Intermittent miss under load where to begin search?

[Richard Shilling]

I thought the ignition analyzer (scope) was the latest thing. Sun Electric had free training classes and I took one. When the shop I worked at was too cheap to buy one, I built a Heathkit ignition analyzer. I used it on every car I tuned up. Reverse polarity is difficult to diagnose without a scope, but with one it's easy - the pattern is upside down. Switching the two wires at the top returned the pattern to normal. Over the 14 years I was a mechanic, I only found two coils that were reversed, one Bosch and one Delco, so it's pretty rare, but it does happen.

[Mike Horton]

Richard, I have one of those Heathkit units. Back in '71, I had a '70 Mercury with a Cleveland engine, and that engine did not have water running through the intake manifold. With the low hood clearance, the plug wires ran under the air cleaner, and laid on the hot manifold, and would "bake out", and leak, and that was hard to find. My HK unit had long wires, and on a winter, or rainy day, I could hook the scope lead to the #1 plug wire, and take the scope into the kitchen, where it was warm, and knowing the firing order,could pinpoint the single failed wire. That engine had 11:1 C/R, and would go through two sets of wires, to a set of plugs. Those experiences, teach us a lot, like using our heads... worked for me, and I taught myself a good troubleshooting trick,

Dan, do you have access to a scope?

[Mike Horton]

...Dan, in troubleshooting, only change one thing at a time, then, you will know the source of your problem, just 'sayin...

[Larry Coreth]

Richard:

OK with an oscilloscope that's cheating but effective !
So you determined the primary labels on top of the coil, +15 and -1 were reversed ? Interesting,
I've seen lots of coils connected incorrectly, i.e. the wires to the +15 & -1 reversed but never the actual coil labels reversed !

Was this a Bosch coil ?

[Jason McCandless]

Just an update... I'm leaving my findings here in case others need to look at their pump.... In short, I have my pump rebuild and working again however, the intermittent hesitation remains. I should however, make some clarifications to my earlier posts.

The new leaf spring (inlet valve, reed valve, etc) from Stoddard is NOT flat, there is a definite, and deliberate, bend to it. The bend provides sealing pressure when screwed in place (face down) and the new spring had a much more pronounced bend. See pic, the new part is on the far right. I suspect you could rebend an old spring to give it more effect. The new part resulted in a small improvement as I could feel a little suction on my finger when over the inlet hole but, the diaphragm would fully return in a couple of seconds.

The biggest improvement in suction was replacing the outer cover fiber seal (or likely a combination of the two changes). The new fiber seal is much snugger fit. Also, see pic new seal ins on the right. The pump cover wasn't leaking fuel prior to this but maybe the rebuild kits are getting more precise/better. Anyway, the diaphragm then held suction very well and I put it back on the motor.

Prior to that, I had also replaced the diaphragm shaft oil seal using a 3/8 inch bolt, a few twists into the seal and pull hard... (I also then saw the 3/16th hole which could be used to drive it out from below) The new seal was pressed in using the diaphragm rod as a guide and long socket.

I took took the car for a long test drive and unfortunately, the intermittent hesitation remain. I couldn't reproduce this consistently, going only uphill or only going downhill. etc. etc. One notable characteristic is that the hesitation usually clears up if I keep my foot in in. I'll have to watch more closely to see if its at a specific rpm range, etc.

So, I didn't have a faulty pump diaphragm but the inlet valve was not doing its job. Re; the hesitation, time to look deeper, per the suggestions above.

[Larry Coreth]

I think you may have saved yourself some time by just measuring the fuel pressure at idle and 2000 RPM.If it was per the specified 2.5 psi you would have known that was most likely not your problem. To have fully tested the fuel you could have done a volume test also, see the shop manual pg. F25.

On the other hand you now have learned some valuable lessons on fuel pump structure and performance !

Now you will need to do more test drives and define more precisely the hesitation problem, RPM, throttle position, load (gear, RPM & throttle position) Happy hunting !

[Brad Ripley]

Dick Weiss mentioned it above, go back to check each plug connector. Take wire off the connection, put you ohm meter leads across and (with your third hand) lightly tap the side of the connector. If the needle jumps around, replace the connector. My .02 cents worth.

[Craig Richter]

If you are lucky enough to have original connectors, and they fail the ohm meter test, please don't throw them away. They are rebuildable. Use a big screwdriver to disassemble. If the resistor inside has failed, replace it with a short piece of welding rod or some such thin metal rod; check again and I'm sure they will be perfect and ready for many more years of service.
The original connectors have a nice knob on the end to grab and are by far the easiest to R&R. VW are too short and have no knob, so I usually end up pulling the wire off the connector. There is a really long connector around these days that looks pretty cool, but it's so long it's hard to line it up with the spark plug and hits Solex manifolds.
More 2 cents...

[David Baugh]

Hi, Dan. I didn't see that anyone mentioned it, but you're welcome to ignore me if they did.

A spark plug that is trying to foul can act like that under load.

Dave

[Dan Epperly]

Hi, Dan. I didn't see that anyone mentioned it, but you're welcome to ignore me if they did.

A spark plug that is trying to foul can act like that under load.

Dave

Well, I still have my uneven running issue at load. I took a break from it since I ended up taking my 912 for the 1000 mile trip to Death Valley, but I didn't want to give up on the B and was at it with a friend until 9pm the preceding night before my wife and I left for our trip. Anyway, to date I have rebuilt the fuel pump, removed both carbs and cleaned them out and tested them to see if they are functioning, replaced the plugs, points, cap, coil and wires all to no avail. Next step is to replace the dis with a new rebuilt unit and see what happens. No change then i will pull one carb at a time and replace with a good one and see if that does it. I hate intermittent issues, it is so much better when the engine just takes a complete dump...

[Steve Harrison]

Hello Dan,

I read through everything here and noticed that the only time the condenser was mentioned was when you said on page two you replaced the points and left the condenser. If I missed where you changed it, then I just overlooked it. Have you changed the condenser?
Also,...I don't think anyone has mentioned a vacuum leak? Wiggle the carbs/manifold. Loose?

Another tactic is to make it do the problem, and really try to get it to do it the worst it can be,...then kill the motor and drift to a stop. Pull the plugs and note any differences between them.

Long shots here,...but seems you've got a bugagoo that won't go away. When you find it, it'll be simple, and weird at the same time. The best thing about chasing these types of things is when you find it, then it's like 'flipping a switch' and the car runs like a scalded dog because you've gotten everything else absolutely perfect while trying to chase the fix.
Good luck and I for one am waiting with curiosity to find out what it turns out to be.

[Larry Coreth]

Dan

It seems you have tried the shotgun approach, to no avail, this is where the real mechanics are separated from the "parts re-placer" and it is time to test and define the problem more thoroughly. I do not mean to make this sound like a personal reproach, but I know I have gotten wrapped up in such problems too and lost sight of the logic of the situation.

Review the description and circumstances of the anomaly, (retest as required)
1. When does the anomaly occur, specific RPM? full load (full throttle), less ? combination ? Test for repeatability.
2. Does the engine run on 3 or less cylinders, constantly or is one cyl. cutting out briefly (< a second) ?
3.Just for grins or if you haven't tried this yet, in 4th gear start at about 40 mph and slowly push the pedal to the floor,
hitting full throttle at about 60 mph, hold as long as road conditions will allow Any hiccups ? Repeat in 3rd gear
but start at lower speed, 30 mph.
If you were to report the answers to the above, the 356 gurus maybe able to lead you to a discovery.

[Dan Epperly]

Dan

It seems you have tried the shotgun approach, to no avail, this is where the real mechanics are separated from the "parts re-placer" and it is time to test and define the problem more thoroughly. I do not mean to make this sound like a personal reproach, but I know I have gotten wrapped up in such problems too and lost sight of the logic of the situation.

Review the description and circumstances of the anomaly, (retest as required)
1. When does the anomaly occur, specific RPM? full load (full throttle), less ? combination ? Test for repeatability.
2. Does the engine run on 3 or less cylinders, constantly or is one cyl. cutting out briefly (< a second) ?
3.Just for grins or if you haven't tried this yet, in 4th gear start at about 40 mph and slowly push the pedal to the floor,
hitting full throttle at about 60 mph, hold as long as road conditions will allow Any hiccups ? Repeat in 3rd gear
but start at lower speed, 30 mph.
If you were to report the answers to the above, the 356 gurus maybe able to lead you to a discovery.

Larry, thanks for the advice and I will give it a try.
Last night I replaced the distributer with a newly rebuilt unit, and the problem persist. I ask replaced a frayed coil wire, problem persists. I tried isolating a bad cylinder but it fires on all four, it seems systemic and not isolated to one or two cylinders. To recap, the car starts easily, idles fine. Up around 3000 rpm under load, as in say its gear on level surface, it begins to miss, acting like it is fuel starved to me, the problem gets worse as I get up to 4000 rpm, there is a very audible miss including balking of the engine, you know surging falling back surging. At this point I have to think its fuel system? I have replaced every component in the ignition system.

[Thomas Sottile]

If you have not given you engine a leak-down test now is the time. pay close attention to #3 and #1 cyl.

[Dick Weiss]

Were the carbs balanced correctly @ 2500-3000RPM--then the idle 850-900RPM?
At around 3000RPM is the crossover point of idle and high speed circuits.

Also, when I rebuild carbs, I make sure to observe the edges of the butterfly plates to match the 1st by-pass hole in the carb's throat(s) which make need a twist-stress of the butterfly shaft. The split-shaft Solexes must have a good resiliant/flex pad between the adjusting blocks unless the shafts are permanently locked together as a solid shaft.

Finally, during the butterfly relationship mentioned above, check the shaft endplay when the butterflies are fully closed (back off the stop screw), there should be no more than .003"/.004" between each end. Worn throats in Solex 40 PII will mainly effect idle or low RPMs, but if a lot of light shows thru (when closed), the throats will need to be bored and newer plates installed.

Dick