Do I need a new engine?

[39topless]

In trying to determine why my engine, with a newly rebuilt Bubba distributor and two newly rebuilt carbs, is starting and running so poorly, I did a compression test. The following are the readings: #1= 80, #2=95, #3=70, #4=90, #5=75, #6=85, #7=85 and #8=90.
This engine does not smoke and doesn't seem to use much oil between changes. I also did a vacuum test for leaks and the reading was a steady 15.5". Would the compression readings above indicate that perhaps my poor running engine needs to be rebuilt?

[Scott H in Wheaton]

You can't declare an engine dead just because you think it runs 'poorly'.

Be more specific, what do you mean by 'poorly'?

Hard to start?
Lack of power?
Backfiring?
Running rough at idle?
Surging at cruising speed?

Distributor and carbs won't be related to compression, they are completely separate issues.

[40cpe]

Those compression readings don't indicate top condition, but the engine should run smoothly if all the other systems are functioning correctly.

[ford38v8]

Something else about a compression test... Do it while the engine is fully warmed up, all spark plugs removed, carb wide open, and battery fully charged. The difference in readings can change your mind about the condition of the engine.

[Bassman/NZ]

Point of interest... why should the engine be warmed up? Just curious.

[ford38v8]

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Hmm.. good question. Cold sluggish oil, rings not fully functional, sluggish valve closure, tolerances not normalized yet, not sure, just what I've always been told.

[Scott H in Wheaton]

Originally Posted by Bassman/NZ

Point of interest... why should the engine be warmed up? Just curious.

Things expand when they get hot and potentially seal better, giving you higher compression.

[Ronnieroadster]

What part number and brand spark plugs are you using? If your using Champion H-10's some times the made in mexaco plugs are bad causing poor running dead cylinders and similar ignition related troubles.

[tubman]

When I was a kid, numbers like that were to die for. Seriously, I don''t think they're bad at all, especially seeing you're in Colorado (altitude and all). If it were mine, I'd make sure it wasn't something else before I started tearing it down.

[39topless]

Originally Posted by Ronnieroadster

What part number and brand spark plugs are you using? If your using Champion H-10's some times the made in mexaco plugs are bad causing poor running dead cylinders and similar ignition related troubles.

Autolite 86

[41panelmark]

Originally Posted by Scott H in Wheaton

You can't declare an engine dead just because you think it runs 'poorly'.

Be more specific, what do you mean by 'poorly'?

Hard to start?
Lack of power?
Backfiring?
Running rough at idle?
Surging at cruising speed?

Distributor and carbs won't be related to compression, they are completely separate issues.

I would be interested to know also.

[tubman]

You might want to go back to a single carb until you figure what's wrong.

[39topless]

The engine is: hard to start, must be primed if sits overnight. Chugs with acceleration. Seems to have a steady miss, (if there is such a thing). Recent upgrades include: rebuilt distributor/Bubbas, rebuilt carbs, new coil and condenser, new intake, new electric fuel pump/filter and regulator (set to 2.5). Test shows no vacuum leaks.
Thank you, Phil

[41ford1]

As Scott asked, what do you mean by "poorly"?

Dual carbs. What manifold? What is the carb set up? Jets? Power Valves?

I've had recent experience with leaking PVs. That caused rough idle and stalling.

[Scott H in Wheaton]

Originally Posted by 39topless

The engine is: hard to start, must be primed if sits overnight. Chugs with acceleration. Seems to have a steady miss, (if there is such a thing). Recent upgrades include: rebuilt distributor/Bubbas, rebuilt carbs, new coil and condenser, new intake, new electric fuel pump/filter and regulator (set to 2.5). Test shows no vacuum leaks.
Thank you, Phil

hard to start, must be primed if sits overnight
might be fuel draining back to the tank
and/or fuel bowls leaking into engine after shut-down. that would make it hard to start and could contribute to fouled plugs. I have a fuel line with an anti-siphon valve in it

[Ross F-1]

Originally Posted by tubman

When I was a kid, numbers like that were to die for. Seriously, I don''t think they're bad at all, especially seeing you're in Colorado (altitude and all). If it were mine, I'd make sure it wasn't something else before I started tearing it down.

Here are the compression minimums for evaluating the need for a rebuild, from a Ford manual. I agree, the numbers posted aren't bad, for the altitude.

[deuce_roadster]

Poorly adjusted valves could cause what you describe. One cylinder with an intake valve that barely opens could make it run really poorly. Just another idea.

[41ford1]

Originally Posted by Scott H in Wheaton

hard to start, must be primed if sits overnight
might be fuel draining back to the tank
and/or fuel bowls leaking into engine after shut-down. that would make it hard to start and could contribute to fouled plugs. I have a fuel line with an anti-siphon valve in it

Are you running Holley 94s or 97s?
By design 94s can't leak back. If the power valve diaphragm fails the bowl will empty into the manifold on shut down. That will cause hard starting and rough running.
97s will drain back only to the inlet level to the tank fuel if the check valves in the fuel pump leak.

[Bolts]

Sounds like a fuel issue to me. Does it smell bad when it runs?
I think a power valve might be leaking causing the chugging. JMHO.

[39topless]

Originally Posted by Ross F-1

Here are the compression minimums for evaluating the need for a rebuild, from a Ford manual. I agree, the numbers posted aren't bad, for the altitude.

Wow, I guess I'm ok.
Thanks for the chart.
Now to check the power valve that several people have suspected.

[39topless]

Originally Posted by deuce_roadster

Poorly adjusted valves could cause what you describe. One cylinder with an intake valve that barely opens could make it run really poorly. Just another idea.

Thanks deuce, You may have hit on the problem. I do have an occasional backfire through the exhaust which would indicate a sticking exhaust valve. If that one sticks, it's likely an intake valve could be sticking as well. I'll try removing the spark plugs and pouring in some kerosene to soak overnight. Maybe that will free up all the valves.
Thanks again.

[BILL WZOREK]

Marvel mystery oil in the gas / should put some every time you gas up.
GM on here is a very / very big supporter of MM oil.
Ask about how to put some down carb while running
Doing this and going for a drive will free up any stuck valves.

[Mike in AZ]

do you have a mechanical fuel pressure gauge to go along with pressure regulator??....to check if 2.5 is 2.5....good luck....Mike

[Tinker]

If you had sticky valves wouldn't that also be indicated in the compression test?

Backfires through exhaust could also be too much fuel or a timing issue.

curious

[redmodelt]

Could it be a timing issue? How about running too rich/lean and the mixture is still burning when the exhaust valve opens, wrong jets.

[39topless]

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Originally Posted by redmodelt

Could it be a timing issue? How about running too rich/lean and the mixture is still burning when the exhaust valve opens, wrong jets.

Newly rebuilt distributor from Bubba, jets are .43 per Maxx

[uncle max]

Originally Posted by 39topless

Newly rebuilt distributor from Bubba, jets are .43 per Maxx

Errr... .043". Did you make sure the emulsion tubes were not crushed and that the highspeed airbleeds are cleared?
Sometimes it's the little things that cause bigger problems.

[39topless]

Originally Posted by uncle max

Errr... .043". Did you make sure the emulsion tubes were not crushed and that the highspeed airbleeds are cleared?
Sometimes it's the little things that cause bigger problems.

Oops, decimal point in the wrong place. Now if I did have .43's I would be getting plenty of fuel! I believe the emulsion tubes are ok, don't know about the highspeed airbleeds. Guess it's time to take these two carbs apart again and start over. Thanks for the reply Uncle.

[39topless]

Originally Posted by tubman

You might want to go back to a single carb until you figure what's wrong.

YES!!! I have been thinking about your suggestion since your post. Last night, while laying in bed awake because, I can't figure out what's wrong with my engine, it came to me. Why not test each of these two carbs by blocking off one and seeing how the other one ran by itself. I did that today. The first carb ran like crap so I figured I was just wasting my time again. Then I hooked up the other carb. It ran great! I just returned from one of the moist enjoyable drives that I've had in this car. Two miles, at incredibly smooth, and occasionally illegal, speeds. Thank you Mr Tubman. You is the MAN!!!!

[JSeery]

I'm guessing it's a carb problem, just a guess mind you! LOL

[Capt Kirk]

Not sure if "moist enjoyable drives that I've had in this car" was a Type-o or intentional. Either way, it's all good. Glad you got it fingered out.