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Discussion Starter · #1 ·
Lumpy off idle throttle response...
Last winter I refurbished my zr7S. Amongst everything I did I cleaned and rebuilt the carbs (OEM jetting), new OEM carb holders, balanced the carbs, new oiled air filter, adjusted the valves.
The bike ran perfect. Smooth throttle response from idle. It didn't exhibit the flat spot and stumbling that I read about in other posts including the excellent Bolserst thread.
But now I have it.
So I just now rebalanced the carbs, they were fine. There are no vacuum leaks anywhere. And currently I have my mixture screws at two turns out but I tried 1 3/4 to 2 1/2 and the stumble is still there.

What I might have missed is the ONE jetting solution that absolutely cured the problem, (or mixture screw turns with the OEM jets). Is there such a thing?
And I might have missed the best mixture screw turn out with the factory pilot jets and needles.

If there is a kit out there that doesn't require drilling the carbs or airbox I'd consider it at this point.
I'm not looking for more power... just smooth throttle. It runs fine up at 9000 RPM on a good rip but I'm seldomly there. Mostly 4-6000 RPM.
But if I remove the carbs again, which I hate, I'll try a jet kit but as I've said, I'm perfectly happy with its power as stock. I want smooth.

I really hate having to post like this, Mark.
 

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Manufacturers have to concede to emission laws or they could not sell. That means; before fuel injections, sensors, computers, and O2 readings,.... stock settings was not necessarily best for performance, or smoothness, but what they had to to do to pass emissions.

I remember from many moons ago a 'Partial Kit" for the ZR-7 which included a 38 pilot jet and one washer under the needles, which claimed to fix the stumble.......that would make it a tad richer across the board. Also, the bowl fuel level is another thing to play with.

........don't have a ZR-7 exactly, thus can't vouch all those "fixes", but thought would mention it.
 

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Discussion Starter · #3 · (Edited)
Thanks Hugo. That's what I gleaned from my reading. I plan on turning the mixture screws out even more to see if that helps. If it does I'll know a lean condition exists.
If I get improvement but not perfect than I'll have to rejet the pilot.
What are the factory pilot jets?
EDIT- Factory pilots are #35
 

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Notice that the 'mixture screws' are only the 'IDLE MIXTURE SCREWS'. Meaning; they grab some fuel from the PILOT JET, rout it through the needle(screw) and feed after the butterfly cracked open. Only affects the idle..of any significance.

The pilot jet, on the other hand, makes a base line all across rev range. Of course, after about 25%, the main jet takes over heavily, but still the pilot raises the base. This of course will reduce fuel economy.

Other thing I've found is significant is making sure the fuel level in bowls, is at least by the seam, by the wet method. This also affects how rich it is. My factory manual with same Kehins 32 says level should be about 1 mm above the seam, yet the 17mm dry method in same factory manual places it too low. For me above the seam is too much, below the seam is too lean. I believe the ZR-7 factory manual says it should be below seam.

....Am I making any sense???

Don't know if you do it like that, but I can remove/replace everything from top or bottom of carbs w/out removing them from engine.....
 

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Discussion Starter · #5 ·
Hugo, yes, you're making sense.
I'm happy with throttle response above 2000rpm or so once it catches, it's the stumbling in between the 1100rpm idle (it idles smoothly there) and 2000 or so RPM that makes for poor drivability coming off the line.
When I rebuilt the carbs I did not set the float heights.,. I know, I know. I've rebuilt a few sets of carbs and have never done it but never had a problem. -> Keeping in mind that the bike ran smooth throughout the rev range right after I put it all together.
It did however feel flat off idle but didn't stumble.

And just recently while attempting the idle drop method with the mixture screws I notice one of the screws had no effect on dropping the idle when fully closed so it's possible my problem is that carb's low speed circuit became crudded up after a few months.

Oh, I just remembered... when I ordered what orings I needed for the carbs I forgot all about the tiny orings for the mixture screws. They still appeared supple so I reused them as I was anxious to get the bike running. Bad, I know. I don't know what effect a bad oring has on the fuel mixture screw. It would let air in but wouldn't gas leak out? There is no gas leaking out.

Me thinks I have to pull the carbs for an idle circuit cleaning. At this time I'll check the float heights using the dry method. I've tried the wet method before with only questionable results.

And since I'll have the carbs pulled do you think it advisable to install the #38 pilots to get rid of the off idle flat spot?
 

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Discussion Starter · #6 ·
Never mind. I found the problem. It was ME. I tried turning the screws out three full turns, which I always feel is always too many, and the bike runs smoothly now off idle. It does feel a little flat just as you pick up the throttle from idle but the stumble is gone. Past 2000rpm or so she's off and running.
Thanks Hugo for the input, it helped. If I ever take the carbs off I will increase the pilot jets to #38 and shim the needles but for now I'm happy.
 

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....the o-rings on the screws needles are only to prevent oil leaking out of the screws. I know they get hardened and flat, but if it does not leak, they are OK. As long as it runs OK as is, and is fine with you, don't do anything. But be aware, only ways of making it richer in a significant way, at all revs, is increasing pilot jet size or float levels. Shimming the needles, will make it richer when the mains take over,....25% or over.

........................and it is possible making changes w/out removing carbs from engine.
 

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Discussion Starter · #8 ·
....the o-rings on the screws needles are only to prevent oil leaking out of the screws. I know they get hardened and flat, but if it does not leak, they are OK. As long as it runs OK as is, and is fine with you, don't do anything. But be aware, only ways of making it richer in a significant way, at all revs, is increasing pilot jet size or float levels. Shimming the needles, will make it richer when the mains take over,....25% or over.

........................and it is possible making changes w/out removing carbs from engine.
I rode it around the neighborhood today and it was fine. And after reading your post I see that it would indeed be possible to change the pilots and needles with the carbs still in the bike. Float bowl heights would be another story tho.
Do you set up a mirror or just go by feel?
 

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...long ago a replaced the bowl screws with allen bolts, and lock washers. They don't need to be very tight, just snuggled. I use tiny 1/4 very small ratchet, 4.5 mm socket and little square bit from a JK-6089 precision tool kit. Hold them together with electrical tape. I use a tiny jewelry screw driver, or paper clip to push the float pin. I can remove bowls, floats, and floats needles in about 5 min flat each, after draining the fuel. Of course, to work on the inboard carbs, I remove first the ones on the outboard ones, so I have the room, and reinstall them last. I can remove and install jets just as easy also with another flat bit from said tool kit.

I cheat a bit here, having a spare ZR-7 carb rack I bought many years ago from a forumer for a song. Remove the float from one of the spare ZR-7 carbs, and install my floats and do my dry height settings on spare rack. I use a precision digital caliper. Even 1 mm, makes a difference. Then when I install the float/bowls back, and reprime, I do the wet clear u-tube level check on bike. To do the inboard carbs, I simply use the outboard carbs seam as reference. If level exactly not at the seam, remove again and repeat, adjusting float tabs, until I get it......and I mean all even.

Let's just say over the years, I have" perfected" the technique. I also remove/replace diaphragm's and needles from the top, through the frame in a matter of minutes.
 

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Discussion Starter · #10 ·
Hugo, that's amazing. I do have the tools necessary for such a task but not the spare carbs.
I did see a video of a guy who pulls the carbs off to the left side of his 7 leaving the throttle cables still attached. That would also allow some cleaning to be done once all the rubber parts have been removed.

I've been thinking about my reluctance to turn the mixture screws out past 2-1/2 turns. I believe I was confusing them with air screws which generally shouldn't come out two turns.
Plus.. On my ZRX1200, the fuel mixture screws only need to come out less than two turns indicating richer pilot jets. The ZRX engine feels smooth like an electric electric motor from idle. I'm spoiled by it.
Maybe in the future, riding the zr7, its weak off idle response will gnaw at me and I'll want a little extra push off the line. At that point I'll rejet but we'll see. I enjoy the bike for what it is... just a good simple bike.
And troof be told, on my last test ride, while now happy again with the throttle response, I got pissed off at the front brakes. Those pin-slider calipers are just crap.
I'll have to revisit the pad material and possibly install stainless braided lines and see if I can't fit a master cylinder with a smaller bore if it already doesn't have one. I can't remember.
 

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One other thought to add to the mix. Two years ago my ZR-7S developed a hiccup/stumble just off idle. I thought it was a card issue but after lots of trouble shooting it turns out one of my ignition coils was bad causing a intermittent misfire. I first noticed that one exhaust header was cooler than the others (odd that it was only one cylinder since there are just 2 coils that fire all 4 in the wasted spark set up we have).

New coils, wires, and fresh plugs and it ran better than new with no hiccups or hesitation.

They are easy to check with a multimeter using the procedure from the shop manual. May be worth checking if the other troubleshooting does not get at the issue.
 
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Discussion Starter · #12 ·
One other thought to add to the mix. Two years ago my ZR-7S developed a hiccup/stumble just off idle. I thought it was a card issue but after lots of trouble shooting it turns out one of my ignition coils was bad causing a intermittent misfire. I first noticed that one exhaust header was cooler than the others (odd that it was only one cylinder since there are just 2 coils that fire all 4 in the wasted spark set up we have).

New coils, wires, and fresh plugs and it ran better than new with no hiccups or hesitation.

They are easy to check with a multimeter using the procedure from the shop manual. May be worth checking if the other troubleshooting does not get at the issue.
Ah, ok. I suspected the same thing but didn't want to mention it to confuse the issue. I felt the header pipes soon after cranking over and all got equally hot together. (I leant my infrared thermometer to my brother to use with his pizza oven and haven't got it back in five years. And I haven't asked for it back, if you know what I mean. His pizzas are gourmet quality. LOL. So I used my hand.)

Also not mentioned was that because I was having trouble hearing the RPM change while doing the 'RPM drop method' with the mixture screws I bought a cheapo induction RPM meter to more accurately see the RPM but it worked like crap.
While idling it would read a steady 1140 RPM which seemed to match the gauge on the dash but would suddenly change to 670, 840, 1280, 1500, back to 1140 then on to 1540 or what ever it felt like doing. It did hold consistent at higher RPM though.
Here it is but don't buy one, it is junk. I'll sell you mine for cheap.

Right now my 7 is running great but I need a 300 mile day to fully assess. I hate just throwing parts at it on a whim. Electrical issues are tough to diagnose. But I did suspect a bad coil. We shall see. I'm just glad I didn't needlessly pull the carbs.
The more time I spend with this ancient bike the more I like it.
 

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Discussion Starter · #13 ·
Alrighty.. the three turns out did the trick. I went on a 160 mile ride today doing slower two lane roads through small towns. I brought my adjustment screwdriver with me but never felt the need to use it. The last thirty miles was 75-85mph highway and all was good there as well. I'm happy. 😊
 

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Alrighty.. the three turns out did the trick. I went on a 160 mile ride today doing slower two lane roads through small towns. I brought my adjustment screwdriver with me but never felt the need to use it. The last thirty miles was 75-85mph highway and all was good there as well. I'm happy. 😊
Great to hear! Glad you got it sorted.
 
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