The dreaded 6 k RPM stumble pick up wires good?

[markb80]

So after 2 years of no major issues with my 80 special, it had to sit for a year as I wasn't able to ride (health issue). Cleaned the carbs all out, got it purring again and carbs synchronized, fuel level and diaphragms all good in carbs. Pick up wires checked pretty thoroughly, no breaks found.

I get a stumble only if I go full throttle up towards red line at 6k. If I go smoothly up to 8k, no problem. My first thought was pickup wires, so I unhooked the vacuum line to run without vacuum advance. Also velcro-ed a volt meter to the gas cap to take some data. I hoped the stumble would be gone or same. Instead, the stumble(cut out) moved up 1kRPM to about 7k RPM. Voltage read 13.8V from 4k and up (RPM). I ohmed out coils, primaries at spec, but secondary measurement was 25k on each. I attribute that to 15k ohm (secondary + 2- 5k ohm resistor caps on wires). So that seemed pretty normal.

I swapped ECU's, no difference. I made sure carb slides are free and diaphragms good, no problems found. The stumble makes all cylinders seem to cut out (or at least 2, feels like all), till RPM comes back down.

Carb manifolds look great. I am not sure whether it is coils/weak spark, or maybe diaphragms bad but look good. I have seen an XS650 do this with cheapo pods causing not enough vacuum at rear of carbs to make the slides stay up (they would fall down with full throttle due to reduced vacuum).

So I could try swapping all the slides, but don't really think that is it. Maybe its coils or corrosion in wire ends? Maybe need to trim a quarter inch of coil wires. I looked at primary connections, very clean. Stator output plug, very clean. Stumped!

[BA80]

Have you put new plugs in it?

[James England]

Have you put new plugs in it?

If it's that that's causing this problem, you'll be up to 20 points.......

[James England]

or maybe diaphragms bad but look good

Have you held the diaphragms over a bright light (preferably in a low-lit room) and seen if there are any pinholes, or tears or creases that let light through?

What exactly did you do when you cleaned the carbs?

[BA80]

I was getting a little bit of stumble and hesitation with my SG all this spring and about a month ago I washed it and like to never got it started. When I finally got it going it would missfire under a load. I let it sit overnight hoping whatever it was would dry out and start working but it didn't.

I put a new set of plugs in it and made sure the caps were getting good connection and EVERYTHING cleared right up. The plugs were a couple years old and had maybe 20K on them.

That's why I asked.

[markb80]

I am leaning toward diaphragms

Yes I tried new plugs, and others. I have a box of good plugs, and a plug cleaner (blasting media connected to air compressor).

I am leaning toward diaphragms because even though I didn't see holes, they were a little worn at sealing surface (outer ring of rubber). I didn't hold them up to light. They didn't look the prettiest, but no obvious tears.

I think its diaphragms because when I get it to start cutting out, it never recovers. It makes me visualize "The Slides Have Fallen, And They Can't Get UP!" : D.

I have a new aftermarket set stashed away, guess its time to use them.

Also I have those "cool looking" chrome plastic diaphragm covers on outside carbs. I don't trust those, time to pull those off and use the steel ones. The outer plastic covers may be warped and/or not sealing well.

If I roll up to redline slower they are probably "staying up" better. Once engine starts cutting out and the butterflies are all the way open, I think you lose some vacuum necessary to keep them up.

I have seen a similar problem on every pre-1980 XS650 I have owned and/or worked on, if cheapo pods were used. With better pods with more flow resistance the slides lift better and stay up better.

I'll let you know, may take 1-2 days to get to it. Maybe I should just not cane the poor old bike so much : D

[BA80]

Maybe I should just not cane the poor old bike so much : D

She loves it and you know it.


Check all your plug in connections and fuse connections. Also the grounds at the regulator under the seat. Time generally causes connection problems due to corrosion.

It wouldn't hurt to recheck those pickups either.

The stumble makes all cylinders seem to cut out (or at least 2, feels like all), till RPM comes back down.

That really sounds like the pickup wires.

[markb80]

It could be a ground also. I checked and regapped the pick ups to the rotor tip, as I had to take the wire holders loose to check the pick up wires throughly.
The thing that makes me think its not electrical is the fact that if I wind it up smoothly with or without advance vacuum hose connected, I can make it up to close to red line without issue. I even polished up the slides and carb bodies to make sure they moved smoothly up and down. Since its a high RPM WOT problem, I think I can generally rule out a slight manifold that I'm not seeing, and also butterfly seals. Battery is good too, emulsion tubes clean.

[cywelchjr]

[QUOTE=markb80;380106 Instead, the stumble(cut out) moved up 1kRPM to about 7k RPM. Voltage read 13.8V from 4k and up (RPM).[/QUOTE]

13.8 from 4k and up is low voltage for the charging system and can cause your spark to be a bit low, you should have a MINIMUM of 14.2 volts at 3.5k and up to 14.8 volts from 3.5k and up. You need to run through ALL the charging system checks, including making sure that your battery is good and fully charged, as a bad battery can make a good charging system look and act bad, so make sure it's fully charged and have it load tested before testing the rest of the charging system, but if the battery is fine, something is wrong with the charging system if your accurately reporting the charging voltage at 4k and higher.

[cywelchjr]

It could be a ground also. I checked and regapped the pick ups to the rotor tip, as I had to take the wire holders loose to check the pick up wires throughly.
The thing that makes me think its not electrical is the fact that if I wind it up smoothly with or without advance vacuum hose connected, I can make it up to close to red line without issue. I even polished up the slides and carb bodies to make sure they moved smoothly up and down. Since its a high RPM WOT problem, I think I can generally rule out a slight manifold that I'm not seeing, and also butterfly seals. Battery is good too, emulsion tubes clean.

One thing to remember is that there is a BIG difference in the amount of spark needed to fire under heavy load VS light load, so WOT will cause a miss that even steady throttle won't. Check that charging voltage cause 13.8 is NOT enough voltage to run the bike a the times it's requiring max out of the system, such as WOT at higher RPM's.

[Rasputin]

My guess is a vacuum leak as I just went thru the same thing. One of my "Blind Plugs" on my carb boots was leaking. Was very puzzling as it did seem to clear up some when everything was warmer.

[BA80]

My guess is a vacuum leak as I just went thru the same thing. One of my "Blind Plugs" on my carb boots was leaking. Was very puzzling as it did seem to clear up some when everything was warmer.

The only thing I really noticed when one of mine blew off was lousy idle and a low end misfire.

[markb80]

Well I don't think it was the diaphragms. Sure reminded me of what a falling diaphragm would do.

I think it IS a weak spark problem. It bothers me that the secondary resistance measurement on coils is 26 kohm across spark plug caps on coil for cyls 2-3, and 29.8 kohm across spark plug caps on coil for cyl 1-4. It is supposed to be 15k. I checked what the resistance of the resistor caps is supposed to be, and it's 1 kohms. So secondary resistance is too high. Well out of spec.

Is a high secondary resistance a common failure mode? Maybe the wires developed higher resistance? I guess a set of Dyna coils might fix it, but I better ohm out charging system stuff first. Will do that tonite.

BTW, aside from this issue, this is a sweet 1980 Midnight Special with gold parts replaced by chrome parts. I am LIKELY TO SELL as I am getting an FJR 1300, and need to make room. I have spare carb diaphragms (excellent used set, and a new set), spare charging system (or two), a couple spare ignitors, 2 good ignition pick up plates, 2 spare voltage regulators, middle and rear drive unit (on swingarm), and a bunch of other parts in boxes. I put up pics on the older website, painted in black lacquer (very nice looks similar to Euro XS1.1 Sport). However, after 4-5 years the lacquer clear coat started cracking. It has about 17 k miles, valves adjusted, nice tires, forks, brakes, speedometer, Maier fairing etc etc, you name it. Transmission excellent, no 2nd gear issue (spare trans parts too). I hate to sell, as its been my cruiser for around town. It has a SWEET and rare Racer One exhaust (4-1 ceramic coated). I accumulated parts from fixing several XS11's over the years and a couple parts bikes that I disassembled. It is located in Dayton Ohio. I will try to get pics up and offer to sell to forum members. It is in very rideable condition.

[crazy steve]

...I think it IS a weak spark problem. It bothers me that the secondary resistance measurement on coils is 26 kohm across spark plug caps on coil for cyls 2-3, and 29.8 kohm across spark plug caps on coil for cyl 1-4. It is supposed to be 15k. I checked what the resistance of the resistor caps is supposed to be, and it's 1 kohms. So secondary resistance is too high. Well out of spec....

Actually, they're not too bad. The 15K ohms number is across the secondary side of the coil, less the resistor caps, which are 5K ohms each. So your 2-3 coil is close to spec, 1-4 is the one you want to look at closely...

[motoman]

Yes I tried new plugs, and others. I have a box of good plugs, and a plug cleaner (blasting media connected to air compressor).

I am leaning toward diaphragms because even though I didn't see holes, they were a little worn at sealing surface (outer ring of rubber). I didn't hold them up to light. They didn't look the prettiest, but no obvious tears.

I think its diaphragms because when I get it to start cutting out, it never recovers. It makes me visualize "The Slides Have Fallen, And They Can't Get UP!" : D.

I have a new aftermarket set stashed away, guess its time to use them.

Also I have those "cool looking" chrome plastic diaphragm covers on outside carbs. I don't trust those, time to pull those off and use the steel ones. The outer plastic covers may be warped and/or not sealing well.

If I roll up to redline slower they are probably "staying up" better. Once engine starts cutting out and the butterflies are all the way open, I think you lose some vacuum necessary to keep them up.

I have seen a similar problem on every pre-1980 XS650 I have owned and/or worked on, if cheapo pods were used. With better pods with more flow resistance the slides lift better and stay up better.

: D

.......just another reason why the vacuum slide set-up and spring used all works together with the stock air-box.....just sayin. I would do one thing at a time, by first replacing the carb tops with the original steel tops, since that IS the easiest test first. Least that would rule out some guessing there. Then I would remove the left-side timing cover and remove the vacuum advance from the advance timing plate and make sure that advance assembly is floppy loose. If its a bit stiff moving, a quick fix is spray some brak-kleen with the plastic straw attached around where the crack in between the fixed part and the moving part while manually rotating that assembly back and forth. Should get some nasty looking black stuff running out the bottom. Then blow it out dry real good with the compressor air. Next, use some light gun-oil by getting it to weep by that seperation at the top and down the sides(it will migrate down on the small ball bearings). That set-up is kinda like a lazy-susan. Nothing spins fast there, just the rotating back and forth from the vacuum can arm. Just a good check and idea to free things up that I've used and had excellent results. Let us know here what the culprit ends up being!