Stumble at 1500-2000 RPM

[JPaganel]

So, I twiddled the idle knob and got the idle to be stable and smooth at 1000 RPM.

Trouble is, between 1500 and 2000 I get a kind of a stumble. This is not a huge problem, it runs great above 2K where most riding is, but it's kind of annoying on takeoff and slow speed parking lot maneuvers. 

It's hard to explain, so I made this video.

https://youtu.be/oiLkunafKJc

What is it, and what do I do about it? Ambulance 1.0 did not do anything like that.

[JPaganel]

I should add that carbs are just cleaned and pilot jets are 40, brand new from Randy.

I am leaning towards thinking this is a mixture screw issue.

[movenon]

I should add that carbs are just cleaned and pilot jets are 40, brand new from Randy.

I am leaning towards thinking this is a mixture screw issue.

You are probably correct on the mixture screws. All the normal questions, are you convinced the screws all have there o rings and washers installed correctly ?

As a rough test I would open each screw 1/2 turn and see if it improves.  If it doesn't then go back to your original setting and adjust in 1/2 turn and again see if it makes any difference.

When you think it is zeroed in I would do a 1/8- 1/4 max throttle run and do a plug read and see how even the plugs look. On mine the last 1/4 turn made a difference.  Mark your grip with some tape or chalk at the 1/4 points.  To RPM readings are nice but throttle position tells me more.

As a note the last week I adjusted my carb mixture screws my plug reading (at 1/8 +- throttle) told me that my outside cylinders 1&4 were on the lean side and my inside cylinders 3&4 were a bit rich.  I adjusted my outside cylinders 1/4 turn richer (out) and the inside cylinders 1/4 turn leaner (in). Now all 4 plugs look close enough to the same. That's running it at less than 1/4 throttle opening.  I also picked up a little more idle RPM when warmed up so I adjusted it back down to 1100 RPM.
George

[Flynt]

between 1500 and 2000 I get a kind of a stumble.

Dirty pilot circuit I'd bet...  new jets are one thing, clean carbs are another.  As George said, try the pilot screws as it can't hurt.  But this sound EXACTLY like my first FJ (a '90) behaved before I got Randy to clean the carbs.

Frank

[JPaganel]

are you convinced the screws all have there o rings and washers installed correctly ?

No, actually.

I have not removed the mixture screws.

As a rough test I would open each screw 1/2 turn and see if it improves.  If it doesn't then go back to your original setting and adjust in 1/2 turn and again see if it makes any difference.

I'll give that a try.

Dirty pilot circuit I'd bet...  new jets are one thing, clean carbs are another.

I did clean the carbs, not just change the jets. I have not dipped them, or broken the rack, but solvent and compressed air, healthy streams out of all orifices.

[Flynt]

"I have not removed the mixture screws"

and

"I did clean the carbs"

These statements are not compatible...  I'm going with dirty carbs still.

Frank

[movenon]

How did you clean the carbs without removing the mixture screws ?  Did you pull out the emulsion tubes ?  My definition of cleaning carbs is to totally disassemble them, thoroughly clean them (then do it again),  install all new o rings and gaskets, new jets as required (for sure pilot jets), new fuel/air screws, new emulsion tubes and needles if required, new needle and seat valves with new o rings etc.

Half measures such as changing jets and squirting some cleaner around is more of a field repair to get you home.

The carbs are pretty simple but require a good degree of paying attention to details and proper inspection of parts.  You can have real clean emulsion tubes or needles but if they are worn out of spec. It is almost a waste of time just cleaning them except for a field repair.  Did you check the choke plunger ends (the rubber seals) ? Diaphragm's for pin holes, dress up the mating surfaces for the diaphragm covers (make sure carb top surface and covers are flat). Float shafts clean, smooth not bent, floats parallel and not hitting the bowl sides and adjusted correctly by tipping the carbs to the correct angle when setting the float height.  Vent hoses routed correctly and not pinched.  Detail, details... I am still learning myself. It never stops.

But before you decide to "back track" play with the fuel air screws first and see how it works. Let us know what happens.  I didn't mean to rant like a "know it all" because I don't.  Here to help if I can and throw out some things to think about.  
George

[JPaganel]

How did you clean the carbs without removing the mixture screws ?  Did you pull out the emulsion tubes ?  My definition of cleaning carbs is to totally disassemble them, thoroughly clean them (then do it again),  install all new o rings and gaskets, new jets as required (for sure pilot jets), new fuel/air screws, new emulsion tubes and needles if required, new needle and seat valves with new o rings etc.

How do you like them tubes?

[Pat Conlon]

I had a stumble in the upper rpm range. I ruled out carbs and it turned out to be my kill switch circuit. Check your run/stop switch.

[movenon]

Tubes look clean. But is the hole in the center elongated ? The needle will bounce or vibrate in the hole and over time elongating the hole. making it harder to get your mid range consistent.  But I am betting problem is in the low end.  Pilot jet, pilot jet circuit, fuel air screw needle.

I do see that one of your pilot jets is a different style. Must be the ones you pulled out originally.

Adjust the fuel air screws first. Don't assume anything. Pat might be correct. inspect/ clean your kill switch.  I don't have a problem but I am thinking about taking my kill switch apart just to look see, clean and water proof as best I can just because it is an old bike. 
George

[JPaganel]

Tubes look clean. But is the hole in the center elongated ? The needle will bounce or vibrate in the hole and over time elongating the hole. making it harder to get your mid range consistent. 

Haven't measured. Looked round to me.

Also, is 1500-2000 really mid-range? I didn't think it was off pilots at that point.

But I am betting problem is in the low end.  Pilot jet, pilot jet circuit, fuel air screw needle.

That's what I'm thinking.

I do see that one of your pilot jets is a different style. Must be the ones you pulled out originally.

Yep. See here: http://www.fjowners.com/index.php?topic=14139.0

Adjust the fuel air screws first. Don't assume anything. Pat might be correct. inspect/ clean your kill switch.  I don't have a problem but I am thinking about taking my kill switch apart just to look see, clean and water proof as best I can just because it is an old bike. 
George

Way ahead of ya. Took apart and cleaned kill switch last week, on general principle.

[movenon]

The info about the emulsion tubes was just for information.

Your problem is low range.

If I were you I would:

Do a visual check to make sure the choke plungers were all doing the same. I would also check that the carbs are all booted and clamps snug. etc.

Check at my plugs to check for any obvious problems with one or more cylinder (clues).

Adjust the fuel/air screws and monitor for effects.  Better / worse / no change. I would think your screws would end up from aprox. 2 1/2 to 3 1/2 turns out as a course adjustment.

If no change I would bite the bullet and pull the carbs for a total tear down and cleaning.
George

[fj1289]

Is the issue at 1500-2000 RPM rich or lean?  If rich, my guess is ovaled emulsion tubes.   

How to tell if it's rich or lean?  Try covering half your air filter(s) with duct tape and see if it gets better or worse.  If better, you were lean there.  If worse, you were rich there.  No change?  Cover half the remaining filter area and repeat.