Is that what a flat spot feels like?

[Macmatic]

On the advice of several people here I was "riding the snot out of" my GS750EF last weekend and I experienced something new to me. I was riding down an on ramp at about 6500rpm in 2nd gear, whacked open the throttle and did kind of a "brace so I don't lay down on the tank and then hold on for dear life" kind of thing. Definite deceleration followed by acceleration.

Now the bike will pull to redline no problem in any gear I've tried (1,2,3) with no detectable hesitation but if I let it sit at 6k or so and open the throttle slowly it'll stumble a little and accelerate, open it quick and it jerks me forward pretty good.

Fuel starvation? Well maybe... but I can ride at least two miles at 9,000rpm and the bike feels steady as a rock, no surge or stumble I can feel at all.

I've never felt this before but I also usually ride pretty easy. Plenty of torque below 5k or so and thats usually where the revs stay unless I'm out on the highway. Most of the time anyway... I DID make a change to the carbs just before this. I have Canadian carbs with five position needles and I moved the clip from the middle to the bottom position but unless I'm getting too much fuel and stumbling on that I dunno. I also recently went from 117.5 to 122.5 mains. ....still trying to find a place to do a high speed plug chop.

Another characteristic that might give someone a clue is that the bike it pretty gutless rolling on in 5th gear at around 65mph. I was riding another forum members '82 GS750 16v and it had WAY more pull than my bike, this is what initially clued me in that something was off with mine and led to the 122.5 mains.

I have a stock airbox with a K&N filter (oiled) and stock header with 'busa cans grafted on. Iwent with the 122.5 mains because I figure the K&N and new mufflers are a little more free flowing than stock and the 117.5 stock jet is known to be a little on the lean side. I think...

Any thoughts?

/\/\ac


Oh yeah, what kind of oil should I use?
(Just kidding)

 

[salty_monk]

I had that same setup on my 1000 with a KN inbox & V&H 4:1. I eventually settled for a 125 main but 122.5 would work ok.

By lowering the e clip like that you changed the mixture quite a lot & I would imagine that is your problem, sounds like it is on transition from pilots to needles the way you describe it.

I also had canadian needles & moved the clip one notch from stock. I think I probably could have got away with a half. Try moving the e clip back closer to stock, one notch richer max, (or even too stock, you may not need to move it at all).

Dan

 

[Macmatic]

Thanks Salty,

Yeah, I'm going to put it back to the middle for now. I moved it down because I was hoping my low power in 5th problem was due to a lean mix, after the bigger mains didn't help I thought I'd try the needles.

I guess I'll go back up to the middle notch and try and find another solution to the lack of power. Previously I've had zero power delivery issues so I'll be happy to get back to that for now. Of course I'll have to ride the snot out of it again just to make sure...

Any clue on why it might be so dead when rolling on? It'll accelerate but not at any useful rate. Carbs are pristine, coil relay mod done and working well, balanced carbs with a Carbtune II at 3k, 4,500 and 7,000rpm and am getting healthy flow from the tank.

/\/\ac

 

[salty_monk]

Low power in 5th... it can't be carb related unless it relates to a particular throttle position & it will be the same at that same position in all gears (although maybe the lower gears help to mask it or you are not at that throttle position very long in other gears).

If your ignition is fine (I had a problem with my ignitor that didn't show up until certain RPM) then it can only be air leaks or starvation or Jetting issues.

You could try the one step up from bottom but I wouldn't think it is going to help.

You need to get it running great at Wide open throttle first on a main jet & then work back from there...

Dan

 

[seuadr]

can you buy the canadian needle and slides? i was thinking of doing the washer thing, but that would be a much easier solution

 

[Macmatic]

can you buy the canadian needle and slides? i was thinking of doing the washer thing, but that would be a much easier solution

I bought a Canadian parts bike as one of the three and a half I built mine from so thats how I ended up with the adjustable needles. I might actually try switching to the US needles because I think they have more of a taper, the Canadian ones are really blunt with just a short taper at the end.


I'll have to try and recreate the low power condition in other gears. Up to now I haven't noticed any hesitation or weak spots at any time other than rolling on in 5th. Hmm... maybe it is these needles? I think it is running pretty great at WOT on the mains....other than in 5th. It feels great pulling to redline in 1st to 3rd, idles great and is awesome around town with wide torque.

/\/\ac

 

[SqDancerLynn1]

If the front turns freely tighten it 1/16 turn at time !!!

 

[Mark M]

I might actually try switching to the US needles because I think they have more of a taper, the Canadian ones are really blunt with just a short taper at the end.

That is weird, the OEM needles in my 1100E have a very fine taper along a fair part of their length. What you describe sounds more like the Dynojet needles.

Mark

 

[Mark M]

You need to get it running great at Wide open throttle first on a main jet & then work back from there...

Dan

Dan,

You have that backwards. He needs to start on the pilots and then work UP. Sorting the mains, then the needles, then the pilots just guarantees having to redo the upper circuits because adjusting a lower circuit affects everything above it.

I would use some tape to mark 1/8, 1/4, 3/8 and so on throttle positions to see if it correlates to a particular throttle position. Since the needles got raised 2 clip positions at once, I would try lowering them 1 notch to see if that sorts some of the problem. If you want to find out if it's rich or lean in the bog, pull the choke on a bit next time you get a chance to get it pulling into the bog. If it pulls better you are lean, if it bogs worse you are rich, if it bogs very slightly you are close on mixture and your problem lies elsewhere.

Mark

 

[salty_monk]

Sorry.. the way I understand it that's bull.... You start with the mains at WOT & work backwards to the pilots... it's worked for me & plenty of others here.

Check here for this info below: http://www.factorypro.com/ click on tech support.

To properly tune, you MUST:
1. Have selected the BEST main jet for full throttle power (not just a "good" main jet - we mean "the BEST main jet" for power at high rpm).
That eliminates the common severe tweaking of the midrange and lower tuning ranges to compensate for a "wrong" main jet.
2. Then - select the BEST needle height / clip position for power at FULL THROTTLE / MIDRANGE after selecting the BEST main jet -
That almost eliminates weird problems at cruise caused by tweaked needle heights that were required because the main jet wasn't correct.......
3. Then, adjust the BEST Float Height for BEST FULL THROTTLE / LOW rpm (many Honda's excluded because floats are not adjustable) -
You should be able to apply FULL THROTTLE at LOW RPM in TOP gear without ANY misfire of bogging or stumble.......
If you follow that order, you will have:
1. Best topend.
2. Best midrange.
3. Best low rpm power.
Then - all you have left is dialing in the pilot circuit - i.e. mixture screw and pilot jet size - That's IT - Don't tweak needle heights and throw away full throttle midrange to try to fix a cruise issue!!! (Unless you want to!)


Changing the mains effects the lower circuits hugely, there's no way you can reset the needles before the mains, makes no sense.

I had a Canadian set of carbs for my 1000G - I measured the tapers on both sets of needles, they are identical apart from the extra notches in the canadian ones. Yours may be different but that's how it was for me....

 

[Macmatic]

If the front turns freely tighten it 1/16 turn at time !!!

If what turns freely? I missed this one.


Thanks all, I'm going to pull and reset the needles to the middle notch today. I'll also measure them and see if I can read the Mikuni number off the side. [Edit] The Canadian needles I've got are Mikuni's, marked 5D10. The manual says the stock needles are 5D12.

 

[Zook]

Your atomizer may also need to be changed. Many people forget about that jet.

Here's a couple tuning guides that can help:
http://greenflag.net/dell/dellorto.html

http://web.tiscali.it/abosco/moto/carb/dellorto/dellorto.html

These are for Dellorto carbs but the principle remains the same.

BTW-the pilot has no effect on the main at WFO and vice versa.
At WFO all the fuel is supplied through the main.

 

[Macmatic]

Your atomizer may also need to be changed. Many people forget about that jet.

I think that by Atomizer you're referring to what I call the needle jet? Maybe also called an emulsion tube. The bit wots got the holes in it between the main jet and the venturi opening which the metering needle goes into.

So if we're talking about the same thing I don't think mine need to be changed. I know they are clean and the stock size for my bike. Since I haven't done any radical changes to the intake/exhaust I've been assuming I'd only need to play with needle height.

Test ride in the mountains tomorrow should give me some more info to go on!

/\/\ac

[Edit] I just took measurements of both the 5D10 and 5D12 needles. I'd say the only real difference between them is the adjustable height. The first number is the #10 and the second is the #12. Taken at eight points along the needle and at the tip. The differences are most likely my error. Stock needle height and the middle position on that adjustable needles is the same height from clip to tip.

1) .0118/.0119
2) .0118/.0119
3) .0117/.0118
4) .0115/.0115
5) .0111/.0112
6) .0108/.0108
7) .0105/.0105
8) .0102/.0102
Tip) .0097/.0097

 

[Mark M]

Sorry.. the way I understand it that's bull.... You start with the mains at WOT & work backwards to the pilots... it's worked for me & plenty of others here.

Check here for this info below: http://www.factorypro.com/ click on tech support.

Yes, that will work. But it is easier to work from the bottom up.

You don't even need main jets in to ride around, the bike will run fine on the pilots and needles until you open the throttle enough to start transitioning onto the mains. It will bog bad at that point because the open jet hole just dumps fuel in. I will also ask - how do you sort the main first if the pilot and needle circuits are so screwed up that you can't get up onto the mains? In that case, you MUST start at the bottom and work up... So why do it any different any other time?

I will stand by my method, it is a long time standard from back when 2 strokes were the race bikes of choice. On those, you couldn't start on the main because if you went lean anywhere, you seized the thing solid. So those tuners would sort from the bottom up, with no main jet (or a huge main) in place to guarantee no lean seizures.

YMMV, of course.

To the TS, be careful and methodical, only make one change at a time and take lots of notes. If you chip away at it, you will get it sorted for sure.

Mark

 

[Macmatic]

Latest update. I pulled the plugs to check the color before starting the bike up again since my last ride and discovered 2&3 looks OK, a little rich maybe and 1&4 are a dry black color. Its doesn't look like oil and since this is a coil pair I'm going to guess I've got a weak/bad coil or poor connections.

I hadn't thought of this until now but I recently did the coil relay mod and some swapping around of coils between two sets. I'm not sure anymore which is which and I probably have one coil from each set paired up now. Grr. I think I'll swap the other 1&4 coil in and see if the plug color evens out. Might be about time to order those Dyna greens....

/\/\ac

 

[Killer2600]

Yes, that will work. But it is easier to work from the bottom up.

You don't even need main jets in to ride around, the bike will run fine on the pilots and needles until you open the throttle enough to start transitioning onto the mains. It will bog bad at that point because the open jet hole just dumps fuel in. I will also ask - how do you sort the main first if the pilot and needle circuits are so screwed up that you can't get up onto the mains? In that case, you MUST start at the bottom and work up... So why do it any different any other time?

I will stand by my method, it is a long time standard from back when 2 strokes were the race bikes of choice. On those, you couldn't start on the main because if you went lean anywhere, you seized the thing solid. So those tuners would sort from the bottom up, with no main jet (or a huge main) in place to guarantee no lean seizures.

YMMV, of course.

To the TS, be careful and methodical, only make one change at a time and take lots of notes. If you chip away at it, you will get it sorted for sure.

Mark

Well things change and our bikes aren't 2-strokes, The whole reason for doing mains, then needles, then pilot is fuel flows to the pilots after going through the mains. Sure if you take the mains out the pilot will get fuel but if you plug it up you can't tune anything.

 

[gearhead13]

My bike is a canadian version and when i pulled the carbs and looked at the needles, they looked like there was no taper at all at first, I havent seen needles with so little taper. Is there an online resource or something where you can compare different needles and maybe order some with more taper?