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Discussion Topic:
Low RPM/high load miss/'buck'
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len47merc |
12-20-2015 @ 6:17 AM
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Senior
Posts: 1168
Joined: Oct 2013
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Gents - need some troubleshooting advice. Took the '47 59AB out for a family Christmas light cruise last eve and for the first time noticed a low rpm/high load miss. Motor and carb are fundamentally new (<3,000 miles, 115+ psi all cylinders)) with all new or rebuilt electricals. Distributor was set up on a Sun machine. Has always ran great with excellent power and especially torque, so much so that I often both shift into 3rd on flats at 20 mph and take flat and/or downhill 90 degree corners without shifting out of 3rd - the motor just presses on smoothly. With the 3.78 rear the car has performed very well in 3rd here in the NC hills with little to no shifting out of third necessary at low-to-moderate rpms.
Last night with crisp sub-40 ambient temps and a warm motor, upon leaving the neighborhood and turning up the relatively steep hill I routinely run up in third gear at 25 mph, the motor for the first time exhibited misses when pressing the car to accelerate in 3rd up the hill. It has never done this this before. Shifting down to 2nd and/or pressing less firmly on the gas pedal to accelerate - this miss disappears. When leaving the first stop I shifted into 3rd at 20 mph on a flat road and attempted to smoothly, with a bit more gas pedal than normal, accelerate and the miss(es) is/are consistently there. Again, letting off the gas a bit and pressing less hard under these conditions the miss disappears. If I run the motor up to higher rpms (1,500+) before shifting and press the gas harder to accelerate in third, at the higher rpms the miss does not show itself. I can make it happen in 2nd as well if I choose to shift at a very low rpm and try to accelerate from the low rpm.
The miss is very similar to that I've seen in other cars (not necessarily EFV8's) with something breaking down electrically, such as a spark plug wire or cracked distributor cap, etc..
This is new for this motor. Again, it runs great as it normally does except under low rpm / high load situations. Fuel mileage has not changed btw and is in the expected range.
Upon returning home I opened the hood in the dark and ran it up 2,500 - 3,000 rpm but could see no arcing anywhere on the motor. Checked the original-style plug wires and they all are tight. Pulled the plugs this morning and they all are burning a medium gray.
Before I go through any further trial-and-error adjustments and/or replacement of parts I'd greatly appreciate some sage troubleshooting process advice that may more quickly and cost effectively identify the root cause.
Thanks in advance -
This message was edited by len47merc on 12-20-15 @ 6:19 AM
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Pat's 52 |
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12-20-2015 @ 6:31 AM
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Member
Posts: 35
Joined: Oct 2009
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I had similar problem with my recently rebuilt 50 Ford convt. I took top off carb and found black dirt in bottom. I cleaned it out and replaced stone filter in fuel bowl with Wix paper filter. Seems to have taken care of problem. I will check gas tank to determine if that is the cause. Just a thought because I did similar checks that you did.
Good luck and Merry Christmas!
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len47merc |
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12-20-2015 @ 6:49 AM
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Senior
Posts: 1168
Joined: Oct 2013
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Thanks Pat's 52 - for me the fuel tank was removed, cleaned and sealed upon car receipt and fuel lines were also thoroughly cleaned and/or replaced. I will go to this step (checking the bowl filter) in a progressive manner from easiest/quickest to more complex after seeing additional thoughts.
Very good suggestion - have to admit my current paradigm has me so focused electrically (given prior experience on other vehicles exhibiting similar similar behavior) that I've given little-to-no attention to fuel considerations thus far.
Steve
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TomO |
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12-20-2015 @ 8:23 AM
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Senior
Posts: 7385
Joined: Oct 2009
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Steve, I'm inclined to believe that the problem might be electrical also. I suspect that the condenser may be breaking down or your ignition resistor is not making good contact at the rivets.
Tom
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len47merc |
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12-20-2015 @ 10:28 AM
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Senior
Posts: 1168
Joined: Oct 2013
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Thanks Tom - cleaning the plugs now and about to post a separate thread on them with a photo.
The condenser you are referring to is that located on the distributor - correct? I've never had one breaking down under load only and work well otherwise - they've always been either all good or all bad with no in-between. Perhaps this will be a first - I'll try that after cleaning and re-gapping the plugs (see new thread I am about to post on this).
Question - by 'ignition resistor' assume you are referring to the resistor under the dash - ? Feel stupid having to ask that question. EDIT - also Tom, does a method exist beyond straight replacement for the condenser and/or resistor that will confirm they are good/bad? Thanks -
Mon, 12/21 EDIT - gapping and installing a new set of plugs today and as well resetting mixture screws with a vacuum gauge (ref other thread on 59AB spark plugs color). Will give this a go as a first step. Will advise of results.
Steve
This message was edited by len47merc on 12-21-15 @ 7:28 AM
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TomO |
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12-21-2015 @ 8:24 AM
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Senior
Posts: 7385
Joined: Oct 2009
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Steve, a weak spark shows up first under load. The insulation between layers of the condenser foil, can begin to break down and change the value of the condenser causing a weak spark. Usually the spark will change color from a nice blue to orange or red when the spark becomes weak. I have also seen cold soldered joints on the grounding tab of the condenser cause this problem.
The ignition resister is is the one under the dash and is checked by checking the voltage at the input to the coil with the points open and closed. Open points should show very close to battery voltage, closed points should show around 4 volts.
You should also check your dwell for 34-36 degrees.
I replied to your other thread.
Tom
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cliftford |
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12-21-2015 @ 11:07 AM
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Senior
Posts: 845
Joined: Jan 2014
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Here is something we all know but we need to remind ourselves of from time to time: A fuel problem often mimics an ignition problem, and vice versa.
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len47merc |
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12-21-2015 @ 12:15 PM
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Senior
Posts: 1168
Joined: Oct 2013
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Thanks Tom & cliftford -
Steve
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oldford2 |
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12-21-2015 @ 12:27 PM
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Member
Posts: 275
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My 2 cents worth because I had this happen a few years ago:
Dirt in the main jets. Easy check to pull the jets and inspect.
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len47merc |
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12-21-2015 @ 1:23 PM
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Senior
Posts: 1168
Joined: Oct 2013
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Ok guys - 'splain something to me. Taking it one step at a time so as to not introduce too many variables at one time, removed as stated above the plugs with ~3,500 miles on them and installed a new set of freshly gapped plugs (.025"). Cranked/warmed it up and just drove it 20 miles with multiple stops & starts. Shifted into 3rd multiple times at 10 mph and pressed it to accelerate up hills and on flats. Took several 90 degree green light turns without shifting out of 3rd and accelerated harder than usual back up to speed. No miss - no buck. Tried several times under heavy uphill low rpm 3rd gear loads and no miss.
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So apparently one of the relatively new plugs (for my more 'modern' expectation) was breaking down. They were all clean as a pin so which one I don't know. The question is how long/how many miles should a set of original style copper plugs last before cleaning/changing is required? What are your sage expectations and experience in this regard? The plugs look too clean and the age/mileage too low IMHO to have to be dealing with this already but this also is my 1st EFV8. And/or are Autolite 216's the problem? Appreciate your thoughts here.
Thanks -
Steve
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