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Vacuum advance Cross Ref. question


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10 replies to this topic

#1 BratRod

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Posted 02 December 2012 - 08:12 PM

http://www.ebay.com/...3e5f288&vxp=mtr

http://www.oreillyau...=vacuum advance

Ovbiously looks can be decieving. I also have no intention of buying rom ebay, BUT... these do look considerably close to the timing advance on my 85 EA81 Brat with hitachi disty. Other than bolting up and looking like its doing its job correctly, should I be concerned with things like the throw? or the rating on the diaphragm or??? I plan on going up to oreillys and sizing them up, but are there things i should be cautioned of before attempting something like this. What do you guys think?

#2 djellum

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Posted 02 December 2012 - 09:39 PM

theres a half dozen different styles of vacuum advances, and there are hundreds more that are different tweeks of the same style.

yes the arm length matters, as well as the speed they advance. you would just have to send it in to be recalibrated to work, which is the same as sending in your old one to have it rebuilt.

subarus have a super high amount of vaccuum advance over other motors that I can think of. the ones I have go up 25 degrees or so, while a standard "350" advace is 10-15 deg. I wouldnt try random ones for multiple reasons, but even if it "fit" its unlikely to give you the right amount of advance.

I know theres a place in portland that rebuilds them for 25ish bucks. probably someone in any major city somewhere that will do the same. you can also rebuild it yourself, it mainly just takes patience to make sure it doesnt leak.

#3 naru

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Posted 03 December 2012 - 01:44 AM

http://www.ebay.com/...3e5f288&vxp=mtr

http://www.oreillyau...=vacuum advance

Ovbiously looks can be decieving. I also have no intention of buying rom ebay, BUT... these do look considerably close to the timing advance on my 85 EA81 Brat with hitachi disty. Other than bolting up and looking like its doing its job correctly, should I be concerned with things like the throw? or the rating on the diaphragm or??? I plan on going up to oreillys and sizing them up, but are there things i should be cautioned of before attempting something like this. What do you guys think?


Other than fit,you need to be concerned with how much total advance it has.
Also how much vacuum it takes to start the advance and how much vacuum it takes for full advance.

Subaru used 3 or 4 different vac advances on ea-81s w/hitachis.
Looking at a 84 FSM shows that the differences are very minor.I would be surprised if you noticed a difference.

I have a good one from an 83 you could have cheap.
Unfortunately,it has no #s for a positive ID.

Upon examination,it appears total advance is controlled by the sleeve under the spring.Advance rate is controlled by the spring.Looks to be a trivial matter to swap the spring and sleeve from one Hitachi vacuum advance to another similar one.(compress the spring and slide the retainer off the shaft)
I would get another similar looking Hitachi vac advance(new like in your links or used) and swap your existing spring and sleeve onto it and call it done.

Another option is a used one from http://car-part.com/ .
84s should be the same as your 85.Compare dizzy numbers.
I bought a complete ea-82 turbo dizzy in order to get an ea-81 turbo vac advance.Spare ignition module was a bonus.
Complete dizzy is probably only a few more bucks.

#4 naru

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Posted 03 December 2012 - 01:49 AM

theres a half dozen different styles of vacuum advances, and there are hundreds more that are different tweeks of the same style.

yes the arm length matters, as well as the speed they advance. you would just have to send it in to be recalibrated to work, which is the same as sending in your old one to have it rebuilt.

subarus have a super high amount of vaccuum advance over other motors that I can think of. the ones I have go up 25 degrees or so, while a standard "350" advace is 10-15 deg. I wouldnt try random ones for multiple reasons, but even if it "fit" its unlikely to give you the right amount of advance.

I know theres a place in portland that rebuilds them for 25ish bucks. probably someone in any major city somewhere that will do the same. you can also rebuild it yourself, it mainly just takes patience to make sure it doesnt leak.


25 degrees? Really? what year is that?
84s have 12 degrees.

Have you rebuilt one yourself?
Where did you get the diaphragm?

#5 djellum

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Posted 03 December 2012 - 03:10 AM

87 carbed ea82.

I have tried 2.

first was already on the car, but was leaking. car did fine but I found one that didn't leak at the JY, so I replaced it (another carbed ea82, same dist). checked again and it was the same.

I must admit though, the car hasn't run right since I replaced the leaky one, I've thought about putting the damn thing back on. I know its just a tuning thing, but it just refuses to dial in right.

I have other problems too, but both cans showed 25 degrees increase when hooked to manifold vac. might be the dist itself, I haven't dug into it yet.

either way I still say get the one you have rebuilt, the settings are all over the place for different cars, and you will spend less that way.

#6 naru

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Posted 03 December 2012 - 10:51 AM

Since similar looking hitachi vacuum advances have diaphragms with identical surface areas,the settings are completely determined by the spring and sleeve which are VERY easily swapped to a unruptured junkyard unit.

You will spend less that way.

12 and 25 degrees sounds suspiciously like the difference between distributor and crankshaft degrees.Too early in the morning to be sure of that.

#7 djellum

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Posted 03 December 2012 - 03:02 PM

for mine its actual vacuum advance.

I hooked up the vac line to manifold to bring it to full advance, dialed in the distributor to 22 deg just so it was on a mark somewhere. pulled the line and checked again and it was at -2ish. plug the vac line back in and back up to 25.

normally its hooked to ported of course, but I wanted to see how much it had.

#8 BratRod

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Posted 03 December 2012 - 03:33 PM

Cool, I'll pick one up then

#9 naru

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Posted 06 December 2012 - 02:03 PM

Just one furthur thought....

Preload on the spring controls when the advance starts so the installed length of the spring needs to stay the same.I`m guessing the diaphragms have arms of the same length,but,you should check.

#10 BratRod

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Posted 06 December 2012 - 06:33 PM

I'll check that. Its on special order, so im waiting.

#11 BratRod

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Posted 22 December 2012 - 02:43 PM

A very late update:

The timing advance i purchased from O'reillys was from a 1982 civic (The one in the link on the first page) Everything checked out, so i brought it home and swapped the sleeve and spring from the old advance. Once bolted up to the Brat it started up, as usual. Upon depressing the accelerator, there seemed to be a sweet spot at which the engine would rev, and only to about 5K. When you would floor it: bogging and backfire. This was due to the arm being longer on the new advance. So, I redrilled the advance tabs to compensate for the difference in length, and volla! Perfect running rubber-burnin' Brat once more. :banana:

It has been two weeks since the new advance has been on and i have yet to experience an issue. Very happy with the results, no missing at all now.

Thanks to everyone who helped out on this issue :drunk:

T.J




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