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very low compression on all cylinders after installing rebuilt 350

G

gfackrell

New member
'79 fiberform 20'
Bought the boat with engine not running several months ago
Put rebuilt 350 in, and it is weak on power
Been trying to figure out why and found out that I have really low compression on all cylinders;
readings are:
CYL 1: 72 psi
CYL 2: 66 psi
CYL 3: 72 psi
CYL 4: 65 psi
CYL 5: 63 psi
CYL 6: 45 psi
CYL 7: 63 psi
CYL 8: 62 psi

The boat has a hard time starting cold, but once it starts it runs fairly well. It just seems underpowered. I'm turning a 15x17 prop, getting 35 mph, at a little over 4000 rpm.
Questions:
How does the engine even run with this low of compression?
What are the easiest quickest ways to figure out what is causing the low compression?

Thanks,
Greg
 
Those compression numbers don't look that bad. Compression numbers are also dependent on throttle position, oil thickness, and cranking speed. Doing the test on a cold engine with a low battery and idle throttle position will yield lower readings than a warm engine with a good battery and wide open throttle.

Also it takes some hours for the rings to seat in a rebuilt engine.

Do you have more than 10 hours or so on the rebuild?
 
I would agree, those numbers are pretty bad.
I just did another test with a completely different tester, with the battery fully charged. The throttle was wide open on both tests. the engine was not warmed up, and it's 65 degrees outside right now here.
New numbers:
Cyl 1 -109
Cyl 2 - 100
Cyl 3 - 110
Cyl 4 - 97
Cyl 5 - 94
Cyl 6 - 85
Cyl 7 - 105
Cyl 8 - 102

Still not good numbers from what I am reading. Any guesses as to why before I do a leakdown test?
 
I just got done re-adjusting all the valves. It increased my compression about 5 psi per cylinder. I have had a couple people say something about cam timing being off. I don't know what that would do. Could the engine run very well with it off? Would the leak down test tell me if that is the problem, or would I just have to pull the timing cover and check? I'm starting to become not very happy with the guy who rebuilt this engine....
 
gfackrell said:
I just got done re-adjusting all the valves. It increased my compression about 5 psi per cylinder.
I have had a couple people say something about cam timing being off. I don't know what that would do. Could the engine run very well with it off?
Short answer.... NO! But it would run, and it would lack power.

Would the leak down test tell me if that is the problem
Not necessarily if the camshaft is out of phase.

or would I just have to pull the timing cover and check?
Yes.

I'm starting to become not very happy with the guy who rebuilt this engine....
Click to expand...

Greg, if the camshaft is indexed correctly, you've got something else going on.
Tell us about the engine build.
What piston profile was used? Flat tops or the full dished pisons?
If full dished, which dish volome?

Which cylinder head combustion chamber volume was used? 64cc, 76cc, other?
Which compressed head gasket thickness was used?

The piston profile must compliment the combustion chamber volume, and/or visa-versa.
Wrong combination here, and it may explain the low cylinder pressures.
Example: -22 cc dish volume pistons with 76 cc chambers, 4.030" bore, 3.480" stroke, .035" compressed gasket thickness, .016" deck clearance, will render a static C/R of just over 7.5:1.




.
 
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Ricardo,

Thanks for the reply. Unfortunately I don't have all of that info from the engine builder. I bought the engine from a guy in Wyoming (I'm in Utah) that builds a lot of marine engines. It's supposed to carry a warranty. I'll be calling him Monday to get a bit of info from him. I would however like to make sure that it is something internal in the engine, not something that I installed, like the carb or distributor. Is there any other external thing to check that could cause this low compression? I'm pretty sure I timed it right, could the timing cause this?

P.S. I recently rebuilt the PDS in the flywheel cover on this same boat purely from posts on this forum with your responses. Thanks for all the free info!
 
gfackrell said:
Ricardo,

Thanks for the reply. Unfortunately I don't have all of that info from the engine builder. I bought the engine from a guy in Wyoming (I'm in Utah) that builds a lot of marine engines. It's supposed to carry a warranty. I'll be calling him Monday to get a bit of info from him.

1..... I would however like to make sure that it is something internal in the engine, not something that I installed, like the carb or distributor. Is there any other external thing to check that could cause this low compression?

2.... I'm pretty sure I timed it right, could the timing cause this?

3.... P.S. I recently rebuilt the PDS in the flywheel cover on this same boat purely from posts on this forum with your responses. Thanks for all the free info!
Click to expand...

1... if built and assembled correctly, the parts that you installed should not have any bearing on this.

2.... ignition timing should have no bearing on your compression readings. Your ignition should be disabled while doing the test.
Note that you'll want to give each cylinder 3 to 4 complete cycles (6 to 8 revolutions) as this pressure is accumulative as a result of the first 1 or 2 cycles. From there, your readings will/should be consistant. IOW, you could do 10 cycles, and the readings will not climb any higher.
The throttle plates should be open above low speed position as to not restrict incoming air.

3.... What free info? We have your credit card on file! :D



Your cylinder pressure numbers are low, and there has to be something that explains why, and in particualar for a fresh engine.
If you did not cycle each cylinder several times, try doing the test again.

A camshaft that is not indexed correctly will:
  • still allow for correct ignition timing, but as mentioned, the engine will not perform well
  • affect cylinder pressures depending on which way the indexing is off


Questions and comment:
Was this engine in long block form when you received it?
Who set the initial cam follower adjustments? (the rebuilder should have done this if "long block")
I believe that cam followers that were set too deep [i.e., too tight], (that would cause cylinder pressure issues), would have given you other tell tale signs.



.
 
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gfackrell said:
I just got done re-adjusting all the valves. It increased my compression about 5 psi per cylinder. I have had a couple people say something about cam timing being off. I don't know what that would do. Could the engine run very well with it off? Would the leak down test tell me if that is the problem, or would I just have to pull the timing cover and check? I'm starting to become not very happy with the guy who rebuilt this engine....
Click to expand...


Ayuh,... How, Exactly are ya adjustin' the valves,..??
 
bondo said:
Ayuh,... How, Exactly are ya adjustin' the valves,..??
Click to expand...
Yes.... I'd ask the same question.

IMO, I'd be using the 8 stop static procedure begining with #1 cylinder @ TDC C/S (C/S = compression stroke).
Adjust both intak/exhaust, and then roll the crankshaft 90* and adjust #8..... and continue right on down the firing order.

images


Keep in mind that we're actually setting the cam follower "plunger" depth once ZERO lash has been established.


But again.... if these were so far out of adjustment that it caused low cylinder pressure readings, I think that you'd have noticed other running issues.
IOW, I doubt this is the issue.


.
 
I'm thinking that his concern has been there from the begining.

22.8 hours since rebuild.
He's turning a 15x17 prop, and is getting 35 mph at a little over 4000 rpm.

We don't know what hull this is.
We don't know if his compression readings were taken properly.
Nor do we yet know how the cam followers were adjusted.
And we don't know what ignition system he's using, nor if he set this up for TA, or just base advance, and called it good.

I think that information is lacking here.

.
 
RicardoMarine said:
I'm thinking that his concern has been there from the begining.

22.8 hours since rebuild.
He's turning a 15x17 prop, and is getting 35 mph at a little over 4000 rpm.

We don't know what hull this is.
'79 pleasure/family type boat. Planing type hull
We don't know if his compression readings were taken properly.
I just did the compression readings again, throttle wide open, ignition disconnected, battery fully charged, let it turn over 5 or so times per cylinder. etc. Pretty sure its correct. :)
Nor do we yet know how the cam followers were adjusted.
I followed a write up from DON on Iboats. I loosened the rocker nuts, cranked the engine to pump the lifters back up, then tightened the nuts till lash was gone (vertical movement) then went 3/4 of a turn more. I did it with number 1 at TDC, then turned it 180, and did it with 6 at TDC. As per the write up.
And we don't know what ignition system he's using, nor if he set this up for TA, or just base advance, and called it good.
I don't know much about this either. Tell me what to do to check this.

I think that information is lacking here.

.
Click to expand...

Yes it was a long block, and yes the valves were adjusted from the builder, when I re-adjusted them, I ended up putting them back to a very similar position.
 
Here are a couple of rare causes...
I messed up head gaskets once during re-assembly :-(
Could the builder have accidently installed two head gaskets on each head?
I've also heard of head bolts being too long and bottoming out before the head is actualy fully down on the block.

still, if the engine was indeed rebuilt, I'd bet the timing chain is off by one tooth.
 
Before you do anything else, did you retest the compression with a different gauge and hose? Not just the gauge?

If the schrader valve in the end of the hose is incorrect (pulled from a tire fill stem) you will get low readings like what you are getting.
 
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Any news for us???

*********************************

Back to the 8 stop -vs- the 2 or 3 stop:


Look at this cut view of the LT1 engine. (it's not our standrad SBC, but close enough for this purpose)

Note where the camshaft profile is while #1 cylinder is @ TDC Compression Stroke.
This is when both #1 cylinder cam follower adjustments can be made.

Next up is cylinder #8 when the crankshaft has been rotated 90*.

Follow #8 cylinder 6 more times (90* rotation for each indexing), and you will have successfully adjusted the cam followers for all 8 cylinders.



valvetrain.gif
 
if a valve adjustment fails to improve your compression, you have no other choice than to pull the timing chain cover and make sure that the marks on the timing chain gears are aligned.

if the cam gears are aligned,

then the mechanic either installed the rings upside down, or did not properly hone the cylinder walls for the type of rings being installed. (chrome...etc.) (btw.....an engine will run just fine with the rings upside down......it will have lots of blowby and low compression as you describe)
 
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