Pontiac 400

General Engine Discussion

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Bruce69Camaro
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Pontiac 400

#1 Post by Bruce69Camaro » Wed Jun 01, 2011 11:15 am

My brother-in-law bought a 78 Firebird with a 1966 400 motor in it, not that it matters.

As far as what has been done to it, who know's except for what we can visually see. It does have a different cam with a noticable sound to it, and it does have an Edelbrock intake and I think it's a Holley 600cfm carb.

I noticed that when he had it running, it stunk with richness, so the kid that had it prior to him had this car all scewed up so I went to adjust the idle mixture screws and didn't have too much luck.

I always thought rule of thumb was, turn them in all the way and back out 1 to 1-1/2 turns and you should be in the ballpark :scratch:

Well when I turn the screws in the engine stumbles and I back it out, maybe 1/2 to 3/4 turn and the engine appears to be at it's highest point. If I go out anymore then that the engines runs rough.

Does this sound right?

Also when you go to drop this thing into gear, in park, the car idles around 1000rpm and when you pull it into gear, the car snaps and drops down to around 600/ 700rpm.

Could this be a carb issue as well? I've heard that if the car is too rich or lean (?) it will cause this.....

Any help would be appreciated and Chris, feel free to chime in buddy because this is a Pontiac we're talking about.

Thanks
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ytnova
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Re: Pontiac 400

#2 Post by ytnova » Thu Jun 02, 2011 6:07 pm

I would start with checking the timing before adjusting the carb anymore. Check total timing first, make sure the centrifical advance works, make sure the vaccum advance works, the diaphram is not ruptured and it is connected below the throttle blades, constant vaccum at idle. Shoot for @32-36 degrees total without any vaccum advance. Also check for any other vaccum leaks and make sure the power valve(s) are not blown. Then I would see if the throttle blades are opened up beyond the ilde mixture slots, they should not be. Does it have a pcv valve? If the throttle blades are opened up past the mixture slots to idle, you may need to install a pcv valve if there is not one, open up the rear throttle blades a little(holley), they are fully closed stock, or even drill a small hole in both primary throttle blades to allow extra air in without opening up the throttle blades. Then you are correct, initial setting for the mixture screws should be around 1 1/2 from fully seated and adjust from there. Hope this helps and let us know-ED
I am not really sure what the question is, but I am pretty sure the answer is Big Block.

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Bruce69Camaro
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Re: Pontiac 400

#3 Post by Bruce69Camaro » Fri Jun 03, 2011 5:36 am

Thanks.

I agree, I'm going to check for some vacuum leaks and also double check the pv's. With the cam this kid stuck in this thing, I highly doubt the stock pv's are the right size.

I'm also going to make sure he has all the vacuum lines going to the carb are where they are suppose to be going.

I do know the bladder in the booster is shot, would that cause a problem? My thoughts on that are, the only time that would leak vacuum is when you hit the brake pedal, yes or no?

When you say train wreck, think of this car....
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ytnova
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Re: Pontiac 400

#4 Post by ytnova » Fri Jun 03, 2011 11:06 am

vaccum brake booster? yes, if the diaphram is ruptured, it is almost always a constant vaccum leak, the back side is usually vented to atmosphere, also the reason cars with low manifold vaccum try to stall when you step on the brake pedal, they are robbing vaccum from the intake. You can just plug it off to troubleshoot.
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Bruce69Camaro
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Re: Pontiac 400

#5 Post by Bruce69Camaro » Fri Jun 03, 2011 11:12 am

He's getting that fixed, after spending 12+ hours working on that thing, I said take it to the shop and let them fix the brakes, booster and m/c.

Like I said, train wreck...........

Hopefully when he gets it back I can check things out better.

Thanks Ed.
Those who think they know it all have no way of finding out they don't......... :scratch:

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