Topic: Carburettor 1974 CB550

[Moleskins]

I’m chasing a hanging idle problem.
I’ve set the float heights to 22mm
I’ve cured an air leak
I’ve balanced the carbs with gauges
I’ve cleaned them
I’ve fitted new o rings throughout
I’ve put the needles down
I’ve put the needles up
I’ve adjusted the air screws
I’ve sprayed carb cleaner to detect further air leaks
I’ve set the static timing
I’ve checked the timing with a strobe
The plugs are sooty so it’s not running lean
All the brass bits are clean but not new
Before I visit the toupee shop because I’m tearing my hair out
What would be the consequence of too much fuel in the bowl?
Or have you solved this another way?

[BenelliSEI]

Way too much fuel will appear running out the overflow pipes. Assuming your throttles are snapping shut, and this is not a cable/ mechanical issue, have you checked your ignition advancer? If it is lazy and slow to retard, the advanced ignition setting will hold the idle up.

To check, watch with your timing light. If the ignition stays fully advanced, after releasing the throttle, and then slowly returns to idle setting, get in there and polish the shaft and lube it all very lightly. Make sure it is totally free.......

[Moleskins]

I’ll get onto that one thanks your reply is much appreciated.

Anything else anyone?

[Deltarider]

Yeah... what bike?

[Moleskins]

A very good question!
It’s a CB550K0 1974.
Sorry about that, I was concentrating on thinking of all the things I’d done

[Bankerdanny]

Are the o-rings in the air screws present and in good shape? Do you have the springs and washers in there too, and in the correct order?

[Scott S]

 Exhaust?
 Stock air box/filter?

 And +1 on checking the advancer.

[Moleskins]

Thank you for the replies.
I found the nut that holds the cam chain adjuster was loose.
That *seems* to have helped with the hanging idle.
I’ve cleaned the advance mechanism and used a bit of light oil. It wasn’t obviously bad.
I’ve obviously re set the timing.
It’s 4 into 1 exhaust with a baffle and the original air box is long gone so mesh filters.
There’s no o rings on the air screws but I couldn’t see any showing on exploded diagrams.
I’ve seen conflicting info on re jetting. Some say the original jets were selected for worldwide use so will cover variations, others say the slightest change and re jet is needed.
I’ve not found any clear info. anywhere.
Things like … put the needle clip on the 4th ring.
Counting from the top or the bottom??
Turn the air screw.
In or out??
Any suggestions where to get decent jets etc in the U.K.?

[Deltarider]

Are the o-rings in the air screws present and in good shape? Do you have the springs and washers in there too, and in the correct order?

There are no O-rings nor washers at the airscrew in the oldstyle carbs like: 022A and 087A, just springs. The O-rings and washers were on the PD carbs.

[Scott S]

 Cleaning the advance will certainly help, but you may need to trim one or both of the advance springs ( one coil on each, then bend the spring over to form a new hook).

 The mixture screws are on the air box side. They control air. Screwing them in decreases the amount of air, making the mixture richer. Screwing them out increases the amount of air, making the mixture leaner.

 The exhaust is probably OK. Most 4-1 exhausts rarely require re-jetting. But we don't know what brand, so....
 Your biggest issue is the pod filters. You just have to experiment, do plug chops, or find a dyno ("rolling road") and jet accordingly. There is no universal setting for pods.

 Moving the clip on the needle up....towards the fat end... will lower the needle. That makes the mixture leaner.
 Moving the clip down on the needle....towards the pointy end....will raise the needle, making the mixture richer.

[flo1014]

Hi guys I have a question, on my carburetor on a CB550/78 I have 2 pipes for fuel on smaller on large and the tank is connected to the big on as per photos, can you please let me know if a second pipe should go there or should be free?
Regards
Flo

[Scott S]

Hi guys I have a question, on my carburetor on a CB550/78 I have 2 pipes for fuel on smaller on large and the tank is connected to the big on as per photos, can you please let me know if a second pipe should go there or should be free?
Regards
Flo

 The smaller one is a vent tube. It should have a hose that goes up and over the air box.

[Deltarider]

Hi guys I have a question, on my carburetor on a CB550/78 I have 2 pipes for fuel on smaller on large and the tank is connected to the big on as per photos, can you please let me know if a second pipe should go there or should be free?
Regards
Flo

What Scott S says. Your bike is probably a K4. Anyway, the air vent tube is supposed to be 4,5x6x350 mm.

[Moleskins]

Cleaning the advance will certainly help, but you may need to trim one or both of the advance springs ( one coil on each, then bend the spring over to form a new hook).

 The mixture screws are on the air box side. They control air. Screwing them in decreases the amount of air, making the mixture richer. Screwing them out increases the amount of air, making the mixture leaner.

 Your biggest issue is the pod filters. You just have to experiment, do plug chops, or find a dyno ("rolling road") and jet accordingly. There is no universal setting for pods.

 Moving the clip on the needle up....towards the fat end... will lower the needle. That makes the mixture leaner.
 Moving the clip down on the needle....towards the pointy end....will raise the needle, making the mixture richer.

Thank you for clear unambiguous advice.

I've fiddled around to the point where I've got it to run on tickover, I'm now in the process of trying to get it to not miss and spit when it goes up the road, progress is there but slow.
The air screws are out ....... I don't know if anyone will believe this ........ 3 1/2 turns
1 1/2 is the stock setting. Any attempt to take them in brings back the hanging tickover problem. Doesn't make sense to me either.
I'm wondering if there's a blockage in one of the holes in the carb body. I've had that before on Triumph bikes, you think you've cleaned them out but it seems to take a few goes

[Scott S]

 Did you pull the emulsion tubes? Dirty emulsion tubes can make it run very rich.

 If you are 3.5 turns out on the mixture screws,  that indicates that you need to change the pilot jets.

[Deltarider]

The air screws are out ....... I don't know if anyone will believe this ........ 3 1/2 turns
1 1/2 is the stock setting. Any attempt to take them in brings back the hanging tickover problem. Doesn't make sense to me either.
I'm wondering if there's a blockage in one of the holes in the carb body. I've had that before on Triumph bikes, you think you've cleaned them out but it seems to take a few goes

First of all, what is the stamped in number on the carbs? (see pic for where to look) What airfilter do you run? In case you run OEM, does removing the filter element make any difference?
3 ¹/₂ turns out is too much. I hope you did not make the (usual) mistake trying to find the highest idle (rpmwise) by turning the airscrews out. On these bikes that is NOT the right way to set the idle.
Make sure the engine idles at a TRUE 1100 rpm.
Are the slowjets the #38 or #40 size? Could it be that a PO has accidentely reamed the slow jets by using an improper tool, like poking in there with anything that is harder than brass? Happens a lot. When in doubt, I'd order new #38 Keihin slowjets. They are not that expensive.

[RAFster122s]

With pods the only way to tune is with plenty of new plugs to do plug chops to determine mixture/running condition at WOT, mid throttle and low throttle settings and of course idle to not foul out the plugs.  Stock motors do not perform well without the air box, only a modified motor would benefit from pods. Alternative tuning method is with handful of jets and time on the dyno, with several sets of new plugs.

You will need to be more precise and specific with your descriptions and share images of your plugs if you want help tuning the motor’s mixture.  We also need to know specifics of the motor build and the jets in the carbs.

If you are missing the air box, then there is always Ebay for picking up another or see if anyone has one via the classifieds.