Topic: New CB550 - diagnosing low vacuum - 8 inHg

[dunnp2]

Hey all! I picked up a CB550 a few months ago and drove it home despite a few issues which I attributed to the carbs needing a rebuild - hesitation on throttle opening being the main one.

I rebuilt the carbs and the engine fired on the first kick but when I went to sync I found low vacuum on all 4 cylinders - 8 inHg after balancing them a little.

First thing to check is the boots, etc. There's no air leak that I found after looking and spraying the connections with carb cleaner.

Next I checked for a jumped cam chain but the cam is perfectly aligned with horizontal when at the T 1-4 mark.

I haven't done a compression test yet but I did drive it to work this morning with plenty of power and good response everywhere except idle. When I pulled the clutch in approaching stop lights it immediately stalled on me 3 times. If I blip the throttle after pulling the clutch it avoids stalling and idles fine, though the idle slowly wanders all over the place. I think the trend is after running the idle is high, then after 10-15 seconds it drops to extremely low but doesn't stall. If I blip the throttle the idle goes back to high for 10-15 seconds.

Any advice on something to check? I'm trying to focus on the low vacuum issue and get that solved because I suspect it's causing the stalling issue. Given the good power and throttle response off idle I doubt it's the rings but I'll do a compression check soon anyways.

[dave500]

go right over the ignition.

[kerryb]

If you do a 3000 mile tune-up, you should catch that problem.  I'm dealing with similar issues on a 77 550 but mine is burdened by cheap pods.  Cleaned the carbs, checked the jets, adjusted the points, at the urging of dave500 fixed my timing light and found the timing was off just a little, synched the carbs again and now it idles great, revs good, just waiting for cold,wind,and rain to let me take it out for a test run.

Are you running all stock parts? Including airbox and filter?  Are the slow jets REALLY clean? Is the timing really correct?  Thats just a few things that made a difference for me.  Don't forget to let us know what year and model too.

How are you measuring the low vacuum condition?

(dave posted while I was checking speling, he's the ignition guru, thanks dave)

[dunnp2]

Thanks for the responses - my bad, it's a 1977 CB550F.

I believe it had pod filters at one point because the choke lever was bent and the jets were different from what came in my rebuild kit. By the time I got it, it was back to mostly stock - stock intake box but missing the top cover piece under the seat, so looking down you see the filter.

My rebuild kit included a #100 main jet and #40 slow jet. I have a 4 into 1 exhaust. Also all of the rubber boots that connect each side of the carburetor are new and the clamps are tight.

The slow jet is brand new but not a genuine Keihin so maybe that's the culprit? I still have everything I removed during the carb rebuild, so I can try putting those back in.

I will go through the ignition system first, thanks Dave. It has a Dyna S ignition kit but stock-looking coils. I don't have a timing light but I'll get one and check that, assumed it was set properly and didn't need adjustment but we all know how assumptions go...

I'm measuring with one of these vacuum gauge kits, which I 'calibrated' (just balanced at a few different points) using a vacuum pump at work. I know it's cheap but it's definitely accurate enough that I know I have a problem.


So to recap, I'll confirm the ignition timing is correct, then do a compression test, then replace the plugs & plug wires, then try swapping the other slow jets back in? Hesitant to fire the parts cannon on plugs and wires given that I'm equally low on all 4, it's likely not an individual component issue...

[dave500]

with the dyna youve only gotta check the timing no points to play with,also watch the timing marks as you increase rpm it should advance the timing as speed increases,if not find out why and fix it,DO NOT crank hard on that big looking nut on the end of the crank,if you need to turn the motor by hand bump the kicker or remove the plugs and gently on that nut,if you remove the smaller bolt in the middle youll see why.

[kerryb]

I'm still curious why you think you have a vacuum problem.  When does your low vacuum occur according to those gauges?
I just finished synching my carbs and it idles better, doesn't die at stop signs, but I still have that hanging idle issue you talk about.  Waiting for a test drive to prove that timing fixed some small issues.

Why do you think you have a vacuum problem?

[bryanj]

Actual vacuum is not that critical just needs to be same on all 4with tickover at minimum 1100 revs

[dunnp2]

Picked up a timing light, it was a little advanced at idle so I adjusted and it helped a bit but still idles rough. Confirmed that the advance works properly.

I did a compression test and got  118 | 108 | 125 | 125

During the compression test after idling I saw all of the plugs were very black. I also noticed during the timing adjustment that if I quickly open the throttle it hangs at a really high RPM (full advance) until I back out the idle screw enough that it drops to a very low RPM - maybe just a couple hundred...the tach was removed before I got the bike 
There doesn't seem to be an in between with the idle speeds, it'll either hang very high or drop to very low, where it smells bad and burns my nose.
So now I'm thinking I just have rich slow jets. I'll double check what I had before the rebuild and probably put those in. I also picked up new plugs but haven't put them in yet. I'll get time to work on in again tomorrow afternoon.

I'm still curious why you think you have a vacuum problem.  When does your low vacuum occur according to those gauges?

The vacuum is 8 inHg at idle, I didn't actually spend time at steady state high RPM. It's in the range labeled "late valve timing or leak in intake" like this one.

[Spanner 1]

I think you messed up your gauges when you attempted to 'balance' them with a vacuum pump ! Try this ; connect gauge #1 ( left ) to cyl #1 ( left ) start bike and read gauge at idle. Use the damper to stabilize the reading. Take note. Now attach gauge #2 to the same #1 cyl and adjust the screw on the gauge face to read exactly as the first gauge. Repeat for gauge #3 and 4. Now try to balance the carbs again. If one carb slide is too high compared to it's 3 mates then 'your off to the races' as the one open slide will carry the other cyls to high revs on an unloaded ( idling ) motor

[dave500]

dont try to apply any car type settings etc with these bikes(especially stinking holleys!),the carbs work differently and the camshaft would be pretty hot compared to a stock type car motor even any old small block chev/ford etc.

with your timing shining the light on the marks,increase the rpm and watch as it advances,does it make it to the second full advance mark?check both 1/4 and 2/3,if not adjust the plate untill it does,where the marks end up at idle dont worry about it,once your riding the bike itll be at full advance all the time anyway,some guys adjust so the idle is correct on that "F" mark,they may be losing out on full advance and good economy and power,forget about the #$%*ing f mark when dynamic timing,its a good ball park setting to fire an engine up,plus with the #$%*ty points these days its a #$%* fight getting them correct,retarded ignition gives a lower vacuum reading,you can even set the advance higher as long as it doesent ping!

[Deltarider]

I fail to understand what the problem is. Doesn't 8 inch equal 20-21 cm? Just read the right scale.

[dunnp2]

I fail to understand what the problem is. Doesn't 8 inch equal 20-21 cm? Just read the right scale.

Yes, but that's in the range labeled red on my gauge, far from the green zone, which originally made me think that was my problem when also experiencing either a runaway high idle or basically stalling out.

forget about the #$%*ing f mark when dynamic timing

Ah shoot, it was set (Dyna S) exactly to the high advance mark and I retarded it to exactly match the F mark at idle. I'll reverse my change and set it to the full advance mark at high rpm. Thanks.

So last night I realized these carbs had keihin 35 slow jets before my rebuild, when I installed the 40s. Duh, that explains the spot on the plugs and all of my problems! ...or so I thought. I swapped the 35s in and no change, still a runaway idle.

I think you messed up your gauges when you attempted to 'balance' them with a vacuum pump ! Try this ; connect gauge #1 ( left ) to cyl #1 ( left ) start bike and read gauge at idle. Use the damper to stabilize the reading. Take note. Now attach gauge #2 to the same #1 cyl and adjust the screw on the gauge face to read exactly as the first gauge. Repeat for gauge #3 and 4. Now try to balance the carbs again. If one carb slide is too high compared to it's 3 mates then 'your off to the races' as the one open slide will carry the other cyls to high revs on an unloaded ( idling ) motor

That's a good idea, I'll try switching the gauges around and see how it compares. Though I did use a throttle valve on a vacuum pump to compare my gauges at a few different vacuum levels so I expected them to work pretty well. We'll see. After this I'm running out of ideas 

[kerryb]

Acouple of comments:  i'm working on the same bike, and am having some success solving the same problems you are describing.  The differences between our bikes are...I have stock ignition, #38 slows &#98 main jets by keyster.  Cheap pods and no aibox, stock ignition.

1. I don't care what the vacuum gauges say, as long as they all equal.  These carbs seem much more sensitive than pd carbs.
2.  Changes that have given the best results have been ignition (mine was too advanced), 
       Carb synch smoothed out the idle.
       Mixture screws and needle clip reduced the plug fouling though still fouling...
3.    Stock jets for these are 38 slows and 98 mains ( from an era that stressed clean burning to limit emissions)
       Dave's recommendation for 40's & 100's is probably a very good one, but my jets are keyster and not keihin so all bets are tainted.  And, I'm still fouling plugs with the small ones...probably the pods!

I haven't done a compression test yet, maybe today, I haven't set my valve clearances yet (forgot to bring my cute little feeler gauges) and at least one is noisy when warmed up.

My suggestion to you is keep working on your ignition and carb synch, with those two I have fixed the idle roughness and most of the hanging idle.  Power to pull away from a stop sign is also much improved.  Still playin'...

[dunnp2]

Thanks Kerryb, glad to hear we're struggling through together. Very confusing.

I will say it runs a lot better than it did before the carb rebuild and I'm learning a lot, so I don't feel like I'm wasting my time and I'm grateful for all the time and advice you guys have given me!

I just have to keep reminding myself that it's just an air pump. Air, fuel, compression, spark. Which one is wrong??

[enwri]

 it hangs at a really high RPM (full advance) until I back out the idle screw enough that it drops to a very low RPM - maybe just a couple hundred...

Check it out with your timing light, watch the timing while you drop the idle. Could be the old "cut a turn off the advance spring" thing. Which I read on here years ago.
Loose advance springs will let it stay advanced too low. Has to be fully retarded around 1500 or at least before chosen idle speed. Otherwise it will never retard fully and always hang, until you drop the idle so low it wont run when it does finally return to full retard.
There's a huge rev difference between idling with advanced timing and retarded timing.
.
Sometimes you do just have to go "full retard".
Edit to add, it'll probably make your vacuum results even worse.

[dunnp2]

Oooh, that's an interesting thought. I just spent some time searching for spring specs, has nobody figured out the replacement spring?

[dunnp2]

I think you messed up your gauges when you attempted to 'balance' them with a vacuum pump ! Try this ; connect gauge #1 ( left ) to cyl #1 ( left ) start bike and read gauge at idle. Use the damper to stabilize the reading. Take note. Now attach gauge #2 to the same #1 cyl and adjust the screw on the gauge face to read exactly as the first gauge. Repeat for gauge #3 and 4. Now try to balance the carbs again. If one carb slide is too high compared to it's 3 mates then 'your off to the races' as the one open slide will carry the other cyls to high revs on an unloaded ( idling ) motor

I swapped vacuum lines around last night and noticed a drastic difference, you're exactly right! Made some quick guess adjustments in the direction of the errors and the bike was idling happier.

I just re-calibrated the gauges in the range where the engine actually runs (8 inHg) instead of the 20 inHg I had calibrated them at before. Very cheap gauges, only good in a very small band. They were off by 5 inHg to each other at the lower vacuum setting!! Feeling good about re-balancing when I get a chance. I thought it was weird that I had to make such large changes away from my bench sync...

Fingers crossed, but hopefully this is the solution!

[dave500]

using tee connectors hook them all together to the same vacuum port,they should all be the same.

[dunnp2]

Yep - that solved it. I was just WAY off with my balance! It's running happy and driving great. I'm still running 35 slow jets and have 38s to swap in, which should fix my little hesitation off idle. But now it quickly returns to a happy idle when I pull in the clutch at a stop light. Just drove a few miles down the street and back!

Kerryb, did you get yours solved? After my issue, it's worth double checking the gauges. Dave500's advice is perfect:

using tee connectors hook them all together to the same vacuum port,they should all be the same.

[Deltarider]

Checking your vacuum meters can be done by connecting them all four to one vacuum source, but that source has to be a stable vacuum pomp and NOT one of your four vac ports of a running engine. That won't work. Not at the same time. One after the other is OK tough.

[dunnp2]

Gotcha. I'm lucky to have an adjustable vacuum pump at work so I used that to set mine.