Engine Stumbles When Picking Up From A Stop

GAPAUTO · May 17, 2014

Hi Guys,

Recently i did a carburetor cleanup using carb cleaner and changing the JETs and gaskets. I did the same 2 years back and after fitting the carb back the engine ran fine. But this time after fixing the carb the idle seem to run smooth. But when i pickup the car in 1st gear the car stumbles a bit (hesitate) and then returns to the rpm which it should according the throttle position. This only happens at certain angle in the throttle movement as i have notice. But pushing Gas pedal beyond that angle accelerate the engine just fine. What could be the reasons to have such symptoms ? I checked the accelerator pump which seem the work as expected.

Engine Model : Mitsubishi 4G12

GAP.

Rumesh88 · May 17, 2014

Does it happen even after the engine is warmed up? If so there could be an issue with your fast idle port. Are you doing a DIY job? If so with the engine fully warmed up open the air cleaner and listen (or observe) to the onset of fuel delivery from the primary venturi while slowly opening the throttle. If the hesitation is just before the primary venturi begins to deliver fuel you may have your fast idle port clogged up. If the hesitation happens only during a cold start you need to check the choke mechanism.

GAPAUTO · May 17, 2014

On 5/17/2014 at 3:43 PM, Rumesh88 said:
Does it happen even after the engine is warmed up? If so there could be an issue with your fast idle port. Are you doing a DIY job? If so with the engine fully warmed up open the air cleaner and listen (or observe) to the onset of fuel delivery from the primary venturi while slowly opening the throttle. If the hesitation is just before the primary venturi begins to deliver fuel you may have your fast idle port clogged up. If the hesitation happens only during a cold start you need to check the choke mechanism.

Thanks Rumesh, It happens even when the engine is warmed up. I will check tomorrow as you have mentioned when is the hesitation is actually occurring.

GAPAUTO · June 1, 2014

On 5/17/2014 at 3:43 PM, Rumesh88 said:
Does it happen even after the engine is warmed up? If so there could be an issue with your fast idle port. Are you doing a DIY job? If so with the engine fully warmed up open the air cleaner and listen (or observe) to the onset of fuel delivery from the primary venturi while slowly opening the throttle. If the hesitation is just before the primary venturi begins to deliver fuel you may have your fast idle port clogged up. If the hesitation happens only during a cold start you need to check the choke mechanism.

Yes i just checked it and it just before the primary venturi starts to deliver fuel. This happens until the primary venturi is fully operational. So i guess its the fast idle port which is clogged as you pointed out. Is the fast idle port is above the throttle plate ? And can this be clogged because of the deposits which flowed when i was spraying card cleaner ?

Rumesh88 · June 1, 2014

On 6/1/2014 at 6:46 AM, GAPAUTO said:
Yes i just checked it and it just before the primary venturi starts to deliver fuel. This happens until the primary venturi is fully operational. So i guess its the fast idle port which is clogged as you pointed out. Is the fast idle port is above the throttle plate ? And can this be clogged because of the deposits which flowed when i was spraying card cleaner ?

Yes. It is above the throttle plate when the carb is on idle. While a clogged up fast idle port is the most likely cause please keep in mind that there could be other things contributing to this hesitation. The transition from idle ports to primary venturi is set by the design so that it happens in a smooth manner. But in some cases you may initially get large droplets of fuel from the venturi. In that case you may have to take a look at primary circuit emulsifying tube too. Also check the idle circuit air bleeder at the top of the carb (you will have to remove the top part to get access to it). BTW instead of taking the carb apart again, try to get it cleaned externally first.

Edit: The easiest way to clean both idle ports is to remove the idle mix adj. screw and insert the carb cleaner nozzle all the way in first. Then spray liberally to clean the area near idle port and fast idle port (both are internally connected). Re-install the mixture screw and adjust as necessary.

Edited June 2, 2014 by Rumesh88

GAPAUTO · June 7, 2014

On 6/1/2014 at 2:49 PM, Rumesh88 said:
Yes. It is above the throttle plate when the carb is on idle. While a clogged up fast idle port is the most likely cause please keep in mind that there could be other things contributing to this hesitation. The transition from idle ports to primary venturi is set by the design so that it happens in a smooth manner. But in some cases you may initially get large droplets of fuel from the venturi. In that case you may have to take a look at primary circuit emulsifying tube too. Also check the idle circuit air bleeder at the top of the carb (you will have to remove the top part to get access to it). BTW instead of taking the carb apart again, try to get it cleaned externally first.
Edit: The easiest way to clean both idle ports is to remove the idle mix adj. screw and insert the carb cleaner nozzle all the way in first. Then spray liberally to clean the area near idle port and fast idle port (both are internally connected). Re-install the mixture screw and adjust as necessary.

I removed the carb today and checked for the port above the plate, and in first spray there was no visibility that the spray came out. but then i close the port below the plat by a finger and sprayed again which made the spray come out of the port above the plate. Another question is the emulsion tube, is there a o-ring ? cause there is a o-ring in the kit i bought but in the old removed parts there was no o-ring with the emulsion tube ?

Rumesh88 · June 8, 2014

Is it a O-ring of about 1cm dia? If so it should be for the electrically operated power valve of a different carb model.

GAPAUTO · June 8, 2014

On 6/8/2014 at 5:49 AM, Rumesh88 said:
Is it a O-ring of about 1cm dia? If so it should be for the electrically operated power valve of a different carb model.

No it was actually 5mm. But the i found out that it doesn't belong to emulsion tube since there was no such o-ring in the removed parts. By the way there were some more air passages which has varnish in them and i sprayed them clean. And i have messed up the balls in the pump cylinder and discharge chamber. i had balls in the same side (small size). But after reading some articles about AISAN carbs i found out that the big ball should be in the discharg chamber. And also the small ball in the pump piston chamber was stuck with varnish. After cleaning and fitting it back, now it runs like a kitten . Thanks for all the help Rumesh

Rumesh88 · June 8, 2014

On 6/8/2014 at 10:02 AM, GAPAUTO said:
No it was actually 5mm. But the i found out that it doesn't belong to emulsion tube since there was no such o-ring in the removed parts. By the way there were some more air passages which has varnish in them and i sprayed them clean. And i have messed up the balls in the pump cylinder and discharge chamber. i had balls in the same side (small size). But after reading some articles about AISAN carbs i found out that the big ball should be in the discharg chamber. And also the small ball in the pump piston chamber was stuck with varnish. After cleaning and fitting it back, now it runs like a kitten . Thanks for all the help Rumesh

Congrats mate! It is really nice to hear from someone who is into DIY carb rebuilding. That's a dying breed in fact. Keep up the good work and keep us posted on your DIY jobs.