Feeling Crabby …

[Admiral]

I desperately need some help getting a crab-style distributor setup correctly. The one I’ve got on the car right now works perfectly. The engine starts in half a turn, idles flawlessly and has plenty of smooth power out on the road. I set this one up myself using feeler gauges to set the points’ gaps.

I’ve been trying to get another crab distributor put together to keep in the trunk, one that’s tested and ready to go should I ever need it. However, for the life of me I cannot get another one of these units to work properly!!!

So far I’ve tested three different distributor bodies, two different shaft assemblies and three or four different sets of points. Carefully setting everything up, I can’t get them to work as well as the one on the car. Overall I’ve had a rebuilt distributor on and off the engine AT LEAST 25 times, which is not easy given how cramped things are.

The latest one I setup is a really nice iron unit I bought not long ago (see picture). I tore it apart, cleaned everything and put new points in. It’s in such good shape I didn’t even bother painting the housing. The lobes are beautiful, the advance weights move freely and the springs aren’t bent; even the backer plate is immaculate. I set the gaps at a perfect 0.016” with a feeler gauge, ensured the initial advance was OK and then installed it on the car.

With this distributor bolted up the engine starts perfectly and idles smoothly but, BUT – as with the other ones I’ve rebuilt – when you drive it the engine sputters/flutters. Cruising at an even 55 MPH it will flutter every few seconds. Other times it will buck and kick at random times and speeds. It’s absolutely maddening.

I’ve experimented with advancing and retarding the timing, I’ve fiddled with the vacuum brake, I’ve done everything I can think of and now I'm TOTALLY out of ideas. As a control I’ve even been using the same rotor, cap and condenser so those variables are out of play.

My car is 12-volts and has a Petronix Flame Thrower coil on it.

Sorry for the long post but what am I doing wrong? I’m ripping my hair out in bloody clumps at this point. Perhaps the next step is to send a core out to Bubba ...



Thanks in advance ….

- Craig

[mfagan]

Bubba's the man!!

[Mart]

The crab units are normally pretty bulletproof. The problems I have had are normally with the cap arcing through to the spring clips. Had this a couple of times now, with the black caps. I also suspect I had a faulty rotor, but can't be 100% sure.

The other thing that can get a bit sticky is the advance mechanism, the outer part of the shaft sometimes doesn't rotate properly on the inner part and can generally give a retarded spark at higher revs due to no advance.

Other than that, you would need to progressively change parts from the good distributor into the bad one until it comes right. Murphys law would probably kick in, though and you'd end up with 2 bad ones.

Without the parts in hand, it's very hard to offer more targeted advice.

Bubba comes highly recommended and has the machines and equipment in place to find out what the problem is.

Mart.

[BUBBAS IGNITION]

Originally Posted by Admiral

I desperately need some help getting a crab-style distributor setup correctly. The one I’ve got on the car right now works perfectly. The engine starts in half a turn, idles flawlessly and has plenty of smooth power out on the road. I set this one up myself using feeler gauges to set the points’ gaps.

I’ve been trying to get another crab distributor put together to keep in the trunk, one that’s tested and ready to go should I ever need it. However, for the life of me I cannot get another one of these units to work properly!!!

So far I’ve tested three different distributor bodies, two different shaft assemblies and three or four different sets of points. Carefully setting everything up, I can’t get them to work as well as the one on the car. Overall I’ve had a rebuilt distributor on and off the engine AT LEAST 25 times, which is not easy given how cramped things are.

The latest one I setup is a really nice iron unit I bought not long ago (see picture). I tore it apart, cleaned everything and put new points in. It’s in such good shape I didn’t even bother painting the housing. The lobes are beautiful, the advance weights move freely and the springs aren’t bent; even the backer plate is immaculate. I set the gaps at a perfect 0.016” with a feeler gauge, ensured the initial advance was OK and then installed it on the car.

With this distributor bolted up the engine starts perfectly and idles smoothly but, BUT – as with the other ones I’ve rebuilt – when you drive it the engine sputters/flutters. Cruising at an even 55 MPH it will flutter every few seconds. Other times it will buck and kick at random times and speeds. It’s absolutely maddening.

I’ve experimented with advancing and retarding the timing, I’ve fiddled with the vacuum brake, I’ve done everything I can think of and now I'm TOTALLY out of ideas. As a control I’ve even been using the same rotor, cap and condenser so those variables are out of play.

My car is 12-volts and has a Petronix Flame Thrower coil on it.

Sorry for the long post but what am I doing wrong? I’m ripping my hair out in bloody clumps at this point. Perhaps the next step is to send a core out to Bubba ...



Thanks in advance ….

- Craig

In the picture the plate is way retarded for timing.......tab should be in the center , this one is way to the left .......

[Mart]

Originally Posted by Mart

Bubba comes highly recommended and has the machines and equipment in place to find out what the problem is.

Mart.

He also has a good eye!

[Admiral]

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Originally Posted by BUBBAS IGNITION

In the picture the plate is way retarded for timing.......tab should be in the center , this one is way to the left .......

Good eye! Yeah, that was after the last test I made with the distributor. I tried retarding the timing fully, which, predictably, made the car run very poorly. However, when I had it fully advanced the engine only ran a little better.

[Terry,OH]

Is the point plate retaining ring properly seated at the bottom of the distributor? I had one that would always throw the ring off after a short run.

[Tony, NY]

Originally Posted by Terry,OH

Is the point plate retaining ring properly seated at the bottom of the distributor? I had one that would always throw the ring off after a short run.

Looks like it's not seated in the pic.

[Terry,OH]

If the ring seats and then comes off after a short run something is bent like the distributor shaft or upper bushing mount, maybe the point plate itself.

[Admiral]

Originally Posted by Mart

The crab units are normally pretty bulletproof. The problems I have had are normally with the cap arcing through to the spring clips. Had this a couple of times now, with the black caps. I also suspect I had a faulty rotor, but can't be 100% sure.

The other thing that can get a bit sticky is the advance mechanism, the outer part of the shaft sometimes doesn't rotate properly on the inner part and can generally give a retarded spark at higher revs due to no advance.

Other than that, you would need to progressively change parts from the good distributor into the bad one until it comes right. Murphys law would probably kick in, though and you'd end up with 2 bad ones.

Without the parts in hand, it's very hard to offer more targeted advice.

Bubba comes highly recommended and has the machines and equipment in place to find out what the problem is.

Mart.

Mart, per your suggestion I pulled the entire advance mechanism apart. It moved pretty freely on its own but I still tore it down then cleaned and oiled everything. It moves slightly smoother now, though not much.

Originally Posted by Terry,OH

Is the point plate retaining ring properly seated at the bottom of the distributor? I had one that would always throw the ring off after a short run.

I think the retaining ring is properly in place ... or at least in place well enough . It's never popped out yet, though in subsequent rebuilds I've made sure it's seated as tightly in the groove as it can go.

In other news I may have found out what the ongoing issue is. I think the leather on the vacuum plunger may have been swelled too much. It had a nice groove worn into it and I suspect what was happening was if it wasn't installed with the groove aligned with the plate it would drag all the time even if the adjuster screw was all the way out. I gently sanded the leather to reduce its thickness and tested it again last night. It seemed to run a lot better though a touch more tuning is probably needed. Does this seem like a plausible explanation for the bucking/fluttering while driving?

[deuce_roadster]

I was going to suggest new leather on the plunger. IF the bore is worn erratic vacuum could cause the piston to flutter and effect the running of the engine. The different bodies you tried could all be worn, they are over 70 years old after all.

[Admiral]

Originally Posted by Mart

The crab units are normally pretty bulletproof. The problems I have had are normally with the cap arcing through to the spring clips. Had this a couple of times now, with the black caps. I also suspect I had a faulty rotor, but can't be 100% sure.

The other thing that can get a bit sticky is the advance mechanism, the outer part of the shaft sometimes doesn't rotate properly on the inner part and can generally give a retarded spark at higher revs due to no advance.

Other than that, you would need to progressively change parts from the good distributor into the bad one until it comes right. Murphys law would probably kick in, though and you'd end up with 2 bad ones.

Without the parts in hand, it's very hard to offer more targeted advice.

Bubba comes highly recommended and has the machines and equipment in place to find out what the problem is.

Mart.

And we have a winner! Mart, you get a gold medal in the diagnostics Olympics. Turns out a DEFECTIVE distributor cap has been causing all of the intermittent issues my car's been having. As you can see in the attached pictures, it was arcing between the leads for cylinders six and three. Yesterday the problem got so bad the car would barely run so I finally pulled the cap and lo and behold, a line right between the two contact points. I can't believe I didn't swap this out sooner . Oh well, at least now I know better and can pull the whole dizzy in about three minutes.

[Mart]

Glad you found the problem. When I was having problems it was Walt Dupont who suggested to look there, so credit should go to him. It's actually the clip that holds the cap on that the spark is trying (and succeeding) to jump to.

Mart.

[48 coupe]

check the plug gap. too much gap will help cause tracking in the cap. Ben there done that. I have a Mallory. Lots of spark. ouch.

[48 coupe]

Or plug wires not all the way in the cap.

[russcc]

That is great picture of an arc line from contact point to point. Is that a black cap, can't tell in the picture. The best ones, but I believe are hard to find are the brown ones with copper contacts. Fred Wilner at SSO may have some NOS, or Ron at Dennis Carpenter NOS.

[Bassman/NZ]

You still need to seat the ring better. Sticking out like that could allow the plate to rock.