having odd vaccum readings

[shady]

I'm running a 79 350 in my 85 k5. Carburated with what I beleive is a mild cam. (bought it this way) Had erratic idle so I set the idle mixture screws even and at 1.5 turns out to get a base line. Idle came around but when I hook up the vaccum gage it bounces violently from 5 -15. Cant get a smooth reading to fine tune the mixture screws. Truck runs ok but I also now have to floor it to start it. I took my vaccum pump and went through all the hoses hooked to carb and intake. When pumped down they all hold a constant reading. I dont know much about carbs or vaccum yet , just learning as I go. I figured the guys here would have some ideas

 

[Cornfield creations]

What kind of carb is it? If it's a quadrajet. Baseline for idle screws is 2 1/4 turns on the corse threaded screws, 4 1/2 on the fine threaded. The cam will throw off the vacuum a bit, but you shouldn't have to floor it to start it. Usually a pump or two.

 

[4X4HIGH]

Irratic vacuum reading means bad valves. Now you need to do a leakdown test to further sustantiate the findings.

 

[shady]

older holley 600, 4160. it faltered and stumbled constantly at idle before. but 2-3 pump starts were normal. did full tune up with no change. then looked for ideas on google. came up with the 1.5 turns but didnt say what carb. so i just tried it. counted turns in and had 1 at 1.25 and 1 at 1.75 so should be close. didnt beleive it till i tried it but changing the pcv valve did the most good on the idle????? i didnt think that could change any thing but i'm told the spring in it regulates the vaccum lost to the crank case. i dont know. runs good now, its just the starting thing that bothers me. and the vaccum thing that boggles me. i'm going to try 1.25 turns on each.

 

[shady]

i wanted to do a leak down any way so i'll try that too. just bought the truck so i'd like to learn as much about the motor as possible.

 

[payback]

some good vacuum info here

http://www.secondchancegarage.com/public/186.cfm

 

[shady]

Thanks for that link to second chance garage. I learned loads in the ten minutes it took to read it. It's now in my favorites list. The visual scenarios are very helpful. Unfortunately for me none are even close to what mine does. My needle bounces from 5-15 like 50 times a second all the time idle or accelerating. Hopefully not a completely trashed valvetrain. Thats what projects are for though. I have a nice complete set of 1970 2.02 double humps laying around they may finally have a home.

 

[British Slave]

Thanks for that link to second chance garage. I learned loads in the ten minutes it took to read it. It's now in my favorites list. The visual scenarios are very helpful. Unfortunately for me none are even close to what mine does. My needle bounces from 5-15 like 50 times a second all the time idle or accelerating. Hopefully not a completely trashed valvetrain. Thats what projects are for though. I have a nice complete set of 1970 2.02 double humps laying around they may finally have a home.

Do a compression check first. I'll bet you find signs of a blown head gasket.

 

[shady]

vaccum problem still there but i did figure out the starting issue. carb is still spraying gas down the hole well after the engine is shut off and pedal back to normal. lots of screw driver marks on the float level adjusters. looks like they got rough with it too. gonna learn a lot more about carbs real soon it looks like.

 

[shady]

gotta borrow a compression tester from a buddy of mine. may be a day or two for that. but its definately in the works.

 

[diesel4me]

Loose valve stems/guides can cause rapid vibration of the needle on the vacuum gauge..but I'd ensure there is no vacuum leaks at the base gasket,intake manifold,and power brake booster,etc,before condemming the engine...if it were me I'd ditch the Holley carb,but I'm no fan of most Holley carbs!..out of 20 or so I've had,only about 5 of them ever ran right or worked good,despite rebuilding them and adjusting them to no avail...many times the mismatch of a Holley (or Edelbrock Performer & Carter AFB) carb on a stock Q-jet intake (or a spread bore aftermarket one) is responsible for vacuum leakage and a myriad of poor running troubles...you need the correct adapter and gaskets to make one work right on a Chevy..

 

[shady]

So far it does seem to point to the valves, seats, guides? in one way or another. May still see about rebuilding my double humps but I'm afraid of the compression ratio. Gonna rebuild the carb in a day or 2 since the seals went south on the float adjuster (gas all over now). Shouldnt have touched emThe intake is a sqaure bore edelbrock, so that part matches at least. Will replace the gasket when I rebuild it of course. I want a new carb but I've spent $6000 on my kids 72 k5 in the last 4 months so it'll have to wait.