Not getting much enough vacuum
Thread Starter
Burning Brakes
Joined: May 2001
Posts: 862
Likes: 2
Not getting much enough vacuum
At 600 RPM, I'm only getting 3" vacuum. At 1000RPM it goes up to 8" and at 2000rpm goes to 20". Vacuum reading is rock steady at each speed and does the expected drop in vacuum when I hit the gas. The vacuum line is coming off the carb. I retightend the manifold and carb bolts and even the valve cover bolts. No improvement.
The idle is pretty lopey at 600 and fairly smooth at 1000. The timing at 600 is 8* with or without vacuum can connected. Vacuum can DOES advance timing at higher RPMs when connected.
The engine is new. It was built by a local shop that does excellent work, but went out of business after assembly but before the first start. I can still contact the owner, but I'd like to avoid dealing with him as there were some pretty bad feelings because I got screwed royally when they went bankrupt.
It's a 350 with Edelbrock intake, aluminum heads, Holley Street Avenger carb, MSD ignition and distributor (with vac advance), mild roller cam with hydraulic lifters.
I should probably hire a real mechanic (which I am definitely not qualified as), but before I do, are they any basics I've overlooked? Any thoughts appreciated! :)
- Mike Greene
The idle is pretty lopey at 600 and fairly smooth at 1000. The timing at 600 is 8* with or without vacuum can connected. Vacuum can DOES advance timing at higher RPMs when connected.
The engine is new. It was built by a local shop that does excellent work, but went out of business after assembly but before the first start. I can still contact the owner, but I'd like to avoid dealing with him as there were some pretty bad feelings because I got screwed royally when they went bankrupt.
It's a 350 with Edelbrock intake, aluminum heads, Holley Street Avenger carb, MSD ignition and distributor (with vac advance), mild roller cam with hydraulic lifters.
I should probably hire a real mechanic (which I am definitely not qualified as), but before I do, are they any basics I've overlooked? Any thoughts appreciated! :)
- Mike Greene
Thread Starter
Burning Brakes
Joined: May 2001
Posts: 862
Likes: 2
Re: Not getting much enough vacuum (58Mike)
Update:
On the carb (Holley Street Avenger), the vacuum was connected to a fitting that was fairly high. There was another one (with a plug on it) that was much lower and right next to where the PCV hose connects. I assume this is a PCV hose at least, it goes from the top of a valve cover to the carb.
Anyway, I switched the connections (now I'm next to the PCV connection) and now the vacuum is around 12' or 13" at idle and around 18" a 2000 rpm.
Naturally, I got no paperwork with the carb or anything else, so I think I'll contact Holley and get a set of instructions for the carb. Any thoughts are of course still welcome.
- Mike Greene
On the carb (Holley Street Avenger), the vacuum was connected to a fitting that was fairly high. There was another one (with a plug on it) that was much lower and right next to where the PCV hose connects. I assume this is a PCV hose at least, it goes from the top of a valve cover to the carb.
Anyway, I switched the connections (now I'm next to the PCV connection) and now the vacuum is around 12' or 13" at idle and around 18" a 2000 rpm.
Naturally, I got no paperwork with the carb or anything else, so I think I'll contact Holley and get a set of instructions for the carb. Any thoughts are of course still welcome.
- Mike Greene
Burning Brakes
Joined: Jan 2002
Posts: 1,206
Likes: 62
From: Redondo Beach CA
C2 of Year Finalist (performance mods) 2019
Re: Not getting much enough vacuum (58Mike)
Mike,
I can go get the paperwork for the carb if need be, but I think you got it right the second time. The lower port is full manifold vaccum, and the upper one is ported(?) vacuum. 12" or 13" is at least close, depending on how radical the cam is...
I can go get the paperwork for the carb if need be, but I think you got it right the second time. The lower port is full manifold vaccum, and the upper one is ported(?) vacuum. 12" or 13" is at least close, depending on how radical the cam is...
Thread Starter
Burning Brakes
Joined: May 2001
Posts: 862
Likes: 2
Re: Not getting much enough vacuum (bcwaller)
Thanks Brad! The cam is supposed to be mild, but compared to 3", I'll settle for 12" or 13"! Mostly I wanted to make sure there are no major problems before I do the final body drop in about a week (if I stay unemployed :) )! Working on the engine with the body off is so much easier, it's hard to imagine working on it any other way! If you could double check that I'm supposed to use the port next to the PCV port, I'd appreciate it! Holley's on the East Coast, so I don't know how soon I'll get a set from them.
- Mike Greene
- Mike Greene
Race Director
Joined: May 2000
Posts: 12,712
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Re: Not getting much enough vacuum (58Mike)
The problem with the first setup, Mike, is that you had the vacuum can connected to a "ported" vacuum source above the throttle plates, so there was no vacuum signal to the vacuum can at idle. This will result in poor idle quality and vacuum.
You should now verify that the engine pulls enough vacuum to pull the vacuum can to the limit at idle. If not, you need a different vacuum can.
It sounds like you might be a candidate for the NAPA/Echlin VC1810 vacuum control, which provides 16 crank degrees at 8".
Duke
Duke
You should now verify that the engine pulls enough vacuum to pull the vacuum can to the limit at idle. If not, you need a different vacuum can.
It sounds like you might be a candidate for the NAPA/Echlin VC1810 vacuum control, which provides 16 crank degrees at 8".
Duke
Duke
Burning Brakes
Joined: Jan 2002
Posts: 1,206
Likes: 62
From: Redondo Beach CA
C2 of Year Finalist (performance mods) 2019
Re: Not getting much enough vacuum (58Mike)
Mike,
I have the picture you need... Page 6 of the manual has a picture identifying everything, and page 7 has one for the vacuum ports. You should call Holley up and see if they can send you a copy anyhow, as it also has information on assembly and adjustments.
o The port at the base by the Electric Choke is listed as the "Power Brake Vacuum Port"
o The large angled port under the Primary bowl is the "PCV Vacuum Port"
o The small angled port under the Primary bowl is the "Full Manifold Vacuum Port"
o The small port above the primary idle mixture screw is the "Timed Spark" vacuum source to go to your distributor vacuum advance.
You should be able to measure vacuum at any of the first three ports. I'm assuming that you have a PCV setup, so you can use the Power Brake or Manifold port to measure.
I have the picture you need... Page 6 of the manual has a picture identifying everything, and page 7 has one for the vacuum ports. You should call Holley up and see if they can send you a copy anyhow, as it also has information on assembly and adjustments.
o The port at the base by the Electric Choke is listed as the "Power Brake Vacuum Port"
o The large angled port under the Primary bowl is the "PCV Vacuum Port"
o The small angled port under the Primary bowl is the "Full Manifold Vacuum Port"
o The small port above the primary idle mixture screw is the "Timed Spark" vacuum source to go to your distributor vacuum advance.
You should be able to measure vacuum at any of the first three ports. I'm assuming that you have a PCV setup, so you can use the Power Brake or Manifold port to measure.
Racer
Joined: Aug 1999
Posts: 337
Likes: 0
From: Azusa, Cal.
Re: Not getting much enough vacuum (58Mike)
Hi Mike. thinking two items. A low idle vacumn and a very good reading higher up makes me think a vacumn leak. Your timing could be a factor but not with 600 rpm and only 3 inches. For starting off I would look at 12 initial, mechanical at 22 and 8 degress vac. All in by 2,800. Total would be 42 and 34 under heavy load. Once you get broken in and on a dyno you can finish dailing her in but this will get you real close and the total 34 gives you a safe cushion. Mel
Thread Starter
Burning Brakes
Joined: May 2001
Posts: 862
Likes: 2
Re: Not getting much enough vacuum (bcwaller)
Well that's odd! The port that was giving me trouble was the one above the idle mixture screw which Holley apparently says is the one I should use. I again hooked up to the small port next to the PCV port and got about 17" vacuum at idle. With the timing light and plugging and unplugging the vacuum can, I confirmed lots of vacuum advance, so it appears that the vacuum can is functioning properly. It had better be, it's all new!
Thanks for the help, Brad and Duke! I'm calling Holley today for a manual. Amazing how useful this forum has been. I wouldn't know about any of this vacuum advance business except for reading the threads about overheating engines! Funny how everything ties together!
Brad, did you find a bellhousing yet?
- Mike Greene
Thanks for the help, Brad and Duke! I'm calling Holley today for a manual. Amazing how useful this forum has been. I wouldn't know about any of this vacuum advance business except for reading the threads about overheating engines! Funny how everything ties together!
Brad, did you find a bellhousing yet?
- Mike Greene
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Thread Starter
Burning Brakes
Joined: May 2001
Posts: 862
Likes: 2
Re: Not getting much enough vacuum (Mel Foye)
Mel-
I guess I started typing right before you posted! Now Brad's whole crew has checked in! :D A leak is possible, maybe even in the hoses I was using as it wasn't very snug at the vacuum can. But since I got good results from the alternate port, I assume I don't have any major problems that need addressing before I drop the body on. I'm sure final tuning the carb and distributor will be a learning experience cuz I don't know nuttin' about this stuff! I've got it close now, but you can expect questions from me when it comes to final tweaking in a couple weeks! :rolleyes:
- Mike Greene
I guess I started typing right before you posted! Now Brad's whole crew has checked in! :D A leak is possible, maybe even in the hoses I was using as it wasn't very snug at the vacuum can. But since I got good results from the alternate port, I assume I don't have any major problems that need addressing before I drop the body on. I'm sure final tuning the carb and distributor will be a learning experience cuz I don't know nuttin' about this stuff! I've got it close now, but you can expect questions from me when it comes to final tweaking in a couple weeks! :rolleyes:
- Mike Greene
Burning Brakes
Joined: Jan 2002
Posts: 1,206
Likes: 62
From: Redondo Beach CA
C2 of Year Finalist (performance mods) 2019
Re: Not getting much enough vacuum (58Mike)
Bellhousing is in. Now I just need the oil pan and matching pump/pickup. At the rate I'm going, you might be driving before me!
