Good idle, smooth revving, sputter, then die

[Steeman01]

Okay, so I rebuilt my carbs and put them on, had trouble getting them to run well. So I took them off and redid the centralizing, and 2.25 turn out of each jet. Now I have the idle set around 900-1000 rpm and it idles nice, revs nice and smooth... Moderate revving, ki issue. High revving cause it to sputter and die when I let off.... Thoughts? I am stumped. Seems to be starved once you let off the throttle on high revs. Do I need to back out the main jets? Am I running too lean? Too rich and flooding the engine?

 

[beamdream]

Okay, so I rebuilt my carbs and put them on, had trouble getting them to run well. So I took them off and redid the centralizing, and 2.25 turn out of each jet. Now I have the idle set around 900-1000 rpm and it idles nice, revs nice and smooth... Moderate revving, ki issue. High revving cause it to sputter and die when I let off.... Thoughts? I am stumped. Seems to be starved once you let off the throttle on high revs. Do I need to back out the main jets? Am I running too lean? Too rich and flooding the engine?

Are we talking Strombergs ?

 

[Steeman01]

Sorry, yes, duel Strombergs.

 

[Gordon Holsinger]

Okay, so I rebuilt my carbs and put them on, had trouble getting them to run well. So I took them off and redid the centralizing, and 2.25 turn out of each jet. Now I have the idle set around 900-1000 rpm and it idles nice, revs nice and smooth... Moderate revving, ki issue. High revving cause it to sputter and die when I let off.... Thoughts? I am stumped. Seems to be starved once you let off the throttle on high revs. Do I need to back out the main jets? Am I running too lean? Too rich and flooding the engine?

Weak fuel pump or bad timing advance!

 

[RootesRooter]

Have you looked at the fuel bowl when the engine is sputtering while revving it? Is the fuel level staying up?

 

[Steeman01]

Thanks @Gordon Holsinger and @RootesRooter for the input. I think the fuel pump and timing is good, only because the other day I had it running okay (decent idle, but not ideal) and it revved flawlessly. However, I ended up pulling the carbs again, as my pops was messing with the main jet to a point where lost track of where we sat in reference to when it was centralized. So i reset those, mat note of where that is on each main jet (making a smal precise paint mark on the jet and the jet body (plus the gap on each was exactly 3mm from the crown of the jet screw head and the bottom of the jet housing). Anyways, I think it is a fuel/air mixture issue now.

Should there really be much need to adjust the main jet (once it is centralized and then each turned out 2.25 turns)?

 

[Gordon Holsinger]

Thanks @Gordon Holsinger and @RootesRooter for the input. I think the fuel pump and timing is good, only because the other day I had it running okay (decent idle, but not ideal) and it revved flawlessly. However, I ended up pulling the carbs again, as my pops was messing with the main jet to a point where lost track of where we sat in reference to when it was centralized. So i reset those, mat note of where that is on each main jet (making a smal precise paint mark on the jet and the jet body (plus the gap on each was exactly 3mm from the crown of the jet screw head and the bottom of the jet housing). Anyways, I think it is a fuel/air mixture issue now.

Should there really be much need to adjust the main jet (once it is centralized and then each turned out 2.25 turns)?

If all else is good the jets don’t require much attention I allways set the rear Stromberg a little richer to help cool the rear cylinders. One trick I have done when doing the initial jet drop I pull the dash pot covers and measure the jet drop.

 

[bernd_st]

Take off both air filters and check whether the pistons move freely and synchronously during idle and part throttle. With engine off you may lift the Pistons completely up by hand and let them loose at the same time. They should move down with the same speed and settle with a soft "clunk"...

 

[hartmandm]

Did you rebuild and calibrate your tachometer to get it accurate? If not, you probably will be setting idle speed incorrectly.

Check the spark plugs' colors to see if you are lean, rich, etc. Now that you are tracking the turns out, if the plugs look lean, turn out the jets in 1/4 turn increments and check the plug colors.

If you have the original 5M needles and are running ethanol, I doubt you will be able to get the fuel-air mixture rich enough. I had rebuilt carbs and I think I was at 4 turns out on the jets and still too lean. I swapped in 6J needles based on other folks experience on this forum. The 6J needles were originally designed for slightly larger engines, and those made a big difference for me.

Mike