unstable idle rpm

[Mr. 427]

hello there, everyone.

i've a 1982 Daihatsu Charmant LE that seemed to have a mysterious never-ending issue with idling. it flat-out refused to hold the idle steady, but accelerates just fine under load even after multiple carb rebuild attempts. the exhaust sounds seemed out-of-rhythm too when that happens.

it's got a stock 4k-j engine and ofc, aisan carb. got that carb cleaned (again and again), and refitted with the repair kit (gaskets, jets, idle screw, steel balls, and etc). the float bowl is adjusted accordingly (lying flat on its own when turned upside down)

note that the distributor, ac fuel pump, carb thermal plate, breather valve are also functioning well. plugs indicate slightly lean mixture, with a very light brownish stain on the ground and the porcelain on each plug)

so, rollaclub fella, what gives?

have a nice day. cheers

[Banjo]

Hi Jasper,

                 Seems like You have checked everything, but the car being quite olde, there are a number of issues.  If You go look on the internet, it does turn up a few possibilities, for your Daihatsu.

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If a 1982 Daihatsu Charmant LE fails to idle properly after all standard manual tests have been performed, the issue typically stems from specific aging components or internal carburetor failures that are not easily detectable through standard diagnostic routines. 

 

Hidden Carburetor and Fuel Issues

Because the 1982 Charmant uses a carbureted system, standard tests may miss internal mechanical wear: 

• Clogged Internal Idle Circuits: The fuel path for the idle circuit is highly convoluted, often traveling above the fuel level in the float bowl before descending. If the car has sat with old fuel, these internal passages can clog in ways that surface cleaning or "blowing through" cannot fix.
• Worn Needle Valve Tip: Microscopic wear on the needle valve tip can allow fuel to leak past into the engine, causing a rich mixture that cannot be corrected by the idle mixture screw.
• Malfunctioning Economizer Valve: If the idle is too rich and cannot be leaned out via adjustment, the economizer valve may be feeding fuel into the engine incorrectly.
• Sticking Automatic Choke: On older carbureted engines, a choke that is even slightly sticking or misadjusted will disrupt the air-fuel ratio enough to prevent a steady idle. 

 

Vacuum and Air Leaks

Standard manual checks often focus on visible hoses, but hidden leaks can be more subtle: 

• Carburetor Base Leak: Vacuum may be leaking around the base of the carburetor or through worn-out gaskets rather than the hoses themselves.
• Intake Manifold Gasket: Worn or damaged gaskets can cause a vacuum leak directly at the engine block, which is difficult to see without specialized testing like a smoke test.
• Clogged EGR Valve: Carbon buildup inside the EGR valve or its passages can cause stalling at idle. 

 

Mechanical and Ignition Timing

• Ignition Timing "Out of Whack": Even if set to manual specs, wear in the distributor or on-board computer components can cause timing to drift, resulting in a poor idle.
• Worn Valve Seals or Piston Rings: In older Daihatsu models, worn internal engine components can lead to reduced performance and erratic idling that external adjustments cannot fix. 
• ____________________________________________________________________________________________________________________________________________________________

 

Hope somewhere in amongst that, You spot something, You've not checked as yet.  Having had a couple of 4K engines, the Intake manifold gasket leaking, is quite a common issue.  I'd be taking off both inlet & exhaust manifold together, & then run a straight edge across them.  As it's been happening for some time; You may even see evidence of the leak, with discoloration of the metal surfaces, each side of the gasket.  Let us know, if any of these suggestions, bring your idling issues to a sucessfull resolution.

 

Here is a post on this forum, written 15 years ago, where the issue, I'm suggesting was discussed.

 

https://www.rollaclub.com/board/topic/38875-4k-intakeexhaust-gasket/

 

Cheers Banjo 

 

 

 

 

 

 

Edited December 16, 2025 by Banjo

[Mr. 427]

yeh.. found the leak at the breather valve inlet on the carb fiber gasket.

now, should i just apply band-aid like gasket sealant or replace the whole thing 

[altezzaclub]

Great that you found it- I'd just seal it until the next time you're doing a big strip and rebuild, but I'm a  real bush mechanic. Other's would strip it all now.

How did you end up with a  Charmant? Is it a 2-door? I only ever see them in SE Asia.

Edited December 17, 2025 by altezzaclub

[Mr. 427]

11 hours ago, altezzaclub said:

Great that you found it- I'd just seal it until the next time you're doing a big strip and rebuild, but I'm a  real bush mechanic. Other's would strip it all now.

How did you end up with a  Charmant? Is it a 2-door? I only ever see them in SE Asia.

nah, just the typical 4-door saloon. got myself a 79 ke36 years before, and recently the 82 charmant. both came equipped with the k50 speed gearbox. however, the charmant had its own charms, with the panhard bar on the rear and the super comfortable luxurious seats. it still got plenty of issues currently, but drivable nonetheless.

also, I'm a heavy equipment mechanic/welder specialising on the hydraulic crawler excavators and cranes, wheel loaders and forklifts. so, after series of tiring and mind-numbing days, the last thing i want to work on is my own cars on the weekends and holidays, ofc xD

but anyway, i got that sorted out. the idling issue persisted, with slightly smoother all around. i guess it's about time to pull out the vacuum gauge and compression tester kit for a more thorough check.

cheers and have a nice day.

 

Edited December 18, 2025 by Mr. 427
mis-spelling

[Banjo]

I asked Google A.I. the following questions.

Quote

What made the 1982 Daihatsu Charmant LE a more attractive vehicle to purchase, than, that the Toyota Corolla of the same year & era, although the Daihatsu shared the Toyota 4K engine ?

 

While the 1982 

Daihatsu Charmant LE shared the reliable 1.3L Toyota 4K engine and chassis components with the Toyota Corolla (E70), it was positioned as a more premium, upmarket alternative. 

The primary features that made the Charmant LE more attractive than a standard Corolla of the same era included: 

• Luxury Positioning: As Daihatsu's flagship model, the Charmant was designed with an "upper-class" feel compared to the entry-level positioning of the base Corolla.
• Upmarket Exterior Design: The Charmant featured a "six-window" saloon body style, providing better visibility and a more sophisticated profile than the four-window Corolla. It was also distinguished by a more ornate front grille and four round headlights ("four-lens dual headlamps"), a feature typically reserved for more expensive 1.6L or 1.8L cars at the time.
• Enhanced Interior Comfort: The LE trim often included "big lush seats" and higher-quality interior materials and "gizmos" that surpassed the basic specifications of the KE70 Corolla.
• Ride Quality: The suspension was specifically tuned for "maximum comfort," featuring softer settings than the more utility-oriented Corolla to provide a smoother, more "charming" ride.
• Exclusive Features: Higher trims like the LE could be equipped with longer bumpers (50mm) featuring shock absorbers and premium options such as power steering, which were less common on 1.3L Corollas of that year.

 

which early model 1980s toyota corolla vehicles were factory equiped with power steering ?

 

Power steering was introduced to the Toyota Corolla lineup during the early 1980s, primarily as an optional feature for higher-trim models before becoming more common toward the end of the decade.

 

The following early 1980s models were factory-equipped with or offered power steering: 

 

E70 Series (1979–1983) 

The fourth-generation Corolla was the first in the model's history to offer power steering as an option. 

• US Market (1982–1983): Power steering was introduced to the U.S. market in 1981 for the 1982 model year. It was typically found on higher-spec models like the TE72.
• Other Regions: While most standard E70s (like the KE70) utilized manual steering racks, power steering gearbox units were manufactured for certain variants during this period. 

 

E80 Series (1983–1987) 

The fifth-generation Corolla transitioned many models to front-wheel drive (FWD) and expanded the availability of power-assisted steering. 

• Corolla LE Sedan (FWD): Power steering was a notable feature on the "Luxury Edition" (LE) trim. In 1984, it was offered as an optional variable-assist power steering system.
• Corolla Sport Coupe (AE86 - RWD): The iconic rear-wheel-drive AE86 (1983–1985) offered power steering as an optional extra. While many enthusiasts later removed it for better road feel, it was a factory-available option, particularly on the GT Apex and SR5 trims.
• Corolla AE82 (FWD): Higher-spec FWD models from 1983 onwards, such as the FX16 and certain sedan variants, were frequently equipped with power steering racks.
• Regional Variations: In markets like Indonesia, late 80s versions of this generation (marketed as "Corolla Twincam") began including power steering as standard on trims like the SE Limited and GTi, though the base SE remained manual.

 

 

 

Cheers Banjo

 

Edited December 19, 2025 by Banjo

[Mr. 427]

done with the compression tests. here are the results for the each cyl with dry test: #1 @ 142psi, #2 @ 145psi, #3 @ 147psi, #4 @ 144psi. with wet test, it only rises for about +/- 3 to 4psi. so, it's still a healthy engine, being rebuilt right after purchase.

next was vacuum test. the needle went berserk between 12 inHg and 17inHg at 850rpm, telling me that there's smth not right with the carb adjustment.

dismantled the carb, shaved off the inlet of the pilot jet till its wide open, resassembled. put it back on the manifold, starts the car, adjust the idle af mix screw accordingly. then and only then, it starts to idle properly. even the vacuum gauge reads more stable too.

guess there's still some stubborn cruds deeply attached in the idle circuit that a steel wire, carb cleaner and diesel couldn't remove. sighh

 

 

[altezzaclub]

Those compressions are good, lots of life still in that motor. I usually get a syringe without a needle and hose petrol through all the little drillings in the carb after its soaked for a while. Once stuff dries out in there its very hard to get out with compressed air.

[Mr. 427]

On 12/21/2025 at 6:27 AM, altezzaclub said:

Those compressions are good, lots of life still in that motor. I usually get a syringe without a needle and hose petrol through all the little drillings in the carb after its soaked for a while. Once stuff dries out in there its very hard to get out with compressed air.

exactly the point.

found out that there was a very minor vacuum leak from the worn throttle shaft bushings when i sprayed brake cleaner around the base. that doesn't affect much, but if not eliminated will make me sleepless throughout the nights. lol..

as the rebuild kit didn't include them in the package, the only rational & pragmatic solution for this issue is to get a brand new MD-192036 Mitsubishi CV carb with a matching adapter plate that I also use for the 4K in my ke36.

any opinions regarding that?

[altezzaclub]

I've never used them, but it looks like a modern version of an SU.  Are they quite economical to run? Do they have a pump jet or a tiny hesitation when you boot it like SUs do?

Of course there is always quad Mikunis off a GSXR Suzuki...

[Banjo]

Hi Jasper,

                 I've never used one; so cannot personally advise good or bad results. In this case, I turned to Google A.I., which scans all available information on the internet, to make it's conclusions, or suggestions.

 

Quote

How good or bad are the replacement model md-192036 Mitsubishi cv carburetors, made in China ?

 

Replacement 

Mitsubishi CV carburetors

 (model MD-192036) made in China are a gamble in quality and performance, often serving as a cheap, temporary solution with potential tuning and durability issues, though some users report positive initial results. 

 

Performance and Reliability

• Inconsistent Quality: The primary issue is a lack of consistent quality control. Some units may work well initially, while others suffer from poor machining, improper assembly, or the use of substandard materials, leading to immediate engine performance problems.
• Tuning Difficulties: These carburetors often have incorrect jet sizing, poor fuel delivery, and improper air-fuel mixture ratios. The jets and bleeds may not be marked, and there is often no documentation, making tuning a "nightmare".
• Performance Issues: While some users report their car running great with no lag, others experience issues like rough idling, difficulty starting in cold weather, or the need for significant adjustments.
• Temporary Fix: Many experts and users consider them a viable low-cost, temporary solution to get a non-running car operational until a higher-quality or rebuilt original unit can be sourced. 

 

Durability and Materials

• Substandard Materials: The carburetors are often made from lower-grade aluminum alloys that corrode faster when exposed to ethanol-blended fuel or condensation compared to high-end options.
• Component Failures: Gaskets and seals are reported to degrade quickly, leading to leaks. Small inconsistencies, like a float being slightly heavier than an OEM one, can cause fuel trickling and other issues.
• Lack of Quality Control: Issues like loose fittings or components, missing check valves, and improperly aligned parts are common due to less stringent manufacturing standards. For example, one source noted a lack of Loctite on the throttle blades, a potentially serious safety issue. 

 

Compatibility and Support

• Fitment Issues: Despite being advertised as direct replacements, some users report fitment issues with mounting points or linkage connections that require modification.
• Limited Support: Most manufacturers offer limited customer support or warranties, making it difficult to return a faulty unit or get a refund. 

Ultimately, while the low initial price is attractive, the potential for hidden costs in tuning, repairs, and premature replacement means they are often a "false economy". 

•  
   
• Cheers Banjo

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited December 22, 2025 by Banjo

[Mr. 427]

@altezzaclubit's quite economical and efficient to run at any altitude, weather conditions, & driving conditions. it's got pump jet too like any other carbs. but mostly without the hesitation when u suddenly open the throttle, since the vacuum-controlled piston barrel immediately lets in the ideal amount of air entering through the carb for the computed quantity of fuel during the.

i intend to complement it with the rebuilt engine as it's got ported/polished, flywheel-crank assembly fully balanced till 2g, re-machined cam with shorter duration, wider lsa & lower exhaust valve lift than stock; remachined valve face & high tension main compression rings.

motorbike carbs are just too much of a hassle for me to setup, tune and maintain during my rest day. so, I'll give it a pass.

@Banjonah. I'm going for the MMC genuine carb manufactured in Japan and made in Thailand. at least it's 85 leagues ahead in terms of proven long-term consistency and longevity. much bigger than the stock pumper too, mind u...