should this suck?
Thread Starter
Melting Slicks
Joined: Jul 2002
Posts: 3,128
Likes: 3
From: Deltona (son of Deland and DAYTONA) FL
should this suck?
There is vacuum coming from my rt valve cover. The other day when I was hooking the breather hose back up to the air cleaner on my 79 I noticed it. Is this an indication of worn valve guides? I've replaced the valve seals already.
I was looking for a vacuum leak because I still get detonation only when I open the secondaries. I've backed the timing way off and I don't get any detonation while accelerating on just the primaries, no matter what the RPM. It's been previously suggested that I change the seconday rods in my Qjet, but I want to make sure I'm not covering up some other problem by doing so.
Team Owner
Joined: May 2001
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From: Exiled to Richmond, VA - Finally sold my house in Murfreesboro, TN ?? Corner of "Bumf*&k and 'You've got a purdy mouth'."
CI 6-7-8 Veteran
CI-VIII Burnout Champ
St. Jude Donor '06-'10, '13
Re: should this suck? (isosceles)
Yeah - if the PCV is working then you should have a suction force there. Make sure it's well filtered.
As far as detination, it could be due to a lean situation (or helped by a lean situation). Also check your total advance both with and without vacuum to make sure it's not going too far.
As far as detination, it could be due to a lean situation (or helped by a lean situation). Also check your total advance both with and without vacuum to make sure it's not going too far.
Pro
Joined: May 2001
Posts: 608
Likes: 0
Re: should this suck? (isosceles)
:iagree:
Vacuum coming from tha valve covers is a good sign. Its when you have air coming OUT that there are signs of trouble, like ring blow-by.
You mention that you are having part-throttle detonation problems.
I had some part-throttle gremlins that actually caused the engine to backfire up through the carb. Turns out I had a broken valvespring, and the engine would run fine at low rpms, but when I went full throttle, and opened up them secondaries, there was some hesitation, and then, a nice big bang. I threw all kinds of new parts at it, in an attempt to fix the problem. Nobody could get a good diagnosis on the problem. It wasn't until I pulled the engine apart, and when the head shop started disassembling the heads, that they found the broken spring.
My driving her around in this condition probably caused the crack that spelled the end of my camel hump heads, and the end of my wimpy 350.
My advice is to try and "borrow" a carb to see if the problem lies with your q-jet tuning or if the problem lies elsewhere.
Vacuum coming from tha valve covers is a good sign. Its when you have air coming OUT that there are signs of trouble, like ring blow-by.
You mention that you are having part-throttle detonation problems.
I had some part-throttle gremlins that actually caused the engine to backfire up through the carb. Turns out I had a broken valvespring, and the engine would run fine at low rpms, but when I went full throttle, and opened up them secondaries, there was some hesitation, and then, a nice big bang. I threw all kinds of new parts at it, in an attempt to fix the problem. Nobody could get a good diagnosis on the problem. It wasn't until I pulled the engine apart, and when the head shop started disassembling the heads, that they found the broken spring.
My driving her around in this condition probably caused the crack that spelled the end of my camel hump heads, and the end of my wimpy 350.
My advice is to try and "borrow" a carb to see if the problem lies with your q-jet tuning or if the problem lies elsewhere.
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Re: should this suck? (isosceles)
:rolleyes: I donknow Iso but from here in California that vacuum leak sounds like it could be from the intake manifold. But this would be felt on both heads/valve covers. Usually there is a small positive pressure inside the valve cover due to blow-by all the way from the pistons. Now PCV allows vacuum from the carburetor sucking in air to remove this positive pressure through that breather tube. But a vacuum inside the valve cover with the breather tube out could mean just a bad intake seal or cracked intake or even cracked head (doubt cracked head as it would be very large crack). I would check for vacuum when opening oil fill cap on other side/valve cover.
Happy hunting, cardo0 ;)
Happy hunting, cardo0 ;)
Race Director
Joined: Mar 2001
Posts: 11,322
Likes: 1
From: Living in the Hartland
Re: should this suck? (cardo0)
Pull the pcv out and plug grommet to see if the vac goes away, that will tell you if the vac is coming from the pcv.
[Modified by Fevre, 8:50 AM 12/5/2003]
[Modified by Fevre, 8:50 AM 12/5/2003]
Instructor
Joined: Mar 2003
Posts: 217
Likes: 1
From: Buffalo MO
Re: should this suck? (isosceles)
:thumbs: Fevre is correct. remove the pvc and plug the hole in the v/c. then check for vac at the other valve cover. you should have pressure at this point. if you do not, you have an internal vaccuum leak. you can also place your hand over the top of the carb to slow down the air flow goin in. if it runs smoother while you are covering the top of the carb, there again, vac leak. you can also spray around the intake and such with some carb cleaner to check for external leaks. the carb cleaner will fill the leak and richen the mix for just a sec. you will hear the difference.
as far as the prob w/ you open the secondaries, fixin the vac leak might do it. also keep in mind that w/ you goto WOT, vaccuum drops and the vac advance moves to change the timing. be sure and check for a broken wire in the pickup coil. i know this my sound wierd, but, the hard vac drop w/ the secondaries open, can cause the damaged wire to "open" for a slight sec. u can check for this easily by either unhooking the vac advance and tryin it, or by pulling vac on the advance while at an idle.
good luck :cheers:
[Modified by duntovlives, 5:56 PM 12/7/2003]
as far as the prob w/ you open the secondaries, fixin the vac leak might do it. also keep in mind that w/ you goto WOT, vaccuum drops and the vac advance moves to change the timing. be sure and check for a broken wire in the pickup coil. i know this my sound wierd, but, the hard vac drop w/ the secondaries open, can cause the damaged wire to "open" for a slight sec. u can check for this easily by either unhooking the vac advance and tryin it, or by pulling vac on the advance while at an idle.
good luck :cheers:
[Modified by duntovlives, 5:56 PM 12/7/2003]
Thread Starter
Melting Slicks
Joined: Jul 2002
Posts: 3,128
Likes: 3
From: Deltona (son of Deland and DAYTONA) FL
Re: should this suck? (duntovlives)
thanks for the help guys. I tried the carb clean around the base of the carb and the intake seal-no difference. I placed my hand over the carb and get improve idle for about 1/2 sec, then it dies as it starves for air. I changed the valve seals over the summer and found the spings to be in one peice. I've had this detonation at high RPM since I've owned the car, I just try to keep the secondaries closed most of the time and it doesn't occur.
I haven't had time to diconnect the PCV yet and test for vac or pressure. I'd imagine a compression check might help to see if carbon deposits might be raising the compression. I ran GM top engine clean through it this summer too, so I doubt that carbon would be causing it.
You've given me more areas to check, so it's back to :smash:
Thanks!
I haven't had time to diconnect the PCV yet and test for vac or pressure. I'd imagine a compression check might help to see if carbon deposits might be raising the compression. I ran GM top engine clean through it this summer too, so I doubt that carbon would be causing it.
You've given me more areas to check, so it's back to :smash:
Thanks!
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