1994 VFR750F Runs OK, but rough idle & stalling

Got a problem with my VFR.

Starts well enough (have to add a touch of throttle when cranking it), with the choke engaged the RPMs are rock solid and don't jump anywhere, and once I'm up and going the bike rides great - I've put 6000km on it since I bought it, with a couple of 12hr days on the Autobahn and over some mountain passes this summer, and it never missed a beat.

but...

It idles very, very poorly, no matter if it's warm or cold. Left alone (with no throttle) the RPM will jump around from 1000 - 2000 at random, constantly surging up and then crashing back down, often causing it to stall out once the RPM drops below 1000. I've been "fixing" this by giving about 1% throttle whenever I'm not moving to keep the RPMs stable at about 1500, but this has become really annoying and I want to fix it!

It wasn't always like this - when I first got the bike it idled just fine at around 1500 RPM, but I had to take it back to the shop where I bought it because it started leaking gas from the carbs, and they had to replace some rubber part because it had cracks (not exactly sure what). Since then, the idle has been getting slowly worse and worse, but this could also just be because I have been riding the bike more and more KMs and might not be directly correlated (?).

With all of the above in mind, I'm thinking it's either:

1. Clogged pilot jets/dirty carbs in general: (although, since I bought it, I have used gas with ethanol a grand total of 4 times, and never a full tank - just enough to get home before I put the good stuff in again.
2. Idle speed set incorrectly: I'm not an expert on carb issues, could this be the cause?
3. Carbs out of balance.
4. All of the above.

Anyone have any idea what it could be based on my description? And what's the first thing I should start with when diagnosing the issue & fixing it?

 

"some rubber part because it had crack"

You should have been told, you should have asked specifically, what part?

Sounds like you might have a small intake leak (probably from the "work" that was done), combined with carbs that absolutely should be properly, COMPLETELY, gone thru.

1. Yes / likely. Absolutely need to verify this. Your fuel choice is not an issue in this scenario.
2. Idle speed on good carbs set incorrectly stays where it was set incorrectly.
3. Maybe. Not your problem here. Do this after #1
4. 1 & 3.

Seen and read this scenario so many times. #1 thing is you need to confidently know that you have properly cleaned and adjusted carbs, properly installed on this new-to-you bike. Guarantee it will run great if in the hands of somebody who knows how, and wants to, do them right. That is usually not the shop you take it back to where you bought it.

If it appears I am missing something, please comment.

Good luck!

 

Yeah, the shop I bought it from wasn't the best - but they stopped it leaking gas for free (as it was less tham 1 month after I bought it), so that was the main motivator. Also, they did tell me which part, but I have forgotten & don't have the chat history since I got a new phone.

I'm also thinking of cleaning the carbs myself (would be my first time). How difficult would it be? Again, mainly motivated by cost as I've been quoted 1000eur ++ to get them done - which is almost what I spent on the entire bike.

 

if youre lucky you might cure the problems simply by pokin a thin wire (,o10") through each idle jet and doin nothing else.



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^^^^^however, it seems like you need an accurate carb synch in addition and possible adjustment to idle mixture screws.

 

You absolutely can do it yourself. Do your research, collect your parts, have a dedicated work space that won't be disturbed if (when) left unattended.

Start a thread ahead of time (in my opinion). Take pictures as you disassemble. Ask questions before doing something you're unsure of (see WHEN left unattended above).

 

try doing the simple things first (as above) rather than have a carb newby to tear everything apart. just tackle the simplest, easiest things first, like cleaning idle jets, which commonly clog during long periods of inactivity without anything else goin wrong.

 

Yes, but... Doing a simple thing first is quite a bit of work to get there.

You've removed the carbs, you removed the bowls, removed the slow jets AND pilot screws (right?....). Shit, at that point you are already more a V4 carb "expert" than most everybody. You don't have to "tear everything apart" and learn to loosen the carbs off of the plenum and remove the two outer carbs off of the choke linkage to replace the fuel tube orings (which are gonna leak next season!). How about float bowl rings? Replace or just slap the bowls back on? They are only 30 years old and have gonna thru countless heat cycles in the center of a hot V4. Same goes with the fuel tube rings.

I'm all about if those carbs are coming out, I want to maximize headache free years and miles of V4 goodness.

Do just enough for now or do the job for your future. Everyone has a choice.

Next season, or perhaps the next one after that, is the price on rebuilding carbs gonna go down? Or will it then be the time to learn.