One too rich, one too lean...

[Alastair]

Hi all, I could do with some advice as this is pissing me right off! I have two carbs, both freshly rebuilt to the same spec, 25mm venturi, 120 main, 1mm air corrector, 1st stage untouched, 2nd 1.2mm. FPR Is set at 5 psi at idle, float vv is std 1.7 and new in both for now. Neither of them will idle properly...

Carb 1
Runs fine when put on, then for no apparent reason goes way lean I have checked the idle jet is clean, it is, and blown through the idle circuit with my air gun, which gets it to run for a second or two then way lean and dies. I have stripped this carb two or three time (immaculately clean) and even changed the throttle plate assy, and still no good. Oh and checked inlet gauze is still there, and clean and also have a new fuel filter.

Carb 2
Idles at 9.7 (rough) and I have no leaks into the venturi now, it is dry. No matter what I do this carb won't lean out, and when driven won't get above 12.5 AFR. I cannot understand how this carb is overfuelling now as I can not see where it is coming through.

Can any one offer some advice! I am getting fed up with hanging carbs over ATM.:coffee:

[Nayls]

mine needed air blasting through it about 7-8 times to be clear.

also,you could have an air leak in the breather system.

[Alastair]

mine needed air blasting through it about 7-8 times to be clear.

also,you could have an air leak in the breather system.

Cheers Nayls, I think that's my only option on carb 1, were you blowing it through from the idle jet with air?


Anyone any suggestions about carb 2?

[Alastair]

Anyone any thoughts on what would cause a carb to run rich even if the back of the throttle was dry? Could the new float hb be leaking and forcing fuel through the idle circuit?

[Alastair]

Anyone?

[Nayls]

is the mix screw in carb 2 ok? not snapped a little of the end?

standard size idle jet??

[Matt Cole]

Get then in an ultrasonic tank for a good clean

[Alastair]

is the mix screw in carb 2 ok? not snapped a little of the end?

standard size idle jet??

Yeah I put a new mixture screw in to be sure, and have changed the idle jet over 3 times with others. Still confused about why carb 2 is running rich
?
Managed to get carb 1 running ok now, loads more blasts with air.

[Sparkie]

the throttle plate assembly can be put in several positions, check to see that the gap between the edge of the throttle plate and the edge of the carb base is the same.

also, if the float chamber is not sealing properly, it will cause fuel to drip onto the top of the throttle plate and cause rough running.

[Alastair]

the throttle plate assembly can be put in several positions, check to see that the gap between the edge of the throttle plate and the edge of the carb base is the same.

also, if the float chamber is not sealing properly, it will cause fuel to drip onto the top of the throttle plate and cause rough running.


Cheers sparkie, just had a look at the assy and the carb base and throttle assy all line up nice and evenly on the mega rich carb. There is no fuel dripping down the throat or from the top of the venturi, the back of the throttle plate is dry... I just can't understand why the carb is idling so rich?

[Brigsy]

i had the lean problem, turned out the hole that is above the mixture screw in carb base was blocked. I had to strip full carb down & check everything to find the fault!

Is the idle jet the correct size in the rich carb? Also what is your idle speed set at, Too high & mine is always rich.

[Sparkie]

ive seen 44 and 45 stamped idle jets...

[James5]

ive seen 44 and 45 stamped idle jets...

So have I along with a 46 aswell

[Brigsy]

I have a .46 but when i tried a .45 drill bit it wouldn't fit:laugh:

[Alastair]

Not sure but I put the lean carb idle jet in the rich one and still no better. Although the lean carb is now running ok!
I'll look at the markings tomorrow...

[fishead]

with the mix screw fully in, how far out does the needle part protrude out ?