Topic: Help! '75 CB550 idles fast and inconsistent

[Joshevelle]

Thank you. I will order new carb rubbers and give it a try without inline filters, however the bike ran great three weeks ago with inline filters 

[uksparky]

I got the same problem start cold warm up 1200 rpm.. ten mile on freeway coming off ramp 3k idle, no cable hang up, turning idle screw ok...town riding, next thing it stalls not enough idle adjustment...78 550

[Joshevelle]

Yeah it is the same problem I am having.

For what other members proposed, I thought it was an air leak, I did a test by spraying carb cleaner to the boots and the intake manifold, the rpms did not change at all 

However I already ordered new boots.

Is there any chance that, although all the bowls have the same fuel level, #1 & #4 could be providing more fuel to the engine?? Those spark plugs are still getting carbonized, and I truly believe that the problem is the fuel delivery at 3K RPM and up.... but how can we explain the sudden rise in the RPM at idle??

I'm so confused, we should have a "Tootired help signal" some sort like batman's.

[BomberMann650]

Joshevelle - as to your last question.  Yes there are.
Make sure the air;fuel mix screws are set the same.
Were the carburetors perfectly vacuum synced?

[calj737]

Josh - With all due respect to our fellow members, much of their advice does not seem to be relevant to your symptoms. If the bike was running fine configured as is, then suddenly is not, it stands to reason that only aspects related to fuel delivery and ignition would be the culprits for carbon fouling.

So, following the logical path of diagnosis:
- Clear Tube done, all four are equal. Good.
- Inline filter has not previously had an adverse effect, and it would not only effect two of the four cylinders anyway. Disregard as culprit, though better to eliminate it long term.
- New points and Condensers. Do you happen to know the brand of these?
- Swapped coils from 1/4 to 2/3, problem remained on 1/4. (When you did this, did you also change the ignition wires to the coils from the plate? Else, your coils would be firing in reverse...)

I'd suggest:
- An idle plug chop, with fresh clean plugs, as a baseline.
- Measure the resistance of the plug caps on all four cylinders. They should be 4.7-5.0K Ohms.
- Are these stock coils? Can you provide some detail on the means by which you swapped coils?
- Can you provide the model number for your carbs please
- And to the best of your knowledge, the needle clip position and brand of jets inside your carbs.

This is to sort out the fouling issue as mentioned. The racing idle is more likely associated with a vacuum leak, or a timing advancer issue, which, could also be a source of problems for your plugs fouling.

[uksparky]

Two ways I can bring rpm down thumbscrew or letting clutch out in gear when it's at idle 3k not moving putting load on engine...I checked advance it moves freely ..

[BomberMann650]

I can bring rpm down.  thumbscrew

And that doesn't keep the idle rpm set where you want it?

[Joshevelle]

calj737, thanks for your help.

- New points and Condensers. Do you happen to know the brand of these?

New points and caps were bought from 4into1, no branding, they are "aftermarket built to meet or exceed OEM specifications"

- Swapped coils from 1/4 to 2/3, problem remained on 1/4. (When you did this, did you also change the ignition wires to the coils from the plate? Else, your coils would be firing in reverse...)

Yes, I did swap cables too. I should have elaborated a bit more into this. I physically swapped ignition coils, left to right and right to left, I had to change the high voltage cables too. 

I'd suggest:
- An idle plug chop, with fresh clean plugs, as a baseline.

ok I already have a box of NGK D7EA, I will try to do this today and send an update.

- Measure the resistance of the plug caps on all four cylinders. They should be 4.7-5.0K Ohms.

I did that, all four caps where around 5K (NKG, can't remember the model number), I also measured resistance between plug caps with the coil connected to them.  It was around 24K. The testing I did is detailed in the following URL:

http://blacksquaremotorcycle.com/ignition-coils-test/

- Are these stock coils? Can you provide some detail on the means by which you swapped coils?
- Can you provide the model number for your carbs please
- And to the best of your knowledge, the needle clip position and brand of jets inside your carbs.

I got the coils from a salvage Honda Nighthawk 1983, perfectly running before a crash (the guy told me that, I assumed it was true since the bike crashed while running xD).

Carbs are Kei-Hin 627B, stamped as 3313 in the rack.

Regarding the needle position, I did insert the clip in the 1st notch from top to bottom. I got the best performance at high RPM in this position (when the bike ran great). All the carburetor repair kit was bought from 4into1 (aftermarket ), except for the slow jets, which are OEM, brand new, honda packaged (I bought them last year).

This is to sort out the fouling issue as mentioned. The racing idle is more likely associated with a vacuum leak, or a timing advancer issue, which, could also be a source of problems for your plugs fouling.

When I changed the points and condensers, I set them up using a multi meter (MM) in continuity mode:

1st: adjust point gap (0.35mm)
2nd: with the coils disconnected, connect one cable of the MM to ground and the other to the terminal of the point under configuration.
3rd: rotate the shaft till the MM beep stops and check the shaft position.
4th: move the point slightly until the beep stops at the F mark.
5: repeat for the other point.

After doing this and running the bike a couple of times, I do not have back fires nor the timing has moved (I double checked this).


Again thanks a bunch for chiming in, I will try to do the idle chop today and let you know the outcome. If you think anything else, please let me know, I would highly appreciate it.

THANK YOU!!

[flybox1]

Joshevelle - as to your last question.  Yes there are.  Agreed
Make sure the air;fuel mix screws are set the same.  Disagree.  They need to be set to individual need, assuming all else is sorted
Were the carburetors perfectly vacuum synced?  Needs to be done correctly

[Joshevelle]

Thank you all for your help. I just did an idle plug chop (see attached image, cyl 1 is most left spark plug, then two, three and four).

First I got the engine to operating temperature, turned it off and installed a new set of NGK D7EA, and turned it on again. I put a fan in front of the engine to keep it from overheating and let it run for 5min, then hit the cut off button.

The bike holded solid 1K RPM and it did not rev up, it tried to die on me a couple of times though.

To me, it looks like I need to close the air screw in cyl 2 and 3. 

@BombberMann650 no, the carbs had only been bench synchronized (drill bit technique), I was going to take it to a shop for vacuum syncing but since the bike ran so good I just didn't.

All four air screws are 1 1/2 turns out, should I start to play with them? At least cyl 2 and 3...

[uksparky]

I can bring rpm down.  thumbscrew

And that doesn't keep the idle rpm set where you want it?

Checked fuel level clear hose ok...Correct still playing with the thumb screw after run out today ,...i did change mixture screws from 1.5 out to 1 turn out after i got back...seems to idle better

[BomberMann650]

If ya think the bike runs good on a bench sync, you'll love the way it runs after a vacuum sync! 
It's worth the investment into a set of sync gauges (if you plan to ride alot) because vacuum sync is the last step of the 3k tune up.  Let the shop do it the first time so you'll know what "perfect" is.  That will be the benchmark.

Sounds like you're getting really close to taming the beast.  Keep it up!

[BomberMann650]

@josh - it's A O K for your bike to idle anywhere between 1100 and 1400 rpm.  The old mechanical tacho's are not always spot on.  The motor could be trying to die because the idle is lower than it likes.

Since we're discussing high idle - I was mind blown to watch a video of an old Moto Guzzi 8 cylinder track bike idle (and race).  It was so hopped up on black magic go fast parts, if the idle dropped below 3.5k, it stalled instantly!

[Joshevelle]

If ya think the bike runs good on a bench sync, you'll love the way it runs after a vacuum sync! 
It's worth the investment into a set of sync gauges (if you plan to ride alot) because vacuum sync is the last step of the 3k tune up.  Let the shop do it the first time so you'll know what "perfect" is.  That will be the benchmark.

Sounds like you're getting really close to taming the beast.  Keep it up!

Thanks for your inputs, I will definitely buy a set of sync gauges, but yes, first I will let a shop do it for me. But I need to get this problem sorted out first.

UPDATE!

After doing the idle plug chop, I was about to adjust #2 & #3 air screws, but I thought... why would have moved in the carbs to require this adjustment? (It ran great at 1 1/2 turns out, all of them). So, with the engine at operating temperature, I tried one more time the air leak test, but I couldn't find my carb cleaner, so I grabbed my WD40 and VOILA!!! The RPMs went crazy up! 

Then, when the revs went down again, I started to spray WD40 very carefully to find out where the leakage is... boot #2 and #3, from the carb side!!!! CRAP! I did this test before and NOTHING HAPPENED BEFORE!!!   

I guess I will have to wait for my new set of carburetor boots.... 

[Joshevelle]

@josh - it's A O K for your bike to idle anywhere between 1100 and 1400 rpm.  The old mechanical tacho's are not always spot on.  The motor could be trying to die because the idle is lower than it likes.

Since we're discussing high idle - I was mind blown to watch a video of an old Moto Guzzi 8 cylinder track bike idle (and race).  It was so hopped up on black magic go fast parts, if the idle dropped below 3.5k, it stalled instantly!

I am using a digital tach, I used to have the mechanical one but it was changed with the front end (CBR600F1, 2006). The bike used to idle at 900RPM and used to purrs  like a kitten!!! Now it is not 

Let's wait for my new boots to arrive soon T.T

[calj737]

Then, when the revs went down again, I started to spray WD40 very carefully to find out where the leakage is... boot #2 and #3, from the carb side!!!! CRAP! I did this test before and NOTHING HAPPENED BEFORE!!!   

And now we are onto the right path... Your #2/3 plugs are lean at idle, #4 is a bit fouled. Tuning a bike with pods is tricky, and it takes more effort than a PO slapping in aftermarket jets and turning a few screws. I suspect the carbs need a thorough tuning, replacing the main jets with Keihin brand jets, somewhere closer to 105s, maybe 110s at the most. But plug chops after a proper vacuum synch and all vacuum leaks resolved.

Your descriptions sounds as though you are perfectly competent to do all the work yourself, properly. Buy a set of Motion Pro gauges, synch the carbs yourself. What you pay a shop to do, is more than the cost of the gauges. And your manifolds should have vacuum screws already. It's actually easier than a pug chop...

A sidebar: ideally, all carbs would be tuned/adjusted equally. It may well be the case that specific cylinders have less compression, thus less vacuum, thus need fine tuning individually. So, as you get close on synchs, chops, you may well tweak one or another small bits in a different direction. Make a note of it so you remember. Also, carbs wear internally if neglected. So that produces minor changes in their tune.

Now, if your plugs are fouling at 1/2 throttle and above, you're way too big on the MAIN jet. Since its an aftermarket jet already (IIRC from your post) then its time to get a Keihin brand and start with a 105. Then a new WOT chop.

[Joshevelle]

And now we are onto the right path... Your #2/3 plugs are lean at idle, #4 is a bit fouled. Tuning a bike with pods is tricky, and it takes more effort than a PO slapping in aftermarket jets and turning a few screws. I suspect the carbs need a thorough tuning, replacing the main jets with Keihin brand jets, somewhere closer to 105s, maybe 110s at the most. But plug chops after a proper vacuum synch and all vacuum leaks resolved.

Yeah, is very tricky to get the bike running "properly" with pods, it took me a while to get the proper settings last year but once I got it right, it ran great but it only gave me 15Km/L (~35MPG) . I believe I still have a set of Keihin mains #100 somewhere in my garage... I will give them a try and I will also need to get the needle up, so I will move the clip one or two notches down. I definitively need to get rid of the vacuum leak first, still waiting for the new carb boots to arrive.

Your descriptions sounds as though you are perfectly competent to do all the work yourself, properly. Buy a set of Motion Pro gauges, synch the carbs yourself. What you pay a shop to do, is more than the cost of the gauges. And your manifolds should have vacuum screws already. It's actually easier than a pug chop...

A sidebar: ideally, all carbs would be tuned/adjusted equally. It may well be the case that specific cylinders have less compression, thus less vacuum, thus need fine tuning individually. So, as you get close on synchs, chops, you may well tweak one or another small bits in a different direction. Make a note of it so you remember. Also, carbs wear internally if neglected. So that produces minor changes in their tune.

Now, if your plugs are fouling at 1/2 throttle and above, you're way too big on the MAIN jet. Since its an aftermarket jet already (IIRC from your post) then its time to get a Keihin brand and start with a 105. Then a new WOT chop.

Thank you calj737. I'm an electronics engineer and my hobby is mechanics, I used to do cars only, but I fall in love of the CB some time ago. I will buy the set of gauges to synch my carbs myself.

I agree with you, the air screws should be set them equally "ideally", but yeah, taking in consideration that this set of carburetors were manufactured in 1974, there must be some wear internally.  If the set of Kehin #100 that I have do not work (after fixing the vacuum leak), I will order a set of #105 and #110. Thanks a bunch for the advice and for taking your time to reply to me.

I'll keep you all updated once I get rid of the vacuum leak!

[BomberMann650]

My bike is a little on the lean side with 100's, and I have a stock airbox.  You may want the 105's cal suggested.

[Joshevelle]

My bike is a little on the lean side with 100's, and I have a stock airbox.  You may want the 105's cal suggested.

OK, I will order a set of Keihin #105 then.  THANK YOU!!!

[Duanob]

Man I'm always late to the party! 

[Joshevelle]

Man I'm always late to the party! 

lol!