Doms CB550F1 project

[K2-K6]

The absolute airscrew setting has always been part of adjustment scheme in reality, rather than solid set and forget.

With variation likely on component now in comparison to production line original, then highly likely that set point is going to be different.

Bore of jet, bleed holes, airflow passages, air filter performance, piston ring sealing, fuel burn characteristic, ethanol inclusion (contains oxygen over and above no e types) will all contribute to a setting that may be different to "book" value originally given.

The cleaning  of the jets and air passages are also a scource of variance too.

Establishment of a set for what you've now got in place should effectively accommodate these.

Fuel injection system do exactly this now, with more or less the same methodology as Honda originally published in their manuals, just automatic in itself via ecu programming, built in sensors etc.

Like these carbs though, if the sensible adjustment fails to yeald a competent combustion before reaching the end of logical steps available, then it'll usually throw emissions/check engine code and warning to request further work in establishing what is the real cause of going out of range.

[DomP]

Thanks Nigel, that's one hell of a reply.  Carbs are coming off this week if I get a chance so that I can eliminate my bench sync from the equation

[DomP]

Is there a way of testing if my standard Keihin pilots (38) are within spec and not worn or over cleaned to a point they are now significantly larger?

[McCabe-Thiele (Ted)]

I believe the concensus is that the screw adjustment range will compensate for normal wear.

That said I know one member used one 38 and four 40 jets to avoid a rich running cylinder. The air screws for the 500 are a very different design  to the 550. My 500 was fitted with the 550 brass solid flat ended screw. Courtesy of Ken I have fitted the correct ones that are hollow with side holes.

[DomP]

That confirms my thoughts Ted, I suppose it would run richer as the pilots wear meaning screws out further to compensate.

[DomP]

Having sorted the rpm on my Koso gauge I set about trying to rectify the hanging idle I've been experiencing. As much as it pains me I removed the carbs (again!) and checked/adjusted the float height being even more anal than the last time, no2 carb had a float with a chunk missing out if it so I replaced it with a used original.  Having refitted the carbs and opened the fuel tap I still have a hanging idle,  it's absolutely fine on start-up but any blip of the throttle won't return to idle, nothing mechanically is sticking and a spray of carb cleaner around the carb boots doesn't change the idle. I'm beginning to lose patience with it.

[Johnny4428]

Are carbs balanced too Dom?

[McCabe-Thiele (Ted)]

I have little experience with no formal qualifications on bikes, with that disclaimer  heres what what I think.

I'm often wrong with a relatively short experience on only two Hondas in the last four years.
I am in my seventh decade of experience on a variety of cars and at the limit of my memory from bikes during the 1960s & 1970s. I do eventually admit when I am wrong.

My perception now seen through the prism of old age & the many posts on this subject is roughly as follows.

These bikes are fitted with four carburetors that at a basic level have two fuel supplies to the engine.

1) The first is what I think of as the idle jet circuit - this supplies fuel not just at idle but at significant amounts as you increase the engine revs with a sort of reducing influence  as the main jets take over.

2) The second fuel supply is the main needle & jets that operate by the sliders.

Assumptions - there are no air leaks on the induction side with standard air cleaners, original jets & needles in good condition. When hot the open & closing throttle cable are working & not binding. The ignition advance/retard is working as it should & timing is spot on at idle & full advance settings.

A hanging throttle is confusing as if the sliders fully  close when you shut off the throttle the engine rpm should fall relatively quickly back to a normal idle as no fuel is being provided by the main jets as the needle is effectively shutting off the fuel supply.

Contrary to logic I believe a hanging throttle is caused by a lack of fuel or conversely too much air supply - a weak mixture at that point as the revs should fall instead  they take a while to drop from say 2,200 rpm to 1200 rpm.

So how does the engine manage to maintain this hanging throttle?

I think it is as a result of a tad too much air passing through the intake under the sliders when the throttle fully closed, this draws fuel via the idler jet circuit as well as an unknown amount from the main jet in some situations.
This might be as a result of 1.2.3 or all 4 sliders being too wide open in the throttle closed position.

It's possible to think you have the air screw spot on when you do the bench sync by reducing the fuel flow by opening the air screw you can get an a false idle. When the engine is up to full running temperature this in my view points to the slider gaps being just that bit too wide. So it's all good when warm but after a fair run the idle gets too fast.

If when the main throttle stop screw is completely undone the sliders are fully closed then as no petrol can get through it will of course not even idle but stop.

I think the trick is getting the least amount of slider gap yet maintaining a tickover contolled by the air screw adjustment only - on a engine that is up to temperature.

I'm waiting for incoming - my shields are up !


Just edited some poor spelling & sentence structure for clarity

[DomP]

I've bench sync'd the carb Johnny, I think there is some good information in Ted's comprehensive write up, I need some good weather to get it upto temperature really.  Not sure when that might be but hopefully this week.  Thanks again Ted!

[Bryanj]

I have had it where the carbs have been bench synced with the slides a tad too high, make sure all the slides can bottom out with the big tickover screw unadjusted

[DomP]

Yes I can kill it if I wind the idle screw out, pretty damn sure I've got no vacuum leaks too as I've gone around it with carb cleaner and not noticed any change to revs.  I need to get a good free day to fully dedicate to going over it with a fine tooth comb.

[McCabe-Thiele (Ted)]

Yes I can kill it if I wind the idle screw out, pretty damn sure I've got no vacuum leaks too as I've gone around it with carb cleaner and not noticed any change to revs.  I need to get a good free day to fully dedicate to going over it with a fine tooth comb.

I've added an additional assumption to my analysis that I omitted -  you need to have the original mechanical advance & retard this needs to be in good working order or an electronic equivalent that works.

[DomP]

One other thing of note is that it is far too easy to start, no choke needed, whether that points to the shield being a little too open or if its just that it's running too rich?

[K2-K6]

If you start it from cold, run it 30 sec, switch off and then check the header pipes for temperature (basic spit on your thumb and touch will do) what have you got ?

Any cold ones  ?

[McCabe-Thiele (Ted)]

I am not saying this as a thing but both my bikes need full choke to start up from cold as a first start of the day. As soon as the engine is running I reduce the choke as necessary. If I switch off after half a minute or so no choke needed to fire her up again if it's within a few minutes. might need some slight choke to ride off