High compression? or?

[78 GMC Jimmy]

So I have a per-igniton problem.

Tried lots of other stuff to fix it.

Next attempt: Determine if its a high compression motor.



Its got plenty of upgrades. Intake, carb, air filter, lots and lots of chrome, etc.

The previous owner put this motor in. Based on when I'm told by others, they say it must have a cam in it. Partly because of the way to sounds and how the exhaust characteristics are.


The vacuum gauge is never steady. It always flutters, but is in the green.


So dies it have high compression pistons? Shaved heads? How do I tell if its a high compression motor?


I'm told that a compression tester wont really help top determine.


Thoughts?

 

[Wes Harden]

if your static compression test reads above 165psi chances are good it has higher than stock emissions engine compression. If the vacuum needle is not steady, this may indicate a mechanical issue with the engine. Valve train, or cam lobe problem.

 

[78 GMC Jimmy]

if your static compression test reads above 165psi chances are good it has higher than stock emissions engine compression. If the vacuum needle is not steady, this may indicate a mechanical issue with the engine. Valve train, or cam lobe problem.

Are there any engine upgrades that could cause the needle to bounce around? Could a cam do that?

 

[Wes Harden]

A vary radical cam but the vacuum wouldn't be in the green at idle.

 

[78 GMC Jimmy]

A vary radical cam but the vacuum wouldn't be in the green at idle.

I'm not sure whats going on with the motor. It might be defective or I might be trying to tune a funny car engine to be a normal street machine.

 

[78 GMC Jimmy]

So only when the engine is hot, occasionally it wont turn over unless I disable to distributor due to pre-ignition, then it turns over just fine. While its spinning I can flip the switch to enable the distributor and it will start. Could this be cause by abnormally high compression?

 

[eodcoduto]

Have you checked and set the timing on it yet? Once thats verified check for intake leaks, and miss adjusted valves.

 

[78 GMC Jimmy]

Ya, timing is good. Changing the timing does not eliminate the problem. As for the valves, how do I adjust the valves if it has an unknown cam in it? Most likely aftermarket cam. Is there some way to identify it without pulling the trans and then the rear plug to see the end of the cam?

 

[Wes Harden]

If it doesn't crank slowly cold, it is more likely poor starter windings, battery cables, connections, or battery.

If it were high compression it would crank slow cold as well.

 

[eodcoduto]

For the valves you’ll have to pull the valve cover and check to see which rocker arms you have and look down the pushrods at the lifters to see if if you have solids or hydraulics. Wes is right too, check all your grounds and see if the starter is getting heat soaked.

 

[Blue85]

So only when the engine is hot, occasionally it wont turn over unless I disable to distributor due to pre-ignition, then it turns over just fine. While its spinning I can flip the switch to enable the distributor and it will start. Could this be cause by abnormally high compression?

How many threads are you going to start for this same problem?

Yes, if turning off spark makes is spin easier, than you have too much spark advance during crank. It's been covered in the other threads. The combustion chamber volume, valve timing and compression are the same whether the distributor is running or not.

 

[78 GMC Jimmy]

If it doesn't crank slowly cold, it is more likely poor starter windings, battery cables, connections, or battery.

If it were high compression it would crank slow cold as well.

You might be right, but how would disabling the distributor factor into it? Engibe barely turning over and I flip the distributor kill switch and it starts tuning over great. When warm, sometimes it starts just fine, but many times it has a ping during the start up. There is definitely something causing pre ignition during startup and its effected by heat, not timing. Unless somehow the timing is changing when the engine is hot. Thats why I was thinking maybe it a high compression motor. The high compression doesn't cause pinging unless its combined with enough heat in the cylinder?

 

[Wes Harden]

400s run hot anyway, could be a bit of carbon causing the ping.
Even a little compression engine can suffer from heat soak. Disabling the ignition fir cranking is band aiding the symptom, not the root cause.
not having the ignition powered does 2 things, gives the starter more voltage, and no spark provided for flame front to slow the pistons.

Is this the engine that was timed by vacuum ? I remember a thread on timing, but not the end results.

 

[78 GMC Jimmy]

400s run hot anyway, could be a bit of carbon causing the ping.
Even a little compression engine can suffer from heat soak. Disabling the ignition fir cranking is band aiding the symptom, not the root cause.
not having the ignition powered does 2 things, gives the starter more voltage, and no spark provided for flame front to slow the pistons.

Is this the engine that was timed by vacuum ? I remember a thread on timing, but not the end results.

Tried timing by ear, tried timing by light, tried timing by vacuum, tried timing by setting it too far advanced and then moving the distributor slightly and retest, and turn slightly and retest and turn slightly and retest until it was too far retarded. Tried lubricating the distributor weights and springs. Tried removing the distributor and installing a complete band new unit. Tried verifying timing marks. Tried using a brand new timing gun. Tried no vacuum advance. Tried timed vacuum advance port. Tried constant vacuum advance port.

 

[78 GMC Jimmy]

400s run hot anyway, could be a bit of carbon causing the ping.
Even a little compression engine can suffer from heat soak. Disabling the ignition fir cranking is band aiding the symptom, not the root cause.
not having the ignition powered does 2 things, gives the starter more voltage, and no spark provided for flame front to slow the pistons.

Is this the engine that was timed by vacuum ? I remember a thread on timing, but not the end results.

I have been thinking about carbon. I bought two packs of this stuff, and I was thinking about trying it. What procedure would you use? What about letting it soak? Use both cans at the same time? Try one one day and the next can the next day?

https://i.ebayimg.com/images/g/sgcAAOSwLHxm9Hhv/s-l960.webp

 

[Wes Harden]

Yes you can let it soak. Follow the directions on the can. Then put the other can in a full tank.
Be careful not to get to much sea foam in the engine at one time, Sea film doesn't compress, engine damage can occur

 

[78 GMC Jimmy]

Yes you can let it soak. Follow the directions on the can. Then put the other can in a full tank.
Be careful not to get to much sea foam in the engine at one time, Sea film doesn't compress, engine damage can occur

Does this stuff de-carbon the engine by adding to the fuel as well?

Also, what about filling the tank with premium gas and then adding 2 bottles of octane booster. Would this fix this dang problem.

 

[Blue85]

Can an engine really ping during start-up or idle?

If it's really spark-knock/ping, then higher octane will help. That's the definition of octane. If it's early spark, octane won't make much difference.

 

[shady]

Are you sure the timing is good?

Find top dead center on the compression stroke of #1 and see how close your timing tab actually lands to the '0' on your balancer.
There are 2-3 different tabs, balancers, etc.
Plus if it's a stock design one the outer wheel can spin on the rubber and your marks will be all off.

If you find TDC and the pointer is actually at '0',
Take the distributor cap off and see if the rotor is pointing at or close to the #1 plug wire post in the cap.
Then you will at least know all of that part is good.

I'd bet one of them is off.

 

[78 GMC Jimmy]

Are you sure the timing is good?

Find top dead center on the compression stroke of #1 and see how close your timing tab actually lands to the '0' on your balancer.
There are 2-3 different tabs, balancers, etc.
Plus if it's a stock design one the outer wheel can spin on the rubber and your marks will be all off.

If you find TDC and the pointer is actually at '0',
Take the distributor cap off and see if the rotor is pointing at or close to the #1 plug wire post in the cap.
Then you will at least know all of that part is good.

I'd bet one of them is off.

I can check them all. Again..... Once again, I will verify everything with the timing. From the piston to the rotor.

Note: this problem is not consistent. It only happens when hot, and it depends on how long its been sitting before restarting. Restart right away and its not there at all. Wait a bit and its there a little. Wait the correct amount of time and it wont even turn over enough to start. Wait longer and the problem is there a little. Wait long enough and its not there at all.