Topic: '78F carb sync

[matt365]

So, background on the bike.

It was stored in a barn most of its life, I bought it off a buddy who vintage races CB750s and was going to use it as parts. 


It has pods, and an open exhaust of unknown origin.  I cut 4'' off a Yoshimura tri-oval pipe I had bought as a track pipe and used the 4'' section as a baffle on the CB.

I've drilled out the stock jets to 1.19mm, so she pulls pretty good at high revs, but the pressed in pilots make it problematic off idle.  I'm going to grab some guitar strings off a buddy, and try and hone a set of pilots i yanked out of a set of '79 CB650 carbs that I've got for parts.  Hopefully that will cure my stumbling issue.


Now for the main question, it's something pretty simple, but as of yet, I've been unable to locate the answer, and trust me I've searched.


I've got a set of Motion Pro guages, and I have attempted to synch the carbs.  I am used to there being one "un-adjustable" screw to synch all the others off of, but the 'ol CB can be adjusted off the board.  I've checked the directions in the Honda service manual, but it's like gibberish to me...

Anybody used a set of Motion-Pro Mercury Vaccuum Guages, and what did you use as a baseline?

[upperlake04]

 Are the carb bodies the stock PD42A?

[eldar]

Why are the press in giving you issues? Just pull them out. Yank the carbs off and flip them over, pull the bowls off, grab the jets with pliers and twist while yanking. Off they come. Then Use the #1 string on a guitar(the high E) and clean them out carefully. OR use a couple strands of copper wire. After cleaning them with wire, spray them with carb cleaner and spray all the fuel paths with cleaner. Re-assemble and bench sync the slides to about 1/8th inch.
Set your idle mix screws to about 1 3/4 turns out. From there, tune the idle mix. once done, then sync the carbs.
The level does not matter much, they just need to be pretty close to each other.

[matt365]

Yeah, there the stock '78 F carbs.



Just a piece of information I've been told in my travels, Confirm, correct, or deny - Somebody set me straight

 Adjusting the needles would be problematic on '78 carbs, so that's why I've decided to hone out the pressed in pilots.  I've been told adjusting the needles will only change when the fuel comes on, not how much fuel is added at low revs, so it's not really a fix to the stumbling issues.

[eldar]

That would be correct but boring the idle jets will probably just cause more issues. These bikes run fine with their jets, with the airbox. The pods are probably causing most of your issues. Pods are hard to tune for. Clean your jets first though. these carbs are quite flexible. The idle mix screws adjust fuel, not air. So they can cover a wider range. Have you cleaned your plugs and idled for a while to see what your plugs look like? Are the tips really white?

[matt365]

Why are the press in giving you issues? Just pull them out. Yank the carbs off and flip them over, pull the bowls off, grab the jets with pliers and twist while yanking. Off they come. Then Use the #1 string on a guitar(the high E) and clean them out carefully. OR use a couple strands of copper wire. After cleaning them with wire, spray them with carb cleaner and spray all the fuel paths with cleaner. Re-assemble and bench sync the slides to about 1/8th inch.
Set your idle mix screws to about 1 3/4 turns out. From there, tune the idle mix. once done, then sync the carbs.
The level does not matter much, they just need to be pretty close to each other.

I yanked the pilots out of the CB650 carbs I had, but it tends to score up the outer body of the pilots pretty good.  Will this cause any real issues?  One pilot bent a bit, but the other three came out pretty clean, just light scoring from the pliers.

I read one of your other posts about the E string and copper wire, pretty informative, thanks Eldar!

1 3/4 it shall be, and I'll sync them to eachother, adn not worry too much about the levels.  -That's really the main thing I needed to know.



I know it's a bit of a crapshoot figuring out jetting, but with pods and an open header, no engine mods, stock ignition, what do you suspect I should be running for jetting?

The drilled out 120s seem to work pretty good, but without fixing my pilot jet issue, I can't really figure out if my jetting is spot on...  What'd be your guess for jetting?

I've got an old jet kit from a '98 Katana 600 I had years ago, and the jets are the same thread as the Honda jets, so I've got 114, 110, &116 on hand as well.  

I seem to recall hearing about running pods that the center pods are restricted more for airflow than the outer pods, and as such, many run different jets for the center carbs compared to the outer?  -Anyone heard this?  Applicable to the CB750?

[upperlake04]

Yeah, there the stock '78 F carbs

 Number 2 carb, second from the left when sitting on the bike, is the non-adjustable one. After the slides have been adjusted as close to each other as possible on the bench, and after all tune-up procedures have been done, adjust 1-3-4 to 2 at idle. The numbers on the vacuum gauges aren't important, that they are all drawing equally is.

[matt365]

That would be correct but boring the idle jets will probably just cause more issues. These bikes run fine with their jets, with the airbox. The pods are probably causing most of your issues. Pods are hard to tune for. Clean your jets first though. these carbs are quite flexible. The idle mix screws adjust fuel, not air. So they can cover a wider range. Have you cleaned your plugs and idled for a while to see what your plugs look like? Are the tips really white?

they look alright when reading the plugs.  When I bought it, the bike had the stock 105 mains, and it wouldn't rev over 5,000 rpm without the choke on full (one of the reasons I got it so cheap  )

when she's hot, it idles alright, but it's always got that nasty stumble off idle.  I'd actually rather have a stock airbox, but I've got the pods now, so I might as well live with it.  If I screw up the pilots, I've got another set, so it's not a huge issue if I hone them out too much.

[matt365]

Yeah, there the stock '78 F carbs

 Number 2 carb, second from the left when sitting on the bike, is the non-adjustable one. After the slides have been adjusted as close to each other as possible on the bench, and after all tune-up procedures have been done, adjust 1-3-4 to 2 at idle. The numbers on the vacuum gauges aren't important, that they are all drawing equally is.

that's what I figured!  But the #2 carb had an adjustment screw just like all the rest! (As far as I remember?)

[eldar]

Any time you run multiple carbs with pods, air flow will differ. Also remember the idle jets are #35. Have you checked to make sure your accel pump is working correctly? If it is not, then you need to fix that first.

As for chewing up the outside of the jets, no biggie. The outside does not matter BUT just wrap a piece cloth between the jet and pliers. Remember to TWIST, do not wiggle back and forth. I did but I was very careful, otherwise you can bend the jet housing.

[matt365]

Thanks for the warning on the wiggling thing. 

How would I know if the Accelerator pump is damaged?  Running issues I should be looking for?

the spare set I have, I've been dissassembling them to get a better idea of the ones on the bike...

My buddy had a CB650 parts bike, and had these carbs in a bucket of water (freakin' idiot I know) throughout much of the early winter last season, so one of the carbs is completely full of calcium & white crust...  The carb body is alright, but the float pin was seized in pretty bad, and the float needle is so corroded, It'll be quite a job to get it out.  The accelerator pumps seemed pretty clean and in good condition on those, so I've got spares if need be.

[upperlake04]

that's what I figured!  But the #2 carb had an adjustment screw just like all the rest! (As far as I remember?)

 Should look like this..

[eldar]

There are 4 brass nozzles in the carb throats. If they have fuel, when you hit the gas, fuel should squirt from each nozzle. Bike does NOT need to be running to do this.

[mlinder]

You have an A/F screw on those carbs, right?

Don't bother with boring out the idle jets. You can make the idle mixture fine with the A/F screw, and more than likely, unless you have a jet drill, you'll have uneven idle jets.

The stumble could simply be unsynched carbs, and may have nothing to do with AF ratio. Get those synched first, then use the A/F screw to fine tune your idle mixture.

[matt365]

that's what I figured!  But the #2 carb had an adjustment screw just like all the rest! (As far as I remember?)

 Should look like this..

okay, then I'm having a brain fart of some kind.  I've adjusted my throttle bodies on my 929 through 4 adjusters attached to the throttle bodies; adjusted the carbs on my buddies '82 CB650, and '81 XS650 Yamaha...

There are brass screws that the adaptors for the guages screw into, and then the adjuster screws on the bottom of each carb.  I just remember there being an adjuster screw on the base of the #2 carb as well, I wasn't thinking about the slides.  Honestly, I haven't even checked them, and until I started disassembling the 650 carbs, I didn't even realize those needed to be adjusted.

I've always considered carbs a little bit of majic, and as all my previous bikes were daily drivers I've never really messed with carbs all that much.

Anything I can learn is much appreciated.

[eldar]

There is the Q. Which to do first. Some manuals, honda, say to tune the idle mix first then do the sync. Which makes the most sense to me.  I think that if you tried to sync first, that the sync may work but when you tune the screws, it will change the amount of gas that gets fed and throw things off.

[eldar]

Matt, you are thinking the wrong screw. Each carb DOES have an idle mix screw. The adjustment for the sync is under the top covers. 2 screws hold on each cover, underneath you will see the locknut and adjuster screw. You loosen the nut and turn the screw to adjust, then cuss like hell as you have to compensate while locking the nut back down. #2 does not have this nut/screw combo under its cover.

[mlinder]

There is the Q. Which to do first. Some manuals, honda, say to tune the idle mix first then do the sync. Which makes the most sense to me.  I think that if you tried to sync first, that the sync may work but when you tune the screws, it will change the amount of gas that gets fed and throw things off.

With modified intake and exhaust, you have to have it running well enough to be able to tell wether your idle mixture is too rich or too lean. If they are too far off balance, you will get different readings on each plug. Doesn't do too much good if you dont get a pretty even result with which to baseline off of.
If he had stock box and exhaust, Id say "turn idle screws to 1.25 or so out, then synch".
But that's not his siuation.

[upperlake04]

 The K8 guy has it right - the idle screw is under the carb on the engine side  and the  port for  synching is in the carb body between the carb and head. Hoses in pics are from synch tool to port.

[eldar]

thats why you bench them first. unless the carbs are real sloppy, that gets you close enough to do what you need to do. If the carbs are that sloppy, a rebuild is in order anyways. On the F bikes though, the fuel screw is turned out 1 3/4 turns though as the stock start point. As he has pods and open exhaust, the start may be better at 2 turns out. Remember, out is richer, in is leaner on the 77/78 k and F and even the A.

[matt365]

Thanks guys, I was a little mixed up about the synch. 

So, so far I've got to turn the mixture screws 2 turns out because of the pods & open exhaust, and then adjust with the synch tool through the top of the carbs.  I'm not used to these slide barrell carbs (that is what they're called? ).  I just assumed I had to synch it up like throttle bodies, or newer carb setups (newer, and I use that term loosely) on my buddies XS & '82 CB650

Consider me a spunge, and I'll soak up all the knowledge I can.

[mlinder]

While I understand what you are saying, and agree that yes, it could work, I do it the other way around, (making an edjumicated guess at the starting point for AF screw), and since I make minor adjustments to idle mixture for about a week, a synch first works for me, though I end up doing a new synch if I need to change the screws more than half a turn, or of course, need to change jets.

[eldar]

you will be hooking 4 leads up, 1 to each carb. Take the tops off all 4 to make them equal. Start bike and go to town. Remember, the bike MUST be are normal operating temps, place a fan in front of the engine to prevent overheating. you will have to remove the tank so either rig up a hoist to hang it from, or make some sort of hanging tank. Also, adjustments on 1 carb will affect the other carbs. They do not have to be perfect, acceptable is generally within 1 inch of each other, if possible, a little closer.

[eldar]

If you bench sync, then tune the idle screws, then final sync is easier. Some people have not even had to do the final sync. Every bike is a bit different of course. this last summer, I never bothered to sync at all. I barely had any clutch noise and they were close enough on the scale.

[matt365]

Speaking of jets, Sirius Consolidated sells jets for the CB750 up to a 120 main.  I've got a buddy that had a 77'F that was running a 130 or 135 main... seems like overkill to me.  I've got the 120 in there right now, but I may want to switch them out for a 116 and see how it goes.  I've got to sort out this pilot jet thing first, or at least clean them out.  I was told the stumble is normal when going to pods, and the only way to get rid of it is new pilots/honed pilots.  It would have been nice if Honda hadn't pressed them in, would have been an easy switch then.  I guess we couldn't just run a tap through the seat for the pilot and use an aftermarket screw-in type pilot jet?