cylinder head problem? help wanted

Dual Carbs?  Sounds like one side was running very very lean which would be synchronization of the dual carbs. 

Most likely, (and this is a bit of a stretch...) you had a slight intake manifold gasket leak which, over time, made that side (both cylinders) run lean.  That would account for the popping and other nonsense while running.  However you could also have had a valve stick slightly open, a defective (cracked) head or a small number of other nasty things.  Drive on a freeway for 30 minutes with a lean burn condition and it'll burn a valve, just like you've seen.

A look at the piston rings and cylinder walls on the cylinder with the burnt valve will either point to a cylinder problem or not, depending on what you see.  You can't really tell until you've at least gone that far and, since the head is already off, you should pull the cylinders and at least check ring health.  That will tell you a lot.

"The mechanic says there might be a problem with the case or cylinders themselves and the problem in the heads would not explain the problem."  Well, I would probably rule out the case (unless your 'mechanic' is trying to sell you a new engine).  Case irregularities don't cause burnt exhaust valves.  Overheating and/or a lean burn in the cylinder causes a burnt valve.  I would get a new head (don't try welding your existing head to "fix" it - there are precious few welders/machinsts left who might get it right), along with a new set of intake manifold gaskets and put it back together.  

If you're going to check the rings, then, depending on how they look you might get a new set of rings sized to your cylinders and wear - make sure they deglaze the cylinder walls, too, if you install new rings.  If the engine has low mileage, I would stop there.  If it's under 20,000 miles I would probably leave the existing rings alone, depending on how they look.  If it has higher mileage (over 50,000 miles) I would go for new cylinders, pistons and rings, too (the package is relatively cheap).

The bottom end (crankshaft and bearings) is pretty strong for a 1776, so I wouldn't worry about that.

Gordon

The Speedstah Guy from Grafton

thanks for the replies. The cylinders and pistons are worn. We checked them out as well. The mechanic says those could be honed. Should I take the case apart as well? Or would you recommend passing to the 94 mm? Is the problem related with overheating when the cylinders are worn out? 

If I upgrade to 94 mm will I need any shims under the cylinders?

I have 5.4 inch I beam rods.

This is hard to help with as I would much prefer to actually BE there to see it, but here's some stuff to think about.

First; How many miles on this engine?  If the engine (NOT the car, necessarily) has over 50,000 miles on it, I would consider new cylinders and pistons if they look worn.

Second; How do you tell if they are worn?  You remove a piston (or all) and, with a cylinder bore gauge (looks like a spring-loaded "T" that can check the diameter way down in there) you check the bore diameter below the top ridge formed at the top of the ring travel, and then above the ridge.  Most VW manuals will tell you the max wear to tolerate before replacing the cylinders, pistons and rings. (I don't remember the spec off the top of my head)

If the cylinders are only worn a slight amount, then they should be de-glazed, at least.  A hone would do that.  If they have a noticeable ridge, I would probably replace them.

Third; The overheating was probably not related to worn cylinders.  From what you described, it was probably related to an intake manifold gasket leak and driving at speed on a hot day (IMO).

Fourth:  I would email Larry Jowdy about upgrading to 94mm cylinders.  Going to that size from your 1,776 might mean boring the case for the larger barrels and for that you'll need a good machine shop (your local mechanic will have to end it out).  If that's the case, it might be cheaper to add up all of the machining, parts and labor costs (you're essentially building a new engine) and then price a new engine of whatever displacement you choose from the builder of your choice and see what the cost delta will be versus doing it locally.  It might, under those circumstances, be better/cheaper to source a new engine, especially if all you buy is a "long block" and then use your existing peripherals (fan shroud, ignition, etc.).

Fifth; Whether or not you'll need shims is dependent on a number of things: cylinder height, rod length, piston size, head configuration, resulting deck height, etc.  All this will become evident as the build progresses, but a competent engine builder will work with all those measurements and go with the flow.

Good luck in your search

Gordon's post above is good advice; just a note on the 4th point. You can buy 94mm cylinders with the bases turned down to the 90.5 spigot size (or have a machine shop do it), so if the short block is okay it doesn't have to come apart. Yes, they get a little thin down there (don't drop one; but that will screw it up no matter what) but it's proven to work without problems, as the ring area doesn't come down far enough to exert any force on the thinner surface. Measure the spigot openings before the work is done; I believe .010" diameter difference is sufficient (I will post this question on the Samba right after this on the "Engines/Transmissions" forum).

The results of my question of cylinder base/spigot clearance on the Samba-

http://www.thesamba.com/vw/for...ewtopic.php?t=517421

Interesting thread on the Samba.  Just underlines the need to find a good machine shop to do the case work.  On the West Coast that's not much of a problem, it seems, but here in New England (and in the Southeast, as well) it's getting pretty hard to find a competent shop that will do a good job on a 1970's vintage VW case.  A lot of them SAY they can do it, but a lot of them have never seen a VW case before.  I've had good luck with shops that work on Harley engines, but usually have a 30 minute chat with them before I give them the work to make sure they know what they're doing.  

One thing I forgot about; we've covered the cylinder bottom/case spigot fit, but the heads will have to be opened up for the 94's as well. The outside diameter of the tops of 90.5's are 98mm, while the tops of the 94's probably measure 101.1mm (measure the cylinders for the proper dimension before talking to your machinist or provide him with a cylinder). It may seem like a lot of extra work, but remember you're adding 139 cc's, and it's worth it. BIGGER IS BETTER!  Al

Thanx guys for the replies.

Al so you mean 0.9mm or 0.85mm thickness instead of 1.6mm  for a spigot is ok?

How long have you been using them? I am not sure. Durability seems to be a problem to me 

Gordon,

the engine ran only 2000 miles and yes the pistons have a noticeable ridge.Do you think I had the inside of case checked?

First off, the pistons shouldn't have a ridge..   Are you talking about a ridge that would form on the inside of the cylinder just above the piston when it's at top dead center??      After only 2,000 miles it's highly unlikely that there would be any ridge on the cylinder.

I mean scratches on the piston and the cylinder, but deep ones

With deep scratches, you may have a broken ring or ring land

those seem ok. there might have been a problem with the gap in between the rings, I am not sure.

should the length of the gas hoses be the same after the T or is not that an issue for dual carbs?

If I was lean on the right side of the engine that might have been the cause. What do you think?

By the way I had a CHT gauge screening cyl 2 which never exceeded 325 F

With such short distances, it makes no difference re: the gas hose length but theoretically regardless of the length, once the hoses are pressurized then both will deliver the same fuel.

325 is what the CHT reads but is it accurate.  Most have been damaged during spark plug removal and installation...  Get a thermometer and measure the oil temp.  That usually gives a pretty accurate blueprint of the engines cooling system.   Not so say that your CHT isn't working just that I don't totally rely on them, they're just a tool.

325 on the freeway is pretty decent but Oil Temps are tied more closely to RPMs. and take a while to move. CHTs are tied closer to engine load and can get change in a second.    The sensor should be on #3 cylinder.

normal operating temp changed between 225-275F. 325 only when I climbed a hill or pushed too hard. (I once saw 110 mph). I forgot to mention that I use a porsche fan shroud. I have checked the cht with boiling water and it was correct.

FYI, the replica Porsche fan shrouds don't work as well as a stone stock VW shroud.  Temp's sound good...