Momentary stumble at 80mph ish...

[Marcus De Grate]

Hi all,

I had a little trouble with my bike about a year ago where it had hesitation at 3000 rpm's but following the advice on here I swapped out my incorrect fuel line, also removing an in-line filter, replaced all my vacuum tubes and tank vent, new petcock, in-tank filter, new jets, cleaned the carbs and ran a little B12 through the tank for good measure and it resolved the issue.

But now after 6000miles more I'm back! 

This has happened all of a sudden.

The bike runs lovely right through the rev range in gears 1-3 at any throttle.
In 4th and 5th when you get to around 85mph on the full-throttle there is a momentary bogging down which lasts maybe 1 or 2 seconds and then no further issues right through to 100mph +

If you steadily increase the speed you can cruise on through this range with no issues at all. It's only at full throttle.
If you are going downhill there is no issue even with full throttle.

I weigt in at 220 so prob 240 max with leathers.

It may be running a little rich. The sparks look fine but the exhaust is a little sooty. (probably not the best gauge of running)

Always grateful for advice and opinions.

[old2soon]

           Once in awhile I fergit and fill my I/S right up into the neck. And when I get back on the highway sometimes she burbles or stumbles a bit. Check tank vent line and is your air cleaner fairly clean? I'm fairly certain others Will chime in. Let us know What ya find and How ya solve the issue. RIDE SAFE.

[sandy]

Check your pilot screws. Even they're for an idle mixture, they still feed fuel at all revs. Run them in lightly and open them 2 turns to start with. What size idle jets do you have? 35s are stock.

[Bagger John - #3785]

What's your air filter look like?

If you've ridden in a lot of dust for those 6k miles, airflow restriction is a possibility. That'll enrich things a bit.

[John Schmidt]

Goes good downhill? Did you check the float levels of the carbs, and do you possibly have a fuel line that's too long and causes a low spot in the line from the petcock to the T? Often that will feed enough gas under low demand but not at high demand....be it constant or sudden but usually affects sudden accel. Just a couple things to consider.

[Marcus De Grate]

Thanks,
 What really screws with my mind is why it only stumbles for a second and then picks up. I can't think how anything could cause that. I would expect a fuelling issue to be constant or get worse if you keep the throttle wide open under load, but it does not. It's just a momentary thing.
I'll of course check everything kindly suggested.

Is there anything on the electrical side that  could trouble under engine load that you can think of. I guess it can't hurt to try new plugs...

[Bagger John - #3785]

I'm betting the low-speed mixture is the culprit.

Especially in light of your "sooty exhaust" comment.

Can you hold the throttle at the setting in 4th or 5th where the power loss occurs? If maintained at that speed/position, does the bike start backfiring or running erratically?

[longrider]

If you still have the OEM petcock run a vacuum test on it.  Easy to do.  Remove the line off #6.  Get down there and apply a little vacuum.  I just put my tongue over the end and wait ten or fifteen seconds.  If it doesn't hold the bike will starve under full throttle.  

[Ricky-D]

Hi all,

I had a little trouble with my bike about a year ago where it had hesitation at 3000 rpm's but following the advice on here I swapped out my incorrect fuel line, also removing an in-line filter, replaced all my vacuum tubes and tank vent, new petcock, in-tank filter, new jets, cleaned the carbs and ran a little B12 through the tank for good measure and it resolved the issue.

But now after 6000miles more I'm back! 

This has happened all of a sudden.

The bike runs lovely right through the rev range in gears 1-3 at any throttle.
In 4th and 5th when you get to around 85mph on the full-throttle there is a momentary bogging down which lasts maybe 1 or 2 seconds and then no further issues right through to 100mph +

If you steadily increase the speed you can cruise on through this range with no issues at all. It's only at full throttle.
If you are going downhill there is no issue even with full throttle.

I weigt in at 220 so prob 240 max with leathers.

It may be running a little rich. The sparks look fine but the exhaust is a little sooty. (probably not the best gauge of running)

Always grateful for advice and opinions.

Will you certify the engine has absolutely stock carburetion? Nothing has been altered or changed? Same with the exhaust Is the exhaust in completely stock condition?

***

[Marcus De Grate]

I believe the bike is all stock. Certainly nothing obvious different from OEM. Carbs and exhaust definitely are stock.
I tested the petcock with a mityvac. Seems fine. The tank breather is clear and fuel flows freely from all carb drains.
Would it make any difference if I switched the vacuum from 6 to another? Would that indicate anything if it improved or got worse?

[Ricky-D]

I believe the bike is all stock. Certainly nothing obvious different from OEM. Carbs and exhaust definitely are stock.
I tested the petcock with a mityvac. Seems fine. The tank breather is clear and fuel flows freely from all carb drains.
Would it make any difference if I switched the vacuum from 6 to another? Would that indicate anything if it improved or got worse?

Is that vacuum line from #6 the only one present? Are there no other vacuum lines?

[Marcus De Grate]

Is that vacuum line from #6 the only one present? Are there no other vacuum lines?

Yes, it still has the smog kit.

[Hook#3287]

I believe the bike is all stock. Certainly nothing obvious different from OEM. Carbs and exhaust definitely are stock.
I tested the petcock with a mityvac. Seems fine. The tank breather is clear and fuel flows freely from all carb drains.
Would it make any difference if I switched the vacuum from 6 to another? Would that indicate anything if it improved or got worse?

the tank breather hose and carb drain hose are two different systems.  I doubt switching the hose from 6 will make any difference.

[Marcus De Grate]

the tank breather hose and carb drain hose are two different systems.  I doubt switching the hose from 6 will make any difference.

Thanks. Yes, I know they are different. Sorry I was unclear.  What I meant that was after checking to make sure the breather wasn't blocked "Air in" I also checked that the fuel was flowing "Fuel Out"

Just doing easy quick stuff until I get some time to pull her apart a bit more. (probably over winter)

[Ricky-D]

Ok, to recap: The bike is completely stock and there is an apparent flat spot when accelerating with a wide open throttle at around 3000 rpms which then goes away and the engine resumes normal acceleration to higher rpms.

First off, I would say that it is not fuel starvation from any problem that is gas tank related or stop rooster related.

Since your at "wide open throttle" it certainly would not suggest it's a transition problem with the fuel metering systems (jets, needles etc.)

So, I think it is a sticky slide in one of the carburetors, or maybe a diaphragm that's not 100%. To troubleshoot the problem I would first examine all the spark plugs together looking for the one that may be different from all the others hopefully revealing the problem carburetor.

Failing that, a more intense investigation looking at the individual slides and diaphragm assemblies would be the next order of business. The sealing surfaces of the towers and all mating surfaces, unusual wear spots on the slides or any kind of hindrance of the motion of the slides.

This is what I'd do next.

***

[3W-lonerider]

i agree with Ricky. you either have a slide thats sticking or a pinhole in one of the diaphrams. had the same thing happen on a 700 honda shadow. if you full throttle it, it felt like it had a lad. but it woulda been more noticable on that v-twin than a six cyl. there might be some gunk build-up down around one of the needles to,