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low vacuum, rough idle

 
71MCSS
I love my Chevy Chevette!
Posts: 7
Joined: 08/13
Posted: 08/02/13 02:23 PM

I recently started working on my 71 Chevy Monte Carlo with a stock 454 in it.
I will try to make this brief but informative.
When I first got the car it ran smooth. After about a month the #8 intake push rod bent on start up,holding the valve open.I pulled the valve covers off to find all the wrong size push rods were installed. The push rod guides were 7/16" the rods were 5/16". The motor was rebuilt before I acquired the car sometime around 1991 according to the receipts. I have no idea how many miles are on the rebuild.The push rods were replaced and the motor reassembled. The motor would not idle smoothly.Rpm's vary from 600-850 at idle and engine shakes erratically.
The following has been done since.
Checked manifold vacuum - 12 {to low}, checked for intake manifold leaks(none). checked carb base for leaks{none}. Replaced all vacuum hoses. Heads were removed and completely rebuilt. Intake was magnafluxed for cracks {none found}. Rebuilt Rochester carb. installed.
Motor reassembled, same idle issue and low vacuum. Compression check, engine hot, high is 152, low 145.
Any ideas would be greatly appreciated.  

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waynep712222
V-6 Camaros rule!
Posts: 122
Joined: 03/13
Posted: 08/02/13 10:34 PM

with the vacuum gauge hooked up with as short a hose as possible to a vacuum port on the intake..  is the vacuum stable at idle.. or does it fluctuate?

if its stable...

when was the last time you rebuilt the carb??? i am taking that you have a quadraJET carb

these are not impossible to rebuild at home in a few hours..  there are a few tricks.. but we can cover those if you decide do dig into the carb.

are the #5 and 7 spark plug wires separated by more than an inch.. so you don't end up with crossfire issues..

these two wires fire 90 degrees of crank rotation apart...  so the #5 fires and the #7 piston is part way up the cylinder with the valves closed.. a full charge of fuel and air.  a ghost spark created by the magnetic field corona induces a spark in the wire from being too close..  this lights off part of the fuel mixture a quarter turn early.. causes a lean misfire when the piston actually reaches the top.. sometimes it will melt the piston and tear up the cylinder wall also..  

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71MCSS
I love my Chevy Chevette!
Posts: 7
Joined: 08/13
Posted: 08/03/13 08:52 AM

I checked the #5 and #7 plug wires and they are separated. The # 5 runs from the cap over the valve cover and to the plug. The #7 runs back between the valve cover and the firewall.
I also checked vacuum like you mentioned and it does fluctuate down to about 11.
Any ideas on what would cause that?

The Quadrajet was a rebuilt model from a local parts store installed after this problem began. Even after this carb was installed there was no change in the engine performance. Not that the carb still isn't the issue. If it was a carb issue, what could be occurring with the carb. that would cause this?

If I had a crossfire issue would it show up in the color of the spark plug electrode?  

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68scott385
I have an SS396 tatoo
Posts: 325
Joined: 07/09
Posted: 08/03/13 09:18 AM

Vacuum Gauge reading chart; this may help identify your problem by what the vacuum gauge is doing/showing.

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm  
68scott385 68scott385 68scott385

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68scott385
I have an SS396 tatoo
Posts: 325
Joined: 07/09
Posted: 08/03/13 09:18 AM

Vacuum Gauge reading chart; this may help identify your problem by what the vacuum gauge is doing/showing.

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm  
68scott385 68scott385 68scott385

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71MCSS
I love my Chevy Chevette!
Posts: 7
Joined: 08/13
Posted: 08/03/13 12:06 PM

I took a look at the vacuum chart. My engine seems to be generally low with a slight fluctuation. This would indicate possible late valve timing. Late valve timing would indicate the timing marks on the cam and crank were not lined up.

If the marks were off by one tooth would this cause the engine to act this way or would I have more erratic vacuum readings? Has anyone ever heard of the timing chain jumping a tooth when a push rod breaks?
Is there any way to check for this without pulling the timing chain cover?  

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waynep712222
V-6 Camaros rule!
Posts: 122
Joined: 03/13
Posted: 08/03/13 12:56 PM

you can get close... pull a valve cover on either side.. look at the rocker on the intake valve.. slowly rotate the crank in normal direction of rotation.. drivers side the second rocker arm.. passenger side the sixth rocker arm.

see when the rocker arm first starts to open..

if its really close to a stock cam.. some of the older service manuals show a intake starts to open at xx degrees..   you might need a degree tape.. of if you can narrow it down.. you can scratch a mark on the damper and use your timing light after you have reinstalled the valve cover..  you can turn the advance knob at idle.. till the scratch lines up and then subtract base timing..  i have not done this in years..  i usually measure from the TDC mark and then measure the diameter of the damper .. and calculate how much each degree on the outside covers when divided by pi and then 360..  something like that..  sometimes it is within the range of the timing pointer..

i have worked in 2 engine rebuilding shops..  there is always a double check by a second person on the timing marks when the chain is installed..  about once a month.. somebody would get it a tooth off..

i will look around as i think i see an early 70's tune up manual that will probably have the intake opening spec..

.....

hint.. you can use a conventional test light with a needle sharp probe to short out individual spark plug wires for a few seconds.. see if this will help narrow down the slight vacuum variation..   you can also .. pull the wires out of the cap one at a time..  for a few seconds.. using i have a pair of plastic fuse puller pliers for this..  i don't like getting zapped.. i am a big chicken..

you did not mention what was done when you found the small pushrods and replaced them with the proper 7/16" versions..  did you pull the heads again?? or just pushrods .. adjustment and valve cover gaskets..

if i were you.. after finding a slight variation in idle vacuum.. is go back and carefully readjust the valves .. this can be done by turning the engine 1/4 of a turn at a time and following the firing order.. verifying that when you move the crank to the next 1/4 turn that the valves you adjusted are NOT moving.. it should be 1/2 a turn of the crank before the valves you just adjusted start moving..

i like 1/4 turn down from just touching..  but i have done 1/2 turn and 3/4 turn..

this gets confusing to some.. as ZERO is where the push rod play is gone.. but you have NOT started depressing the plunger seat on the lifter.. 1/4 turn down.. moves the plunger seat away from the snap ring a fraction of an inch..

things move around..   usually there is a slight amount of cam wear.. but valve seat errosion is an issue.. so you normally have to loosen the valve adjustments one at a time till the rockers make noise and take them to zero.. then go a quarter turn.. best is with the engine running but i have taught a lot of techs in person how to cold set the rockers.. so they don't have to pull the valve covers off after the engine is in..  

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71MCSS
I love my Chevy Chevette!
Posts: 7
Joined: 08/13
Posted: 08/04/13 06:58 AM

I am going to go back and check the valve setting while the engine is running to see if that changes the vacuum reading.Due to work commitments I will not get to this until later this week or next weekend. I will keep you posted on what I find.
Thank you for all the help so far.  

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waynep712222
V-6 Camaros rule!
Posts: 122
Joined: 03/13
Posted: 08/04/13 11:58 PM

thats OK... we will be around..  

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71MCSS
I love my Chevy Chevette!
Posts: 7
Joined: 08/13
Posted: 08/15/13 09:02 PM

ok
So the frustration continues. I re-adjusted the valves with the engine running. The engine seemed to smooth out slightly but there is no improvement in vacuum. After zero on the valves I went 1/4 of a turn. Now at 600rpm I am at 11.5-12 vacuum with a slight fluctuation. When I put the air cleaner back on, vacuum dropped to almost 10. I put the car in drive and it drops to 7.5-8 at idle.
At 1000rpm I am only at 15.
The heads have been completely rebuilt. New valves, springs, seats, guides and seals. Compression is 140-150.
The engine still shakes like it wants to kill when in gear.
Any thoughts?  

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waynep712222
V-6 Camaros rule!
Posts: 122
Joined: 03/13
Posted: 08/15/13 10:57 PM

were you able to identify either by turning the crank slowly.. and watching for the intake valve to open???   to note where the crank shaft timing mark is????


which carb do you have...

you could have the idle transition slots uncovered at idle..  and flooding the engine with too much fuel....

Quadrajet Primary Circuits

if your manifold vacuum is SO low.. that it cannot pull down the primary rod holder.. its going to go dead rich as you come off the idle feed ports below the throttle blade..  and with a big cam low vacuum.. you could have that issue...

i will have to go out in the friday and grab a quadrajet and see which hole you can check primary rod holder height on one..

Transferslot
yes.. i know this is not a quadrajet..

i will have more pictures tomorrow night if i get a few minutes..

i know that one can use a dipstick to measure the height of the primary rod holder on the later quadrajets.. by dipping down the vent tube..  going into the slotted opening near the air cleaner hold down screw is where you can measure the float level at...

532Or5mmislowestaptsett


please post your carb #.. so i can get the proper info for you..  

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cruisebutton
I love my Chevy Chevette!
Posts: 4
Joined: 08/13
Posted: 08/17/13 02:40 AM

I had the same problem and found a vacume leak in the break boost canister. with the car running spray some wd40 around the boost canister and that will tell you if you have a leak. I changed the canister and it solved the idle problem.  

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71MCSS
I love my Chevy Chevette!
Posts: 7
Joined: 08/13
Posted: 08/18/13 07:18 AM

I disconnected the break booster line from the carb completely and plug the port on the carb. No change in vacuum at idle.
Before we get too involved with the carb.(which I am not real knowledgeable in.)
shouldn't the vacuum be higher?
I do not have a big cam in this engine. It is a Crane Cam Fireball II 290H.(No.134801) lift is 515, advertised duration 290. rpm range 1800-4400.
Shouldn't I be more in the 15-21 range for vacuum at idle?
The port off the carb for the vacuum advance was only pulling 7 in of vacuum at 600rpm.
Like I mentioned earlier, I am not that strong in carb knowledge. So, is it possible that the carb is causing the vacuum issues? I do not seem to have any leaks in the intake manifold or around the carb base gasket.
At higher rpm's the engine seems to run pretty well.  

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waynep712222
V-6 Camaros rule!
Posts: 122
Joined: 03/13
Posted: 08/18/13 09:40 AM

basic checks..
does the vacuum advance work...  it should not leak vacuum... when you apply vacuum to it.. does the breaker plate rotate and snap back when you release the vacuum..??

grab the ignition rotor..  twist it against the spring tension.. release.. does it snap back fully.. you might want to remove the rotor.. see if the advance weights are free to operate.. once in a while the advance will hang up because of dry grease and rust on the upper shaft.. it will usually hold the centrifical advance in an advanced position..

how about base timing.. got a timing light??? where is it set at..

how about PCV flow??  could it be TOO MUCH..

what kind of manifold..

~~~~~~~~~~~~

carb adjustments..

since you seem to have a vacuum gauge...  have you adjusted the carb while using it..

the 2 screws in the base are the idle mixture screws...  

the screw on the drivers side front of the carb is the idle speed screw..

with the engine warmed up..  the choke fully open and the fast idle cam NOT holding the throttle open...

with the vacuum gauge hooked up to manifold vacuum..

turn the idle mixture screws in one at a time. to verify the engine stumbles when you do so..  turn the idle mixture screw in the baseplate  out till you reach the highest manifold vacuum.. turn it in slowly till the manifold vacuum starts to drop.. stop and go back just till you get to the point where it just started to drop..

adjust the other idle mixture screw...  in till the engine stumbles.. then out to reach the highest vacuum..  then in till the vacuum starts to drop. then back slightly till you reach the point where it just started to drop...




go back to the first idle mixture screw.. turn it backwards till you get to the highest vacuum.. then in till it just starts to drop again... perform this on the second screw.. this gets you to the a proper mixture to drop the idle speed as low as it will go ..  so slow the idle down if you can with the screw on the front drivers side of the carb..

reset the idle mixture screws again... now the idle speed is reset..

then turn them in a fraction of a turn.. just enough to drop the manifold vacuum 1/2" on the gauge. this is lean best idle mixture.. so it won't foul out the spark plugs at idle...

in theory.. you should have the throttle open only enough to barely expose the idle transition slot..

this used to be learned in auto shop.. it is a standard way of setting idle mixture..

while you have the vacuum gauge hooked to manifold vacuum.. can you slowly increase the RPMs and see that the manifold vacuum increases  not decreases..

post what happens.

with an 1/8" allen wrench or piece of wire about the same size.. 3 or 4 inches long.. lower it into the vent tube... see if you can feel something spring loaded about an inch and half to 2 inches down..   it will move down about 5/16 of an inch when you depress it with the engine off..  mark the tool with the component all the way up...    when you start the engine.. does the manifold vacuum pull the device all the way down???

this is the primary rod holder...

the quadrajets have very small primaries..  the qjet comes off the idle transition circuit very early..  if the rod holder is not pulled all the way down by the vacuum.. it will be very rich as you come up off idle..  

if the carb adjustments are way out of whack..  you could have the primary throttle so far open that its starting to pull fuel from the idle transition circuit or the mains..


there could be several issues with other engine components..

have you verified the timing pointer and the harmonic damper are accurate...
by rotating the engine with a tight fitting breaker bar on the crank bolt..
hint a plastic store bag over the square on the breaker bar will tighten up the fit of the socket..  the same plastic over the crank nut hex will tighten up the socket fit .. this allows you to rock the crank back and forth while you have a coat hanger  i(i like copper solid conductor wire better) into the  #1 or #6 spark plug hole.. to see if the damper passes TDC at the top of the stroke.. this is a feel type of thing..  

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68scott385
I have an SS396 tatoo
Posts: 325
Joined: 07/09
Posted: 08/18/13 03:55 PM

Icon Quote71MCSS:
I disconnected the break booster line from the carb completely and plug the port on the carb. No change in vacuum at idle.
Before we get too involved with the carb.(which I am not real knowledgeable in.)
shouldn't the vacuum be higher?
I do not have a big cam in this engine. It is a Crane Cam Fireball II 290H.(No.134801) lift is 515, advertised duration 290. rpm range 1800-4400.
Shouldn't I be more in the 15-21 range for vacuum at idle?
The port off the carb for the vacuum advance was only pulling 7 in of vacuum at 600rpm.
Like I mentioned earlier, I am not that strong in carb knowledge. So, is it possible that the carb is causing the vacuum issues? I do not seem to have any leaks in the intake manifold or around the carb base gasket.
At higher rpm's the engine seems to run pretty well.




Can you post the cam's specs?

Is this it? ...crane cams fireball II 290h (290/290 advertised duration) 234/234 @.050 .494/.494 lift 112/107 LSA)  
68scott385 68scott385 68scott385

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