High Idle and slow throttle return...

My 90' vfr750f just got the carbs cleaned and new gaskets and has never started easier BUT... The idle is somewhere around 1800rpm. The throttle cable is not bound up, the idle screw is adjusted out as far as it will go and no longer touches the throttle plate, nor is the choke or choke cable too tight. I think i must have changed something when reassembling the carbs (i.e. pilot screws, throttle plate adjustment). Also, when you roll the throttle and release it the bike takes 4 or 5 seconds to fall back to the idle position. I don't know if this is something associated with the high idle or completely independent. Is it just a matter of adjusting the pilot screws while the bike is running or am I missing something? (Do I have two problems or one?)

Also, the screws for the throttle shaft adjustments (there's three of them located in the center of the carb assembly) were not touched unless just the nature of taking them apart can cause them to fall out of place.

 

How is the carb sync? I would set the pilot screws 3 turns out from lightly seated.Are the carbs fully seated in the boots? Almost sound like excessive air intake.

 

Ok boots are tight. Idle drops correctly after reving but idle is still about 1700-1800 rpm with idle adjustment knob backed out all the way.

 

Choke cable/linkage is hung up causing a high idle. Make sure all chokes are fully closed. If they move AT ALL, then they are not closed.

 

Ditto (more-or-less) ...

Hello:

I just went through this on my '89 FK.

The choke "system" (lever, cables, linkage and adjustment) is completely separate from the throttle "system” (cables, linkage and adjustments).

So, you gotta make sure that the choke linkage is "tuned" at each carb - that they ALL operate simultaneously.

When I got my carbs back together, I simply could not figure-out a way to reassemble them without disturbing the balancing screws. There may be a way to do it, but I could not see how to do it. So, I purchased a CarbTune, and balanced/synced the carbs - the last step. Until they were balanced, the RPM would not drop down to idle. It was only when I was getting close to having them balanced that the idle-adjustment then made contact with the "wheel" on the carbs again, and I could properly set the idle RPM.

So, it sounds to me like you gotta balance/sync the carbs.

By the way, I took her for a 173 mile test ride yesterday; she's running like a dream.

Gray Market

 

Well, the choke linkage after the cable is all hooks that pull the choke assemblies (or starters) out simultaneously. The starters are all spring loaded to return to the closed position. The only adjustment is the set screw on the starter link shaft that connects one choke (starter) to the one next to it (across; LF to RF, LB to RB). Unless in the closed position something is getting bound up, which from what I could tell wasn't the case when I put it back together, then could it be an increase in gas causing the high idle. My pops who's a car guy says he thinks it's running to rich. Would an imbalance in pilot screw adjustments cause this or is this a throttle plate adjustment issue?? I'll recheck all the linkage tonight to make sure there's no way those starters are open.

 

Don't understand. Had the carbs off twice last night and choke and throttle lines are fine. It must need a carb sync because the pilot screws are all backed out to 1-1/2 turns which should be the basis for setting them right but I can't get that idle down. What does a carb sync consist of? I heard of the 3/32 paper trick but isn't that going to leave the throttle plates way open and raise the idle?

 

The carb sync ensures that the vacuum for all carbs match. If they don't, one or more carbs can be supplying the engine more or less fuel/air which will raise or lower the idle. (the one or more carbs drag the others with it)

The 3/32 slip of paper as a feeler gauge is to try and get the throttle plates as equal as possible before putting them on the bike. They still won't be "synced" until you perform a carb sync on a running bike.

Carb Synch Overview:
The carb sync device measure the vacuum on each carb. For optimum performance and efficiency, all carbs must be pulling the same vacuum (actually, the vacuum in indicator of how much the throttle plates are open - which meters air/fuel into the engine).

One carb is usually set and cannot be changed. This is usually the carb with the idle adjust knob on it.

Since this carb is set, the other 3 carbs must be adjusted to match it. Turning the sync screw changes the angle of the throttle plate for that carb.

As all the throttle plates are linked, the carbs must be synced in a specific order (see manual). Typically, it is the order of firing.

Once all the carbs are synced, and idle drop procedure is performed.

As you can see, since the carb sync depends on vacuum, any air leak (air box, carb boots, etc...) can cause the sync to be incorrect.

 

So I see a need for a vacuum gauge... Carb 2 has the throttle wheel and from my observations all the other carbs "fall" onto it, so that when you move carb 2's throttle plate they all move. So when idle all carbs' throttle plates should be equal distance from the bore with some adjustments to make sure all the vacuum pressures should be the same... But no throttle plate should be fully closed?? If any other this is incorrect set me straight. So my next question is where do you get 3/32 paper?!?!?

 

You are correct in regard to the reason and purpose of the sync.

You misunderstand the 3/32 paper method. Get a piece of copy paper and cut a slip of paper 3/32" wide. Cut it about 4" long.

You will use the paper as a feeler gauge in the #2 carb between the throttle plate and the bore. Adjust the idle screw until you get a consistent pull. Manually pull the throttle linkage and allow it to snap back. Measure again. Repeat until you get a consistent pull. Be sure to place the paper feeler gauge as far from the throttle plate shaft as possible. If it is off to the side, the tolerances will be tighter.

Now go to the next carb in the sync order and do the same thing - this time adjust the sync screw for that carb. Repeat on each carb in order until all carbs have the same pull. Go back and check each carb again.

That was a bench sync.

Now back out your idle screw until it barely touches. Now screw it in 2 complete rotations. This should give you a "good-enough" idle for starting the bike.

 

Ok... (sorry you have to spell all this out for me but I have to completely understand the how and WHY before I get stuff...) By consistent pull you mean... I can pull the slip of paper out or not? I adjust the idle screw and it changes the angle of the plate in the bore. Should I avoid compressing the paper against the bore? Are we going for the clearance the 3/32 piece of paper makes placed lightly on the bore? Crap, I'm drawing a pic... Check my profile. I've loaded up 2 pics...maybe I got this...maybe I don't.
test2 - VFRworld Photo Gallery

 

Are you sure the slides are returning to full closed (i.e.: hitting the stops), when you release the throttle? sounds like something is hindering the slide's ability to completely close, or there is an air leak somewhere. Just my .02 worth of thought....

 

Sorry, I thought you knew how to use a feeler gauge.

Here is a pic of a set of feeler gauges:


A feeler gauge is used to adjust to small tolerances. For example, if you wanted the distance between two adjustable objects to be .005", then you would use a feeler gauge that has a width of .005".

Stick the feeler gauge between the two objects and adjust them until they touch the feeler gauge. You can adjust them too tight and you are unable to pull the feeler gauge out. You can adjust them too loose and not feel any friction at all when you pull the feeler gauge out.

You want to adjust them tight enough so that when you pull the feeler gauge out, it feels like pulling a sheet of paper out from under a heavy phone book.


Now say you have 8 objects (4 tolerances) you need to set. You would use the same feeler gauge in all objects and adjust until you felt the same friction on the feeler gauge for all four tolerances.

As for this particular instance, your drawing is correct (although not to scale).

Since we don't need to have a set tolerance - and only need to ensure all 4 tolerances are the same - any "feeler gauge" can be used.

This bench synch is not an exact science. In fact, the manual probably states to eyeball the throttle plates so they matches the non-adjustable carb - in reference to where the plate stops near the small holes in the bore.

The 3/32" slip of paper method is just to get you closer so you don't have to do as much adjusting when doing a true carb synch.

 

Ok, I'll do it and shut up. I have used feeler gauges before but just on spark plugs. Thanks for your patience.

 

Lots of help here ...

Hello:

I've not visited this thread for a while, and I see you are getting lots-o-help herein. Nonetheless, I thought I too would try to help-out again.

With all due respect, I am not certain that you are "getting" that the use-a-piece-of-paper-as-a-feeler-gauge "trick" is simply a method, a technique, which will only get the balance/synchronization of your carbs in the ballpark. That is to say, it is not the final adjustment, by any means.

You will not get these carbs accurately synchronized unless you use a proper tool to do the job. I purchased the CarbTune ( http://www.carbtune.com/ ), which I found to be extremely accurate and easy to use. Having just done this, the bike is performing extremely well; great throttle response, especially when "blipping" for a down-shift.

In my first response to you, I tried to explain that my bike was exhibiting exactly the same symptoms as yours in your first post, until I used the CarbTune and got the carbs accurately balanced/synchronized. And, yes, I used the "bench technique" of using a thin piece of paper to get 'em in the ballpark first.

It wasn't until each of the other three carbs (other than #2) began to come into "range," that the idle adjustment screw (knob) made contact with the throttle "wheel" on the outside of #2.

By the way, when it came to actually balancing the carbs, I used a long (deep) 7mm socket, as opposed as attempting to use a flat-head screwdriver. I found it easier to count the turns as the carbs were coming into balance.

Another thing to know is that, when it comes to balancing the carbs, there is no "actual" value to the readings. That is to say, all you are attempting to do is get 'em all to read the same - to be balanced the same - and the value of the reading does not really matter.

Gray Market

 

Well, nothing as of yet has solved the high idle issue. All adjustments to the throttle plates, pilot screws or idle screw will not drop the idle (1800rpm). In fact, it is so consistent even after removing the carbs 3 times that it has to be some other issue that's causing this high idle. I know of someone who was a mechanic on these 80's and 90's Hondas and I think it's time I have him take a look and see if he can fix it. At this point I just want it done so I can ride! Plus, I'd love to hear what the real problem is.

 

Ok I can't stop thinking about it... Could not putting enough twists on the throttle shafts cause erratic or high idle? Since the plates pivot on a shaft what effect does a vacuum have on them (I assume they want to close?) But a high idle acts just like having the choke open and I wonder if a vacuum leak could be the culprit. Guess I should try the old carb cleaner trick... When I took the carbs off the second time I also noticed some deposits of something on the throttle plate. Looked like a few grains of pepper. Any thing to worry about? Also, what effect does adjusting the float have? Sorry but this is as frustrating as having a 20oz T-Bone and no charcoal!!!

 

Ok, you all can take turns slapping me in the face. Turns out it was a bound up throttle cable, go figure! Bike now runs smoother than I ever could imagine. Going to go through the pilot adjustment procedure tonight. Thanks for all the help. I guess if you can't figure out the problem then you're thinking too hard.