Flat spot at 3500 RPM's

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Track T 2411
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Flat spot at 3500 RPM's

#1

Post by Track T 2411 »

Miss Scarlett, 76 with freshly rebuilt carbs (Randakk's kit), stock air box and filter, stock exhaust, older Dyna ignition, new plugs. Fuel tank was split, media blasted, and sealed several years ago, and shows no signs of issues. Starts almost instantly, responds to throttle well, idles well at 1200 RPM'S but a little slow returning to idle the last 500 RPM'S or so. Worst issue is a very obvious flat spot at 3500-4000 RPM'S. It's not noticeable when accelerating through, but at 3500-4000, steady throttle, giving her a twist, she bogs down big time. Above 5k she rips again. Feels like a fuel starvation issue, but that doesn't make sense to me if it's only at mid-range... I should mention I did not do the 'off idle fix' ... Ideas or suggestions?
"I have no special talent. I am only passionately curious." - Albert Einstein

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Current Rides:
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'Layla' - '81 Standard w/dealer installed fairing and Hondaline bags.
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In The Shed:
'81 gl1100I barn find aka "Josie, the farmer's daughter." (almost comatose build)
'77 gl1000, roller parts bike.
'82 gl1100I, 'Old Crusty' titled roller parts bike (free!)
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Re: Flat spot at 3500 RPM's

#2

Post by robin1731 »

You should do the off idle fix. ;)

But I don't think that is your problem. It does sound lean by your description. Could also be ignition related. You sure the advancer is working correctly? Go through a tune up procedure. Including valve adjustment. Check timing at various rpm.

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Re: Flat spot at 3500 RPM's

#3

Post by Track T 2411 »

We adjusted the valves when we did the belts.
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Pics of the plugs. Black and sooty. The bike had been idling a lot beforehand. We have since done a compression check, 92-95 PSI across all four (the bike has been sitting 2 years). Synced the carbs and did some minor adjustments to the idle mix screws, which smoothed out the idle better. We did have issues with a couple of the plug wires. We'll start with checking the valves and the advance tomorrow, along with cleaning the the plugs, then go from there. Thanks!
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"I have no special talent. I am only passionately curious." - Albert Einstein

"He that is good with a hammer tends to think everything is a nail" - Abraham Maslow

"If you can't take the time to do it right the first time, how are you ever going to find the time to do it over?" -Unknown

Current Rides:
'Grumpy' - '81 Standard, now fully dressed.
'Layla' - '81 Standard w/dealer installed fairing and Hondaline bags.
'Scarlett' '76 'Survivor' nekkid as a j-bird!

Under Construction:
The 'Jalopy' '78-'79 Mash-up
'Quikie' '81 gl1100I back on the lift, project with the step-son!

In The Shed:
'81 gl1100I barn find aka "Josie, the farmer's daughter." (almost comatose build)
'77 gl1000, roller parts bike.
'82 gl1100I, 'Old Crusty' titled roller parts bike (free!)
'82 gl1100I, My first 'Wing, and an expensive lesson!
New2U Bike? Read Me.
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Re: Flat spot at 3500 RPM's

#4

Post by todd54219 »

You might think about throttle shaft felts. I did mine last year and it really helped with the slow return to idle. It also helped it start easier.

Todd
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Track T 2411
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Re: Flat spot at 3500 RPM's

#5

Post by Track T 2411 »

Update... Reset the valves, cleaned the plugs, then decided replace the Dyna III and stock coils with a new Dyna S complete system I had for my '78 project as we couldn't get clean wire from the stock plug wires. The advance seems to work fine (haven't done a dynamic test yet, though). Finally got it out for a couple of short runs (maybe 10 miles total, various RPM) then pulled the plugs. Sooty black again, except for #3 plug which looked quite normal (slight tannish white). We were running out of daylight, so we couldn't investigate further.
I'm curious about the change in plug #3 compared to the others. Next step will be to check dynamic timing, clean the plugs yet again, check for fuel at #3, and do a proper 'chop'. Any other suggestions?
"I have no special talent. I am only passionately curious." - Albert Einstein

"He that is good with a hammer tends to think everything is a nail" - Abraham Maslow

"If you can't take the time to do it right the first time, how are you ever going to find the time to do it over?" -Unknown

Current Rides:
'Grumpy' - '81 Standard, now fully dressed.
'Layla' - '81 Standard w/dealer installed fairing and Hondaline bags.
'Scarlett' '76 'Survivor' nekkid as a j-bird!

Under Construction:
The 'Jalopy' '78-'79 Mash-up
'Quikie' '81 gl1100I back on the lift, project with the step-son!

In The Shed:
'81 gl1100I barn find aka "Josie, the farmer's daughter." (almost comatose build)
'77 gl1000, roller parts bike.
'82 gl1100I, 'Old Crusty' titled roller parts bike (free!)
'82 gl1100I, My first 'Wing, and an expensive lesson!
New2U Bike? Read Me.
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Re: Flat spot at 3500 RPM's

#6

Post by Track T 2411 »

Still sorting out stuff, but haven't had much hands - on time due to weather and other more pressing tasks. :mrgreen: My unfamiliarity with the 1000's became even more apparent when I realized it had the wrong air filter in it. Must be from a 1200, which is shorter (among other differences), so it wasn't even being held in place by the air box cover. I also failed to fully tighten the sync port screws after the carb rebuild, which probably contributed to the slow return to idle, which is much better now. Reading through another thread, I realized I haven't rechecked the intake clamps either, so that is on the list as well.

I've also found that it is over - charging (15v+ @idle), but haven't investigated it further. I have a spare regulator off the '78, I've read a lot about the black sense wire bypass, as well as upgrading to a mofset type reg/ rec combo. Just haven't decided which way to go...

BTW, I still have that dang flat spot at 3500...
"I have no special talent. I am only passionately curious." - Albert Einstein

"He that is good with a hammer tends to think everything is a nail" - Abraham Maslow

"If you can't take the time to do it right the first time, how are you ever going to find the time to do it over?" -Unknown

Current Rides:
'Grumpy' - '81 Standard, now fully dressed.
'Layla' - '81 Standard w/dealer installed fairing and Hondaline bags.
'Scarlett' '76 'Survivor' nekkid as a j-bird!

Under Construction:
The 'Jalopy' '78-'79 Mash-up
'Quikie' '81 gl1100I back on the lift, project with the step-son!

In The Shed:
'81 gl1100I barn find aka "Josie, the farmer's daughter." (almost comatose build)
'77 gl1000, roller parts bike.
'82 gl1100I, 'Old Crusty' titled roller parts bike (free!)
'82 gl1100I, My first 'Wing, and an expensive lesson!
New2U Bike? Read Me.
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Re: Flat spot at 3500 RPM's

#7

Post by desertrefugee »

It sounds like you're really getting it sorted. I usually don't mind the sort of problems that don't prevent operating the machine. The overcharging would be a bit of a concern and likely ought to take priority.
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Re: Flat spot at 3500 RPM's

#8

Post by Whiskerfish »

I think if this were mine I would pull the carbs again. Double check the Air jets in the top and make certain they are in the correct places and check the operation of the CV slides. I use a vacuum cleaner. With the rack hanging off the edge of the work bench I put the cleaner hose in the bottom of the runner of each carb one at a time and then manually open the throttle and monitor the air flow through the plenum. As you open the throttle you will hear the air flow increase dramatically as the slide goes up. Once you have done it a couple of times you can easily notice the difference once the slide goes up. Repeat for all 4 to ensure they are working evenly and smoothly. Want to make sure the bowls are empty first to reduce and fuel being ingested by the vacuum.
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Re: Flat spot at 3500 RPM's

#9

Post by Track T 2411 »

Thanks for the vacuum tip, WF. I was wondering how I might go about checking the slides for proper operation. I'm also not sure about the condition of the float needles, as the tips seemed a bit hard and all had VERY slight marks where they would seal to the seat. That said, I've tested and checked several times for leaks but found none. I don't really want to spring for new needles and seats unless it's truly an issue.
"I have no special talent. I am only passionately curious." - Albert Einstein

"He that is good with a hammer tends to think everything is a nail" - Abraham Maslow

"If you can't take the time to do it right the first time, how are you ever going to find the time to do it over?" -Unknown

Current Rides:
'Grumpy' - '81 Standard, now fully dressed.
'Layla' - '81 Standard w/dealer installed fairing and Hondaline bags.
'Scarlett' '76 'Survivor' nekkid as a j-bird!

Under Construction:
The 'Jalopy' '78-'79 Mash-up
'Quikie' '81 gl1100I back on the lift, project with the step-son!

In The Shed:
'81 gl1100I barn find aka "Josie, the farmer's daughter." (almost comatose build)
'77 gl1000, roller parts bike.
'82 gl1100I, 'Old Crusty' titled roller parts bike (free!)
'82 gl1100I, My first 'Wing, and an expensive lesson!
New2U Bike? Read Me.
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Re: Flat spot at 3500 RPM's

#10

Post by Whiskerfish »

Bad float valves will usually show themselves quickly with fuel on the floor of the plenum. That said when I rebuild a rack I flip the whole thing upside down with the bowls removed and using a long piece of fuel line I blow into the barb where the fuel goes in. There should be no flow. Then I gently lift each set of floats individually and as they pass through level the air should start to flow and you can feel it and hear it. Release and it should stop. I do this several times for each float. Quick and easy check.
"Agreement is not a requirement for Respect" CDR Michael Smith USN (Ret) 2017
"The book is wrong, this whole Conclusion is Fallacious" River Tam
2008 GL1800 IIIA "TH3DOG"
1975/6/7/8/9 Arthur Fulmer Dressed Road bike
1975 Naked Noisy and Nasty in town bike
and a whole garage full of possibilities!!

Psst. oh and by the way CHANGE YOUR BELTS!!!!
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Re: Flat spot at 3500 RPM's

#11

Post by BlueThunder »

As for the over charging... ground the regulator to the battery. At least that is one less thing to sort out.
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Re: Flat spot at 3500 RPM's

#12

Post by OCR »

Seats may be OK but if the needle tips show indents then they should be replaced.
Be sure to use Viton tips, they last longer and will not harden with ethanol exposure.
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Re: Flat spot at 3500 RPM's

#13

Post by Lucien Harpress »

I'm going to throw this out there- slides.

Namely because my '76 had the EXACT same issue you describe, and a new set of (polished up) slides did the trick. Well, truth be told I'm still dialing in some minor, unrelated issues, but of all the messing around I did with the carbs, replacing the slides made the biggest positive difference.

It was also to the point where checking the slides manually didn't make much difference- working my old "junk" slides after removing them from the carbs seemed to feel just as good as the "new" ones I replaced them with. But, for whatever reason, I think after a bit of operation SOMETHING would cause them to bind up just enough to matter in that 3500-5000 rpm range.

So that would be my recommendation. Either polish the crap out of the ones you have, or if you can find a set cheap replace them. That's my two cents anyway.
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Re: Flat spot at 3500 RPM's

#14

Post by Whiskerfish »

The slides on my dresser were acting up on the run to and from SD but only after the first 50 miles or so and she was really warm. Creates a nasty flat spot. WOT work great.
"Agreement is not a requirement for Respect" CDR Michael Smith USN (Ret) 2017
"The book is wrong, this whole Conclusion is Fallacious" River Tam
2008 GL1800 IIIA "TH3DOG"
1975/6/7/8/9 Arthur Fulmer Dressed Road bike
1975 Naked Noisy and Nasty in town bike
and a whole garage full of possibilities!!

Psst. oh and by the way CHANGE YOUR BELTS!!!!
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Re: Flat spot at 3500 RPM's

#15

Post by ericheath »

A simple check for leaking floats or seals is turn the petcock off while running at that 3500 range. If within a minute it begins to run good and then begins starving from a lack of fuel, you know you’re getting too much fuel. There is that point when the levels in the bowls will be right and it will run good for sixty seconds or so.

Simple check to cross off the list.
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