Carb balancing driving me insane

Hi,

I have a 97 VFR 750. I have spent the best part of 2 days trying to balance the carbs on it. I'm using a Morgan carbtune manomoeter. I have it now that the 3 non-reference cylinders are pretty much even. However the reference cylinder is at the top of the range. It idles at about 3K RPM.

HELP !!!! How do I get the reference one back down????

Ironically when I measured them at the start they weren't out much.

Thanks

 

have you changed any jets or linkage lengths? also have you returned the carbs to factory spec for reference? basically tell us what you have changed and then we can suggest things to try.

 

I first tried manually syncing them on the bike with a manometer and I ended up with this condition. Then after that the front right cylinder wouldn't idle at all. Part of the problem is the screws are SO HARD to access. I even have a special tool for it but it's still hard.

So I took the carbs off the bike. I used Post #5 from this thread: http://vfrworld.com/forums/showthread.php/34319-Can-you-sync-carbs-on-the-bench to bench sync the carbs.

Put everything back on the bike. Mercury on the reference cylinder was nearly off the scale, and on the others was near the bottom. That's where I'm stuck.

Sorry, other than that, didn't change anything.

 

Yah you are way out to lunch there.

I wrote a who bunch of crap and then erased it all to start again.

The trick here is to bring the other carbs up to the reference not bring the reference carb down.

Once you have them in sync, you can adjust the idle know to bring them all down together but it sounds like your idle knob is turned in so back it down first and then adjust the three carbs up to match the reference.

 

Remember that a high reading can be a throttle plate that is too open or a vacuum leak!!!

 

I had one kick my ass all over before I finally took it apart to realize some dumb nut before me had damaged the linkage and had a wrong sized pin in the linkage. May want to take a look.

 

I been to Cork! Nice country, wana do a motorcycle tour!

I had your problem and ended up taking the carbs off and bench synchronizing them. You basically play "peek-ah-boo" with the holes until you cant see them with the butterflies. I have found with these bikes, if its close, its good. Sometimes going after perfection kicks your ass. It helps if the bike is way up on a lift and your looking up at it.

I had an ok manometer with fake mercury that drove me nuts and ended up buying your Morgan carb tool, have not used it yet, but anything is gona be better than the crappy motion-pro unit that they wanted calibrated before each use.

My bike idles and takes a cunt hair of throttle through my gated community like a champ. Thats how you know if the carbs are on the money. Pictures are appreciated too :wink:

 

Let me know if you make it over here !

Ok that's the strange thing, when I had them off earlier today, the other guy was going on about lining up the holes with the butterflies too. But the holes were nowhere near the butterflies, I'd say about 1/4 of an inch below them?

I did have the carbs upside down on the "bench" (kitchen table - wife was not impressed :tyrannosaurus: ) - maybe that's why?

 

I find that cleaning stuff in the kitchen sink usually ensures a fun filled evening in our tee-pee. :cower:

 

The easiest and accurate way to bench sync carbs is to get at least two drill bits of the exact same size. For example get two bits that are .025". use one to set the fixed carb with the main idle adjustment screw. Set it to the .025" using the drill bit like a feeler gauge and leave in place so you can double check reference carb after each carb adj. Now take the other drill bit and adjust the other three the same way using the 2nd .025 drill bit as a feeler gauge . Once all four are set then you can back down the main idle adjustment screw and all four carbs will be the same idle opening. If you do this correctly you will be real close on the vac adjustment.


addition:

The above will get you real close THEN you can vacuum adjust or check the sync with some type of guage. Yes as Squirrelman says bounce/blip the throttle to let carb set.

 

Seems you have some very good answers. Remember that your manometer is a balance tool across 3 carbs with the fulcrum on the reference carb. Make sure you put the reference in the right spot on the meter then bring the others up to the reference (which will come down as the others share vacuum pull). Do not try to balance a carb to reference then add another. It wont work. You must balance them together making small changes to all three then checking how much it has moved on the meter. 1/6th of a turn is a lot of adjustment. Last, but most important, make sure you have blocked off all vacuum lines, this is important as all your adjustments are based on the vacuum present.

 

It's essential to follow the proppa SEQUENCE (see FSM for details) of adjustment, starting with the reference carb (idle adjust screw) or things WILL go wrong. I fear differing with Grey750 cuz he's so often absolutely right, but i use visual methods only when setting synch, referencing the throttle plate to the first exposure of the outermost progression hole, and rocking the setting back and forth with the idle adjust screw to hone in on more and more exact settings all around. Also, adjusting any one screw may slightly throw off earlier adjustments cuz they all interact to a small amount.:eek-new:

For best, most consistent results, always blip the throttle a few times after any screw adjustment to allow the linkages and throttle plates to settle into their new position.:star: And keep the idle speed within 1000-1200 rpms during adjustment.

 

i like to use the drill bit trick with 1/4" bits cause they're easier to grab with fat fingers, but i've done the line-it-up-to-the-holes trick too. Either will get you back into the ballpark so it will run, but after using the drill bits you usually won't need to do any adjustments on the mercury.

The idle speed adjust screw sets the reference carb opening, but don't forget that idle speed is actually adjusted using the mixture screws with the rpm-drop procedure. So before you synch with the mercury you must adjust your idle using the mixture screws.

1. Bench synch, drills or holes
2. Adjust Idle with mixture screws rpm-drop procedure
3. Check synch with mercury
4. Repeat step 2 if plates were adjusted in step 3

 

Hi All,

Thanks for the great replies. I think the first thing I need to do is get my idle speed down. So a stupid question first - the main idle adjuster do I turn this in or out to slow down the idle? Should it be possible to get it to idle on the reference carb only - i.e. the other 3 not pulling any vacuum - or is it normal that it won't idle on that?

If that doesn't work, I think I need to look at my throttle linkage - as I may have tightened that too much perhaps ...

Then I'm going to look at how the carb is seated - maybe there's some air getting in?

Finally if none of that works, I'm going to take it off the bike again (groan) and take some pics - as I don't get the lining up to the hole thing.

I'll keep you posted. Have to work too unfortunately so don't have much time for this !

 

The 'holes' are the tiny transition ports in the carb throat covered by the lower edge of the big brass butterfly valve. As the throttle is cracked open from idle these ports feed the fuel mixture necessary to let the engine transition from idle to partially-opened-throttle [POT] operation.

Not a great photo but figure 5 shows them here: http://vfrworld.com/forums/showthre...-Fuel-and-Air-Circuit-Identification-and-Porn

At idle these ports should be covered by the closed throttle valve. All air and fuel for idle comes thru the idle air port and the fuel mixture screw port, but sometimes you might need just a tad of air and have to crack the throttle open by turning the idle speed adjust screw 'in' to raise idle. It opens the butterfly valve gap and lets more air in, and uncovers the 'holes' which pulls fuel thru the transition ports.

 

If the carbs are seated and the clamps are tight then no air should get in.

That's why I use drill bits. Depending on your eyes and all, the holes can be hard to judge at times. You just align the edge of the throttle plate with the same hole in each carb. This will get you close. That's all a bench sync is, to get things close for a starting point to vac adjust the carbs.

As far as the idle screw just turn it, one way raises and the opposite lowers. That's your only options lol. Can't you tell I forgot which is which. I think rightie tightie will increase. I'll bet squirrelmans life on it!!!

 

Guys,

Got it working. Turned out that I had the 3 carb sync screws turned out too far. I was afraid before turning them in that I was doing damage, so turned them out.

Now it pulls like a train. It might be idling a bit rich, but I will have a look at that over the weekend.

Thanks for everyone's help.

 

Now that makes sense

i didn't know what that meant before, but now it makes sense that the valves were synched open too far.

There once was a rider from Cork,
Who's synch screws were lacking of torque,
He said with a grin,
As he turned them back in,
I shouldn't have screwed with a fork.

 

^ funny, your a poet and you may have known it. Thinking aboot a woman from Nantucket that used to like to do something. Anwya, Happy Endings are always nice.

 

Or the one from Wheeling

or from Dallas..........................or never mind.........................................................