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jays55

MC454SS Vacuum Leak, or.....?

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Semi-skilled mechanic looking for some help!  Stock 1970 SS454, except for Holley 650cfm carb (#80555-1).  I came back from a drive and, pulling into the driveway, noticed the power brakes were hard and the engine was idling rough.  Didn't notice anything while driving at speed.  Vacuum Leak, right?  Went under the hood and checked all vacuum lines to the carb (dist. advance, choke, PCV) and intake manifold (brakes, trans, canister, all attached behind carb).  Pulled each hose off and plugged it (1 at a time) and saw no change.  Noted that there was vacuum to the brake booster.  Also, the PCV valve is pulsating, rather than being pulled open.  Low vacuum at idle, right?  Sprayed some carb cleaner around the carb and intake manifold, noticed no change.  I am hearing a hissing sound from the carb (with the air cleaner off), but don't know if that's normal.

A couple of other observations;  I pulled the plugs and they all look even.  However, I noticed the passenger side exhaust at the tailpipe is cool, while the driver side in hot.  There is a heat riser on the pass side, which is functioning.

That's about as far as I know to go.  I don't have a vacuum gauge, but seems obvious there is low vacuum.  Any ideas would be appreciated.  Your help is appreciated, in advance......

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Can't answer your specific question, sorry but will add Harbor Freight is your friend. Vacuum issues are a PITA so I would definitely get a gauge so you can get readings. Also get a digital temp gauge, you can read the temps off the exhaust manifold on each port. I say HF as I assume your not a mechanic by trade and their prices won't break the bank. I never remember a Holly hissing (which sounds like an air leak) but as with your original question hopefully someone with more experience can chime in 

 

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if you bend a valve it would make a sucking noise at the carb, the Pcv will chatter. you vacuum gauge will be erratic. you should notice a miss if that's a case.

vacuum leak would make a sucking sound and could raise the idle speed, but you said you checked all the hoses. check the booster they can leak internally. 

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Thanks for the great feedback!  I will pick up a vacuum gauge to have on hand.  Since I disconnected and plugged all of the vacuum lines and there was no change, I felt an external component failure could be ruled out.  The "bent valve" suggestion is interesting.  I have always felt a slight miss at idle, but it never felt significant.  I just did a drive around the block and I do feel a more pronounced miss/flat cylinder.  The issue seems confined to the carb, or engine.  But, I don't want to overlook something simple, before going into the engine....... Thanks for any suggestions!

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Find the dead cylinder first, either by an IR gun as suggested above or pulling a plug wire at a time. Then I'd probably pull the valve covers and look for loose or broken rocker arms or broken valve springs. If you find a loose one, it may suggest a bad cam lobe too. You'd maybe hear some type of ticking noise though if it was sloppy enough. A compression test may point you in a direction as well. Just a couple of thoughts.

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I had a mechanic show me a trick ,but have a fire extinguisher handy, take a propane bottle and open it up, with the torch UNLIT ,and while the car is running at idle slowly wave the propane gas ,around the base of the carb, and see if the idle changes, i also had vacuum issues for a very long time and could not figure it out. the propane trick indicated i had a bad carb ,base gasket ,changed that now car runs great , i must have overtightened my carb bolts,

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Any ideas about whether the differing exhaust temps (at the tailpipe) on the passenger side (cool) vs the drivers side (hot) means anything?

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I would look for a dead cylinder on that side. The colder side. 

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do you have stock exhaust or headers. if you start the car when it's cool and touch by the manifold or headers, they should warm up pretty even. I have seen where and exhaust gets plugged and on piper is cooler or super hot. 

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Thanks for the insights.  A little more sleuthing today, based on feedback here.  I put in a new set of spark plugs and checked the wires.  Started the engine.  As the stock exhaust manifolds heated up, the #8 cylinder remained touchable, as the others got too hot to touch.  Pulled the #8 plug wire and confirmed fire to the cylinder.  Looks like I probably have a dead #8 cylinder.  UGH!

Leaving on vacation shortly and returning with only a couple of weeks before the FGMCC Western Meet.  I will pull the valve cover and hopefully find something obvious.  If I have to pull the head, not likely I'll have it back together in time for the trip ......

Fingers crossed!  Any other insights will be appreciated ...... thanks!

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5 hours ago, imr said:

how did the number 8 plug look? hopefully its something simple. 

All of the plugs on the pass. side looked the same, that has me stumped!  The driver side plugs were also consistent, but slightly cleaner......

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well I guess either check compression or pull the valve cover.  Let us know 

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