Topic: '75 CB550 carb sync issue

[c.tran]

Yet another carb sync issue for the forums.  So I've been rebuilding a non-running CB550 I got from the PO.  I finally got the engine to turn over and run, and am at the point where I am vacuum syncing the carbs.  Here's what I've done to the bike so far:

-set the valve clearance
-set the static timing and checked the dynamic timing
-installed a MAC 4-1 exhaust, Hondaman's Transistorized ignition, Dyna 3 ohm coils, Hondaman's 2 ohm resistor pack, new spark plugs, and Stilltime's air intake adaptor for a K&N RD-0710 filter
-cleaned the carbs with simple green, carb cleaner, an ultrasonic cleaner, and an air compressor.  Sprayed out all of the passages with carb cleaner afterwards.
-set the float height
-bench synced carbs

I bough a motion pro carb synchronizer and calibrated it before attempting my sync.  I warmed up the engine first (with a fan on it) and it seemed to hold steady at about 1100 rpms.  However, when I started it up again, it wouldn't keep to 1100; the rpms kept dipping and the engine sounded like it was about to die at around 900-1000 rpms.  At this point I gave it a bit of throttle and it stayed on a little longer.  Also, sometimes when I give it some throttle or adjust the main idle control knob, the rpms keep increasing or stay high (anywhere from 2000-4000 rpms) without coming back down for quite a while unless i give the main idle knob a few turns out.  I checked my throttle cables and they return just fine.  I'm not sure what is causing the bike to increase the engine speed.

I'm pretty sure I managed to sync the carbs at about 2000-2200 rpms (all of this is with the choke closed), but when I tried to bring the rpms down to sync again, the bike would eventually die after a couple of minutes.  The engine has been pretty temperamental.  Each start up is always slightly different and requires me to adjust the main idle knob or blip the throttle to keep it going.

TL;DR:
Bike won't idle at the appropriate rpms long enough for me to vacuum sync the carbs.  Every now and then, a blip of the throttle or the tightening of the main idle knob will cause the rpms to continue to rise to about 2000-4000 rpms.

[Flyin900]

I don't believe you can sync the carbs at that high an RPM (2K-4K) you are out of the idle circuit at that point.
I would check for a vacuum leak, as that is a sure sign of some of the symptoms you are having. Make sure there are no leaks on the front or back side of the intake runners or the carb air box.

Usually something like a flammable mixture WD 40 sprayed around the carb boots will show up as a change in the running speed of the engine if there is a leak.

[reddyvv]

Other than air leaks between the carbs and the engine, the idle rising after blipping the throttle is almost always too rich on the idle circuit. What do you have your idle mixture screws set to and what size are the jets?

If you used a wire a little too vigorously in the idle jets to clean them they can easily get to a size too large (very easy to do). Don't ask how I know!!  I drove myself crazy chasing other causes because I was convinced I hadn't been too rough when cleaning the jets.

[c.tran]

Hmm I'll try out the wd-40 to see if I have any leaks and report back. I have my idle mixture screws set to 1 1/2 or 2 turns out from lightly seated. I followed the service manual, but I can't remember exactly what it said now.

[Duanob]

You had the choke closed? Do you mean off/open? You can't do a good sync with the choke closed.

You have the type one carbs 022As? You will have to go back and forth as you set one carb others with change. If you have to set the idle knob high to start with then so be it, get it to idle lower and lower until you can sync the carbs at ~ 1100 - 1300 RPMs. It takes patience.

Also as mentioned check for vacuum leaks around the boots and intakes.

[LesterPiglet]

What's this idling at x rpm's to sync. Funny I manage to do mine without a rev counter. While you are syncing, the revs will either rise or fall. Adjust the idle screw to compensate and continue synchronising.

[Duanob]

What's this idling at x rpm's to sync. Funny I manage to do mine without a rev counter. While you are syncing, the revs will either rise or fall. Adjust the idle screw to compensate and continue synchronising.

They should be synced at the lowest RPMs possible to idle speed to get the most out of it. Syncing over 2500 probably doesn't do a bit of good.

[c.tran]

When I start to open the choke even a few millimeters, (by open I mean the butterfly valve is opening up, allowing more air in), the rpms shoot up to about 4000rpms.  It's ridiculous.  I try to compensate by backing off the main idle knob, but it's not very effective.  I have to twist the knob a few times in order to lower it from 4000 to just below 2000.  I haven't tried opening the choke fully yet; I have my doubts that I would be able to back off the main idle knob enough to compensate.

Umm, I'm not sure of the model number of the carbs, but it's not the ones with that uses a hinge to clamp the float bowls on.  I've been going back and forth with the carbs, but when trying to sync at below 1500 rpms, even small adjustments tend to drop the rpms too low to where the engine dies before I have a chance to adjust the others to compensate.

I checked for vacuum leaks today by spraying PB Blaster (I don't have WD-40) at the intake manifold and the carb boots.  There was no change in RPMs.  I noticed today when trying again that I got the RPMs to hold at about 1200 rpms for a minute or two, but it started to lower, and eventually the engine dies unless I blip the throttle again.  It has consistently done this.

[LesterPiglet]

They should be synced at the lowest RPMs possible to idle speed to get the most out of it. Syncing over 2500 probably doesn't do a bit of good.

Exactly. That's why I said adjust the idle screw. If it's going to die raise the revs, if it's screaming turn it down.

[LesterPiglet]

If your bike will not run properly with the choke off (butterfly open) there is something badly wrong with your carb settings. I suggest you recheck everything you have done.

[harisuluv]

FYI the carbs that use hinge clamps to secure the float bowls are for a CB750, so good thing you don't have those.

To be honest, what it sounds like is that your bench sync was not done in accord with the idle adjust screw (not air screws).

For example, it is possible to bench sync all the slides at an identical height but where the idle screw is either too far in or out, therefore causing a very limited range of adjustment, and sometimes not at all.  This can result in something like trying to turn down the idle to no effect, because the screw is not even making contact with the linkage.

Pictures would do a lot to help explain what is going on, you would be surprised what can be quite apparent to an experienced eye.

[DaveBarbier]

I checked for vacuum leaks today by spraying PB Blaster (I don't have WD-40) at the intake manifold and the carb boots.

Have any propane? Carb cleaner? PB Blaster and WD40 isn't really that flammable. You can soak a rag in that stuff and it doesn't light that well. I like to use a small bottle of propane because carb cleaner eats paint.


---
1978 Honda CB550K

[reddyvv]

I haven't tried opening the choke fully yet; I have my doubts that I would be able to back off the main idle knob enough to compensate.

It sounds like harisuluv has got it nailed.

The carb sync should be done with the engine at operating temperature - ergo the choke should be OFF, butterflies fully open. If you can't get the idle down far enough with the Idle adjustment screw you'll have to take the carbs off and redo the bench sync. Keep the idle screw about halfway in and then adjust the slides so they are just cracked open a smidge. That should give you enough adjustment either way.

[Duanob]

I checked for vacuum leaks today by spraying PB Blaster (I don't have WD-40) at the intake manifold and the carb boots.

Have any propane? Carb cleaner? PB Blaster and WD40 isn't really that flammable. You can soak a rag in that stuff and it doesn't light that well. I like to use a small bottle of propane because carb cleaner eats paint.


---
1978 Honda CB550K

I always thought the idea of spraying a light oil was to momentarily plug the vac leak. When plugged it will run correctly until the oil gets sucked into the leak.

[DaveBarbier]

I checked for vacuum leaks today by spraying PB Blaster (I don't have WD-40) at the intake manifold and the carb boots.

Have any propane? Carb cleaner? PB Blaster and WD40 isn't really that flammable. You can soak a rag in that stuff and it doesn't light that well. I like to use a small bottle of propane because carb cleaner eats paint.


---
1978 Honda CB550K

I always thought the idea of spraying a light oil was to momentarily plug the vac leak. When plugged it will run correctly until the oil gets sucked into the leak.

Interesting, I always though you were enriching the fuel mixture with another flammable thing. Both theoretically work?


---
1978 Honda CB550K

[Duanob]

Any expert opinions on this? Interesting topic.

[reddyvv]

I have never heard of momentarily plugging a leak with sprayed oil, besides the fact that it will make a big mess.

The idea of a flammable spray is so that it burns along with the fuel and causes a change in engine rpm (due to the change in richness) thereby confirming that there is an air leak.

[harisuluv]

I have never heard of momentarily plugging a leak with sprayed oil, besides the fact that it will make a big mess.

The idea of a flammable spray is so that it burns along with the fuel and causes a change in engine rpm (due to the change in richness) thereby confirming that there is an air leak.

Yup.  As further support, you can use propane gas to the same effect.  Gas that is entering via vacuum leak isn't going to "close off" any leaking areas.

[DaveBarbier]

I have never heard of momentarily plugging a leak with sprayed oil, besides the fact that it will make a big mess.

The idea of a flammable spray is so that it burns along with the fuel and causes a change in engine rpm (due to the change in richness) thereby confirming that there is an air leak.

Yup.  As further support, you can use propane gas to the same effect.  Gas that is entering via vacuum leak isn't going to "close off" any leaking areas.

Beat ya to it!

Have any propane?

---
1978 Honda CB550K

[Duanob]

I have never heard of momentarily plugging a leak with sprayed oil, besides the fact that it will make a big mess.

The idea of a flammable spray is so that it burns along with the fuel and causes a change in engine rpm (due to the change in richness) thereby confirming that there is an air leak.

Yup.  As further support, you can use propane gas to the same effect.  Gas that is entering via vacuum leak isn't going to "close off" any leaking areas.

Using propane around anything that sparks just doesn't sound like a good idea to me. Ether doesn't have the same explosiveness as propane. Seems to find vac leaks good enough.

[c.tran]

Hey guys thanks for all of your input so far--sorry I've been a little busy so I haven't had time to get back to this thread.

Harisluv you make a good point.  When I bench synced the carbs, I had the main idle screw backed out pretty far (maybe 3-4 turns from not making contact with the linkage).  I found out that cylinders 1 and 2 weren't firing properly.  Compression turned out okay (all around 140-155 psi), no leaks were found (I used carb cleaner to check), and spark was good.  Thinking it had to be the fuel, I took off the carbs and discovered that the idle jets for 1 and 2 were clogged...I guess some junk entered the system.  My o-rings were a bit torn up, so I am also going to replace those so they don't tear off and go downstream.  Hopefully this will alleviate my idling issue.

[flybox1]

Hey guys thanks for all of your input so far--sorry I've been a little busy so I haven't had time to get back to this thread.

Harisluv you make a good point. When I bench synced the carbs, I had the main idle screw backed out pretty far (maybe 3-4 turns from not making contact with the linkage).  I found out that cylinders 1 and 2 weren't firing properly.  Compression turned out okay (all around 140-155 psi), no leaks were found (I used carb cleaner to check), and spark was good.  Thinking it had to be the fuel, I took off the carbs and discovered that the idle jets for 1 and 2 were clogged...I guess some junk entered the system.  My o-rings were a bit torn up, so I am also going to replace those so they don't tear off and go downstream.  Hopefully this will alleviate my idling issue.

Wrong.
if you bench sync'd your slides with the idle screw backed out, how can your slides close all the way?  you've just created NO idle adjustment lower than your bench sync tool (drill bit) OD 
you need to bench sync them with the idle knob turned IN a few turns so you can further CLOSE them, lowering the idle....

[c.tran]

Sorry, I think I miswrote what I meant to say.  I bench synced with the screw in a few turns. After the sync I backed out the screw until the slides closed as far as they would go (at this point the main is screw was still putting pressure on the linkage).

[flybox1]

Ensure that your slides completely drop, allowing no light to shine thru from the other side, when the idle knob is turned all the way out.