle jet o-rings, (and checked needle jets), adjusted float height per manual (which was on the lean side). Needle is still on the richest position.
Also, replaced plugs, wires and boots, adjusted the points gaps and had the timing done (I don't have a strobe), and the carbs have been synced but only a bench sync.
Overall, it runs fine to slightly lean still, even with these changes and will not run at a leaner needle setting. The left side seems to run great but the right still misses a little at idle. Weird that I haven't been able to richen it up.
Maybe timing or sync isn't perfect, leading to the right side issue?
The hanging idle got better when I went richer but still happens. I can usually remedy by lowering the idle speed once the bike is warm. This I believe to be related to worn advancer springs as well as the advance rod needing a clean and lube. The weights didn't snap back like they should.
If anyone has any suggestions please let me know, I would love to sort everything out in the spring. I am especially interested in reasons why I could still be lean.

 

[5twins]

Having to run the needle in it's richest position is indicating something is still amiss here, especially on this carb set. A little background - Yamaha increased the mains on this set quite a bit from the previous set (122.5 to 135). That's a very big jump (5 sizes). In order to do that, they needed to reduce (lean) the needle jet. They reduced it from a Z-8 to a Z-2. Still, that left the midrange right on the verge of being too rich. Usually any increase in main jet size, even just one up, requires the needle setting be leaned. So like I said, your carbs are not acting like a '78-'79 set usually does.

Are you sure you have the needle assembled properly? There are several washers and a small preload spring that must be in the proper spots or the needle will sit at the wrong height. Here's the needle washers, thin metal one under the e-clip, thick plastic one above it .....

And here's the whole slide assembly. Note the little preload spring should be under all the washers and e-clip .....

If you have the little spring on top of the washers, that would lower the needle too much and may explain why you have to set it at it's richest slot. You also made no mention of your mix screw settings. The spec for this set is 2.25 turns out but 2.5 to 2 5/8 is not uncommon.

The jetting recommendations you've been given are usually what work in this carb set with minor mods - one or two up on the pilots, 1 to 3 on the mains, needles leaned a step.

For these bikes to run best, EVERYTHING needs to be set right. They really like their timing right on. With a points system, that can be difficult to achieve. You're actually setting two little separate ignitions, one for each cylinder. Besides timing them correctly, you want to match them cylinder to cylinder (both timing and points gap). It can be a real chore to get it perfect. That's why so many of us use the Pamco.

 

[D Stroy]

Thank you very much for the help. Mixture screws were 2.25 turns out. The last time I was in there I did check to make sure that the assembly order was correct and I had a friend double check before reassembly. Just to make sure I will pull it apart in the spring. Hypothetically, if everything is correct, is there some other area that could be the source of an air leak? I have checked for leaks with the carb holders with wd-40 and carb cleaner and can't find leaks from the normal sources.

 

[5twins]

Well, bad butterfly shaft seals can be a leak source, but that usually has it's biggest effect on idle. But then again, you are having some wandering idle problems. The stalling when you try to let it idle could be a bad float needle and/or seat. It may be leaking enough to over-fill the bowl while idling which causes the mix to go real rich and chokes the bike out. At speed, the bike is able to burn the extra fuel being leaked by. Besides that, I would verify that you have all the correct parts in there. The needle should be a 5O2 and a the needle jet a Z-2. The pilots should be the VM22/210 type. Running the needles in the richest #5 slot (lowest slot from the top) should be giving you very bad break-up in the upper midrange under heavy throttle applications, even with a stock main, and much more so with larger than stock sizes.

 

[retiredgentleman]

As for the hanging idle:

Service the ATU, well lubed and weights should snap back freely.

A carbs sync is important............make yourself a manometer, its cheap,easy, and very accurate.

2.25 turns is OK for a starting point, but each side can have some differences, so a little fine tuning may help. I suggest you use the dead cylinder method. With just the one side running, adjust the mixture screw each way from the 2.25, and rev the engine between each adjustment. You may find the sweet spot where it no longer hangs.