What have you done with your bike today?

[Brockett]

VX800:- I spoke too soon. At breakfast it came to me that I have replaced the tank without locking the carb balance screw  Having done that I thougth it best to check the setting again. I warmed the engine and reattched the carbtune. With a reading difference within 2 it looks good at idle, 1100 and at 1500 RPM but as I opened the throttle to 2500 to 3000 RPM the no 2 cylinder vacuum went up to 34 and the no 1 cylinder vacuum went down to 14. minor adjustments on the air screw made no differnce. I would imagine the vacuum should increase with engine speed which generates low pressure at the inlets so I would expect the vacuum to increase. Even if the no 2 cylinder was not firing at all the vacuum should still increase. So I'm wondering now if the #2carburettor slide is not opening properly . Considerable strip down of bike to get at the #2 carb. oh! well ... what else is there to do?

It's this sort of thing that reminds me that I know so very little about how all this stuff works. 

[Mr Nick]

It might just be a cable adjustment issue: some differential stretch over the years?
I had to source longer cable adjusters for the V50 to get the slides opening in sync: it has an aftermarket Tomaselli twin pull throttle setup and the cables aren't quite the right lengths.

[kwackboy]

Sounds like a split diaphragm or the cap is leaking air ..

[Brockett]

Do you think I am on the right track in
a) thinking the vacuum goes up as RPM increases?
b) being concerned about the difference of indicated vacuum readings?

[kwackboy]

A) yes you are correct

B) yes, I believe they should be similar.

I presume the vacuum connection is on the inlet ?, as this could be leaking /split.

To detect a leak, let it run on idle then spray WD40 or similar around the carbs / inlets.
The idle should raise if there's a leak .

[Brockett]

thanks.
While I am busy spraying wd40 here is a vid of what the carbtune shows

https://www.youtube.com/watch?v=j5GXWEnANCc&feature=youtu.be

[Brockett]

The WD40 did not reveal any leaks but it does stink when it gets hot. And I did check the top and the  diaphram for leaks - all was good.
I am thinking now that I will have to strip the carb and make sure there is no dirt got in it. I stripped and cleaned both of them but it is possible that dirt got in via the fuel pump which is not something to take apart for fun as it's vacuum driven from the front carb to engine manifold. It's the rear carb that suffers low vacuum pressure.

[Mr Nick]

I can't think of any reason dirty fuel would affect vacuum: it's the air side that is being measured.

That video looks more like there's an imbalance in the throttle actuation - the rear is being opened more than the front for a given input allowing more air flow (reducing vacuum), and goes straight back to spot on as soon as you release, so idle is fine at least.

Check both cables for being seated correctly first (rear could be lifted out a little) and check there's the same free play. Turn the throttle slowly & gently whilst watching the respective throttle plate linkages & see if they both move the same amount at the same time.

[Brockett]

I removed the rear carb to check the socket connection to the cylinder and the other to the air box. They are sound as far as I can see. I removed the float bowl and checked the float cut off height and the cut off needle valve. The screen of which was fairly dirty with what looks like "fluff". I have jetted all the passageways with carb cleaner and found no trace of any other detritus. I will reassemble it and refit tomorrow and go to slack cable settings and start again. Thanks for the tips.

[kwackboy]

I agree with Mr Nick , the gauges suggest to me that the carbs are somehow imbalanced once you open the throttle.

Is there anyway you can run it up without the airbox connected ..?.
You can then check both the slides are moving as they should, then you can also check the butterflies are doing their job.

[Brockett]

Yes KB that is the plan for  first job in the morning.  It has two air boxes one for each carb and a balance hose between them. With one carb over the front cylinder and the other behind the rear cylinder it will not be possible to see both butterflys at same time. And I hope I never meet the girl who can !!

[Rusty Nuts]

You need my friend to help. Isaac Imbo.

[Mr Nick]

That's a bugger. Can you at least see the 2 carbs from somewhere you can turn the throttle? Even if all you're doing is seeing how much you turn each before the throttle plate shaft begins to turn: a marker pen to indicate a point on the grip and put a dot on the switch housing where one just starts to move, then do the other & compare against the dot. It sounds like one is opening earlier I.e. the rear one.

[Brockett]

I try it in the morning . Thanks for the advice.

[Mr Nick]

New axle arrived yesterday & all back together again. Went with the 'in from the left' setup that seems to be how it came from the factory: can't see any reason to reverse it other than just getting it wrong.

[Rusty Nuts]

In from the left?

[Mr Nick]

Yup, put the axle in from the sprocket side as opposed to it going in from the brake side. Nothing more sinister.

[Rusty Nuts]

OK,  it just looks like the threaded end coming through the block there, which would suggest the end with the hex is on the brake side and the spindle went in from the right. (definitely 'sinister' if the left according to the ancient Romans)




Edit.. Just seen how they work - didn't realise one side was fixed in one of the adjuster blocks. (Not done enough miles to need to adjust my chain yet)

[Mr Nick]

Aye, the axle itself is threaded both ends: one is locked into the block and you pass the other through the opposite side adjuster and pop the nut on. This is the axle as it comes:

[Rusty Nuts]

Seems even stranger that it could have split like that.