Ongoing PD46 carburettor woes (possibly)

[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.

[keithtraffic]

I did think about that but I can't make the No4 lead reach to the No 1 cylinder - is there a quick way round this?

[K2-K6]

You'd have to dismount the coils and physically flip them over to facilitate reaching the plugs.

Maybe of benefit though, run it in dark conditions and carefully view the 1 lead closely to see if you have any leakage.  You can usually see a spark but also hear one too,  like a loose tappet.
If there's obvious spark you can move the lead with a stick to see if the cylinder chimes in. Where they pass rocker cover is normal suspect.

[keithtraffic]

I will give that a go tomorrow - although I am not sure that would explain the situation where I can spray carb cleaner into the vacuum tube screw hole and the cylinder picks up and runs pretty cleanly (for a second or two).

[K2-K6]

Yes was thinking the same as you,  that it fires with spray cleaner.

But to explore what's happening,  cleaner is much more volatile than fuel mix and will fire with veeeery weak spark.  So is it cutting in with cleaner for the above reason (comprised spark),  or because it had no fuelling from carburettor. 

Guess you could go either way to prove / disprove,  one of which involves carb removal 

I had one recently that took an embarrassing time to get it.  On a V8 had similar symptoms of dead cylinder on cold start,  chimed in after a while, same again if left for 1/2  hour, exhaust temperature confirmed the cylinder # ended up finding a break of HT lead just inside the rubber boot entering plug cap.  You could see a carbon track where the spark was travelling too (not ngk caps) once fixed it's absolutely perfect.

Just another quick reality check,  have you swapped the plug caps over between 1 and 4 ?

[keithtraffic]

Well its all a bit bizarre!

I played around with trying to get the coils off and turning them round but that seemed to be bit difficult in practice and as the no-4 cylinder lead almost reached the no-1 spark plug in any case, I carefully put the coils back in place and fashioned a small 'bare wire' extension, just long enough so it would reach the spark plug from the plug cap, taking care to make sure it could touch any of the cylinder metal work.

But before trying this out I though I would put everything back properly, just to re-check the baseline - and lo and behold the engine fire up and after about a minute was ticking over fairly smoothly - a touch fast at 1600 revs but with no indication of the backfire - and cylinder 1 was definitely working as it got as hot as the others very quickly.

Nevertheless did then swap round the plug leads but this made no difference that I could perceive - so put them back properly again and the engine fired up easily on the starter button with no throttle - and then just sat comfortably at 1600 revs.

So - either disturbing the coils and plug leads has done something or perhaps what ever blockage there might have been in the No1 carb has moved - but I really have no idea which it is!

Assuming the weather here tomorrow is OK I will the bike out again and see how it goes - but any thoughts on the above?

[philward]

I agree with K2-K6 ref the maybe a duff lead. I had a break in a lead and worked fine most of the time but on start up and initial slow running, run intermittantly on that cylinder. New coil/leads and instantly cured.

[K2-K6]

Appears that you've got the response needed.

Although not absolutely conclusive,  it certainly is most of the way to confirming that the HT supply to number 1 is compromised intermittently.

The carbs just seem to work as they should with the response you're getting with mixture screw etc. Your diligent cleaning here seems to have payed off.

Coil 1 and 4 obviously fired by same points so low tension triggering not suspect,  but route out from coil down to #1 would be the definite location.  If original coils they are extremely reliable, but certainly not infallible.

Wouldn't be a surprise to see it come back though if it's just got it by moving the leads around.

The fact it "chuffs" now and again when cylinder 1 wasn't firing shows fuel is present in the combustion chamber,  just unlit and passes into exhaust to be later ignited.

[keithtraffic]

Thanks for all the help - took the bike out today and generally ok, but did again develop a little chuffing and reluctant No1 after stopping for a while.

I agree tick over at 1800 is definitely too high. I thought the proper figure was about 1000 according to the shop manual?

I have looked at replacement coils and plug caps and they are pretty expensive for original types -  over £100 each with VAT for the coils and the plug caps are another £25 each.

David Silver has repro ones at a more palatable £30 + VAT for the coils although they don't seem to be handed (same one for both the left and the right side as far as I can see) - has anyone had experience of these?

[deltarider]

... the 600 was to show that with an engine in fine fettle they will go much lower...

You beat me. Once I was able to have it idling at 800. Not that I recommend it. You want that oil up there. Most bikes of that era wore up there, not at the bottom.

I'm toying with the idea of changing the HT leads to red instead of black, they'll match the NGK caps and look a little different.

Long time ago I went for silicone HT leads and dito plugcaps. Great stuff. At the time there was yellow only. Now even Louis has black.

[JezzaPeach]

Hi Keith. Not that you need to but I’ve used a small right-angled Torx bar with a JIS or Philips driver tip to undo all bowls and remove/clean the main and idle jets with a set of abrasive wires/brushes off eBay, but fiddly!
Re the coils cylinders 2&3 had been slower to warm up.
I bought DSS copy coils and just needed to fit female bullet connectors to I think the yellow adapter leads. There is + and - so note. They then fit fine. Cool screw mounts 11cm apart unlike a set I bought on eBay and returned.
Old ones screws tight to undo. A decent JIS T bar driver useful.
Might be possible to recoup some £ by selling the old coils for refurb.
Other issues, a faulty no 2 plug of all things. Have replaced all plugs with non resistor ones, replaced plug caps with NGK resistor ones, and all working well. CB500/4 K1)