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Old 08-28-2021, 03:03 PM   #1
aermotor
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Default camshaft (Long)

I have never had an A engine totally apart. My question is - can a model A "jump" timing if the camshaft thrust plunger is not doing it's job because of a weak spring or some other reason? Reason I ask is because after complete trouble shooting the timing and entire ignition system the engine runs fine at idle and a little above. However when the advance lever is pulled down the engine will "bog" down and eventually quit. The distributor plate lever is touching both sides of the housing at full advance and full retard. Note, again, that this is happening with the advance lever being pulled down from full retard. I did pull the distributor shaft housing and shaft gear and could see no problems. The steering box is 7 tooth so no adjustment is available there but the dist. plate arm adjustment is okay as stated above. Timed with NewRex tool and once again the entire ignition system was trouble "shooted" and no problems noted. The carb was taken apart and rebuilt 2 times so carb. problems should not exist as the condition is all related to the spark advance - so I think. I have no way to know if it is going more retarded or advanced when it bogs down. I know how to determine with a timing light but I don't believe they even existed in the model A days and this problem should be fixable without one. The problem started with an erratic miss which could not be pin pointed to any one cylinder and occasional back fire when rapidly letting up on the gas pedal at higher RPM's when it was running. So I fixed it till it's really broke. O, yes the point gap is .017 and rotor to housing contacts all the same @ .025.

John

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Old 08-28-2021, 04:01 PM   #2
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Default Re: camshaft (Long)

Point gap may be too close for stock. I run mine at .020. Service Bulletins from April 1930 say .018-.022. I dunno about any other system.
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Last edited by 700rpm; 08-28-2021 at 04:08 PM.
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Old 08-28-2021, 04:33 PM   #3
Patrick L.
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Default Re: camshaft (Long)

Is this a continuation of the previous, 'fixed till broke' thread ?
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Old 08-28-2021, 04:38 PM   #4
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Default Re: camshaft (Long)

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Point gap may be too close for stock. I run mine at .020. Service Bulletins from April 1930 say .018-.022. I dunno about any other system.
I have tried everything from .017 to .020

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Old 08-28-2021, 04:39 PM   #5
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Default Re: camshaft (Long)

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Is this a continuation of the previous, 'fixed till broke' thread ?
Yep. John
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Old 08-28-2021, 04:45 PM   #6
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Default Re: camshaft (Long)

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It is unlikely that the timing gear has jumped a tooth or two. But there is an easy way to check without taking the engine apart. First a little theory. The piston is at top dead center (TDC) for two times during the 4 stroke cycle. The first time the fuel is igniting and pushing down on the piston to start the power stroke. The second time the cylinder is switching from the exhaust stroke to the intake stroke. At that TDC both valves are slightly open.

The timing detent in the timing gear on the camshaft is used to locate TCD for the power stroke for the #1 cylinder. At the same time the #4 piston is at TCD switching from the exhaust stroke to the intake stroke.

Take all the plugs out. Leave the ignition and fuel off. Rotate the engine until the timing pin drops into the detent in the camshaft timing gear. Use a bore scope or other tool to check that the #1 piston is at TDC. The timing can be off a few degrees and the piston will still look like it is at TDC. Now check the valves on the #4 cylinder. The both should be just slightly open. If the cam timing is off, one valve will be closed and the other will be open about 1/4 inch.

The timing gears are helical cut, not straight cut, so I guess it is possible for the timing to change if the cam shaft is moving forward. If the cam plunger and spring are completely missing then I guess the timing could change to the point where the engine performance would be compromised.
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Old 08-28-2021, 04:56 PM   #7
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Default Re: camshaft (Long)

If the engine bogs down when you pull down on the timing lever then either the timing is way too advanced or there is a fault with the distributor. I know you said that you have checked the ignition system, but it is still ignition related, so something is still wrong. Pulling down on the timing lever will advance the timing.

Check to see if the top plate in the distributor is not loose so that when you move it the point gap is changed. Also check that the wire that leads to the plate underneath does not have a bare spot that is rubbing against something.

The timing method on a Model A is pretty fool proof. Assuming the cam is timed correctly, the points should just be opening when the timing lever is full up and the timing pin is in the detent. If not then it is not timed correctly.

Lots of people use a timing light and degree wheel on a Model A to check the timing. The mechanics are the same as a modern engine. You can buy the parts from the vendors. Instructions come with the parts.
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A is for apple, green as the sky.
Step on the gas, for tomorrow I die.
Forget the brakes, they really don't work.
The clutch always sticks, and starts with a jerk.
My car grows red hair, and flies through the air.
Driving's a blast, a blast from the past.

Last edited by nkaminar; 08-28-2021 at 05:07 PM.
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Old 08-28-2021, 05:04 PM   #8
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Default Re: camshaft (Long)

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Originally Posted by nkaminar View Post
It is unlikely that the timing gear has jumped a tooth or two. But there is an easy way to check without taking the engine apart. First a little theory. The piston is at top dead center (TDC) for two time during the 4 stroke cycle. The first time the fuel is igniting and pushing down on the piston to start the power stroke. The second time the cylinder is switching from the exhaust stroke to the intake stroke. At that TDC both valves are slightly open.

The timing detent in the timing gear on the camshaft is used to locate TCD for the power stroke for the #1 cylinder. At the same time the #4 piston is at TCD switching from the exhaust stroke to the intake stroke.

Take all the plugs out. Leave the ignition and fuel off. Rotate the engine until the timing pin drops into the detent in the camshaft timing gear. Use a bore scope or other tool to check that the #1 piston is at TDC. The timing can be off a few degrees and the piston will still look like it is at TDC. Now check the valves on the #4 cylinder. The both should be just slightly open. If the cam timing is off, one valve will be closed and the other will be open about 1/4 inch.
Did all that. I'm talking about the gear on the shaft that goes from the oil pump to the distributor. The shafts are all slip tight at the unions. I realize that top dead center observation in the spark plug hole could have a few degrees of crankshaft rotation but that should be a non-issue. I don't mean to be a smart a** on any replay - some may take it that way, I just need help.

Thanks, John
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Old 08-28-2021, 05:17 PM   #9
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Default Re: camshaft (Long)

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Originally Posted by nkaminar View Post
If the engine bogs down when you pull down on the timing lever then either the timing is way too advanced or there is a fault with the distributor. I know you said that you have checked the ignition system, but it is still ignition related, so something is still wrong. Pulling down on the timing lever will advance the timing.

Check to see if the top plate in the distributor is not loose so that when you move it the point gap is changed. Also check that the wire that leads to the plate underneath does not have a bare spot that is rubbing against something.

The timing method on a Model A is pretty fool proof. Assuming the cam is timed correctly, the points should just be opening when the timing lever is full up and the timing pin is in the detent. If not then it is not timed correctly.
Got all this checked and even replaced the distributor with one out of my
speedster that ran great on the road 10 mins. before the swap and all the problems are the same. I know I am missing something somewhere but two other A people have looked over my shoulder, we are all stumped. BTW, I replaced the cam gear a couple years ago because the old one had a lot of slack between the phenolic center and the ring gear. I did check the points opening with a trouble light.

Thanks, John
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Old 08-28-2021, 05:39 PM   #10
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Default Re: camshaft (Long)

Well, John, It sounds like you have done everything I can think of and done it correctly. I am stumped too. I will have to think about it.

If the engine is getting the right fuel/air mixture, the compression is good, and the spark is at the right time and a hot spark, the engine should run well. Have you tried a different coil and high tension wire? A marginal spark will not jump the gap at the plug under compression. It will look good at atmospheric pressure but not under compression.
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Step on the gas, for tomorrow I die.
Forget the brakes, they really don't work.
The clutch always sticks, and starts with a jerk.
My car grows red hair, and flies through the air.
Driving's a blast, a blast from the past.

Last edited by nkaminar; 08-28-2021 at 05:46 PM.
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Old 08-28-2021, 07:15 PM   #11
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Default Re: camshaft (Long)

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- Well, John, It sounds like you have done everything I can think of and done it correctly. I am stumped too. I will have to think about it.

If the engine is getting the right fuel/air mixture, the compression is good, and the spark is at the right time and a hot spark, the engine should run well. Have you tried a different coil and high tension wire? A marginal spark will not jump the gap at the plug under compression. It will look good at atmospheric pressure but not under compression.
I did try a different coil and wire. I only checked the compression the old time way with my thumb in the spark plug hole. As I said every thing I changed I took off my good running speedster. As an aside I have had the engine running smooth on one cylinder taking one plug wire off at a time, I didn't think a 4 cylinder engine would do that but when tuned properly it will. I fool with hit & miss engines and some will run good by pulling the flywheel through with my thumb and fore finger, I doubt more than 3 or 4 to 1 compression ratio - if that much. I also run 2 hit & miss 8 cycle engines - figure that one out. I am really stumped.

Thanks, John
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Old 08-28-2021, 07:20 PM   #12
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Default Re: camshaft (Long)

Can you borrow a compression gauge and get some accurate numbers? You may want to use a vacuum gauge too. See the thread about the vacuum gauge.
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Step on the gas, for tomorrow I die.
Forget the brakes, they really don't work.
The clutch always sticks, and starts with a jerk.
My car grows red hair, and flies through the air.
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Old 08-28-2021, 08:27 PM   #13
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Can you borrow a compression gauge and get some accurate numbers? You may want to use a vacuum gauge too. See the thread about the vacuum gauge.
I will do that, will take a couple days due to other commitments and will need help with the results as related to this problem.

Thanks, John
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Old 08-28-2021, 09:41 PM   #14
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Default Re: camshaft (Long)

Let's return to the OP's original question: "My question is - can a model A "jump" timing if the camshaft thrust plunger is not doing it's job because of a weak spring or some other reason?"

The answer is Hell, yes! The spring may be weak or simply broken. I chased this all over the ignition, timing, compression, and fuel delivery universe before discovering a broken camshaft thrust plunger spring. I kept checking and adjusting every parameter of normal running, the engine would start and run correctly until I braked, made a sharp turn, shifted gears or scratched my arse, then it all fell apart, with backfiring, loss of power, etc.

Pull that front timing cover and tell us what you found.
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Old 08-28-2021, 10:12 PM   #15
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I have tried everything from .017 to .020

John
But are you running stock?
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Old 08-28-2021, 10:39 PM   #16
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Default Re: camshaft (Long)

When I read "jump timing", I thought you meant can the gears change due to the cam gear moving forward and out of mesh with the crank gear. The answer to that is no, but the spark timing will change due to the camshaft moving forward and rearward some if the spring is broken.
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Old 08-29-2021, 12:53 AM   #17
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But are you running stock?
Yes, John
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Old 08-29-2021, 01:02 AM   #18
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When I read "jump timing", I thought you meant can the gears change due to the cam gear moving forward and out of mesh with the crank gear. The answer to that is no, but the spark timing will change due to the camshaft moving forward and rearward some if the spring is broken.
Thanks Tom, on the way to the shop to put things together so I keep from tripping over parts (in the AM). Then unto the timing cover, any thing else to check with the cover off for good measures?

John
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Old 08-29-2021, 01:10 AM   #19
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Default Re: camshaft (Long)

ursus was on the spot with several symptoms, so timing cover next - nice to have Tom's advice to go along. I may have been demoted to Newbie. What size is that slippery little roll pin that was a booger to get out with the engine in the car and is it hardware store quality?

Thanks, John

Last edited by aermotor; 08-29-2021 at 04:39 AM.
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Old 08-29-2021, 01:18 AM   #20
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Default Re: camshaft (Long)

Mouse nests clogging the exhaust? Or a backfire dislodging baffling in the muffler? That can mess up how an engine will run. I know that doesn't jibe with all the symptoms but if all else fails.....
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