Balancing carbs and adjusting idle

[Omegasteve]

I'll see if I can get a pic of where I set mine to

[bluedog59]

Was there much difference between your "by eye" settings and the ones using the gauges 

[Omegasteve]

10 minutes ago, bluedog59 said:

Was there much difference between your "by eye" settings and the ones using the gauges 

Oh absolutely

[bluedog59]

Ok, theory time (and we all know theory and real world don't always tally up).

Carbs butterflies set evenly by eye should give roughly the same amount of resistance to flow (vacuum).

Butterflies set to achieve equal vacuum at point of measurement at carb vastly different to each other.

That suggests (in theory) that there may be an air leak between carb and head on the cylinder(s) that needed the butterflies closing to balance them using the gauges.

[Omegasteve]

4 minutes ago, bluedog59 said:

Ok, theory time (and we all know theory and real world don't always tally up).

Carbs butterflies set evenly by eye should give roughly the same amount of resistance to flow (vacuum).

Butterflies set to achieve equal vacuum at point of measurement at carb vastly different to each other.

That suggests (in theory) that there may be an air leak between carb and head on the cylinder(s) that needed the butterflies closing to balance them using the gauges.

That sounds very reasonable to me, so we are pointing fingers at the manifold seals again? I have now ordered new seals and I'll inspect the couplings thoroughly (although they look fine to me) just to put mind at ease. 

[Omegasteve]

Just now, Omegasteve said:

That sounds very reasonable to me, so we are pointing fingers at the manifold seals again? I have now ordered new seals and I'll inspect the couplings thoroughly (although they look fine to me) just to put mind at ease. 

And that would cause high idle and hard starting?

[bluedog59]

If the cylinder is drawing air other than through the carb there is less flow to active the pilot system and high idle is classic air leak.

[Omegasteve]

15 minutes ago, bluedog59 said:

If the cylinder is drawing air other than through the carb there is less flow to active the pilot system and high idle is classic air leak.

Sounds reasonable to me!

[bluedog59]

If the cylinder is drawing air from before the carb there will be little or no depression/vacuum/Venturi effect over the jet. Carbs don't suck fuel through the jets, it is pushed through by the atmospheric pressure in the float chamber against the low pressure in the carb bore. Air coming in before the carb will result in less via the carb = less depression etc = no fuel flow.  At higher rpm air flow via leak falls as % of total flow creating some depression to allow fuel flow but not enough to feed air intake of carb and leak combined. Result ? Weak mixture.

A bit long winded but an explanation of the thinking behind my theory.

[Omegasteve]

1 hour ago, bluedog59 said:

If the cylinder is drawing air from before the carb there will be little or no depression/vacuum/Venturi effect over the jet. Carbs don't suck fuel through the jets, it is pushed through by the atmospheric pressure in the float chamber against the low pressure in the carb bore. Air coming in before the carb will result in less via the carb = less depression etc = no fuel flow.  At higher rpm air flow via leak falls as % of total flow creating some depression to allow fuel flow but not enough to feed air intake of carb and leak combined. Result ? Weak mixture.

A bit long winded but an explanation of the thinking behind my theory.

So when you say before the carb I'm guessing you mean any air that does not travel through the body of the carburettor itself to reach the cylinder? 

[Buzuki]

6 hours ago, davecara said:

Unless theyre damaged, you wont need to replace the carb rubbers themselves, just the O rings. Just be dead careful taking the JIS screws out that hold them on. Dont try and do it with a Phillips screwdriver as it'll cam out and damage the head of the screw

If you cant find the Lidl ones, you'll want 4 of 0928040020 from Suzuki. Theyre only about £4 a throw. might be a quid cheaper off Grumpys

 

 

O Ring(d:2.4,id:43.2)  is the size btw

 

Many times I have successfully solved that chronic false vacuum leak because of those bad O-rings which is hidden behind each rubber boot just by adding layer of quality silicone kit over that flat surface where those O-ring seats , without to replace old ones , my favorite silicone kit is the high temperature Victor Reinz , typ Reinzosil 300 , it is fuel resistant and it is almost as some kind of rubber type of sealing kit which is also very resistant to high temperatures , it is ideal for that position ,

of course the best way to stop vacuum leak is to replace those bad four O-rings with quality new ones altogether using some quality silicone kit ,

and yes special care have to be paid when those eight JIS screws need to be removed , since aluminium cylinder head can be damaged very easy .

Edited December 14, 2022 by Buzuki

[bluedog59]

1 hour ago, Omegasteve said:

So when you say before the carb I'm guessing you mean any air that does not travel through the body of the carburettor itself to reach the cylinder? 

Correct. Where the carb enters the rubber, between rubber and head or through cracks in the rubber.  Have you got the standard clips at the carb/inlet rubber joint ? If you're using Jubilee clips it's possible to overtighten them and create a leak.

Edited December 14, 2022 by bluedog59
Typo.

[Omegasteve]

12 minutes ago, bluedog59 said:

Correct. Where the carb enters the rubber, between rubber and head or through cracks in the rubber.  Have you got the standard clips at the carb/inlet rubber joint ? If you're using Jubilee clips it's possible to overtighten them and create a leak.

No I am using jubilees, but the fit of the carbs into the rubbers is very very snug even before I nip the hose clips up 

[coombehouse]

2 hours ago, Omegasteve said:

No I am using jubilees, but the fit of the carbs into the rubbers is very very snug even before I nip the hose clips up 

Have you tried spraying WD-40 around the rubber boots & the joint with the motor running. The engine note will change if there is a leak as the spray is sucked in.

[Omegasteve]

On 12/14/2022 at 4:00 PM, TonyGee said:

I set the butterflies up on the bench first, open the idle screw a fare bit so you can get a thin drill bit under one, then set the rest the same.  then you can do them on the bike with the gauges.  (dont forget to adjust the idle screw before starting)  

Not the best pics but this is where I set them to before fitting them to the bike. 

Edited December 18, 2022 by Omegasteve

[Gixer1460]

Those / that mixture screw looks like its projecting too far and is possibly screwed in too far - I had a set of RS carbs like that and had to literally dig the screws out cos the tips broke off the rest of the screw. May or may not be anything but could be an issue?

[george 1100]

Yep, doesn't look right. From what I know, that should not be able to protrude like that unless the orifice has been drilled out

[davecara]

13 hours ago, george 1100 said:

Yep, doesn't look right. From what I know, that should not be able to protrude like that unless the orifice has been drilled out

Or the tip of the air screw has snapped off and is jammed in there. @Omegastevewhip the screws out mate and stick a pic up of them 

[Omegasteve]

13 hours ago, Gixer1460 said:

Those / that mixture screw looks like its projecting too far and is possibly screwed in too far - I had a set of RS carbs like that and had to literally dig the screws out cos the tips broke off the rest of the screw. May or may not be anything but could be an issue?

Gotta clarify, that's not the tip of a mix screw. That's just the edge of the larger hole. There's nothing sticking through at all. 

[Gixer1460]

It's a f**king big hole then! Most are, sizewise, like the staggered two at the top !

[Omegasteve]

Just now, Gixer1460 said:

It's a f**king big hole then! Most are, sizewise, like the staggered two at the top !

Looks the same as the holes in both sets of carbs I have? 

[TonyGee]

2 hours ago, Omegasteve said:

Looks the same as the holes in both sets of carbs I have? 

looks ok to me, at the end of the day its the size/shape of the mixture screw that matters. 

this is the hole in a 34mm carb body.