big block won't rev
Thread Starter
Heel & Toe
Joined: Dec 2008
Posts: 17
Likes: 0
big block won't rev
I recently picked up a 70 LS5 4 speed. the motor is fresh, has headman headers, holley 750, transistor ignition. It was real slow when I got it, found it had a broken plug. I set the timing at six degrees, disconnected the TCS, it has no smog pump. The car will burn rubber like no tommorrow, however it does not seem to have the power it should on top, in fact it does not seem to want to rev past 4000 rpm, not that it cuts out or anything, just seems flat. I tried differnt secondary springs in the carb, but did not help much.
Thanks in advance for your help.
Thanks in advance for your help.
Team Owner
Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
You might just have a truck cam and an intake that's targeted at the lower RPM band. Maybe it was built for torque and not for winding up.
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Had a similar problem with my big block. It was a bad coil.
Also I would check your heat riser for proper operation. Check your float bowl levels. Might be running out of fuel at higher RPM's. Good luck.
Also I would check your heat riser for proper operation. Check your float bowl levels. Might be running out of fuel at higher RPM's. Good luck.
Safety Car
Joined: Oct 1999
Posts: 4,101
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From: NorCal
The fact that it has great torque and runs out of steam leads me to finger the cam and/or lower than stock compression.
Since it doesn't miss or hesitate when it stops pulling, check the total advance and see what your getting around 3500 rpm. If it is the cam or compression, you should be able to run quite a bit of timing and still not experience any detonation. Yuo have plenty of carb and exhaust on paper, and without knowing exactly which heads and pistons you have that my best guess for now. If you have access to a boroscope, you can verify the pistons, and the head numbers should fill in the picture a bit as well.
Hans
Since it doesn't miss or hesitate when it stops pulling, check the total advance and see what your getting around 3500 rpm. If it is the cam or compression, you should be able to run quite a bit of timing and still not experience any detonation. Yuo have plenty of carb and exhaust on paper, and without knowing exactly which heads and pistons you have that my best guess for now. If you have access to a boroscope, you can verify the pistons, and the head numbers should fill in the picture a bit as well.
Hans
Drifting
Joined: Apr 2008
Posts: 1,540
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From: Lexington Park Maryland
Put a high quality fuel pressure guage on it where it can be read when the motor starts to "fall on it's face"......, in other words, on the outside of the windshield in front of your face so you can read it when you're a WOT in high gear (not under the hood). Big blocks need alot of fuel. I have a mid 10 second '67 Nova I drag race that had a similar problem until I put a second Holley "blue" elec. pump on it. One pump just wasn't getting the job done.
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Melting Slicks
Joined: Nov 2001
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From: Eastern US XX
Also, make sure your dwell is set correctly (can cause symptoms you describe). 6 degrees timing is low - I agree with Wrencher - adjust TOTAL timing (in by around 2-2.5K).
Read Lars or Barry's papers for more specifics on timing.
Read Lars or Barry's papers for more specifics on timing.
Thread Starter
Heel & Toe
Joined: Dec 2008
Posts: 17
Likes: 0
The fact that it has great torque and runs out of steam leads me to finger the cam and/or lower than stock compression.
Since it doesn't miss or hesitate when it stops pulling, check the total advance and see what your getting around 3500 rpm. If it is the cam or compression, you should be able to run quite a bit of timing and still not experience any detonation. Yuo have plenty of carb and exhaust on paper, and without knowing exactly which heads and pistons you have that my best guess for now. If you have access to a boroscope, you can verify the pistons, and the head numbers should fill in the picture a bit as well.
Hans
Since it doesn't miss or hesitate when it stops pulling, check the total advance and see what your getting around 3500 rpm. If it is the cam or compression, you should be able to run quite a bit of timing and still not experience any detonation. Yuo have plenty of carb and exhaust on paper, and without knowing exactly which heads and pistons you have that my best guess for now. If you have access to a boroscope, you can verify the pistons, and the head numbers should fill in the picture a bit as well.
Hans
Thread Starter
Heel & Toe
Joined: Dec 2008
Posts: 17
Likes: 0
I thought of this also, it has the correct intake but I don't know about the cam, as I said in another reply the shop that did the motor for the previous owner said they did an exchange, so I thought maybe they put a truck motor in it, however the casting numbers are correct for the 70 LS5.
Team Owner
Joined: Jul 2004
Posts: 40,338
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From: Redondo Beach, California
Put a high quality fuel pressure guage on it where it can be read when the motor starts to "fall on it's face"......, I have a mid 10 second '67 Nova I drag race that had a similar problem until I put a second Holley "blue" elec. pump on it. One pump just wasn't getting the job done.
I'd be suspicious, among other things, of a lack of fuel pressure also. I've had a 69 and 70 BB. I've always been suspicious that the factory designed the cars to run out of fuel pressure at high loads/high rpms. A motive would be liability problems of the 1970's. No real substantive reason for this reason other than the engine dying off in power was consistent with a lack of fuel.
Thread Starter
Heel & Toe
Joined: Dec 2008
Posts: 17
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No dwell, the distributor looks like it has the electronic conversion kit I see in mid americas catalog.
Thread Starter
Heel & Toe
Joined: Dec 2008
Posts: 17
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I have the adjustable timing light, what should the timing be set at 3000 RPM?
Safety Car
Joined: Mar 2007
Posts: 3,652
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From: Bridgewater nj
Sasha, I have a different though for you to check out. If you have brutal low end torque and it runs strong when cold but it just hits a wall at @ 4000rpm. Your timing chain is off by 1 tooth. I had a worn timing chain jump and that sucker ran like a duramax on steriods up to 3000rpm than nothing. Good Luck
PS if it was a fuel pressure issue it would surge. IMHO
PS if it was a fuel pressure issue it would surge. IMHO
Last edited by hugie82; Jan 20, 2009 at 07:11 PM. Reason: miss spelling
