Carb synch got complicated, need an advice.

Hi. My bike is vf700f 1984. I was trying to synch carbs and run into the following problem:base non adjustable carburator (on cylinder 4) is showing very little vaccuum on my Carbtune tool. So little, that it only moves a bit, muchc less then other 3, when engine is revved. Does this mean that I have a vaccuum leak somewhere? If yes, then what would be a wise course of action?

 

I would start by looking at two major things. one of them being your intake boots. are they hardened, cracked? if so, those are a common and VERY likely source of leaking air. replace em.

beyond that, I cant really suggest anything as these motors are pretty simple, once you understand them.

 

Before pulling the bike apart do a compression check on that cylinder first. If it's good no need to take things apart. Could be a bad diaphragm in that carb. Make sure the side is moving up/down when running. Could be the idle is turned way down on that carb only.

#4 only works off the idle screw. If the other carbs are adjusted (out of whack) so the bike runs they may have dropped the over all idle. Which in turn kills the #4 carb.

Just some more guesses for you to look at.

 

Thank you everybody for fast reply. I did switch rubber tube on carbtool few times thus eliminated possible problem there. I do get #4 gauge moving when under throttle. One more thing: it seems that throttle response is not immediate ( looking at the carbtool) as matter of fact I see a momentarly drop in vaccuum on all four cylinders prior to followed surge when throttle apllied. I just got a bike, never owned a motorcycle before, therefore it is hard for me to judge its performance. Also I do hear a knock/tick when at the idle ( idles correctly at 1250 rpm).

 

I have to ask a dumb question...... did you use the idle adjust screw to set the #4 carb first and then match the others to it?

 

Nope. Honda shop manual is not very detailed on this. What is suggested vaccuum at the idle for vf700 in youR opinion? As low as possible?.

 

Don't know about the vacuum at idle for the 700. The value for the 500 is all carbs should be within 40-60 mm Hg at idle depending on the year. I would think that the 700 would be similar, I'm sure someone will chime in with the correct figure. The procedure is to set the idle at 1200-1500 RPM with the idle adjust screw on #4 carb. Then, adjust the remaining 3 carbs to match the reading on #4.

 

Hi all,

I purchased some vacuum adapters from Motion Pro
Motion Pro - Vacuum Adapter, Brass w/Cap 5mm (Ea)

The first one that I installed broke off. Can you buy SS ones anywhere?

 

Quick way to check carb boots for leak. While engine is running spray a LITTLE carcleaner on the boots one at a time. Remember this stuff is explosive so use it carefully. If engine speed increases whrn this is done you have found the leaking boot. Iknow everyone will say I am crazy using carb cleaner on a warm engine but I have used this method for many mnay years with good results. Be carefull and do it in well ventelated area. Others have said cam wear, etc, this could be the problemm but eliminate the most common easily to reach parts first then move on to compression tes and rocker arm adjustments after that. You can use a dial indicator to check valve lift also, compare a good running cylinder to the one not running good to see if cam is worn. Good luck.

 

carb cleaner will lower RPM when sprayed on the boot, starter fluid will raise RPM, go easy if you use starter fluid on hot engine be carefull not to start a fire.

 

My experience with starting fluid is that if it does ignite, it burns away so fast there is no harm done..... :kaboom: ...... hopefully....

 

We use Honda aersol Carb Cleaner in the shopo as STARTING FKUID and CARB CLEANER in the shop. It is very flammable just like starting fluid, so it raises RPM unlike the carb cleaner that comes in a can that you simply put the parts in to soak

 

Hey, Im preparing to do a carb sync on a 82-83 VF750S. Too late for a bench test (didnt know about it till just now) I already re-installed the carbs after giving them a good cleaning. Can I still do a sync or do I have to take the carbs back off AGAIN? WHERE DO I START?

Im confused after reading this THREAD because you say #4 works off the idle screw and then in a posts after this one it says,

(The procedure is to set the idle at 1200-1500 RPM with the idle adjust screw on #4 carb. Then, adjust the remaining 3 carbs to match the reading on #4.)

Well if i adjust my idle with the #4 then play with the idle screw adjust which is #1. Wont everything be out of wack?

Please tell me how to start synchronizing and how to adjust my idle?

*BTW i have no guages on my bike so I will have to adjust the idle by ear.

 

HEY DUDE U CAN ALWAYS BE COUNTED ON Well i think i messed up!!!!!

Just had my carbs off for cleaning and adjustment and popped em back in not knowing about this Bench Sync Thing!!! My Carbtune Pro doesnt even show any readings I can work with now and I imagine my idles outta wack. My bike doesnt have any guages so I'll be adjusting by ear. How do I get back on track?

IS IT POSSIBLE WITHOUT PULLING THEM CARBS AGAIN???

My bikes the same old European 84 VF750 Sabre but the same as the 82-83 American Models!!!