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302 Thumpr Cam Won't Idle

73azbronco

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Replace the camshaft. I'm not recalling saying don't do this, but I would have. Go with the 250-260 versions of compcams.

Read compcams own description, need a 2000rpm plus stall converter. High performance vehicle. You have either of those? This cam was designed for a light Camaro or mustang to do burnouts on the street, not be a drivable truck cam pulling 4000 lbs.


However, it should idle. Some others mentioned one nagging issue why it might not, not enough compression to run with the performance cam, resulting in as you noted 4 inches of mercury. 4, I would expect no less than 8-12 with a performance cam.


I don't think you have a vacuum leak or a carb issue, but, if you have another known good to you carb, might try running with that. Or take your carb and run it on another motor and prove it works or does not.

Then replace the camshaft with an RV type camshaft.
 
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pennyduke

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The Dyno shows that weekness off in the first 2000 rpms. The XE 256H shows a little better. I have 3.50 gears with 33" tires. I'm still looking at that VooDoo.
 

pcf_mark

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You are going to go cam crazy if you keep reading everything on the internet! 256 or 262 both are going to be fine or the VooDoo. All three are a little bit older designs but will simply beat the pants off the cam you have now.

I bet if you do a cranking compression ratio you have low #s (80-100) because all your power is going out the tailpipe. Any of the above cams will get you to 130-150 psi.

Comp Cams has a lot of engineering time on their designs that is why recommend them. In the end the installation and tune are equally important.
 
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pennyduke

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Y'all are right. I screwed up and now I'm going Cam Crazy. I going to order the XE 256H and do all the spring and valve testing. Thanks for all the help. I will update soon.
 

68ford

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The voodoo cam will have less low end and mid-range than both of those comp cams but will idle better and smell better. The peak torque will be at a higher RPM than the comp cams. They will probably idle fine but lope noticeably more.
 
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pennyduke

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This may sound crazy but was I supposed to block the small center EGR port on my Edelbrock Intake Manifold?
 

DirtDonk

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The large two-port thingy with two adjacent threaded holes? Yes...

Paul
 

DirtDonk

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Did the intake come with a block-off plate? I forget how that works. Maybe you have to buy one?

Paul
 
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pennyduke

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My intake has the 4 ports that mate to the head then a small port in the middle that I thought was a EGR port.
 

gr8scott

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This may sound crazy but was I supposed to block the small center EGR port on my Edelbrock Intake Manifold?

You can, but it's not necessary. Most don't.
 

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pennyduke

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Ok thanks, Someone said that I may have had a large Vacuum leak and I thought this port could have been it. The Thumpr is out and I'm waiting on Summit for the new XE256H Kit. Thanks for all the help
 

rmk57

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The port your referring to is the heat riser for an automatic choke.

If you don't use an automatic choke you block it off with a piece of tin or brass shim stock.
 

tirewater

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Concerning the EGR port. What did you use for a carburetor base plate? There certainly could be an EGR related vacuum leak on the carburetor side if things aren't blocked off or connected properly at that end. Did you use an EGR valve or plate with facilities for one?
 

73azbronco

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That 256 is a great choice of cam, might ask them if it would be better to advance it a few degrees
 
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pennyduke

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I will ask Comp about advancing the Cam. I never really understood what that does for a engine. Here are some pictures of my Intake.
 

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DirtDonk

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Perhaps with some limitations and parameters, if you take any came and adjust the timing gears/chains/belts so that the cam is rotated slightly in advance of it's as-tested position, you are going to lower the rpm range at which peak torque is developed.
Conversely, if you retard the cam timing you will raise the rpm range at which peak torque is developed.

Generally if you choose the right cam for your needs, you don't actually "need" to tweak the timing of the cam events. But for fine-tuning it to better fit your needs, advancing or retarding the cam timing is a great tool.
Racers might go either way, but I could see retarding it for two reasons. One would be to get more of it's power up at a higher rpm range based on a track that they know what engine speed their gearing and tire sizes and track length is going to be beneficial.
Another would be that if they are developing too much torque down low where it might induce tire spin while accelerating out of a corner, changing that curve to be later in the rpm range might get them through the corners faster. Or safer.

For a truck engine, or street engine, or whatever reason is thought reasonable, you might want to lower the rpm range that the peak occurs just to get better acceleration off the line. Or, in another street scenario, you might be able to gain a little better fuel economy by putting the torque peak in a more favorable rpm level.

That's all it is...%);)

Paul
 

DirtDonk

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Can you tell if that vacuum fitting/hose barb on the backside is drilled into the common plenum area of the intake? Or is it connected to that one runner only?
If just the one runner, make sure that's used only for power brakes or other accessories, but NOT for the PCV valve.

Just FYI in case you were not already aware.

Paul
 
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pennyduke

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It goes to the open plenum below the carb. I use that barb for my transmission module. It seems to work really good.The pcv is hooked to the front of the carbs large port. I don't run a vacuum brake booster. I'm gathering parts for hydroboost. So should I be good leaving those middle small ports open?
 

DirtDonk

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You talking about the odd shaped heat riser ports on the underside? Sure, you can do it. Or you can plug them if the gasket kit you buy comes with plates for that. I know that most Chevy based gasket kits include little block-off plates.

The colder your climate, the better it is to leave them open to avoid icing or other cold-weather issues with carburetors. In my experience the worst thing they usually do is discolor your new blue paint on the topside of the intake from the heat.

If you live in a generally warmer climate, you can block them off if you want.

Paul
 
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