Bad compression on number 6 (1 Viewer)

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Joined
Jul 26, 2017
Threads
23
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280
Location
Castle Rock, CO
Ok so here I go. I set the timing today, then did another lean drop on the carb. Once I did that I then went back and advanced the timing to about 10 degrees since I am at 6500 feet elevation. I tested the HAC and it was working great, cleaned it out a bit and put on some new vacuum hoses. So at this point i have rebuilt the carb, tuned it, checked almost all of the emissions systems, replaced most of the vaccum lines with new silicone lines, and replaced the PCV valve and grommet. I’ve also got pretty good vacuum. I’m seeing about 20 here at this high altitude.

The PO put on a new Cardone air pump, which I know are kind of hit or miss as far as working well. When I ran all the AI system tests from the FSM the air came out or did not come out of the right hoses, so it seems to be working, however I can't tell a flow rate. It doesn't seem to be blowing very strongly, but I don't know how strongly it needs to blow.

I went back and got my truck retested at the emissions place today and I failed again with high HC and CO. NOx passed. I drove it pretty hard before hand to heat up the CAT, (not the original CAT, but I don't know when this one was installed, it doesn't look terribly old) I had also put 3 bottles of heet in it before I tested it on friday, when it also did not pass, but that was in a full tank of premium, I had not had a chance to drive it around to run the tank down. I don't know the effectiveness of heet after 4 days.

So I came back and the first thing I decided to do is a compression test. Here are my numbers
1. dry 135
2. dry 130
3. dry 120
4. dry 130
5. dry 125
6. dry 70 wet 90

I didn't do a wet on the first 5 because i was pretty happy with those numbers, or maybe I shouldn't be? Soooooo...

What is the next step? I'm pretty bummed, but I def want to get it fixed. Never rebuilt an engine before, but I am not scared to try. The 2f seems pretty simple and I have the FSMs. This forum is also such a great resource. Thanks for any and all input!!!
 
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I am planning on checking the valve lash, but wouldn't the difference in wet and dry indicate a ring problem, not a valve problem? I guess it is a good thing to do regardless.
 
You’ll get a different wet and dry reading on any cylinder regardless of age/wear.

I was more hoping you had a tight valve that once adjusted got you back within 10% of the others.
 
Also, is this probably one of my problems with passing smog?
 
If the valve lash on #6 isn't too tight, the exhaust valve is likely burned (warped). Common.
Usually that's caused by not keeping the valve adjusted. It gets too tight, overheats, then warps, then leaks.
But even a valve train that's properly adjusted can get burned valves (usually from desmogged engines).

After you check/adjust #6, you can do a leak down test to determine if it's the valve (likely) or the rings (unlikely). Search leak down test. It's easy.

Removing a 2F head is a rite of passage for any long time 60 owner. It's just a matter of time before the head needs to be rebuilt.
 
That’s likely why my #5 got locally thinned.

5C4852DD-843C-43BE-98A2-90C0104C16EA.jpeg
 
Ok so I finally got the time to adjust the valves today, for the most part they were pretty good, some a little loose, but not much. Unfortunately, the valves on 6 were pretty good, definitely not to tight. So I am moving on to a leak down test. Quick question to see if I am thinking about this right. For a leak down test I need to have the cylinder in question at TDC right? I am thinking for number six this would be 360 degrees from #1 TDC. Is that right?
 
For leak down you need to have both valves closed. If the valve cover is off you should be able to determine this pretty easily.
 
Here is the leak down test on number 6. Good right? :doh::bang::frown:
IMG_2967.jpg


I used a piece vacuum tubing to listen for leaks and i heard most of the air coming out of the oil dipstick tube. Rings right? I didn't hear any out of the exhaust or carb. None out of the radiator cap or bubbles or anything. I had the valve cover off and I heard a little bit near the pushrods but none near the valve springs. Looks like I'm gonna be starting a new thread called inexperienced one banana mechanic rebuilds an engine for the first time.
 
Looks like cylinder 6 is toast.

Options:
  1. Find another used inexpensive 2F & toss yours.
  2. Remove the head and see the big gouge in #6 cylinder wall (I've seen that) then have the block rebuilt or choose option 1.
  3. Remove the head and see nothing then have the block rebuilt or choose option 1.
 

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