4000 rpm stumble

[corey]

I know I've been posting a lot here lately, but hopefully I'm getting this wrapped up. Would have continued this on the last thread, but I think its different enough to warrant a new topic.

Just got my bike going again after some ignition problems. It runs, if just a little rough (pops and gurgles from the pipes, mostly from the right [4-2 exhaust]). When I roll the throttle, however, there is plenty of power... until 4000 rpm. It cuts out for a second, then continues on the upper revs with problem, just as smooth as before 4000 rpm. I guess you could say it stumbles over the 4000 mark. Now does that sounds like another ignition problem, or is it a carb/jetting thing? See, I just added cone filters and I'm not 100% sure I have the jetting dead on, even though I followed the jetting tips. Perhaps its lean up in that part of the power band?

[tab1324]

If it were lean because of jetting, I would think it wound't just stumble, but continue to lack power past 4000 rpms - but you say it kicks in again after 4000? This seems to me like an ignition issue. Try warming the bike up on the center stand (or ride for a while) then leaving it in nuetral and slowly revving to 4000. Does it still stumble or does this just happen under load?

Tom B.

[corey]

It just happens under load. While revving it after it is warmed up it climbs as high as you let it. No burble or stumble whatsoever.

Ignition problems would produce that problem all the time at that rpm, no?

[TopCatGr58]

Hey Corey,

First, don't worry about the posts, that's what this site is for! Secondly, I'm not a carb tuning guru, or "any" guru for that matter, but I found this in the carb FAQ section. The Stumble may actually be the bike getting too much fuel too soon, until the rpms get up a bit, then it can handle it up to redline. That rpm is where the slide needles start rising and allowing the mains to kick in. The following may explain about why:

Q: What about the springs that push the slides down? Do they wear out?

A: The short answer is that stiffer springs seem to cause the mixture to be leaner and softer springs seem to be richer. I believe they do soften up over time and I think it's important that all four carbs have matching springs. Here is my experiment:
I have several XS650 carbs as well as XS11 carbs. Many of the parts are interchangable. I noticed that the XS650 slide springs are about 10mm shorter than the XS11 springs (both from '80 models). I have been fighting an "off idle" stumble on the XS11 since I've owned the bike. I got to thinking that shorter springs would let the slides open quicker and probably improve the performance (WRONG!) -- it's a good thing I had the shorter 650 springs to try instead of cutting my XS11 springs shorter. I discovered, to my surprise, that the engine didn't run well at all with the shorter springs - it wouldn't take any throttle, just sort of gagged on it unless opened very gradually - it seemed to exaggerate the problems I had been having before. So, I decided to try lengthening the springs instead and started wondering how much tension they might have lost being compressed for 20 years. I carefully stretched the stock springs out about 8mm longer and reassembled the carbs. It accepts the throttle much better than it ever has. NOTE: my bike has low restriction air filters and exhaust and perhaps this is unique to my setup and not something required on a stock system. My impression is that the springs had softened over time and were allowing the slides to lift too quickly, causing an overly rich mixture at low rpms - but it may just mean my exhaust setup requires slightly stiffer springs than stock.

Aside from adjusting the length of the springs, you can adjust the position of the slide needles so that they stick downward further=leaner, placing the rings on the upper notches of the needle vs. the middle=oem placement. HTH?
T.C.

[randy]

Then again, you may have the dreaded pickup wire problem. Or, your vacuum advance unit and / or mechanical advance unit may be sticky.

[corey]

Randy: I investigated the pickup wires, and they appeared to be fine. When looking at them, I'm supposed to check for rubbed spots, etc.? While in there, I also turned the advance unit by hand. It moved freely and I could hear it creating a vaccum. Any further ways to test it? I knwo I can look in my book, but I'm lazy!

TC: I tried to set the needles the other day, and to no success! i couldn't figure out where to adjust the clip. I saw the E clip, but I didn't see any other intentions on the needle. Could you run through that with me? Remember, my carbs are from an 80.

I think I will try stretching the springs, as well. I put the air filters on it, and it had pipes before, so I figure my setup is quite a bit like the authors. I doubt it could hurt!

[Ken Talbot]

Corey - when you're checking your pick-up coil wires, you don't just look at them looking for rubbed spots. With the engine running, you pull and tug quite forcefully at the wires. You're looking for a spot where the wire has broken inside the insulation and it only occasionally comes apart enough to break contact. If you can pull any one of the wires enough to stretch the insulation, or break it completely with just your fingers, it was already broken. When the engine is running and you find a break, the effect is immediate.

[prometheus578]

Interesting concept, TopCat.
Aren't 25 yr old bikes a hoot to experiment on!

[TopCatGr58]

Hey Corey,

Sorry for the misinfo regarding your carbs, was looking at your signature, says 79Sf, and forgot you had swapped the 80 carbs in there. You're right, the 80's don't have any "easy" adjustability of the slide needles!

Pull the vacuum advance hose off of the carb, and apply suction to it while watching the timing plate, ensuring that it turns easily and returns to it's original position without any snags!

Aside from pulling on the wires, you can use your meter checking for resistance of the P/U coils, states ~760ohms, however, while you're measuring it, rotate the timing plate, and see if the values change, like Infinite showing a Break in the wire!
T.C.
Prom, shame Yamaha removed the adjustability of the needles in the later years. Just thankful mine behave as good as it does.

[corey]

Alright. Been busy but I'm about to get to it with this bike again.

I checked the pickup wires. They had some smooshed spots, so I repaired them. Seem to be working well now, though the stumble was still present. I then took apart both advance mechanisms. The mechanical system was fine, just dry, so I lubed it. The vaccum mechanism tested well with the vaccum hand pump, but when I turned the unit through its travel by hand, I could feel a slight click about 2/3 of the way through its travel. It felt almost like a spring was being oversprung or something. Just a soft click and "twang". Not supposed to be like that, no?

Because of this, I believe the vaccum advance is the source of my problems. I'm going to tear the carbs down tomorrow and try the spring thing, but I'm pretty sure that won't cure my problem. In the mean time, does anyone know a good place to get the vaccum advance mechanism for my 79SF???

[TopCatGr58]

Hey Corey,

I just did a search for that part number:3H5-81683-09-00 It's a discontinued item , so BikeBandit, nor OldBikeBarn lists it!

You said it was working, holding vacuum. You can see where it connects to the timing plate. Disconnect it, and then turn the timing plate and see if you can still feel that "snag". If yes, then you know, it's not coming from the vacuum unit, reinspect all surfaces, look for wires snagging on parts, etc.. Also, a fellow Xsive found a worn spot where the vacuum plunger afixes to the timing plate, either put some washers there, or filed of something to stop it from hitting/snagging against whatever it was hitting!?

Good Luck!

[corey]

The unavailability of this item is a bit discouraging, though I saw one on eBay the other day for around $80. Might end up having to go that route.

In the meantime, I'll take your advice to heart and get after that thing sometime soon. I'll inspect it closer this time, as well. Thanks!

[LoHo]

Hey Corey, I've just encountered the same sort of problem. Starts fine, seems to rev fine, and runs fine with lots of throttle, but stumbles or hesitates in the mid-range, @4000 or so.

I took the the tops off of my carbs and examined the diaphragms of the vacuum pistons/needle jet...three of them had pin holes from wear, one having two holes. This seems to be a logical solution, as at idle they don't come into play, and at higher revs the excess vacuum would overcome the small leaks. In the mid-range, though, the carb opening would be erratic as vacuum and leakage would bounce them around. I've done the "PlastiDip" fix and will re-assemble Friday. We'll see...

[corey]

Let me know how that goes. I just had the tops off and examined mine last weekend... the diaphragms appeared fine. It does sound like it will fix your problem though.

Recent developement/observation: when I rev up the problem area slowly, the bike works through it. How's that for a clue?

[LoHo]

I seem to have solved my problem. Corey, when you hold it at 4000, does it hesitate, or is it only during the transition?

FYI, my bike is about as stock as can be, all original.

On the other hand, while I was out blasting away with my new-found power, my fuel guage quit...