Help - My A is barely running

[saintjoelarry]

I've been struggling with my A for some time now, and I'm not stumped.

My 31 Roadster ran fine the first 2 years after I purchased it. Then out of the blue one time while driving, it started loosing power, and my speed dropped. Then dropped further, until I limped home the last couple miles at no more than 20 MPH.

Here are the symptoms, followed by what I've done to date.

Sypmtoms:
- Very little power
- Runs fine at idle, when I accelerate, it pops/backfires out of the carburetor
- Also, while running even at idle, there is a loud whining noise. I cannot pinpoint its location. Is pretty clear echoing out of the exhaust pipe, but at the engine, I cannot pinpoint the source.

What I've Done
- Ignition? - Started with the easiest things first, right on the side of the road that day. I've now timed the distributor probably 20 times since this whole thing started, with no change. I've tried 3 different condensors, 3 different coils, and 2 different coil wires. No change.
- Checked for Burned Valve or Hung Up Valve - First, I ran compression check on all cylinders. They were all 60 to 62 psi compression, which seems to be within range.
- Check Valve Timing - Although it seems unlikely that valve timing would change on a model A (giving the steel timing gears - more on that later). I checked that both valves of each cylinder are closed when the piston is at TDC on compression stroke. Using a flashlight, I can see the valves are closed.
- "Static" Compression Check - not sure what else to call this test, but with each piston at TDC on compression stroke (both valves closed), I attached my compression test hose, then set my air compressor hose pressure very low at 20 psi (low enough so that it didn't push the cylinder down), attached the hose to my compression hose, and listened for air movement. I heard no air coming from the carburetor or out of the exhaust. There is some air flowing past the rings into the oil pan (heard clearly through oil fill tube). Although that's not great, it certainly is not unusual in my experience for a Model A to have ring blow-by. Also, this should not cause back fire through carburetor.
- Bad Head Gasket? - I thought maybe I had a bad head gasket between 2 cylinders. But, when I did the compression check, all spark plugs were out. If compressed air was escaping from one cylinder to the next, it would have shown essentially nothing on the compression gauge. Also, the static compression test would have had air leaking out of neighboring sparkplug hole.
- Bad Timing Gear? - As I also had a pretty decent oil pan leak, while I was stumped, I decided to go ahead and replace the pan gasket. While down there, I could clearly see the timing chain gears - so I watched the gears as a friend turned the engine over. The gears look great. No missing teath.


Now what? I'm stumped. Could a gaket between the exhaust and intake manifold have failed, causing exhaust gas to be pushed into the carburetor (thus the back firing)? Seems unlikely, but who knows.

Any help would be greatly appreciated!


Larry

[Bob C]

Have you tried a different distributor body, they've been known to short between cylinders or to ground? Also vacuum leak.



Bob

[saintjoelarry]

Thanks. I just ordered a new distributor. The only spare I have has bad threads for popout switch.

Regarding vacuum leak, where would an A have a vacuum leak? Intake manifold?

[Bill G]

I would definitely check the manifold gasket. The exhaust and intake gaskets are connected and the two manifolds come off at the same time. Check nuts/bolts, to make sure one or more haven't worked themselves loose.


That might explain both the whining noise and loss of power.



With the car running, use a length of rubber or plastic tubing that you can hold one end up to your ear and the other that you can point around at various places. Chances are you might find the whining loud and clear around the manifold area.

[RandyinUtah]

I would take a close look at the intake manifold. I had one crack down by the stud and nut area and had similar systems like you describe.

[ryanheacox]

Sponsored Links (Register now to hide all advertisements)

Might be overkill for an A but you could stick a vacuum gauge on the wiper port if you have one.

[Bob C]

Originally Posted by saintjoelarry

Thanks. I just ordered a new distributor. The only spare I have has bad threads for popout switch.

I would have tried a new body first unless your distributor has other issues.


Bob

[SeaSlugs]

Originally Posted by ryanheacox

Might be overkill for an A but you could stick a vacuum gauge on the wiper port if you have one.

Most certantly not overkill - to me it goes leakdown test (static as OP called it), then vacuum gauge, then compression.

Yes the ring blowby is normal on a leakdown test - once the air pushes the oil past all the rings theres air gaps everywhere in the rings on any engine - same reason as you slowly hand turn the engine you can hear air getting squished squash noises coming from oil pan.

All good things to check especially manifold gasket and any air leaks from carb to intake ports.

Does it rev up in neutral or does it need the load of the car driving to act up? When it starts acting up pull the choke about 1/2 out (forces it to suck more fuel in) if it magically gets better its fuel related or vacuum related.

If it doesn't affect anything or run worse (flooding) id guess electrical and check that wire between upper and lower plates but usually that shorts/flakes out the car one minute and not the next or moving the spark rod causes the issue to arrise.

Since it idles OK as you say and your leakdown test showed no bad head gasket or internal engine sealing problems i would do a vacuum gauge test and try the fuel thing i suggested.

I assume it makes no difference if engine is cold or hot?

[mightynova63]

Try spraying carb cleaner on your intake gasket and the intake itself while the engine is idling. If you have any leaks the engine will increase in RPM.

Sent from my SM-G930V using Tapatalk

[Tom Wesenberg]

Idles good but won't rev up, so I'd look for a blocked exhaust.
Bent pipe, rodents nest, etc.
Vacuum gauge will drop quite a bit with a blocked exhaust.

[dpson]

I've had similar frustrations with a model a chassis I'm working on and after retightening the manifold nuts, which helped some, I adjusted the float level in the carburetor, the fuel level was too high which was fouling the plugs, and it seems to run much better now.

[JBill]

Even something as simple as a leaky carburetor gasket can makes some strange sounds and cause the symptoms you describe. To check it, use same procedure as in #9 above.

[katy]

Backfiring sounds like it's running lean. Have you checked fuel flow? Cleaned/checked carburetor?

[diomed]

I was thinking the same thing.

Originally Posted by Tom Wesenberg

Idles good but won't rev up, so I'd look for a blocked exhaust.
Bent pipe, rodents nest, etc.
Vacuum gauge will drop quite a bit with a blocked exhaust.

[Phil Brown]

Sounds like it's running out of gas, plugged carb jets or fuel line ?

[saintjoelarry]

OK, I'm heading out there now. I don't actually have a vacuum gauge, but will try with carb cleaner/starting fluid spray. But, I may run to store and buy a vacuum gauge.

I've thought about the exhaust blocked too. I thought about disconnecting exhaust from manifold and trying it, but worried about carb and exhaust being too close together with backfiring issues - that could make for a hot situation. I guess I could keep extinguisher nearby and a friend at hand just in case!

[saintjoelarry]

If anyone knows how to add video, I can upload a 30 second video so you can hear the sound and the engine missing. Not sure if we can do it or how to do it.

[kimlinh]

If it was a blocked exhaust why would the trouble start when the car was being driven, wouldn't it have been blocked from the time the car started? Did you try opening the GAV while the car was backfiring? If the engine was running lean this might show some improvement. It does sound like you are running lean. Good luck with it and let everyone know what it was when you find it.

[saintjoelarry]

FIXED!!!

It was plugged exhaust. A more modern muffler was on the car when I bought it, and the baffles inside the muffler collapsed and plugged the muffler. Not enough to stop it from running completely, but enough to stop it running well when any pressure more than an idle was going through it.

Thanks again everyone for the great suggestions. I learned a lot from our chats.

Larry

[SeaSlugs]

Originally Posted by saintjoelarry

FIXED!!!

It was plugged exhaust. A more modern muffler was on the car when I bought it, and the baffles inside the muffler collapsed and plugged the muffler. Not enough to stop it from running completely, but enough to stop it running well when any pressure more than an idle was going through it.

Thanks again everyone for the great suggestions. I learned a lot from our chats.

Larry

Awesome good to know - fyi id still get a vacuum gauge and look up below what different readings show. It would have shown a blocked (chocked) exhaust in a few seconds.

https://d2culxnxbccemt.cloudfront.ne...cuum-image.png