Sticky Exhaust valve on no3 causes?

Dave Byron

I have just rebuilt the 36hp motor in our 57. Its been running ok on test and first fire up I and set it up properly the other day and took it for its first shakedown run. It started popping and generally sounded rough after a few minuets driving. Limped it home and investigated.

After all the initial checks spark fuel etc I popped the valve cover off for a look.
I noticed No3 exhaust pushrod had jumped out the rocker socket. so I pulled off the rocker assembly and removed the push rod and was greeted with this.....

Image may have been reduced in size. Click image to view fullscreen.

I then noticed No3 exhaust was stuck open. quick spray with some lubricant and a tap with a hammer it sprang shut. happy days I thought. Gave a bit of lube reassembled everything (straightened the pushrod) re-did all the valve gaps which are set at 0.06"

Gave it a few test spins with the coil unplugged and the cover off and all looked good opening and closing. replaced the cover and fired the motor up. ran well on idle for about ten minuets then the tell tale sign of the stuck exhaust valve again.

pulled the cover again and yep its stuck open. quick tap with the hammer snaps shut. There OG rebuilt heads so new valve guides new valves, new springs but OG retainers. Im a bit miffed as to whats causing it. I have got to pull the motor but don't want it happening again. for the record the other head is the same spec with no issue. anybody got any ideas?

Cheers Dave

This is the motor running on its first fire up before I set the idle and everything.

Link

Joe Bence · Posted: Thu Apr 16, 2015 12:45 pm Post subject:

Not enough stem to guide clearance, guide needs to be honed/reamed to size.

Dave Byron · Posted: Fri Apr 17, 2015 9:36 am Post subject:

Ah thought that maybe the case. Worth lubing them up with graphite lube as well as an extra precaution or just hone them? Is there a specific tolerance as to what they should be honed to?

Cheers for the reply.

splitjunkie · Samba Member Joined: April 04, 2006 Posts: 4091

Do it correctly and have the guides reamed to size. Lubrication won't fix the problem. The Bentley manuals have all of the specs.

If you didn't have all of the guides checked this would be a good time to have them checked and reamed if they are new and too tight or if they are old have them checked to make sure they are not too loose and have them replaced and reamed as necessary.
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Chris

You know, a lot of these scratches will buff right out... Jerry Seinfeld

Dave Byron · Posted: Fri Apr 17, 2015 2:38 pm Post subject:

To be honest as they were new guides and new valves I simply overlooked the tolerances between the two. Im pulling the motor tomorrow and will strip the heads down and check them all. They will be reamed out as needed and I will lube them as well. every little helps and all that.....

Cheers
Dave

Zwitterkafer · Posted: Sat Apr 18, 2015 9:32 am Post subject:

The act of installing replacement guides can distort them, which is why they need to be reamed to prevent valve sticking.

Here's a question: Is it safe to assume that guides in NOS bare cylinder heads have already been reamed and measured by the factory?
_________________
"Criticism comes easier than Craftsmanship"
- Zeuxius, 400 BC

Dave Byron · Posted: Sat Apr 18, 2015 9:41 am Post subject:

wimkever · Posted: Sat Apr 18, 2015 11:14 am Post subject:

wimkever · Posted: Sat Apr 18, 2015 11:20 am Post subject:

Maybe you want to know the spec's for intake valves as well :

Stem diameter new : 7.94 to 7.95 mm
Stem diameter wear limit : 7.90 mm
guide inside diameter : 8.00 to 8.02 mm
_________________
a beetle is like a cocker spaniel, always funny but sometimes not listening

Dave Byron · Posted: Sat Apr 18, 2015 1:23 pm Post subject:

wimkever · Posted: Sat Apr 18, 2015 1:44 pm Post subject:


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