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4g15 carburetor problem


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Well hello all

I do not know if it is right forum to ask question so mods feel free to move it to proper section if needed.

I have old 4g15 carburetor  mitsubishi lancer station.I know the car is history but brings me lots of memory.

Anyway i have very bothering problem with the car . The car starts fine and run for sometime but after about 15-20 of driving when engine is fully warm the rpm starts to fall lower and lower at idle this continue until the engine starts surging very badly at idle and if i dont play with gas pedal it will eventually die. 

I did not notice any problem off idle and while driving and all seems good.

One extra point if i start driving when this is happening and then comes to stop again the idle seems to become ok but again after 1-2 minutes it starts fluctuating again.

I had took the carb apart and cleaned it thoroughly and changed float/needle and some gasket and adjusted the float but it did not cure the problem.

 By the way it is aisan variable god damn venturi carb .

Thank you for any help

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Welcome to the forum. If you search the forum with string bothal carb you will find a few previous posts on the subject. The term bothal carb BTW is the local jargon for variable venturi carb.  Does the engine idle speed rev up at the cold start and then gradually come down after a few minutes? Can you identify the automatic choke mechanism (IACV) which is attached to the side of the carb with two coolant lines connected? If so you should be able to visually observe its operation beginning with cold start to fully warmed up by keeping an eye on its cam mechanism. Either you have an issue with IACV or you have a vacuum leak somewhere (that may be why idle returns to normal when you stop). Are all your vacuum connections in tact ie they are properly connected and not by passed or blocked? Also check if there is a vacuum leak in your brake booster (How to perform a DYI check is explained here http://forum.autolanka.com/topic/15409-break-issue/#comment-245590.)


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Thank you Twin Turbo and Rumesh88

I forgot to mention something i had another carb in garage and swapped them for a test that other carb does exact same thing

Maybe the other one has the same problem as i had played too much with that one too.

 Twin Turbo  does the coil problem just happens at idle ? should not it have any other symptoms when driving too ?

Rumesh88 i have observed the choke and the cam goes out when it is hot , but there is one gotcha when starting engine cold the choke does not seems to be acting correctly at first the idle is very low after starting to drive the idle gradually starts going up  to 1200-300 and remains there for sometime then it starts falling and the main problem as i described starts to happen.

Some how the choke seems to act opposite  first it is low then goes up and it took a little long time to comes down again.

I had the choked removed and cleaned .

I used some carb cleaner to check for vacuum leak and could not find anything but i have not checked the brake booster you mentioned , i should check it.can it be a rich condition as i can smell fuel from exhaust clearly  ? or maybe lean ?

One another point which is little hard to explain , when im in parking ramp which is steep the same things starts to happen and car starts surging very badly i doubted may be its float and needle but i have changed them anyway.


Best regards


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13 hours ago, dimebag said:


One another point which is little hard to explain , when im in parking ramp which is steep the same things starts to happen and car starts surging very badly i doubted may be its float and needle but i have changed them anyway.


Best regards


Park on level ground and move the carb piston by hand a few times and note the faint noise it make when hitting the its travel limit (home position). Repeat the same on a ramp and see if the piston stops before its limit. You can easily do it with the spare carb. If the piston fails come to the limit each time you may have an issue with the linear bearing or it could be due to lack of tension in return spring etc. However, you seem to have an issue with IACV. Check if the vacuum line connected to IACV is leaky or connected to the wrong point. Do you have a vacuum hose diagram? (I have one but it is at the office and decided to stay at home today because of fuel shortage).

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You are really a great help Rumesh88

I was not able to find anyone with enough knowledge about these carbs.

Thank you again

yesterday i went ahead and removed the carb again I had noticed the piston had some issue and get stuck at different position randomly.

I cleaned it again and it still had issue i decided to take the main jet out , after removing the piston movement become totally smooth then i noticed the needle is somehow deformed and it gets stuck in main jet .I do not know how it did happen as i had never touched the needle .

I decided to do a fool thing and added second spring for the piston but after mounting it on a car it was black smocking like hell at idle and after one minute the carb flooded and engine died also killed my new sparks in a matter of minutes they all became fully black.

Im going to test again with the spare carb i have .

Just if you can take me out of confusion regarding 3 things :

1- that air/fuel screw what does it actually control fuel or air ? i cant find anything about it

2- does main jet affects idle too ? as it seems so

3- what exactly causing black smoke on these carbs ? i tighten both main jet and air/fuel screw all the way in and car was still running and smoking black i think the engine should die when both screws are fully closed. can it be because of orings ?

About  IACV  where is it located ? i can not find it.

If you can give me vacuum lines diagram that would a great great help as these carbs have really no documents over the web.

Best regards

Edited by dimebag
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It is really nice to see someone doing  DIY on these carbs. In fact I have a few documents in my laptop which will provide answers to most of your questions. Please send me a PM with your email. I have another document with more descriptive vacuum diagrams which I will send tomorrow when I get back at the office.

In these carbs fuel flow is controlled by the main jet and the needle. If your needle is bent or worn out you will not get the required results. Also the needle should not come into contact with the jet over the whole length of its travel. The DIY method I follow to check linearity and alignment is to set the piston on a turntable (which is an old hard disk with the cover removed but the disk still on its good bearings and mounted on a heavy wooden block) and project its enlarged shadow on to a wall with a point source of light (nothing but a small LED - SMD one or one  with its lens filed off and polished). With this set up you can check if the needle is bent by rotating the turn table. You need not have the same set up but can improvise on the general idea. 

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Dear Rumesh88

Thank you so much for the docs.

I went ahead and started to test some more , i started to think what can really be wrong as i had tested with two carbs and both had same issue and the last one was not even starting even the one i had rebuilt totally with new parts had the same issue.

Guess what i had nearly swapped , renewed anything but i forgot to test idle valve solenoid and i was using the same for both carbs i went searching through my old stuff and found another one it was not in good shape and i had to solder the wires as they were cut .i removed the one from carburetor and used the old one and BAM it started working without a problem i even tested the steep ramp and it was ok .

Strange thing is i had checked the solenoid and i could here it clicking maybe it had some internal issue which caused it to connect/disconnect .

For now everything seems good though i still need some fine tuning as i can smell a fuel but it should not be that hard .

I really hope this topic helps someone as i had been struggling with this issue for months and never suspected that damn solenoid.

You have been great help man.

Wish you best .

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  • 1 year later...
On 4/25/2017 at 9:13 PM, Rumesh88 said:

Glad you could sort out the issue. I also totally forgot about the idle fuel cutoff solenoid. I have already emailed one document and will sent the other one tomorrow.

Hi rumesh. Would you like to help me also about vaccum diagram lanver 4g15 soch carburetor? Thanks [email protected]

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