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23° Distributor Cam Arm

Posted by RobAK64 
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Rob Tirpack
Anchorage, Alaska, USA   usa

Hello - first time MG owner as of 1 month ago. A 1964 MG Midget with the stock 1098 engine. Was barely running when I purchased it. I am in the middle of a basic tune-up and decided to clean up the distributor. Upon dis-assembly, I noticed that the cam arm has 23° stamped on it. After much research on this site and a couple of other sites, I am pretty sure that this indicated the amount of distributor degrees advance, which in turn equates to crankshaft degrees advance (2X). I also learned that most 948-1098-1275 engines like 32-36° total advance (initial plus mechanical).

Am I correct that this cam will give 46° (23 x 2) of advance? If I need to end up at 36° total, then I would have to retard initial timing by 10°. All of the literature I have found shows that this engine should be set at 3° or 5° initial advance. If I do this, then at RPM's, I would end up at 49° to 51° total adnvance. This is way too much and would surely ping.

Seems like an arm with 13-15° (26-30°) would be ideal.

Am I missing something?

On a side note, one of the springs looks kind of bent and maybe stretched alittle. I don't trust it and would like to replace the springs.

Besides sending my distributor off for a rebuild, are distributor cam arms and springs available?

Thanks...

Robert Kirk
Davenport, Iowa, USA   usa

Rob, Welcome to the forums! smileys with beer
You've got a somewhat rare Midget that will be a lot of fun. I believe you are over thinking all this. The engine will "tell" you where the dizzy is best set and at speed if you need to make minor adjustments to the idle setting. There used to be a sundries kit that may have included the springs but I don't see them anymore. There is a 5 pack of "universal" springs available PN TT1903 but a "cam arm" would mostly likely have to come off a used unit. If I can help with the springs let me know.



Regards,
Robert Kirk
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Trevor Jessie
Louisville KY, USA   usa

If you want to tinker with it yourself, you can slip something over the stop post to limit the travel of the cam. But then you still have the questionable spring issue. Or, if you just want it fixed you can send it off to Jeff at Advanced Distributors for a rebuild/recurve.



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Mike Miller
Upstate SC, USA   usa

The 23 degree stamp on the distibutor cam is indeed distributor degrees. That is a bit high for an A-series in-line engine. Don't remember seeing one that high. The ones I've seen vary from 8 degrees up to 19 degrees. Springs vary from application to application as well. Without putting the distributor on a machine, or doing some actual in engine tests, it is hard to predict where the ignition advance is at a specific rpm. Soft springs will allow quicker advance while stiffer holds the ignition advance out longer.

If your distributor is mechanical advance it might have a higher advance cam than vacuum assisted advance distributors. Whatever the situation, the 23 degrees just sounds excessive to me. I would recommend dropping back to something more along the lines of a 13-15 degree cam like you state. This would allow initial static advance to be set to 6-8 degrees for easier starting.

Good used distributor cams can be found via this site or others. I have boxes full of them (maybe a 23 is hiding in there and I don't know it), since I don't use distributors on the race car (Electromotive wasted spark). Some of the American car spring packs sold via the big box vendors will work in the distributor. I use the softest ones if restricted to a distributor fired ignition.

Racers typically will weld up the end of the distributor cam to reduce the mechanical advance. Usually get starting timing at about 10-12 degrees and 30-35 total depending upon application.

Unless the distributor was a NOS replacement or recently rebuilt (bush replaced too), then there is likely quite a bit of spark scatter in it. If the timing light shows the timing marks jumping all over the place, then there is scatter.

HTH,
Mike Miller

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Jeff Schlemmer
Minnesota, USA   usa
1950 Willys Jeep Pickup "Ratrod"
1955 Chevrolet Thriftmaster
1958 MG magnette
1971 MG MGB "POS"
1973 MG Midget MkII   → more

The place you'll normally see the 23 degree cam arm is an early Land Rover. My guess is that they didn't want the engines to rev over 3000 rpms, so by throwing the timing way beyond the correct power range, it would flood and misfire. I always weld old cam arms when doing a rebuild. Its necessary in 99% of all rebuilds to get the advance curve optimal. Even if you found a 10 degree cam arm, it'll offer from 11-14 degrees of advance, not 10.

As for springs, yes the Chevy aftermarket spring kits fit, but the spring pressures aren't correct until you start cutting springs and testing combinations of mismatched springs. You could play with a set of 6 springs for an entire day and not get the advance curve right unless you get lucky. You have 36-72 scenarios to try if you start cutting loops off the springs which will likely be necessary.



jeff@advanceddistributors.com

Rob Tirpack
Anchorage, Alaska, USA   usa

Thanks for the input and advice. Considering the general poor condition of the distributor (end play and radial play), the bad spring and unfavorable 23° arm, I decided to send the entire unit to Jeff for a rebuild/recurve. Jeff did mention the Range Rover and the obscure 23° arm.

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