400 run-on
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400 run-on
I have a brand new 400. 362 HP 472 TQ. Timing is at 12 BTDC adv curve around 34 at 2500 RPM. I am running 90 eth free and 93 at a 50/50 mix. I am using a brand new Holley 80670. I am using double throttle springs but the idle changes sometimes it's 950 RPM, where I want it. Sometimes it drops to 600,650 where it's to low. linkage is tight not sure what to think here. 1 in 10 shut downs it runs on. Any thought's ?
Grumpy goat
Joined: May 2017
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I'd suspect that high idle speed. That's a lot! Btw, I sure hope you're not running that high idle with an automatic transmission that has a stock type torque converter. Do you have a camshaft that requires you to idle that high in order to stay running?
That aside, if it's not running hot, bad fuel and timing are my first thoughts. I only run non-ethanol fuel also. But I don't trust the octane ratings at the stations. So I run 87 octane fuel and I add Klotz every time I fuel up. I would try that first. If that doesn't stop it, then maybe your carburetor is still dropping fuel after shut off.
Causes after those above are probably more serious. Hot spots/carbon deposits in the combustion chamber, intake valves not sealing....
That aside, if it's not running hot, bad fuel and timing are my first thoughts. I only run non-ethanol fuel also. But I don't trust the octane ratings at the stations. So I run 87 octane fuel and I add Klotz every time I fuel up. I would try that first. If that doesn't stop it, then maybe your carburetor is still dropping fuel after shut off.
Causes after those above are probably more serious. Hot spots/carbon deposits in the combustion chamber, intake valves not sealing....
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It doesn't run hot. Normally around 180. It seems to idle best at 900. Seems high but any thing less than 800 its's really rough. Using a Hughes 2500 stall. I think it's something with the carb as the idle changes. I'm just not sure how to check this. It should not have this swing in idle settings.
Lead Driver
Joined: Oct 2018
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Sounds like maybe you're hooked to manifold vacuum on your advance. manifold is best but only if your distributor is set up for it and none are anymore.
It could also be your throttle plates are tight and stick sometimes, my new 4160 was doing this at first. sometimes it idled at 1800.
The dieseling could be because your plates are open too far, they need to be open a lot or none. it could also be you need more octane or your idle mix is too rich .
It could also be your throttle plates are tight and stick sometimes, my new 4160 was doing this at first. sometimes it idled at 1800.
The dieseling could be because your plates are open too far, they need to be open a lot or none. it could also be you need more octane or your idle mix is too rich .
Lead Driver
Joined: Oct 2018
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Mine quickly wore in but some guys pull the plates out and sand them for a better fit. seldom have I heard of this needing to be done.
Before you get to carried away make sure that's the problem, the other possibilities can cause a high idle even if your plates are closed properly.
Are you sure your high idle cam is fully backed off ? sure you have no vacuum leaks ?
Before you get to carried away make sure that's the problem, the other possibilities can cause a high idle even if your plates are closed properly.
Are you sure your high idle cam is fully backed off ? sure you have no vacuum leaks ?
More Turbo
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From: Oregon
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vacuum is steady at 17 psi, 900 rpm. Snap the throttle open, 17 down to 0. You must remember it only runs-on 1 of 10 times. it's not every time. Start truck it idles at 900. Come back 3hrs later, start truck it idle's at 650 drive truck a little and it straightens up. I can lay 15 ft patches down my driveway.
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Yes, that seems to be when it happens. Run into town 8 miles shut it off acts normal. Start it up after a few minutes in the store, run back home 8 miles shut it off and sometimes it runs on for a second or two. It does not do it every time. I can drive 35 miles to the hardware store and it's fine. I did notice that the High idle/choke operates after it has sat for a few minutes with the key off. Engine temp is 180 and the choke operates as soon as I turn the key. Idles at 1500 wait a few seconds, kick throttle and it drops back to regular idle
Lead Driver
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With it idling slowly cover the air horn with a rag and see if it speeds up or dies. if closing the plates more makes it worse that sounds like a vacuum leak.
Vacuum leaks don't effect top end much because at WOT you have no vacuum anyway. idle is where it shows up worst.
Vacuum leaks don't effect top end much because at WOT you have no vacuum anyway. idle is where it shows up worst.
Post Fiend
Joined: Apr 2002
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From: Waterloo, Iowa
Here's an idle mixture setting routine that should work good, make sure transition slot is not over-exposed:
"Here, some idle-speed experimentation is worthwhile. Try adjusting the RPM to the point the engine is just short of stalling. When you have established this RPM, set the idle speed to about 100 rpm higher. Continue progressively adjusting the mixture screws in until you achieve the highest manifold vacuum. As the mixture nears optimal, you may need to readjust the idle speed again, as an optimal mixture also allows a lower, stable idle RPM. If the idle mixture screws make little difference to the idle from the full-in to the full-out position, it’s a fair bet that the butterflies are too far into the transition slot. If the idle jetting is okay, the idle mixture screws should ideally be about one turn out, although 1/2 to 11 ⁄2 turns is acceptable."
(From David Vizard's "How to Tune and Super Modify Holley Carburetors.")
"Here, some idle-speed experimentation is worthwhile. Try adjusting the RPM to the point the engine is just short of stalling. When you have established this RPM, set the idle speed to about 100 rpm higher. Continue progressively adjusting the mixture screws in until you achieve the highest manifold vacuum. As the mixture nears optimal, you may need to readjust the idle speed again, as an optimal mixture also allows a lower, stable idle RPM. If the idle mixture screws make little difference to the idle from the full-in to the full-out position, it’s a fair bet that the butterflies are too far into the transition slot. If the idle jetting is okay, the idle mixture screws should ideally be about one turn out, although 1/2 to 11 ⁄2 turns is acceptable."
(From David Vizard's "How to Tune and Super Modify Holley Carburetors.")