Well the car is falling on its face so rpm is very low but i cant get an exact number.
Edelbrock 1407 off idle bog
- Thread starter Baker7888
- Start date
You need to base line.
For now just do some easy stuff. Unplug line to distributor and plug it off.
Drive it and see if different.
For now just do some easy stuff. Unplug line to distributor and plug it off.
Drive it and see if different.
Just unplug vac advance and drive it ??
I’ll try it and get back to you. What is your theory here?
Driving with part choke on (mechanical choke knob remember) improved drivability and completely got rid of the off idle stumble. To me, this tells me I am lean across the board.
Hopefully my jets and rods will be here in the next day or two.
There are a lot of threads online about this exact issue but nobody seems to find a solution. Lots of focus on the accelerator pump. My shot is instant (as described that i should verify this) but seems to hit the primary butterflies as they are basically not even open yet. Almost like the shot is timed wrong in my opinion. My measurement was 1/2” plunger at rest which supposedly is correct. Some explain the linkage settings are volume versus longer duration. (I think top hole is volume and bottom hole is duration). I don’t see a hell of a difference either way if the fuel is hitting the butterflies. Just a thought. I was hoping to find a slack in the linkage but it was spot on, according to everything i have read ….
Same problem I ran into in the early days.
Running bad ... You need new carburetor...
Running bad ... You need new distributor ...
Can't fix it... We don't know what's wrong ... You need new car...
No you don't. You need to find root cause of it running bad.
Runnability and drivability is the number one cause of customer complaints. So as a technician it is the number one thing to master.
Everything is a system and you have to understand the system.
If you have 10:1 pistons and good rings, proper compression that is your starting point.
18 degrees on 10:1 is too much, then you add full vacuum advance and now, it is way too much. The engine is fighting itself. Don't get too hung up on 36 all in at 3000. That is easy to do with proper setup of springs and weights.
Yours is off idle stumble so get the idle right, then transition to cruise and then acceleration to WOT.
Timing will be smooth all ranges and stumble goes away.
Running bad ... You need new carburetor...
Running bad ... You need new distributor ...
Can't fix it... We don't know what's wrong ... You need new car...
No you don't. You need to find root cause of it running bad.
Runnability and drivability is the number one cause of customer complaints. So as a technician it is the number one thing to master.
Everything is a system and you have to understand the system.
If you have 10:1 pistons and good rings, proper compression that is your starting point.
18 degrees on 10:1 is too much, then you add full vacuum advance and now, it is way too much. The engine is fighting itself. Don't get too hung up on 36 all in at 3000. That is easy to do with proper setup of springs and weights.
Yours is off idle stumble so get the idle right, then transition to cruise and then acceleration to WOT.
Timing will be smooth all ranges and stumble goes away.
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The only issue I have with lowering the timing is, this past spring when i started road testing the car. I started out around 10-12* initial, the distributor out of the box gave something like 24* mechanical at 5000 rpm. I dont remember exactly. But the car was a dog. I am almost certain the off idle bog was still there. I was more focused on other things. Dropping the timing is easy and i will try hopefully tonight. Was raining last night. I certainly
don’t mind ruling it out.
Let me ask you if I may. Where do you think timing SHOULD be at on this 10.4:1 LT1
Back to that distributor for a minute or so here. Have you ever popped the cap on it and taken a peek at the weights and springs it came with? This is a very old school subject but back when Sun offered a line of distributor service machines it was possible to take a timer and set it up out of the car using the Sun unit to check and adjust dwell, gap, and advance.
So, I guess my first question ought to be, When you acquired that distributor, did it come with a tuning kit to swap out the weights and springs? Why I ask is that, with a pair of light advance weights and a set of "heavy" advance springs, the timing is not going to advance until almost WOT. If your stumble is not the carburetor but the when and where the advance start to open, then you may need to be looking at keeping the existing weights but changing out the springs to lighter ones so the advance comes in sooner. You could possibly go with a lighter weight set that takes less centrifugual force to open but then you would have to swap in lighter springs to let them move more quickly as well.
There are weight and spring kits available, Moroso for starters, but the thing with them is that they are most valuable for the varying tension of the springs that come in them. The weights and center cam, when compared to the factory versions, are more shapeless and lack the finished shape of what the factory versions possess.
Nick
So, I guess my first question ought to be, When you acquired that distributor, did it come with a tuning kit to swap out the weights and springs? Why I ask is that, with a pair of light advance weights and a set of "heavy" advance springs, the timing is not going to advance until almost WOT. If your stumble is not the carburetor but the when and where the advance start to open, then you may need to be looking at keeping the existing weights but changing out the springs to lighter ones so the advance comes in sooner. You could possibly go with a lighter weight set that takes less centrifugual force to open but then you would have to swap in lighter springs to let them move more quickly as well.
There are weight and spring kits available, Moroso for starters, but the thing with them is that they are most valuable for the varying tension of the springs that come in them. The weights and center cam, when compared to the factory versions, are more shapeless and lack the finished shape of what the factory versions possess.
Nick
Thanks Nick. Yes I recurved the distributor. Timing is 18 degrees base 16 mechanical all in at 3000-3200. Vac can ran to manifold port. I am not sure how this would cause a stumble on takeoff (which disappears when using about 1/2 mechanical choke - i have a mechanical choke that is cable operated to a choke knob) but runs great everywhere else. Timing keeps coming up in this thread, with the suggestion to retard it even more. I did mention last night that i’d give it a shot but i didn’t get home till late today. I did run the distributor briefly last spring (first road tests) “out of the box” at something like, 12 degrees base, 24 mechanical all in at like 4800+. Ran like a dog. Still had the same stumble.
Sorry didn't get back to this sooner. Unfortunately I don't have a project to show what I'm talking about. The caprice is TBI in process of going to carb, every thing else I have is Olds 307 CCC or EFI, and nothing in the 10:1 range, the 327 is still in pieces and only 9.2 at best.
You got some issues and still not sure of true cause, plus you said you got 16 inch at idle then 25 as RPM increased?? That is definitely not right.
You might have hit on one possibility and that is being over carbed. If you run the CFM calculator, you are nowhere near 750 (1407), you are much closer to 600 which is the 1405 or 1406.
One thing I see, and did some reading and talked to few folks is that Edelbrock carb is notorious for off idle lean stumble.
On the timing issue which actually is more related to dynamic compression, it sounds like base compression wet and dry results is still an unknown. That's an easy fix just by doing a compression test series.
Basically you are chasing your tail and it is getting very frustrating. At some point it's time to stop the merry-go-round and get off.
Basically you have to know what you have. Find the problem, fix it, and back to happy motoring.
You didn't build the engine so you are at the mercy of what you have been told.
So, "Trust but Verify" is very much at play here... Is it actually 10:1? How did they set the cam? Straight up, or 4° advance? ? Or 4° Retard. Is Zero TDC actually Zero?? Or just "close"?? If so, how close??
You know that choking it down gets the bog to go away. This eludes being lean and street talk is the Edelbrock is notorious for this. So at least that's a start point.
So now puzzle pieces... Do you have a Mityvac? Have you checked to make sure the vac can on distributor is working properly, is it smooth and doesn't stick? Verify brake booster doesn't have a leak? Verify the line going to vac can is actually drawing vacuum? Verify intake manifold or carb mounting doesn't have a leak? Tested the module? Check distributor shaft for wobble (excessive run out)? All good plugs, wires etc??
Sounds like a lot, but just do one thing at a time and do simple first. I think you already answered most of the above.
Seems to me maybe richer primary jets might help, still need to check timing, and possibly got to 600 or 650 carb instead of 750.
You got some issues and still not sure of true cause, plus you said you got 16 inch at idle then 25 as RPM increased?? That is definitely not right.
You might have hit on one possibility and that is being over carbed. If you run the CFM calculator, you are nowhere near 750 (1407), you are much closer to 600 which is the 1405 or 1406.
One thing I see, and did some reading and talked to few folks is that Edelbrock carb is notorious for off idle lean stumble.
On the timing issue which actually is more related to dynamic compression, it sounds like base compression wet and dry results is still an unknown. That's an easy fix just by doing a compression test series.
Basically you are chasing your tail and it is getting very frustrating. At some point it's time to stop the merry-go-round and get off.
Basically you have to know what you have. Find the problem, fix it, and back to happy motoring.
You didn't build the engine so you are at the mercy of what you have been told.
So, "Trust but Verify" is very much at play here... Is it actually 10:1? How did they set the cam? Straight up, or 4° advance? ? Or 4° Retard. Is Zero TDC actually Zero?? Or just "close"?? If so, how close??
You know that choking it down gets the bog to go away. This eludes being lean and street talk is the Edelbrock is notorious for this. So at least that's a start point.
So now puzzle pieces... Do you have a Mityvac? Have you checked to make sure the vac can on distributor is working properly, is it smooth and doesn't stick? Verify brake booster doesn't have a leak? Verify the line going to vac can is actually drawing vacuum? Verify intake manifold or carb mounting doesn't have a leak? Tested the module? Check distributor shaft for wobble (excessive run out)? All good plugs, wires etc??
Sounds like a lot, but just do one thing at a time and do simple first. I think you already answered most of the above.
Seems to me maybe richer primary jets might help, still need to check timing, and possibly got to 600 or 650 carb instead of 750.
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