Compression check...How am I doing?
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,352
Likes: 654
From: Tampa, Florida
Compression check...How am I doing?
Engine is a 350 with unknown internals and stock '70 steel heads.
To help understand my setup:
1. Rebuilt '76 Q-jet
2. Reshimmed stock distributor
3. MSD custom cut wires with Champion plugs.
4. Dwell is correct and timing is within reason
5. ~13mmHg vacuum at best
6. Idle is ~950-1000 I cant get to idle any lower and its still choppy
7. My right exhaust is more 'rumbly' than the left. It has stock manifolds on the wrong sides. The heat riser valve is on the left and stays mostyl closed.
8. I have no idea if the vlaves are properly adjusted.
Cylinders:
1. 140
2. 150
3. 140
4. 150
5. 140
6. 140
7. 135-ish
8. 150
Any thoughts or comments?
To help understand my setup:
1. Rebuilt '76 Q-jet
2. Reshimmed stock distributor
3. MSD custom cut wires with Champion plugs.
4. Dwell is correct and timing is within reason
5. ~13mmHg vacuum at best
6. Idle is ~950-1000 I cant get to idle any lower and its still choppy
7. My right exhaust is more 'rumbly' than the left. It has stock manifolds on the wrong sides. The heat riser valve is on the left and stays mostyl closed.
8. I have no idea if the vlaves are properly adjusted.
Cylinders:
1. 140
2. 150
3. 140
4. 150
5. 140
6. 140
7. 135-ish
8. 150
Any thoughts or comments?
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
How does it run ? Looks like a motor with a few miles on it the cranking compression is a little low but within tolerances 
As for the idle you should be able to fix that with timing and carb adjustments
As for the idle you should be able to fix that with timing and carb adjustments
Advanced
Joined: Jan 2007
Posts: 96
Likes: 0
From: Sandnes Norway
My 1974 BB is in the same range as yours, it runs good and idle is a little rough but i think someone put a bigger than original cam in there. If you have a big cam I have learned that cranking compression will go down but I'm not sure on this.
Also my idle vacuum is in the low 10-12 range, yours at 13 and that's low too. A big cam will produce such numbers.
Am I wrong on this?
Also my idle vacuum is in the low 10-12 range, yours at 13 and that's low too. A big cam will produce such numbers.
Am I wrong on this?
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,352
Likes: 654
From: Tampa, Florida
I'm thinking it may have a bigger cam but really have no idea.
As far as timimg I dont know what more I can do. I have 36 (initial plus mechanical) then the vacuum on top of it.
As far as timimg I dont know what more I can do. I have 36 (initial plus mechanical) then the vacuum on top of it.
Drifting
Joined: Apr 2007
Posts: 1,970
Likes: 45
From: Cameron Park CA
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Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,352
Likes: 654
From: Tampa, Florida
Before reshimming I was at .035".
Melting Slicks
Joined: Sep 2007
Posts: 2,429
Likes: 0
From: Tucson AZ
You said that the left side has the heat riser on it? And it stays mostly closed? That's the #1 problem to clear. The #2 situation to clear is the valve adsjustment. After those are cleared and you're happy about that, try running the engine with no vacuum advance, some engines like it, some don't. What are the spark plugs gapped at? Try .005 more or less and see how it idles. Do that a few times to see if it smooths out.
The vacuum issue could be a leak in either the intake somewhere, or with headlights or windshiled wiper door? Mine was about 9" vacuum at idle in gear at about 850 RPM's. Since all vacuum leaks are fixed, timing is good, carb adjusted, and I have 13-14" vacuum now. Methodicly is the answer to all problems.
The vacuum issue could be a leak in either the intake somewhere, or with headlights or windshiled wiper door? Mine was about 9" vacuum at idle in gear at about 850 RPM's. Since all vacuum leaks are fixed, timing is good, carb adjusted, and I have 13-14" vacuum now. Methodicly is the answer to all problems.
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,352
Likes: 654
From: Tampa, Florida
You said that the left side has the heat riser on it? And it stays mostly closed? That's the #1 problem to clear. The #2 situation to clear is the valve adsjustment. After those are cleared and you're happy about that, try running the engine with no vacuum advance, some engines like it, some don't. What are the spark plugs gapped at? Try .005 more or less and see how it idles. Do that a few times to see if it smooths out.
The vacuum issue could be a leak in either the intake somewhere, or with headlights or windshiled wiper door? Mine was about 9" vacuum at idle in gear at about 850 RPM's. Since all vacuum leaks are fixed, timing is good, carb adjusted, and I have 13-14" vacuum now. Methodicly is the answer to all problems.
The vacuum issue could be a leak in either the intake somewhere, or with headlights or windshiled wiper door? Mine was about 9" vacuum at idle in gear at about 850 RPM's. Since all vacuum leaks are fixed, timing is good, carb adjusted, and I have 13-14" vacuum now. Methodicly is the answer to all problems.
Valves...They could be properly adjusted but I just dont know. I'll leave that for a different thread
Plugs are gapped at whatever the book says for a '70.
Vacuum system is tight. All new components and hoses. Wiper system has been isolated (tubing is clamped).
Melting Slicks
Joined: Dec 2006
Posts: 2,669
Likes: 3
From: Kingston Ontario
A compression test is good for a quick check, but a better test is a leak-down check. The leak-down test will isolate a problem better such as partially stuck intake/exhaust valve etc. The following link gives an excellent description of the procedure and expected results.
http://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm
This check will give you a definite answer to how your engine is operating.
http://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm
This check will give you a definite answer to how your engine is operating.
Melting Slicks
Joined: Sep 2007
Posts: 2,429
Likes: 0
From: Tucson AZ
A compression test is good for a quick check, but a better test is a leak-down check. The leak-down test will isolate a problem better such as partially stuck intake/exhaust valve etc. The following link gives an excellent description of the procedure and expected results.
http://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm
This check will give you a definite answer to how your engine is operating.
http://www.geocities.com/dsmgrrrl/FAQs/leakdown.htm
This check will give you a definite answer to how your engine is operating.
Beat me to it.
Melting Slicks
Joined: Dec 2006
Posts: 2,669
Likes: 3
From: Kingston Ontario
You may be passing up a chance to buy tools for the garage!
You can build a leak down tester;
http://www.xs11.com/tips/misc/misc3.shtml
and then buy a cheap compressor (inflate flat tires or kids bike tires).
You can build a leak down tester;
http://www.xs11.com/tips/misc/misc3.shtml
and then buy a cheap compressor (inflate flat tires or kids bike tires).