Weird Performance Problem.. I Could Use Your Advice

Saturn · December 9, 2009

I drive an old Mitsubishi with a 4G15 carb engine.

Here's how the problem goes:

when I start the car, it performs really well, but after using for about an hour or so, the performance drops a bit:

such that I have to accelerate/rev more -

for example, I have to rev to 2k to 3k RPM to get reasonable performance in gears 2 - 4 - performance with AC is particularly sluggish.

(when the car is started up I can use any gear comfortably under 2k RPM).

Its like the car suffers a loss of power after it is used a bit.

If I pull over and leave the engine running idle, or even off for about 5 minutes, the power sometimes returns to normal.

Notes:

car does not overheat, engine temp management is good/radiator etc, good

I got the carb serviced by a garage (or at least, they claimed they did) and various other stuff done (fuel filter, plug wires, etc)

I've tried various mechanics, none seem to have any suggestions. One guy believes its something to do with vacuum or something (don't know what he means)

Personally I strongly suspect it is something to do with the carb, as - if I tweak the carb, a bit (say, adjust the mixture nut a tiny bit) suddenly it works well.

I remember someone here mentioning someone who fixes carbs... could someone tell me where that guy is located?

also, could the problem be timing related? tweaking the timing a bit sometimes helps, but then the car slowly gets worse again.

Edited December 9, 2009 by Saturn

Tikiri · December 9, 2009

This could be well a electrical problem even rather than a carb issue.

How is your Ignition coil, or CDI unit, condenser, plugs, hi-tention wires and the distributor.

Even with slightly craked cub which may not be well visible there could be a current leak that give rise to

weak performance. Or Distributor is warn off.

Just few things to check, I am sure others will help u.

Supra_Natural · December 9, 2009

Tikiri said:
This could be well a electrical problem even rather than a carb issue.
How is your Ignition coil, or CDI unit, condenser, plugs, hi-tention wires and the distributor.

Even with slightly craked cub which may not be well visible there could be a current leak that give rise to

weak performance. Or Distributor is warn off.

Just few things to check, I am sure others will help u.

Im inclined to think this could be an electrical issue as well, symptoms seem to indicate a worn ignition coil, had almost exactly the same experience a while bak and it did turn out to be the coil. Of course it COULD be a combination of issues as well, but my first instinct is that it sounds like the coil, so you could maybe swap out a newer coil and see?

Sumiya · December 10, 2009

Hi Saturn

It doesn't look any issue in the carb, just see whether you can see the float level of the carb through the glass(if your carb is having it only)if it sits in the middle while your car is in start there will not be any issue on it, and not even vacuum problem

if it is vacuum problem you will always have it while your car is in start,if the carb lower needle is blocked you may have similar issues but as long as you try to move your car it tends to stop or knocking,Just check your ignition coil it might not supply correct amount of voltage when it is hot or check your ignition coil condenser .tell me where are you staying thn ill tell you agud guy for carb adjustment.

Thanks

regards

sumith

Saturn · December 10, 2009

Supra_Natural said:
Im inclined to think this could be an electrical issue as well, symptoms seem to indicate a worn ignition coil, had almost exactly the same experience a while bak and it did turn out to be the coil. Of course it COULD be a combination of issues as well, but my first instinct is that it sounds like the coil, so you could maybe swap out a newer coil and see?

Thank you (and everyone else here) for the helpful suggestions!

Will get those checked out. By the way, do all cars have ignition coils? as I don't recall seeing one in this car.

Also, the weirdest thing is, tweaking the carb mixture a little tends to cure the problem!

still, I will get all the electrical's checked out.

Ripper · December 11, 2009

Saturn said:
Thank you (and everyone else here) for the helpful suggestions!
Will get those checked out. By the way, do all cars have ignition coils? as I don't recall seeing one in this car.

Also, the weirdest thing is, tweaking the carb mixture a little tends to cure the problem!

still, I will get all the electrical's checked out.

yep...all cars and all internal combustion engines have to have a ignition coil...

to generate a hot enough spark to burn fuel...and also get enough current to jump the gap in a sparkplug, you need to have a huge amount of volts.... the dc system in the car is hovering little over 12v and this is not enough to get you a spark...

this is where the ignition coil comes into play...

it takes you 12volts and and convert it into 1000's of volts at lower amperage...

I'm not too sure how a weaker Ign coil issue get resolved by messing about with the fuel mix...

i would guess that when the fuel mix is rich enough, even a weaker spark generated by a weaker coil would get your motor to startup and work ok

Ripper · December 11, 2009

I once had a similar issue with my XR

it would run perfectly but at times it starts to loose power and gets bogged down... sometimes the motor would die and not fire up for a while...

after going through all fuel related issues and general electrical stuff... my mechanic at the time wanted to replace the ignition coil... not too cheap on that particular model...

luckily for me another XR came to the same garage for a minor matter and he offered to test run my one with his Ignition coil...

Even with a perfectly working ignition coil... my problem didn't go away...

then we hooked back my Ig coil and got the other dude's ECM to run my XR...and whallah!

Problem solved...

I think it's best if you can test you car with a known working Ig coil if you can so you can at least eliminate that factor if it doesn't solve your issues