Jump to content

Balancing carbs and adjusting idle


Omegasteve

Recommended Posts

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.

Link to comment
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. 

Link to comment

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.

  • Like 2
Link to comment
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? 

Link to comment
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 .

Reinzosil 300.jpg

Edited by Buzuki
  • Like 1
Link to comment
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 by bluedog59
Typo.
Link to comment
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 

Link to comment
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. 

20221113_162509.jpg

20221113_162522.jpg

Edited by Omegasteve
  • Like 1
Link to comment
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. 

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...