Tracking a miss - check my work

[02Pilot]

Recently developed a slight, intermittent miss at light, steady throttle. Idle OK, WOT OK, so naturally I'm looking at ignition first. I haven't found anything that seems overtly wrong, but I could use some extra eyes on it. Have a look and see if I've missed anything.

 

Centrifugal advance distributor, Pertronix, stock cap & rotor, correct NGK plugs, Bosch wires & red coil, resistance wire intact, no ballast

 

- Plug appearance consistent, gaps good.

- Plug wires resistance consistent.

- Cap & rotor clean, no measured resistance.

 

- (Meter resistance 0.4 ohm - values below are corrected)

- Resistance wire 1.4 ohm

- Coil primary resistance 1.4 ohm

- Coil secondary resistance 10k ohm

- System operating voltage 13.8v

- Voltage at coil + at idle 10.5v

- Voltage at coil + running 11.8v

 

Nothing seems wrong to me. The coil is not overheating, no wires melting. The one thing I cannot pin down is the resistance spec for the Bosch coil (p/n 0221119050). I've found listings for the 030 red coil at 1.6-1.8 ohm primary resistance, but nothing for the 050. Nothing has been changed in the ignition for several years and thousands of miles while the miss is new in the last few weeks, so it's clearly something failing rather than incompatibility. Any thoughts?

['76mintgrün'02]

If you don't mind my asking, what is your current carburetion situation?

 

Tom

[02Pilot]

Dual Weber 40 DCOEs, Carter rotary electric pump. Had that setup for 20+ years. They're clearly getting enough fuel, since I can run at WOT without fuel starvation, but I have considered the possibility of a plugged progression hole affecting light throttle running on one cylinder, which might be just enough to create the slight miss I'm feeling. Ignition seems more likely, however, so I'm running through that side of things first.

 

[Brandon]

It might be the pertronix, I went through about three of them when I was racing my Tii...two failures resulted in misses in the engine, one just flat out died and left me on the side of the track.  

[02Pilot]

Yep, it might be. I just can't figure out why the miss is only at light, constant throttle. If it were the Pertronix, I would think it would be constant, or related to a variable like heat, or even totally random. But I can't figure out how to correlate an intermittent miss that only occurs under particular non-strenuous driving conditions with a failure of a magnetic trigger.

[02Pilot]

OK, update. After dip-tank cleaning all the bits, I rebuilt my spare 002 distributor (same as the one in the car) to dead-on .005 axial play. Installed with new cap, rotor, points, condenser, and BP5ES; all the same as what was there except they sent me a 1K rotor instead of the 5K that was in there. Red coil is only a few years old, as are the wires.

 

Power actually seems a little better (probably just the placebo effect), but the miss (or perhaps better described as hesitation) is still there under the same conditions. At this point I'm calling it a fuel problem. I'll suck it up for a month or two, then yank the carbs and rebuild them over the winter. I'm guessing one of the progression holes is plugged.

Edited September 15, 2018 by Andrej

[Healey3000]

Check the axial play in your distributor shaft.  At light throttle, the shaft may be hunting up and down, unlike when loaded.

[02Pilot]

29 minutes ago, Healey3000 said:

Check the axial play in your distributor shaft.  At light throttle, the shaft may be hunting up and down, unlike when loaded.

 

Perhaps you missed the part about "I rebuilt my spare 002 distributor (same as the one in the car) to dead-on .005 axial play."

Edited September 16, 2018 by Andrej

[Healey3000]

17 minutes ago, Andrej said:

 

Perhaps you missed the part about "I rebuilt my spare 002 distributor (same as the one in the car) to dead-on .005 axial play."

Indeed!  Sorry about that.  Perhaps I should send you mine to fix as it has almost 1 mm of play

[02Pilot]

1 minute ago, Healey3000 said:

Indeed!  Sorry about that.  Perhaps I should send you mine to fix as it has almost 1 mm of play

 

No worries. The rebuild is pretty straight-forward. If you've got that much play, the fiber washers are probably gone. I did have to put in a slightly thicker shim to get it perfect, but the fiber washers were source of the majority of the play (which was about 0.4mm before the rebuild).

[2002Scoob]

Yep, it might be. I just can't figure out why the miss is only at light, constant throttle. If it were the Pertronix, I would think it would be constant, or related to a variable like heat, or even totally random. But I can't figure out how to correlate an intermittent miss that only occurs under particular non-strenuous driving conditions with a failure of a magnetic trigger.

DCOE’s huh? I might have your same problem too. Under light throttle, low RPM’s there’s a sweet(or sour, rather) spot that every once in a while I’ll hit and the motor hesitates, looses power, and does a muffled rapid fire stuttery sound.

For me, usually it happens when giving it not enough beans combined with slipping the clutch too quick when pulling away from a stop. I’ve kinda learned to drive around the issue by popping the throttle a tad when pulling away. It’s as easy to replicate the issue as it is to avoid it. It happens maybe once a month when I’m not thinking.

I kinda just attribute my issue to temperamental DCOE’s, and I’m probably ever-so-slightly off on my jetting somewhere, or that I’m running stubby velocity stacks, or I didn’t perform some ritual properly. But maybe it’s something else we both have in common. I haven’t done a solid cleaning on my carbs for a few years now...

But like you, all other aspects are great. Idle, mid, and WOT are fine. Once I have TUV and can drive to Munich I’m having my Tuning Guru put in some bigger chokes, and dial it in right.


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[2002Scoob]

On 9/15/2018 at 8:54 PM, Andrej said:

OK, update. After dip-tank cleaning all the bits, I rebuilt my spare 002 distributor (same as the one in the car) to dead-on .005 axial play. Installed with new cap, rotor, points, condenser, and BP5ES; all the same as what was there except they sent me a 1K rotor instead of the 5K that was in there. Red coil is only a few years old, as are the wires.

 

Power actually seems a little better (probably just the placebo effect), but the miss (or perhaps better described as hesitation) is still there under the same conditions. At this point I'm calling it a fuel problem. I'll suck it up for a month or two, then yank the carbs and rebuild them over the winter. I'm guessing one of the progression holes is plugged.

 

Also... if a progression hole is clogged, you're 5 minutes and a flathead screwdriver away from solving the problem, or at least eliminating another potential cause.

 

The progression holes have a small brass cap covering them thats easy to remove, and you'll be able to visually see if there's blockage by shining a light down the chokes. From there, some carb-cleaner and/or a GENTLE prodding with a fine pick should clear things up. 

Edited September 17, 2018 by 2002Scoob

[2002Scoob]

Fyi, progression-hole covers are these little guys.

 

[AustrianVespaGuy]

Does the problem stick around when you leave the throttle at the spot where it bogs, or is it just brief when you first put the throttle in the right spot and then picks back up again after a the stumble?  Labeling this as a transient or steady-state problem I think will help to better pinpoint it.

[02Pilot]

It's transient and intermittent, if not downright random. Let's say I'm doing ~45-55mph steady, 4th gear, flat road - every once in a while there will be a slight hesitation, not directly related to anything (seemingly). Accel or decel, or full-on highway speeds, and it's not there, so figure under 3k RPM.

 

I'll pull the progression hole plugs and look, but one way or another the carbs are getting their first rebuild in maybe 20 years over the winter.