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85 K15 chev silverado misses

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My grandson has a restored 1985 silverado p/u that has an HEI a 400 hp. small block with roller rockers/ dart heads etc. Just out of the blue it developed a miss/cuts out. What it does is he can put his brakes on and give it gas and it will turn the tires over although as he accelerates at appx. 45 mph it developes a miss/ cuts out. It also has been starting hard recently. It does have an electric choke and has power to it. I did check that and the butterfly on the top of the carb stays open when you go to start it in the cold. Does not close / choke it. Can it be this or a fuel pump? Need some ideas guys.... THX!

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The choke not closing seems like it could be the hard starting when cold problem, from what I recall about carbs.  

The question is what is making the choke not close, and could that also cause the miss?   Some sort of temp sensor thing? 

What keeps the choke open when starting warm?  

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   The electric choke dose not open and close according to engine temp.    The electric choke starts to warm up when you turn the ignition key on.  If you leave the key on but don't start the engine the choke will continue to warm and eventual open.  So a normal cold start with any carb regardless of choke type is to pump the peddle to get a shot of fuel from the accelerator pump.   If you had an automatic choke that peddle pump would set the choke too.   Now crank over the engine.

If your electric choke is open before you've put power to it it'll need to be adjusted.  Loosen the set screws at the round black cap and rotate the cap till the choke closes.  Snug the screws back up.   Chances are you'll have to go back and fine tune this along with the high idle speed.     I'm sure you can find that procedure on the net somewhere.      The miss and cut out at higher RPMs could be a lot of things but could be related to the choke and specifically a choke butterfly closing by the increased vacuum.  Thats contrary to a choke being pulled open by vacuum but I've seen it happen when the choke isn't right when under load and vacuum drops with throttle butterflies wide open.    At any rate adjust the choke to fix the cold start issue and that might clear up the cut out.   If your still bogging, wire the choke open after the engine has warmed and try it under load.

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I think ST is right on the vacuum part.

With some carbs, you have to maintain a certain amount of vacuum for it to operate right. Thus making it necessary to run some sort of airbox or a lid. It takes resistance on the filter side of the carb to pull the slide up. I am not sure how it works exactly but from what I can gather it has to do with velocity through the carb that pulls the vacuum to operate the slide etc.

Some carbs are just very finicky to how they pull in air. Not sure if this restoration would have changed any of that

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On ‎12‎/‎10‎/‎2017 at 9:50 AM, Surface Tension said:

   The electric choke dose not open and close according to engine temp.    The electric choke starts to warm up when you turn the ignition key on.  If you leave the key on but don't start the engine the choke will continue to warm and eventual open.  So a normal cold start with any carb regardless of choke type is to pump the peddle to get a shot of fuel from the accelerator pump.   If you had an automatic choke that peddle pump would set the choke too.   Now crank over the engine.

If your electric choke is open before you've put power to it it'll need to be adjusted.  Loosen the set screws at the round black cap and rotate the cap till the choke closes.  Snug the screws back up.   Chances are you'll have to go back and fine tune this along with the high idle speed.     I'm sure you can find that procedure on the net somewhere.      The miss and cut out at higher RPMs could be a lot of things but could be related to the choke and specifically a choke butterfly closing by the increased vacuum.  Thats contrary to a choke being pulled open by vacuum but I've seen it happen when the choke isn't right when under load and vacuum drops with throttle butterflies wide open.    At any rate adjust the choke to fix the cold start issue and that might clear up the cut out.   If your still bogging, wire the choke open after the engine has warmed and try it under load.

Tried wiring the choke open at operating temp and took for a drive. Still misses at appx 45 mph. Always seem to miss about 90% of the time at that speed although sometimes at around 30mph. Next question,,, fuel pump maybe or possibly the EGR VALVE. Dont have a way to test the fuel pump but thinking with the way its hard to start I was thinking possibly low fuel pressure. 

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Get the choke adjusted that should take care of the cold start issue.

A weak fuel pump usually starts to show up at high RPMs.   If left to idle the fuel pump fills the carb bowl, as you accelerate the weak pump can't keep up and the engine bogs and bucks.  A bad fuel filter will do the same thing and is often overlooked. You probably have 2 filters,  one on the gas line and one inside the inlet of the carb.  Replace them, don't clean them.   A  fuel pump pressure test would be the next thing to do.

  If your getting the bog throughout the power band then I'd suspect something electrical but it could show up just at higher RPMs or load as well.  Condition of plug wires, dist cap, rotor, pickup coil, and vacuum advance.  1st check all the vacuum lines for cracks and loose connections.  Something that old is bound to have bad lines.  While your at it clean the PCV vavle.  Google HEI distributor to test the pickup coil and ohm values.   Check the linkage to vacuum advance.  You can do a visual on the cap and rotor but its hard to find a hairline crack.   Keep us updated.

 

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6 hours ago, Walleyehooker said:

How hard has he been running this 400 HP rig? Might be as simple as a plug wire or might require a compression test. 

Went ahead and changed the fuel pump guys. The spring on the mechanical pump was gone. That solved the cut outs and will start now with a touch of the key. Thanks to all!

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