All you guys with big cams

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All you guys with big cams

Post  Foxfire85 on May 18th 2014, 9:42 pm

All you guys with big cams (mainly duration), how bad does your vacuum gauge dance around while setting your idle and 4 corner mixture screws? My cam is 530/530 @280 and it's dancing like crazy. Varying at times up to 4" of fluctuation. This is around 650, but of course it cleans up a little around 850 or 900 but still fluctuates pretty bad. Is this normal to you all, or is this indicative of a vacuum leak or possibly valve lash out of adjustment?

Aaaalso, after ripping off a pretty smokey patch or blacktop yesterday, I coasted back in the driveway to a motor that now wants to idle about 400rpm higher than it had been set for the last 6 months...... I have t checked anything on it yet because I was just disgusted with it, shut it off and called it a day. My thoughts on things to check for include timing, if timing is out pulling the dizzy and checking the roll pin and all that jive, vacuum leaks (as it was rich, I don't know about a 400 rpm gain worth of rich but it's an idea), and throttle linkage and springs. This is the the easy stuff that pops to mind. Anybody have any other ideas?

Thanks fellas
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Re: All you guys with big cams

Post  maverick on May 18th 2014, 11:31 pm

I think it's pretty normal for a big cam to cause a fluctuating vacuum reading at low rpm.  Also, the type of intake manifold and the location of the vacuum port can either help or aggravate the condition.  I wouldn't worry too much unless I saw some other symptoms to indicate a problem, i.e. compression checks indicating a valvetrain or ring seal problem, etc.  The fact that the needle settles down with a little rpm increase sounds right, too.

As for the rpm increase after the burnout...It's also pretty normal to see such an increase as you build heat in the engine, especially those with aluminum heads. My stuff does that, so I expect a crappy idle when the engine's cold.   Again, if no other symptoms indicate trouble, then I think you should set idle mixtures and idle speed with the engine at N.O.T.  Of course, that means you'll have to put up with an engine that's a little balky until warmed up, but such is life with a carbureted hotrod.  And besides, who flogs a cold engine?? Warm it up.
Just my $.02.

Double check the timing thing you mentioned and be sure the distributor hold-down clamp is snug.

Has this car been driven much?
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Re: All you guys with big cams

Post  Foxfire85 on May 19th 2014, 9:18 am

Thanks for the reply man. The vacuum bouncing I thought would've been fairly normal, I just wanted to check. Although this cam isn't huge, it's the biggest I've dealt with so I was unsure on the area.

As for the rpm increase, the motor was already at 190 and had been driven for a good portion of the day before the increase had occurred. My stuff gets warmed up before it ever gets moved because it doesn't get driven too much. The cold idle is rather rough but was set to 850 at normal operating temperature which is smooth(er). After I ripped off the burnout it was idling around 1300. I'll tinker with it today and report my findings.
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Re: All you guys with big cams

Post  maverick on May 19th 2014, 9:32 am

Make sure your throttle return spring(s) is returning the throttle all the way to the stop.
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Re: All you guys with big cams

Post  BigBlockRanger on May 19th 2014, 4:03 pm

Make sure the mechanical advance is not hanging up in the dizzy.
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Re: All you guys with big cams

Post  maverick on May 19th 2014, 5:27 pm

BigBlockRanger wrote:Make sure the mechanical advance is not hanging up in the dizzy.  

Good thought.
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Re: All you guys with big cams

Post  Foxfire85 on May 22nd 2014, 10:58 pm

It's been a crazy week at work but I finally got around to checking some things. First off, the car is no longer idling high. How that happened is beyond me. I checked timing, still at 13 initial, 34 total right where I set it. Throttle return spring, and throttle cable are all in proper working order. I sprayed around with starting fluid for vacuum leaks to no avail. Only thing I havent checked is the mechanical advance on the dizzy. One should hope that's not the issue as the thing is only about 50 miles old but I will give it the once over when i get home. After that I suppose I'll just have to wait for it to happen again to diagnose as it's occurring. I appreciate the ideas fellas and by all means if you have anything else lay it on me!
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