Stumbling

[BA80]

Thank You BA80! I have been fighting a serious off idle hesitation/stumble for weeks now on my new to me 79SF. The bike came with a mac 4 into 2 exhaust, and I put pod filters on it. I figured it had to be a matter of getting the jetting right. I tried every combination that I could think of without much improvement and sometimes making it worse. Then I read your post about the slide vent hole orientation, something that I never thought of before... and sure enough, the holes were pointing in 4 different random directions. The rubber diaphragms were loose and could easily be turned on the slide. I aligned the holes facing the engine and put a little RTV around the rubber and plastic joint to prevent the rubber from from rotating. I also found aftermarket needles in the slides so I put in the correct OEM needles. What a huge difference! Thank you for your brilliant insight!

Thanks for calling me brilliant. Nice change from the usual description.

Do the diaphragms have the stock hard plastic rings mounting the rubber to the slide or is it gone? If it is gone that means someone has replaced the diaphragms.

If they have been replaced a little super glue in the groove on the slide is all you need. If not, I just lightly spiked the top edge of the slide with a center punch to tighten up the plastic rings on the set I encountered.

[HalfCentury]

My carbs diaphragms have the tab that locates the diaphragm correctly.

I spent much of today disassembling the carbs, taking the rack outside to spray carb cleaner out of every hole/opening.

This is the first set of Mikunis that I have owned. My XJ650 had Hitachi carbs.

I think I missed some passages the first time through. This time I sprayed until I got a good stream out the exit of every opening.

Now I have to buy a small metal metric ruler. It will be good to have one in the toolbox. Once I check/adjust the floats dry, I will put the bowls on and take a wet measurement. Just looking at the floats with a critical eye, the tang on float #4 appears to be way more bent than #1 through #3.

I also sanded all of the bowl gaskets to remove the gasket material hanging into the bowls.

I will let the measurements guide any needed adjusting.

[BA80]

My carbs diaphragms have the tab that locates the diaphragm correctly.

So did the set I came across. The plastic rings that mount them to the slides were loose and they had spun on the slides. Not saying that's your issue, just saying.

[Ranger_xs1100]

BA80,
Yes they have the original plastic rings, which I tried tapping together but was afraid of breaking something, so I chose to RTV the top plastic ring to the top of the metal slide, using a minimal amount....I think pinging the metal might have worked, but the chance of breaking old plastic was not worth the risk to me... My locating tabs were in place but I would have never guessed that the vent hole orientation underneath the slides was not...Thanks again!

[HalfCentury]

Here is an update.

The stumbling remains after carefully going through the carbs.

Removed the valve cover to re-visit the valve clearances. I adjusted the clearances many months ago.

The re-check of the clearances shows that one of eight is too loose. Two other ones are too tight.

I have a spare shim that fills the need on one of the three. The other two had to be ordered. After the re-shimming the remaining two, the clearances should be good.

Progress may be happening. I pulled the plugs. Cylinder #1 and #2 are now both fouled. This leads me to believe that the left side coil may have been marginal and/or intermittent and is now just bad on both plugs. I will investigate ordering at least one coil for the left side to see if that cures the fouling problem.

If anyone has an old but completely functional coil to let me try that would be helpful. Otherwise I will have to order one.

[Bonz]

One and two are served by different coils. 1,4 and 2,3 is how they fire. With a wasted spark on the one that's not firing.

Silly question, you do have the plugs being fired with those respective coils?

[HalfCentury]

One and two are served by different coils. 1,4 and 2,3 is how they fire. With a wasted spark on the one that's not firing.

Silly question, you do have the plugs being fired with those respective coils?

The plug wires are numbered and still readable after 35 years. So, yes, the plugs wires are in the correct locations.

And, I realized after posting that if its a coils problems, both coils will need to be replaced.

[Bonz]

The plug wires are numbered and still readable after 35 years. So, yes, the plugs wires are in the correct locations.

And, I realized after posting that if its a coils problems, both coils will need to be replaced.

Ok, good. I wasn't questioning if the correct numbered wire was on the same numbered cylinder.

When you said "cylinder #1 and #2 are now both fouled", Left side coil" ,"order at least one coil for the left side"... That wasn't adding up.

[skids]

Well, it still does not add up to me. If either coil is bad, both of its plug wires would be problematic, UNLESS one of its plug wires is shot. What is the chance that BOTH coils have one plug wire that is shot? Pretty slim. If you are going to buy a new coil, then yes, buy a pair and upgrade to Dynas or the like.

[BA80]

I've had a coil go bad internally so one tower didn't get fire. Not so much a mystery.

[Schming]

Here is an update.

The stumbling remains after carefully going through the carbs.

Removed the valve cover to re-visit the valve clearances. I adjusted the clearances many months ago.

The re-check of the clearances shows that one of eight is too loose. Two other ones are too tight.

I have a spare shim that fills the need on one of the three. The other two had to be ordered. After the re-shimming the remaining two, the clearances should be good.

Progress may be happening. I pulled the plugs. Cylinder #1 and #2 are now both fouled. This leads me to believe that the left side coil may have been marginal and/or intermittent and is now just bad on both plugs. I will investigate ordering at least one coil for the left side to see if that cures the fouling problem.

If anyone has an old but completely functional coil to let me try that would be helpful. Otherwise I will have to order one.

If I may, test the coils and caps before replacing. I'll look in my stash and if I still have a good set I'll send 'em to ya.

Are you sure the jets in the float bowls are clean?

Have you removed the emulsion tubes and made sure the small holes are clear?

I know it takes time and effort but I'm a firm believer that a carb overhaul is not complete without replacing the 30 some year old shaft seals.

[BA80]

I know it takes time and effort but I'm a firm believer that a carb overhaul is not complete without replacing the 30 some year old shaft seals.

In the numerous sets I've overhauled I've never had to replace any shaft seals.

[Bonz]

Greg, earlier he said he soaked the carbs when he did a major cleaning years ago. The thought TC shared is the carb dip may have damaged the seals. Are you saying carb dip shouldn't cause a problem? I have always soaked my carbs in Pine-Sol, it doesn't damage anything.

[skids]

Also, the tomato can fix for the exhaust may be indicative that the baffles are shot. It is real difficult to compensate for this with carb adjustments.

[TopCatGr58]

Greg, earlier he said he soaked the carbs when he did a major cleaning years ago. The thought TC shared is the carb dip may have damaged the seals. Are you saying carb dip shouldn't cause a problem? I have always soaked my carbs in Pine-Sol, it doesn't damage anything.

Hey Bonz,

I reread this thread just to make sure, but I didn't see where he said he soaked the carbs. I think you may be crossing threads/posters.....the other fellow DakotaFlying/Tim was the one that had cleaned/soaked his carbs back in 2000.

T.C.