I screwed up my carb adjustment

Well I screwed up. Yesterday I started drinking and thought it'd be a good idea to work on the bike. I just got the Morgan carbtune Pro and I guess I figured I should sync the carbs. I've never synced my own carbs before.

I hooked up my Carbtune and started fiddling with the pilot screws. I wasn't getting any noticeable changes in vacuum pressure no matter what I did so I figured.. what the hell I'll turn that screw until the gauge starts moving.

Well it turns out the hoses provided weren't sealing on the carb vacuum nipples and now my adjustment is so far off from where it was the bike is idling horribly. :dread: Actually the idle doesn't drop past 4k. I think something is binding on the throttle plate cams. I'm not worried about that though.

So I'm going to start over from scratch. According to what I can see in the Clymer manual my 1996 Canadian VFR750's pilot screws should be turned out 1 and 3/8ths turns after they lightly seat.

I however cannot find the initial starting point for the idle speed screw.

I haven't screwed the pooch this bad in a VERY long time and I'm feeling pretty silly about the whole thing. :cower:

Please help me get this thing back in a usable state.

 

Unless I have misunderstood you Matt, I think you have attempted to adjust the vacuum levels using pilot screws. Pilot screws don't do that, they set the amount of fuel flow through the pilot jets, and so affect mixture at the lower rev range. Setting them to the stock initial setting is the right thing to do (well, not messing with in the first place would have been better) until you get the carbs balanced up. Once that is done, there is a procedure called idle drop that is used for a final pilot screw setting.

The carb synch screws are on the linkages between the carbs, you have three screws that adjust to the fixed fourth reference carb. The screws basically set the throttle plate positions so the amount of air passing through each (and thus vacuum generated) at idle is exactly the same. You can bench set these if needed so that they all have the same physical gap e.g. using a fine feeler gauge, before using the vacuum gauge on a running motor.

So find the idle adjuster and back that out until the idle settles down to something reasonable (1400 say). Now synch the carbs using the Carbtune, so that vacuum from the three adjustable carbs lines up with the fixed one. You may need to reset the idle as you get closer to balanced, and recheck the synch again.

 

Drinking, while making fine adjustments on a motorcycle's carburetors...... What could possibly go wrong?

 

^ Haha right? I remember being so happy while doing it to.

No Cadbury, I'm the confused one. I'm not familiar with the terminology of the carbs. The screws I adjusted are the carb sync screws (3 of them). Somehow the idle now rests at 4000rpm even with the idle speed screw completely loosened off.

I'll take everything apart on my next couple of days off and see what's going on. I was just wondering if anyone knew what the base starting point was for the idle screw and the carb sync screws just to get me back into the ballpark.

 

A-ha!

Well 1 3/8 turns won't be any good for the carb synch screws, but I now understand why the idle is so high. Good that you left the pilot screws alone.

The idle screw holds the throttle butterflies just a little open, so the engine gets just enough airflow to run. And the balance adjusters keep the three other butterflies opened to the exact same amount as the reference carb. By winding the balance adjusters in, you are holding the other 3 butterflies really open so you get lots of revs.

If you can get a good look down the carb throats you should be able to adjust the three other butterflies by eye so they are evenly open with the fixed butterfly. I know from recent experience the butterflies should just have a sliver of clearance when they are at idle, so move the idle adjuster first to get the reference carb just starting to open, then eyeball or feeler gauge the others so they are opened the same amount.

I would suggest that you get your carb synch tool back on there (sealing properly), get the bike running, then start tweaking the adjustment screws until you get back to a balance. The adjusters are usually extremely sensitive, 1/16th of a turn will make a big difference. Good luck, and lay off the drinkies this time.

 

Actually I just looked in the manual and found this, which should get you rolling again.

 

Chocolate Lad's info is A-OK. If needed you caN JUST REMOVE CARBS and use the advice above to align all the throttle plates. In prACTICE, IF THE BIKE IDLES ANYWHERE NEAr properly, no synch screw would need more than A 1/4 TURN....OR LESS !

 

^ Yeah. Not my finest moment.

Hopefully I'll fix it up by Saturday afternoon.

 

I re-jetted the 599. That wasn't too bad. Took about 4 hours. I prefer a good set of carbs to FI. I like the feel. But on my '01, Interceptor, I'm glad it's FI. The 599 is 5 years newer; with carbs. I love that little bike BTW.

 

I got back at it today.

[video=youtube_share;3PophqxErxY]http://youtu.be/3PophqxErxY[/video]


For anyone in the future reading this.

On a 94-97 VFR 750 your base carb is the back left one while sitting on the bike. The one in front is 2, rear right is 3, front right is 4.

Turning the adjustment screws clockwise *tightening* increases vacuum pressure and lowers the idle a minuscule amount per each adjustment. Counter clockwise *lefty loosey* on the adjustment screws lowers vacuum pressure and increases idle. Which makes sense. As the throttle plate butterflies open vacuum pressure decreases. By tightening the adjustment screw you're depressing the spring that attaches to the throttle plate cam and lowering idle and increasing vacuum pressure.

A decent base starting position (if you screwed up your adjustment like me) is turning the 3 screws almost all the way tight. If they're too loose it'll put too much pressure on the throttle plate cam and your idle will sit anywhere between 1000-5000rpm even with the idle adjustment completely backed off.

My engine pulled roughly 18cmHg once the carbs were all balanced so slowly adjust your screws until you start to see the carb get close to that reading though your bike may balance out at a different cmHg reading. As it's overall vacuum pressure you're after adjusting one carb will affect the others. I must have made dozens of small adjustments on each carb until they all started lining up pretty close to Carb 1.

Now that I've done it I can't believe how easy it is. I did use a 90deg screw driver and that helped a lot, though carb 3 screw was tough to get at due to the angle. I did not however require an auxiliary fuel tank and kept overall disassembly to a minimum.

Hopefully this helps someone.

Thanks for the all the positive input guys! Stay safe out there.

 

Yeh, its a snap, helps to raise bike up on a lift and use your headlight. Angled screwdriver is imperative.

 

It's best to snap the throttle wider a couple of times after each small adjustment so the butterflies settle firmly into the new position.

 

^ Yes. Excellent advice as always.

I rotated the throttle cable connection between cyl #1 and #2 to snap the throttle so I didn't have to get up each time.

I also didn't have to worry about vacuum gauge fluid issues with the carbtune, which was nice.