Engine power

[Bob Johnson]

I just installed a rebuilt short block. It did not have the power I thought it should have. One of the tests I did was to short out each spark plug one at a time while the engine was at idle. Shorting out plugs 1 or 2 would slow the engine down a little. Shorting out 3 or 4 would cause the engine to stop.

The short block is a complete rebuild. 0.040 under pistons, new babbitt, new valves, adjustable lifter, reground cam, Snyder 5.5 head. The carburetor and distributor are from the old engine, which ran good.

I have checked the following;
Compression -- 85-90
Timing -- Spot on
Valve timing -- Cyl 1 and 4 have almost identical valve timing profiles.
Manifold leaks -- Check intake and exhaust NO Leaks
Spark plugs -- New plugs, swaped them 3<>2, 1<>4 no difference
Distributor -- I set timing to 180 off and swapped spark plug wires 1<>4, 2<>3 no difference

I am out of ideas. Do you have any?

Bob

[gweilbaker]

Distributor cam or shaft wear wobble?

High voltage leak, rotor and or cap?

Throttle linkage motion, any loss due to wear or bent?

[CarlG]

Originally Posted by Bob Johnson

... 0.040 under pistons,...

How did you manage that?

[31Abone]

Engine tight ?,..I had a31 tudor with police head then I took it off and could not tell much difference in power a little maybe..sounds like you expecting too much with only a hi comp head..I picked up a 31 with a Skokie motor now that thing really moved, it had a balanced crank,inserts,cam,head,chev valves and b carb..60 to 70 HP I think?

[Tom Wesenberg]

Try another distributor top just in case you have crossfire between the front two cylinders. I usually find #3 goes out, as it did on mine the other day.

[Mitch//pa]

Sponsored Links (Register now to hide all advertisements)

Recheck the valve adjustment...
That can give you all the symptoms you have

[harleytoprock]

Intake manifold leak at 1 and 2 ?

[Tom Wesenberg]

"Distributor -- I set timing to 180 off and swapped spark plug wires 1<>4, 2<>3 no difference

I am out of ideas. Do you have any?"

So, you must have a modern cap and wires. Then forget swapping caps as I suggested. I would connect a vacuum gauge and report the reading.

[Bob Johnson]

Sorry pistons were 0.040" OVER

The issue is that at idle I can short out 1 or 2 and the engine only slows down a little. But shorting out 3 or 4 kills the engine. To me that suggests that I am not getting as much power from 1 and 2 as I am from 3 and 4.

gweilbaker: I swapped out the distributor --- same results. Throttle linkage is fine and would not cause above problem.

Mitch//pa: I measured the valve timing, every 0.010" of lift for valves 1,2,7,8. They were very close (1 and 8, 2 and 7).

harleytoprock: I checked no leak at manifold.

Tom Wesenberg: I do not have a modern cap. I just ran small jumper wires instead of the metal strips. So I could reroute the sparks.

Any other suggestions?

Bob

[tbirdtbird]

what does the rebuilder say?
what piston clearance was used?
what ring gap was used?

"Mitch//pa: I measured the valve timing, every 0.010" of lift for valves 1,2,7,8."
Not sure how common this measurement is done. The cam and lash control the lift.
So, again, what is the lash for each valve?

"reground cam"
what exactly does this mean? ground back to stock specs or something better

Is it hard to turn the motor over with the hand crank?

a stock motor should be able to run on any one cylinder, with the other 3 shorted; in fact anytime the stock 64# flywheel is used.....

[Mitch//pa]

Not sure what u mean.. Recheck the valve clearance / lash with a feeler gauge.. Intakes and exhaust

[kwisor]

how many miles on motor maybe tight and needs broke in I just got 775 on my motor with a hc head and the more I drive it the better it runs for a model a motor runs strong I curse 45 to 50 and have had it up to 60
kevin
1930 tudor

[1crosscut]

What are your "small jumper wires"?
Too small??

[Bob Johnson]

tbirdtbird: I purchased the engine from someone who had it as a spare. It had never been run. So I do not have access to the builder. I checked the lash before I put the engine in the car. The intakes were set to 0.011" and exhaust at 0.013". The compression test showed 85 to 90 so the valves are closing. I did a profile for the cam and the regrind has a little more duration than stock so the lift is not as much as stock (you can not have both). The motor is tight but not real tight. I can turn it over by hand but a doubt that I could start it by hand.

PLEASE note the following. The issue is that cylinder 1 and 2 do not seem to be as strong as 3 and 4. The cam profile (and lash) for cylinder 1 and cylinder 4 are very close.


1crosscut: jumper wires were just used to test swapping 1-4, 2-3 wires. I am using regular metal connectors.


Tom Wesenberg: vacuum test appears OK. http://www.modelahouse.com/images/vac01.mp4
Bob

[Tom Wesenberg]

The only causes that come to mind are:

1. Vacuum leak on front intake runner, but you said "no leaks".

2. Someone left a rag in the front runner, but you said vacuum gauge showed good.

3. Spark problem with cap, but you switched wires with no change.

4. Slow closing valves, but you said vacuum gauge showed normal.

When I had my distributor problem with #3 shorting internally the other day, the spark from the screwdriver to the plug base was very small, compared to the others with about 1/4" spark while shorting.

[1crosscut]

Try switching plugs 1 & 2 to cylinders 3 & 4 to see if the trouble follows the plugs.

[tbirdtbird]

"PLEASE note the following. The issue is that cylinder 1 and 2 do not seem to be as strong as 3 and 4. The cam profile (and lash) for cylinder 1 and cylinder 4 are very close. "

1. The reason I asked about piston clearancing, ring gaps, and can you turn the motor with the hand crank is because if you have a tight cylinder or two, (ie cyl 1 and/or cyl 2), then they will be binding and not pulling their weight as it were; so shorting them out won't slow the motor much. And then when you short # 3 or #4, 1&2 are binding enough to bog the engine down. This was and still is my suspicion.
2. remove all 4 plugs, and try to turn the motor with the handcrank. It should not take much effort at all if the clearances are all OK
3. I am not sure just what you mean by your measurement of 'cam profiling'. Neither does Mitch. Whatever you are referring to does not sound like a measurement normally taken by a builder and prolly not by a troubleshooter, either. I am not sure if this 'profiling' is providing helpful info. And whatever this profiling is supposed to mean, why are not all 4 cyls the same if the cam was reground.
4. Surely the seller knows who rebuilt the motor for him
5. Are you absolutely certain this motor was never run; or was it run and the owner decided it was too shakey a rebuild job and he decided to unload it

Be aware there are a lot of wannabe engine builders out there.........most of my work is correcting the 'rebuilds' of 2 builders not far from me......

[Bob Johnson]

Below is an example of cam profiling. I take measurements of the lift vs crankshaft angle. This graph shows the profiles for 5 different "used" Model A camshafts. I did not plot the data for the engine I am asking about but the profiles for all valves are similar.

I did not remove the pistons before I installed the engine so I do not know the fit of the pistons/rings. I did look at the cylinder bores and they looked freshly honed. The pistons, valves and everything else looked brand new. There were no signs that the engine had been run before.

If it is tight pistons in 1 and 2 will they loosen by running the engine. Or will they just seize ? Do I have to take off the head and/or the pan to check?

[Mitch//pa]

Is the engine running smooth at idle (normal) or does it have an uneven rough idle as your shorting of the cylinders indicate.
How did you check for vacuum leaks?
How does the spark intensity compare on all cylinders?
How about a leak down percentage of al cylinders

[gweilbaker]

Just for curiosity, does the distributor seat on the head? I've seen a few that didn't and would wobble whilst running.