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Engine won't run under 2000 rpm

This is a discussion on Engine won't run under 2000 rpm within the Internal Engine forums, part of the LSx Technical Help Section category; Not sure if this should be posted here since it isn't a LS series engine but any help would be ...

  1. #1
    Junior Member kempfer99ta's Avatar
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    Engine won't run under 2000 rpm

    Not sure if this should be posted here since it isn't a LS series engine but any help would be great. I have a 421 SBC that was just built and I can't get to run under 2000 rpms. It has all forged internals a lunati cam with .480 lift and 230 degrees duration at .050, with 1.6 roller rockers. It has 9.8 to 1 compression vortec iron heads with the modification to accept more lift. The intake is an rpm air gap from edelbrock, and a 750 holley double pumper carb. The distributor is a DUI hei unit with 24 degrees advance. The car runs great at 2000 and up. The total timing is set at 35 degrees and is all in at 3000 rpm. At 2000 rpm there is 23 inches of vacuum. When I try to idle the engine down to normal 800-900 the engine cuts off just under 2000 rpm. I have tried 6.5, 4.5, and 3.5 power valves in the carb. If I set the idle speed screw to where the normal idle should be the engine just fires a few cylinders then quits. The reading on the vacuum guage when it is doing this is 10 inches of vacuum. I have tried the different power valves and adjusting the timing with no luck. Any help would be great. Thank you guys. I love the site

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    Member cuzimoto's Avatar
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    Are you sure about those cam specs. .480 sounds small for 230 duration @50. That duration on a small block won't give much vacuum at idle and would require high idle to keep running. Check for vacuum leaks also. Check that rockers arn't to tight also.

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    Junior Member kempfer99ta's Avatar
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    The cam specs are right. It is A Lunati Bracket Master II 292/292 part number 000110. I have done a compression test and all looked good. The 421 was put together by an engine builder. I have all vacuum ports plugged. I can't imagine where there would be a vacuum leak. I can see an idle around 1000 to 1200 but not 2000

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    Member cuzimoto's Avatar
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    A vacuum leak at the intake gasket to the head would cause nightmares in tuning and you would'nt see it. Smoke test best way to check for it although I know most people don't have one.

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    Junior Member kempfer99ta's Avatar
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    any other good way to check for that besides removing the intake

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    Member BKHgto04's Avatar
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    Quote Originally Posted by kempfer99ta View Post
    any other good way to check for that besides removing the intake
    I always used WD40 to find a leak. Spray it around the intake lip and if it gets sucked in you have found your leak. At idle it would normally rev up a little too,but with yours idleing at 2000 i dont know if it will idle up or not. I have also heard of people using ether to but i would think it might be too flammable. Good luck and i hope this helps.

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    Member cuzimoto's Avatar
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    Yeah, I've used brake clean butt with such a high idle it would be hard to notice the change in rpm and getting to ambitous with it can make things exciting

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    Junior Member kempfer99ta's Avatar
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    I was able to get it to idle somewhat yesterday. It was at about 500 rpm max, it didn't sound quite right. but I sprayed carb cleaner around the intake ports and at the base of the carb and it didn't change anything. I checked the timing it it was like 4 degrees after tdc, which makes no sense to me. When i tried to advance the timing the engine quit and when i tried to increse idle speed the engine would quit too.

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    Senior Member bigrondownhiller's Avatar
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    Do you have a vacuum advance or mechanical advance distributer? Make sure the advance is disconnected when doing the timing. What balancer are you using? What size carb are you using? What intake gaskets did you use with the manifold?

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    Member cuzimoto's Avatar
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    Hmmmmmmmmmm. Starting to accumulate theories. You said base timing was at 24 degrees butt at what rpm if you can't get it to idle under 2k. Mechanical advance will kick in before 2K. Also you could have a bad idle circuit in the carb. Would be nice if you had access to a differant carb for testing purposes.

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    Junior Member kempfer99ta's Avatar
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    The distributor has a vacuum advance, which is not hoooked up. The caskets that are used are the felpro gaskets for the vortec heads, the plastic kind with the silicone inbedded in them . The carb is a 750 holley double pumper, brand new, I was using a 650 speed demon that I had from my 350 to get it going but was having the same issue, thought it was the carb so I bought a new one. The engine builder balanced the engine internally and used a balancer from a 283 due to the engine being internally balanced. The distributor has 24 degrees of mechanical advance in it, I should have been more clear about this. I had set the total timing at 35 degrees at 3000 rpm. I have already tried the different carb thing and didn't help.

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    Senior Member bigrondownhiller's Avatar
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    The reason I mentioned the balancer is if it was an old one the ring could have spun on the hub. This will give you wrong timing readings.

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    Member cuzimoto's Avatar
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    I'm thinking

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    Junior Member kempfer99ta's Avatar
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    I checked to make sure the mark on the balancer and the mark on the timing timed lined up properly, they do, the rotor on the distributor is pointing at the 4 o clock position. Any thoughts.

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    Senior Member bigrondownhiller's Avatar
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    Quote Originally Posted by kempfer99ta View Post
    I checked to make sure the mark on the balancer and the mark on the timing timed lined up properly, they do, the rotor on the distributor is pointing at the 4 o clock position. Any thoughts.
    Sounds right just as long as at 4 o'clock is the number 1 plug wire. reason I said this is if it was an old balancer the ring on the outside could have spun on the hub. If it did then when you think #1 is at tdc it won't be. Make sure you have your firing order right too.

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    Junior Member kempfer99ta's Avatar
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    I am going to triple check everything this weekend

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    Junior Member kempfer99ta's Avatar
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    Everything is correct. I had the engine builder over on Monday to take a peak. Come to find out the plugs were fouled again. We are going to try to get to it this weekend He suggested letting the cylinders air out as well and with new plugs we will give it a shot again.

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    Senior Member JaycenK's Avatar
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    You sure that carb is good? Years ago My father and I had 55 F-100 with a built 454 with what was supposed to be a mild cam in it and that thing would not idle to save it's life. It would spit and sputter and not run untill it got warmed up all the way to temp. You would have to keep it up above 1600 untill it got warm. We thought old John (the engine builder) F'd up and put a real hot cam in it. One day we took the carb off of the road race car and dropped it on the 454 and that thing started cold with no choke and the idle was perfect when it was 30 deg out side. See even though the other carb ran fine once it was warmed up, for some reason that damn thing would not idle cold and even after it got warmed up the idle was rough. We went round and round on timing for months on that thing as well as monifold gaskets. By the way the carb that would not idle, It had just been rebuilt by a guy that had done 7 or 8 other carbs for my dad and I over the years. Might want to queston that thing if you can't come up with anything else that makes sense.

  19. #19
    Junior Member kempfer99ta's Avatar
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    I thought it was the carb as well. I had bought a 650 speed demon with electric choke and vacuum secondaries for the 350 that use to be in the car. It sat for about a year while the 421 was being built. This was only on the 350 for a few months. I wasn't planning on buying a new carb since this one was fairly new. Once I got the 421 in the car and everything hooked up and primed the engine I fired it and had the idle set high so it would be close to 2000 for the initial break in. after running the engine for about 20-25 minutes as 2000 with the engine at a steady 170 degrees I went it idle it down and it cut off right around 1800. So i got it running again at 2000 brought it up to 3000 set the timing at 35 degrees and made sure it was all in at that rpm I tried to idle it down again and the same thing happened. Thought the idle circuit may have had stuff in it. took the carb apart cleaned everything and the same thing happened. took it apart again to make sure I had all the gaskets on correct and tried again. same thing. bought the new holley 750 double pumper with manual choke and it does the same thing. tried lowering the power valve incase the low vacuum at idle was opening the power vavle. and the same thing happens. but above 2000 it sounds great and crisp. The spark plugs are black. When I try to fired where the idle speed is for idle and it only fires a few cylinder and then cuts off. After a few tries of this the plugs get fouled.


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