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Holley 390 tuning on a 258 - advice wanted
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Bart R. Orlans View Drop Down
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Points: 214 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Bart R. Orlans Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 6:43pm
Defective or binding vacuum advance?
Bart R. Orlans

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Points: 67 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurley Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 6:51pm
That is something I havent considered.... time to get a vacuum pump and start testing.  The mechanical advance is definitely working.
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Points: 349 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Pdok Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 6:57pm
I haven't seen it mentioned, but have you ruled out vacuum leaks?  Take  some starting fluid spray around all the intake gasket ports and the carb base/and carb throttle shafts to see if you have some leakage there.  If you still have any of the old vacuum hoses routed to the various EGR and/or temp sensor, start there.  Might be one of the t-fittings has an open end you can't see from up on top?

Remember that there could be a vacuum line running inside to the heater controls, too.

The PCV system should also be free of leakage, so when it's hooked up it there aren't leaks around the grommets or in the hoses.  The valve thingy inside should be free to move against the spring.  I'd recommend keeping the PCV hooked up to the vacuum source.

Sorry, I'm not much help on the tuning parts, but the vacuum leakage thing has had me chasing my tail before so I thought I'd mention it.
76 Grem X 258/904 w/ Cheetah VBody/Jeep intake/MC2150/ Clifford Header
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Points: 67 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurley Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 7:07pm
I have not ruled out a leak, however as of right now, there's only one vacuum hose connected to the carburetor, and it's the one going to the distributor (new).  The PCV hose and valve are new, and are not leaking.... I unhooked it the other day but have returned it to factory plumbing.
 
As a side note...I dread the day I try to re-incorporate the heater control vacuum line into all of the mess hhaha.
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Points: 349 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Pdok Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 7:33pm
tubes sound good then, but my vac leak problem was mainly at the intake gaskets.  I'd used a crummy quality one during an intake change, and it wasn't thick enough I guess.

I don't know if that could cause the symptoms, but I think most slightly off-idle problems are going to be affected by the initial idle mixture settings somewhat, and the mixture can't be right if another source of air is making it into the intake stream without going through the carb.  The transition from the idle circuit to the mains is different from carb to carb, and generally speaking Holley has a pretty easy to tune design.

Throttle shafts are a potential source of leaks, too.  If your carb was new, then no problems there.  It's easy to check with a shot of ether.
76 Grem X 258/904 w/ Cheetah VBody/Jeep intake/MC2150/ Clifford Header
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Points: 67 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurley Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 7:36pm
I've got some, ill go check it out.  The carb was bought used (and you know where the intake came from haha).
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Points: 6525 		Post Options Post Options   Thanks (0) Thanks(0)   Quote FuzzFace2 Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2012 at 9:13pm
If you did not get any change in the timing with the hose on (carb  or manifold) or off then it sounds like the vac advance is a no show. Get that fixed and I bet that stumble will be gone.
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Points: 67 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Hurley Quote  Post ReplyReply Direct Link To This Post Posted: Jul/02/2012 at 10:15pm
I tested the vacuum advance tonight - it works.  with 10.5" of vacuum I am getting 8deg of advance, and verified that my vacuum line is sealed in the process (i.e. didnt drop vacuum).  I went back and checked the vacuum signal from the metering block, and it's getting up to 20" by 'wicking' the throttle.
However, when I work the throttle by hand, watching the timing light at the same time, the timing does the following as the engine does its stumbling thing:  retard a few degrees, advance a few more degrees, retard a few, then advance on up as the engine takes off.
 
 
 
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Vac advance drops out under demand, and vice versa.  So its normal you see it decrease when u blip throttle.  Vac advance wont be your problem here. You could go to a dyno or find a tuner with an A/F ratio tool and verify the mixture. 
Have u tried tightening  the acc pump link "too" tight to see what would happen?  I doubt itll hurt diapragm.  Maybe dont floor it and it wont hurt anything.
Yes, by all means check for loose/leaking intake.  Ive seen this put some amcs in the boneyard.
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My 292 had a similar problem, and it was a leaking intake gasket.  Being as they are a shared intake exhaust gasket, it also caused popping out of one of the tail pipes.  Try pouring some water over the intake/head mating surface immediately after startup.  Great way to find intake gasket leaks on inline motors.
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