1981 cb650z - carbs

[blackeagles]

I suspect I know the answer to this but I thought I would post my problem as there may be something I’m missing.

I have never had much luck with 4 cylinder carbs (3 too many) but I spotted a tatty 1981 cb650z so against my better judgement I thought “how hard could this one be?” so I have spent the last few months cleaning it up and now I have got to the carbs.

OK, the problem, it ran and ticked over well without any smoke/hunting or choking so I was going to leave it but being a meddler I had to check it and the carb sync showed 1 and 4 twice the value of the base carb and 3 just a little high but the real thing I could not live with was the slow running screws were set between 3 ½ and 4 ½ turns out ( the correct setting is, I think, 1 ½ out).

So anyway I re-synchronised them with the screws at still out and it ran hwhen hot so I reset the screws to 1 ½ and big surprise it no longer runs. It starts on full choke but if I touch the throttle it stops even when hot.

I’m going to guess they will have to come off but before I do that any other suggestions or things to check?

[SteveW]

If it’s PD50 carbs like on my 1980 650 the accelerator pump jets are blocked.
That’s exactly the symptoms I had, open the throttle and the engine dies.

[blackeagles]

It is the pd50 carb and I had forgotten the acc. pump.  I will have to drill out the screw in the clamps (rusted) and check.

[SteveW]

I was on the point of giving up on my 650 and dumping it in the back of the shed.
My carbs must have been on and off a dozen times.
The only way I found to unblock the accelerator pump jet was to separate all 4 carbs. The hole left by the aluminium tubes between each carb, connect a piece of hose connected to a large syringe.
In the syringe caustic soda mixed with water.
Apply pressure, nothing will happen then it will suddenly start to flow as the blockage clears.
Immediately rinse with water as aluminium doesn’t like caustic soda.

If you do this method ALWAYS add the caustic soda to the water, never water to caustic soda.

[blackeagles]

well, I'm not sure if its good or bad but I borrowed one of those cameras of a flexi connection and the acc. pump jets that point into the carb are clear as I can see a jet of petrol firing out of them into the carbs.

Its an odd thing but it was running well with the sync and pilot jet settings so far out originally - 1 and 4 were twice the reading of 2 and 3 AND the pilot screws had 1 and 2 set at 4 1/2 out  and 3 and 4 set at 3 1/2 out so you would have thought it was running very rich at tickover but it was not!!
 

[SteveW]

Sure my pilot screws are 2 1/2 turns out.

[Oddjob]

Sounds like there is a blockage in the pilot circuit, having the screws so far out partly corrects it.

[SteveW]

Have you removed the pilot jets? They are pressed in but come out pretty easily with. Hot air gun.

[blackeagles]

I'm hoping to try anything other than taking them off as the air box is built onto the rear mudguard and there is very limited space when you loosen the connector from it to the air chamber - maybe 5mm - which is supposed to be enough to 'wangle' the carbs off.

as I said earlier, it does run allbeit with odd settings so I'm firstly going to try taking the pilot screws out and spray carb cleaner into them and reset it all to a basic setting and see what happens

[blackeagles]

Sure my pilot screws are 2 1/2 turns out.

well the carb cleaner may have worked - not really possible to tell - but if I set the pilot screw to 2 1/2 out (like you have) it runs well enough allbeit with a 'spit back' or two when its cold but needless to say now number two overflow is pouring petrol out so the needle is jammed down again.

talk about a step forward and then one back!!

[McCabe-Thiele (Ted)]

If it's like the 400 the needles can stick if the bowl is low on fuel. I find a shake left & right fixes it until I run the bowl dry again or similar.

[blackeagles]

Well, just an update - I have finally taken the carbs off (what a struggle) and they are all clean, not even dirt in the bowl.  All the jets including the pilot (which I took out) are clear.

Sure my pilot screws are 2 1/2 turns out.

I'm going to set them at 2 turns out for the time being (I am assuming the more out it goes the richer it is?!?)

Anyone else with a PD51a on a 650/650z and know what setting the pilot screw is?