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I had my 356 rebuilt about 2000 miles ago. While addressing a rough idle and some tappet noise I found:
1. way too much choke - adjusted, but still rough idle.
2. compression test revealed No. 8 is low. 90 on 1-7, 40 on number 8. Leak down revealed exhaust valve leak.

Pulled the head. I think this much carbon is excessive and could be the cause of the valve issue. No. 8 is gunked up the worst.

I'm attributing the carbon to the choke issue now resolved because:
I have a good mix of highway vs. city miles. Occasional 70mi highway trips.
Using a relative hot plug - Autolite 3136.

Do you folks think I am missing something here?

Thanks,
Mark

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Use a MULTI grade oil such as 10w-30 or 10w-40.

The straight weight oils such as just sae 30 or sae 40 will cause thecarbon build up.

Also change brands of gasoline. Gas varies from region to region of the US. But here in mid-Tn i used Marathon almost exclusively for several years 2004 - 20012. I had alot of exhaust valve STEm carbon in an 88 2.0 ranger. Fought it for 4 or 5 years.

I've switched to Shell gasoline. PROBEM SOLVED!!!

Also noticed other engines would give some minor problems.

So try the detergent 10w-30 or 40 and switch gasolines to maybe Shell.

Thanks V8.

After break-in, been using Rotella 10-30.

I use either a Chevron or Shell product probably from the same refinery - same stuff as far as I am concerned.

Mark - I'm not sure I buy the gas and oil explanation. I suggest you consider the following:

1. A chronic rich mixture condition, for example a high float level setting, incorrect (oversize) main jets, incorrectly set automatic choke, high fuel pump pressure, restricted air filter, etc.

2. A diet of mostly local low speed driving, and or very short driving trips.

Thanks OD for confirming that I am on the right track and haven't missed something obvious. I may have addressed the rich mixture with the choke adjustment. Going forward I'll read the plugs frequently.
Mark

I've got a couple of follow ups:

I had planned to disassemble the valves on No. 8 only - is that advisable?

Cleaning the carbon from the head was kinda fun, but what would be a good method to clean the engine side? I'm thinking a rag with solvent- (carb cleaner worked well on the head). Bringing each cylinder to TDC for access would make things easier. Sound good?
Thanks,
Mark

I had planned to disassemble the valves on No. 8 only - is that advisable?

I guess that depends entirely on what you find in #8. A valve-related compression loss only 2000 miles into an engine job raises the question of whether it's some odd, isolated cause or whether the valve work in total is questionable and problems with other cylinders are lurking in the near future. Your first clue on how to proceed will be your analysis of what you find in #8.

I wonder how some of the micro lubricant products would do with the deposits or if they are just another in the snake oil category.

Some of the automotive shows that tout the various products have done engine teardowns showing how the tiny molecules supposedly works into the metal and provides a slick surface carbon cannot attach to. They claim regular oil molecules are larger and only sit on the surface so are pushed aside and let carbon attach.

Here is one of the products that several shows have promoted.zmax.com

A recent buildup of carbon is unlikely to be "baked on" and thus easy to remove. I've never needed anything more than a 1" or so paint scraper of a gasket scraper with a plastic handle. Bring each piston to TDC and in 4 minutes the piston and area around it is done. Some carbon will always build up, anything left will not be a problem. Make sure to remove any scrapings from the head gasket seal area.

If you fix the cause of the buildup (choke ??) and drive the car while gunning it under hard acceleration you will see black smoke, that will be the carbon being blasted off. About the only effect the carbon will have if left there is to boost the compression a bit and on a low compression Packard that's not a bad thing.

I know it feels real good to clean it off, enjoy.

The education process continues. I appreciate your help with this simple stuff...

My mechanic/machinist will be over on Saturday to help pull the valve. I proceeded to clean the carbon on the engine side. I also did some research on burned valves and their causes, etc.

The offending exhaust valve looks to me like it may be burned, at least to some extent. It looks scorched compared to the others.

I understand we will know for sure when we examine the valve, but a burned valve appears probable, and likely caused by the carbon interfering with the seating process.

Would you folks agree with this observation and preliminary conclusion?

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