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Discussion Starter #1
Since my last (much needed) carb sync, the engine seems to run too rich at idle and low rpm's. The idle rpm goes to 3000 RPM when the engine is warmed up, and hitting the choke lever kills it right away, suggesting to me that it is running way too rich. I've checked the rubber boots, and they are ok, spraying them didn't change anything.
Also, i've got a lot of popping and sometimes even firing through the exhaust (a Viper racing can, open).

The strange thing is, when I force the throttle handle back (putting pressure on the return cable), I can get the rpm's down. So my first thought was that the throttle cable was stuck, and I cleaned and lubed it, but this is not the cause.

So I cleaned and re-oiled my air filter, which was a lot dirtier then i thought. I got most of the dirt out, but it still looks brownish. It didn't help.

The pilot screws are 2.5 turns out. The strange thing is that this didn't come gradually, but sudden as a result of my last carb sync. I must say that it had been maintained by my dealer before, and I doubt that he ever did a carb sync, because the difference in smoothness and throttle respons is huge!!! It now runs very very smooth.

So my question is : can a long overdue carb sync reveal other problems? Do I need a new air filter, and if so, which one? Stock? K&N?


I don't want to invest too much in this bike, because it is for sale right now (moving and kids force me to temporarily collect money).
 

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We could all use a new air filter.

I've been running a K&N for years and years and have finally gone back to the stock and/or aftermarket filters. They are cheap to replace and just as good as a K&N when clean (although I'm sure we all have an opinion on this).

Carbs are scarey but simple things.

Chances are your dealer did a particular crappy job on your bike.

It would be advisable to open up the carbs and clean each one out individually. Then reset all settings to default. Check your needle valves for wear around the cone, and also your pilot screws for grooving.

Things are not as expensive as they may seem. I managed to pick up some quality carb repair items at a huge discount from Motorcycle Spares Warehouse. Products are made in Japan NOT China which gave me a level of comfort. Delivery was very quick, although it is an Australian company delivering locally.

Carb Repair & Parts Kit
which includes:
Chamber Gasket, Needle Valve, Air Screw, Air Spring, “O” Ring, Washer



or the trusty Needle Valves alone for $8ea, not $45ea. I am using these at the moment and can recommend the quality. I have found that they are about 1mm shorter than the OEM but you can just reset your float heights after installation.

 

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Have you tried lowering the idle by turning the knob?

Sync'ing the carbs has nothing to do with the mix, is entirely mechanic, basically you are making sure each individual carb has same throttle plate opening at idle using vacuum as reference. Still, after the carbs are about the same, usually is necessary adjusting the common knob to adjust the engine idle.

The stock air filter is supposed to be replaced sometimes.
 

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Not certain...

Dunewar, you mentioned much in your original post.
I keyed on the fact that "when I force the throttle handle back (putting pressure on the return cable), I can get the rpm's down."
So, I'm wondering.... Have you checked the Idle adjustment screw?? I have to fiddle with mine over the course of the year depending on temperatures. Or the interaction with the choke and/or throttle sometimes cause me issues.

I realize that this is simple and you've probably considered it. However, you did not mention it, so, I thought I would.
Keep us posted.

[late edit] P.S. I like the way Hugo thinks!
 

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Discussion Starter #6
I did try that knob, that was my first call.

At first I thought it didn't do anything at all. But what happens is that something else is making my engine rev at 3000 rpm, So when the idle rpm is set lower then this treshold (which is normally is), the engine doesn't react to this knob. But when raising it over the treshold of 3000 rpm, the engine follows. When I force the throttle control to close down, I can get a decent RPM setting with this knob, as soon as I release the tension from the return cable it goes back to 3000 rpm.

so the idle rpm knob is most surely not the culprit.

Can a carb sync cause this? Basically it just balances the mechanical linkage between carbs. I have found that turning the adjustement screws too much upsets the RPM's a lot (at one point I had 6000 idle RPM), can this be happening? The gauges show a perfectly syncrhonised bike, and it runs very very smooth.
 

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Who did the synch? If it wasn't you I'd take it back to whoever did it. Perhaps the tech got things very wrong, or perhaps a cable is binding from fraying or mis-routing?

I can't imagine a clogged-up air filter could cause increased idle speed; I'd expect the opposite.

BTW, I thought a "run away" idle, if mixture-related, is caused by an overly-lean condition; maybe some of our carb experts can clarify.
 

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Hi Dunewar

I'm not saying your carbs are shot, I said they should be cleaned out to elimiate any other causes. At the same time, I would check things for wear. There could easily be wear in places of your carbs but that is merely an assumption and me trying to isolate any problem. I'm just trying to understand what you're trying to say in your post.

I do think that whoever worked on your bike has done something strange, which seems to be the consensus here also.

I am also wondering if your throttle "butterflies" close when they are supposed to. If you adjust your return cable so it is slacker would there still be a revving problem.

Is the return mechanism being interferred with by one of the fuel pipes, vacuum pipes or anything else?
 

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Discussion Starter #9
It is me who has done the sync, but I don't rule out anything went wrong here. I'm not a mechanic, but i want to do everything myself even if I do make a few mistakes along the way. I've done carb work on another bike, i've done valve adjustements, and i'm doing an engine rebuild of a honda CJ360t, so i'm not new to this, but I probably still make mistakes.

The butterfly valves looks like they are returning ok, and loosening the return and throttle cable (at first I thought the throttle cable was stuck, but it is loose and can move freely) makes no difference. When i pull the return cable, I can make the rpm's go down, but I have to put some pressure on it, which doesn't feel right.

The reason i'm looking at my air filter right now is two facts :
- The bike needs no choke to start, even when cold
- Choke kills it when hot, immediately. Before, the bike would increase in rpm's when choked, but not stall. It does now.

So it feels like i'm running with a permanent choke. I've checked the choke slide and it returns smoothly and the cable is free. The air filter is old (15k kilometers, supposedly 3 years but I'm not sure if the tech at that time replaced it as he was supposed to).

The only thing that startles me is that I can get rpm's down by forcing the throttle to close. Can you do that on a well set bike as well? One other option would be that something got in to the throttle return mechanism blocking a proper return, but from where I can see it without taking the carbs off, it looks good.
 

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Well what happens when you run the bike with no air filter?
If the problem goes away you've soved it, if it doesn't it's not the filter.
 

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Have you tried loosening the return throttle cable? It does have adjustment. You could even try disconnecting it altogether to eliminate this possibility. It is there only for safety.
 

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If you want to see if it is a cable problem I would disconnect the cables from the handlebars and see if the revs drop down.

Hitting the choke on on a warmed up bike should always try to kill it as the engine is being starved of air (or is it fuel?)

It is not normal to have the revs drop when you force the throttle twist grip closed unless the cables are binding or not adjusted correctly and are inhibiting the butterflies form closing.
 

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Johnny Blue Lightnin'
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It's not the air filter. it could probably stand to be replaced but it is not causing the revving.

It sounds to me like you have done something that is causing it to rev. Maybe you've got the syncing screws so far in the throttle plates are all open.
 

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Discussion Starter #14
It's not the air filter. it could probably stand to be replaced but it is not causing the revving.

It sounds to me like you have done something that is causing it to rev. Maybe you've got the syncing screws so far in the throttle plates are all open.
That would be my most plausible guess, since the effect only occured after the last carb sync. A clogged air filter would present itself much more gradually. i do have a lot of popping in the exhaust, much more then before, but then again this could be down to extreme inbalances in carb sync, right?

Wouldn't the carbs be terribly out of sync then? Or at least one of them.

This could be one scenario : I sync carb one vs carb two, but I get it terribly wrong, and I screw it in way too much....Carb two is opened up, carb one is not, but because of the big difference, cylinder one is dragged along in a heightened rpm. I then sync 3+4 to 1+2, but since carb 2 is opened up too much, I have to open up 3+4 as well. This would lead to 3 of the 4 carbs having too much opening compared to one.

could this be? Theoretically, it could I think, because the sync was soo far off I had to adjust it a lot. And i mean a lot! At one point I had 6k idle rpm, and I had to find a way back out by adjusting the sync screws.

So it seems I have a carb sync which is totally wacked by my own fault. Any way how to reset all this? I knew that doing my own maintenance was fun....:silly:
 

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I also had this problem on my 77 KZ650, what ended up being the problem was the return cable was broken in the throttle assembly on the handle bar. The end had broken off an was just sitting in the twist assembly. removed the entire cable and no more problem.
 

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That would be my most plausible guess, since the effect only occured after the last carb sync. A clogged air filter would present itself much more gradually. i do have a lot of popping in the exhaust, much more then before, but then again this could be down to extreme inbalances in carb sync, right?

Wouldn't the carbs be terribly out of sync then? Or at least one of them.

This could be one scenario : I sync carb one vs carb two, but I get it terribly wrong, and I screw it in way too much....Carb two is opened up, carb one is not, but because of the big difference, cylinder one is dragged along in a heightened rpm. I then sync 3+4 to 1+2, but since carb 2 is opened up too much, I have to open up 3+4 as well. This would lead to 3 of the 4 carbs having too much opening compared to one.

could this be? Theoretically, it could I think, because the sync was soo far off I had to adjust it a lot. And i mean a lot! At one point I had 6k idle rpm, and I had to find a way back out by adjusting the sync screws.

So it seems I have a carb sync which is totally wacked by my own fault. Any way how to reset all this? I knew that doing my own maintenance was fun....:silly:

Now we're getting somewhere.

Sounds like Occam's Razor to me :)
 

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Johnny Blue Lightnin'
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That would be my most plausible guess, since the effect only occured after the last carb sync. A clogged air filter would present itself much more gradually. i do have a lot of popping in the exhaust, much more then before, but then again this could be down to extreme inbalances in carb sync, right?

Wouldn't the carbs be terribly out of sync then? Or at least one of them.

This could be one scenario : I sync carb one vs carb two, but I get it terribly wrong, and I screw it in way too much....Carb two is opened up, carb one is not, but because of the big difference, cylinder one is dragged along in a heightened rpm. I then sync 3+4 to 1+2, but since carb 2 is opened up too much, I have to open up 3+4 as well. This would lead to 3 of the 4 carbs having too much opening compared to one.

could this be? Theoretically, it could I think, because the sync was soo far off I had to adjust it a lot. And i mean a lot! At one point I had 6k idle rpm, and I had to find a way back out by adjusting the sync screws.

So it seems I have a carb sync which is totally wacked by my own fault. Any way how to reset all this? I knew that doing my own maintenance was fun....:silly:
Yes. The carb sync typically requires less than 1/8 turn per screw to bring back into spec. I'm not quite sure how to start over. Trial and error I guess. There is no specific procedure in the shop manual. Maybe you can find a link online somewhere... When I look up info like this I try to find 2-3 links that basically say the same thing. Then I can be reasonably sure the info is fairly accurate.
 

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Maintenance Overlord
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carb adjust

set 1 and 2 (LH adjustment screw) then 3 and 4(RH adjustment screw) and finally group 1+2 to group 3+4 (centre adjustment screw) easy easy as you go,

gg likes the final adjustment to be done screwing inwards as this takes up any slack in the screw adjustment.

[email protected], ava good day
 

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set 1 and 2 (LH adjustment screw) then 3 and 4(RH adjustment screw) and finally group 1+2 to group 3+4 (centre adjustment screw) easy easy as you go,

gg likes the final adjustment to be done screwing inwards as this takes up any slack in the screw adjustment.

[email protected], ava good day

Any don't forget to blip the throttle after each change, although I think gg knows this.
 

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Discussion Starter #20
The problem seems to be resolved. First I checked the pilot screws and behold : they were very very out! Which is surprising since I didn't touch them for years. I had set them at 2.5 turns out 2 years ago and never touched them, but they were definately way way out. Maybe because of vibrations?

So I reset those. Then I warmed the engine up with a testride, and the RPM stayed at 1250 idle speed. I then checked the sync and it was all good. The bike felt good as well, no vibrations, very smooth. I must add that it was -3° celcius here, so maybe that's interfering with it as well. I did let the bike warm up real good, slow testride, letting it sit turning idle for about 10 minutes in the garage etc...

I'm glad it is sorted, because............it's sold!!

I sold the ZR-7s and I will go hunting for another bike in spring. I'm looking for a used BMW R1100s. I sold it because it had done it's share of commute work and salt and water had gotten the best of it. I was expecting problems sooner or later.


But I had to get this right before the buyers came to pick it up. Anyway, thanx for all the help!
 
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