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Vacuum retard

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donandmax

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The 1500 engines do not use vacuum advance they use vacuum retard. Anyone ever disconnect this and just leave it off. What are the effects of leaving it off. Poorer gas mileage ?
Don
 
Correction, the 1500s do have vacuum advance (spark fires earlier as vacuum increases), and there are specifications for how much advance the vacuum is supposed to provide. Can't say I've tried disconnecting it, mainly because I would have to plug the vacuum line. A buddy of mine's vacuum advance had a ruptured diaphragm, but seemed to run okay once we plugged the leak.
-Dave
 
I think the springs in the distributor do more than the vacuum assisted advance. I've heard of people not running a vacuum advance but I can't see where there would be any benefit in it. (Unless the diaphragm is busted)
JC
 
I'm not familiar with what dizzy is on a 1500. However, if your car has a working vacuum advance it generally improves your fuel economy while cruising. With gas prices where they are, I'm not sure this is the time to disconnect it.

You can disconnect the vacuum advance on a carb so equipped, but the only benefit you MAY gain is that IF you also lock the advance plate in the distributor you MAY reduce timing scatter.
 
Hello Don,

the vacuum advance is, as stated earlier, an economy device that allowed the manufacturer set a weaker mixture and still run well. It is irrelevant if you drive hard and you can disconnect and plug the line if you wish

"but the only benefit you MAY gain is that IF you also lock the advance plate in the distributor you MAY reduce timing scatter."

I would say that is a definite way to gain poor running. Without the mechanical advance, it will only run well at one part of the rev band, so I suggest that you don't even try it.

Alec
 
I second that, I wouldn't lock your mechanical advance. However, if you tune your car to run well, you can adjust timing slightly to compensate for the lack of vacuum advance if you so desire. Since optimum advance is hard to determine, it's difficult to recommend a particular timing figure. Also, you'll probably never notice a difference in driving of the car if the carbs are properly tuned and the spark is set as per factory settings. My car runs about 34 degrees total advance, which amounts to a heck of a lot at idle, but it runs pretty much fine at as much as 38 degrees advanced (except for a slightly over-advanced idle due to the limited range of mechanical advance in my distributor, about 24 degrees). My recommendation is to follow the factory spec unless you have a hotter engine. Vacuum advance won't make a whole lot of difference, but it might improve the gas mileage slightly. And, as mentioned before, if you lean it out a bit (the way the manufacturer intended), the advance makes it run better (but not as well as if the carbs are tuned properly).
 
I replaced the stock dist. w/vac. adv. with the strictly mechanical Mallory unit from Moss (Dual point). Used the vacuum fitting on the manifold to install a vacuum guage on the dash which is a valuable tool for tune-ups.
 
I did not suggest in any way shape or form that he should lock the mechanical advance. I suggested that if he disconnect the vacuum advance he should lock the advance (breaker) plate.

The points are on a breaker plate that advances when the vacuum unit pulls on it. The mechanical advance has NOTHING to do with that. Mechanical advance works by weights pulling against springs and camming the center post of the dizzy "forward".

The A-series mechanical advance dizzy of most fame is the 23D4. Take a look inside this dizzy and you'll see the points are mounted on a rigidly fixed plate. All I was suggesting is that if the vacuum advance is disconnected the advance plate be locked to keep it from bouncing around.
 
Sparky Dave and all. NOT SO..According to Bentley manual 1975-1976 1500 engines used vacuum retard.Ca, 1977 engines also used retard. Now heres a good one 1977 Fed and Ca. used advance. Apparently it was a mixed bag between advance and retard depending on where the vacuum was hooked up. ie
manifold or carb.However I must admit after reading on another discussion board I belong to (archives) It is all very complicated and confusing (at least for me)but it is for sure that 75-76 1500 engines did use the retard system. Any one care to jump in..Whoops the cat just jumped out of the cracker barrel..
 
I'd love to test this theory on my '76 Federal NonCatalyst Midget this weekend but the PO put a 25d in it when I'm assuming the OPUS ignition died. So with that I don't have a dog in this hunt but it sounds like it could be a good debate. Don't have the Bentley manual but I'll check my workshop manual and see what information I can dig up. I'd love to put that cat back in the barrel for ya...
JC
 
He probably put the 25d in it because that uses vacuum advance..But you have to mess around getting the correct degree of advance using different springs. Its a pain in the neck to get it right..The engineering in these 1500's is a total abomination.But at the time the engineers were fighting U.S. smog laws which were changing every year and they finally tired of it and thus ended the MG as we know it.I find the 1500's to be the worst engine ever made. If I knew this before I bought (e-bay) the thing I never would have gone this route.But I wanted a small 4 cyl. car because I saw this gas situation on the horizon. I enjoy driving it and tinkering with it but to be honest with you at 70 yrs. old I'm growing weary of the task !And dread the day I have to take the engine out for overhaul. My original quest was for a 70's Beetle..The problems with this MG and the oil leaks are somewhat overpowering.I've replaced all the seals that have anything to do with oil and the darn thing still leaks. But we are having fun arn't we ?
Cheers and safety fast
Don
 
I stand corrected Don, I went paging through my Midget 1500 factory workshop book, and sure enough, some of the years did have vacuum retard. I didn't realize they had different distributors on them, because mine has vacuum advance. I'll have to agree they did some peculiar things to meet the emissions requirements. I guess my only beefs I can think of with the 1500 are the weak bearings and the fine Zenith carburetor.
-Dave
 
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