Carb Gurus Please Help!

Hi All,

I'm restoring / rebuilding a 1991 VFR 750 and I really need advice about some recent carb work I've been performing. I never intended to work on the carbs but after letting the bike sit for about four months while I worked on other systems a float needle got stuck and fuel poured out of the carb past a worn out float bowl gasket. I was pretty bummed at the time but I now realize that was a blessing in disguise as I'm now very familiar with the "heart" of the bike and a system that before now intimidated me. For anyone concerned I did change my oil after the overflow happened and checked there was no fuel in the cylinders.

After reading how common this problem is I sucked it up and purchased four sets of OEM gaskets and o-rings as well as a new valve and seat for the stuck carb. The new valve was probably overkill but better safe than sorry! The carbs themselves were pretty clean but I disassembled the carbs one at a time and used a can of carb. cleaner per carb to carefully blow out each passageway as well as each jet. I installed new float bowl gaskets, new pilot screw (fuel / air mixture screw) o-rings, and replaced one of the carb boots that was cracking. I set the float hight per my Haines manual and reassembled everything. I casually did a bench sync but figured the linkage should be in sync since I didn't disassemble any of it. One butterfly valve seemed a bit tighter than the others so I backed the screw out about a half turn.

The bike fires right up now with no leakage. Great! It sounds pretty rough but smooths out a bit after warming up. I figured I was way ahead of the game at this point because I had completely removed the pilot scerws to clean them and replace the o-rings so I figured a carb sync would fine tune the bike and I'd be all set. I set all the pilot screws to 1 1/2 turns from tight as a starting point and hooked up the Carbtune tool I borrowed from a friend to the ports just below the carb boots. I was dissapointed to see that my front two cylinders we at about 11 / 12 (not sure what units this tool uses) and the rear two were at about 16 / 17. At least this explains the rough running... So I start adjusting the pilot screws to see if I can raise or lower the bar to start to match the cylinders to each other. And here's the problem: when I adjusted the pilot screws even from totally tight to almost totally out it had little effect on the meter. What the hell?!?!

So now I'm pretty stumped. This is my first time working on carbs so please don't assume that I checked all the easy stuff.... I probably don't know what that would be! I do however have a list of questions:

-The Haynes manual says there are plastic limiter caps on the pilot screws. I only see caps on the rear two. Has anyone removed these? Is there any reason to? Any reason not to? One of the caps seems to spin about quarter turn before the screw starts to turn. The other seems solid.
-The Haynes manual says the stock non-California carbs should have their pilot screws set to 1 1/2 turns from tight. Mine were set at 2 1/2 to 2 3/4 from tight when I removed them. Also, my bike has a K&N air filter and a Two Brothers slip on. what do you think I should use as a starting point? Two turns from tight?
-Why would adjusting the pilot screws from fully tight to almost falling out not make a difference? Do I need to pull the carbs back off and bench sync again? Replace the screws? Something else...
-My assumption is that you should set one pilot screw to the correct turns from tight and then adjust the other carbs to match it on the Carbtune. Is this correct? Is it really true that there is no correct hight on the Carbtune and you really just want them all to match the first carb's height? I'd really like to set the carbs properly to avoid rich / lean running and all the problems that brings.

Any help or links to similar posts would be greatly appeciated! I'm so close to being done with the restoration I'm getting very antsy but I realize all good things take time.

Thanks for helping!

IF you are adjusting the pilots that much w/ no effect, then most likely some of the passages are still blocked.

I think there might be a little confusion here, and it could be on my part. But if you hooked your tool up to the vacuum ports on the intake, it is not the pilot screws you should be adjusting, it is the adjustments to the adjusting screws for the throttle butterflies.  It sounds like you need to adjust them front to rear to sync them.  If memory serves me right, to adjust the pilot screws, you need a digital tach, you adjust #2 out a half turn then see if the idle raises by 50 rpms or more, if so turn them all out by half a turn.  The turn the #2 in until the rpms drop 50 rpms, when it does, turn it back out 1/8 turn, then do cyl 1, 3, then 4 the same way.

Thanks for the reply Sam! I was adjusting item #5 on the diagram.

http://www.bikebandi...m151579#sch7762

You're saying that's not the correct screw to adjust when doing a carb sync?

The link did not take me to the diagram, but you can download the manual off this site, section 3-11 gives you the carb sync, end of section 6 gives you the pilot screw adjustment.

http://www.vfrdiscus...service-manual/

Awesome! I was definitely sync'ing the carbs incorrectly. I'll give it another shot tonight and post the results. What are people using for digital tachs?

[image: View Post]tylerstock, on 13 September 2012 - 03:43 PM, said:

What are people using for digital tachs?

Now that is a good question, I have a multi meter that has a tach function I use when tuning my old Jeep carb, it should work.  I have never tried to hook it up to my VFR, but it is just two connections, one to the negative battery post and one to the coil, negative side I think (Jeep coil has a tach label).  To be honest, I did my pilot screws the initial setting, sync the carbs, it idled great, I left the pilots alone.

Thanks again for the help Sam! I attacked the carbs last night after work with great success! At first I couldn't really see the adjustment screws so I ended pulling the carb assembembly back off my bike and adjusted the linkage to what I thought should be correct based on the initial Carbtune readings. I ended up sending them farther out of sync but I was able to figure out where all the screws were and which size socket fits them. I'm glad a socket fits because there was no damn way I was going to adjust them with a flathead once they were re-installed! After they were re-installed (and farther out of sync) I convinced myself it must be possible to adjust them on the bike and set about making it so. After about an hour and a half of work (with a few pauses for adult beverages) everything was synced and the bike ran great! It's such a relief to hear it run properly!

I've got a multi meter which I believe has a tach. I'll start working on adjusting the pilot screws using the tach feature and post the results.

Thanks SamW and everyone on this site. VFRD ROCKS!!!!!

Good to hear! Let us know how the multi meter tach works out, maybe I will go back and tune my pilot screws if it works.

Anyone in the ATL area have carb sync tools?  We are trying to get my other sons 3rd gen back on the road after a lengthy period of sitting with the po.

This is great info here and I am sure it will help us greatly.

Bring it up with you next week and we can sync them at my house, then you can take it for a test run in Shady Valley!

Good you got it sorted. Also remember that pilot screws are for idle only ( and to a small degree just off idle). Alot of folks get stumped on this and mess with the screws to fix open throttle conditions.

[image: View Post]SamW, on 14 September 2012 - 03:44 PM, said:

Bring it up with you next week and we can sync them at my house, then you can take it for a test run in Shady Valley!

New riding son plans to ride it up...hope the carbs will allow the ride up.  Looking for some type of carb sticks tomorrow or a trip to a shop on Monday before we head north on Tuesday....

Hi,
You should take a look at this guide with pics:
http://www.bikersora...ead.php?t=87239

Greetings from Denmark :)

Thanks for the link.  We still cannot get the bike to idle at the correct speed and the idle adjustment does not raise the idle far enough.

The bike will stall without "help" from the throttle hand.

I guess we are taking it to a shop this morning.

Perhaps this answer is a little to simple:
But did you rember to adjust the idle by using the plastic knob attached to the cable at the left hand side of the carbs?
It's function is like the "stop position" of the carb intake butteflys :)

Plus one on the above, and also check for air leaks around the boots or maybe check to see if the screws are back in the vacuum ports.

yes, we adjusted that plastic knob to the fullest and the bike still stumbles and shuts off at an indicated 900 rpm, the plastic knob wont bring the idle any higher....no air leaks as well.

just dropped it off at a new hsop in town, the guy seemes knowledgable and said he could get to it today because he is waiting on parts for the other bikes in the shop.

they also have a bike dyno in a dedicated dyno room

Hopefully he will get it fixed, sometimes if the idle jets are clogged, it will idle on half choke, but not without choke.  Those can sometimes be a pain to clean out, they are the smallest of the passages and there are no jets to pull out and soak, you just have to tap a little wire through them.  That was the last thing I cleaned out to get my '95 running right.