HELP engine vacuum

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HELP engine vacuum

Post by a-body on Fri Apr 15, 2016 11:41 pm

I saw another topic but it wasn't quite the problem I was having. So for a while I have had these nice heads on my 77, 041 x heads (69/70 vette heads l48 I think) so recently I decided to get a 70 vette cam. Melling part number 22210. Got the car running with it and when we first adjusted valves one of the studs backed it's way out cause it was a screw in stud. It cause a backfire, it was an exhaust rocker, the rocker wasn't engaging the pushrod. Somehow the rocker right next to it got pushed sideways and bent the pushrod. So two pushrods later (replaced the other just in case) the car is back running. I can't get vacuum no matter what I do though. The only way I get near 20 inches at idle is if I loosen the rocker arms till they clatter. It's not a clatter you can deal with either. Timing doesn't help. Carburetor is right. I have no idea what to do, any tips?
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Re: HELP engine vacuum

Post by bitterfitter on Sat Apr 16, 2016 5:46 pm

Did you change pushrods when you changed the valve train?

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Re: HELP engine vacuum

Post by a-body on Sat Apr 16, 2016 6:01 pm

No, just lifters
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Re: HELP engine vacuum

Post by bitterfitter on Sat Apr 16, 2016 7:48 pm

I asked because the bent rod sounded like they might have been too long and had no where to go when the cam lobe came around. And if they were long then the valves won't close completely until the rockers were loosened.
I'm just trying to brainstorm here. Sometimes that triggers that "hey wait a minute " moment.

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Re: HELP engine vacuum

Post by a-body on Sat Apr 16, 2016 9:02 pm

That makes sense. Are there differences in lengths of pushrods? I'm really lost as to where to go next, there are no vaccum leaks.
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Re: HELP engine vacuum

Post by clanceman427 on Sat Apr 16, 2016 11:09 pm

Sorry you're having troubles. Sounds like you have good heads and a good cam. Outside of the bent pushrod issue, I looked up that Melling cam and it is pretty stout. I looked up the specs and it's got more than .450 valve lift and a good amount of duration, which tells me you're never going to get 20inches of mercury at idle. I had a similar but slightly bigger (.480 valve lift Intake and exhaust, 280 intake and exhaust duration, not too far from yours) in a 400 SBC and it was a lumpy idle, and vacuum was low enough at idle to affect the power brakes, so maybe 12 in. mercury?

I think your idle vacuum expectations with this cam are unrealistic but I'm curious what others think
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Re: HELP engine vacuum

Post by pila on Sat Apr 16, 2016 11:55 pm

X2 on what Kevin said about the cam you have, and the amount of vacuum you have. I have a Magnum 280 in my Malibu, 408 small block, and I went with Rhoads lifters, to get decent vacuum for brakes etc..This cam has a nice lope at idle with those lifters.

You could also use a vacuum tank for more capacity for brakes etc....

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Re: HELP engine vacuum

Post by a-body on Sun Apr 17, 2016 12:28 am

I honestly didn't expect that. I mean it'd crossed my mind but did a 1979 corvette really not get that much vacuum at idle? That's what the cam is a repro of. Maybe gas quality is the difference? Thank you guys for the help
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Re: HELP engine vacuum

Post by clanceman427 on Sun Apr 17, 2016 10:29 am

Well I think it's time to verify all parts are correct. Correct length pushrods as previously mentioned. Correct ratio rocker arms (1.5:1). I'm assuming cam and lifters are correct. Was it installed correct? Did you degree it in or just line up the dots on the timing chain gears? Then look at the spring assemblies, are you getting coil bind at max lift maybe? Rotate engine until an intake is at max opening and see if you can get a feeler gage between the spring coils. Could also be binding at the valve seal/retainer/ keeper area too so shine a light in that area and see if the valveguide/seal/retainer look like they've been smashing together
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Re: HELP engine vacuum

Post by clanceman427 on Sun Apr 17, 2016 10:44 am

And I know that everything SHOULD work together I'd you have 69/70 heads and a 79 L82 cam with factory specs but did you yourself rebuild the heads knowing that the parts used were correct? If not they might have been rebuilt by someone else who didn't use correct parts. Just trying to think of possibilities. Tell us the casting numbers and date codes of your heads. All that info should be visible once the valve covers are off
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Re: HELP engine vacuum

Post by a-body on Sun Apr 17, 2016 10:58 am

Meant to day 70, not 79. My bad. I really think everything was done correctly, however I did not degree the camshaft.
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Re: HELP engine vacuum

Post by a-body on Sun Apr 17, 2016 2:50 pm

Okay so I work at O'Reilly and I've been checking part numbers on the cam components and stuff. Lifters, pushrods, rocker arms, all the same part number. Not sure what I'm doin wrong. So did they really not get 21 inches of vacuum in 1970 with the vette? Do I need to degree the camshaft, I thought it wasn't necessary with a factory cam... thank you guys for helping by the way.
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Re: HELP engine vacuum

Post by 73ss on Sun Apr 17, 2016 8:53 pm

MAYBE 18 inches withe the 300 horse cam. I'm assuming you have the 350 horse cam? If it is you may get 13~14.
I have a 68 vette with the 350 horse 327. Engine was freshened up in the 90's. Still has original pistons, heads, crank, distributor. cam was replaced with a crane factory blueprint duplicate. I don't have the specs handy but it looks similar to yours. It Barely makes 14 inches at idle. Has a slight lope. All vacuum accessories & pwr brakes work great. vettes also have a large vacuum storage tank. In 68 you couldn't get this engine with auto trans or A/C because of the cam. These cams were designed for 10~11:1 CR also. Won't work well with much less than that.
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Re: HELP engine vacuum

Post by a-body on Sun Apr 17, 2016 8:58 pm

I have 041x heads, I'm hoping that's good enough for the cam, correct me if I'm wrong though. Thank you man that is exactly what I wanted to hear, that clears everything up
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Re: HELP engine vacuum

Post by 73ss on Sun Apr 17, 2016 9:10 pm

Glad to help. I used to be up on the head numbers but not so much anymore. You can google the casting number and find it. If they are the "good" heads they should be 64cc. 76cc are alot more common. My 68 has the 64cc heads with domed pistons to get the 11:1 squeeze.
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Re: HELP engine vacuum

Post by bracketchev1221 on Mon Apr 18, 2016 8:18 am

I would be looking for 15-18". 20 is optimistic by a lot.
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Re: HELP engine vacuum

Post by a-body on Mon Apr 18, 2016 11:06 pm

Got the timing down where it needs to be, no sparkknock. Revs great. Gonna put my tachomter to see exactly how high its revving without pushing it too hard. 15 inches in drive, drives good, need to get the carb straightened out and it will be great. Thank all you guys for the help.
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Re: HELP engine vacuum

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