Need help: backfiring, black sooty #3 intake port (pic)

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John Berners
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Need help: backfiring, black sooty #3 intake port (pic)

#1 Post by John Berners »

Hello! Glad to be back on 356talk after some lapsed time. I hope someone can suggest what might be causing severe backfiring through the carbs and making my #3 intake port black with soot. Here's a picture:
IMG_3569.jpg
My car is a 1964 C.
It has Solexes and a street cam. It sits too long each year over the winter.
Some time ago it started backfiring severely and continuously (for no apparent reason, just when I started it one day). Many, many backfires, like popcorn popping, except a lot of the pops were explosive bangs through the carbs. In this condition, it still ran and had decent power. But I was afraid to drive it of course because I thought it might incinerate.
The valve clearance is fine all around and valve springs do not seem to be broken.
I did the normal tuneup, timing, etc. but could not improve the severe backfiring.
When I removed the Solexes and drained them, gooey brown syrupy stuff came out of one of the bowls.
I decided that must be the problem and started to change back to the Zeniths, which I'd wanted to do anyway. When I got the manifolds off, I saw this soot in the #3 port. All the others are nice and clean. What could be causing it? Could this come from a carb or ignition problem, or would this have to be a valve leaking or open at the wrong time?
Many Thanks for any insight!
Best,
John

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Ron LaDow
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#2 Post by Ron LaDow »

John,
Thanks for noting you've done the tune up.
Probably a carb problem at this point. Pretty sure you can swap Solexes side-to-side by swapping the throttle lever parts. Do that; if the problem follows the carb, you're golden. Put the Zeniths on and go for a ride.
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Martin Benade
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#3 Post by Martin Benade »

A compression test would be the next step in my book. Not many carb problems will turn the intake port black. Continuous backfiring sure sounds like a leaking intake valve to me.
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#4 Post by Ron LaDow »

Martin Benade wrote:A compression test would be the next step in my book. Not many carb problems will turn the intake port black. Continuous backfiring sure sounds like a leaking intake valve to me.
Good point, and easier than swapping carbs.
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#5 Post by C J Murray »

It sounds like a valve problem. Do a complete cylinder head service/valve job.
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#6 Post by Dick Weiss »

Could be a clearance problem w/the valve stem to the guide ID; Stem straightness is critical for the valve to seat correctly along w/the bore of the guide(s) are also straight w/in specs.

W/the carb 'popping's, it takes a couple of minutes for the carbs to 'settle down' after start-up.
I never 'instant start' an engine w/o oil-priming it (w/the starter) after a long sit; Of course, the battery is OK(?).

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Re: Need help: backfiring, black sooty #3 intake port (pic)

#7 Post by Martin Benade »

He mentioned continuous popping, so probably not the. occasional sneezing on startup that many healthy 356s do.
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John Berners
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#8 Post by John Berners »

Hello! Thank you for these replies, which I have read with great interest. As soon as my battery is charged up (may be 8 hours, or may have to replace it), I am going to do a compression test, as suggested. I'll let you know what I find!
Gratefully,
John

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Re: Need help: backfiring, black sooty #3 intake port (pic)

#9 Post by Thomas Sottile »

I would Pull the head you have a valve problem.

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Re: Need help: backfiring, black sooty #3 intake port (pic)

#10 Post by Martin Benade »

There is still a chance of a broken valve spring that escaped detection. That could be changed with the head on.
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#11 Post by John Berners »

Hello!
Thanks again. Compression readings are:

#1 150psi
#2 151
#3 155 (sooty intake)
#4 160

Also, with the horizontal shroud off I can see all around the #3 intake valve spring and still didn't detect a break. I flexed each coil gently with a screwdriver and nothing seemed cracked or wrong in any way. With #3 valve clearance at .004 - .005" where I set it, does that mean the valve is not hanging up on debris or anything? So what are some other possible valve problems -- damage to the valve or seat doesn't show in the compression test?
Many Thanks for your help.
Best,
John

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Re: Need help: backfiring, black sooty #3 intake port (pic)

#12 Post by David Jones »

As you already did a tune up I would switch plugs from side to side and switch #2 and #3 plug wires at both distributor and plug and that way you are substituting known good items to the faulty cylinder. From the compression test it now sounds like a carb issue and you do not solve the problem you can also switch carbs side to side and see if the problem transfers. You do not need the throttle linkage of course to run the car at idle.
As you already started to change carbs you should have no problem now doing side to side switches and you can even do it with two different sets of carbs which should really allow you to nail down the problem item.
If I had known I would live this long I would have pushed the envelope a little harder.
Cymru am byth
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#13 Post by C J Murray »

Intake valve sticking while running but not when cranking?

Valve spring issue?

Which century was the last valve job done? Do a proper valve job with new springs and other parts as needed.
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#14 Post by Ron LaDow »

OK, you checked the easier stuff. Now try swapping the carbs.
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Re: Need help: backfiring, black sooty #3 intake port (pic)

#15 Post by C J Murray »

How does a carb make exhaust come up the intake port?
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