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Engine idles smooth but revs rough
https://slantsix.org/forum/viewtopic.php?t=43995		 Page 1 of 1
Author:  Nongan [ Fri Mar 11, 2011 10:45 pm ]
Post subject:  Engine idles smooth but revs rough
Ok, so I got my dart down to my high school autoshop, barely. I put a new Holley 1920 carb on it last week and now it idles great!! :D But, on the other hand when I step on the gas I can watch the tach twitch to-and-fro from 550 to 650 back to 550 up to 750 back to 650 so on and so forth, it did this with the old carb also. I have a constant 16 lbs of vacuum from the intake manifold. It is leaking exhaust at the manifold (I have new 2.25" pipe but I've been to busy with my ford truck to bend it and weld it). I have the cheap Bosch plugs and I just put fresh gas in (the tank was dry, I siphon it if it sits longer than a month and also I changed the carb after I put the new gas in). So my question is this, what is causing it to rev so hard. The sooner I figure this out, the sooner this becomes my new daily driver :D

MANY thanks in advance to everyone here,
Seth
Author:  ceej [ Fri Mar 11, 2011 10:58 pm ]
Post subject: 
16 inches of Mercury (16" HG) is pretty low vacuum if that is taken at idle in park. Check for vacuum leaks around the intake manifold and carburetor base. A good number on an otherwise stock engine is 19-21" HG.
What is your timing set at now? 10° BTDC is a good number most of the time.
Set the mixture screw at warm idle for Peak vacuum reading.
Double check valve lash.

CJ
Author:  Nongan [ Fri Mar 11, 2011 11:25 pm ]
Post subject: 
ok, I have checked for vac leaks both with carb clean and propane, both of which I found no leak. I've been too busy with my diesel motor swap to even time the motor yet (some idiot dropped our timing light so I have to use the giant tune up machine, which is behind my truck). But with the idle mixture screw I can turn it all the way to bottomed out and it doesn't affect the car at all.
Author:  ceej [ Sat Mar 12, 2011 12:05 am ]
Post subject: 
OK, either your timing is retarded, or you need to rebuild the carb. Even new, that's an old carburetor. :lol:

Advance the timing, then decrease the idle screw setting to drop the idle.
Without a light it takes a fine touch and ear to time a slant dynamically. It can be done, but the location of the distributor makes it a bit of a pain.
Until you have done it a few times, your best off with a timing light.

CJ
Author:  Nongan [ Sat Mar 12, 2011 12:12 am ]
Post subject: 
Well the tune up machine has a timing light, but its a lot to set up. And I do think that the timing is retarded. But tomorrow I am getting new plugs, the NGK ZFR5N's that you recommended, but that's going to be about it because I have my family's truck to finish (just have the water pump, down pipe, radiator, trans cooler, intercooler, body left before it leaves) but once it leaves I'm going to devote all my time to MY car, so probably right after spring break, sadly, but I'm still looking for input on this problem so thanks.
Author:  Fopar [ Sat Mar 12, 2011 8:08 am ]
Post subject: 
Sticky or slow to react advance mechanism, check to see if the weights move freely.

Richard
Author:  Nongan [ Wed Apr 20, 2011 7:48 pm ]
Post subject: 
Got it pretty much resolved at this point. Got half of my new 2 1/4" pipe on today and it helped immensely. In this video I forgot to block off my intake manifold vacuum port :oops: http://www.youtube.com/watch?v=mRV0elbiD-k
Author:  Doc [ Thu Apr 21, 2011 8:17 am ]
Post subject: 
Quote:
16 inches of Mercury (16" HG) is pretty low vacuum ...
Double check valve lash.
CJ
Vote # 2 for a lash check... then trouble-shoot from there.
DD
Author:  Nongan [ Thu Apr 21, 2011 3:47 pm ]
Post subject: 
I did recheck lash, it's now at 19 hg after new exhaust and tune up
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