Question about timing.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Careful.. Even when connected to a ported source the vacuum advance will start to kick in very quickly above factory idle RPM. So will mechanical advance for that matter. Consequently the base or initial timing ahould be checked/set at a slow idle and vacuum advance disconnected and plugged. Maybe you know all this already, but for people following along... For all that the initial timing # really isn't critical, something more than 10° and less than 20°. The total timing is what's important.
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Mountain Pass
Joined: Jul 2016
Posts: 146
Likes: 0
Yeah, I figured that at 800 rpm with mech and vac going being at 12* would be a conservative/safe number for break in as long as it starts, not a long term setting. I will have someone rev to 3500 while I time it this evening just to make sure. I've read 38* ish at 3500 rpm is the goal for a moderately built 400. Not sure if that is true or not. I know it's whatever your engine can take but that seems to be a safe and common number.
Logistics Pro
Joined: May 2010
Posts: 4,436
Likes: 19
From: Regina, Saskatchewan
Yeah, I figured that at 800 rpm with mech and vac going being at 12* would be a conservative/safe number for break in as long as it starts, not a long term setting. I will have someone rev to 3500 while I time it this evening just to make sure. I've read 38* ish at 3500 rpm is the goal for a moderately built 400. Not sure if that is true or not. I know it's whatever your engine can take but that seems to be a safe and common number.
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Mountain Pass
Joined: Jul 2016
Posts: 146
Likes: 0
So, you guys were right to check the vac advance. It was ramping very fast, 25-30 right of idle, a short stall then up to 45ish at 3500 rpm. I'm sure it never saw that high under load since I've been keeping it under 3000 rpm. I disconnected the vac advance and plugged the carb port, with mechanical alone it has a nice flat increase from 14* to 38* and feels good on the road. It will do me well for now.
Now to find my new post build voltage leak...
Now to find my new post build voltage leak...
Lead Driver
Joined: Mar 2004
Posts: 8,922
Likes: 153
Spring pressure has no effect on hydro lifters,
"In a modern hydraulic lifter, a hardened steel push rod cup sits on top of a plunger mounted inside the hollow lifter body. A lock ring in the top of the lifter holds the assembly together. Under the plunger is a spring that holds the plunger up so oil can fill the cavity between the plunger and lifter body. A one-way check valve in the bottom of the plunger allows oil to enter the plunger cavity but traps the oil inside when the lifter moves up. This prevents the lifter from collapsing, which would not allow it to open the valve fully.
The clearance between the plunger and lifter body is extremely tight, typically 0.0002? or less. This is done to limit oil loss from inside the lifter (called the “bleed down” rate) when the valve opens and closes. A small amount of leakage (bleed down) must be allowed with each valve cycle so the lifter can readjust itself to maintain zero valve lash."
"With flat tappet cams, too much initial spring pressure can have an adverse effect on the cam lobes and lifters until the cam is broken in. For the initial break-in, one cam supplier said open seat valve spring pressure should be limited to no more than 280 lbs. If the engine has double springs, temporarily remove the inner spring for break-in, and use a lower rocker arm ratio to reduce the load on the cam. Some racers go even lower, using no more than 225 lbs. of open spring pressure for break-in. "
You can read about lifters here, Hydraulic Camshafts and Lifters 101
"In a modern hydraulic lifter, a hardened steel push rod cup sits on top of a plunger mounted inside the hollow lifter body. A lock ring in the top of the lifter holds the assembly together. Under the plunger is a spring that holds the plunger up so oil can fill the cavity between the plunger and lifter body. A one-way check valve in the bottom of the plunger allows oil to enter the plunger cavity but traps the oil inside when the lifter moves up. This prevents the lifter from collapsing, which would not allow it to open the valve fully.
The clearance between the plunger and lifter body is extremely tight, typically 0.0002? or less. This is done to limit oil loss from inside the lifter (called the “bleed down” rate) when the valve opens and closes. A small amount of leakage (bleed down) must be allowed with each valve cycle so the lifter can readjust itself to maintain zero valve lash."
"With flat tappet cams, too much initial spring pressure can have an adverse effect on the cam lobes and lifters until the cam is broken in. For the initial break-in, one cam supplier said open seat valve spring pressure should be limited to no more than 280 lbs. If the engine has double springs, temporarily remove the inner spring for break-in, and use a lower rocker arm ratio to reduce the load on the cam. Some racers go even lower, using no more than 225 lbs. of open spring pressure for break-in. "
You can read about lifters here, Hydraulic Camshafts and Lifters 101
Laughing Gas
Joined: Feb 2003
Posts: 1,234
Likes: 3
From: In the woods of Oregon
I'm pretty sure the reason your getting single springs, is because you can get the spring rate and open and closed spring pressures with those springs, no need to get doubles if singles will work.There are now double springs available that provide over 1000#open pressure, way more than many of us would ever use with a hydraulic flat tappet, definitely solid roller territory. Regardless of the spring pressure, the check valve in the lifter prevents the lifter from losing pressure. I just got done replacing my lifters because two of them collapsed during initial break in. Without the check valve working even the near stock springs used for the 252H would cause the lifters to bleed down and lose pressure. If it wasn't for this recent experience even I wouldn't have known how lifters work exactly. That's whats so awesome about this forum. We can all share or learned knowledge.
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