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vacuum advance miss
https://slantsix.org/forum/viewtopic.php?t=42698		 Page 1 of 3
Author:  mopardean [ Fri Nov 26, 2010 7:48 am ]
Post subject:  vacuum advance miss
I have an engine miss on a freshly rebuilt slant out of a 75 duster. the miss is noticeable at idle and at cruising speed and at high rpm's and especially when coasting down. About the only time it seems to run right is during acceleration up thru the gears.

After trying virtually everything and with the good help from people on this board i have been able to improve my situation by loosening the valves (the cam i am using is non stock- it is a comp cam grind number cr6 264s-10...part number 64-241-4) which helped, hooked up the vacuum line to the proper carb port which also helped (i had it hooked to a carb port that had continuous vacuum instead of one that only started pulling at about 1300 rpm). The engine now pulls about 18'' and i have it timed to achieve max vacuum

Wednesday evening before i installed my third distributor i noticed the motor ran much better when i disconnected and plugged the dizzy vacuum advance altogether. After doing this i am left with only about 20% of my original problem.

Any ideas re: what this should be telling me or what to do next would be greatly appreciated.

Dean
Author:  ceej [ Fri Nov 26, 2010 8:50 am ]
Post subject: 
What is your intial advance?

When you hook up the vacuum advance at idle does the timing change?

CJ
Author:  Joshie225 [ Fri Nov 26, 2010 10:53 am ]
Post subject: 
If the vacuum advance is causing missing you've probably got a rotor-to-cap phasing problem. If you have a spare distributor cap you can sacrifice drill a ~3/8" hole just inboard of one of the distributor terminals. Connect a timing light to that plug wire and you can observe the rotor position when that cylinder fires. At idle the rotor should appear on the far side of the terminal and move to the leading side at maximum advance.

Also, if you have vacuum at the distributor port at idle then the throttle plate is open too far. This is a common problem with performance cams especially if the compression ratio is too low or there is too little initial ignition advance.
Author:  mopardean [ Fri Nov 26, 2010 4:54 pm ]
Post subject: 
The dizzy advance port on the carb does not alter the advance at idle and like i said the port does not start pulling vacuum until about 1300 rpm so i guess the initial advance would be zero.

Josh i will try your recommendation but it is not clear to me how to alter the rotor to cap positioning should a problem appear. it appears to me the cap can only fit one way in the little slot on the dizzy.

i am about ready to run the thing in the Ohio River. i have done nothing but work on this since i got the motor back from the machine shop and installed in early August !!
Author:  ceej [ Fri Nov 26, 2010 5:09 pm ]
Post subject: 
It's possible to stab the distributor a tooth off, and have the cap and rotor "close enough" to make it run. Your distributor may need to be reinstalled. Check the bottom distributor timing bolt to see if it is set to one or another extreme.

Adjust as necessary to get your ignition point to center on the cap properly as Josh outlined above.

CJ
Author:  mopardean [ Fri Nov 26, 2010 5:32 pm ]
Post subject: 
Ceej

not to appear as an idiot but what and where is the bottom distributor timing bolt?

dean
Author:  ceej [ Fri Nov 26, 2010 5:43 pm ]
Post subject: 
Not easy to see!

The bolt that holds the timing plate to the bottom of the distributor is also adjustable. When the bolt is loosened, you can move the plate to allow more rotation of the distributor where the top adjuster attaches to the block.

CJ
Author:  mopardean [ Fri Nov 26, 2010 5:50 pm ]
Post subject: 
Ceej
the way i always installed the distributor is to put the timing mark at tdc on #1 and line up the rotor to the #1 position. for a slant #1 is at about the 4:00 position. correct?

could i be a tooth off under these circumstances??

Dean
Author:  ceej [ Fri Nov 26, 2010 6:30 pm ]
Post subject: 
Sounds right to me. You can be off, yes. Check the bottom adjuster and see if you can get the timing to a more manageable location. If you find you are at one extreme or another with the top adjustment, you can move the plate by loosening the bottom bolt. They are hard to get at with the distributor in, but it's not impossible.

CJ
Author:  mopardean [ Fri Nov 26, 2010 6:40 pm ]
Post subject: 
i will try

i've done about everything else.

incidentally for the second time i must have the machine shop remove the head and replace the gasket. bad oil leak

today i spent my day off hooking up an aftermarket oil pressure guage as the factory unit showed no oil pressure. guess what ...the aftermarket guage shows zero also and only starts creeping up at about 2000rpm's. the motor sounds good....absolutely no knocking....another mystery....

i am just about at wits end....i just wish the damn thing could fight

:evil: :evil:
Author:  Joshie225 [ Fri Nov 26, 2010 8:18 pm ]
Post subject: 
If you have no oil pressure until about 2000 RPM it's highly likely the pressure relief valve in the oil pump is stuck open.
Author:  mopardean [ Fri Nov 26, 2010 9:36 pm ]
Post subject: 
has this damaged the engine ?
Author:  Joshie225 [ Fri Nov 26, 2010 9:54 pm ]
Post subject: 
Maybe. You'd have to go invasive to know.
Author:  ceej [ Fri Nov 26, 2010 10:01 pm ]
Post subject: 
Have you been running it long like this? As long as you haven't been running it except to test, you may be OK. This is clearly not a good thing.

Pull the cap on the bottom of the oil pump. It's a square bolt looking thing that points down toward the front of the engine at the bottom of the oil pump. There is a spring in there, and on top of that is the relief plug.
Is this a used oil pump? Sometimes the relief plug can be hard to remove if it's got a bunch of build up in there. Get it out and clean it up.

Put everything back together and test again. You should see oil pressure at idle over ten pounds.

If you don't have oil pressure, stop running it and replace the oil pump. Not much fun in the car, but it can be done. You will have to lift the engine off the mounts in an A-Body car to do it.

CJ
Author:  kipamore [ Tue Nov 30, 2010 6:14 pm ]
Post subject: 
Been thinking about this one a bit. I might have a clue on your engine miss. I had a lot of problems with mine due to slightly different caps and rotors. I'm sure this is covered in detail somewhere on the board, but some rotors are longer and some are shorter, and the caps correspond to this. This problem plauged me for a while with my engine, so it's worth looking at. In theory, a too short rotor would still fire the engine, but it might miss and it might miss randomly.

After that, I'd be looking for an arcing plug wire or a wire that isn't snug, or some such. After that, I'd be looking for a vacuume leak, which I'd try to chase with a can of starting fluid.

As to oil pressure, it's possible that your gauge is reading low because there's air in the line to the gauge. If you have the clear nylon oil line, you should at least be able to see some kind of movement of the oil right at startup. Regardless, I'd pull the drain plug and try to get a sample of the oil living at the bottom of the pan.
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