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Guest Message by DevFuse
 

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Is vaccum hose retarding or advancing my distributor?


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12 replies to this topic

#1 jdub

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Posted 25 November 2003 - 04:17 PM

After careful research last night I thought I understood that the hose from my manifold, to the distibutor diaphram pod, was pulling on the internal distributor plate to cause retardation.

Can that be so? I have rarely seen any vaccum that was used to dampen a mechanically-activated advance. Is that the case for my Hitachi dist. on an '87 GL Wagon 5-speed Carb'd model?

Reason for this query is to put to rest, once and for all, that the rattling I am hearing is pinging from a too advanced state. The weird thing is that I checked timing last night and plate movement via hose (works fine) and all was spot on (8degreeBTDC@700rpm, hose plugged). I left the hose plugged and drove in this AM and the car did not rattle that I can remember...

John

#2 Goatboy

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Posted 25 November 2003 - 04:21 PM

Should advance as manifold vacuum increases. Not retard. That way the spark can ignite the chamber earlier since the pistons are moving faster. -- GoatBoy

#3 archemitis

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Posted 25 November 2003 - 04:23 PM

when the hose gets closer to atmospheric pressure, it tells your disty to advance the timing.

at idle you have lots of vaccume and when you floor it, it drops to aroung 1or2 lbs of vac.

#4 jdub

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Posted 25 November 2003 - 04:36 PM

that lets the dog run out a bit, but holds at the very limit!

At idle, I have mucho vacuum, and this limits the advance of the mechanical advance plate mechanism. As I hit the pedal, vaccum decreases (closer to atmospheric) and the plate is allowed to run out via centrifugal force.

So, if my engine is pinging, it would stand to reason that the vaccum is not enough to prevent the mechanism from advancing too much, too early.

Hmmm. How can I possibly effect this (if I have this right) unless I were to "fool" the car by retarding the timing from 8degrees to 6degrees or more? Seems I would lose some performance.

John

#5 GeneralDisorder

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Posted 25 November 2003 - 04:47 PM

Vacuum advance units do fail - they can be rebuilt.

These guys can do it:

http://www.philbingroup.com

GD

#6 jdub

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Posted 25 November 2003 - 04:50 PM

Maybe mine is falling out of range - sucking on the hose does cause a nice shift in the plate, but then again is it enough?

Thanks to all for this information. I must say, as slow as the carb model is, it does get me around the ECU and whatnot. After messing with the Digifant II setup on VWs I can really appreciate the (relative) simplicity of this car.

John

#7 GeneralDisorder

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Posted 25 November 2003 - 04:53 PM

You think that's simple? Get an EA81 car. hehe

GD

#8 beauregaardhooligan

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Posted 25 November 2003 - 06:47 PM

Try the next higher octane gas, or retard the timing some.
Those catalytic converters are famous for rattling.

#9 GeneralDisorder

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Posted 25 November 2003 - 06:54 PM

The carb model really shouldn't be that slow. Your making 15 more HP than my Brat, which is about 20% more I would say. Your car weighs 20% more or maybe a little less. My Brat is not slow. Doesn't even have a weber on it yet. I mean - it's not a speed demon, but it does get up and move - even with a 4 speed.

Your engine may need some tune up done on it. Vacuum advance units are notorious for getting cracks in the rubber, thus not working as well as they used to. Cost to rebuild is about $25 from the guys I linked to above. If you going to do it, you might as well have the disty shaft re-bushed at the same time - about $35 if you dissasemble it for them.

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#10 edrach

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Posted 25 November 2003 - 10:44 PM

Your 8 degrees with the hose plugged is spot on. As the engine rpm increases the vacuum should cause your advance to climb to around 20 degees advanced. It's possible the advance is not stopping at the maximum advance and that's what's causing the pinging. A timing light will tell you what's happening. Also, whose gas are you buying? I generally use Chevron, Shell or Unocal regular. Whenever I try to save a few cents per gallon (like Arco, Costco, or any of the other "off brands") my car pings like crazy.

#11 rallyruss

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Posted 26 November 2003 - 01:55 AM

my old carbed 85 gl needed to run at 5deg. to keep from pinging. could have been a disty problem but it ran just fine at that timing.
no not a lot of power but thats why I now have a turbo car (or two)

#12 Guest_taprackready_*

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Posted 26 November 2003 - 02:12 AM

Originally posted by jdub

At idle, I have mucho vacuum,

1. At idle you should have no vacuum at the ported vacuum off of the carb. You will only have vacuum at idle on non-ported or full manifold vacuum.

and this limits the advance of the mechanical advance plate mechanism.

2. No, vacuum advance is not supposed to limit the mechanical advance.

As I hit the pedal, vaccum decreases (closer to atmospheric)

3. True for full manifold vacuum but ported vacuum continues on to higher rpm levels.

and the plate is allowed to run out via centrifugal force.

4. The plate changes advance due to the weights. Springs hold the weights in at lower RPM's and centrifugal force overcomes the strength of the springs and forces the weights out to full advance.

So, if my engine is pinging, it would stand to reason that the vaccum is not enough to prevent the mechanism from advancing too much, too early.

5. I think the vacuum advance may be hooked up to the wrong port on the carb. Should have no vaccum at idle on the correct port.

Hmmm. How can I possibly effect this (if I have this right) unless I were to "fool" the car by retarding the timing from 8degrees to 6degrees or more? Seems I would lose some performance.

6. Try a ported area on the carb and see what happens.

John


Bill

#13 archemitis

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Posted 26 November 2003 - 08:43 AM

im not sure what you mean by ported, but you have to get vaccum from behind the throttle plate. i think all the ports on the carb will be vac ones. you might need to try something else. what if its carbon buildup in the cylinders?




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