Stumbling at around 5500 rpms

Hey all,

Just on the road and there's already questions eh? I seem to have a problem around 5,500-6000RPM where it will stumble, then be fine past that and pull well. Shouldn't be a fuel supply problem right, where the mains are engaged well before that...

I did swap coils and tci from the 650 (which are the same IIRC), and on the 650 i had no problems. Think it's worth swapping over the old TCI? Both coil packs tested fine. Anyone else see this issue? Will check my plugs over lunch, but at last glance they all looked pretty close to brown.

Marcel

 

Question, if you creep up on that rpm is it as bad or kind of go right through it ?

 

Looks like I can keep it there at around 5500 and she'll sound terrible but if i load it up it will miss. Honestly I checked my plugs and one seems pretty black, so it might be completely fouled, although Polock, I'm still curious as to what you think it might be, based on your question.

Marcel

 

those were the same symptoms i had with a MAC 4/1 and the thing MAC passes off as a muffler. 1-3 weren't as bad because it just blows past 5K but 4 and 5 sputtered and stumbled at 5K or so then once past that, about 6 or 6.5 K it took off. Now if i went slowly, say half throttle up to 5K, it would go through the 5 to 6.5 range without too much problem.
long story short, i put on a SuperTrapp muffler and kept taking plates out, adding back pressure, and the problem improved and finally went away all together.
i think a different needle jet"emulsion tube" or a slide with a different cut away might have worked too

 

I had the same prob with a Mac 4-2
5K?
Stumble.. It was AWFUL... Made me sell the bike.
I played and played and played with jets, shimmed the needles, stock paper filter... No avail..
Did BOTH TCIs do this?
Have you added or removed antyhing else since?
Do you know your jet sizes? Needles? Any shims under the needles? Air filter?
-Chris

 

Thanks for the replies guys,

Other than the mac, i have +1 the main jets (122s now IIRC). I have swapped the TCI for the original. The ignitor on it was from the 650, and it was the same model, but had a higher number stamped on the label. Just fired it up and sounded ok, but wasn't under any load.

Funny thing, on the secas side cover it calls for 91 Octane, but the Maxim called for regular... Not on topic I know, but Ethanol sucks! Plugs look way nicer running the good stuff.

 

first - this should be in modifications - that's not a 650
You messed around with alot of stuff and we have to guess
You are hitting a flat spot in the fuel delivery
My first Q is did you swap all the air jets, and the needle jets
Then you got the wrong TCI?? or the right one,
This can be sorted with the right info - - trying to help !!

 

I didn't think i did that much fooling around. Other than jets and the mac it's stock, mechanically speaking.

I swapped all jets and needles from 650 - 750, so they should be in effect 750 carbs.

The TCI was the same model, but a different batch I'm guessing based on the stamp on it. Non issue now where i put the original back on... At this point i'll see what it behaves like the next time i'm out on her; tomorrow night or sunday depending on the weather...

 

Well, original TCI and... No change.

I went for about a 40 mile run and halfway through I noticed some bogging (way too rich on plug #1), so I swapped out for a fresher plug and turned in my mixture screw 1/4 turns. That seemed to help my missing on take off but i still noticed the stumble at 5K.

For now I'll keep dialing in my mixture (may even use the color-tune) to see if I can get closer to the sweet spot. I guess any supertrap muffler would have to be adapted to fit onto the MAC header.

Marcel

 

When you close-in on getting the Engine "Fine Tuned" ... you have to abandon the ... 1/2 and 1/4 Turn adjusting.

When you get close to being Dialed-in, ... the adjustment you need to do to the Pilot Screw might very-well be minute (my~nute).

There are situations where the Mixture Screw may only require a nudge that is about the width of a Nickel or a Dime.

 

Don't worry about the octane. Unless your riding at altitude (>5000 ft) then 89-91 is fine. Sounds like you have an out of synch or a fuel mixture issue. If all four aren't on the same page they have always shown that to me right about where your stumble is-Mains on-line, torque high, air-flow midrange.

I would check your mix carefully-and try for a better match.

 

Borrow a standard exhaust from somebody if possible. At least we can then eliminate that then probably move onto carbs

 

I wish it were that easy to swap exhausts, however....

Pulled the rack and swapped the original 750 Floats, which appear to never have been set. Bike seems to run better and when i checked my plugs, one was pretty much fouled and the others were a little on the lean side... I'm keeping an eye on them, and making minute adjustments (seeing improvements for sure).

Will keep this thread posted but basically i think i may have messed up my floats and bent where it shouldn't have... we'll see if the hypothesis holds...

M

 

Chasing your own tail dude...

 

If you didn't WET-SET the swapped-in floats you won't get it completely right.

 

I know i tend to chase my own tail from time to time. Lets just call this a controlled experiment. Understood BF...

My 650 floats hung way way higher than the ones on it now and it was blowing fuel out the tail pipe... I know I may lose some XJ street cred here but trust me its not hard for me to pull a rack of carbs.

 

Not hard.
Tedious.

Make the job easier.

Replace stock Clamp and Boot Fasteners with Stainless Screws.
"Dress" (Sand) the Inside Diameters of the Airbox Boot Holes to be ever-so-slightly larger, ... allowing for the Boot to be rotated a bit more easily.
Tape Finishing Paper to a Tomato Paste Can. Enlarge slightly.
Finish with 1200 for a smooth surface.
Treat the Boot Locating Channels with a "Lubricating Gel".

Treat yourself to a set of these Tools, ... which are indispensable for manipulating the Boots and fitting them to the Horns on the Carbs.
• Shorten and "Dull" the end on the right-angled tool.

http://www.electricaladvantage.net/pick ... cekit.aspx

 

Thanks for the tips Rick, the boots are tough to spin that's for sure!

Here's the current state of the plugs (after a 5 mile run):


Would there be one that I should be aiming for here?

Marcel

 

Mine look like #3.
I run it that way.

#1 is Textbook.
Cool-running and Aluminum safe.

 

Hi all,

been a while since i touched this thread, but here's an update:

wet set floats
shimmed the needles with a .250mm washers
swapped idle jets out for 45s (originals were 40s or 41s.

It runs ok, but still have that stumble. Fast forward to today and i just got a mid pipe made at meinike so i can mount my 90 buck 2nd hand supertrapp exhaust (for free nonetheless!). will advise if that helps any, once it's mounted. Will try with 6 plates and take it from there...

Marcel

 

When I had a bad sync on my 550 it behaved as you described, stuttered in the 5k range and the plugs looked j just like yours, 3 was fine and 1 black.

I still get a hint of hesitation in that range, but I'm hoping a new airfilter will remove that.

 

I shouldn't have said that, still have quite a lot of the hesitation left in the 5-6k range.. But It disappears momentarily when I've refuelled it, So I'm guessing I've got dirt in the tank that is clogging up the filter inside the tank..