Intermittent miss under load where to begin search?

[Steve Harrison]

I remember there was a fellow one time who had an untraceable problem and he found eventually that there was foreign matter "floating" in the bottom of the fuel tank and when the car was not running, or just getting going the stuff wasn't a problem, then as the car drove, the stuff got sucked up against the screen and starved for fuel.

Here's a thread about a fellow who was chasing a problem and all it turned out to be was bad gas.
viewtopic.php?f=1&t=38581

And finally,...I'm still wondering about a "vacuum leak".

[Larry Coreth]

Steve,

It has been my experience that a vacuum leak will affect idle and low speed running the most when the vacuum is at its greatest. Don't forget that as the throttle gets to full open the manifold pressure approaches atmospheric (0" Hg vacuum)


Dan,

Steve does bring up an easily verified potential cause , i.e., trash or rust flakes in the fuel tank clogging up the fuel pick up screen, #6 below. Even more elementary would be if the bowel #5 and/or filter screen #4 in the fuel cock are clogged with foreign material ! I have experienced this problem, although in a'73 911T, the principle is the same.
If you have a top sender fuel tank you can remove the sender and with a flash light peek into the tank to see what if anything is swirling around in there.

[Pascal Giai]

Dan,
I think your camshaft is worn and needs to be replaced...My 54 coupe had the same symptoms and with the help of Richard Miller ( I am talking in 1997 ) after changing Carbs, Coil, Regulator, Fuel Pump we finally agreed to look thru the oil sump cover to find one camshaft lobe worn...not fun.
I hope I am wrong

Pascal

[Phil Planck]

You could also try hooking up a voltmeter to the hot side of the coil and extend it into the passenger compartment. With a passenger watching the meter maybe a change in voltage at the coil during the intermittent miss would reveal a possible bad electrical connection up-stream.

[Dan Epperly]

Dan,
I think your camshaft is worn and needs to be replaced...My 54 coupe had the same symptoms and with the help of Richard Miller ( I am talking in 1997 ) after changing Carbs, Coil, Regulator, Fuel Pump we finally agreed to look thru the oil sump cover to find one camshaft lobe worn...not fun.
I hope I am wrong

Pascal

Pascal,
I will check the valves, but with a worn cam I would suspect I would have some serious clattering in the valve train and that is not the case. I have enough here to keep me busy over the weekend. If it is the cam maybe I plug my $600 industrial engine in with the single carb manifold and all. Vroooooommmmmm

[Mike Horton]

...I had an ever so slightly cracked spark plug porcelain on a new out of the box plug I installed in a 289 Hi-Po Ford engine once, and it only broke down under load...that took a while to find, same symptoms,

[Thomas Sottile]

Joined: Wed Mar 26, 2008 3:02 pm
Posts: 644
If you have not given you engine a leak-down test now is the time. pay close attention to #3 and #1 cyl.

[Alan Hall]

May be a long shot but I have run into a situation where the rotating rubber seal/valve piece in the fuel cock had swollen so much that the holes were nearly closed off thus starving the engine for fuel under load. Maybe a try in the reserve position would see if there was any difference in running. Also, there may be some way to temporarily bypass the fuel cock for testing purposes, but I have never tried to do it.

[Mike Horton]

...are you running the 10 %ethanol?

[Larry Coreth]

Mike,
Why do you ask about the 10% ethanol ? Do you men to suggest this may be the root of Dan's problem?

I have been forced to use it for years and have never had a problem that was traced to the fuel. I have had my 356A since long before the Feds ever got the ethanol idea into their heads and have found that except for a slight adjustment of the float level, if the engine is setup per the factory specs it will run correctly.

Everyone seems to love blaming all sorts of motor ills on that Sh...y fuel of today ! But then again I heard that excuse 30 years ago when they took the lead out of the fuel too !

That's my rant for today and I'm sticking to it ! Stepping off the soapbox now.

[Dan Epperly]

...are you running the 10 %ethanol?

Pretty much running whatever they are putting in the tank at the local gas station ever since I had the car and with no issues, so that is a constant.

[Mike Horton]

Mike,
Why do you ask about the 10% ethanol ? Do you men to suggest this may be the root of Dan's problem?

I have been forced to use it for years and have never had a problem that was traced to the fuel. I have had my 356A since long before the Feds ever got the ethanol idea into their heads and have found that except for a slight adjustment of the float level, if the engine is setup per the factory specs it will run correctly.

Everyone seems to love blaming all sorts of motor ills on that Sh...y fuel of today ! But then again I heard that excuse 30 years ago when they took the lead out of the fuel too !

That's my rant for today and I'm sticking to it ! Stepping off the soapbox now.

Larry, that question followed Alan's comment on the swollen rubber disc in the fuel source selector, curious on the fuel/rubber compatibility? OK?

BTW, in my '68 912, original engine, with split shaft Solex 40PII 4 carbs, 3300' elevation, 10% ethanol, I had to go to the solid shaft 57.5 idle fuel jets, up from 55s, and up to 127.5 mains, up from 125s, to get my oil temps back to a normal range, in 100+* hot West TX summer weather. Running the richer jets (per our David Jones' excellent old tech article on the ethanol fuels, which I suggest you review, a good treatise, thanks, David!), that change also returned the 32 mpg highway cruise mileage. Sample size. (1), from my experience, my engine did not run correctly, jetted as stock, for the later split shafts, it ran much better after the larger jets were installed, and no flat spots. Other than the 022 distributor (in lieu of the stock 061), the engine is otherwise stock, 82.5 mm Biral P/C (not my faves). I can't speak for others, just from my 5 year, 15,000 mile experiences in one car. I never had this on my old 356s, both with 32PBIC Solexes on the '57 T1 N, or, the Zeniths on my T5B, but...that was on real gas. The A T1, stock, except for the Abarth, would consistently get 33.5 mpg, highway, in the mid '60s, same part of the country, @ 80 mph, BBBC gears.

[Mike Horton]

I'm sorry, Dan, that question was really for Alan, I thought I ought to clear up my intentions...

[Larry Coreth]

Mike ,

If memory serves the seal in the fuel cock selector is made of cork. The one in my car has been there since 1984. As long as it stays wet (with fuel) it will last a long time.

The '68 912 came from the factory to run very lean to meet the Fed air regulations (new for that year) . Back in the day (leaded fuel, no ethanol) I changed many a' 912 set of mains & idle jets to make it run richer which improved the drivability greatly !

Now my previous post should not be interpreted to mean that all is copasetic with the introduction of the 10% methanol. Yes lower fuel mileage is to be expected, greater evaporation, etc The point was that before one blames the sh...ty fuel of today make sure the motor is setup per the specs. Just one of my pet peeves along with a latent "Quixotic" complex !

[Mike Horton]

Larry,...Alan's post contained the reference to rubber, I haven't had a 356 fuel cock apart in decades, myself. Yes, I recall the cork, as in my old Army canteen caps, same wet sealing issue. My situation, and real world experience with ethanol on my '68 stands, and I didn't even mention the vapor lock issue in really hot summer TX weather, not present in more mild weather, I'll deal with that in my overhaul. I never experienced that in a 356 on real gas. It really gets hot out here in the TX South Plains in summer months, and stays hot for 4-5 months. In the same time frame, in part, I also used a '96 Chevy P/U for that same commute, Lubbock to Amarillo on I27, and just with the change to the 10% from real gas, it lost from 18 mpg, to 16 mph, consistently, do the math... I'm not trying to be argumentative, just sharing my own real world experiences. Brazil went to ethanol long ago, of necessity. I have a low pain tolerance with the ethanol savings myth... let's agree to disagree on this issue, and yes, the '68, & '69 912s were not optimized, due to the then new environmental issues. Do go back and read David's articles, he is one of us, and professionally blended fuels at refineries, and makes a lot of sense. Today's fuels are a real issue, with which we each must deal/compromise.

Dan, these intermittent miss issues are not easy, and the culprit is where you find it. IIRC, you have some spare parts , you might keep interchanging systems until you see a change. Swap plugs, to a known good set, plug wires, etc. Just change one thing at a time, you'll find it. Try to find someone with an oscilloscope, it will eliminate many ignition issues for you.