WHY low vacuum on freshly rebuilt motor?!

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lucahemi

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1972 Dodge Demon 340 with: X heads, Kieth Black Pistons 72cc 9.5:1
Comp Cam:
Grind - CRS 280H PP474
Lobe Separation - 110
Duration- .050
Intake - 233
Exhaust - 240
Lift - .474 for intake and exhaust hydraulic
Intake Center Line - 106

Engine has no bottom end torque and idles terribly due to about 7-8 vacuum. Checked thoroughly for vacuum leaks. Thermoquad carb runs good on other motors. Valves are adjusted correctly. Compression is 145-150 on all cylinders. Any suggestions? Thanks
 
Well, low compression + big cam w/lots of overlap = low vacuum.

Did you have this cam in the engine before the rebuild?

Where's your initial timing at?
 
65Val said:
Well, low compression + big cam w/lots of overlap = low vacuum.

Did you have this cam in the engine before the rebuild?

Where's your initial timing at?
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Cam was reccomended by comp cams for a very mild street cam. Do you agree with their reccomendation? Timing is 36 all in. Vacuum does not change no matter what i do to the distributor. No this is a new cam, everything in the motor is new.
 
lucahemi said:
Cam was reccomended by comp cams for a very mild street cam. Do you agree with their reccomendation? Timing is 36 all in. Vacuum does not change no matter what i do to the distributor. No this is a new cam, everything in the motor is new.
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Its an old design...they say in their write-up..."Nostalgia cam...for that 60's/70's sound"" ...kinda like the old Purple Shafts Mopar used to sell. There are much better profiles available today.

110*/106* does indicate a higher RPM cam...not much good power or torque at lower (under 2500) rpm's.

EDIT: Try running more initial. Does the engine ping at all?
 
65Val said:
Its an old design...they say in their write-up..."Nostalgia cam...for that 60's/70's sound"" ...kinda like the old Purple Shafts Mopar used to sell. There are much better profiles available today.

110*/106* does indicate a higher RPM cam...not much good power or torque at lower (under 2500) rpm's.

EDIT: Try running more initial. Does the engine ping at all?
Click to expand...

No pinging in engine , tried adjusting the timing all over the map at initial, does not change the vacuum at all. Why would Comp Cams reccomend this cam. Can i salvage this motor with this cam? It idles horribly to the point that the tti headers are banging on the steering column. Or will i have to change the cam?
 
lucahemi said:
initial timing is at 8 degrees advanced
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8* seems low. Try 12-16* initial in increments. If your low end and vacuum does not increase, I would venture to wonder if the cam was degree'd in wrong "retarded". Did you degree the cam or just go dot to dot on the gear marks when you installed the timing set?
 
lucahemi said:
initial timing is at 8 degrees advanced
Click to expand...

That's your problem.

At least double it to 16 and see how it runs at idle. It may want more depending on if it kicks back on the starter.

You'll need to limit mechanical to hit your 36 total number.

When you change the initial timing, you need to readjust the carb. Just changing the timing will not necessarily cure what's wrong. That engine should idle at 800-850 without any issue.
 
Mad Dart said:
8* seems low. Try 12-16* initial in increments. If your low end and vacuum does not increase, I would venture to wonder if the cam was degree'd in wrong "retarded". Did you degree the cam or just go dot to dot on the gear marks when you installed the timing set?
Click to expand...

Unfortunately i had a family member that is supposedly a mopar gear head assemble the engine. I assumed the timing marks were lined up but have no idea. What cam would you guys reccomend with the pistons im running?
 
I would tune it first before giving up. You say no bottom end torque. Are you running an auto trans, what converter, what rear gear. I changed the cam in my engine and it was a flat out dog off the line. I changed rear gears and that helped, converter is next.
 
lucahemi said:
Unfortunately i had a family member that is supposedly a mopar gear head assemble the engine. I assumed the timing marks were lined up but have no idea. What cam would you guys reccomend with the pistons im running?
Click to expand...

Thats probably a good assumption...you trusted him, now you have to verify for yourself.

I'd try what cracked has said. Keep advancing it til you get starter kickback or pinging. If you still can't get it to run properly, you may have to go in and verify cam install position.

I'll let someone else with more knowledge chime in on cam selection.
 
Bad Sport said:
I would tune it first before giving up. You say no bottom end torque. Are you running an auto trans, what converter, what rear gear. I changed the cam in my engine and it was a flat out dog off the line. I changed rear gears and that helped, converter is next.
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4 speed. 3:55 gears. Car is a dog from bottom to top. There is no power anywhere. Also set distributor by ear with no vacuum advance hooked up
 
crackedback said:
that's your problem.

At least double it to 16 and see how it runs at idle. It may want more depending on if it kicks back on the starter.

You'll need to limit mechanical to hit your 36 total number.

When you change the initial timing, you need to readjust the carb. Just changing the timing will not necessarily cure what's wrong. That engine should idle at 800-850 without any issue.
Click to expand...

x2
 
Put a timing light on it. No response to the vacuum. If i have to pull the motor... any cam recomendations?
 
crackedback said:
That's your problem.

At least double it to 16 and see how it runs at idle. It may want more depending on if it kicks back on the starter.

You'll need to limit mechanical to hit your 36 total number.

When you change the initial timing, you need to readjust the carb. Just changing the timing will not necessarily cure what's wrong. That engine should idle at 800-850 without any issue.
Click to expand...
x3,it needs alot more initial.with a 4 speed an 3.55 gears it should have plenty of power.i dont think that the cam itself is to blame.id start from the beginning,verify firing order,initial timing,carb adjustment,etc.maybe event try a "Proven good carb".if it cant be solved by basic tuning id look at how the cams installed.it almost sounds like the timing gears are off a tooth.
 
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