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Carb Problem?
http://slantsix.org/forum/viewtopic.php?f=19&t=63593
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Author:  Kerosene [ Tue May 07, 2019 10:56 am ]
Post subject:  Carb Problem?

I am back again. My Dart got back burnered for a while. But I am back trying to make it work.

My problem is when you give the car throttle. Slow or fast the car tries to die about 20% throttle, and will usually flame out the intake. You can fiddle with the throttle and occasionally get past this dead spot and the car sounds nice and revs well. No back fires or anything above this dead spot.

I've got the engine running and idling well. The car will start with a slight bump on the starter once its warm and sounds good at idle. With the vacuum advance blocked off the car is timed to 5 BDC with a light, idle seems at a normal speed 800-900 rpm. The vac and mechanical advance both seem to work(no idea if its correctly, but they both can move the timing). Mechanical seems to add about 10 and vac seems to add about 15 to BDC. I'd guess total timing is about 35 BDC, when the car idles the happiest. Valves are all lashed to .10 and .20. Ignition is all new except for the distributor, new cap, wire, coil, plugs. The carb was rebuilt by me, but all passages seem clear and to my novice eyes seems fine. All the intake side gaskets were changed, so there are no obvious vacuum leaks.


I've moved timing around, but the dead spot in throttle remains no matter where I start the timing. It also remains with vac advance disconnected. I am tempted to throw another carb at it, but would like someone else's opinion that the timing seems to be functioning correctly before I do.

Author:  THOR [ Tue May 07, 2019 11:15 am ]
Post subject:  Re: Carb Problem?

First thing: What carb model do you have?

Second thing is this:
Kerosene wrote:
I'd guess total timing is about 35 BDC, when the car idles the happiest.
'

We need to make sure what exactly you mean here. 5 degrees of initial timing at idle without vacuum advance should be fine for testing. At idle, mechanical should not be adding any advance. If your vacuum is adding timing when you hook it up, it's either being hooked to the wrong vacuum port, or the carb is open so far at idle that it's pulling vacuum, which it shouldn't be.

Once we've determined that your settings are correct on the ignition advance, then it's time to start looking at the carb.

Why did you rebuild the carb? Was it having this problem before the rebuild, or it is new since you've rebuilt it? Have you put a vacuum gauge on it to get a reading of vacuum at idle?

We'll get to the bottom of this!

~THOR~

Author:  Kerosene [ Tue May 07, 2019 1:32 pm ]
Post subject:  Re: Carb Problem?

The carb is a Holley single barrel.

The car is currently set at 5 BDC with the vacuum line disconnected. If the vac line is connected it instantly advances about 10 degrees to 15-20BDC(by timing light). The vacuum advance seems very much like its on/off, no gradual changes. The mechanical advance seems more gradual and floats back and forth, and seems to add another 10-15 degrees.

The line is connected to the port on the left side of the carb (inboard) near the idle mixture screw. The mityvac says there is 11 inches at idle and it drops as it revs off this port. I did notice the distributor side of this doesn't hold vacuum according to the pump(should it?). The carb only has 3 vac ports, a big one for the valve cover vent, and another small one on the right that is connected to vacuum pot on the choke.

The whole car came to me as a basket case. The carb was supposedly rebuilt recently before I got the car, but leaked fuel from the bowl, so I bought rebuild kit and mostly replaced the gaskets ect that came in the kit. The carb was very clean, and the check balls all seemed to move freely. The PO also cranked the car with the valves stuck, so I've also replaced all the push rods, since many were S shaped. Compression was equal on all 6 cylinders, so I assume no serious damage was done.

Author:  THOR [ Tue May 07, 2019 3:25 pm ]
Post subject:  Re: Carb Problem?

Kerosene wrote:
The carb is a Holley single barrel.


I'm going to assume here it's a Holley 1920.

Kerosene wrote:
The car is currently set at 5 BDC with the vacuum line disconnected. If the vac line is connected it instantly advances about 10 degrees to 15-20BDC(by timing light). The vacuum advance seems very much like its on/off, no gradual changes.

Kerosene wrote:
The line is connected to the port on the left side of the carb (inboard) near the idle mixture screw. The mityvac says there is 11 inches at idle and it drops as it revs off this port. The carb only has 3 vac ports, a big one for the valve cover vent, and another small one on the right that is connected to vacuum pot on the choke.


This tells me that you are connecting to manifold vacuum with the distributor line. The choke vacuum line should be going to manifold vacuum (vacuum all the time). I suspect it's currently hooked to ported vacuum (vacuum only above idle) which is above the throttle plates and where you want the distributor hooked up.

Kerosene wrote:
The mechanical advance seems more gradual and floats back and forth, and seems to add another 10-15 degrees.


It should be doing that only above idle and maxing out about 2500rpm or so. If it is wandering back and forth 10 to 15 degrees at idle, I seriously doubt you are seeing the mechanical advance unless the springs inside are missing/broken. Floating timing at idle is usually a symptom of a timing chain/gear set which has worn out. Based on your vacuum reading of 11 inHg, I suspect this very well may be the case.

Rotate the engine to TDC, take off the distributor cap and rotate the engine backwards by hand until the rotor starts to turn in the distributor. Once it moves, check and see how many degrees of crank rotation you got before the rotor moved.

Kerosene wrote:
I did notice the distributor side of this doesn't hold vacuum according to the pump(should it?).


If you put a vacuum pump on the distributor vacuum line and it isn't holding vacuum, you may likely have a leaky vacuum diaphragm assuming the rubber hose is in good shape. Not the end of the world, but you will want to replace it.

Kerosene wrote:
The whole car came to me as a basket case. The carb was supposedly rebuilt recently before I got the car, but leaked fuel from the bowl, so I bought rebuild kit and mostly replaced the gaskets ect that came in the kit. The carb was very clean, and the check balls all seemed to move freely. The PO also cranked the car with the valves stuck, so I've also replaced all the push rods, since many were S shaped. Compression was equal on all 6 cylinders, so I assume no serious damage was done.


Sounds like it. What is the result of the compression test? Did you set the valve lash hot or cold? If you set them cold, they are going to be too tight when it warms up.

~THOR~

Author:  Kerosene [ Wed May 08, 2019 1:15 pm ]
Post subject:  Re: Carb Problem?

It is correct I have a 1920. I'll check for slack in the timing as soon as I have a chance. I also lucked out and found another carb locally for $50. It was pulled from a working car that got an upgrade. So I'll have what should be a known good carb to play with tomorrow as well. Sounds like rain all weekend, so there will be nothing else to do.

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