Vac guages for carb synchronization

Hi,

I have a 1972 CB750 (CB750E-2012610) that I am restoring. I have
replaced the ig. coils, rear brakes, tach cable, battery, and plugs so far.
I am starting to work on the carbs and am looking for a cheap alternative to
the 300 bucks in tools that honda will rape me for in order to buy vac
guages to synchronize the carbs. I realize I could have a shop do it for me,
but I am really interested in tinkering/learningm myself.

-Has anyone purchased the guages from JC whitneys
-Will they work with the honda adapters
-What is the fitting size to mate up to the carbs?

Also, has anyone ever tried to convert this bike to modern EFI? (not looking
at cost necessarily, just performance and uniqueness)

Thanks,
Kyle.

 

Make your own carb-sync tool for 2 USD:
http://www.powerchutes.com/manometer.asp

 

These work well and do not require any adjustment. It is the
only kind I have ever used. You match the height of the
mercury column in each tube and you are in sync. Doesn't
get any more accurate. See 'em here for one place. There
are several places on thenet to get this type tool - maybe
for even less.

http://www.z1enterprises.com/detail.aspx?ID=1689

Lugnut

 

I recommend staying away from the types with dial gauges. I've tried
several models over the years, and none came anywhere close to being as
accurate and easy to use as the "mercury" style (although not all of them
use mercury--it's the vertical-tube-with-liquid style I'm referring to).
That's the type in Mike's suggestion, although that one will be difficult to
use with a 4-carb engine, and you'll still need to find the adapters
somewhere. I use an old "Carbstix" model, but I don't think it's been sold
for many years now. If I were looking for something today, I'd go for the
CarbTune.

The CB750's Keihin carbs have a small screw that plugs a hole in each carb,
near the rubber boot to the engine head. You need four adapters that screw
into those holes. (You can see the adapters in the JC Whitney photo, one
pair each of short and long adapters for the outside and inside carbs). The
adapters are hollow down the center, and you slip the rubber tubing to the
gauges over the end of the adapter. (I actually tried the JC Whitney gauges
many years ago, but didn't like them so I threw them away and kept only the
adapters from the kit.)

 

The first reply with the 2$ DIY tool sounds like a really good idea!
It might make the whole thing a little bit complicated for 4 carbs
though...
-Ajust the two carbs on one side,
-ajust the two other ones
-and then ajust one carb of one pair against one carb from the other
pair.

I personnaly bought the Morgan Carbtune II.
=> http://www.carbtune.com
Its works really well and besides it's not using mercury as their
previous model (Carbtune I) so there's no restriction on the shipping
and you don't have to worry about spilling the fluid...
I've ordered mine on a friday and they were in my mailbox on the next
monday!!!
It's about a 100$.

 

Except that this leaves you with #4 carb to synch still. You're better
off synching three of your carbs against the "master" carb (typically
one carb does not have a synchronizing adjustment, and is the master).

I'd definitely head over to the Factory pro website and check out their
tools. They're not necessarily the cheapest, but they are very good.
They've also got a comprehensive carb synchronization guide.

Like PrizM, I ended up with a Carbtune II and am quite satisfied.
Another tool that the BMW crowd speaks highly of is the TwinMax. I've
seen one of those in operation and it sure seems both quick and
accurate. I haven't seen one set up for 4 carbs, but it would be no
harder than the homemade one.

There are better candidates for swapping EFI to, unless you just really
want to do the job. I'd skip it myself.

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

I built one and used it on my Bandit 1200. It's really not as tough as it
sounds. You just end up starting and stopping the engine a lot to swap the
tubes around.

And, with the way that the linkages are between the carbs, on the Bandit at
least, doing a pair of carbs at a time works out really well.

I used to have a Morgan Carbtune II. If you're going to be doing regular
carb syncs, I highly recommend one. All of the accuracy of mercury sticks,
with none of the high toxicity.

(I ended up giving the Morgan to a friend who does a *lot* more fiddling
with carbs than I do.)

 

If he says his manometer needs to have each leg attached to a different
carb its just measuring the pressure differential between the two. Is
that what its really supposed to be or is one supposed to be set as the
reference on first and not readjusted?

Ian Singer

--


=========================================================================
See my homepage at http://www.iansinger.com
hosted on http://www.1and1.com/?k_id=10623894
All genealogy is stored in TMG from http://www.whollygenes.com
Charts and searching using TNG from http://www.tngsitebuilding.com
I am near Toronto Canada, can I tell where you are from your reply?
=========================================================================

 

Typically there's one carb that cannot be adjusted, and you match the
rest to that one. So, it's basically the same thing.

If your "master" carb isn't pulling the correct amount of vacuum, that
means that there's something wrong internally (and cue B-12 to tell you
how to clean it out).

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

One improvement you can make on this design
is to add a crossover tube with shutoff valve.
A crossover will probably cost another $2, but makes
the tool way more user friendly as it allows you to
gradually adjust the sensitivity.

With the crossover wide open, there's zero differential
vacuum. As you gradually close it, the vacuum will
increase. This prevents an out of balance engine from
sucking all the fluid into one cylinder. It's especially
useful when starting the engine and adjusting the carbs.

When I made this gadget, I used a shutoff valve and
tee fittings from the drip irrigation section of my local
hardware store. Use two tee fittings at the top of the
U to make the crossover and install the shutoff valve
in the middle of the crossover tube.

You may also want to add some restrictors to the
two ends of the U to reduce the oscillations.

 

I was fixing a vacuum leak when I used a set-up like the one in the link,
so when I would first start the engine, the carbs were so far out of sync
it would try to suck the fluid (I used ATF) into one carb. Luckily, I
had really long lines and had hung the manometer over the rafters in the
carport, so I could shut down the engine before the ATF made it over the
hump.

After two starts I had the carbs close enough to leave the engine running
while I did adjustments.

For maintenance syncing the carbs should never be different enough to
cause a problem. But, yeah, what you suggest would be an improvement.

Personally, I've been pondering getting some T fittings and more tubing
so that I hook up all four carbs at once.

 

I'd had some problems with fluid getting sucked in during startup.
A lot of these were probably because I didn't make the legs
long enough, but I still like being able to turn the sucker on
and off.

For a four carb setup, I guess maybe you'd have all four lines
running into some kind of common reservoir ? I'm not sure what
if any shutoff or crossover arrangement would work best.

Given the simplicity of the gadget, it'd probably make sense
to cobble up 2, 3 and 4 cyclinder versions as needed rather
than trying to make one that would work for different kinds
of engines.

 

Where's the fun in that?

I'm not a Cheap Bastard because of the money, I'm a Cheap Bastard because
of the challenge. Saving money is just a bonus... and a way of keeping
score.

(Though, actually, I used an ATF delta-manometer recently because of the
convenience. There's a good hardware store within a couple miles of my
home. The nearest good moto store is hundreds of miles away--or a week's
post.)

 

I've just made something similar for my SV1000S but it cost a bit more
than $2 - something like 4 GBP. The most expensive item was a whole
bottle of automatic transmission fluid at 2.49 GBP, I only need a few
cc. I'm using 4 mm ID fish tank air tubing.

Leon

 

Motion Pro sells an economy mercury balancer for under 50 usd. Comes
with the adapters for the Honda. I use mine once every couple of months
on several bikes. Works fine.
The guages are all right IF they are accurate or matched, but are hard
to read as opposed to a straight level line across 4 tubes.

--
Gopher Greg
'77 CB750K Stock '78 CB750K AHRMA
'00 ZG1000 Stock '96 Ducati 900SS Former track bike
'01 GSXR750 Current race bike
**********pull 'mychain' to reply***********
("I've abandoned the idea of trying to appear a normal, pleasant person.
I had to accept myself as I was, even if no one else could accept me.
For the rest of my life I would continue to say precisely the wrong
thing, touch people in the raw and be generally unpopular. I had a
natural gift for it" W. F. Temple)

 

I don't see it anymore on http://www.motionpro.com. Do you know where
it's listed? The electronic ones are only good to 0.5cm of mercury.

Ian Singer

--


=========================================================================
See my homepage at http://www.iansinger.com
hosted on http://www.1and1.com/?k_id=10623894
All genealogy is stored in TMG from http://www.whollygenes.com
Charts and searching using TNG from http://www.tngsitebuilding.com
I am near Toronto Canada, can I tell where you are from your reply?
=========================================================================

 

Thanks. You would think that the description could at least include the
number of tubes it has.

Ian Singer


--


=========================================================================
See my homepage at http://www.iansinger.com
hosted on http://www.1and1.com/?k_id=10623894
All genealogy is stored in TMG from http://www.whollygenes.com
Charts and searching using TNG from http://www.tngsitebuilding.com
I am near Toronto Canada, can I tell where you are from your reply?
=========================================================================

 

Gunson do a carb sync for under £20. Might be worth a try.

John