Engine stumble at highway cruise?
07-21-2018, 01:51 AM
#1
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Thread Starter
Engine stumble at highway cruise?
Hi everyone, it has been a long while since I posted! Overall, my car has been very good and is in the final (cosmetic) stages of restoration....
Currently, when cruising at any speed 65 MPH and over which equates to 3500 RPM and up, the engine hesitates/stumbles with both audible and physical attributes. The car feels like it is shaking back/forth almost like the engine stops running and restarts but very rapidly and constantly. If you go below 65, the issues are non existent, and if you go WOT itll pull fine to 100 no problem as if there was no issue. Cold starts fine, hot starts are normal Holley nonsense. Idles fine, drives fine at any speed lower than 65. Any thoughts?
-Will
Currently, when cruising at any speed 65 MPH and over which equates to 3500 RPM and up, the engine hesitates/stumbles with both audible and physical attributes. The car feels like it is shaking back/forth almost like the engine stops running and restarts but very rapidly and constantly. If you go below 65, the issues are non existent, and if you go WOT itll pull fine to 100 no problem as if there was no issue. Cold starts fine, hot starts are normal Holley nonsense. Idles fine, drives fine at any speed lower than 65. Any thoughts?
-Will
07-21-2018, 09:40 AM
#3
Le Mans Master
Member Since: Aug 2017
Location: Cool Northern Michigan
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So basically you have a "stumble" in the middle of the RPM range. Isn't that about when the secondaries are all ready beginning to open? Vac or mech?
I hear you about the Holley hot starts, just part of the game some days. But I would question the secondary side of your Holley:
Secondary:
Float level.
Squirters and pump if mech sec..
Spring adjustment if Vac sec.
Secondary throttle blades actually opening or you just think they are?
Maybe passages blocked in metering block?
I hear you about the Holley hot starts, just part of the game some days. But I would question the secondary side of your Holley:
Secondary:
Float level.
Squirters and pump if mech sec..
Spring adjustment if Vac sec.
Secondary throttle blades actually opening or you just think they are?
Maybe passages blocked in metering block?
Last edited by HeadsU.P.; 07-25-2018 at 06:22 AM.
07-21-2018, 02:03 PM
#4
Le Mans Master
I would look into the timing as well. By 3500 rpm you're likely fully advanced mechanically and when in cruise now it's fully advanced on the vacuum advance. So you may be running too much timing.
07-22-2018, 01:50 PM
#6
Le Mans Master
07-22-2018, 02:00 PM
#7
Burning Brakes
VS
07-22-2018, 02:17 PM
#8
Drifting
Just offering alternative possibilities.. Have you not seen pumps that will pump long enough to get you to speed but lose pressure under sustained steady driving? that's why I suggested checking pressure at 3500 rpm where his engine dying is.. but , no doubt you have it solved for him .. no need for other ideas...Its simple enough to check vacuum canister...UNPLUG it, drive 3500rpm. if problem goes away, that's it...if not...back to a fuel issue
Last edited by fishslayer143; 07-22-2018 at 02:46 PM. Reason: spelling
07-22-2018, 04:30 PM
#9
Le Mans Master
Just offering alternative possibilities.. Have you not seen pumps that will pump long enough to get you to speed but lose pressure under sustained steady driving? that's why I suggested checking pressure at 3500 rpm where his engine dying is.. but , no doubt you have it solved for him .. no need for other ideas...Its simple enough to check vacuum canister...UNPLUG it, drive 3500rpm. if problem goes away, that's it...if not...back to a fuel issue
07-22-2018, 04:51 PM
#10
Le Mans Master
Member Since: Aug 2017
Location: Cool Northern Michigan
Posts: 6,879
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To the owner:
Would you happen to know what color coded springs are under the dizzy rotor? Light? Medium? Heavy?
Reelav8R brought up a good point that at 3500 RPMs the weights are LIKELY, (and that being the key word here) in fully advanced position.
If the springs or even the weights have any sticky-ness to them they will hesitate at middle cruising speeds. At WOT they will fly out without effort and will return at idle. But at med throttle, who knows where the advance is? Is it sticking?
You could pull the rotor and glance at the advance. What springs are there? Smear a little White Lithium grease under the weights. Install rotor and see if it snaps into position after advancing it by hand. It should release quickly and easily EVERY time.
Would you happen to know what color coded springs are under the dizzy rotor? Light? Medium? Heavy?
Reelav8R brought up a good point that at 3500 RPMs the weights are LIKELY, (and that being the key word here) in fully advanced position.
If the springs or even the weights have any sticky-ness to them they will hesitate at middle cruising speeds. At WOT they will fly out without effort and will return at idle. But at med throttle, who knows where the advance is? Is it sticking?
You could pull the rotor and glance at the advance. What springs are there? Smear a little White Lithium grease under the weights. Install rotor and see if it snaps into position after advancing it by hand. It should release quickly and easily EVERY time.
Last edited by HeadsU.P.; 07-25-2018 at 06:23 AM.
07-22-2018, 06:47 PM
#11
Pro
Thread Starter
07-22-2018, 06:48 PM
#12
Pro
Thread Starter
To the owner:
Would you happen to know what color coded springs are under the dizzy rotor? Light? Medium? Heavy?
Reelav8R brought up a good point that at 3500 RPMs the weights are LIKELY, (and that being the key word here) in fully advanced position.
If the springs or even the weights have any sticky-ness to them they will hesitate at middle cruising speeds. At WOT they will fly out without effort and will return at idle. But at med throttle, who knows where the advance is? Is it sticking?
You could pull the rotor and glance at the advance. What springs are there? Smear a little White Lithium grease under the weights. Install rotor and see if it snaps into position after advancing it by hand. It should release quickly and easily EVERY time.
Would you happen to know what color coded springs are under the dizzy rotor? Light? Medium? Heavy?
Reelav8R brought up a good point that at 3500 RPMs the weights are LIKELY, (and that being the key word here) in fully advanced position.
If the springs or even the weights have any sticky-ness to them they will hesitate at middle cruising speeds. At WOT they will fly out without effort and will return at idle. But at med throttle, who knows where the advance is? Is it sticking?
You could pull the rotor and glance at the advance. What springs are there? Smear a little White Lithium grease under the weights. Install rotor and see if it snaps into position after advancing it by hand. It should release quickly and easily EVERY time.
-Will
07-22-2018, 06:49 PM
#13
Pro
Thread Starter
07-22-2018, 06:50 PM
#14
Pro
Thread Starter
Just offering alternative possibilities.. Have you not seen pumps that will pump long enough to get you to speed but lose pressure under sustained steady driving? that's why I suggested checking pressure at 3500 rpm where his engine dying is.. but , no doubt you have it solved for him .. no need for other ideas...Its simple enough to check vacuum canister...UNPLUG it, drive 3500rpm. if problem goes away, that's it...if not...back to a fuel issue
-Will
07-22-2018, 06:52 PM
#15
Pro
Thread Starter
Having read Lars paper on timing a few dozen times I would think it possible you are getting too much vacuum advance, like reelav8r suggests. Perhaps a different vacuum can is in order. Classic symptoms while at cruising speed are: feels like a bit of a jerk, no power, then everything is back to normal.
VS
VS
-Will
07-22-2018, 06:54 PM
#16
Pro
Thread Starter
So a summary of suggestions - could be vacuum advance or vacuum secondary issue? I will try eliminating the vacuum advance and see if that changes anything.
-Will
-Will
07-23-2018, 01:29 PM
#18
Le Mans Master
The thing I find most concerning about your description is that it shakes and stumbles enough that you can hear AND feel it. It sounds like it could be as bad as pre-ignition. Usually pre-ignition is preceded by detonation, however it's possible that the detonation may not be audible to you or the sound you are hearing IS detonation. It often sounds like a small hammer on metal or like small rocks being shaken in a can.
Lars describes excessive ignition lead at cruise as "trailer hitching". It jerks back and forth like your towing a trailer.
Too much ignition lead and the fuel/air charge is igniting and burning too early creating lots of positive pressure for the rising piston to fight still in the combustion stroke phase.
Pre-ignition is a source of ignition that ignites the fuel/air charge even earlier than the ignition spark creating destructive pressure and heat conditions while the piston is still trying to get near TDC on the combustion stroke.
Once you go to WOT the manifold vacuum drops off and so does you vacuum advance timing, leaving you only with initial and mechanical advance.
Yes, you understand correctly.
Lars describes excessive ignition lead at cruise as "trailer hitching". It jerks back and forth like your towing a trailer.
Too much ignition lead and the fuel/air charge is igniting and burning too early creating lots of positive pressure for the rising piston to fight still in the combustion stroke phase.
Pre-ignition is a source of ignition that ignites the fuel/air charge even earlier than the ignition spark creating destructive pressure and heat conditions while the piston is still trying to get near TDC on the combustion stroke.
Once you go to WOT the manifold vacuum drops off and so does you vacuum advance timing, leaving you only with initial and mechanical advance.
Oh wait this does make sense. Pulls no problem if I stay in it, has no problem pulling to 5500 RPM
-Will
-Will
Last edited by REELAV8R; 07-23-2018 at 01:33 PM.
07-24-2018, 11:57 AM
#19
Pro
Thread Starter
07-24-2018, 11:59 AM
#20
Pro
Thread Starter
The thing I find most concerning about your description is that it shakes and stumbles enough that you can hear AND feel it. It sounds like it could be as bad as pre-ignition. Usually pre-ignition is preceded by detonation, however it's possible that the detonation may not be audible to you or the sound you are hearing IS detonation. It often sounds like a small hammer on metal or like small rocks being shaken in a can.
Lars describes excessive ignition lead at cruise as "trailer hitching". It jerks back and forth like your towing a trailer.
Too much ignition lead and the fuel/air charge is igniting and burning too early creating lots of positive pressure for the rising piston to fight still in the combustion stroke phase.
Pre-ignition is a source of ignition that ignites the fuel/air charge even earlier than the ignition spark creating destructive pressure and heat conditions while the piston is still trying to get near TDC on the combustion stroke.
Once you go to WOT the manifold vacuum drops off and so does you vacuum advance timing, leaving you only with initial and mechanical advance.
Yes, you understand correctly.
Lars describes excessive ignition lead at cruise as "trailer hitching". It jerks back and forth like your towing a trailer.
Too much ignition lead and the fuel/air charge is igniting and burning too early creating lots of positive pressure for the rising piston to fight still in the combustion stroke phase.
Pre-ignition is a source of ignition that ignites the fuel/air charge even earlier than the ignition spark creating destructive pressure and heat conditions while the piston is still trying to get near TDC on the combustion stroke.
Once you go to WOT the manifold vacuum drops off and so does you vacuum advance timing, leaving you only with initial and mechanical advance.
Yes, you understand correctly.
I suppose that means I have too much timing....
-Will