Rough starts in high-mileage M20B23 motors

[JBasham]

Hi. I'm still new to auto repair and maintenance. I'm learning slowly from nice people on the internet, the same way I learned tube amplification.

I have an M20B23, one of the early ones that came with K-Jet. The K-Jet is gone now and a previous owner put on an intake manifold that I think came off a first-generation M20B20 with the Solex. Throttle came from that too, but I don't know where they got the fuel pump. The carb is a Weber 38/38.

Basic stuff like the wires, cap, rotor, distributor have been replaced but the iginition module looks original.

The trans is a 5 speed from an E30, and the transmission seal leaks a tiny bit, but otherwise, the car's in good nick. The motor seems to pull pretty hard, though I don't have much to compare it to and haven't dynoed it. The odo stopped at 160k due to the transmission swap so I don't know how many miles I have on the motor beyond that.

ANYWAY,

what kind of things cause an old motor like this to be a hard-starter from cold (10-20 seconds crank while I constantly flapper the throttle pedal), and then want to sputtter and die unless I keep the revs up to 1,500-1,800 for however many minutes it takes to get the whole unit up to normal op temp, or at least 75% of the way up to normal? Once I get it up there, it's fine.

My "cold choke" is a half-thickness masonry brick that I lean on the accelerator pedal for five minutes to warm up before setting off.

Thanks in advance for any advice you can offer.

Jamey

[Tom D]

My "cold choke" is a half-thickness masonry brick that I lean on the accelerator pedal for five minutes to warm up before setting off. Jamey

well there you go, you need a proper choke, or learn how to use it properly.

im not versed in the weber 38/38, but unlike an injected engine you first have to set the choke on a carb. you do this by pushing the accelerator to the floor before attempting to start it. with the engine cold stomp on the throttle and look down the throat of the carb, there should be a flap covering most of the opening, that is the choke.

[JBasham]

well there you go, you need a proper choke, or learn how to use it properly.

im not versed in the weber 38/38, but unlike an injected engine you first have to set the choke on a carb. you do this by pushing the accelerator to the floor before attempting to start it. with the engine cold stomp on the throttle and look down the throat of the carb, there should be a flap covering most of the opening, that is the choke.

Thanks Tom. I'm off to search the details of the 38/38 and to pull the air filter . . . .

[NIS323i]

I don’t know anything about configuring the Carbs on these cars, but I do have a pdf file of the factory manual on how to tune and troubleshoot the fuel system on the 323 if you want it (K-Jet and Carb.)
Just PM me and I will email it to you.
NS

[Tom D]

here is the top of a weber 38/38. the two flaps in the throats are the chokes and they are currently in a cold start position. it also appears that the choke is electric/ mechanical and not thermo/mechanical.



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here is the bottom of a weber 38/38. the two flaps in this picture are the throttle plates and are in an idle position.



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the black disc on the far right side of the above picture is the choke control, the metal spade is the curcuit connection. with the engine cold and not running if you push the throttle to the floor the linkage will release the choke and a bi-metal spring in the black choke control will pull the choke close or part close depending on how cold it is. then when you start it the electrical circuit will heat the spring and after a certain period of time pull the choke open. this way the car isn't sitting idling with a rich mixture. also when you 'set the choke' the throttle plates will be ramped open for a higher idle speed.

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you'll also notice that under the throttle plate on the right there is a lever attached to a cover on the far right. this is the accelerator pump. the throttle shaft goes through the carb body and at the end of it there is a cam. this cam is position right above the accelerator pump's lever. you the throttle is opened wide or almost wide open the cam on the throttle shaft will press on the lever and a small amount of gas is 'pumped' into the throat from a camber under the cover of the accelerator pump.

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here's a side view, you can see the black cover that houses the choke spring and electric input, you can also see the linkage from the choke control at the choke flaps.

to the right is the accelerator pump.



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in this picture you can see a camber on the front right that appears to be connected to the choke control, I'm will to bet there is a diaphragm inside and is seeing ported vacuum on one side and the other side has linkage that pulls the choke open during acceleration.

also right to the left of this piece and to the right of the throttle gears you can see a gold wire link. this is probably the link that ramps the throttle plates partially open when you 'set' the choke.

see the brass ring and three screws that hold it around the black cover of the choke? if you loosen the screws you can rotate the black cover, this is how you adjust the cold choke flap's position if they're not closing completely, or adjust the hot position of the flaps if they don't open fully when the engine is hot.

[gsumzzz]

Tom, are you on a carb diet now?

[Tom D]

I screwed with carbs for the past four decades and although I have no experience with webers the principles ( and the construction) are all the same. this is why I say that carbs are not an improvement over KJet. if you want an upgrade then go to EFI. carbs are nothing more then a controlled gas leak.

[JBasham]

Wow, now THAT's what I'm talking about!

Thanks Tom D. It may be a controlled gas leak, but it's the controlled gas leak I'm working with. I think I prefer to call it a leaque.

So this was pretty helpful on the adjustment: http://forum.roadfly.com/threads/937...8-tuning-guide

The choke is working now. But the electric heater in the choke that warms up the coil to release the choke needs 12 volts. P.O. sourced the voltage from one terminal of a thing on the firewall that looks like a large capacitor, or a can of Iron City beer, that says Bosch on it. I'm getting 7-8 volts off that terminal. The other terminal on the cap thing is only about 5-6 volts. I need more voltage because what I have isn't enough to heat the choke relax/release coil when the car is warming up, so the cold RPMs keep climbing from where I set them (1800) to 3000, until I manually kick down the choke.

Where's a handy place to tap 12 switched volts under the hood?

TIA,

Jamey

[Twelvizm]

Start with the simple things and work your way towards the carb. Check the plugs, plug wires, etc. The biggest mistake many make when tinkering is over thinking the problem.