Topic: Oh boy, another issue: 80# pressure in #4 cylinder

[BlackMax]

I have a feeling this $400 bike may not end up quite as "cheap" as I thought.   Okay, it's a '78 SOHC "F".  It fires up on the first kick (if you kick it) and sounds pretty strong.  However, it definitely smokes a little too much.

Now, I tested all cylinders, warm and dry.  The results:

#1  160
#2  160
#3  160
#4    82  (ouch)

With a little oil in each cylinder, the readings move 7-10 psi, that's it.

Also, the plugs, especially #4, are smooth black.  I assume that's oil.   So, I assume it's the top of the motor (which is fine, as I planned on doing a complete valve job).  But, the black plugs make me wonder.  Could the plugs be getting black just from a problem on the top-end?  Or, does this surely indicate oil getting around the cylinders too?

[WHALEMAN]

I assume you have checked the tappet clearance on #4 as a too tight valve would show up like this also. How many miles on the engine? If it was me and I was going to do a valve job I would not even think twice about honing and installing new rings. Dan

[BlackMax]

okay, that's kind of what i was hoping for.  The open valve thing (mechanical as a result of an adjustment) crossed my mind. 

Now, I don't know how to adjust??  I know, it's simple and basic, but I've just never done it.  I know you break the lock nut lose, and adjust with a screwdriver.  I assume backing it out would tend to allow the valves to close if in fact they are being kept open slightly?  Rather than using a tool to measure, I thought I'd just back it off 1/2 turn or 1 turn and simply see if compression returns.  If so, I leave it as I'm getting a valve job anyway.

Do I have the adjustment procedure about right, or am I in left field?

[Duke McDukiedook]

You need to read the manual on this proceedure, it is quite clear on what to do. Download the manual from the faqs, read, read again then do.
If you have any other questions after reading the manual, feel free to do so.
You'l want to buy a set of tappet adjustment tools and feeler gauges for the clearances you are looking for, just backing it out a hair here or there won't cut it.

[WHALEMAN]

With all due respect if you do not even know how to adjust the valves how the heck do you know it even needs a valve job? Dan

[BlackMax]

Huh?

Well first, knowing exactly how an ic engine works is different that knowing step by step how to do the work without making a mistake.   The actual hands on, engine open, hands greasy thing, that's not really my favorite part.  I can teach a class on camshaft theory and design, but I have never installed one.  I can lecture on ic engine thermodynamic efficiency, but have never rebuilt one with my hands.   There's a difference.  And yes, I know when a valve job is likely needed (i.g., when it's burning oil and you're somewhat sure that much or all of it is not a result of oil getting around the rings).

More often than not, I pay people who care about HOW to do it while I make money doing something else.  When I don't know how, but still want to do, I ask questions.....which brings us here.

[BlackMax]

You need to read the manual on this proceedure, it is quite clear on what to do. Download the manual from the faqs, read, read again then do.
If you have any other questions after reading the manual, feel free to do so.
You'l want to buy a set of tappet adjustment tools and feeler gauges for the clearances you are looking for, just backing it out a hair here or there won't cut it.

well, no actually I don't.  Like I said, the heads are going to be re-done anyway (tomorrow); so, I just want to know if, in addition to a top-end job, I am or am not likely also looking at rings or a possibly a total rebuild. That's all I need to know.  It's not being driven.

Anyway, I backed out the adjustment enough to know that it had no effect on pressure.  The valves are closing as much as they are going to.   Hopefully it's just a burnt valve or chunk of carbon. 

[Duke McDukiedook]

What are you having done as part of this complete valve job you speak of? What led you to believe you needed a complete valve job?

If compression reading (dry) does not differ between cylinders by more than 10 psi, the rings and valves are in good condition. #4 does not meet this criteria.

If a low reading (10% or more) is measured on one cylinder, it indicates valve or ring trouble. If a wet test didn't yield significantly higher compression values, odds are you have a bad ring on #4. A valve job won't help this.

[BlackMax]

What are you having done as part of this complete valve job you speak of? What led you to believe you needed a complete valve job?


If a low reading (10% or more) is measured on one cylinder, it indicates valve or ring trouble. If a wet test didn't yield significantly higher compression values, odds are you have a bad ring on #4. A valve job won't help this.

1-  it smokes.  the smoke isn't black (improperly burnt fuel); and it's air cooled.  So, we know it's oil.  It smokes less under hard throttle, so the assumption is that oil is getting past the seals and/or guides and not the rings (or at least less than is getting past the stem seals), which usually produces more smoke under load.  this is how I'm pretty sure the valves need to be done (basic valve job, new seals, maybe guides, springs, retainers.....probably a new cam while I'm there).

2.  If a wet test didn't yield significantly higher compression values, odds are NOT that it's a bad ring.  A wet test that doesn't improve things, indicates the problem is NOT the ring, but something on top, such as an improperly seated (burnt, worn seat, bent, carbon) valve, or head gasket.  Since #3 has good compression, but #4 does not, I doubt (but can not totally rule out) a bad head gasket. At least it's not between the two.

Who knows, perhaps I'm really lucky and it's a combination of valve seals, valve guides, a burnt valve, seats, rings and worn cylinders.    I'm hoping for a single chunk of carbon on #4, with no other issues.....but there's just too much smoke for it not to be at least one other issue.

[ofreen]

A leak down test can be more informative than a compression check.  Charge the cylinder with compressed air and listen to where the air is leaking from.

[Duke McDukiedook]

You are correct about the valves, I re-read what I posted before.

Ofreen is right about the leak down test being more informative than a regular comp test.

New cam, are you sure you want to spend this much money on an old bike like this?
I don't know about how much money you want to spend on this bike but you can get into thousands of dollars very easily when you get into engine repair on these bikes.

[upperlake04]

 The 77-78 Fs are noted for exhaust valve guide wear. Your bike sounds very similar to mine when I got it. A cylinder hone, valve lap, exhaust guides and new gaskets and rubber pieces was all it needed. 15,000 miles now and still going strong.  Good luck.
  Edit> Oh yeah, I forgot, the valve tips were worn but were fixed with lash caps.