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My 90 Ninja 750R (ZX750F4CA) bike idles rough like its fuel starved and then dies on me if I have the bike idling at anything less than 1250-1500 rpm after the bike has warmed up. When I first start it, it starts fine with a little choke, and then it runs great until the engine is warm. After that it idles pretty rough, and if I don't pay attention to the rpms, it will stall.

The carbs were recently overhauled, and completely cleaned. A few weekends ago, I took the carbs out again, and cleaned them, and everything looks good. My bike is completely stock.

Right now the only problem with the bike is the idling/stalls, other than that its got great power and is very responsive above idle.

The only other thing that I can think of right now is when I first overhauled the carbs, my buddy and I thought that the book called for the idle screws to be set to 2 turns out. Well that was for the UK model of the bike, not the US which the manual says is preset. We put the screws to 2 turns out, but is it possible that the screws not being out enough could cause this? If so, does anyone know what the screws should be turned out to? The screws were originally set to 3.5 turns out, but one of the carbs was 3.25 turns out. We figured that the idle jets were incorrectly set, so we reset them. This bike has been under repair for quite a while, but it did run correctly before.

I read that most carbed bikes usually set the jet needle to between 2 and 2.5 turns out, so the 3.5 turns did seem excessive. What do you all think?

Thanks
 

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My 90 Ninja 750R (ZX750F4CA) bike idles rough like its fuel starved and then dies on me if I have the bike idling at anything less than 1250-1500 rpm after the bike has warmed up. When I first start it, it starts fine with a little choke, and then it runs great until the engine is warm. After that it idles pretty rough, and if I don't pay attention to the rpms, it will stall.

The carbs were recently overhauled, and completely cleaned. A few weekends ago, I took the carbs out again, and cleaned them, and everything looks good. My bike is completely stock.

Right now the only problem with the bike is the idling/stalls, other than that its got great power and is very responsive above idle.

The only other thing that I can think of right now is when I first overhauled the carbs, my buddy and I thought that the book called for the idle screws to be set to 2 turns out. Well that was for the UK model of the bike, not the US which the manual says is preset. We put the screws to 2 turns out, but is it possible that the screws not being out enough could cause this? If so, does anyone know what the screws should be turned out to? The screws were originally set to 3.5 turns out, but one of the carbs was 3.25 turns out. We figured that the idle jets were incorrectly set, so we reset them. This bike has been under repair for quite a while, but it did run correctly before.

I read that most carbed bikes usually set the jet needle to between 2 and 2.5 turns out, so the 3.5 turns did seem excessive. What do you all think?

Thanks
If it was at 3.25/5 and it ran fine then that's clearly what she wants. I would run it at 3 turns to see what that does. 3.5 is a little on the excessive side.
 

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Discussion Starter #3
Thanks for the reply.

That's what I'm thinking, and I will most likely pull the carbs tonight to reset the jet, but what throws me is how it starts up fine and idles fine until it's warm (it will run perfect until after about 20 minutes of riding time).

Before I pull the carbs again, does anyone with this bike (stock) know what their jets are set to?
 

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36mm CVK carbs. I don't have these but have worked on many of them. Floats at 14mm, pilot jet size 35, pilots out 2.5 turns, main jet 112's. No issues. Try those settings (14mm float height, pilots out 2.5 turns and make sure the jet inside the carb is 35, and seated).
 

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Discussion Starter #5
36mm CVK carbs. I don't have these but have worked on many of them. Floats at 14mm, pilot jet size 35, pilots out 2.5 turns, main jet 112's. No issues. Try those settings (14mm float height, pilots out 2.5 turns and make sure the jet inside the carb is 35, and seated).
ChrisitKitty,

Thanks for the tip. Before I read your post, I came across this site that was linked from another post..http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

This post calls out exactly what my bike was doing, and indicated that it was running rich. So before I pulled the carbs again, I pulled the plugs, and all 4 plugs were coated with dry dark powder.

I pulled the carbs and turned the idle jet into to 1.5 turns, and ran the bike for 20 mins. It seemed to run a little better, but it did begin to idle a little rough (nowhere near as rough as it did before), but then it died while idling.

I pulled the plugs, and now they all look fine with grey ash. I should have checked the floats while i had the carbs out, but I didn't so I think I will pull the carbs again tomorrow and recheck the floats.

Since the plugs now indicate that I am running the correct fuel/air mixture, would the gas in the bowls cause me to start running rich again and idle rough. I think I start idling rich after a while because I start to smell gas in the exhaust, and that is when the bike starts to sputter a little and sound like its missing a spark here and there.

I also did not re-sync because I don't have my gauge back from my friend. I will resync on Monday, but would re-syncing the carbs really have an impact. As it is right now, when its idling before it gets rough after 20 mins, the bike sounds fine.
 

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Carb sync is super critical. 1.5 turns out on the carbs is a little lean actually so I would get into the float height and see where that is. Also check the float seat o-rings in the carbs. I had to replace mine as they were leaking pretty bad.
 

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Discussion Starter #7
So here's a quick update.

I did not pull the carbs because I wanted the bike running so that I could retake my MSF course today (3 year requirement for the military). But after turning the idle screw in to 1.5 turns, and resyncing the carbs, the bike runs much better now.

When the bike fully warms up after about 20 mins of running, it doesn't idle rough anymore (tends to fluctuate a little but nothing extreme like before) and I do not smell gas during idle anymore.

The bike still has a tendency to want to stall if I let the rpm around 1K. The bike will be fine, and then it will sound like its fuel starved a bit, and its missing a spark or two, and then it just dies. As long as I keep the idle around 1250 I don't really have the problem, but obviously I would like for the bike to idle closer to 1k.

Anyway, would the floats possibly being out of adjustment cause this? Or does anyone have any other suggestions.

I initially thought the bike was running too rich, which it was, but now I still stall out if I am not careful. Also, if the bike starts bogging before it stalls, I can give it a little throttle to keep it from stalling..what would cause that?

Thanks for the help. I am leaning to pulling the carbs again to measure the floats, but I don't want to go through the hassle if it would be a waste of time.
 

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Discussion Starter #8
While I'm brainstorming, does anyone know if coils or spark plugs would cause this, or would I see sluggish peformance/stalling throughout the throttle range and not just at idle?
 

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I'd check your float level first. he best way to do this is to connect a clear tube to the bowl drain, and open the screw to see where the fuel level is.
 

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Having the EXACT same problem with my ZXR (ZX750) 1993! Not as good mechanicly as you guys so if I was to bring the bike in the shop for that, what should I tell these guys to avoid unnecessary inspections and fees?
 

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3.5 turns out is too rich, 1.5 turns out too lean (generally).

I would go 2.5 turns out on all carbs, with a 14mm float height. And as others have said, carb synchronisation is CRITICAL (will run like crap if it's out).

Carb settings are about subtlety.

The fact you're seeing grey at 1.5 turns out shows you're running lean. Be careful or you'll burn valves. I've found that 2.5 turns is bang on where you want to be, just a tiny bit rich - but that's good.
 

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Discussion Starter #12
I still haven't gotten around to pulling my carbs yet to adjust the floats! I've been extremely busy at work, but hopefully I can get to it this weekend.

Krugerrand, I'll look at the plugs again this weekend. I thought that ash grey was what you wanted to see on the plugs though.

Jeanmi, I have started to notice some more symptoms with my idle/stall problem. If I am on a decline, the bike starts to rev high, and if I am on an incline (even a slight one), that is when I have the biggest chance of stalling out completely. So I definitely need to get these floats adjusted corrected.
 

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No I havent noticed the incline\decline idleing. Its really only when the bike gets warm (about 20 min in) that the idle starts dropping very low and then stalls, and playing around with the idle knob on the side of the bike dosent help either... I'll talk about it with my guy about the jet screw ajustement and see if it resolves the problem... lemme know if it works for you, if you end up taking it a part this weekend
 

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If it was at 3.25/5 and it ran fine then that's clearly what she wants. I would run it at 3 turns to see what that does. 3.5 is a little on the excessive side.
If it is 3.25-3.5 turns out, then he needs a size larger on the pilots. Which is easy to remedy.

However, if it runs worse as it heats up, then it is too rich somewhere, and those pilot settings might not be related (as you leaned it out)
 

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As long as I keep the idle around 1250 I don't really have the problem, but obviously I would like for the bike to idle closer to 1k.
Uhh, why?

The bike should be at 1300 +/- 50. So 1250 is just fine. What reason do you have for keeping it at 1k? I personally think my bike idling at 1k sounds and feels like shit and results in poor charging, the HIDs and LED indicators I have flicker slightly.
 

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Discussion Starter #16
Uhh, why?

The bike should be at 1300 +/- 50. So 1250 is just fine. What reason do you have for keeping it at 1k? I personally think my bike idling at 1k sounds and feels like shit and results in poor charging, the HIDs and LED indicators I have flicker slightly.
I guess the main reason is fuel economy at idle, but I can't even get it too run reliably at 1k anyway. If it idled at 1K, but it was rough, I could deal with that. But once it's warm, anything below 1250-1500 stalls the bike.
 

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I guess the main reason is fuel economy at idle, but I can't even get it too run reliably at 1k anyway. If it idled at 1K, but it was rough, I could deal with that. But once it's warm, anything below 1250-1500 stalls the bike.
Unless you are sitting around with the bike idling for hours on end, I wouldn't bother. Either way you are still only on the pilot circuit, and a couple hundred RPM isn't going to be a huge kick in the wallet.
 

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Discussion Starter #18
I can see your point, but the bike used to idle just fine around 1k - 1100. What I am trying to avoid is the stalling issue. Even at the low end of 1250, it will occasionally stall, especially if I'm on an incline. That's not normal at all. Sometimes it doesn't even idle rough before it stalls, it just cuts out like there is no gas flowing at all. When that times I have to give it a little choke to get it started again.

I do agree that I should keep it around 1250, which I would love to do if it would run reliably when its warm/on an incline.
 

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Yeah check the float level. Set them at 14mm, then reset the pilot screws while running to attain max idle per cylinder. If you get too far out past 2.5 turns then something else is wrong.
 

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Discussion Starter #20
I have the carbs out and the floats were set about 15mm. The book calls for them to be at 17mm, so should I set it to factory or set it to 14mm? What would the effect be if I set it lower versus higher?

Thanks again for all the help.
 
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