After changing out the diaphragms and jet needles, adjusting the fuel levels in the bowls, and replacing the float needles and seats, and setting the slow-running control valve to out 2-1/2 turns, and the mixture to 3-1/2 turns after the seat begins to drop, I started the engine. The outside temp was 75 degrees F. I switched on the thremo for a few seconds and the engine caught. Very quickly the rpm went to 2,500, steady. I made sure the foot throttle wasn't sticking. Under the bonnet I checked the linkage and that the throttle plates were closed. I screwed down the front slow-running screw almost shut. The revs did not drop. Then I screwed down the rear slow-running screw almost shut. Still no change in rpm. By now the revs were about 3,000, and the engine was getting very hot. I shut it down.

After it cools down I am going to reset the slow-running screws to 2-1/2 turns and check the mixture settings because I turned the front one in a little with no response. Before starting I had checked the thermo solenoid by turning it on and off, listening for the clicks, which were audible. My only current suspect is the thermo carb. Perhaps disconnecting the pipe that comes out the back of it will test if that is it.

Any advice will be appreciated. Regards, Morgan

 

Also check the vac takeoff on the rear of the manifold Did you do any other work while you were working on the carbs. Sure sounds like a larger air leak.

 

When the carbs were off the car, did you check that the throttle plates can completely close?

 

As with Jagboi check that the throttle linkage arms are tight and have not slipped on the throttle shaft/s. The butterflies (throttle plates) must be able to close completely when you lift your foot off the pedal.

This happened to me at first start up after total restoration & just required the shaft arm clamps tightened sufficiently with plates fully closed. We were ready to cut ignition should anything go awry. As it picked up revs we just turned off at the key. Probably went to 3000RPM before shut down. We had cranked the engine with plugs out & no ignition until oil flow was achieved at the cams & full pressure indicated on the gauge.

 

It is easy to forget the large spring under the carbs that pulls the throttle spindle to the closed position.

 

Yes ~ both springs must be at full tension which is pretty light.

 

Where is the "vac takeoff on the rear of the manifold"? The only vacuum takeoff I know about is from the distributor to the top of the front carb.

 

I lifted the throttle lever and made sure that both throttle plates were closed before I tried the slow-running screws. I only ran it for a few minutes because it was getting very hot... The indicted temperature was getting close to 100 degrees C. When I switched off the motor dieseled so I stopped it by holding the brake and releasing the clutch.

 

Is it possible that the thermo solenoid is sticking?

 

There is a major vacuum take-off for the power brake servo. If this line is dislodged you will get a major air intake leak and high RPM.
I presume the 340 intake manifold is much the same as the MK2 3.4&3.8 manifold.
The vacuum take-off point is underneath or on the bottom side of the manifold

 

Vacuum Takeoff to booster/servo. ~ Transitions from metal to a reinforced rubber pipe/hose.

 

Thank you Glyn for the location of the brake vacuum line. From the engine, it dips under the rear carb, but by feel it is intact.

I loosened the throttle linkage, checked for closure, and checked the adjustment on the intermediate throttle stop.

The starting pipe will not come out of the thermo unless the carb comes free from the intake manifold. My thought is to take the solenoid off and stopper it from there.

Here are the before and after pictures of the carbs on the car.


Before

Today 11Jul22

 

That rear carb is heavily deposited ~ front not too good either. The carbs really need to come off for a thorough clean up.

Any vacuum line issues will be in the flexible pipes to the booster/servo.

There are small rubber pipes under the manifold that need to be checked as well.

 

Today, the solenoid came off the thermo, and the top of an eye dropper poked down to shut off the fuel from that source. The idle screws on both carbs were screwed shut, and the closure of the throttle plates was confirmed. The temperature in the shop was 80 degrees F, and the car started right up. The revs rapidly climbed to 2,200. After about ten seconds, I shut it off.

I can't see nor feel under the carbs. I have an appointment at the shop with a lift for Monday a week. I have an endoscope that should be available tomorrow so I can look at the brake booster and the thermo distribution pipes. I don't understand where the fuel is coming from to run the car at about one-third throttle, given that the throttles plates are shut and the thermo definitely out.

I would like to find the problem so that I can do a minority engineering fix to avoid the roll-back charges.

 

And clean those carbs. I hate to think what the jets & needles look like. Put a bottle of Techron in your fuel tank.

 

Because it is difficult to see below the carbs but you can just get your hand under there with the air filter housing off, I use blue paper towel to detect any leaks. Either lie the paper towel under the carbs or wipe the bottom of the carbs with the towel and any fuel leak will show up by turning the light blue paper to a dark colour as the fuel is mopped up. Try doing this in small areas first checking the paper towel as you go along before moving onto the next point. A really common point for fuel to leak is from the joint between the bottom of the front carb to the starter carb. The joint is held in place with two banjo bolts with copper washers and is difficult to get a spanner on but you do not want to over tighten these banjos as the are steel in an alloy thread.

 

Today I finished tuning the carbs and putting the air cleaner back on. It turns out I did"t have the carb mounting bolts tight enough. I set the idle screw to 2-1/2 out and the mixture screws to 3-1/2. The idle was slow, but the pipe listening test indicated that the carbs were synchronized. Small mixture adjustments going back and forth between the front and rear completed the carb tuneup, and the rpm was about 600.
The car is running very well, and no fuel leaks. I may tackle the heater next because I want the defrosters to work.
Here are my notes from the thermo control circuit.
Thanks to all for the advice. Posts pointed out early on that the problem was an induction leak, and indeed it was.
Here are my notes for installing the Otter eliminating circuit... Use at your own risk.
Regards, Morgan

 

"Posts pointed out early on that the problem was an induction leak"

I'm delighted for you that you have it sorted out ~ Now you can enjoy the car.