Vaccum gauge bouncing ...

Posted By: JohnRR

Vaccum gauge bouncing ... - 09/29/14 05:27 AM

Friend that I am helping out 383 has all of a sudden started to run poorly. After checking to make sure the cam isn't going flat he got it back together and I start see whats going on , he adjusted the timing without a light

Anyway when it's idling at about 900 rpm the vac gauge at the base of the carb has the needle fluctuating wildly, 10" or more, bring the RPM up to about 2k and the needle smooths out ?

The engine is fresh and it was running well till the other day it just started running poorly, carb is a fresh rebuilt Holley 650 DP that's about 30 years old.

I'm a little stumped and don't want to tear the thing apart further than necessary, a search online says sticky valves or a vac leak??? I sprayed starting fluid around the carb and intake thinking big vac leak but that turned up nothing ? There are paper gaskets on either side of the valley tin.

Posted By: 383man

Re: Vaccum gauge bouncing ... - 09/29/14 06:18 AM

I know I have always found anytime the vacum gauge needle bounces alot like 10 or more inches it is usually a mechanical problem. If it moves a good bit but drifts and dont bounce then it may be fuel. I know I had one once that drove me crazy as it was a 318 with a BBD carb and it would run bad and the vacum gauge bounce back and forth alot until it hit about 165 and then it smoothed out and the gauge got steady. Turned out to be the top of the hydaulic lifter popped above the retaining spring kinda making it act like a solid lifter and holding the valve off its seat. After it warmed up enough from the heat exspansion the lifter then let the valve close enough to run fine. I know its not just like your problem but its kinda in the idea ballpark. But you say yours smooths out with rpm no matter what temp ? Ron

Posted By: RapidRobert

Re: Vaccum gauge bouncing ... - 09/29/14 06:30 AM

Quote:

383 has all of a sudden started to run poorly.

Anyway when it's idling at about 900 rpm the vac gauge at the base of the carb has the needle fluctuating wildly, 10" or more, bring the RPM up to about 2k and the needle smooths out ?

The engine is fresh and it was running well till the other day it just started running poorly,

Hello double R. Blown head gasket. Do a compression test

Posted By: Mcode69

Re: Vaccum gauge bouncing ... - 09/29/14 08:13 AM

Pull the rocker covers and check to see is a pushrod has punched through a rocker or two.

Posted By: moparmarks

Re: Vaccum gauge bouncing ... - 09/29/14 01:26 PM

Or maybe a broken valve spring.

Posted By: TJP

Re: Vaccum gauge bouncing ... - 09/29/14 04:07 PM

Is it missing at low RPM IE: dead cylinder????
IF so you may be able to locate it by pulling the plugs wires one at a time the culprit will show no change in RPM, I would then pull the that valve cover and investigate further

Posted By: GTX MATT

Re: Vaccum gauge bouncing ... - 09/29/14 04:14 PM

If it smooths out when you rev it up common wisdom is that its really loose valve guides. Never seen an engine do it though, anything I've seen is a burned valve or another issue and it never smooths out. I'm not sure how loose they would need to be to cause this.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/29/14 04:32 PM

Quote:

If it smooths out when you rev it up common wisdom is that its really loose valve guides. Never seen an engine do it though, anything I've seen is a burned valve or another issue and it never smooths out. I'm not sure how loose they would need to be to cause this.

Heads have new valves and bronze liners, Comp springs, so I can't see it being the heads, especially consider who I had do the heads. It does have adjustable pushrods I can to adjust them again to rule out a valve hanging open. The weird thing is it was running so good, cam break in was perfect without any hiccups ...

I had the valve covers and the intake off and everything looked good, rotated the engine over to make sure everything was working fine.

Decks on both the heads and the block are fresh, Felpro 8519 gaskets. I hope it's not a head gasket because the block or head surface is probably torched by now ... uuugggghh

Posted By: GTX MATT

Re: Vaccum gauge bouncing ... - 09/29/14 04:53 PM

Head gasket should just read erratic no matter what in my opinion.

If you trust the machine work I'd say look into ignition and fuel, another carb if you can try it. Smell rich/funny??

Posted By: RapidRobert

Re: Vaccum gauge bouncing ... - 09/29/14 05:14 PM

Assuming that is the problem (HG), At higher RPM's the head gasket is still blown obviously but the firing pulses being closer together masks/distorts the reading showing on the gauge

Posted By: HemiRick

Re: Vaccum gauge bouncing ... - 09/29/14 05:49 PM

Do a compression test, it will find the problem.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/29/14 06:16 PM

Quote:

Head gasket should just read erratic no matter what in my opinion.

If you trust the machine work I'd say look into ignition and fuel, another carb if you can try it. Smell rich/funny??

Changed out the distributor from a pertronics to points , didn't change anything, I moved the timing light onto all the plug wires and everyone is firing. I have a set of firecore wires I'm going to swap on to rule that out, bypassed the ballast and didn't change anything.

All he has is old, ugly carbs, it might be the carb on is so I guess I'm either going to have to buy one to try or use my new Thunder series 800. He's retired/fixed income and can't afford to buy new parts.

Compression test is first order of business.

Posted By: stumpy

Re: Vaccum gauge bouncing ... - 09/29/14 06:50 PM

Did you try pulling the plug wires one at a time as was suggested earlier? Throwing parts at it isn't the cure. You need to do the basic diagnosis first.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/29/14 08:11 PM

Quote:

Did you try pulling the plug wires one at a time as was suggested earlier? Throwing parts at it isn't the cure. You need to do the basic diagnosis first.

Thanks for your help Stumpy , I know I can always count on you

Posted By: RJS

Re: Vaccum gauge bouncing ... - 09/29/14 09:21 PM

John no harm intended but this is kinda refreshing seeing you with a problem and not a solution!!!!

Ron

Posted By: feets

Re: Vaccum gauge bouncing ... - 09/29/14 09:30 PM

Quote:

John no harm intended but this is kinda refreshing seeing you with a problem and not a solution!!!!

Ron

A common issue is being too close to the problem.
Sometimes the answers come when you haven't been working on it.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/29/14 09:58 PM

Quote:

Quote:

John no harm intended but this is kinda refreshing seeing you with a problem and not a solution!!!!

Ron

A common issue is being too close to the problem.
Sometimes the answers come when you haven't been working on it.

I've got a good idea as to what it may be , the problem is in order to change out the offending part to prove that it is bad I'm the one that will have to supply the parts ... he's worked in a junkyard most his life and that is where his spares .... and sadly his storage techniques .... come from ... I asked for an iron intake to put on the motor to cover it up before we could get it installed in the car and he brings be one that is full of dirt

...

Posted By: stumpy

Re: Vaccum gauge bouncing ... - 09/29/14 11:05 PM

JRR I was just wondering if you had isolated the problem because you didn't say in your post. You just listed parts you had changed.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/29/14 11:57 PM

Quote:

JRR I was just wondering if you had isolated the problem because you didn't say in your post. You just listed parts you had changed.

I listed a bunch of parts ... the engine is a fresh build with maybe a couple hundred miles on it ... but the only part that was changed out was the distributor and that was because the owner is stuck in the 60's and doesn't like electronic ignition. I was concerned that maybe the the Pertronics and/or the Pertronics coil was damaged because it was being run with the ballast resistor still in place.

If you know of a way to test the Pertronics without replacing it with something else I'm all ears ...

Posted By: herkamer

Re: Vaccum gauge bouncing ... - 09/30/14 12:53 AM

My brother put a Pertronix conversion in his M37 and pinched a wire direct to ground. He was worried about it and called tech support. Guy told him if it was working, then all is well. They generally work or don't.

Posted By: stumpy

Re: Vaccum gauge bouncing ... - 09/30/14 02:13 AM

You still haven't told us what happened when you unplugged one wire at a time which will isolate a bad cylinder in case it isn't a carb problem. Just because all the parts are new doesn't mean one couldn't have failed.

Posted By: TJP

Re: Vaccum gauge bouncing ... - 09/30/14 02:37 AM

Quote:

You still haven't told us what happened when you unplugged one wire at a time which will isolate a bad cylinder in case it isn't a carb problem. Just because all the parts are new doesn't mean one couldn't have failed.

Posted By: ahy

Re: Vaccum gauge bouncing ... - 09/30/14 02:54 AM

I wonder if its possible you got ported vacuum? Sounds silly but wouldn't hurt to check another port... maybe same one as the brake booster.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/30/14 08:31 AM

Quote:

Just because all the parts are new doesn't mean one couldn't have failed.

No [Edited by Moparts - Family Friendly Site - Keep it clean]'

again , thanks for nothing

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/30/14 08:32 AM

Quote:

I wonder if its possible you got ported vacuum? Sounds silly but wouldn't hurt to check another port... maybe same one as the brake booster.

No , it's a holley carb , it was on the base.

Thanks to everyone that was helpful, it'll be a few weeks before I have the time to get back to it.

Posted By: HP2

Re: Vaccum gauge bouncing ... - 09/30/14 10:46 AM

An useful site for vacuum reading is http://www.secondchancegarage.com/public/186.cfm
If you are running a hot cam things changes. It's impossible to get a steady needle at lower rpm's, so how do you determine what power valve to put in your Holley?

Posted By: RapidRobert

Re: Vaccum gauge bouncing ... - 09/30/14 02:56 PM

Quote:

If you are running a hot cam things changes. It's impossible to get a steady needle at lower rpm's, so how do you determine what power valve to put in your Holley?

I would think a person would use the lowest (number) of PV available. Cuz you'd wanna stay out of the power circuit until you nail it. OT At WOT if the in hg is at or more than (1) in hg then the carb selection is too small. & either way (CFM) you'd still need a low number to stay out of "power" until you floor it. Back to the problem at hand, Double R we'll be waitin & for your sake it hope I'm wrong about it being a HG

Posted By: scratchnfotraction

Re: Vaccum gauge bouncing ... - 09/30/14 04:05 PM

Quote:

An useful site for vacuum reading is http://www.secondchancegarage.com/public/186.cfm
If you are running a hot cam things changes. It's impossible to get a steady needle at lower rpm's, so how do you determine what power valve to put in your Holley?

thanks, that was indeed an interesting read on the vac readings.

Posted By: stumpy

Re: Vaccum gauge bouncing ... - 09/30/14 06:03 PM

Quote:

Quote:

Just because all the parts are new doesn't mean one couldn't have failed.

No [Edited by Moparts - Family Friendly Site - Keep it clean]'

again , thanks for nothing

What is your problem?. I have done nothing but try to suggest things to help and you come back with garbage aimed at me. If my advice bugs you so much then put me on ignore.

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/30/14 08:19 PM

Quote:

An useful site for vacuum reading is http://www.secondchancegarage.com/public/186.cfm
If you are running a hot cam things changes. It's impossible to get a steady needle at lower rpm's, so how do you determine what power valve to put in your Holley?

Thanks I'll check it out.

As I was trying to fall asleep I remembered something that happened while turning the engine over to inspect the cam while the intake was off , it's either a head gasket or a bent intake valve.

Posted By: RJS

Re: Vaccum gauge bouncing ... - 09/30/14 09:06 PM

Quote:

Quote:

An useful site for vacuum reading is http://www.secondchancegarage.com/public/186.cfm
If you are running a hot cam things changes. It's impossible to get a steady needle at lower rpm's, so how do you determine what power valve to put in your Holley?

Thanks I'll check it out.

As I was trying to fall asleep I remembered something that happened while turning the engine over to inspect the cam while the intake was off , it's either a head gasket or a bent intake valve.

I was poking you with a stick just for fun, You know your still the coolest right???
Ron

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 09/30/14 11:03 PM

Quote:

I was poking you with a stick just for fun, You know your still the coolest right???
Ron

I'm far from the coolest ...

Posted By: RapidRobert

Re: Vaccum gauge bouncing ... - 10/01/14 01:46 AM

Quote:

I've got a good idea as to what it may be , the problem is in order to change out the offending part to prove that it is bad I'm the one that will have to supply the parts

Double R what are you thinking it is?

Posted By: JohnRR

Re: Vaccum gauge bouncing ... - 10/16/14 10:15 PM

Not that it matters but the spark plug wire test pointed to cylinder #5 according to my friend when he did it but compression check was ok on 3, 5 and 7 ....

Funny part was the broken valve spring was the exh. on cylinder #1