Vacuum reading

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Vacuum reading

Post  Rabbit on April 25th 2015, 10:45 am

Ok im getting fluctuating at idle of 10"-12" of VAC steady between 11"-12" for two three sec but then drops to 10" then jumps back up to 11"-12" my set up is compcams XE284H 240/248 @ .050 LSA 110 584in/588Lift, carb AED850HO, C6 3000 stall, 10.5:1 comp there is no popping noise from the pipes last question with a cam like this is my motor going to shake a little my timing is 20 & 37 total thanks for the help in advance P.S this is my frist motor that I built and I did good at the dyno 562hp @ 5800 now I got to get the tuning right
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Re: Vacuum reading

Post  maverick on April 25th 2015, 11:04 am

Timing sounds like a bit much for most BBF's.  If you're sure you have no vacuum leaks, try 32* total and see if things improve...and leave the vacuum gauge hooked up while you do it.  You'll be surprised at how much timing affects vacuum.  Once that's done, work with idle mixture a bit.  A lean condition can cause a "surge" that'll give a vacuum gauge fits. Also, idling too low with lumpy cams can make a vacuum gauge useless.
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Re: Vacuum reading

Post  Rabbit on April 25th 2015, 11:18 am

I have it at 1000rpm at idle and I plugged up all vacuum ports except the line to the trans in which I will check as soon as it stops raining I even pumped up the idle to 1200rpm more steady but dose the same thing but slower
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Re: Vacuum reading

Post  Rabbit on April 25th 2015, 11:20 am

P.S I will play with the timing a little to
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Re: Vacuum reading

Post  butterbean on April 25th 2015, 11:33 am

maverick is rite, a little less timing will be better, 32-34 total is all you need
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Re: Vacuum reading

Post  Rabbit on April 25th 2015, 11:10 pm

so how much dose a motor with this type of cam idle at I have it at 1150 rpms I have it down to 15* at idle but it still hard to start when hot it dose not like nothing under 1000rpms
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Re: Vacuum reading

Post  butterbean on April 25th 2015, 11:18 pm

that cam is kinda big for being street friendly, it may not idle under 1000 rpm anyway but with 15* timing it should start fine hot, you need to check your GROUNDS and be sure they are ok
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Re: Vacuum reading

Post  Rabbit on April 26th 2015, 1:44 am

well ill get out my Ohm Meter im going to check my wires any ways the MSD box red light is on but ill check
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Re: Vacuum reading

Post  Dave De on April 26th 2015, 7:32 am

What heads are on this engine?
what is the compression ratio?
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Re: Vacuum reading

Post  maverick on April 26th 2015, 8:01 am

What distributor are you using? This sounds like maybe advance springs are too soft.
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Re: Vacuum reading

Post  supervel45 on April 26th 2015, 11:53 am

How much lifter preload are you running? If you have more than a 1/4 turn down from zero lash, you might try backing the rockers off some. The 20 degree's intial timing should help smooth out the idle, but you may want to take some out of the total timing, as was mentioned. The cam you have is not going to idle very low or be smooth. Set your idle mixture screws with the engine warmed up, for highest vacuum. A carb. with a adjustable 4 corner idle circuit may help also, with a big cam.

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Re: Vacuum reading

Post  supervel45 on April 26th 2015, 12:03 pm

butterbean wrote:that cam is kinda big for being street friendly, it may not idle under 1000 rpm anyway but with 15* timing it should start fine hot, you need to check your GROUNDS and be sure they are ok
This, and check and make sure terminals are clean and tight battery is fully charged and in good condition, starter solenoid good and connections are clean and tight. I had the same problem and went to heavy duty diesel battery cables to fix the problem. Are you running forged pistons?

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Re: Vacuum reading

Post  Rabbit on April 26th 2015, 6:31 pm

Eddy heads 10.5:1 15cc dish forged pistons AED850HO 4 corner idle circuit im going to stiffen the springs in the MSD dizzy thanks for the help guys your a big help really what else I have found is if I keep the light on it will advance 5* from 15* initial just small surge every four sec
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Re: Vacuum reading

Post  maverick on April 26th 2015, 7:16 pm

Soft springs? A slight change in rpm (surge) changes timing?
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Re: Vacuum reading

Post  Dave De on April 26th 2015, 7:48 pm

maverick wrote:Soft springs?  A slight change in rpm (surge) changes timing?

Agreed
I have a 521 edel head 10.5 compression pump gas engine as well. It likes initial and runs at 22 degrees with 38 total at the track.
If your measuring vacuum at the runner its going to fluctuate. The best place to measure is the plenum. I get fluctuation as well but my cam is bigger than yours.
Engine cranking can be a problem from small starter wires and the older starter motor. Use #2 or larger wires on all four wires.
If your engine cranks good cold it should also hot regardless of the timing setup.
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Re: Vacuum reading

Post  Rabbit on April 26th 2015, 10:12 pm

Nope nothing still doing small surging but its easier to start one silver light spring/ one heavy silver spring, 18* black bushing 16* initial= 34* comes in at 3000rpm it had two blue springs at first going to replace all carb gaskets tomorrow and put a flat edge on intake and carb base to see if warp if that ant it then going to do valve adjustment probably need to drive the damn thing engine temp 120* need to get power steering hose first get it up to temp 175-185*
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Re: Vacuum reading

Post  supervel45 on April 26th 2015, 10:48 pm

You could spray some carb cleaner around the intake gaskets and carb with it running and see if the idle changes. Taking it for a drive and getting everything warmed up is a good idea, as long as nothing major is out of line. New rebuilt engines put out alot of heat, and headers close by the starter can cause hard start issues sometimes. Factory battery wires and small or weak batteries will not get it with BB engines especially with higher compression.

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Re: Vacuum reading

Post  Rabbit on April 26th 2015, 11:07 pm

I have done the carb cleaner trick if it revved I could not here it
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Re: Vacuum reading

Post  supervel45 on April 26th 2015, 11:22 pm

I don't think you ever mentioned what cylinder heads you have? I assume you have an adjustable valvetrain? Did you set the valves yourself? If you are not positive about the valve adjustment, I would check it before you roll off a cam lobe. If its all OK thats great.

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Re: Vacuum reading

Post  Rabbit on April 27th 2015, 8:31 am

elddenbrock  RPMs 75cc Harland sharp rockers 1/2 turn going back and doing a 1/4 turn what ever that might do
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Re: Vacuum reading

Post  Rabbit on July 5th 2015, 8:56 pm

update I had leaned out the idle circuit problem went away my bad that way a long time ago
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Re: Vacuum reading

Post  butterbean on July 6th 2015, 2:14 pm

well how does it run??? and where did you put the timing??? how's the vacuum at idle now???
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