Carburetor Balancing for the VX800 (and other things carb-related)

As promised a couple months ago, I decided to balance the carbs on the VX with mercury sticks.

Here is the ordeal in all its gory detail; hopefully someone will gain some knowledge that is useful here.

(I amended this file in March, 2000 to add some very useful and quite a bit more informative and accurate information pertaining to carb balancing. The information was provided by Steve G. – aka “Big Steve” from the VX800 mailing list. Much thanks to you Steve for taking the time to send me the info and give your permission to use it in this document. You’ll see Steve’s notes inserted in this document where they apply to the current process. Thanks again Steve!)

This will actually walk through more than a balance. I pulled the front and rear air filters and replaced them, and cleaned the carb sliders first. All of this followed an oil and filter change as well.

Here are the notes from my maintenance log:

29,700 miles (01 January 2000)

Changed oil (Castrol 20w50)

Oil filter (stock Suzuki)

Air filters (front and rear with stock Suzuki)

Replaced spark plugs with regular DPR8EA-9’s (NGK) front and rear

Cleaned front and rear carb sliders with carb spray, cleaned needles

Tried to replace stock nylon needle spacer with 2-#4 washers on each per FAQ but the rear carb 1 of 2 screws in slider would not unscrew (though I rounded the Philips head on the brass screw while trying.

Abandoned the idea for now.

Adjusted idle mix pilot screws to optimal

Balanced front and rear carbs with mercury sticks

Rear was lower at idle, adjusted, but was then higher at 2000 rpm, I split the difference out to balanced at 1500-1800 rpm.

Replaced lower engine mount bolt and nut (bolt had sheared and lost the nut) with stock Suzuki parts.

Noticed a stress crack on right passenger footpeg mount bracket, ordered a new stock Suzuki part to replace it.

After the carb balance, here are some notes I made in my logs later on.

30,000 miles

Rear cylinder backfired after tank fill and then stopped firing (over a 4-day period)

Rear plug was fouled, replaced with NGK DPR7EA-9 hotter plug in rear, still running DPR8EA-9 in front and both now seem fine.

Noticed rear tire seems to lose pressure down to 20-25 psi over the course of a week or so, front tire has no similar pressure loss. Will look into replacing rear and maybe front for spring/summer 2000

30,060 miles (13 January 2000)

Rear plug seems fine, no fouling, will leave it for now with hotter rear and regular front plugs

also replaced footpeg bracket per previous entry.

Ok with that covered to give you a synopsis of what I did, here is the procedure I followed up to and including the carb balance with pictures to help. (sorry for the picture quality on some, I need to be farther away with my camera since it’s fixed focal length…found out after the photos were developed…I used PhotoShop to enhance some of them to be useful here and fixed the upside-down picture in the “carb balancing” section of the VX service manual...you’ll see in the picture)

Here we go:

First I replaced filters and cleaned the carbs after the oil change and plug replacement.

You have to remove the seat, sides (disconnect brakelight), and tank (remove the fuel line from the petcock and the petcock vacuum hose…I labeled them with a pen just for grins, ball-point works well on the rubber) to get at all the stuff including the rear and front airboxes.

Here’s the VX with the tank. Seat, and sides off:

Next I pulled the rear air box. As you look at it there are 4 screws to remove the filter like this:

(it’s the square assembly that holds the actual filter, on the side of the airbox; I left the snorkel part of the filter assembly out of the drawing on purpose)

I noted the lower left screw was longer than the others.

Then I pulled the rear filter assembly, and disconnected the airbox to clean it and get to the rear carb intake. The rear carb connection is hard to get off after loosening the hose clamp, I pushed some of the rubber mount into the airbox to make room to get it off the carb, this made it easier, and just pushed it out again later when reassembling. The top air hose comes off easy after the hose clamp is loosened and there’s a bolt on the bottom of the airbox that you remove from the right hand side of the bike.

This filter was dirty! (I also cleaned the inside of the airbox with carb spray):

See the four screw holes for my drawing above?

I set the rear airbox off to the side, leaving its lower air hose attached:

Left view and view from right side

Another View

I replaced the rear filter and airbox, then pulled the front on per the service manual, it was much easier to do, beware there are little “spacer” collars on both sides of the bolt that holds the front airbox to the frame, don’t lose them! And put them back when you reinstall the front filter assembly and bolt it to the frame. (sorry, no picture on that part)

Next, after the new filters were installed, I pulled the carb sliders and proceeded to clean them and made my attempt at the #4 washer as a spacer for the needle per the FAQ.

Be very careful on the carbs, there is a rubber diaphragm and a spring that comes out of the slider throat assembly when you remove the cover. See the service manual for the exploded view, or look at my scans on the next page:

Here are the front and rear carb exploded views: (rear on left and front on right)

The diagonal line on the rear carb diagram (coming up from the spring) points to the screw at the top of the slider assembly (there are actually two little screws on mine though the diagram only shows one here) that I could not get to turn. It’s brass, and even with a lot of pressure I could only loosen one of the two, and the hard one got the Philips head stripped trying. It’s also about 4 or 5 inches down inside that slider tube, so it’s hard to get at or I’d think of tapping the screw and replacing it. I checked and the slider assembly is over $100(US) so don’t torque it too hard and break the thing if you try this! Mine wasn’t damaged except that I stripped the head and the carb works fine…just beware this happened to me. (The picture was too blurry to include here)

Here’s a nice blurry picture of the clean rear carb throat that the slider and needle go into though!

Be sure to seat the rubber diaphragm’s back properly in their seating groove per the service manual when you reassemble. It can get tricky with the rubber, spring, and cover to get them all to fit right the first time, especially if you are like me and have no garage and ended up having to do it by flashlight because it got dark outside (not this time, but I did last time I cleaned them).

Carb Balancing (what you’ve been waiting for!)

Prep and tools:

Mercury sticks, I got mine at the Suzuki dealer, about $50(US), they are the Motion Pro ones

Throttle lock (my bike had one when I bought it which I removed but comes in handy for this and long trips I would guess)

Your trusty sockets!

A “T” valve adapter for the vacuum port that comes off the rear carb to the petcock. I got them in the HELP! Auto parts section at Auto Zone (a ¼” T-adapter) and used the remaining clear plastic hose I had from replacing the overflow hose on my battery, it’s the same size as that one. The hose I think is also ¼” vinyl, same as comes with the mercury sticks.

Small needle nose pliers or the appropriate small box end wrench to fit the cable loosening nut labeled as “A” in the service manual section on balancing carbs (see below)

(wrench sizes from Steve G.)

[5) The wrench sizes are 8mm and 10mm. The 10mm wrench could be modified to make access to the locknuts easier. The head needs to at least as thin as the locknut, and it helps to make the wrench shaft narrow behind the head to get more turning clearance to the frame. I did this with a cheap tool kit wrench and a bench grinder. Also note that the locknut only needs to be snug, which helps prevent your adjustment from changing while tightening.] (see wrench.jpg at bottom)

I also did all this with the seat off but the tank on and hooked up to the fuel lines normally (hence the need for the T-adapter for vacuum to the rear carb mercury port and to keep the fuel petcock open with the engine running)

I prepped also by loosening the vacuum hose to the rear carb that hooks to the petcock, it was on pretty tight from not being removed very often and being dry.

I also removed the coolant overflow box (that little silvery plastic box on the right side below the tank, to get to the front carb vacuum port, your mercury sticks come with a 5mm adapter for the screw port to hook the front carb vacuum line to) You can remove the overflow box without removing the tank by lifting the tank a bit to get to the bolt that hold it, I used a folded up rag and a long wrench to pry the tank up against the frame without damaging it to get enough room to remove the bolt with my other hand.

***Note from Steve G.!!!***

[2) I've always done the procedure with the fuel tank in place, because throttle cable position & routing will affect synchronization. (Test this the next time you are balancing your carbs. Raise the throttle to 2000 r.p.m. and then move a throttle cable for the rear carb sideways an inch. See how much the column of mercury moves?) I usually leave the rear mounting bolts loose, so I can raise and lower the tank if needed.]

Test your throttle lock first to make sure it holds properly and that you can turn it down quickly if you accidentally rev it (DON’T, you will suck mercury into the engine if you rev quickly or too far ~ 3500-3900 rpm) Do all this before hooking up the sticks and calibrating the two you will use of the 4 mercury ports. Also set your idle mix pilot screws now, and then get all your stuff together to give the engine time to cool down a bit. During the balancing process I shut it off and took breaks to cool it off (it gets hot adjusting the balance cable nut when that fan comes on and blows on your arm). Use a fan if you are lucky and have a garage.

Another tip from Steve G.!

[4) Use a box fan turned on high and placed in front of the front wheel (and to the side a bit), because balance is to be done on a warmed up engine. Warmed up means at least 10 minutes of running (from a cold engine.) The fan also minimizes "yellowing" of the pipes.]

Ok now for some pics to explain all of this:

Left to right: mercury sticks, the T-adapter I made, and the throttle lock installed oddly on the right handlebar (odd because I clamped it onto the mirror stalk to secure it just for this use)

Here’s the prep and hookup stuff:

left to right: the coolant overflow and my rag/wrench prybar, overflow off and pointing to the front carb’s vacuum port screw you remove and replace with the adapter that comes with the mercury sticks for this.

Here are better pics from the service manual: (note I fixed the upside-down pic in the service manual on one of these scans)

left to right: from service manual, front carb vacuum port screw (#1) and screw replaced with the hose adapter (#2) (same thing I’m pointing at in my picture, just not as blurry), and the carb balance cable nut (labeled “a” on cable “A” near the front carb) looking at it from a left side of bike viewpoint.

Here are larger views: (from service manual)

Here’s the balance adjust nut picture to go with the one from the manual (“a” of cable “A”)

It’s the brass-looking thing way up in there… and pic at right shows the hose hookup to the mercury stick

And a second hose hanging on the right near the spark plug there to calibrate the second stick.

Ok, though the pictures don’t show it, I calibrated the mercury sticks per the instructions by doing it all off the front carb vacuum port adapter (BEFORE I hooked up the T-adapter rig to the rear carb vacuum port)

Basically, you hook the line up to one of the 4 tubes on the stick from the front carb, idle the engine and mark the level, then shut off, hook the front carb to the second of 4 tubes (you’ll use this tube for the rear momentarily) and idle it again. If the little restrictors you had to jam down into the top of the tubes per the instructions have about the same holes in them, you should idle the front carb at the same mark level on both sticks, if not mark the second (I used electrical tape…and mine were perfectly even) and that’s your “even” balance point even though they won’t look level. Remember that’s only if yours didn’t calibrate exactly the same like mine did.

***Note(s) from Steve G.!!!***

[1) There's no need to calibrate mercury manometers. The size of the hole in the jet is of no consequence. The jet exists to dampen pulses and make the column of mercury stable enough to give a consistent reading. The calibration part of the procedure is for the Suzuki vacuum gauge only. It consists of steel balls in glass tubes, and since there is no inherent "sealing" of the system, the calibration procedure is needed.]

[3) Attach the #1 vacuum hose to the rear carb (it is the #1 carb; it has the idle speed screw!) When balancing the carbs, it is easiest to think in terms of adjusting the #2 (front) carb to the #1 carb (because, in fact, that's what you're doing)]

(I'd heed Steve's advice here regarding hookups, I did it the non-logical way. Steve's makes more sense according to the service manual and how the carbs are numbered.)

Time to hook the front carb back to stick #1 and use your T-adapter and hook stick #2 to the rear carb and start your balancing act! Here are more pics:

I hung the mercury sticks off the left bars. (note I tied the clutch lever down later – not in this picture - to be able to idle with the centerstand and sidestand both down) I circled my calibration mark for front idle on stick #1 (electrical tape) in red. The mercury reservoir cap is in black at the bottom of the photo.

Here is the hookup for the T-adapter:

Left to right: the T-adapter, and close-up hooked to the rear carb vacuum port, black hose goes to the petcock.

Now we are ready to balance the carbs!

(I’ll put Steve G.’s notes here first…)

[6) I suggest that Suzuki's recommended balancing speeds be used. When the engine speed is held at 2000 r.p.m., this is at the point where both throttle cables are definitely opening the throttle plates under tension, and at the point where the idle circuit transitions to the needle/jet circuit. Smooth running here is important for good low speed driveability, and smooth steady cruise. In general, an accurate carb synch has no real effect on performance above about 1/3 throttle.

7) In the normal course of balancing the carbs, I warm up the engine (this is to ensure that the idle speed remains stable.) Balance the front carb's idle to be the same as the rear by turning the cable that comes from the rear carb. Adjust idle speed. Lightly "blip" the throttle a couple of times to seat cables/throttle plates. Readjust as necessary. Adjust idle speed. Then raise engine speed to 2000 (by opening throttle by hand, don't use a throttle tensioner!), check synch. If it's OK, you're done. If it needs adjustment, release the throttle, adjust the cable on the front carb that goes to the front,"blip" the throttle a couple of times, raise the speed & check, repeat as needed. It usually shouldn't be necessary to re-adjust idle speed after adjusting the "2000 r.p.m." balance (unless you've just installed removed carbs, or replaced the cables, or had previously been confused by the procedure.) At a normal service the synch should take no more than 3 to 5 minutes after the engine is warmed up.

8) Turning the cables out (away from the carb) brings the height of the mercury down.

9) Other Notes:

When checking the 2000 r.p.m. synch, bring up the engine speed slowly. You may have noticed that while doing this, the carbs may "go out of synch" but stabilize when you hold them open at 2000 r.p.m. This is where they should be balanced. Don't bother looking for a "happy medium" between 3500 r.p.m. and idle.

The pilot screws should be adjusted after the carbs are balanced, and absolutely when the engine is hot. If idle balance changes, re-adjust.

It is not recommended to change the idle speed without the synchronization gauges attached, because the idle synch will probably change. This is why it is important to do these carb adjustments at engine operating temperature, and leave them alone until the next service.]

Here’s Steve G.’s wrench diagram if you want to make your own…

The procedure is to set (or balance) at 1500 rpm, adjusting the cable as needed to balance the carbs out. This usually raises the idle on the bike so you need to also occasionally adjust the idle speed screw at the rear carb (the big screw you turn by hand) to get it back down a bit. Then set your rpms at 3000 and balance the carbs again by adjusting the balance cable “A” by turning the adjusting nut “a”. (Note there is a lock nut you need to loosen first to be able to adjust the nut, read your service manual, it covers it well!) This is where you will use your needle-nose pliers or the proper small box end wrench. Not much big fits in that space (since I did mine with the tank on) but if you do yours with an auxiliary fuel source and the tank off, access would be much better.

It can get hot while adjusting this so be careful! You are very near the pipe at it’s connection into the cylinder and it’s very hot, don’t burn yourself!

You may want to check mixture settings as well, they may need adjusting afterward.

Here’s my notes:

Starting out, rear carb was 3” of mercury lower than the front carb at idle

Synched them at cable by adjusting the nut

Idle increased to about 2300 rpm

Adjusted idle down to 1250 rpm (don’t forget here when adjusting, especially above idle speed to turn the throttle down - since it’s locked – before you adjust the idle!)

Once idle was adjusted, front and rear were again out of balance (rear was higher than front this time)

Readjusted to balance, idle was too low now (850 rpm or so), adjusted idle back up

Synched again at 3000 rpm but mine would even up at 3000 rpm with adjusting, and be out of synch when I brought the throttle back down again

(Note: the mercury stick manual states that if you can’t balance the carbs so they are close at idle AND at 3000 rpm, then you have other issues with carburation – either vacuum, seal, or other things affecting what you are trying to do, I expected it since my bike has had rear carb issues and has 30,000 miles on it, yours most likely will balance out just fine!)

Since mine was not going to balance out, I split the difference and got them to balance pretty well at around 2000 rpm and they stayed close back down to idle. I had to play around a LOT adjusting the balance nut, then the idle screw, and had to stop 4 or 5 times to let the bike cool off. DON’T forget when you have it right to tighten the lock nut on the balancing nut and cable!!! You don’t want to lose all your hard work! Please don’t give up if it looks difficult, it reads harder than it is in this how-to document– it’s really not hard at all to do! Trust me!

That’s it! Afterwards I reconnected the petcock vacuum hose to the rear carb, and removed the front vacuum port hose adapter and replaced the screw where it was and remounted the coolant overflow box once again using my rag and wrench prybar method.

I did notice a difference in performance, you’ll probably notice a greater one if your carbs were really out of balance. I think the difference shows in the performance you get at 4000 – 7000 rpms with acceleration, it seemed that way to me. You may notice other improvements.

I must say that now that I’ve done this, I would readily do it again, it’s really easy to do and would really only eat up part of an afternoon (2-3 hours tops) to do.

I’ll end with a couple pics I took earlier of my VX, good luck to you and ride safe!

Craig

(pics below…)