Sick Engine

Thom IV

Thom IV

Senior Member
Need some advice and things to check for.

My 31 Rdstr isn't running well. Lack of throttle response, backfiring through the carb. Necessary to have GAV open almost a full turn in order to get engine to run smooth. Last time I drove it best speed was 35 MPG with throttle to the floor. Starts good. What should I be checking?

Thanks in Advance for your thoughts
 
I woiuld check for:

Fuel restriction (crud in line or carb somewhere), Bad Coil, Bad Condenser, or timing problem. One of these would probably be my first guesses.
 
If it turns out to not be a fuel delivery or ignition/timing problem, check for a possible bad intake manifold to block gasket or a head gasket leak between 1&2 or 3&4. Do you have a vacuum gauge and a compression tester?
 
For sure, check the fuel supply. Disconnect the line going into the carb, Either put an appropriate size hose on it or hold it over a can and have someone turn on the gas valve on and check the flow. It should be a very solid stream. Also check that the gasket in the gas cap hasn't plugged the vent. Let us know what you find!
 
Thom IV said:
Need some advice and things to check for.

My 31 Rdstr isn't running well. Lack of throttle response, backfiring through the carb. Necessary to have GAV open almost a full turn in order to get engine to run smooth. Last time I drove it best speed was 35 MPG with throttle to the floor. Starts good. What should I be checking?

Thanks in Advance for your thoughts
Click to expand...

You have a massive vacuum leak, probably at the intake to block gasket. This will require truing the manifolds and intalling a new gasket. The symptoms are classic, starts good and idles fine. Will not rev up or make power. GAV must be opened to compensate for unwanted air. Could also be caused by a warped flange on the tillotson carb or a cracked intake manifold. An intake valve stuck open will cause similar problems but usually be hard starting also.
Rainmaker Ron
 
Thom, Try turning the small mixture screw on carb when engine is idling, if you don't see or hear any difference, Rainmaker Ron hit it right on the head--- Air leak!
 
Rainmaker Ron said:
You have a massive vacuum leak, probably at the intake to block gasket. This will require truing the manifolds and intalling a new gasket. The symptoms are classic, starts good and idles fine. Will not rev up or make power. GAV must be opened to compensate for unwanted air. Could also be caused by a warped flange on the tillotson carb or a cracked intake manifold. An intake valve stuck open will cause similar problems but usually be hard starting also.
Rainmaker Ron
Click to expand...

Ron,

In this case would someone try to use a propane torch (not lit) and wave it around the area where the leak is suspected to see it engine increases speed?
 
1931 flamingo said:
I would think a spray can of WD40 would be A LOT SAFER! JMO
Paul in CT
Click to expand...
Paul, I agree, WD40 use to work good for such tests and was really some wonderful stuff. They changed the formula and WD 40 is no longer flamable. Now that WD is no longer flamable it will no longer let the engine speed up when sprayed over a vacume leak. When I last tried is on my 29 strip down it ran down the side of the valve cover and left dark ugly streaks from around the manifold on my purdy fresh painted engine.

Though it is very flamable I think that starting fluid works best, it evaporates quick and won't stain the engine. If there is a vacume leak it will tell the story right now.
 
Mike V. Florida said:
Ron,

In this case would someone try to use a propane torch (not lit) and wave it around the area where the leak is suspected to see it engine increases speed?
Click to expand...

Mike, I used that idea for years but lately found it had no effect. I see from Purdy Swoft post that WD 40 is now non flamable..no wonder it stopped working as a test!

Rainmaker
 
Possibly h/gasket , but more likely weak mixture. as well as above suggestions I've been caught out by having no vent on gas tank cap. Whoopsy.. swapped gas cap (which is vented) and rad cap (which isn't vented)! Doh!
 
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It may be something simple as the main jet stopped up. Especially if the car has not been driven for a while. Gas will evaporate through the jet and leave a residue. The new gas is nasty stuff.
 
Here is a easy way to test for vac leaks hold you finger over the carb, do not completely cover it slowly close you fingers covering the opening if a vacuum leak is there the motor will rev up, if not will choke down
 
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