INAT carb auto choke
 

Hi everyone

I'm recomissioning a 1971 230/8 with twin INAT carbs. The choke plates are supposed to open after a few minutes with the ignition on, but mine don't move. They stay completely shut. I measure 12v at the terminal on the front carb and about 0.2v at the rear, which suggests that the rear carb is activated by a temp sensor on the block. However the front should still move! I checked the actuating arm and it is connected. The car seems to run very rich - will start and run for a short while then stalls. Any ideas?

Thanks for a great forum, by the way.

Andrew

(sorry - previously sent to the Technical forum by mistake)

AFAIK there is current sent to the chokes at all times when the key is turned to the run position. The choke release is a thermal spring which is supposed to open when the heat from the electric heaters gets warm enough. You ought to have 12V at both terminals and they should branch off from the same wire so you need to check where they branch to make sure the splice is good. Do the choke plates open if you push them or are they stuck due to crud holding them closed? If they are at all sticky buy some gumout and spray it well to free them up. The reason the car stalls after a couple of minutes is because the chokes are staying shut and the engine is running way too rich...so you need to make sure those choke plates are free.

You can also remove the wires and test for resistance on the terminal on the choke (from ground or "earth" in the UK to that terminal) which will tell you if the heaters are good or not. You also might want to confirm the bodies of the carbs have a good ground to them too by checking resistance from them to the negative terminal on the battery. Do the resistance tests and post your results.

Thank you
 

That is one informative post, nhdoc.

Thanks - more soon!
 

Thanks!

I'll test the resistance of the chokes tonight and post the answer. I'm hoping that the problem is just disuse/bad contacts/needs lubrication, rather than failed part!

Andrew

Test results
 

Hi again

I tested the chokes. The front carb is 20 ohms, the rear open circuit. Both plates move freely with a damped return when you push them. The carb bodies are earthed just fine. SO, one mechanism appears electrically sound but physically wrong, and the other is at least electrically wrong. I'm just going to have to dismantle the choke units and take a look.

You know what? I still love old cars. Maybe I can get some medication for it?

Thanks for support

Andrew

well, you're on to it now! Try applying +12V directly from the battery to the good choke and see if it opens up. You may need to open the throttle while you wait for it to open to trip it but it should open in 5 minutes or so. That will tell you if the choke itself is working or if it is the wiring to it that's bad.

While you have the covers off of the chokes you should check to see if the cold idle cam on the rear carb is clean and moving free. I overlooked that little bugger in the past and had some issues getting it just right. If you can get it properly set, the chokes and cold idle work together - the result is that you can start in sub zero temp and hit the road in no time.

At a minimum, you will want to verify that all parts are there and clean. Use some dry lube and tighten the return spring (which is a coil spring that keys to the brass bushing #37 in diagram). Then when you put the choke plate back on, make sure to catch #35 with the bimetal spring and rotate into place.

Here is a link to the diagram for reference:

http://i151.photobucket.com/albums/s131/Blue72W114/CarbINAT32.jpg

good luck

Missing 12 Volts, Also
 

I am currently trying to trouble-shoot my missing 12 volts.
I have to find the relay(s) that supplies the 12 volts;
I suspect I have a bad ground to the relay(s).

Strip-down approaches...
 

Thanks guys! Seems I have a few jobs this weekend. If I find anything useful I'll try to link a photo.

nhdoc - thanks for great support.;)

Munichtaxi - depending on the age of your car, I believe there might be two relays. Later cars have one carb choke controlled by a temp sensor of the block - won't fully open until the engine is warmed up. I think that was post '71 for the M180 engine in most markets, not sure about others including US which had different emission control stuff. So there must be two relays for that system.:book2:

Blue 72 250 - thanks for advice - great diagram.:)

I'll keep you posted. Thanks again

Andrew

Possible conversion to manual choke
 

I called my local carb shop and they have researched this. They believe they have a compatible kit (intended for an Opel Commodore) to convert the INAT carbs to manual choke. It'll need two kits as the Opel was a single INAT carb, but their workshop believe they have worked out an appropriate linkage. They've even said they'll lend me one to see if it looks OK!:clap:

If it works out I'll post details/photos.

Andrew

Manual choke is not a bad option. It's basically a cable operated system and the pre-64 220 had it standard. I prefer manual to auto choke actually...good luck with the conversion.

I am looking forward to the pictures. Manual is the way to go.

There is no relay. Check for an open in the wiring and fix the connection.

Just a minute
 

The Front carb on my 1970 280S doesn't have a relay.

But, the MB Shop Manual states the Rear carb derives voltage from the 'Emissions Control' relay.
This relay is in a black box, right next to the speed control relay, to the left side of the radiator.
I've determined this relay is the source for the rear carb voltage.
I either have a bad 'Emissions Control' relay, or the 65 degree C sensor is bad.

BTW, I now have 12 volts at the Front carb. There is no relay for the front carb.

Most owners have bypassed the relay and wire the front and rear choke together. I cant think of a situation where I would want the two working at different intervals. If a delay in opening of chokes was desired, a timed relay could be wired, or a simple throw switch that is opperated by the driver.