I didnt build the engine. It’s used. But i did do a leakdown and also compression. All seemed well and i proceeded with a complete gasket job. I pulled the heads and had good cross hatching on cylinder walls. I did throw a cam and springs at it. And removed efi in favor of a carb setup.
Edelbrock 1407 off idle bog
- Thread starter Baker7888
- Start date
On a basic OEM street car you are 125 to 150 psi. On high compression you are 175 to sometimes over 210 psi.
When you have timing coming in too soon you are actually forcing the engine to fight the compression you build.
When emissions took over in the late 70s... Early 80s, one of the solutions from OEM was to lean out and increase timing to 14°. That's fine when you are at 8.5 to 9.2. It is not fine when you are at 10 or more. Look up the old specs from the 60s when factory made 10 to 1 cars. You will see the specs were zero to 4° BTDC as initial timing at 800 to 950 curb idle.
You see this when you build and engine and set initial mechanical timing. You want the flame front to just start as piston moves up, compresses and moves down. On high compression engine you can't have too much timing because it will build pressure way too much and too soon and starts to push the piston down before it gets to the end of it's upward stroke.
On top of that you have distributor hooked to manifold vacuum.
If your cam has enough overlap and base vacuum is around 12 which would be common for LSA 104 to 106 you might be ok. But with an LSA of 114 and if you are around 20 inches of vacuum, it's way too much.
Vacuum will pull your plate forward, full travel immediately after start up. For sake of example... If full travel is 12 and you add your 18, that is 30 and everything I've seen that is way too much.
You really need to see what 30 looks like to the piston in the bore. The piston has too move up 30 degrees of travel with flame on top of it and expanding and pushing down. Now imagine if full travel is 20. You are asking the engine to run idle at 38°BTC???
Now when you go off idle vacuum reduces, plate returns and timing drops. That's going the wrong direction, then light springs and heavy weights increases timing as RPM goes up.
You see the roller coaster here?
When you have timing coming in too soon you are actually forcing the engine to fight the compression you build.
When emissions took over in the late 70s... Early 80s, one of the solutions from OEM was to lean out and increase timing to 14°. That's fine when you are at 8.5 to 9.2. It is not fine when you are at 10 or more. Look up the old specs from the 60s when factory made 10 to 1 cars. You will see the specs were zero to 4° BTDC as initial timing at 800 to 950 curb idle.
You see this when you build and engine and set initial mechanical timing. You want the flame front to just start as piston moves up, compresses and moves down. On high compression engine you can't have too much timing because it will build pressure way too much and too soon and starts to push the piston down before it gets to the end of it's upward stroke.
On top of that you have distributor hooked to manifold vacuum.
If your cam has enough overlap and base vacuum is around 12 which would be common for LSA 104 to 106 you might be ok. But with an LSA of 114 and if you are around 20 inches of vacuum, it's way too much.
Vacuum will pull your plate forward, full travel immediately after start up. For sake of example... If full travel is 12 and you add your 18, that is 30 and everything I've seen that is way too much.
You really need to see what 30 looks like to the piston in the bore. The piston has too move up 30 degrees of travel with flame on top of it and expanding and pushing down. Now imagine if full travel is 20. You are asking the engine to run idle at 38°BTC???
Now when you go off idle vacuum reduces, plate returns and timing drops. That's going the wrong direction, then light springs and heavy weights increases timing as RPM goes up.
You see the roller coaster here?
I do not unfortunately.
In reading the manual it sounds like i should try a smaller rod. There is really only 1 “rod only” option.
The other scenario. Which I am not sure if I am on board with. But if closing the choke is infact increasing vacuum, then the vacuum would be pulling the rod down (indicating i am actually too rich) and should try a lighter spring. I am not on board with this scenario because either way it is choking off air. But worth entertaining because i have springs on hand and they are an easy swap. So i think this is an easy place to start.
I have some rods but will have to see if they are the right ones. I dont think this warrants a jet change. So it would be #9 on the chart for a 1407.
Thats a lot to unpack so i will have to go over it a few times. But everything i’ve read is an sbc likes 34-36 all in. And to get it all in around 3000rpm roughly. Dropping timing is leaving power on the table. And i can confirm this by road testing the vehicle as i was getting it dialed in. I wouldnt be willing to do this just to alleviate an off idle bog that’s only bad in certain road conditions.
Yep, I don't want to get off track and into the weeds. I think you're going the right thought path.
On cold engine pop the the accelerator and then go check where the choke is. Choke should be closed and there should be a small crack just to let a little air pass through. If engines fires then your good. Go drive maybe 5 minutes then again check the choke position. It should be fully open. If it is then it's not a choke problem.
On cold engine pop the the accelerator and then go check where the choke is. Choke should be closed and there should be a small crack just to let a little air pass through. If engines fires then your good. Go drive maybe 5 minutes then again check the choke position. It should be fully open. If it is then it's not a choke problem.
Just should add real quick, increased base timing allowed me to close my throttle plate down which alleviated a dieseling issue i had when running lower base timing
Yes sorry thinking auto choke/electric. On manual just pull it shut and see where it is. Do the same fully open and just verify the install of the manual unit.
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