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-   -   How much compression is enough? (https://forums.off-road.com/jeep-short-wheelbase/156220-how-much-compression-enough.html)

**DONOTDELETE** 03-02-2003 09:54 PM

How much compression is enough?
 
Can't get the Jeep started. 1968 CJ-5 with the 225 C.I. V6. The last time it was running it developed some serious backfire, mostly out the tailpipe. I was close to home so I just limped in. I've been through LEVE's list. Fuel OK, even has a new Holley 4160. Spark is good and new wires and plugs. Timing is dead on. I've checked it all.

The one area of concern is compression. The jugs are getting up to 140 and within about 10% of that, except #1 & #3 which only getting 55 PSI. OK, so I need a head gasket. But, my question is would low compression on two cylinders be enough to keep it from starting?
It just seem like it should still fire.

ozarkjeep 03-02-2003 10:39 PM

Re: How much compression is enough?
 
not on any 2 stroke or 4 stroke ive ever messed with, it should start and run if everything else is in order.

have you verified the cam timing? aybe the backfiring was due to a slipped timing chain?

LEVE 03-03-2003 12:15 AM

Re: How much compression is enough?
 
1. Cylinder's 1 & 3 are, of course low,
2. Doing the compression test wet, and achiving the same results,
3. Indicate a problem in the valves.
4. The previous problem, with the backfire in the tailpipe,
5. Could also confirm that:
a. The exaust valves are burned,
b. Carbon's keeping them open,
c. They're bent.
6. But something's wrong in those two cylinders.
7. Would that preven starting?
8. No,
9. It would attempt to start.
10. Differnet cylinders when dead affect an engine differently,
11. They don't always develope the same power or
12. Share the load equally.
13. I'd be looking at:
a. The distributor timing, it could be 180* out, if you moved it.
b. The timing chain/gear wear.
14. Have you attempted to start with stating fluid?
15. There was a post a few months ago,
16. The engine wouldn't start unless it was bump started...
17. That was a new one on me.
18. Have you attempted to Jump start it,
19. To eliminate low voltage?
20. What's the spark look like?


RRich 03-03-2003 02:35 AM

Re: How much compression is enough?
 
It's entirely possible that if the head gasket was blown between
the 2 cylinders and the water jacket, it may have blown water
back into the other cylinders through the intake manifold,
drowning out the rest of the plugs.

Try the tow truck driver's trick - pull the coil wire out
about 1/4" from the coil and start it up. The extra gap
increases the voltage hitting the plugs, and gets the curve
on the steeper rise time portion - virtually blowing any
crud off the plug.

But - you are taking a chance of hydrauliking the engine if
water's getting in those cylinders. Best to take it down
and fix it before trying to start it.
New rods for the 225 odd-fire are getting hard to find.

To pull the head you've already pulled the water pump, so
might as well pop the cover and take a look at the chain
and tensioners. There should be 2 tensioners in there.

And, surprisingly, Pep Boys has the rubbing blocks in
stock.

highmtnjeepn 03-03-2003 01:18 PM

Re: How much compression is enough?
 
This certainly won't help your compressions problems, but I had a '71 with the 225 and started backfiring like crazy (flames out the sidepipes!). It turned out to be a bad condenser. Took me a while to figure it out, since it was so simple. If you still have points in it, try a new condenser (and good luck with the rest of the motor!)

LEVE 03-03-2003 02:07 PM

Re: How much compression is enough?
 
1. A condensor, if bad,
2. would not be selective between
3. cylinders 1 & 3,
4. It would be failing accross all 6 cylinders.

highmtnjeepn 03-04-2003 01:32 PM

Re: How much compression is enough?
 
Leve
How do you know that it was only backfiring on two cylinders? He may have symptoms of two separate problems. I was merely pointing out my experience with a 225 backfiring through the tailpipe. Obviously, he has a leaking head gasket between 1&3.

**DONOTDELETE** 03-04-2003 02:32 PM

Re: How much compression is enough?
 
Well thanks for your input guys. BTW the condenser, points (Mallory Dual) plugs & wires are all new. At any rate I pulled the timing cover last night. The crank & cam sprockets are lined up. Tonight, I'm pulling the heads. Since I'm tearing down this far I'll go ahead and have the heads serviced. I'll post again to let you know what I find

LEVE 03-04-2003 03:35 PM

Re: How much compression is enough?
 
</font><blockquote><font class="small">In reply to:</font><hr />
How do you know that it was only backfiring on two cylinders?

[/ QUOTE ]1. I don't know.
2. I'm only taking information a applying logic to it.
3. There's been no indication of where the bacfiring is occuring.
4. However, if the exhaust valves are burned on 1 &amp; 3 enough,
5. To keep them open,
6. Bacfiring could occur in the exhaust manifold.
7. It may very well be backfiring intermittenly.
8. And I missed the fact it's a V engine,
9. Which would make one think the head gakset could be blown,
10. Between 1 &amp; 3.

lindy80cj7 03-04-2003 05:11 PM

Re: How much compression is enough?
 
Remember this is a odd-fire engine, it is a differant beast.


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