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Stumbling in the mid-range

  • Thread starter Thread starter Roostabunny
  • Start date Start date
R

Roostabunny

Guest
Hey fellas, searched around and found a couple of threads of interest on my problem, but want some help isolating/troubleshooting before I start randomly shimming things.

It's an '81 850G and my daily driver, stock with all standard fuel/air fixes in place, carb rebuilt with Robert Barr's o-rings about 2 years ago, airbox sealed, intake o-rings and boots replaced, airbox boots replaced, carbs adjusted and synced using a Morgan carbtune about three months ago.

Plugs are a pretty uniform light grey with a tinge of tan on the strap, and black at the rim of the threads.

Starts fine and normal with some choke, idles smooth, accelerates well (though not as crazy as it has in the past), and generally rides well, but I'm noticing some stumbling and bogging at various points, usually at cruising speed and between 4000-6000 rpm. I'm guessing this is at about half throttle. When I crank it up to full throttle, it smooths out, but if I ease off it gets worse. Held it at around 7000-8000 for about 1/4 mile, and it pulled well, but when I let off I got some slight popping.

Noticed some faint whitish puffs from the left hand exhaust while it was warming up at idle. Not a cloud, just puffs that disperse right away. But they're only on the one side.

Just wondering if this could be something simple like a clogged main jet, or what you might check first from this point. Thanks!
 
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Nessism said:
Valves adjusted properly?
Click to expand...

Yessir, forgot to mention that. Adjusted the valves right before I did that mixture and sync adjustment three months ago.

I'm open to the possibility that I made a mistake on that mixture and sync that led to this problem. I can't pinpoint when it started, to be honest. I think it's new within the last month, but it was only on yesterday's test ride that I really tried to pay attention and characterize the issue.
 
If it ran well before, it has to be related to a vairable, ie. fuel, air, spark.

Air is mechanical via the valves and intake pieces. All seems to have been covered there.
Fuel and spark would be the likely places to look. Have you cleaned up the plug wire caps and tested for resistance? Plug reads sound to be about right so...fuel in the midrange sounds short. Filter screens in the float valves? Petcock filter screen?
 
From what I have read and the knowledge I have gained on this site from the senior members, the mid range is controlled by the needle, main jet only regulates at WOT.
If the problem appeared and the bike was running well, something has changed, maybe some dirt in one or more of the needle jets/tube thingies.
maybe a pinhole or two in one or more of the diaphrams, affecting the lifting of the needle?
Did you do the plug chops at 4000-6000rpm, where you are getting the problem, to see if it is running lean or rich when it is buggering around?

The fact that it idles and pulls away would indicate your slow running circuit is good, and the fact that it clears up at WOT would indicate that the main jets are correct and clear.
 
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Flyboy said:
From what I have read and the knowledge I have gained on this site from the senior members, the mid range is controlled by the needle, main jet only regulates at WOT.
If the problem appeared and the bike was running well, something has changed, maybe some dirt in one or more of the needle jets/tube thingies.
maybe a pinhole or two in one or more of the diaphrams, affecting the lifting of the needle?
Did you do the plug chops at 4000-6000rpm, where you are getting the problem, to see if it is running lean or rich when it is buggering around?

The fact that it idles and pulls away would indicate your slow running circuit is good, and the fact that it clears up at WOT would indicate that the main jets are correct and clear.
Click to expand...

OK, I'm starting to put this together.

  • Idle Mixture Screw - Off Idle
  • Needle Jet - Mid-throttle (like when I'm cruising along)
  • Main Jet - WOT

I think there's overlap, but is this how it basically goes?

I'm going to need to run a search for the plug chops procedure - I think it's ride with the throttle where you want to measure, hit the kill switch, stop the bike, read the plugs, repeat, but I'm just not sure about durations and how to do it safely (aside from not doing it in traffic), cause it sounds sketchy.
 
Dave8338 said:
If it ran well before, it has to be related to a vairable, ie. fuel, air, spark.

Air is mechanical via the valves and intake pieces. All seems to have been covered there.
Fuel and spark would be the likely places to look. Have you cleaned up the plug wire caps and tested for resistance? Plug reads sound to be about right so...fuel in the midrange sounds short. Filter screens in the float valves? Petcock filter screen?
Click to expand...

I've been afraid to mess with my plug wires overly much, as they're original and I'm afraid I'll crack a boot or some-such. Not that they're flaking or dry rotted, just old. Can they be disconnected from the coils, and is that how you test resistance?

The short fuel in the midrange rings true, mainly because it literally feels like it's running out of gas. My gas gauge has never worked, but I first discovered the problem after a couple of times thinking I was out, switching to reserve, pulling up to get gas and finding the tank 1/2 full.
 
By the way, Flyboy, hopefully not a sore subject, but I was following your Jennifer rebuild thread. Hope to see more someday, esp after what you did with those carbs.
 
Flyboy said:
From what I have read and the knowledge I have gained on this site from the senior members, the mid range is controlled by the needle, main jet only regulates at WOT.
If the problem appeared and the bike was running well, something has changed, maybe some dirt in one or more of the needle jets/tube thingies.
maybe a pinhole or two in one or more of the diaphrams, affecting the lifting of the needle?
Did you do the plug chops at 4000-6000rpm, where you are getting the problem, to see if it is running lean or rich when it is buggering around?

The fact that it idles and pulls away would indicate your slow running circuit is good, and the fact that it clears up at WOT would indicate that the main jets are correct and clear.
Click to expand...
Yes, I'd check the carb diaphrams for pinholes as this would interfere with mid range throttle.
 
The first time my wire caps got wet they broke down, replace those at least before you go chasing your tail like I did. When the Bakelite caps get dull they are suspect especially with age. Generally bogging down in the mid range is too rich on the needles, but if it ran right before, I would eliminate other things like caps or wires before tearing into the carbs.
 
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Yes I would agree with Oldvet, if you have doubts about some other components, get them sorted first, work from easiest to most PIA.
If your wires can be removed, then I wouls just replace them, they probably are not insulating as well as they should if they are as old as you say. Quick way to check is remove the tank, hook up a temp tank as you would for cab sync park bike in dark garage at night and close the door.
Start her up and watch the light show all over the plug wires.
There was a thread here recently on how to rejuvinate the wires with silicone spray, basically, get a can of 100% silicone spray and soak the wires, let stand over night to draw in, repeat plenty times, the silicone draws into the wires, saling them and making them soft and pliable again.
Personally for the price of a good quality can of silicone spray, you could buy 2m of plug wire, cut to size and just replace.
replace the caps as a matter of course, then you know all is good.

Thanks for the interest in the Jennifer rebuild thread, yes, not much has happened, been busy pushing 18 hour days so it has had to go on the back burner for a while, but I am determined to get the fram and associated bits and pieces ready for the powder coaters this weekend, so watch this space, no my, space, thread, whatever........ ;)
 
BassCliff said:
Hi,

Is is possible that you have worn needle jets (emulsion tubes) or jet needles?

http://www.factorypro.com/tech/needle1.html
http://www.factorypro.com/products/Mikuni_needle_jet_buy.html

Spark plug caps are easy and cheap to replace. There's a new guide on my website with pictures.


Thank you for your indulgence,


BassCliff
Click to expand...

OK, easy, cheap, and overdue. We'll start with that. Cliff where'd you pick this things up. Local or online?

EDIT: OK, nevermind, found them on Bikebandit for <$4 each and adding to an existing order.

Thanks fellas!
 
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OK, waiting for new spark plug wire caps to arrive (among other things), but I have an odd question.

I'll take this to the search as well, but since I already have this thread... has anyone heard anything about these bikes being effected by wind?

My ride home two days ago was really windy, and I could have sworn the bogging problem was worse. In fact, it brought to mind a really windy ride a few months back where I thought I was running out of gas, pulled over, and found more than half a tank.

Any thoughts?

Edit: Ok, I knew I'd read something in here somewhere. Found this issue mentioned here - http://www.thegsresources.com/_forum/showthread.php?t=95073 and here - http://www.thegsresources.com/_forum/showthread.php?t=166927
 
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I've had that happen twice and never could find a reason. Both times I was riding by marshy areas. I have been told methane will do that.
 
Yes, a side wind can cause a positive pressure zone at the snorkel, which will lower the carb slides. The side panels of the airbox may also not be completely sealed. Not much you can do about it, maybe hold your legs in tight to block some of the wind.
 
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