Itinerant Air-Cooled

Okay. A long time ago Colin posted about his Pertronix magnets being off slightly and it caused his timing to be off on some cylinders. I checked mine and it was off also. I changed the Pertronix out for a Compufire and still had the same problem. The bus went in storage for awhile and now it's back on the road, but it still has the same timing problem. I wracked my brain and decided it must be the distributor cap. I put a new cap on today, but the timing is still off.

#1 - 7.5 BTDC
#2 - 6 BTDC
#3 - 7 BTDC
#4 - 4 BTDC

Any suggestions? I'm Baffled!!!

I also noticed that my cooling fan is scraping the fan housing on one side.

My honest answer is 'I have no idea', but just thinking about it, the problem would almost certainly have to be one of the following:

A) A worn distributor drive gear

B) A worn/wobbly distributor shaft

C) A worn/distorted distributor cam

How does it run with points instead of that fancy Compufire? Do you have the spring in place that goes between the distributor and the drive gear? If the spring isn't there, the dist. shaft may wobble a bit. Do you have a different distributor to see if the problem is reproduced there?

Tim

satchmo wrote:My honest answer is 'I have no idea', but just thinking about it, the problem would almost certainly have to be one of the following:

A) A worn distributor drive gear

B) A worn/wobbly distributor shaft

C) A worn/distorted distributor cam

How does it run with points instead of that fancy Compufire? Do you have the spring in place that goes between the distributor and the drive gear? If the spring isn't there, the dist. shaft may wobble a bit. Do you have a different distributor to see if the problem is reproduced there?

Tim

A) My drive gear has less than 8000 miles on it since it passed inspection.

B & C)It's an Aircooled.net SVDA with less than 8000 miles on it. I guess it could have come like that.

The spring is in place.

I have a 009 and 050 I could pop in when I have time (working 7 days a week right now).

Sluggo wrote:

satchmo wrote:My honest answer is 'I have no idea', but just thinking about it, the problem would almost certainly have to be one of the following:

A) A worn distributor drive gear

B) A worn/wobbly distributor shaft

C) A worn/distorted distributor cam

How does it run with points instead of that fancy Compufire? Do you have the spring in place that goes between the distributor and the drive gear? If the spring isn't there, the dist. shaft may wobble a bit. Do you have a different distributor to see if the problem is reproduced there?

Tim

A) My drive gear has less than 8000 miles on it since it passed inspection.

B & C)It's an Aircooled.net SVDA with less than 8000 miles on it. I guess it could have come like that.

The spring is in place.

I have a 009 and 050 I could pop in when I have time (working 7 days a week right now).

You need to distinguish if this is consistent between cylinders or intermittant variation. Mine was deadnuts always advanced in relation to the others, all of whom showed some intermittant "scatter."

Please recall that I also mentioned the quick effect of crankshaft endplay on timing scatter. The brass gear is a worm type gear that has a quick effect on the distributor drive gear as the crankshaft moves axially. A .006" endplay can change the timing 3-4º lickety split.

Satchmo's call on distributor bushings is a good one for people without brand-new SVDAs, the merest sideways movement of the breaker cam can advance or retard the instant that the points open, and it is THIS that affects the timing. Randy, the cap has no bearing on timing ever. It just sits there.
Colin

I think it may be the bushings or a bent shaft. Upon closer inspection of my original cap & rotor I found that there were kinda deep scrape marks on one side of the inside of the cap and on the top of the rotor. Plus the rotor looks like the protrusions on the right and left of the brass contact may have broken off.

Timing does scatter just slightly on 2 & 4.

I'll try dropping in my 050 on Sunday to see if it is more steady and accurate.

Sluggo wrote: #1 - 7.5 BTDC
#2 - 6 BTDC
#3 - 7 BTDC
#4 - 4 BTDC

Any suggestions? I'm Baffled!!!

I'm curious what index mark you are using to determine the timing for cylinders 2 & 4. Did you notch your pulley 180 degrees from the factory notch? If so, how certain are you that the notch is at exactly 180 degrees? Even 1 degree off would give consistently different readings for 2 & 4 compared to 1 & 3.

SlowLane wrote:

Sluggo wrote: #1 - 7.5 BTDC
#2 - 6 BTDC
#3 - 7 BTDC
#4 - 4 BTDC

Any suggestions? I'm Baffled!!!

I'm curious what index mark you are using to determine the timing for cylinders 2 & 4. Did you notch your pulley 180 degrees from the factory notch? If so, how certain are you that the notch is at exactly 180 degrees? Even 1 degree off would give consistently different readings for 2 & 4 compared to 1 & 3.

During the rebuild I placed #2 at TDC and marked it against the scale. Even if I was slightly off, the timing on 1 & 3 and 2 & 4 should match.

Sluggo wrote:

SlowLane wrote:

Sluggo wrote: #1 - 7.5 BTDC
#2 - 6 BTDC
#3 - 7 BTDC
#4 - 4 BTDC

Any suggestions? I'm Baffled!!!

I'm curious what index mark you are using to determine the timing for cylinders 2 & 4. Did you notch your pulley 180 degrees from the factory notch? If so, how certain are you that the notch is at exactly 180 degrees? Even 1 degree off would give consistently different readings for 2 & 4 compared to 1 & 3.

During the rebuild I placed #2 at TDC and marked it against the scale. Even if I was slightly off, the timing on 1 & 3 and 2 & 4 should match.

Agreed, but say you were off by one degree on your marking. That could then mean that your numbers are actually:
#1 - 7.5 BTDC
#2 - 7 BTDC
#3 - 7 BTDC
#4 - 5 BTDC

Except for #4, that doesn't look quite so bad now. So the question then is: why is #4 so far off?

Sluggo wrote:I also noticed that my cooling fan is scraping the fan housing on one side.

This may have been my problem. A cheesehead screw had somehow gotten trapped in between some fins. It threw the fan out of balance and caused it to rub the fan housing on one side. Removed the screw and it was almost worse. I could actually see the offset of the fan. I replaced the fan with the one from my new 77. Timing was now dead on at 4 BTDC on 1 & 3. I reset it to 7.5 and she purred. I don't have a mark on the new pulley for the 2 & 4 side but using a random scrape on the edge I could see that they were even with each other and not scattering.

Could it really have been this simple? Off balance fan causing the crank to wobble slightly which transferred to the dizzy?

Sluggo wrote:Could it really have been this simple? Off balance fan causing the crank to wobble slightly which transferred to the cam and then to the dizzy?

No, I do not think so. But you might have had a visual effect with a stroboscopic light flash catching the engine mid-vibration a millimeter over. Engine is smooooth now? That is a good thing.
Colin

Amskeptic wrote:

Sluggo wrote:Could it really have been this simple? Off balance fan causing the crank to wobble slightly which transferred to the cam and then to the dizzy?

No, I do not think so. But you might have had a visual effect with a stroboscopic light flash catching the engine mid-vibration a millimeter over. Engine is smooooth now? That is a good thing.
Colin

Damn smooth! So basically the timing was not errant at all. The fan & notch had moved out of center/balance and gave false readings.

Either way, I'm glad it's solved.