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Old 07-27-2013, 07:16 PM
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454 misfire figured out...sorta

Hey ive been chasing a misfire on my 59 chevy for about 6 months now and finally found the source. Its got 1980ish 454 motor. It has a heavy misfire and lack of power on acceleration, ran fine at idle. I rebuilt the carb, changed the plugs, wires, intake manifold gasket, wired up an MSD dist (without box) and MSD coil, set timing to approx 10btdc, no change at all. Disconnected the pcv valve, plugged from carb nipple where pcv was connected and removed pcv from driver valve cover, misfire gone.

I had a breather in the pass VC, PCV with baffled grommet in the driver VC routed to the front of the carb and vacuum advance on manifold vacuum (ran like crap when advance was on ported vacuum).

Why would removing the PCV system help? I did also notice an oily substance on the top of the carb around where the accelerator pump is if its related. What should I do to fix the issue and should I even run a PCV system? Thanks guys

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Old 07-27-2013, 08:14 PM
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Originally Posted by chevyrestore View Post
Hey ive been chasing a misfire on my 59 chevy for about 6 months now and finally found the source. Its got 1980ish 454 motor. It has a heavy misfire and lack of power on acceleration, ran fine at idle. I rebuilt the carb, changed the plugs, wires, intake manifold gasket, wired up an MSD dist (without box) and MSD coil, set timing to approx 10btdc, no change at all. Disconnected the pcv valve, plugged from carb nipple where pcv was connected and removed pcv from driver valve cover, misfire gone.

I had a breather in the pass VC, PCV with baffled grommet in the driver VC routed to the front of the carb and vacuum advance on manifold vacuum (ran like crap when advance was on ported vacuum).

Why would removing the PCV system help? I did also notice an oily substance on the top of the carb around where the accelerator pump is if its related. What should I do to fix the issue and should I even run a PCV system? Thanks guys
Unless you've recalibrated the Q-jet, it should use a PCV. Not to mention the other benefits of using a PCV. Are you sure the old PCV valve is any good? I'd try it again w/a new PCV valve to see what the results were- they're cheap.

Depending on the cam, you may need more initial timing. That's shown by it running poorly on ported vacuum- there's no initial advance increase when using ported. Manifold vacuum advances the vacuum advance at idle, making it seemingly run better.

More on this here.
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Old 07-27-2013, 08:19 PM
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these problems seemed to happen when I converted it to PCV, before the PCV I just had a breather on both sides, did I have to routed properly? do i need to install a catch can?
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Old 07-27-2013, 08:22 PM
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these problems seemed to happen when I converted it to PCV, before the PCV I just had a breather on both sides, did I have to routed properly? do i need to install a catch can?
I posted this on your other thread, but the PCV goes to the front large vacuum port. No catch can should be needed- this engine isn't old w/a lot of leakdown, is it? If the engine isn't sealing well at the rings, you'll have probs.
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Old 07-27-2013, 08:23 PM
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the motor isnt old but the valve covers didnt have baffles so I used a baffled grommet. Ive got the msd dist setup the way it was from the factory, what initial timing should I set it at?
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Old 07-27-2013, 08:25 PM
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the motor isnt old but the valve covers didnt have baffles so I used a baffled grommet
Better than nothing, but you'll need to see if oil is getting past the baffled grommet. If it is and you're not going to install a baffle you might need that catch can.
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Old 07-27-2013, 08:26 PM
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You can still run the PCV w/a can.
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Old 07-27-2013, 08:31 PM
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I guess ill buy a can then or baffled covers, what should I change the initial timing to? Would I have a big power gain if I brought the truck to a hotrod shop to have them put the different springs in for the timing curve that were provided with the dist?
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Old 07-27-2013, 08:38 PM
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I guess ill buy a can then or baffled covers, what should I change the initial timing to? Would I have a big power gain if I brought the truck to a hotrod shop to have them put the different springs in for the timing curve that were provided with the dist?
If you have a timing light, you can do this yourself. It's a good skill to have and dialing the timing curve in correctly can help drivability as well as give a real world performance increase if the timing is off much.
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Old 07-27-2013, 08:41 PM
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Ill play with the springs and see what happens, what would you recommend if I replace the PCV and the problem happens again?

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Old 07-27-2013, 08:55 PM
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ill try a new valve from kragen and let you know. Thanks for the input
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Old 07-27-2013, 08:59 PM
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Fram 184 will be fine.
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Old 07-31-2013, 09:19 PM
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I threw on that new PCV valve, back to misfiring, ran both sides with a breather and plugged the front carb port where the pcv went and it still misfired but it wasnt nearly as bad, I think if I were to keep driving it with 2 breathers the misfire would have gone away once it burned through whatever was in the carb and on the plugs. Last time when I removed the PCV for testing it misfired for a minute or two and went away and drove normal. What suggestions do you guys have because im at a loss. Everything else has already been replaced

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Old 08-01-2013, 01:24 PM
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Hey I think I got it! I switched to ported vacuum on the distributor, idled like crap, advanced it until I liked the idle quality, put on one light silver and one light blue spring under the MSD rotor and test drove...totally different truck and no more misfire.

Im going to bring home my timing light from work sometime soon and check the timing with a light though, im assuming I need to readjust the carb now as well?
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