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MG MGB Technical - red rotors

I'm curious. How much difference would a red rotor for a 25D4 work on a 45D4? I decided to replace rotor caps and found them to be different. The car did run with both but I've had a mean misfire lately. Would this be the culprit?
Keith Yarbrough(TD 1275)

Keith, if you have checked all else, I recommend changing the condenser. Read my post about a "What causes a Miss". After weeks of issues, my problem turned out to be a faulty condenser (brand new one)

Unsure on if there is a difference between the 25D and the 45D rotor.
Bruce Cunha

Surely the rotor has to "match" the cap. In turn the cap has to physically fit the dizzy body. If the cap for a 25D fits the 45D body and the leads are in the same locations, then the 25D rotor should work. Surely it's simpler to replace the cap with the one that does fit and then use the corresponding rotor?

I think I must have missed the point!
Michael Beswick

Caps and rotors are different for 25D and 45D
Dave O'Neill 2

The bizarre thing about it is that I got it through a supplier, figured it was right and slapped it on. Obviously, I'm not a chief mechanic by any means. The car ran fine but only started acting out recently. Both rotors were the red premium rotors. I should have taken a picture to show the difference but that car has me so spooked to even go near the engine. HAha
Keith Yarbrough(TD 1275)

The 25D and 45D use different rotors. The red rotors are made from a special plastic that is low in carbon content, unlike all the rest that can cause arcing and short out. RAY
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well, if that wasn't confusing enough, there was a huge debate at our recent tech session regarding timing. Vacuum connected or disconnected. I've always been told disconnected but the powers that be at this tech session said to keep it connected. Sheesh!
Keith Yarbrough(TD 1275)

Vacuum advance gives better cruising mileage and sharper part-throttle acceleration.

There is a tendency to believe that running without vacuum advance gives more performance, not helped by 'performance' distributors being supplied without a vacuum advance facility. Yes they are for performance applications, as cruising economy is not a requirement, and doing away with vacuum advance makes tailoring the centrifugal advance curve much easier - 2 dimensions and not 3 - as well as doing away with a potential source of component failure. Slapping one on to your standard engine may or may make a difference - positive or negative.

It may be necessary to disconnect and plug the manifold or carb port when setting the timing though.
paulh4

I have a red rotor arm in a 45D dissi. It cured an intermittent misfire and has run for about 5 years with no problems. And stick with vacuum advance for a road car, it will go a lot further on a gallon.
Stan Best

thanks to everyone for the comments. It's just hard for your backyard mechanic who gets conflicting info. There seems to be a 10 degree difference between vacuum advance connected or not so I'm sure this plays into the endgame.
Keith Yarbrough(TD 1275)

Tuning specifications are always listed for setting the timing with the vacuum advance disconnected. I've never seen one where it says to leave it connected when setting the timing. A whole slew of reasons not to have it connected. RAY
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"A whole slew of reasons not to have it connected."

Tuning or running? Two very different scenarios. Manufacturers didn't provide it for fun.

Often there is no instruction to disconnect (and plug the carb port) with carb vacuum when setting mixture, whereas there always should be for manifold vacuum. But the throttle doesn't have to be open very far before even carb vacuum starts advancing the timing, especially on the later models with higher idle speeds, even more so today with funny fuels. So it's best to do it on carb vacuum as well.
paulh4

This thread was discussed between 04/03/2017 and 07/03/2017

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