Home | Register | Login | Contact Us

 
Auto Math
Classifieds
Forum
Gallery
Games
Garage
Tech Articles
Utilites
 
FAQFAQ    SearchSearch    RegisterRegister    Log inLog in    Log in to check your private messagesLog in to check your private messages    Forum Subscriptions1/4 Mile Table 

low vacuum

 
Post new topic   Reply to topic    SMOKEmUP.com Forum Index -> Other - Engine Discussion
View previous topic :: View next topic  
Author Message
10sec.et
Member


Joined: 29 Mar 2006
Posts: 3472
Location: Houston,Texas
346624.78 points


1969 Oldsmobile Cutlass

PostPosted: Sat May 11, 2013 1:39 am    Post subject: low vacuum Reply with quote

i have a car in the shop with a late 80s sbc. has the aluminum angle plug heads on it. it is carbed. it is supposed to be completely stock. doesnt sound like it has anything other than a stock cam. at idle, it has about 8" of vacuum. it also likes a lot of timing. i have it at 45 total 25 base right now. i get a lean "pop" through the intake with an agressive tap on the throttle. its worse with less timing. bigger pump shot helps. it idles and cruises well but doesnt like to take any quick throttle movements. i have not yet done a leakdown test (i dont have the tester Embarassed ). compression on a random cylinder was 150. i have readjusted the valves. carb is new. its a little big (750) but i dont think its so big that its causing vacuum to be THAT low. i put pressure in the cylinders on the left bank and could find no sign of leaking valves. this is kinda weird. ive only had two cars with this issue... one needed the valves adjusted (loose, almost to the point of clattering), the other didnt have any compression. any thoughts on what i am overlooking ?
_________________
af2 wrote:
It seems we can look at our magical Balls and come up with a fix?

Back to top
View user's profile Send private message
Big Dave
Moderator


Joined: 04 Dec 2005
Posts: 2644
Location: Tampa Florida
119910.22 points



PostPosted: Sat May 11, 2013 8:34 am    Post subject: Reply with quote

My first impression is a vacuum leak.

Those are not Chevy heads by description because I do not believe Chevy ever cast an angled plug aluminum head. As such there could be a port mis-match with a raised head port and a non-raised port runner in the intake manifold allowing in air out of the lifter valley.

Sounds to be running lean. Have you looked at the EGT with an infrared thermometer?

As a mid 80's it might have a narrow band oxygen sensor, but with a carb I doubt if it is still working. Reason I assume this is based upon my stupidity. When I was younger I ripped off everything I found on an engine that didn't belong there. What belonged on an engine was defined by what I knew from the good old days so I ripped off anything I didn't recognize as being installed on a 1963 motor, which often had a deleterious affect on a lot of post 1972 smog motors.

He may have converted to a carb because it wasn't running worth a darn with the factory TBI due to a sticking EGR valve or something as uncommon as a sticking heat riser blocking off the exhaust on one bank of cylinders.

I recognize your expertise in tuning. (I don't even know how to spell EFI so I only run carbs). Just trying to point out that besides any mechanical issues you may have, you also have to deal with stupid car owners. Being a little older I can testify that I have seen motors that crawled into my shop held together with barbed wire and duct tape (got love a redneck rancher that grew up with a Model T Ford).

Big Dave
Back to top
View user's profile Send private message
10sec.et
Member


Joined: 29 Mar 2006
Posts: 3472
Location: Houston,Texas
346624.78 points


1969 Oldsmobile Cutlass

PostPosted: Sat May 11, 2013 11:25 am    Post subject: Reply with quote

thanks Dave. one thing i havent checked for is a vacuum leak from under the intake. i should add that it was like this with the crusty old Ebrock carb and we put a new Holley on it that we had in the shop.
_________________
af2 wrote:
It seems we can look at our magical Balls and come up with a fix?

Back to top
View user's profile Send private message
clay
Moderator


Joined: 24 Nov 2002
Posts: 3209
Location: South Carolina
318129.23 points


1972 Chevrolet Nova

PostPosted: Sat May 11, 2013 6:07 pm    Post subject: Reply with quote

Cam lobe going flat or either a cam installed incorrectly? Clay
_________________
I have done so much with so little for so long, I can now do anything with nothing.
Back to top
View user's profile Send private message
af2
Member


Joined: 01 Sep 2003
Posts: 5557
Location: grassvalley, ca
71191.50 points


1933 Willys Coupe

PostPosted: Sat May 11, 2013 7:45 pm    Post subject: Reply with quote

Valve springs were the problem with my son's l-98 heads that are angle plug and aluminum Laughing ( sorry Dave I had to)

Worth a check for the hell of it. I pulled the valve covers and could push a few exhaust valves by hand.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    SMOKEmUP.com Forum Index -> Other - Engine Discussion All times are GMT - 5 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You cannot download files in this forum
Home | Contact Us | Automath | Cam Files | Dyno Search | Forum | Garage | Picture Gallery | Reaction Timer

Copyright 1997-2016 SMOKEmUP.com All rights reserved.
Advertising Info     Disclaimer     Privacy Policy