Stumble
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Elder User
Joined: Oct 2016
Posts: 712
Likes: 1
From: Mesa, AZ
Stumble
Truck is a 77 351 2v with what appears to be a vacuum actuated choke? No wires....
Truck starts and idles wonderfully.
Has good power if I give it half throttle or less
When I stomp it down it falls on its face.....then chugs a bit, then finally goes.
I've tried different adjustments on the accelerator pump with no noticeable difference
I've searched for vacuum leaks and while I did find one, I repaired it. There are no more vacuum leaks that I can find.
Timing was set by PO's mechanic right before I bought the truck (for what that's worth).
Truck has OEM electronic ignition.
About 45k miles on engine.
Possible causes:
Fuel delivery / weak pump? Doesn't seem likely.
Accel Pump in carb bad? Hmmm
Where should timing be set? Are the timing marks easy to see on these?
What else could it be?
Truck starts and idles wonderfully.
Has good power if I give it half throttle or less
When I stomp it down it falls on its face.....then chugs a bit, then finally goes.
I've tried different adjustments on the accelerator pump with no noticeable difference
I've searched for vacuum leaks and while I did find one, I repaired it. There are no more vacuum leaks that I can find.
Timing was set by PO's mechanic right before I bought the truck (for what that's worth).
Truck has OEM electronic ignition.
About 45k miles on engine.
Possible causes:
Fuel delivery / weak pump? Doesn't seem likely.
Accel Pump in carb bad? Hmmm
Where should timing be set? Are the timing marks easy to see on these?
What else could it be?
Cargo Master
Joined: Nov 2011
Posts: 3,354
Likes: 48
From: Great American SouthWest
Truck is a 77 351 2v with what appears to be a vacuum actuated choke? No wires....
Truck starts and idles wonderfully.
Has good power if I give it half throttle or less
When I stomp it down it falls on its face.....then chugs a bit, then finally goes.
I've tried different adjustments on the accelerator pump with no noticeable difference
I've searched for vacuum leaks and while I did find one, I repaired it. There are no more vacuum leaks that I can find.
Timing was set by PO's mechanic right before I bought the truck (for what that's worth).
Truck has OEM electronic ignition.
About 45k miles on engine.
Possible causes:
Fuel delivery / weak pump? Doesn't seem likely.
Accel Pump in carb bad? Hmmm
Where should timing be set? Are the timing marks easy to see on these?
What else could it be?
Truck starts and idles wonderfully.
Has good power if I give it half throttle or less
When I stomp it down it falls on its face.....then chugs a bit, then finally goes.
I've tried different adjustments on the accelerator pump with no noticeable difference
I've searched for vacuum leaks and while I did find one, I repaired it. There are no more vacuum leaks that I can find.
Timing was set by PO's mechanic right before I bought the truck (for what that's worth).
Truck has OEM electronic ignition.
About 45k miles on engine.
Possible causes:
Fuel delivery / weak pump? Doesn't seem likely.
Accel Pump in carb bad? Hmmm
Where should timing be set? Are the timing marks easy to see on these?
What else could it be?
I don't believe there is a "vacuum-operated" choke for our trucks. There is a choke that runs off the heat stove, but if the choke is working, I wouldn't worry about that.
The timing marks "can" be easy to see - but not usually, especially under 40 years of rust! I cleaned up mine with a wire brush and carburetor cleaner, then painted over the mark with "white-out".
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
Initial timing should be ~ 12 degrees BTDC, presupposing the engine is stock.
Pull the distributor cap and make sure the vacuum advance plate is not bound up....in other words, it freely moves.
While you're in there, apply suction to the distributor's vacuum advance hose - pull it off the carb and apply suction to it. Observe the advance can's arm while you're doing this. You should see it move. If it doesn't move with suction or you can blow through the line, the advance can's diaphragm is pooched and it needs replacing.
Check the wires on the plate for brittleness and good connection points while you're in there, too.
Pull the distributor cap and make sure the vacuum advance plate is not bound up....in other words, it freely moves.
While you're in there, apply suction to the distributor's vacuum advance hose - pull it off the carb and apply suction to it. Observe the advance can's arm while you're doing this. You should see it move. If it doesn't move with suction or you can blow through the line, the advance can's diaphragm is pooched and it needs replacing.
Check the wires on the plate for brittleness and good connection points while you're in there, too.
Post Fiend
Joined: Feb 2008
Posts: 5,831
Likes: 121
From: St Albert, Alberta
Truck is a 77 351 2v with what appears to be a vacuum actuated choke? No wires....
Truck starts and idles wonderfully.
Has good power if I give it half throttle or less
When I stomp it down it falls on its face.....then chugs a bit, then finally goes.
I've tried different adjustments on the accelerator pump with no noticeable difference
I've searched for vacuum leaks and while I did find one, I repaired it. There are no more vacuum leaks that I can find.
Timing was set by PO's mechanic right before I bought the truck (for what that's worth).
Truck has OEM electronic ignition.
About 45k miles on engine.
Possible causes:
Fuel delivery / weak pump? Doesn't seem likely.
Accel Pump in carb bad? Hmmm
Where should timing be set? Are the timing marks easy to see on these?
What else could it be?
Truck starts and idles wonderfully.
Has good power if I give it half throttle or less
When I stomp it down it falls on its face.....then chugs a bit, then finally goes.
I've tried different adjustments on the accelerator pump with no noticeable difference
I've searched for vacuum leaks and while I did find one, I repaired it. There are no more vacuum leaks that I can find.
Timing was set by PO's mechanic right before I bought the truck (for what that's worth).
Truck has OEM electronic ignition.
About 45k miles on engine.
Possible causes:
Fuel delivery / weak pump? Doesn't seem likely.
Accel Pump in carb bad? Hmmm
Where should timing be set? Are the timing marks easy to see on these?
What else could it be?
Easy enough to do with the truck up to operating temp and shut off pull the air cleaner and look down the carb then open the throttle as quick as you can. You should have a good strong stream of fuel that lasts for a 2 or 3 seconds.
Thread Starter
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Elder User
Joined: Oct 2016
Posts: 712
Likes: 1
From: Mesa, AZ
I would check the accelerator pump first.
Easy enough to do with the truck up to operating temp and shut off pull the air cleaner and look down the carb then open the throttle as quick as you can. You should have a good strong stream of fuel that lasts for a 2 or 3 seconds.
Easy enough to do with the truck up to operating temp and shut off pull the air cleaner and look down the carb then open the throttle as quick as you can. You should have a good strong stream of fuel that lasts for a 2 or 3 seconds.
Was doing this Saturday. Seems to be good there. Revs quickly and freely when not under load.
I haven't pulled the distributor cap yet.
I thought I'd tracked down all vacuum leaks but kept at it on Saturday (used better than a half can of carb cleaner...LOL). Replaced the line to the PCV valve. That was splitting at the valve end. Added hose clamps to that line as an additional precaution.
Maybe fuel pressure is an issue.
Thread Starter
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Elder User
Joined: Oct 2016
Posts: 712
Likes: 1
From: Mesa, AZ
Initial timing should be ~ 12 degrees BTDC, presupposing the engine is stock.
Pull the distributor cap and make sure the vacuum advance plate is not bound up....in other words, it freely moves.
While you're in there, apply suction to the distributor's vacuum advance hose - pull it off the carb and apply suction to it. Observe the advance can's arm while you're doing this. You should see it move. If it doesn't move with suction or you can blow through the line, the advance can's diaphragm is pooched and it needs replacing.
Check the wires on the plate for brittleness and good connection points while you're in there, too.
Pull the distributor cap and make sure the vacuum advance plate is not bound up....in other words, it freely moves.
While you're in there, apply suction to the distributor's vacuum advance hose - pull it off the carb and apply suction to it. Observe the advance can's arm while you're doing this. You should see it move. If it doesn't move with suction or you can blow through the line, the advance can's diaphragm is pooched and it needs replacing.
Check the wires on the plate for brittleness and good connection points while you're in there, too.
that 12 degrees is at idle, yes? I'm idling smoothly at about 900rpm currently.
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
That 12 BTDC is "ball park".....pretty close, though. It actually should be what makes you and your engine happy.
900 RPMs at idle seems high. Do you get a "CHUNK" when put in gear at that idle? What's the RPMs in gear?
I'd like to see ~750-800 RPM at idle and when in "DRIVE" ~ 650-750 RPM. Can you get her to idle any lower without problems?
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Thread Starter
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Elder User
Joined: Oct 2016
Posts: 712
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From: Mesa, AZ
Yes, at idle, vacuum advance plugged at the carb and line when checking or timing her.
That 12 BTDC is "ball park".....pretty close, though. It actually should be what makes you and your engine happy.
900 RPMs at idle seems high. Do you get a "CHUNK" when put in gear at that idle? What's the RPMs in gear?
I'd like to see ~750-800 RPM at idle and when in "DRIVE" ~ 650-750 RPM. Can you get her to idle any lower without problems?
That 12 BTDC is "ball park".....pretty close, though. It actually should be what makes you and your engine happy.
900 RPMs at idle seems high. Do you get a "CHUNK" when put in gear at that idle? What's the RPMs in gear?
I'd like to see ~750-800 RPM at idle and when in "DRIVE" ~ 650-750 RPM. Can you get her to idle any lower without problems?
I have a manual, so no chunk from that, but there is some noise when I ease out the clutch at times. (Ujoints?) I haven't messed with the idle yet. The truck has aftermarket AC, and I suspect that's why the idle is a little higher to compensate when the compressor kicks in.
I may try to turn it down a smidge and see if it still runs ok.
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Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
OOOOPS! I thought with that high an idle you had a C6 fo' sho' 
.
In that case, 900 RPM at idle is waaaaay to high for a stick. You want to be around 650 at idle, when warm, and most things running.
An aftermarket A/C system shouldn't raise the idle a whole lot....especially with a stick.
Mebbe time for some carb tuning/checkup....
. In that case, 900 RPM at idle is waaaaay to high for a stick. You want to be around 650 at idle, when warm, and most things running.
An aftermarket A/C system shouldn't raise the idle a whole lot....especially with a stick.
Mebbe time for some carb tuning/checkup....
Thread Starter
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Elder User
Joined: Oct 2016
Posts: 712
Likes: 1
From: Mesa, AZ
OOOOPS! I thought with that high an idle you had a C6 fo' sho' .
In that case, 900 RPM at idle is waaaaay to high for a stick. You want to be around 650 at idle, when warm, and most things running.
An aftermarket A/C system shouldn't raise the idle a whole lot....especially with a stick.
Mebbe time for some carb tuning/checkup....
.In that case, 900 RPM at idle is waaaaay to high for a stick. You want to be around 650 at idle, when warm, and most things running.
An aftermarket A/C system shouldn't raise the idle a whole lot....especially with a stick.
Mebbe time for some carb tuning/checkup....
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
Ok. The "stumble"....have you adjusted the mixture using a vacuum gauge?
To do this - vehicle at normal operating temperature....vacuum gauge hooked to manifold vacuum....adjust the mixture screws one at a time, no more than a quarter turn each, waiting a bit between turns such that you obtain the highest vacuum reading in inches Hg. Reset the curb idle when done.
She'll be a tad rich but you'll have the best lean idle this way.
Have you checked what I suggested in post #3?
To do this - vehicle at normal operating temperature....vacuum gauge hooked to manifold vacuum....adjust the mixture screws one at a time, no more than a quarter turn each, waiting a bit between turns such that you obtain the highest vacuum reading in inches Hg. Reset the curb idle when done.
She'll be a tad rich but you'll have the best lean idle this way.
Have you checked what I suggested in post #3?
Thread Starter
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Elder User
Joined: Oct 2016
Posts: 712
Likes: 1
From: Mesa, AZ
Ok. The "stumble"....have you adjusted the mixture using a vacuum gauge?
To do this - vehicle at normal operating temperature....vacuum gauge hooked to manifold vacuum....adjust the mixture screws one at a time, no more than a quarter turn each, waiting a bit between turns such that you obtain the highest vacuum reading in inches Hg. Reset the curb idle when done.
She'll be a tad rich but you'll have the best lean idle this way.
Have you checked what I suggested in post #3?
To do this - vehicle at normal operating temperature....vacuum gauge hooked to manifold vacuum....adjust the mixture screws one at a time, no more than a quarter turn each, waiting a bit between turns such that you obtain the highest vacuum reading in inches Hg. Reset the curb idle when done.
She'll be a tad rich but you'll have the best lean idle this way.
Have you checked what I suggested in post #3?
I haven't yet. Saw all the replies starting yesterday and only have time on the weekends to do the tinkering. So will be out there Saturday morning trying your tips and tricks.
I'm taking notes though. Which vacuum line do you suggest I pull to check that vacuum?
Postmaster
Joined: Mar 2011
Posts: 2,698
Likes: 7
From: Taxachusetts
I'm taking notes though. Which vacuum line do you suggest I pull to check that vacuum?
For checking the distributor's vacuum advance can's diaphragm and breaker plate.... you can pull the line off the can's nipple and attach the vacuum gauge's line to it, or, pull the vacuum line off the ported nipple on the carb and use it. The engine doesn't have to be running to do this.
