Intermittent miss under load where to begin search?

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Alan Hall
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Re: Intermittent miss under load where to begin search?

#61 Post by Alan Hall »

The situation of the swollen fuel cock rubber was on my brother's '53 back in the early 60's. So had nothing to do with ethanol, but sounds like it was before a change to cork for that part, probably changed for reason of swelling. Don't know when the change was made, or how old Dan's fuel cock is.

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Martin Benade
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Re: Intermittent miss under load where to begin search?

#62 Post by Martin Benade »

Mike, I agree that growing corn to make fuel is a waste of time (and fuel). However the 10% ethanol fuel is only lacking 2% of the BTU s of summer gasoline, so if you lose slightly more than 10% of your gas mileage I bet there is some other factor.
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Dan Epperly
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Re: Intermittent miss under load where to begin search?

#63 Post by Dan Epperly »

Reviving this Thread after a eight month gap...anyway, I never could Identify the problem, so I took the fine apart. It was a complete unknown to me what I had as the owner didn't know much, so I was curious as to what sort of condition it was in. It ran strong since I owned it but was smoking a bit, using some oil, had some leaks and then that problem emerged, so A partial tear down was in order if anything to resell the engine and check wear.
Turns out it looks like there is little wear on the engine. The cross hatching is still strong on the cylinders and when I spray carb cleaner in the head the valves don't leak. It's got a big bore kit, 912 rods and pushrods, and a counter weighted crank. I can find no serial numbers on the case, other than 25 stamped near the diz. so I guess a 912 case? Also looks to have early S90 heads.
The only oddity is that the heads on 1 and 2 cylinders have exhaust valves that have white hard deposits on them and thick black deposits on the intakes. The other head is much cleaner.
I only looked at one cam lobe so far and it doesn't look ground down at all. I plan on taking the heads into Dennis and have him look them over, maybe I'm missing something obvious.
If the heads check out I'll just reseal and bolt it all up, clean it up, paint the tins, etc. i hope he finds something amiss in the heads so I can ID the problem.

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C J Murray
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Re: Intermittent miss under load where to begin search?

#64 Post by C J Murray »

The top right cylinder had a significant compression leak and the others have traces of the same problem. Exhaust valves should be lighter in color than intakes due to temperature and I would like to see lighter color in the combustion chamber if possible. Are the heads 22* and the pistons 30*? BAD BURN! Multiple shims or more than .040" total? BAD BURN! DETONATION due to trapped charge caused by 22*/30* mismatch. Your heads need work. You need to match up the angles via machining or new pistons.....Unless I am wrong and the pistons already match the heads, then you still have to address the compression leaks and most likely other problems.
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Craig Richter
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Re: Intermittent miss under load where to begin search?

#65 Post by Craig Richter »

Good call on the suspicious mismatch. I pulled many mismatches apart back when, and the heads aways looked like that. As long as Dennis doesn't find any cracks, those heads look like an easy rebuild. A new big-bore kit is most likely needed, and for the best chance at a good running and long lived motor, use cast iron cylinders. I know there's a lot of trick stuff on the market these days, but good 'ol cast iron is cheap and has proven to give the best support. I really like Shasta forged pistons too, if in budget. Mat, you seem to know what you're doing, so if end-play and a few other things check out, you're going to get off easy and be very pleased with the results. Oh, and as long as you're into things, you should take a look at those carbs, especially that bad side. Could be mis-marked jets or sinking float or something going on there. More pictures get more chatter...
 

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Larry Coreth
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Re: Intermittent miss under load where to begin search?

#66 Post by Larry Coreth »

Let me underscore CJ's analyses of the heads, the leaks at the cyl./ head seal is critical ! After verifying that the sealing surfaces are clean and not pitting I like to "hone" the two pieces with one another by using valve grinding paste , rough to fine, and rotate them against each other like valves, observing the pattern left on each surface (head and cyl) if both come out an even dull color with no "blank" spots you are right . Otherwise you need to investigate the discontinuities.
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Mike Horton
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Re: Intermittent miss under load where to begin search?

#67 Post by Mike Horton »

Larry, in the lapping process, l've tried to use machinist's blue dye on steel, and red dye on aluminum, which as you'd expect, erodes away in the process. In the home garage, rather than in a shop, permanent markers substitute well, and aid the analysis, good process on little damaged machined surfaces. The alternative will become obvious, requiring the fly cutting process...
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Larry Coreth
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Re: Intermittent miss under load where to begin search?

#68 Post by Larry Coreth »

Erick,
BTW a digital copy of the shop manual , if a good copy, is just as good as an original. I have both and rarely us the original, except for maybe a picture occasionally where even the original is not too definite.

I have found it always prudent to thoroughly consult the parts and shop manuals before trying to assemble something which I had not taken apart. Even then a refresh of the memory is always good, prevents stupid mistakes !!

GlÜckliche Bauzeit !
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Dan Epperly
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Re: Intermittent miss under load where to begin search?

#69 Post by Dan Epperly »

I tore the engine down to the case just to see what it looked like inside and to figure out if this thing was rebuilt recently or not.
I am no expert, but the engine looks pretty fresh to me. The seals are not hardened and the bearing surfaces on the crank are nice and shiny and smooth to my eye.
Definitely a miss match of parts though. Looks to be a replacement case with a 1961 timing cover, with a 912 crank in it using three 504 rods and one SC I believe rod! Looks to be a fat lobe cam with little wear again to my eye. End play is in specs. Haven't looked at the oil pump gears yet.
The rod bearings showed some wear but no copper showing at all. Pin bushing is even all the away around.
Cylinders show hatching but I am not sure of that rise form honing, indicating a ring in or is the pattern seen on newish cylinders?
I do plan on bugging Dennis over Christmas break but I respect the opinions of those on the list so if have any please feel free to chime in.
I do have a set of 912 rods and good 912 blank heads I could use, as well as an extra 504 rod to balance out the existing three. Not sure what route to go. I know the 912 rods are superior than the 504s so I am inclined to use them given that this engine is built to 912 big bore specs other than the S90 heads, but if the heads check out fine is it worth it to upgrade to the 912 heads? What would my HP gain be?

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Mark Pribanic
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Re: Intermittent miss under load where to begin search?

#70 Post by Mark Pribanic »

Any update to your source?
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Brian R Adams
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Re: Intermittent miss under load where to begin search?

#71 Post by Brian R Adams »

charlesblankinship wrote:one or more bad spark plug wires. Check ohm's of each wire. Check to see if one of the wires is
arcing. Wait till it is getting dark if car is outside or turn off the garage light start engine open rear engine deck lid
use throttle linkage to bring engine RPM's until intermittent starts and look at all spark plug wire to see if wire is arcing to
some metal part of engine .
In the 1990s I had a '68 Buick with what looked to be original plug wires. On a road trip to Seattle, the car started running horribly. After checking all the usual regular tune-up items, including replacing cap/rotor, we happened to run it in a dark garage, and it looked like the monster revitalization scene from "Young Frankenstein" - "Give my creation ... Liiiiiife!". Quite an impressive blue arc light show. Replaced the wires, and it ran like new.

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