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Low vacuum and miss-304 |
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Boris Badanov 
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Joined: Dec/14/2013 Location: NH USA Status: Offline Points: 4209 |
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 Posted: Jul/13/2014 at 11:48am |
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I looked up that camshaft, it's fairly hot for a stock 304, but not too much at all.
try a condensor and try running the motor with no vacuum advance hooked up. I would expect higher vaccum at 2000 rpm + than idle with any cam, but even more so with that one.
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Gremlin Dreams
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ollie09
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Joined: Jun/29/2014 Location: ann arbor, mi Status: Offline Points: 68 |
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Posted: Jul/13/2014 at 11:50am |
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Would an ignition problem result in low vacuum? [/QUOTE] Yes. Low idle vacuum can be the result of bad initial timing and bad condensor. At higher engine speeds the combination of mech advance weights and the vacuum advance will bring the advance up along with the manifold vacuum. "hot" cam timing will have the same result. I sense the whole problem here is in the ignition system or vacuum plumbing. [/QUOTE] There is no condenser, it has the stock electronic ignition. I set the timing for the highest vacuum I could get, 12". It has a 'new' after market ignition module. Unfortunately, I didn't try the old one before I pulled the front cover off. I figured if it runs it must be good. |
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Boris Badanov
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Posted: Jul/13/2014 at 11:51am |
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This from a website running a 9.25:1 SBC 350 with that same cam grind
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Gremlin Dreams
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Boris Badanov
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Posted: Jul/13/2014 at 11:59am |
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Yes. Low idle vacuum can be the result of bad initial timing and bad condensor. At higher engine speeds the combination of mech advance weights and the vacuum advance will bring the advance up along with the manifold vacuum. "hot" cam timing will have the same result. I sense the whole problem here is in the ignition system or vacuum plumbing. [/QUOTE] There is no condenser, it has the stock electronic ignition. I set the timing for the highest vacuum I could get, 12". It has a 'new' after market ignition module. Unfortunately, I didn't try the old one before I pulled the front cover off. I figured if it runs it must be good. [/QUOTE] You are in a tough spot. I now think you need a drastic distributer re-curve. That cam and compression combo needs about 16-18 degrees initial but the distrubiter may have up to 38 degrees in the mechanical advance alone. I think this a job for a guy that knows that distributer better than I. But I will open the circus with this comment. For a test only. I would try to lock out all advance in the distrubuter and set it for 32 degrees initial. No mech advance, no vaccum. Run it. Work from there, maybe 15 degrees mech advance in the dist, no vacuum advance till you get the curve correct. In my old AMC rigs I used to run 15-25 degrees initial depending on cam and compression. Never more than 36 total not counting the vacuum advance. Stock was around "0" degrees initial. |
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ollie09
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Posted: Jul/13/2014 at 12:19pm |
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Plugs are Champion 3405 set at .035". |
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Boris Badanov
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Posted: Jul/13/2014 at 12:21pm |
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Plugs are fine, I now think it's excessive total advance after
setting the engine for max idle vacuum.
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ollie09
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Posted: Jul/13/2014 at 12:29pm |
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This is my first AMC V8 rebuild. Done many SBC over the years so this has been a new experience for me. On SBC, we never bothered with initial timing but rather set max timing using a dial type timing light to get 38 degrees total advance at 3500 RPM.
Before I put it back together to try some of the other suggestions, does everyone agree that the cam looks to be installed correctly? I appreciate everyone's input since I'm new to AMC's. I have a 360 to build after this one, so I hope to learn from my mistakes. |
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rockAMX
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Joined: Sep/16/2011 Location: Lethbridge AB Status: Offline Points: 985 |
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Posted: Jul/13/2014 at 1:25pm |
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A stock 304 is around 5 degrees advanced at curb idle. Since you have the stock distributor, this should be about right. Adding the cam won't change that but upgrading to a higher performance distributor will. I would just start with 5-7 degrees at curb idle and rev her up. Make small changes in timing from there. You will get it. Some recommend getting the engine warm to curb idle then advance the distributor to achieve the highest idle, then retard it just a touch, then drive it and test.
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DWR
1968 AMX  |
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ollie09
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Joined: Jun/29/2014 Location: ann arbor, mi Status: Offline Points: 68 |
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Posted: Jul/13/2014 at 1:27pm |
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Not sure if I mentioned this, but if I slow the idle way down (500 RPM), the timing mark comes into view. This would seem to be the advance weight springs, but I've had 2 different Dizzy's in it and both do the same thing. I see one heavy spring and one light spring, does it matter which goes where?
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FuzzFace2
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Joined: Jul/05/2007 Location: Angier, N.C. Status: Offline Points: 10356 |
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Posted: Jul/13/2014 at 1:28pm |
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To me a motor is a motor and other than where #1 might be, how they number the cly per bank and firing order what works for one should work for them all.
At first I thought you did not have enought timing to start with but yo usaid you set it with a vac. gauge to the highest then I dont know what it could be?
The other thing that got me is yo usaid you could not use a timing light because the mark did not show up?? You did check the old & new dampers and they were the same so now why no timing mark with the light and no vac or RPM?
Did I also read you had this same thing before the rebuild?? If so what are you using that is the same from the old motor to new motor? Being others think timing and the dist might be new or was new to the old motor trying to fix this issue it might not be the dist but what about the IGN box?
Dave ----
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ollie09 wrote: