Vacuum Gauge ?

[clar2001]

Vacum Gauge ?

Hello I hooked up a small vacum guage to my intake manifold to ajust my carb. becouse I have a rought idle I can't get ride of. The vacum was about 15lbs. at idle 700rpm, but the gauge flutters about 2lbs is this bad . I hooked it up in back were the tranny vacum is same port.

75stringray 350/400 stock build except for Edelbrock intake and carb Q-Jet 1903 new,been thure all the normal stuff plugs wires cap rotor new intake gasket, carb gasket
Runs tops except for a rought idle ,but it will rocket off the line and runs very smooth no hesitation

so why the fluttering on the gauge problem ?
Rought Idle ?

Any Injput would be great TKS..........

 

[igasper10]

15lbs of vacuum with fluttering I'm pretty sure indicates a sticking valve. A good engine with no leaks pulls between 17 & 20lbs steady.

 

[clar2001]

Woulden't a sticky valve make noise clicking or tapping ,I have no valve train noise,If I increase my rpm a little the vacum will go up higher............

 

[MaineShark]

I don't think all vacuum ports are created equally. As I recall, some ports supply different flavors of vaccum. I'd try a few different ports, and see if the results are the same on each.

Joe

 

[theo]

Does the vac needle settle out at higher rpm?
I have a similar symptom with my 1980, but it sounds like my needle bounce is a bit more pronounced than yours...
I've read that if the needle settles better at higher rpm, it points to worn valve guides. I'm fairly confident with that reason, since i do get a bit of smoke at startup after a long rest with my motor.

 

[vettenick]

If you're sure the engine is OK, try another vacuum gauge. Also, if you're location is very high above sea level (such as being in the mountains) your vacuum reading will be lower than 18 inches. It doesn't matter where you hook up the gauge, just make sure it's directly to the manifold with nothing in between.
How many miles on the motor?

 

[clar2001]

I not sure how many miles are on the motor the speedo says 95k the engine is very clean and does not burn oil or smoke,when I replaced my valve cover gaskets and intake manifold gasket due to a small oil leak, the inside of the engine was very clean no sludge or dark build up so that kind of tells me its fairly new proberly a rebuild ,I live in Ca. so I am at sea level timming is at 8* I have ajusted timming a couple times but allways end up back at 8* The rough idle is pretty bad once your off idle it just cruises

(Theo) my experience with smoke is your correct with your problem worn valve guides. you can drive a really long time before that really becomes a issue . If you don,t mind some smoke at start up..............

(Nick) I did try anouther gauge and it does the same thing it,s hooked directly to the manifold behind the carb. its on a double post,one for the guage and the other for the tranmission.

 

[vettenick]

Your vacuum reading is on the low end of the scale, but it's acceptable. If your not sure about the motor, do a compression test. But after reading what you've experienced, it sounds like worn valve guides.
I'm also assuming that you no longer have an EGR valve since you're using an aftermarket intake. If you still have an EGR valve, maybe it's hooked up wrong and its getting some vacuum at idle, causing it to open slightly. Or it has some carbon build up causing it to not fully close.

 

[clar2001]

well I'll do a compression check this weekend ,yes I still have a egr valve. I put a new intake manifold gasket on that does not have a port for egr so it doesen,t work .
Vacum is almost steady about 2k rpm but any thing higher it bounces harder and the engine does have a rought spot at any rpm. fells almost like a miss but plugs and wires,cap,rotor,ect all new carbs 10 mounths old ..........????????
At higher rpm vacum is about 17/18lbs.

 

[MaineShark]

Couldn't the tranny line be influencing the vacuum signal? I'd definitely try a direct connection to the manifold, away from other fittings, before looking for damage. If something else it drawing off part of the vacuum, you could see a low and/or unstable signal.

Joe

 

[vettenick]

It's possible, but then the engine still has a flat spot. If the modulator line was leaking, then the tranns wouldn't shift right.

 

[clar2001]

I have no leaks I made sure of that ,I will disconect the tranny so there is nothing else drawing from the manifold do anouther test and let u know .............

 

[theo]

Just a thought, but it may be a quick way to work it out.....
If it is worn valve guides, and OK that ain't necessarily so, then what about temporarily blocking any outlets from the rocker covers? The vacuum fluctuation from worn guides would have to come about from air being sucked in from the valve deck each time a valve opens. This air has gotta come from somewhere, and that somewhere would mainly be the vents in the filler cap and the PCV line. If you remove all lines/cap from the rocker covers and plug the holes, then i wonder if you would see the needle steady out? I've not tried this with mine yet, since it's in bits for new gaskets at the moment....
I'll let you know what i find out when its running again.

Anyone feel free to enlighten me if i'm wrong about this, as it's pure speculation!

Of course, i'm not proposing you do that just to get a steady needle to make tuning easier - since it's not good to block PCV valves for long - but at least it might actually isolate the problem.

 

[vettenick]

The fact that the vacuum hold steady at 2K but then bounces when raised beyond that may also be a weak/broken valve spring. Regardless, there's something going on. Pull the rocker covers off and take a quick look the the springs. You may find a crack.
Are all of the plugs coming out the same? You're going to have to take them out to do the compression test any way. If there's one that looks a little different, I'd start with that cylinder.
I wouldn't drive the car until I know what's going on. You don't want to drop a valve in the middle of all of this mess.
Good luck!

 

[clar2001]

OK more fun
Removed the spark plugs 7 were a nice white with just a small brown spot on one side and one showed a little oil resin not quite white,the plugs are only about 2mounths old.

Did a compression check all 8 cylinders read 160lbs. on each one.

Just cause I replaced my allready pretty new plug wires thinking maby heat damage becouse they go over the valve cover.

Still have a very rought idle and a miss at any rpm.

Vacum bounces 1-2lbs at any rpm at higher rpm it does plane out a bit.

Set the Valve lash about a mounth ago was a little lose not to bad,did not notice any problems when the covers were off and was very clean good oil flow.

when driving you only notice the miss at idle(and its pretty bad) any other speed is ok...........although there is a miss at any rpm.

So next steep get some carb cleaner and spray in my less than a year old Q-Jet 1903 Edelbrock.

Maby do a new Distrubitor Mallery Hei $159 summit. but no funds for that for a couple weeks ( new Headlighs pending, exchange for recessed none pop up $150 rik's ;help

 

[vettenick]

Instead of spending all that good money on a guess. Have a shop scope the secondary firing voltages. The bad cylinder, if there is one will show up. Then concentrate on that cylinder. If the guy using the scope is good, he'll probably figure what's the problem too!

 

[clar2001]

Good Idea, butt I'm the only person who works on the car, I would half to take time off work and arange to leave the car at the shop get a ride home and at $80 labor.I would rather just buy parts until I fiqure it out, I'm restoring the car any way (MOD) so as frustrating as it is,I'm hoping between this forum and a little trial and error to figure it out, althouth I am tired of the vette keeping me broke,I bought it about a year ago and have just porred money into it ............fun to drive and the attention is great ................

tryed carb cleaner now I'm thinking Distributor..

Input is welcome thanks ..................

 

[clar2001]

I have a heat riser that is tied open could this create a rought idle

 

[vettenick]

All the heat riser does is direct some exhaust gas up to the intake manifold to heat it up so you don't have driveability problems like hesitation and poor fuel distribution when the engine is cold.

 

[vettenick]

Forgive me for not asking...but..did the car have this miss since you've had it, or did it surface all of a sudden?
Do you have a true dual exhaust, or are you still running a cat?