Mechanics

MODELS CAN´T ACHIEVE ENGINE IDLE UNDE

1981 Mitsubishi Other

Engine Performance problem
1981 Other Mitsubishi Models 4 cyl Two Wheel Drive Manual 200k+ miles

hi there,

Was hoping you might have some ideas. I ´m a self taught hobby mechanic. I ´ve been travelling through some tough conditions in south america through the atacama desert, the los andes mountains to the amazon and beyond - more than 10,000kms. My vehicle for the adventure is a mitsubishi l100, which is very similar to the minicab models - its a chile/argentina specific fabrication I believe.

Its 2cyl 550cc!

OK. On my travels I have more or less worked on everything but now I ´m stumped.

The latest leg of my journey, it drives, but its rough and it wouldnt idle. Spark plug colour was perfect. I checked points and timing. I didnt have a timing light so I was doing it static styles but after having no results I lost faith in the static method and imported a timing light which showed it was running advanced by about 25deg. So I checked the points gap again, then retimed to just under 10deg. The built in time scale is 10 to -10 so I assume it needs to be in the usual 7-10 deg adv range. One mechanic told me to set it 20 or so. But I believe this was because we were at extreme altitude (the highest city in the world infact - bolivia).

I have rebuilt the carb numerous times. I believe it to be absolutely clean but I ´m stumped on the air fuel mixture screws. There are two (fuel and air pressumably - idle is another screw). I ´ve mucked around with them and can now achieve idle, but its way high. I ´m conscious of not applying too much idle and having a resulting problem with the mixture. There is currently hesitation on accel. Currently, after acceleration it will idle nicely, v high but nicely for about 10seconds and then drop further. If the idle is set lower the engine dies.

the other problem is its a little rough. A little misfire here and there, enough to be upsetting a 2cyl. I havent been able to sort it out in months of travelling (origially wasnt like this). Staring into the timing light it seemed ALMOST perfectly consistent (my way of trying to detect if its an ignition problem!).

On acceleration and high rpms the roughness is barely detectable - if at all.

I ´ve sprayed in search of a vacuum leak - nothing of note. Valves are set and even (but I dont know the official gap). I took an average of all, not tight, not flapping around noisily). I have had previous problems with the distributor. The metal bushing for the dist. Axel was sloppy causing the rotor to loose more than occasional proximity for a spark. When replaced, I had a normal idle but it was still a little rough.

There are leaks in the exhaust around the silencer chamber.

my current thoughts are the problem may lie with the carb, ignition system or lately I ´ve been thinking about compression. Haha! I think thats everything!

my plan now is to investigate the ignition system & distributor (i have replacements for all - not the dist.), Then on monday go in for a compression test.

Its helped me a lot just writing this down. I think I might have two seperate problems.

If you have any ideas that would be great. What would really help is if there is a way I can rule out the carb?

thanks,

Mike
Avatar
Mj007nz
July 3, 2010.



Hi Mike,

We do not have any data on this vehicle so I have nothing to fall back on. What I have is only the years of experience I have on vehicles and the last time I handled 2 cylinder engines should be more than a dacade ago.

From the symptom description, the carb is the most likely cause. The chances of ignition problem is not high.

1. The idle air fuel mixture is out of specs. If carb is equipped with fuel cut solenoid, check the solenoid for proper operations.

2. Fuel float level. If the level is too low or high, the idling would be affected. You mentioned being at the highest city in the worls, carbs of earlier era are moslt ly unable to perform accordingly without reradjustment.

3. Vacumn leakages. If any vacumn leakage occurs, the idling woud be affected. You can test this by partially covering the intake to see if the idling improves.

4. A compression test would determine if there is any fault with the compression to affect the idling. Tight valve clearances can affect idling speed.

KHLow2008
Jul 4, 2010.
Thanks KHLow,

Still struggling here but have some further observations.

Have been trying to get some improvement via adjustments to the idle air fuel mixtures- but no significant improvement. Theres no solenoid (fyi: Carb is a mikuni without a model no. On it)

Float level is spot on. Tested fuel return system - no probs.

Tested again for leaks. Thought for one moment I had found something spraying around but couldn ´t replicate it to no avail. Tried covering the intake various amounts - no improvement.

A few things I observed which might help.

* Hand over the intake makes running more rough (too rich in general?)
* if I put my foot down - engine will die (accel. Pump is adjustable. Maxed at moment. Will back it off)
* no performance difference without air filter

two questions.

* for the idle circuit to function does it need to be opening the butterfly valve? Just a tad? Or a few millimeters. I have the carb off for inspection.

*fuel disappears from the float chamber overnight (2/3rds). Nothing is obviously leaking. Put a piece paper underneath. No marks. The chamber gaskets appear to be sealed. Any experience with this!

No compression test yet.

Thanks for your help.

-mike

Tiny
Mj007nz
Jul 12, 2010.
The throttle valve has to be closed almost completely for idling. If it is opened, then the idling circuit would be affected. By looking through the throttle barrel, you should be able to just note a slight gap.

If fuel is disappearing, then there must be a leak somewhere, unless the weather is very hot then it can vaporise.

Sudden accelerating causing the engine to stall indicates the accelerating pump is not functioning correctly.

With fule in chamber, open the throttle and note if the pump is spraying. Spray must be even and with minimal opening of the throttle, it must at least spray a little.

KHLow2008
Jul 12, 2010.
Thanks KHLow,

In terms of understanding more about my carb, do you think my Mikuni will have more in common with a carb from a large motorbike (remember I have a 2cyl 550cc) than a carb from a 3 or 4 cyl engine with much more displacement. For example, if I had to replace it (i ´m not going to but if I had to). Would I be looking for a motorbike carb or a small car carb?

Thanks again.

-Mike

Tiny
Mj007nz
Jul 12, 2010.
I don't think you would be able to get any small car type to fit as such carbs are no more in use. A bike carb might be more appropriate considering the capacity of the engine.

KHLow2008
Jul 13, 2010.