"Flat Spot" in Aisan Carburetor

[Tham]

My current 3K carb had been giving erratic idle
problems, so I have just reinstalled my older 4K
Aisan carburetor, which had been lying idle in my
room for a year. This carb was actually purchased
new two years ago.

The older carb idles somewhat better, but it tends
to have what I believe is called a "flat spot". As one
opens up the throttle pedal just above idle speed,
the engine will hesitate, sputter and shiver a bit, idle
speed dropping way down for a moment, before it
revs back up as the throttle is opened further.

Might this be caused by a dirty or faulty accelerator
pump ? Or perhaps also a dirty "progression hole"
like the type found in Hitachi carburetors ?

http://dimequarterly.tierranet.com/a...ing_guide.html


I wonder if spraying carburetor cleaner copiously
into the throat might help ?

I've also thought about pouring 3 or 4 bottles of those
tank-type carb/injector cleaners into the fuel tank at
one go to raise their concentration in the petrol, and
hopefully improve the cleaning power enough to
flush the carb. Might this be safe to the fuel rubber
and metal piping and fuel pump ?

These Aisan carburetors are really problematic.

[Pure_In_Sanity]

Check to see if the accelerator pump lever is not bent. I recall a few people having a similar problem where the lever was already down before the throttle was opened.

Just a thought

Cheers
-Phil

[love ke70]

its your tune
mine used to do that, and i cant for the life of me remember exactly why but try tuning it yourself or getting it tuned
it may be that throttle lever arm that gives a little squirt of fuel but i dont think so

[toddw]

Hi

With the car not running remove the air filter lid and shine a torch down the carby. You should be able to see the accellerator pump squirting a stream of fuel when you open the throttle. This is required because the initial air stream reacts faster when the throttle butterfly is opened and leaves the fuel behind that is metered through the primary main jet. So the accellerator pump tries to account for this delay. This sounds as though this is your problem especially that the carby has been sitting around. Have a look as described and see if this is the problem. I hope this helps.

Cheers

Todd

[sam-131]

sounds to me like it's running too rich
off the top of my head i think that the idle jets that are too big

[love ke70]

yeah, i think rich is right makes it bog down then frees up

what motor is this on tham?

[sam-131]

best bet to not to even think about those heap of poo Aisan carbs and buy your self a weber 32/36 there worth there weight in gold fully tuneable i've got one and i've never had a drama

[love ke70]

32/36 is usually a bit big on a 4k and needs jetting down...wat motor r u using it on man'?

[Tham]

Check to see if the accelerator pump lever is not bent. I recall a few people having a similar problem where the lever was already down before the throttle was opened.

Just a thought

Cheers
-Phil

Thanks. I checked and the accelerator pump lever
didn't appear to be bent - it looks fully up at the
closed throttle position.

[sam-131]

32/36 is usually a bit big on a 4k and needs jetting down...wat motor r u using it on man'?

32/36 is good for 1.2 to 2.4lts i got mine on a rona motor (2sc) bog stock bar a shimed head and an exhust

you should allways rejet a carbie if it wasnt jetted for specific engine my carbie was on a l20 and it need the primarys and seconderys fixed up it was allover the shop

[Tham]

Hi

With the car not running remove the air filter lid and shine a torch down the carby. You should be able to see the accellerator pump squirting a stream of fuel when you open the throttle. This is required because the initial air stream reacts faster when the throttle butterfly is opened and leaves the fuel behind that is metered through the primary main jet. So the accellerator pump tries to account for this delay. This sounds as though this is your problem especially that the carby has been sitting around. Have a look as described and see if this is the problem. I hope this helps.

Cheers

Todd

Thanks, Todd.

I went to have a look down the carb's throat
just now. It did squirt fuel when the throttle
was opened, but the stream seems extremely
small and thin.

The squirt response appeared a bit slow in
response to the amount of throttle movement.

I'm not sure if it was due to my finger opening
the throttle lever inconsistently each time, but
sometimes the squirt was lesser, others more,
and yet others none at all.

[Tham]

sounds to me like it's running too rich

yeah, i think rich is right makes it bog down then frees up

what motor is this on tham?

Thanks, Sam and Love KE. It's on a 4K motor.

I opened up the bonnet just now and tried turning in
(clockwise) the idle mixture screw two full 360 degree
turns (any more than that and the idle starts getting
uncontrollably erratic) to make the mixture as lean as
I could, but it doesn't seem to help much.

[love ke70]

yeah my 4k was a bitch to tune.
it may not be the idle mixture screw as the issue is happening as your opening the throttle
]
the jet of fuel is always rather thin so dont stress there

when you were playing with it. when you turned the throttle just a little bit, did u notice the lever arm move and pump in a shot of fuel then the main stream comes in and continues as u open the throttle more
try playing with the other mixture screws and see wat happens
make sure you always remember what you did to each screw so you can go back to square one and try it all again!
g/l mate

[Hurricane]

ok well to set the mixture setting run the car turn the needle in all the way till it stopss, then turn it two turns out and see what its like then? but yeah also see if the accleitaror pump is workin ok, becouse that will mostly be it, me thinks but what ya can do it just pull the carby apart (not that hard) just make a note of were every thing came from then clean all the jets out and make shore that there not block and that, and most of th time ive found that if ya good enough ya can still use tha old gasket to unless ya rip it too bad

OH and ya dont need to seal the gasket with any thing ether.
C how ya go man?

Cheers, DAN

[toddw]

Hey Tham

It should start squirting as soon as you move the throttle. Check the linkages as some of them thad some small amount of adjustment.