SOHC.co.uk Forums > CB500/550

Ongoing PD46 carburettor woes (possibly)

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K2-K6:
Obviously from a remote perspective,  but it seems to me that the  carbs are doing what they should which is why I feel the need to understand if the cyclinder is ok.

Carbs in situ you could check that you've got decent filling of #1 if you can get the float bowl off.  Slow filling would ordinarily give the opposite problem though of reaching level while standing and then depleting while running faster,  the inverse of your experience here.

Thinking aloud,  if it was overfill while sitting to make it too rich when you start it,  then improving as you ride faster,  then possibly the fuel leveling out too high in #1.

A question about the carb on #1.....if you get the overflow pipe from that float bowl,  can you blow air into the carb?  Just clean the end of it and blow with your mouth. Looking to see if there's any impairment to float bowl venting.

Reason for compression test,  if you've a significantly compromised cylinder operation then you'll chase the carbs forever and not get it.  Psi for each cylinder will at least give a baseline overview to chase or eliminate.

keithtraffic:
Thanks - I should have access to a compression tester at the weekend so will check that out (as you say it will help to eliminate one possible area of concern).

I did take great trouble to check that the overflows were working and clear before I reassembled the carbs and great care in ensuring that the floats were set to the right height, but will go round the loop again as you suggest just to make doubly sure.

Does using a clear tube connected to the overflow and holding it up to see what level the fuel comes up to give a proper indication of fuel level on the carbs?

K2-K6:
"Does using a clear tube connected to the overflow and holding it up to see what level the fuel comes up to give a proper indication of fuel level on the carbs?"

No,  the internal pipe sits above the normal fuel level of the carbs (essentially like a wier) so that should the levels rise too far they would go out that exit and not run into the carb throat to the engine.

There's usually a screw on base of float bowl that you'd have to connect clear pipe to in order to monitor live fuel level.

keithtraffic:
Have now checked the compression after warming up the engine and with the throttle open...

Cylinder 1  165psi
Cylinder 2  155psi
Cylinder 3  165psi
Cylinder 4  160psi

(With the throttle closed all cylinders are the same at roughly 90psi)

So all a fraction low compared to the manual figure of 170psi but I wouldn't have thought the numbers are anything to worry about - and cylinder 1 definitely does not have low compression compared to the rest. I did try squirting in a little oil into the cylinders and that did raise the readings across the board by 20-30 psi.

Another symptom I have just noted is that at tick-over and at speeds up to 2000-3000 revs No1 cylinder is softly backfiring / emitting a sort of sputting sound.

I also tried the trick of blowing into the overflow tube and its definitely clear.

So not sure if this sheds any more light or if really the next stage is to (reluctantly) remove the carbs again and have another close look - is there a diagram or anything of the PD carbs showing all the airways etc and where it should be possible to blow / direct cleaner through to ensure all the fine passageways are definitely clear.

K2-K6:
I'd have confidence in those compression numbers, all appears reasonably in order.

"Actually no-1 cylinder is hardly firing - its exhaust  pipe is still cool after a couple of minutes whilst the other three cylinders are all hot. just to be sure this wasn't anything to do with the plus I have swapped 1 & 4 plugs over but to no avail.

To explore a bit further I did remove the screw where you would attach a vacuum gauge and spray some carb cleaner in and immediately the revs picked and No1 cylinder fired up, perhaps suggesting an issue with the fuel delivery to that cylinder. I have also checked that there is fuel in the cab (by opening the drain screw) and confirmed there is a regular flow of fuel"

Before reluctantly investigating the carbs further and reacting to the above,  it's worth flipping the coils so that 1 and 4 leads are swapped across the bike to eliminate anything in the number one lead ( does missfire swap to 4) to make sure that's not catching you out. They're obviously fired at the same time but possible to have impairment on one lead and not the other.

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