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1984 corvette
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33 Posts
Discussion Starter #1
Me and my son are building a mild street car from a 84 vette. We removed the old 350 and replaced it with a 400/406, it starts rite up and idle a little rough and when I step on it it seems to lose power? Here is a list of all that he and I have done
400sbc .30 over
Summit cam (part# sum-1065) 458 lift
Edelbrock rpm intake with a 1 inch spacer
487x heads with 192/150 valves
Roller rockers 1.5
Summit 600 cfm carb
New distributor (vac advance) hei
Double roller timing set, set dot to dot
Ignation timing set to 12 degrees. Btdc
Stock converter on a 700r4
Rear gear is a 3.08
Plug gap is .35
Compression is 135-145 +- 5psi on all 8
Felpro gaskets
Fresh gas
Idle at 500 rpm
Headman long tube headers
No oil or water mixing
When I rev it up it seems to be skipping and bogging down no signs of flooding and I have the float levels set. I did find that my altanator was not charging at 13 volts but was only giving 10 volts. Would this cause a problem with my hei? And is the cam I have ment to have a bad idle? Thanks for any help, this is my first small block chevy so I'm really a New guy to all this
 

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5,475 Posts
Did you set the timing with the line to the vac pot plugged off?
Did you use a 400 flexplate or is your 406 internally balanced?
 

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1 tdc and 6 tdc. 0 lash and 1/2 turn
One and six are companion cylinders with the piston in the same position in the bore at all times, same as 8-5, 4-7 and 3-2 but the cam lobes for each cylinder are on a different plan and cannot be matched up as being in the same position for any two cylinders at any given time.
If the cam had two cylinders matched up as the same, the motor would be firing two cylinders at a time.

Do you know the squish?
Do you know the static compression ratio?
Did you verify the inertia ring on the damper as being valid?
How far down in the bore are the pistons at TDC?
Did you degree the camshaft on installation?
Have you tried it without the carb spacer?
What's the part number of the head gasket used?
What's the part number of the pistons used?

You're using heads with a 161cc intake runner on a motor with 406 cubic inches? What were your expectations?

Please read through these tutorials and see if anything touches a nerve.....
http://www.crankshaftcoalition.com/wiki/Valve_adjustment_SBC/BBC

http://www.crankshaftcoalition.com/wiki/Determining_top_dead_center

http://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

http://www.crankshaftcoalition.com/wiki/Camshaft_install_tips_and_tricks
 

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1984 corvette
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33 Posts
Discussion Starter #8
That's a great article. I did twist and wiggle each push rod to find zero lash but I did not do each cylinder one at a time plus I only turn the preload a half turn
 

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1984 corvette
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33 Posts
Discussion Starter #10
One and six are companion cylinders with the piston in the same position in the bore at all times, same as 8-5, 4-7 and 3-2 but the cam lobes for each cylinder are on a different plan and cannot be matched up as being in the same position for any two cylinders at any given time.
If the cam had two cylinders matched up as the same, the motor would be firing two cylinders at a time.

Do you know the squish?
Do you know the static compression ratio?
Did you verify the inertia ring on the damper as being valid?
How far down in the bore are the pistons at TDC?
Did you degree the camshaft on installation?
Have you tried it without the carb spacer?
What's the part number of the head gasket used?
What's the part number of the pistons used?

You're using heads with a 161cc intake runner on a motor with 406 cubic inches? What were your expectations?

Please read through these tutorials and see if anything touches a nerve.....
http://www.crankshaftcoalition.com/wiki/Valve_adjustment_SBC/BBC

http://www.crankshaftcoalition.com/wiki/Determining_top_dead_center

http://www.crankshaftcoalition.com/wiki/Cam_and_compression_ratio_compatibility

http://www.crankshaftcoalition.com/wiki/Camshaft_install_tips_and_tricks



What is "squish" also what would be the best heads for my set up and how will the heads I have now effect the performance of my engine?
 

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What is "squish" also what would be the best heads for my set up and how will the heads I have now effect the performance of my engine?
Squish/Quench is the distance from the crown of the piston to the underside of the cylinder head with the piston at top dead center and the head gasket in place. Savvy builders will use a piston that has a generous flat deck on it, to mate up with the flat underside of the cylinder head opposite the chamber. It has been found that bringing the piston up into proximity with the cylinder head will "squish" or "jet" the fuel/air mixture that is above the piston, over into the combustion chamber. This jetting of the mixture homogenizes rich and lean pockets of mixture in the chamber, contributing to a more complete burn, more horsepower, less propensity to detonation and the ability to ease up on the octane of fuel that the motor will burn without complaining.

Quench is getting the piston crown in close proximity to a cooler surface to relieve heat from the piston, like bringing the crown of the piston up close to the underside of the cylinder head.

Most builders of street motors will shoot for a squish dimension of 0.035" to 0.045". This includes the thickness of the head gasket, so most times, the block has to be decked. Ideally, a builder would measure the stack of parts that he intends to use in the block and cut the decks accordingly.

The "stack" is the sum of the piston compression height, the rod center to center length and half the crankshaft stroke. For instance, on a 350 Chevy, the piston compression height would be 1.560", the rod would be 5.7" and the crankshaft radius would be 1.74". Add these values together and find a stack of 9.000". Although I don't think anyone has actually ever found a block that measures 9.025" on the block deck height, that is the blueprint dimension from the factory for any Gen I small block Chevy, from 1955 until the Gen II motor was introduced in 1992. The Gen I design was still produced all the way to 2000 or better, but the handwriting was on the wall.

Anyway, if you have a 9.000" stack and you want to arrive at a, for instance, 0.040" squish with a 0.040" thick head gasket, then you would want to cut the block deck height to 9.000". If you wanted a 0.040" squish with a 0.028" gasket, then you would cut the block to 9.012" block deck height. If you wanted to use a 0.020" shim gasket, you would cut the block to 9.020" block deck height.

Problems arise when a builder uses a piston that has a shorter compression height dimension that puts the piston down in the hole too far to effect any squish and the motor will be detonation-prone if the static compression ratio exceeds the anti-detonation properties of the fuel. For instance, let's say the block deck height is 9.025, the blueprint dimension from the factory. Now, you rebuild the motor with a "rebuilder piston" that has a compression height of 1.540", for instance. Your "stack" is 8.980" and you have decided to use a 0.039" compressed thickness gasket because that's what came in the rebuild gasket kit. Now, to find the squish, you add the piston deck height (0.045") to the gasket thickness (0.039") and you find you have a squish (or lack thereof) of 0.084" This squish measurement will generate no squish or quench and the motor will be load-sensitive and fuel-sensitive.

The best heads are a combination of what you want the motor to do and how thick your checkbook is. The heads that are on the motor now were designed by the factory to be used on a 350 cubic inch motor and to haul grandma to bingo and back once a week or to haul the family to Walmart for shopping once a week. The were not designed to feed 400 cubic inches and they were designed to make torque from idle to about 4500 rpm's tops.

If you want fast on a 400, you need heads with at least 195 cc intake runners that will flow 250/260/270/280 cfm at ~0.550" valve lift.

Here are a few examples of pistons that have a nice, flat squish surface on their crown...the first two are dished pistons to lower static compression ratio....
http://static.summitracing.com/global/images/prod/mediumlarge/SRP-139632-8_ml.jpg
https://www.uempistons.com/index.php?main_page=product_piston_info&cPath=3_4_22&products_id=494
This is a pop-up for high compression, with the squish pad on your right....
http://www.probeindustries.com/v/vspfiles/photos/13726-3.jpg
Here is a piston that has insufficient flat area to effect a good squish...
http://www244.pair.com/oldguys/1487b.jpg

It makes sense to me to cut the block decks so that you can use a 0.039", 0.040" or 0.041" thickness head gasket. The aluminum head manufacturers specify a thicker gasket with a pre-flattened fire ring, so even if you're currently building for iron heads, you should be looking down the road and setting the block up for use with aluminum heads so you don't have to disassemble the motor and cut the block later when you change to aluminum heads. Iron heads will do fine with the thicker gaskets, as long as you properly engineer the squish.

Oh, and by the way, all 400's need the heads drilled for steam holes, no matter whose heads you use.... and use head gaskets with steam holes in them.....
http://www.gregsengine.com/converting-350-heads-to-400.html
http://www.summitracing.com/parts/fel-1014/overview/make/chevrolet

.
 

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Cars, Trucks, Boats, Motorcycl
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7,392 Posts
. Despite all of the above... your engine should start and run perfectly... although not especially powerfully for its size... and it will have a mildly lopey idle with that 218/218 cam... don't expect a smooth idle... but it should idle reliably and not stall... if using the same vacuum secondaries carb., the 400 will open them at a lower RPMs than the 350 did, maybe too low an RPMs, may need spring tightened...

. Most likely several of the valves aren't adjusted properly... also, good chance some wires on the wrong sparkplugs, really need to verify them about 3 times, they have a 'magic' way of getting on the wrong plugs... we've all had that happen...

. Charging system should maintain about 14-14.2 volts at the battery all the time... no matter what you turn on like lights or A/C... although some alternators won't 'kick in' at a low idle RPM, but should with a slight rev... and some will even kick back out at a slow idle... don't idle a lot with them...
 

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1984 corvette
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33 Posts
Discussion Starter #13
Thanks for all of the great advice. To me it's ok RITE now if it's not a speed demon he is 16 and don't need all the speed, this car is more of a learning experience for the both of us.we read all of the articles that techinspecto1 shared with us and we will be following it one step at a time. How will I tell if my secondary. Is opening to soon? And what rpm should it idle at with this cam?
 

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Cars, Trucks, Boats, Motorcycl
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7,392 Posts
. If hear the secondaries open before about 1500 RPMs, prolly too low an RPM... disconnect the vacuum secondaries arm and see how it drives without them... may need #76 -#82 secondaries jets... front primary jets should prolly be about #68 - #74 in size... they're prolly currently smaller...

. Should warm idle around 700 RPMs... 600 in gear...
 

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1984 corvette
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33 Posts
Discussion Starter #16
Cfm:600
Fuel:Gasoline
New or Remanufactured:New
Number of Barrels:4
Carburetor Flange:Square bore
Choke:Electric
Secondary Type:Vacuum
Fuel Inlet:Dual
Primary Jet Size:67
Primary Power Valve (Hg):6.5 Hg
Primary Discharge Nozzle Size (in):0.035 in.
Secondary Jet Size:73
 

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1984 corvette
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33 Posts
Discussion Starter #18
Update.. I adjusted each valve one cylinder at a time on each firing order just as the article said. Base timing is set to 12 degrees, my secondarys. Are open around 1300 rpm and my warm idle is at 700 rpm, I check all plug wires. They are correct
It seems to run a lot better but it just does not turn up from a idle I have check my vacuum at idle it's at 20hg but its all over the scale from 15-25. Any ideas
 

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Cars, Trucks, Boats, Motorcycl
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7,392 Posts
. Did you increase the primary jet size?

. Vacuum sounds about normal for a 218/218 cam... bouncing around at lopey idle...

. Secondaries may be coming in at a tad low RPM... but whatever works...
 

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1984 corvette
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33 Posts
Discussion Starter #20
I could not find any Holley Jets at any of the local Parts store. But I have notice it runs a lot better with the electric choke was removed. Do you know what my red line should be ,, thanks for all ur help
 
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