1974 620 DCH340 has "too much" choke: Solution?

[Cardinal Grammeter]

Has anyone encountered this problem?

 

I'm near Pittsburgh and yearly temps range conservatively from the "teens" to the 90's.

 

The carb is a basket case:  parts were "like new" and there was a metal tag with "74 MT" stamped on it.  I checked all the jets and bleeds and they were correct.  I also accumulated 2 other 340 carbs.  I can't determine if I have the right electric heater choke cap.

 

If I adjust my choke for summer, when winter comes, the choke is too rich and the fast idle is too high.

If I adjust my choke for winter, when summer comes, the choke is too lean and the fast idle is too low - engine will actually "flame out" in 90* weather due to too low a fast idle.

 

The reason for this is the bimetal spring is moving too much from its Start (ambient temp) to Run (heated temp.)

 

SOLUTION:  (maybe):  Reduce the bimetal spring movement: 

1) Reduce electric heater current:  insert series resistor - this could also be a power rated rheostat that would be adjustable.

2) Cool the choke housing:  introduce ambient air venting of the choke chamber.  Obvious way would be to drill hole(s) in the plastic choke cover.  

3) Find different bimetal - one with fewer tuns.  

 

 

EDIT UPDATE 2 Years Later:  Just discovered the choke pulloff was not working.  This was probably the cause because w/o the pulloff, you'd have to adjust the choke for the hot and cold temperature extremes.  In cold weather, the choke can be adjusted nice and tight and the pulloff will be sure the choke is opened enough for cold engine requirements.

 

 

 

 

Edited March 29, 2022 by Cardinal Grammeter

[DanielC]

If you could get hold of the original 1974 owners manual, you would find a service interval of every 3000 miles for minor tune ups.

This is why cars went to electronic engine management systems, because people did not service their cars every 3000 miles, or 3 months.

You have probably figured out the clocking of the choke thermostat is adjustable, and if you do your tune ups at 1974 factory suggested intervals, you are adjusting the choke more or less depending on the season.

You can also adjust the amount of fast idle to choke plate position by bending the linkage rod between the choke mechanism, and the throttle shaft.

[Cardinal Grammeter]

EDIT:  Here is a better way to understand what is going on:

 

Choke temperature correction has always been approximate and minimal.

When you adjust a choke in hot weather, it's because you have too much choke.

When you adjust a choke in cold weather, it's because you don't have enough choke.

 

My situation is the opposite:  too much temp correction resulting in too much choke in winter and not enough in summer.

END-EDIT

 

I know exactly what you are saying.

 

What I'm saying is that what I have going on is something out of said envelope.

 

I suspect wrong bimetal or heating element.  As I said, carb was a basket case and frankly I don't know if the black choke cap was included or the proper one since I don't think there is a FSM test for it or a means of IDing it.     .... or is there?

 

 

Edited August 9, 2020 by Cardinal Grammeter

[DanielC]

Are you running an internal regulated alternator?   The reason I am asking is on a 1972 521 Datsun switched from a manual choke to a automatic one, the bimetal coil, heated by an electric coil, when the engine was running.   To let the system know the engine was running, there was a relay that took a signal on the "N" wire on the alternator, and if there was voltage there, the relay closed, and allowed 12 volts to the choke coil heater.

 

I have two running 521 trucks.   Normally, in summer, I do not use the choke to start them, and I pump the gas a few times, and after the engine has oil pressure, then I pull the choke to establish a reliable fast idle for warm up.  A little bit of choke helps with cold engine throttle response.   If I have driven truck within the last half hour, no choke is used.

 

 

When was the last time you did a tune up on the engine?   Does the engine have good compression?  When was the last time you adjusted the valves?  Ignition system in good condition?  Datsun recommended replacing points at 12,000 miles, and spark plugs too.  Point gap OK?  I recommend using a dwell meter to set point gap.   Obviously, check the timing.   Factory recommendation in initial advance is 10 degrees BTDC on my engines, I am actually running closer to 12.5 or so.  Does the vacuum advance and mechanical advance in the distributor work normally?   If the advance mechanisms in the distributor have not been cleaned, the old grease on the parts in the distributor may not be working correctly, and this could affect both the initial timing, and how the timing advances when the engine is running.  Any vacuum leaks?   The point I am trying to make is all these other factors that are given attention to in a tune up affect the carburetor.

 

Finally the carburetor. In the tune up service, the carburetor is the last thing you work on, because everything else affects the carburetor.   However, the carburetor has to have the idle mixture and speed set on a fully hot engine.   I suggest you get a vacuum meter to set the idle mixture.   You also need a tachometer that measures low RPMS.  If your engine idle is a little fast, that started to add mechanical advance.  If the throttle has to be held open more because the timing is off, that may add some vacuum advance.  Generally, when setting initial timing, unplug the vacuum advance from the distributor, and plug the line going to the carb.

 

 

It is possible the wrong parts are on the carb, your truck is over 45 years old, and somebody somewhere may have changed something.   I would confirm you are getting 12 volts to the choke heater after  the engine starts.   The choke should set itself on a cold engine after you step on the gas pedal one time.   Confirm that happens.   After the cold engine starts, the choke should open partially.   Confirm you are adjusting the choke heater the correct way.   Smetimes the markings are confusing, and adjusting what you think is more choke (richer when cold) is actually putting less choke on the engine.

[Cardinal Grammeter]

I've been through everything - I even bushed the throttle plate shaft on the DCH340.  (It has an electric heated choke with the proper inner fender relay that detects alternator charge to determine engine running.

 

As far as centrifugal and vacuum advance, the engine has too much power and fuel economy for either to be a glaring issue - I get over 20 mpg around town and it's not even a full mile to town - 1/2 my runs the engine doesn't get fully warmed up.

 

There is one glaring omission regarding the 620 (and not the 52x trucks), the BCDD.   It was working OK but the last time I had the carb off, there is something wonky with it.  (a quick "test" is to take the plug out and push on the adjusting screw, it should make the engine speed up.)

 

RE:  Timing - I keep the timing as far advanced as the fuel will allow.

Edited August 9, 2020 by Cardinal Grammeter

[datzenmike]

The heating element only turns the choke off at a set rate. You could speed it up or slow it down but why bother? Many of the later 720 carbs the choke adjustment is gone and the choke cover riveted on to prevent owners from meddling with the factory setting. Adjust twice a year.

[Cardinal Grammeter]

Since I might have the wrong choke spring in my carb, the question is, "Does anyone else have this problem - needing more choke in the summer / less in the winter?"

 

 

[datzenmike]

My 521 did so did my new '76 B-210. My '78 620 did and my 710. I'd say they all do. This is why they all say  LEAN<<<<< >>>> RICH on them.

[banzai510(hainz)]

I replaced the auto with a manual,put a manual pull cable wire on there BAM   DONE!!!!!!!!!!!

[datzenmike]

That too.

[Cardinal Grammeter]

So the Datsun choke cal was off in the "opposite direction" of American Iron?

 

NOTE:  Maybe my problem is that I've never dealt with American electric chokes - only the ones that used manifold vacuum to bring hot manifold air into the choke spring chamber.  I could see transitioning from that "wimpy" system which always never produced enough correction, to the electric which was like a blast furnace and produced too much correction.  

 

Hey wait a minute, I had a 1972 510 and a 1974 (or 76?) 4-dr 1400 B210 and don't recall this problem.  I drove them until 1983 when I moved from PA to FL.  My dad drove the B210 until the frame broke in half from rust.  We talked a lot and he never mentioned any problems with the car.

 

I need to go through my other 340 parts carbs and see what springs I have but they are buried on a work bench and will take some time to unearth them... 

[datzenmike]

I don't recall any manifold vacuum used on the old chokes. Exhaust is under pressure and will find it's own way to the choke. Chrysler has the coil mounted in a well in the exhaust manifold with a linkage up to the choke flap.

 

Datsun cars after the Fairlady were semi-monocoque or unibody construction. Trucks had frames.

[Cardinal Grammeter]

Chrysler had those wells in the manifolds that housed the bimetal spring.  There was a rod that went to the choke shaft arm.  Simple system.

 

Studebaker had various tubing layouts that were inserted into the exhaust port that ran under the intake manifold.  (Carters and Strombergs were used in the 50's and 60's)  One end of the tube went to the top of the air horn to obtain air after the air cleaner, the other went to a fitting on the choke cap.  The choke chamber had an internet port to manifold vacuum to draw in the hot air.

 

The more I think about this, the more I think there may be a long forgotten consensus among mechanic friends that those electric chokes never worked right.  But it is all a dense fog nowadays.

 

I'm going to have to "break open" one of those 340 sealed chokes...

 

... you know, the issue isn't with the choke since it always starts the engine, it could be there is "too much" fast idle cam. 

I wonder what would happen if I adjusted the FI cam so that the next to the highest step was the starting step thereby reducing the range of the FI???

Edited August 10, 2020 by Cardinal Grammeter

[DanielC]

... you know, the issue isn't with the choke since it always starts the engine, it could be there is "too much" fast idle cam. 

 

You can also adjust the amount of fast idle to choke plate position by bending the linkage rod between the choke mechanism, and the throttle shaft.

[Cardinal Grammeter]

Well yes it could be the fast idle cam.  But again, the hotter it gets in the summer the lower the fast idle becomes to the point engine will stall.  Then on really cold winter days, the fast idle is way to high for cold oil/engine.

 

If I remember correctly, I had a choice of bimetals to install in the heated choke cover.  I have a fixed unit from late model 340 where I know the correct bimetal/cap combination  is used.  I'm going to try that.

 

I was thinking if it was indeed too much fast idle compensation, I could bend arm to even eliminate the 1st and tallest step.

 

I'm in the midst of getting maintenance done for winter.

[datzenmike]

These engines have tons of bearing clearances unlike today's engines. They won't starve on cold oil.