Comp Cams 268H and vacuum level
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
Comp Cams 268H and vacuum level
I have this cam in my '81 with stock intake and E4ME carb, 882 heads with 2.02/1.6 valves, 74cc chamber, zero deck block, 2 valve relief pistons, stock crank, bored 0.030 over.
I have very low vacuum at idle (600rpm), about 15 - 17 at 1500 rpm or so. Is this just life with this cam or do I have a problem?
Headlights will not operate at idle, have to increase RPM to get them to move. Just replaced all hoses and checked for leaks....found none.
Thanks
More details below..........................
I have very low vacuum at idle (600rpm), about 15 - 17 at 1500 rpm or so. Is this just life with this cam or do I have a problem?
Headlights will not operate at idle, have to increase RPM to get them to move. Just replaced all hoses and checked for leaks....found none.
Thanks
More details below..........................
Last edited by LannyL81; Oct 21, 2008 at 06:46 PM.
Pro
Joined: Jul 2008
Posts: 699
Likes: 3
From: Millersville, MD
I have this cam in my '81 with stock intake and E4ME carb, 882 heads with 2.02/1.6 valves, 74cc chamber, zero deck block, 2 valve relief pistons, stock crank, bored 0.030 over.
I have very low vacuum at idle (600rpm), about 15 - 17 at 1500 rpm or so. Is this just life with this cam or do I have a problem?
Headlights will not operate at idle, have to increase RPM to get them to move. Just replaced all hoses and checked for leaks....found none.
Thanks
I have very low vacuum at idle (600rpm), about 15 - 17 at 1500 rpm or so. Is this just life with this cam or do I have a problem?
Headlights will not operate at idle, have to increase RPM to get them to move. Just replaced all hoses and checked for leaks....found none.
Thanks
Pro
Joined: Jul 2008
Posts: 699
Likes: 3
From: Millersville, MD
Try and isloate the potential problem by eliminating the headlights from the equation and then maybe the emissions, etc. Something is definitely wrong. Was it like this prior to you replacing all the hoses? You should have that vacuum reading at idle and headlights should work.
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
I measured the vacuum level at the intake vacuum port with all headlights and temp controls disconnected;
I have three of the five ports on the carb capped off.
Sounds like I have a leak somewhere, either in the carb or around the intake.
Guess its time to get the starter fluid out and go hunting......
Thanks for the replies.
I have three of the five ports on the carb capped off.
Sounds like I have a leak somewhere, either in the carb or around the intake.
Guess its time to get the starter fluid out and go hunting......
Thanks for the replies.
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Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
Spraying starter fluid around possible vacuum leak areas will find the leak by a change in engine RPM when the starter fluid is drawn in. At least that is the method that I have used in the past.
Advanced
Joined: May 2006
Posts: 69
Likes: 1
From: West Linn Oregon
Whoops - looks like Lanny beat me to it!
Last edited by C3BB4SPD; Oct 20, 2008 at 07:30 PM.
Melting Slicks
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From: Nashville TN
St. Jude Donor '09
Le Mans Master
Joined: Jun 2008
Posts: 5,284
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From: Rockledge FL
isolate everything first.
unplug and cap the headlights, tranny, brake booster, everything that draws engine vacuum.
check your vacuum reading at idle. that will give you an idea of what you should be working with. then start plugging systems back in. check headlights at each junction with a vacuum gauge. when you see a significant drop, the leak is close.
unplug and cap the headlights, tranny, brake booster, everything that draws engine vacuum.
check your vacuum reading at idle. that will give you an idea of what you should be working with. then start plugging systems back in. check headlights at each junction with a vacuum gauge. when you see a significant drop, the leak is close.
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
isolate everything first.
unplug and cap the headlights, tranny, brake booster, everything that draws engine vacuum.
check your vacuum reading at idle. that will give you an idea of what you should be working with. then start plugging systems back in. check headlights at each junction with a vacuum gauge. when you see a significant drop, the leak is close.
unplug and cap the headlights, tranny, brake booster, everything that draws engine vacuum.
check your vacuum reading at idle. that will give you an idea of what you should be working with. then start plugging systems back in. check headlights at each junction with a vacuum gauge. when you see a significant drop, the leak is close.
The current reading is just barely moving the needle on the vacuum gauge. I really do not want to replace the brake booster. Hope to get a look at it tonight.
Oh and this is a manual trans.
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
unless it is a valve not sealing issue this has to be it.
my bet is brake booster.
what happens to your gauge when you unhook the booster and cap it off.
check your one way valve.
also hook up a gauge, with the engine running watch the gauge when you press the brake pedal
It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphram is a hissing when the pedal is depressed
my bet is brake booster.
what happens to your gauge when you unhook the booster and cap it off.
check your one way valve.
also hook up a gauge, with the engine running watch the gauge when you press the brake pedal
It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphram is a hissing when the pedal is depressed
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
unless it is a valve not sealing issue this has to be it.
my bet is brake booster.
what happens to your gauge when you unhook the booster and cap it off.
check your one way valve.
also hook up a gauge, with the engine running watch the gauge when you press the brake pedal
It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphram is a hissing when the pedal is depressed
my bet is brake booster.
what happens to your gauge when you unhook the booster and cap it off.
check your one way valve.
also hook up a gauge, with the engine running watch the gauge when you press the brake pedal
It should dip as the pedal is depressed and return to the base reading. If it drops slowly, but returns, you have a blockage/collapse in the vac line. If it falls and does not return, you have a torn diaphram in the booster. Especially indicative of a torn diaphram is a hissing when the pedal is depressed
I will disconnected and cap-off the brake booster along with sealing the threads of the manifold vacuum fitting.
Having low, actually next to nothing vacuum at idle and then 15 - 17" Hg when I increase RPM is puzzling me. This is on a newly rebuilt engine with only a few hours of operating time on it. I sure hope its the brake booster rather than something with the engine.
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
Latest info
Measured vacuum at the intake manifold after putting teflon tape around the fitting. The brake booster, PCV and MAP sensor remained connected. (Did find a leak in the gauge hose).
Vacuum was about 10" at 600rpm, needle was not steady. Capped off brake booster - no difference, reconnected. Capping MAP caused idle to decrease and vacuum - reconnected. Did not cap PCV.
Adjusted idle mixture screws in and out with little if any difference in vacuum reading or RPM. Increased idle to about 700rpm, vacuum reading more steady, but still about 10". Hold engine at 1000rpm....15" Hg.
Seems that the engine either needs more air or less fuel. I am thinking that there is something wrong in the carb. May be the float level is too high as I have never been able to get any change with the idle mixture screws unless I either take one out or screw both all the way in. Is screw in = more air or less air? May be I should send this carb off and have it rebuilt at a shop that can wet flow it.
Comments???????????????????????
Vacuum was about 10" at 600rpm, needle was not steady. Capped off brake booster - no difference, reconnected. Capping MAP caused idle to decrease and vacuum - reconnected. Did not cap PCV.
Adjusted idle mixture screws in and out with little if any difference in vacuum reading or RPM. Increased idle to about 700rpm, vacuum reading more steady, but still about 10". Hold engine at 1000rpm....15" Hg.
Seems that the engine either needs more air or less fuel. I am thinking that there is something wrong in the carb. May be the float level is too high as I have never been able to get any change with the idle mixture screws unless I either take one out or screw both all the way in. Is screw in = more air or less air? May be I should send this carb off and have it rebuilt at a shop that can wet flow it.
Comments???????????????????????
Melting Slicks
Joined: Apr 2001
Posts: 2,030
Likes: 363
From: Lubbock Texas
I chased a low vacuum reading at idle for a month or so before I figured out the problem was with my brand new vacuum gauge. It was reading at least 5" too low (9-10" at idle). After replacing the gauge with another new one, my readings improved to around 15" at idle which the mechanic that did my dyno tune said was about right for my engine and cam.
Here's a couple of vacuum related links that may help you:
http://www.therangerstation.com/Maga...acuumLeaks.htm
http://www.international-auto.com/fi...uum-gauges.cfm
Good luck.
DC
Here's a couple of vacuum related links that may help you:
http://www.therangerstation.com/Maga...acuumLeaks.htm
http://www.international-auto.com/fi...uum-gauges.cfm
Good luck.
DC
Thread Starter
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
Did a quick test of the vacuum gauge using my MightyVac...which is how I did find the leak in the gauge hose. Gauge appears to be working fine.
I am wondering if the vacuum port on the intake between the carb and distributor is true manifold vacuum, cause it appears that this is only the rear four cylinders if not just the rear two cylinders (7,8). Is this correct?
I am wondering if the vacuum port on the intake between the carb and distributor is true manifold vacuum, cause it appears that this is only the rear four cylinders if not just the rear two cylinders (7,8). Is this correct?
Pro
Joined: Jul 2008
Posts: 699
Likes: 3
From: Millersville, MD
Did a quick test of the vacuum gauge using my MightyVac...which is how I did find the leak in the gauge hose. Gauge appears to be working fine.
I am wondering if the vacuum port on the intake between the carb and distributor is true manifold vacuum, cause it appears that this is only the rear four cylinders if not just the rear two cylinders (7,8). Is this correct?
I am wondering if the vacuum port on the intake between the carb and distributor is true manifold vacuum, cause it appears that this is only the rear four cylinders if not just the rear two cylinders (7,8). Is this correct?