1970 GTO compression check - Pontiac GTO Forum
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post #1 of 4 (permalink) Old 12-26-2017, 10:59 PM Thread Starter
 
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1970 GTO compression check

Hello. Trying to get a handle on the current state of my new car that seems a little tired and has a slight miss at idle. Did "dry" compression test today and the numbers seem good (low 155- high 165). 1970 455 with 6 4 heads. Is this pretty solid? The plugs looked good. When I bought the car the distributor had an recent HEI kit installed. I notice the distributor was clocked funny. Yup put my timing light on it and it was off one tooth. I set it correctly and put the base timing to 12 BTDC as a guess. The light I have is basic I also do not have a tachometer so I do not know what my total timing is. I sprayed around carb and intake looking for vacuum leak with no luck. Only other clue is exhaust smells like unburned gas. Suggestions on where to hunt next? Thanks
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post #2 of 4 (permalink) Old 12-31-2017, 10:23 AM
 
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Originally Posted by slider98208 View Post
Hello. Trying to get a handle on the current state of my new car that seems a little tired and has a slight miss at idle. Did "dry" compression test today and the numbers seem good (low 155- high 165). 1970 455 with 6 4 heads. Is this pretty solid? The plugs looked good. When I bought the car the distributor had an recent HEI kit installed. I notice the distributor was clocked funny. Yup put my timing light on it and it was off one tooth. I set it correctly and put the base timing to 12 BTDC as a guess. The light I have is basic I also do not have a tachometer so I do not know what my total timing is. I sprayed around carb and intake looking for vacuum leak with no luck. Only other clue is exhaust smells like unburned gas. Suggestions on where to hunt next? Thanks
Well, first thing to remember is that these older cars are not like todays fuel injection/electronic laden cars which most are so familiar with. The smell of gasoline from the exhaust and a slight miss at idle can all be "normal."

First, you can't guess as to what is wrong. You need facts. A vacuum gauge is a great tool to diagnose engine problems. They can be bought at your local auto parts store. A "Dial-back" timing light is really needed to dial in not only your timing, but your timing curve - mechanical/vacuum advance.

Compression numbers look fine because there is little difference in the numbers.

Next up is the carb. Many things can cause the carb to run rich and give you that "gas smell." Choke incorrectly setup, sticking, not working. Idle mixture screws incorrectly adjusted. Carb leaking internally. Carb running rich due to larger primary jets or power piston/rods stuck. Float setting too high. Aftermarket cam.

Slight miss can be cause by any one of the above from the carb, to include a worn throttle shaft allowing air in. Needs a complete tune-up. Spark plugs shot or incorrectly gapped. Bad plug wires. Corrosion in the cap at the contacts - often happens with the cheap aluminum contacts. Bad/worn rotor button inside the cap. Bad/worn rotor. Bad HEI. Bad HEI kit. Bad HEI module. Bad/weak coil. Bad power wire to the HEI kit. Incorrect voltage to the HEI. Sticking weights inside the HEI. Bad/weak vacuum advance can on the distributor. Bad/weak vacuum signal to the vacuum can. Poor gas. Poor engine ground. Spark plug wires touching or grounding out. Timing incorrect. Rocker arm coming loose or adjusted too tight. Weak valve spring. Intake heat crossover blocked. PCV valve bad or sucking air. Aluminum intake being too cold and not thoroughly heating the carb to provide even fuel/air distribution. Fuel puddling on the intake floor. Exhaust damper valve ( if you have one on the exhaust manifold) stuck or not operating correctly. Exhaust pipe/muffler collapsed internally or restricted flow.

I think that should cover it and give you a few things to check.
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post #3 of 4 (permalink) Old 12-31-2017, 11:09 AM
 
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Great list.. I copied and saved it.

I like to diagnose instead of parts swap - learn and save money... and no chance of putting on a bad new part or (for me) messing something else up while doing it.

A vacuum gauge is cheap and indispensable - though I read it gets tricky on cammed engines.
If needle's not in the 'green zone' (~20") I like to know why. To check for hose leaks, I'll plug lines (hidden headlights and heater controls have lines going everywhere) while watching the gauge - also plug the line going to vacuum modulator (checking it for fluid/leaking modulator when pulled from carb).

Another good tool is DVM for checking wires (and battery voltage/other stuff when you need to). There are specs for the type wire you use. It goes fast because you don't have to pull/reroute wires (though won't show worn insulation that arcs to ground). You can get a cheap DVM, or often free with coupon, at Harbor Freight.

For ignition misses (bad cap, crossfire), etc, I've put a timing light on all cylinders - sometimes the light will misfire or double-fire (good when correlates to sound of engine) or it jumps around if you look at the damper (if crossfire). I don't think this is definitive - my light jumps around when the engine's fine, but I've found a few problems like this.

Ultimate is using a scope (I had one from work - picked up cheap automotive inductive lead on inet). I've read you can see ignition problems and lean-rich fuel - I don't know how to do this yet, but can compare signals between cylinders. I had a bad miss on 89 Cavalier (distributorless) and a bad coil pack showed up. The big dashed spike in photo showed with an intermittent misfire - other signals are normal.
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post #4 of 4 (permalink) Old 01-02-2018, 02:40 AM Thread Starter
 
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Thanks for the reply, there is a few things on your list I did not think of i can go after.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

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