Burnt Exhaust Valve

[SAJ]

Fellow Barners.
I have a 1931 roadster with 10000 miles on a rebuilt engine from Snyders (Schwalms built). It has stainless valves, alloy seats, reground “touring” cam, 5.5:1 Snyders head, B carb. Fitted about 3000 miles ago (jets adjusted to water flow: idle52. Main146. Cap 470. Comp 126. Power 144) with drilled-out A manifold (1-5/16inch). Compressions were 94-98-98- 95. about 5000 miles ago. About 900 miles ago I noticed compressions were down slightly to around 85 psi and no. 3 was lower at about 75 psi.
A leak-down test showed no.3 leaking at TDC into no 4, and no 4 not leaking at TDC. So I pulled the head and found exhaust valves 2 and 3 a light tan colour and no. 1 and 4 darker -almost black. No 3 exhaust valve is pitted and burned, though OK when I refaced it. The other exhausts also look slightly eroded, though not as bad as No.3. Seats (inserts) were not too bad and No. 3 cleaned up by hand with a 45 Deg stone and just a few manual “twiddles “ of the stone holder.
Valve clearances were all .012 as specified by Ora Landis, and had been checked 3 times during the 10000 miles.
Pistons all had black carbon deposits, bores are shiny and unmarked. The carbon looked a little oily to me, though the car uses very little oil (Mobil1 20W-50).
I need help with two questions. Firstly cylinders 2 and 3 are running leaner than 1 and 4, judging by the colour of plugs and exhaust valves. 2 and 3 mixtures look OK to me and 1 and 4 just a little rich, but I am used to diagnosing leaded gas and have not looked at combustion deposits since we changed to unleaded some years ago. (my daily runners are a Harley and diesel Jeep wrangler and the model A is the first engine I have had the head off for 30 years ). The light brown colour of leaded gas deposits seems quite different from the deposits left by our current unleaded gas (no ethanol in it).
Secondly, did the exhaust valves burn out from being too lean and, if so, how do I richen without making 1 and 4 too rich. Why do the middle two cylinders run leaner than the outside two?.
I had noticed the plugs were almost white in 2 and 3 when I did full- speed plug chops, just after fitting the B carb., so I opened the GAV ¼ turn on the carb and this gave them a browner tan colour about 3000 mles ago. I am aware the B carb is supposed to run with GAV shut, but the jetting I gave it already seemed on the rich side according to other experts figures I researched. So I run GAV open ¼ turn.
Engine has a thermostat in the top pipe (160Deg F) and runs very cool. It sits on about 155 deg F (sensor in the transfer casting bolted to the head and checked often with an IR thermometer). It never gets above 165 Deg F.
Car has a Mitchell overdrive and 3.78 diff. and I cruise at 55 mph and sometimes up to 60 for short periods. (GPS)
Photos are as attached.
The head gasket was a copper laminate and I could not see any signs of leakage.
Please post advice on how to ensure the valves don’t burn again, with thoughts on what might be the cause of the burning and uneven mixtures
Regards SAJ in NZ

 

[Tudortomnz]

SAJ, you have given a very thorough view of your problem.
In 1969, i got my Model A & it had a burnt out valve. Put another old original in & away she went until the engine was rebuilt some years later. In the late '90's got another burnt valve ; actually it had split too. Used another good original & had some of the others built up to give correct clearances [ original non adjust.]
Engine does not have inserts & I do not think a low comp. engine needs them. When these cars were new, there was no lead in the fuel ; ie. it was unleaded .
I have found with the modern fuel [ in NZ no lead & no ethanol] it is hard to know the state of play as plugs can carbon up especially if the engine does not get hot or long idle time.
Model A's have been burning out the odd exhaust valve since new.
I would suggest using upper cylinder lub. but can not really say what caused your problem. I did not set them to 12 thou., but 13- 14 as recommended.

 

[Patrick L.]

#3 leaking into #4 sounds like head gasket, how did it look ?
That valve and seat are just starting to get 'nicked up', you made a good catch. The exhaust valve lash sounds tight to me, but, Ora should know. I think I would run them looser though.

 

[Kurt in NJ]

#3 can seem to leak into #4 if #4 ex valve is open and #3 under pressure leaks ---the air will go into the manifold and go into #4 through the open valve.

 

[H. L. Chauvin]

Hi SAJ,

1. Not being present to actually see the colors of your pistons, valves, & head gasket; actually see & touch the valve pits, etc., etc.; & in looking at the photo of the head gasket, which appears to have a dark color between 3 & 4, it appears you have a unique situation that may be described with a non-mechanical term; i. e, cross ventilation of air between #3 & # 4, whereby the head gasket may be compressed less in this area than in others. (Again, even though you may not be losing lots of water, just looking at unique photos from miles away).

2. Knowing that SS valves expand only a minute amount more than regular valves, increased tappet clearance is never suggested; however, the 0.012" exhaust clearance is not working & it appears 0.012" is "almost" enough clearance because it takes a long time to cause a valve to fail.

3. Seeing that you have a high compression head, you are generating more heat in the combustion chamber than with an original head.

4. For example, in looking back in a 1934 Motors Manual, (even though many recommend .012" exhaust valve clearance today), Ford 4 cylinder intake/exhaust valve clearance in 1934 was: A -- 1931/1932: 0.013"/0.013"; B -- 1933: 0.013"/0.018"; whereby it appears that because of higher compression with a B head, it was assumed more heat required more exhaust clearance.

5. Taking into consideration that one could spend a week or two performing a forensic review of your engine cause(s) of burning a SS valve & uneven carbon sometimes caused by slight steam cleaning from slightly weeping head gaskets;
& furthermore, definitely not being a fan of doing things twice or more; & not an having an abundance of spare time to assemble & disassemble engines, (again, only my suggestion from miles away while looking at photos), here it what I would do:

A. Bring your head to a machine shop to check for flatness & you perform a check on your block with a steel straight edge & feeler gauge for flatness & correct both block & head if required.

B. Valve timing, tappets, higher heat, increased air flow, metallurgy specifications, & what ever can cause valves to burn, never had a problem setting exhaust valves at 0.013"; but, in your case I would set exhaust valves at 0.014" even if it causes my neighborhood mechanics to scream & foam at the mouth!

C. Next, get a silicone gasket & install while providing a "very thin" layer non-hardening Permatex No. 2 on the surface of the head & block, even if not recommended by the gasket manufacturer who is in the business of selling gaskets when they leak.

D. Carefully torque head in 20 ft.-lb. increments to no more than 55 ft.-lbs. with "new" regular studs & nuts. (But first call Mr. Ron Kelley, Model A & B engine builder, (advertised in the Restorer Magazine), at (972-771-1911 to inquire what he sees day after day with Model A/B block upward stress in the small block area between cylinder 3 & 4 when heads are torqued over 55 ft.-lbs. Appears between 3 & 4 is where many have problems.

If I were in NZ to see what you have, may suggest something else, but reading what you have so well described, just offering a suggestion to try to help.

 

[Rich in Tucson]

Could be as simple as opening the GAV to enrichen the mixture while cruising at 55mph.

 

[glenn in camino]

How do you get compression in the mid 90s with a stock head?

 

[SAJ]

Thanks for all the replies. Answering as they came, the car has stainless valves and the recommended clearance is .012 (Ora Landis at Schwalms) for these valves. I used to manufacture Redex upper cylinder lube. I don't think it would help valve erosion, knowing what was in it. It had lubricant and carbon scavengers mainly.
The head gasket did not seem to be leaking. Kurt explained it. When #3 ex. is closed , #4 ex. is open so the leaking #3 travels into #4 cylinder through the manifold. #4 had reasonable compression and a "satisfactory" leakdown on my tester.
I will run with the GAV more open, but cyls. 1 and 4 will then be too rich, as far as I can see. I always open the GAV another 1/4 on long pulls and at high revs for extra cooling anyway, even though I cannot feel any better power.
I will increase the valve clearances to .014 and .013.
I have several surface tables, both granite and cast iron so I will check flatness out and machine if necessary.
Glenn, as I mentioned, I do not have a standard head , it is a 5.5 Snyder. Bores are out to exactly 4.000 inch, which increases compression too.
I guess I put in too much information in my first post, which puts people off reading it all, but I thought this would be helpful. (plus scientists and engineers tend to turn even love letters into research reports, so I couldn't help it!)
There were no comments on whether it is usual for cylinders 1 and 4 to run richer than 2 and 3. Or what I could do to lessen this.
I have noted the colour of Kurt's plugs and how he can burn them white. Mine will burn a light colour with GAV closed, but not white. But I never run like this, having seen it once on a full-speed plug-chop when I first set the B carb up.
For most of its 10000 miles I had a flow-jetted A Zenith on the engine, set a bit richer than the B.
Again, many thanks for the advice so far.
Regards
SAJ in NZ