CB550 Tuning Advice

[Neilw]

Rather than continue this topic in my Highs and Lows thread, I thought it would make more sense to start a new topic specfic one.

Now I have the carbs in 069A, and the bike is starting and just about idling. I'd like to start tuning the old girl.

For reference so far, I've set the valve clearances and points, both static and dynamic. I've bench-synced the carbs per Bryan's instructions and set the float heights with lots of testing. The air screws are one and a half turns out. Circlip on the needle is in position 2.
All rubbers are new; stock air box, new filter.

I've got a vacuum tester.

I've checked with ChatGPT, and it gave me a list of things to do, but I'm wary of diving in without checking what's worked for you.

Where do I start, and in what order should I proceed? I should mention that the top end had a re-bore, and I have new pistons & rings, +.25

I've looked in the common service manual, and no single topic covers it, so even a summary would help me dig deeper.

Many thanks, guys.

[K2-K6]

First, and fundamental, is to individually turn each airscrew in isolation with it running once warm and with choke fully off.

If there's not reasonable/perceptible shift in running, for that cylinder, then most/ further settings to optimise will be invalid.

This checks the normal, acceptable presence of the idle circuit function for air and fuel modulation, without which it'll never really run correctly. 

Any fault with this and you'll need to identify exactly what the cause is.

[Neilw]

So my current default position is.

Choke on, bike starts, and I can adjust the throttle screw to idle, and it will happily idle and rev when I twist the throttle.

As i take the choke off, no change, but when I do the last bit, it dies- no sputtering, just dies. I try holding the throttle open with the same result.

I've tried a few things suggested by ChatGPT, but made no major changes - it wants me to take the carbs off and check the pilot jets and passages, as it suspects a blockage.

I thought before i do that I'd drain the float bowls, and lo and behold, there are little bits of rubber floating around, and in carb 1 there's some sediment. The rubber bits are tiny but are present in all the bowls; see pics, so I guess it's possible these may be responsible?, it seems reasonable.

The rubber hoses are good ones; well i thought they were FS > NT - 006 DIN 73379 Type C1 NBR and 003.5 DIN. 

So before I start taking them off and stripping down again, do you agree with this course of action and if so, can anyone recommend some decent replacement fuel pipes, please? Both sizes.

I should say that the contamination isn't coming from the temp fuel tank or the fuel as i've checked both.

[K2-K6]

The suggested action is correct.

Essentially the engine idles  almost exclusively on the fuel metered through those idle jets, the main, plus needle fuel route virtually closed with throttle closed.

Using choke, causes the vacuum in intake tract to rise, leading to fuel being pulled through both main jet and whatever it can get via idle jets .... that fuel will not be pulled through those routes (if partly blocked ) without the increase in vacuum the choke gives. Hence the stall when choke is fully opened.

Sound logical?

Your description, experience and the gpt effectively agree on cause.

[Neilw]

The suggested action is correct.

Essentially the engine idles  almost exclusively on the fuel metered through those idle jets, the main, plus needle fuel route virtually closed with throttle closed.

Using choke, causes the vacuum in intake tract to rise, leading to fuel being pulled through both main jet and whatever it can get via idle jets .... that fuel will not be pulled through those routes (if partly blocked ) without the increase in vacuum the choke gives. Hence the stall when choke is fully opened.

Sound logical?

Your description, experience and the gpt effectively agree on cause.

Thank you, i'm floundering around in the dark tbh, so your opinion and explanation is very helpful. I'm getting a master at whipping the carbs off/on, having done it so often, so that's my job tomorrow.

[Oddjob]

NBR on the fuel lines means Nitrile IIRC. I would change them to Viton if possible. The rubber debris could be coming from the inside of the fuel lines and that amount will be blocking some of the jets I'd imagine.

[McCabe-Thiele (Ted)]

When I first fired up my 500 after it's full rebuild it ran great, next time it ran like a pig.
The temporary petrol tank that I bought online came with petrol pipe that I used.
The pipe went sticky to the touch over a few days.
Carbs were full of black bits from the pipe.

I was gutted and demoralised to have to remove the carbs, dismantle them and clean them all over again.
I had to leave it for a couple of weeks as I was so ***** off  having  to do it all again.

[Sesman]

Yup, same thing happened to me. I sourced mine from the attached and the pipes broke up depositing bits just like that blocking the jets. Claimed to be Nitrile. I am neither recommending nor condemning the supplier.

Does anybody out there know of another supplier with whom they have not had a similar experience?

[K2-K6]

I've bought and used fuel pipe from here

https://www.ebay.co.uk/itm/405924654488?var=676233395444

Honestly described and so far it has not been affected by deterioration.  It needs more evaluation from anyone on here to back that up, more specifically in leaving it containing fuel over greater length of static/low use scenario in providing more extensive feedback.

So far, no observable failure for me.

[McCabe-Thiele (Ted)]

I used Gates Fuel Pipe as it was fitted to  my 500 by Gerben iirc. .

https://www.ebay.co.uk/itm/334357613189