BS38 1979 Carburetor Stumble - Low Speed

[glennpm]

Hi,

I've recently finished my 1979 XS650F and am having bad low speed stumbling, i.e da ..da..da, off-on-off-on stumbling and missing off of idle to about 2300 -2500 RPM. The first couple of short rides I made a few weeks ago were fine without this problem. The bike sat for about three weeks with about a half tank of gas and then on Monday morning I took a short trip and it was bad right out of the driveway.

I have a completely rebuilt 277 re-phased motor. The carbs were completely overhauled and I have new diaphragms. I'm running Uni pods. I have the small glass bowl with bronze filters on both card gas inlets. I have Norton Commando mufflers and no cross-over pipe. I port matched and polished the heads. I deleted the lower barrel to crankcase gasket, raising the compression ratio a bit, maybe .4 or so. I have a vacuum hose connecting barbs from both carbs. The carbs are set up this way (post quoted from 5Twins):

"Re: Bs38 Carb Re Jet Question.??
" ... For the '78-'79 carb set, start with one up on the pilots (27.5 to 30), one to three up on the mains (137.5, 140, or 142.5), needle clips raised one step to slot #2 (lowering the needle), and mix screws about 2.5 turns out. It's very easy to over-jet these CV carbs because of their forgiving nature. They work on engine demand and the slide only lifts as much as the engine can use. That's why the bike may seem to run OK with a main jet several sizes too large. The slide isn't lifting all the way and that large main isn't flowing at it's full potential.

To properly test the main jet size in these CV carbs, you need to use absolute wide open, full throttle (just twist the grip open as much as it will go). In 2nd or 3rd gear and starting at about 3K, run the bike up to near redline. Watch for glitches (stumbles, flat spots, break-up) and note what RPM they occur at. This will tell you what needs tweaking (needle, main, or maybe both). Full details on this test and also one for the idle/lower mid-range are in the carb guide. In fact, pretty much everything I just told you is in there, lol."

I started with the pilots out 2 1/4 turns, adjusted both sides listening to RPMs and feeling exhaust pulses. I ended up at 2 3/4 out on both carbs. I have not balanced the carbs yet. It did seem that there was more vacuum on the right side then the left, based on feeling the vacuum hos. It was actually visibly partially closing the hose on the right. The valves are adjusted properly, TDC accurately established and timing correct. Both carbs needles seemed insensitive to change on the richer side, i.e. more open. Color for both spark plus is identical and good light tan color.

After adjusting the needles there was improvement but still not real good. High speed response is awesome. I'm surprised at how strong and fast the bike pulls up to 5k from 2500 with NO flat spots or hesitation.

I dosed my gas with 5oz of Seafoam last night and topped it off in the garage but won't be able to test again until Saturday.

Since it ran good, albeit short trial rides and now isn't, I'm suspecting a bad gas problem. Any suggestions?

 

[5twins]

Yes, I guess it could be bad gas, or maybe some dirt has made its way in and plugged the idle circuit(s). Here are a couple other steps I would take though. First, get rid of the interconnecting hose between the vacuum barbs and just cap them off separately for now. Yes, this is a little mod many do to smooth out low running speeds, but it's something you do AFTER the carbs are sorted, tuned, and synced, as a final step. Right now, it's combining possible problems in one carb with the other and not allowing you to isolate them.

Next would be a good, or at least better, sync. If you don't have a manometer, make up one of these .....

http://www.powerchutes.com/manometer.asp

..... or pull the carbs off and do a bench sync.

Your problem does seem related to the low speed or idle circuit. Porting can change your jetting needs. If the carbs, in particular the idle circuits, are clean, then you might try different pilot jet sizes. Go one up and down from where you are now to see if it helps any. Re-setting, or at least checking the mix screws will be required for each jet size change. They may need a slightly different setting with a new jet size.

 

[glennpm]

I was hoping you'd see my post and respond and I appreciate it!

Yes, I have a mercury manometer, bought it for my K100RS Beemer a number of years ago. I plan to do a sync this Saturday. I'll plug each vacuum port independently and see what that indicates.

I'm really hoping is a bad gas thing.

Thanks!

 

[5twins]

I also have an old mercury "CarbStix" from the '70s. It works very well. Like yours probably is, mine is made for a four cylinder, with four hoses. That's no problem, you just connect two. No need to block the others off, just drape them over the bars out of the way.

 

[glennpm]

Yes that is exactly what I have and the way I plan to use it. I'm hoping the vacuum isn't crazy variable so will have my finger poised over the kill switch ;-)

 

[5twins]

Stop at an auto parts store and pick up a pack of these 7/32" vacuum caps. They're the perfect size for capping off the vacuum nipples on your manifolds ......

If you plan to run like this for a while, you'll need some sort of retainer clip on the caps. They're easily blown off if the bike farts or backfires.

 

[chizler62]

1st- I'm jealous that you're that close to lake Norman! I was recently there to sell (finalize build sheet) your neighbor a custom XS650.
Tells me he's was out on the boat the other day.

Anyways.
I'd suggest after doing carb sync doing an idle drop. At idle back out fuel screw 1/4 turn until you don't see a difference. Then the other side. Then turn one side in 1/4 turn wait a few seconds. If the idle doesn't drop a bit. Turn 1/4 in again and wait a few seconds.... repeat until you can physically hear it slightly drop. Once it does back out a half turn then repeat on other side.

If you can turn all the way in......your pilots are too big.
If you can turn way too far out and idle continues to increase pilots too small.
theoretically

 

[glennpm]

Yes, I'm close to Lake Norman but like most lakes is not that accessible unless you own property on it. there is a State aprk at the north end which is nto that close for me and I'm not that interested anyway. Lots of people have fun on their boats there though and the weather is getting good with a day or two of real nice days each week now.

Thanks for the carb sync drop test instructions. I wouldn't have done it that way but the opposite, adjusting each carb for the slightly rich side of the sweet spot and then syncing the vacuum between the two by adjusting the center butterfly position/throttle shaft adjuster. I'm going to try it your way.

the other day I tried screwing in the needle alternately on both carbs, bike stalled out so that confirmed the pilots were not oversize. As I mentioned that on the rich side for each, there wasn't an obvious rpm drop. I'm thinking that may have been to the fact that I had vacuum ports cross tied between the two carb holders. I've already removed that and plugged each.

Good tip on the vacuum caps 5Twins.

I'll post as I work my way out of this problem.

 

[glennpm]

So I synched the carbs with my manometer and reset the idle screws late this afternoon and also went out for a couple of test rides. The right side idle was a little too high RPM. Independently adjusting the screws and then checking, I'm at about 2 1/2 turns out. I bumped them both up to 2 3/4 for a slight improvement.

The stuttering is much better but it still there and annoying of course. One thing I mentioned and noted again today, is that after getting a faster idle with the screws out as I mention, that there is no noticeable change if I screw them out further. Do you think this may be an indication of too small a primary jet? I'm one up from stock now.

When I first tried to start it today, I had high compression and couldn't kick it. It also kicked back on me a couple of times which it hasn't done before.

I guess I'll check the valve gaps and timing again tomorrow but I just set them very recently.

 

[5twins]

One of the first rules of jetting is that when you think you're right, try the next size above and below it to confirm your choice. Sometimes the '78-'79 carb set does like 2 sizes up on the pilots. That would be a 32.5. This is a rather new size offering by Mikuni so may not be listed everywhere, but they do make it. The sizes used to jump from 30 right to 35 and many places still list the pilot jet size progression as such.

No change when adjusting the mix screws can be a sign that your float levels are set wrong. You may have set/checked them but if you only measured one side of each float, they could be off. Many times the floats are twisted on their mount and one side sits higher or lower than the other. If you don't measure each side of the float, you won't catch this, and the level setting won't be what you think it is.

 

[glennpm]

Thanks for helping again 5Twins!

Way better today. I did have to reset the valves this morning which rather surprised me. Pretty consistently they had opened up by .002". I have them set at .006 and .004 with elephant foot adjusters.

I mentioned that I had dosed a tank of gas with SeaFoam and ran four trips today. After the first of about 30 miles including about three miles on the interstate, 85, it improved. Half way back on the trip the stumble at around 2300 - 2500 was just about gone. I still think I may get the larger primary jet you mentioned.

I was extremely careful with my float heights but did not check them side to side for each. When I adjusted, I did it from the middle and didn't think to check as you mentioned. Didn't notice cocking and was looking close but not looking for cocking either. This may be worth checking out.

On my third-fourth trip, I went about 5 miles each way to borrow my buddies timing light and show them my bike which they loved ;-) I have two lights but none here. The timing was advanced maybe 5-7° or so which I thought would explain the hard kick starting today. However even after getting it back on the mark, it was very hard to start. Not sure what's going on there. Should have been easier retarded and don't know if it was too rich and flooded.

Good thing was that it was real fun on the bike today!!

 

[glennpm]

Hi,

an update.

I bought a set of 32.5 pilot jets, installed them, adjusted and synced with my manometer and tried it out. I had #30s in before the change, one up on the needle (leaner) and one up on the mains.

It still stumbles at low speed transition most obvious at around 2300RPM. Timing is right on. I have the pilot screws turned out about 2 1/2 turns. High speed WOT is fine and great. I tried many adjustments and maybe six trial runs yesterday but although a little better, it still stumbles ... aggravating. It is starting easier now than last weekend.

I notice form the Carb Guide this,

"In 2nd or 3rd gear, slow down to near stalling then grab a big handful of throttle. You don't need absolute full throttle like for the main jet tests, just a good sized handful. This will reveal if you've cured your flat spot and, if you've gone up several sizes and get some stumbling, that you've gone too big on the pilot jet size. Once you have the pilot jet size narrowed down, there's another test you can perform. Ride around in a parking lot in about 2nd gear with the throttle just barely cracked. Watch for minor surging (too lean) or rough running and stumbling (too rich). You may be able to correct this with your mix screw setting or you might need to change pilots again."

I have a number of mods, pods, head work, free flowing mufflers, but I'm wondering if I'm over jetted. I an not accelerate smoothly through this RPM zone of badness from a stop, i.e., its more than just a quick throttle turn that's giving me problems.

Comments?

Thanks

 

[5twins]

You mentioned head work. By that I assume you mean porting. Porting can alter your jetting needs because it can change the flow rate. After I ported mine, I had to go down a size on my pilots. I had heard that better flow could require smaller jets because that better, harder flow would pull more fuel through the jets. This was the first time I ever experienced it. However, it was only the pilots that I reduced. The mains, in fact, were able to be bumped another size up after the porting.

 

[glennpm]

Yes 5Twins, I ported my heads. I decided this morning that I am going to pull my carbs and check float level again and condition first. From your comment, I may go back to the original size of the pilots and hope for the best!

Thanks for responding.

 

[5twins]

Yes, I meant to ask about the floats. These bikes are very picky about that setting, more so than most other bikes I've worked on. I'm not sure how you check your floats but I like to use a 6" steel rule graduated in MM on one side. With the mains and pilots mounted in the float bowl on the BS38s, that means there is no jet tower sticking out the bottom of the carb body between the float bulbs. This makes measuring them with a ruler pretty easy. Set the ruler on the far side of the float and sight in line across the tops of both bulbs. Then do it from the other direction and compare the readings .....

As I mentioned, it's quite common for the float to be tweaked of twisted, making one bulb sit higher or lower than the other. Probably near half the ones I check are like this. Fixing it is simple. Grab one bulb in each hand and twist the assembly back the way it needs to go to make it sit level. Be gentle, these tweak easily, which is illustrated by the fact you find so many twisted to begin with.

 

[glennpm]

Appreciate the picture. Your earlier comment about the two not matching was what got me thinking. I only checked from one side. I also pulled the bowls to change the pilots and although gentle may have changed the setting.

What would you start with for the pilots considering the port job i did? I have 27.5, 30.0 and 32.5 now.

 

[5twins]

I would say check the floats first. If they're off, correct them and try the #30 pilots again. If they're OK, try the 27.5 pilots because the 30s and 32.5s aren't working for you now. The spec for the '78-'79 carb set is 24mm ± 1mm. I experimented with mine at all three settings (23, 24, and 25mm) and found 24 to work best.

You may be wondering about the pic above, looking to be set at 25mm. They are, those are a set of '76-'77 carbs and 25mm is their spec. I'm not sure why Yamaha changed the spec by 1mm between the two carb sets. My best guess is the newer set had bigger mains (135) and needed a bit more fuel in the bowls to supply them. The '76-'77 set only used 122.5 mains.

 

[glennpm]

Yes, I noticed the 25mm right away but wasn't going to say anything. Sometimes camera perspective throws the 'view" off. I've done that a few times. But now I know that you really did set them that way ;-)

 

[5twins]

Yes, that's what's known as a "parallax" effect. Your readings will be incorrect unless you sight directly from the side and with the tops of both bulbs in line. Here's a couple more shots of those same carbs, first looking from too low .....

..... and then from too high of an angle .....

As you see, it can really throw your readings off.

 

[glennpm]

Okay, I've got them reset they were too high with carbs upside down and tweaked. wasn't as careful as I thought I was. I also put the .30 pilots back in. More work tomorrow putting the carbs back in.

One thing that bothered me though was some oil in the intakes on both sides. Think this is too much?

A very little smoke on right side right after I start cold but then gone. Motor had 7k on it, I lapped the valves but didn't replace the seals and wish I had now. I'm not sure if I replaced them or not three years ago. I replaced the seals during the engine rebuild, found the receipt and a record that I did them. I'm surprised that I have the thin coating of oil though.

This fix may be in my future ... darn it

http://www.650central.com/smoking_due_to_worn_valve_seals.htm

http://xs650temp.proboards.com/thread/12269