4 cyl Carb bike hesitant from idle when throttle opened.

[spannermonkey...]

Hi all.

Once my engine is warm my zx7r is hesitant when you open the throttle from idle (fast or slow throttle twist). Carbs are balanced, pilot air screws set 2 1/4 out.
New pilot air screw rubber o rings. Each header pipe is the same temp. No leaks in air box or inlet track.

Bike rides smooth and pulls fine just hesitant on pulling away. Guessing at the pilot air circuit still.

Any ideas? Clean carbs again with a ultrasonic machine?

I haven't checked compression or valve clearances but it just under 500 miles ago I carried out a top end rebuild. Hone, new rings, rings gapped, head pressure checked, valve seats cuts to match valves and head checked for flatness. Head torqued down correctly. Fuel pump works and puts out correct pressure and air and fuel filters are new.

I've checked the ohms on the coils, ignitor unit and we are good. Plugs are new as well as the leads and caps.

Can a leak in the vacuum/emission system cause issues? all the hoses are good and tight.

Any ideas would be great. Really start to scratch my head at the moment.
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Current machines - Aprilia RS250. Kawasaki H1A 500. Past machines - Kawasaki ZX7r P6, Yamaha RD250E (the one that got away), BSA Racing bantam (revs over 10'000 sweet as), Yamaha RXS100, Suzuki GP100, Suzuki GS500K2 - best all year machine ever - come rain, sleet and snow just epic.

[MCN.]

Dyno time ftw. Saves guessing. If it runs ok everywhere else I can't see it being the pilot circuit because it wouldn't idle if there was an issue there.

[RhynoCZ]

I'm not sure about yours, by my superbike doesn't really run below 2.500-3.000rpm. I think it's not broken, it's a feature.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[spannermonkey...]

I think you could be right, but once I have fully run it in.
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Current machines - Aprilia RS250. Kawasaki H1A 500. Past machines - Kawasaki ZX7r P6, Yamaha RD250E (the one that got away), BSA Racing bantam (revs over 10'000 sweet as), Yamaha RXS100, Suzuki GP100, Suzuki GS500K2 - best all year machine ever - come rain, sleet and snow just epic.

[Robby]

Assuming the bike is oldish - and being on carbs, that now qualifies as oldish.

A problem with the mixture at small throttle openings is likely - and being CV carbs, the throttle opening is as small as the carb wants, not the size your hand chooses.

You mentioned setting the pilot screws - have you tried removing them, squirting in some carb cleaner, then setting them? Depending on the make and model of carbs, pilot circuits can be an absolute pain in the arse for getting blocked up. Carb cleaner from a can using the little straw does the job of blasting out any crap in there.

After that, does the idle also fluctuate a fair bit between cold and hot? If so, my next thing to look at would be throttle shaft seals, particularly if the bike has spent some time standing. The seals fail, carbs can suck in air through them. It's a small gap, so small amounts of air, just enough to screw up the mixture in a high vacuum draw/small throttle opening situation.

I personally avoid ultrasonic cleaning, I've never had a carb behave itself afterwards. You can clean a set perfectly with a can of carb cleaner.

[Rncv]

Is it the same when cold?
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Current: 01' Bandit 600, 92' Goose 350
Previous: Lots!

[spannermonkey...]

Yep, CV carb bike and yep to standing around. Yep to being old.

Yep from cold also.

Are these the seals that sit on the shaft between carbs with all the springs and adjusters on and the butterfly valve.


From cold, you open the throttle and it struggles also, but it is fine if you control the revs on the choke lever and blip it that way but only when it's very cold (the engine is cold not outside air temp).
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Current machines - Aprilia RS250. Kawasaki H1A 500. Past machines - Kawasaki ZX7r P6, Yamaha RD250E (the one that got away), BSA Racing bantam (revs over 10'000 sweet as), Yamaha RXS100, Suzuki GP100, Suzuki GS500K2 - best all year machine ever - come rain, sleet and snow just epic.

[spannermonkey...]

These seals, are they any thing fancy or just o ring seals in a recess on the carb sitting on the shaft that runs through.
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Current machines - Aprilia RS250. Kawasaki H1A 500. Past machines - Kawasaki ZX7r P6, Yamaha RD250E (the one that got away), BSA Racing bantam (revs over 10'000 sweet as), Yamaha RXS100, Suzuki GP100, Suzuki GS500K2 - best all year machine ever - come rain, sleet and snow just epic.

[MCN.]

[Rncv]

pilot circuit or there's an air leak.

Dyno is the easiest option, failing that you can try and move the needle clip up and see if it makes a difference. If it does it's the jets, if it doesn't there's a leak.
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Current: 01' Bandit 600, 92' Goose 350
Previous: Lots!

[spannermonkey...]

For now replacement carbs from a running machine was the fix.

Bike starts good, runs well, pulls from low down, problem solved.

So the problem is carbs, so I'm going to strip them down and blast the heck with carb cleaner and then look at those seals on the butterfly axle.

But that's a job for another day.
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Current machines - Aprilia RS250. Kawasaki H1A 500. Past machines - Kawasaki ZX7r P6, Yamaha RD250E (the one that got away), BSA Racing bantam (revs over 10'000 sweet as), Yamaha RXS100, Suzuki GP100, Suzuki GS500K2 - best all year machine ever - come rain, sleet and snow just epic.

[MCN]

My auld Thundercat had a penchant for trying to throw me off when pootling around roundabouts on wet roads because of that (The bitch).
I think most bikes suffer from it a little. Not so much cars where your foot is fixed to the floor.

To help control it a bit I grip the bar-end with the edge of my pinky-finger as I open the throttle. It sort of helps to retard the throttle and prevents it 'jerking' open. That is also my 'Cruise-Control.
It doesn't work on Ghey bikes with the those 'free-spinning' bar ends though. e.g. Aprilia's Tuono (Tuonos aren't really ghey. )

It may be a simple physical factor. Where your body is not moving but then movement of the bike 'from under' you when you crack open the throttle appears to shift your weight backward. That will pull your arm and hand attached to the bar. And therefore turn the throttle on moar.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.