Author Topic: Carb sync (Read 51087 times)

Jeremy · « **on:** March 26, 2005, 07:12:41 AM »

Have a 77 550 I'm trying to put on the road but I am having some problems at idle. I get a knocking sound at anything under 1800 rpm, also notice when I start it #4cylinder is not firing. At 2000 rpm no noise and all 4 pipes get hot. I have not synced the carbs yet, could this be my problem or is it a slow jet plugged. I hope you don't say pull the carbs again.

Dennis · « **Reply #1 on:** March 26, 2005, 12:08:29 PM »

Hate to tell you this but you should have bench sync'd them before installation. At least you would be close that way.
Now if you can get the airbox out of your way so you can see into the carbs you might want to do a rough sync now. Simply find the lowest slide and see what drillfits the space. Adjust the remaing slides to the same height, start engine and adjust idle speed. See if #4 warms up.
Unless #2 is not adjustable then adjust all slides to match #2.
Email me if you need me to walk you through it, DScia@aol.com.

Ibsen · « **Reply #2 on:** March 27, 2005, 04:59:34 AM »

I agree about bench syncing the carbs first.
There is also an excellent guide about balancing carbs in the last issue (April) of Classic Motorcycle Mechanics.

And Morgan have got another excellent guide on their website:

http://www.carbtune.com/inst.html

mwvachon · « **Reply #3 on:** March 28, 2005, 04:16:27 AM »

Jeremy,

I am also concerned about the fact that you said #4 cyl not firing at low RPM. Have you pulled all the spark plugs and checked condition? How old are the sparkplug caps? If they're original, you'll want to replace them as they do breakdown (build up resistance to current) over time. NGK makes some nice replacement units that go on easily.

Jeremy · « **Reply #4 on:** March 28, 2005, 05:33:55 PM »

Crappy Tire sells single vacuum guages, can one of these be used to sync carbs?

Dennis · « **Reply #5 on:** March 28, 2005, 08:08:36 PM »

Anything is possible, but it would be very difficult.
You will still need the adapters to connect the gauge to each carb mount and then some method of quickly switching from one cylinder to another to determine that they all match. All this must be done before the engine gets too hot standing still. If I were you I'd bench synch them for now, work out your major bugs and worry about gauges later.

SteveD CB500F · « **Reply #6 on:** March 29, 2005, 02:43:04 AM »

I can definitely recommend the Morgan Carbtune. I bought mine on eBay for about £30.
The metal sliders in the tubes damp out the occillations that you may see on a dial-type guage.
And no they don't use mercury!

Paul · « **Reply #7 on:** April 01, 2005, 03:40:58 PM »

I think thats spelled correct! Anyway.....missing the first page(s) of this operation in my manual, but from what I can gather it appears the manual shows approx 25CmHg as being a good vacuum reading. I can only get approx 17~ (22Kpa) on any of the four carbs, if I turn down any one of the four the other three all rise to approx 40 KPa, but again to get them all equal I have to settle on 17CmHg (22KPa). Is this Ok, RPM at approx 1000. Am I missing something from the pages I don't have?....bike runs fine but a bit slower or more "relaxed" than my 550. In other words , at the end of the day, if they are all the same is this good enough.? $:-\$ I'm not paranoid...it's just that they are all looking at me.

Ibsen · « **Reply #8 on:** April 01, 2005, 05:22:57 PM »

The reading you get on each carb is not so important. What's important is that you adjust/balance/synch the carbs till you get the same reading on all 4.

Warlock · « **Reply #9 on:** April 01, 2005, 06:12:48 PM »

You can relax. According to the CB500 factory manual, readings between 16-24cm Hg are acceptable figures. It also states that the variation between carbs should not be more than 3.0cm Hg. Variation between carbs is more critical than absolute values as mentioned earlier. Ride on...

Stormhunter · « **Reply #10 on:** April 02, 2005, 11:42:31 AM »

Hi. I have stobe set timming and changed plugs,also rebuild carbs with repair kit as seals were worn. Have tried to synck carbs using Carbtune 2. i can get the first 3 carbs from the left as you sit on the bike to line up abut 14 on the scale but cannot get the last carb above 8.If i screw it in anymore it all cuts as it is against the stops. i have set the air jets at 2 1/2 turns out is this correct and can someone tell what the problem could be with the last carb.The only thing i did not replace were the Jet.
Chris

cben750f0 · « **Reply #11 on:** April 02, 2005, 02:57:53 PM »

air screws should be one turn out, from lightly seated, idle should be as the book says about 1000rpm. you should bring the other carbs down to the lowest carb noth the other way.... hope this helps... if there is anything wrong with my explaination someone let me know... peace

Stormhunter · « **Reply #12 on:** April 03, 2005, 01:09:11 AM »

thanks will try this as the manual says 2 1/2 times out at 1200 revs.problem is that it wont tick over below 1500.
thanks cben 750f1

CHUNG · « **Reply #13 on:** April 03, 2005, 01:36:00 AM »

Set all the valves and Ign first. Get them as close to perfect as you can stand to do it, on three tries. Then check the compression. If all pass this test then try to balance the rack again. If #4 fails then check the carb again. Use some sort of tool to physicaly rod jets to the proper size. Torch tip cleaners, muisic string ect. If it'll pass through one jet, it should pass through them all with the same resistance/slack. If all this fails, sell the bike. (just kidding, keep reloading till you hit it) LOL

antinora · « **Reply #14 on:** April 09, 2005, 01:51:53 PM »

I've read the post and visited the website listed but am still having issues with my 75 CB550.

My #2 adjusting screw was all the way tight. So I backed it off and the rpms went through the roof to 5000. So I lowered the idle set screw to about 1200 rpms.

I then balanced all 4 cylinders to about 20...sounds good.

Took bike out for a spin and bike then spiked to 5000 rpms...when I lowered idle using set screw bike died.

Had to drive it home at 5000....sounds like there is a leak somewhere...

Can anyone give me some insight on synching and locating air leaks???

Thanks,

JA

Harry · « **Reply #15 on:** April 09, 2005, 07:01:04 PM »

Locating air leaks is easy - spray some WD40 or Quick-start around the carb boots etc and hear if the engine note changes. Syncing requires a vacuum meter with 4 columns - screws into fittings on the carbs, just follow the instructions that come with the equipment. Check the FAQ for tips on bench syncing. Make sure your timing, camchain and valves are set BEFORE syncing.

Stormhunter · « **Reply #16 on:** April 10, 2005, 02:46:14 PM »

I have just tried tuning mine useing the Carbtune 2 and can get carbs 1,2 & 3 to line up but not 4 if i screw 4 right down it comes up and bike stalls.as soon as i unscrew it slighty bike runs but vacuum drops off.any thoughts anyone.

Jeremy · « **Reply #17 on:** April 10, 2005, 03:55:11 PM »

I tried syncing my carbs using vacuum gauges, and had the same problem, I'd can get 123 the same, when I raise the vacuum in 4 the bike stalls? Bike runs great, but idle<1400 rpm and it start to knock!

Jeremy · « **Reply #18 on:** April 11, 2005, 06:47:46 AM »

"bump"

Ibsen · « **Reply #19 on:** April 11, 2005, 07:21:31 AM »

I can only refere to the 1979 CB650 round slide carbs, but on that carb rack the #2 carb is the reference carb that the other three carbs should be adjusted to match when you synch the carbs.

Mark M · « **Reply #20 on:** April 11, 2005, 07:33:53 AM »

This sounds like a typical problem after a carb rebuild.

I don't know what the set up on your carbs is either but what can happen is you can sync 3 of the 4 and the other is closed but
holding the others up, so when you adjust it instead of the one you are adjusting moving the other 3 are allowed to drop down. A simple test for this would be to allow it to rev at 5k (or whatever high revs it will hold at) then use the idle speed adjuster screw to try and drop the revs, if this has no (or little) effect then something else is holding most of the carbs open. To resolve it you can bench sync the carbs with a bit of heavy duty wire (or a small drill bit) just to get them all close together before you start. Then set the idle speed first before adjusting any of the cabs, sync a bit then rest idle again on the screw adjuster. Each time round you should be getting closer and closer.

Don't forget that you should have checked your valves, points, airfilter plugs etc before you even started, syncing is always the last step in a tuneup.

Stormhunter · « **Reply #21 on:** April 11, 2005, 01:52:13 PM »

did timing and bench synched carbs before i started.Have spoken to a mechanic at taylor raceing and he suggests doing a compressing test to see if a piston ring has gone as this would affect the vacuum on the carb.Have just borrowed a tester and will do that tommorow.

Zeke · « **Reply #22 on:** April 11, 2005, 08:37:18 PM »

Hey Guys:

If you did just overhaul your carbs, shouldn't you do the Pilot Screw adjustment first? My honda manual calls it the "Idle Drop" procedure, and I believe it sets the idle mixture in each carb for a smooth idle at any speed.

I wouldn't know, I'm a few steps away from completing my first rebuild but as someone else mentioned the carb sync is the last thing you should do.

Zeke

SCJIM · « **Reply #23 on:** April 14, 2005, 09:17:59 AM »

with all the problems of "getting it right" ive heard here,

I'm gonna leave mine alone...

if it aint broke dont fix it.

Zip · « **Reply #24 on:** April 17, 2005, 05:38:22 PM »

I have a '75 CB750 and didn't bench sync the carbs before installation. I rotated up the adjusters and set the lock nuts at the tops of the rods and worked from there, hooking up the Carbtune, and making adjustments. Worked fine.

Zip

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What is the best method you have found to synch carbs on a 750?

Author Topic: Carb sync (Read 51087 times)

Jeremy

Ibsen

Jeremy

Ibsen

Stormhunter

Stormhunter

CHUNG

antinora

Stormhunter

Jeremy

Jeremy

Ibsen

Stormhunter

Zip