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Discussion Starter #1
Hi all, this is my first post.
I purchased a 1986 E350 class C RV powered by a 460 carbureted engine and in searching for information I came across this site and what great information contained in the archives. I just finished my first major engine modification, installed a later FI injected cam chain set to 0 the cam timing and purchased a recurved distributor by Scotty from Parkland Automotive. Vacuum advance was adjusted for EGR delete. Scotty recommended ported vacuum for the vacuum advance and this is where I ran into issues with the Hooley 4180 carburetor. Just a plug for Scotty – great to deal with!
On my RV vacuum advance from the factory is manifold vacuum and it is restricted when the engine is at operating temperature. The RV has 21 inches of manifold vacuum at idle and when the engine is at operating temperature the vacuum to the distributor is decreased to 14 inches. I observed it happen, running 21 inches vacuum to the distributor and when the coolant temperature reached the trigger point the vacuum was cut to 14 inches. There is a port on the carburetor for ported vacuum but the signal is weak and the engine RPMs need to be quite high to reach 15 inches. Presently I am using unrestricted manifold vacuum for the advance.
Am I missing something on the ported vacuum?
I can say there is quite a difference with the recurve distributor and cam timing change at part throttle acceleration but I did not notice large difference at WOT throttle acceleration until today when test driving unit. I felt the secondaries open – when that happened the 9,000 plus pounds stated to move, so I suspect my secondaries are not operating has they should. I
Any relation to the low ported vacuum and my secondaries not opening?
 

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At WOT,the vacuum advance is out of play on manifold or ported.Can't really see where that would affect the secondaries unless the port was bleeding off too much vacuum.Personally,I prefer manifold as you can set initial timing higher without pinging on acceleration.I consider ported to be an emissions thing to keep initial lower and temps higher.Lots of discussion on this everywhere.Ported will advance timing more at cruise for reported better fuel economy but never noticed much difference.Not all that much cruising in an RV unless big tail wind or down hill.
 

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While I usually use direct manifold vacuum advance to correct the timing at part-throttle and idle, but requires the distributor to be calibrated for it. If yours is calibrated for ported vacuum, you should use that. Ported vacuum should read the same as manifold vacuum, except at idle with throttle closed. With your other symptom of delayed secondaries, I would check for restriction or covering of any portion or passages of the vacuum system. Using the wrong or generic gasket is a common cause for blocking vacuum. Secondary operation should be invisible, as too early will bog the engine, and too late will feel like a sudden power boost, with neither symptom being good for efficiency or performance.


BTW, vacuum advance is a winner for most uses, as it has no effect on WOT power, and positive effects on part-throttle torque and efficiency by correcting ignition timing. Else the engine is effectively operating with retarded timing for the conditions. Without it some smaller losses are obvious, but also some that aren't, like reducing valve and seat life. Credit due - extra thanks to AERA years ago for real-world results data related to ignition timing.


David
 

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Hi all, this is my first post.
I purchased a 1986 E350 class C RV powered by a 460 carbureted engine and in searching for information I came across this site and what great information contained in the archives. I just finished my first major engine modification, installed a later FI injected cam chain set to 0 the cam timing and purchased a recurved distributor by Scotty from Parkland Automotive. Vacuum advance was adjusted for EGR delete. Scotty recommended ported vacuum for the vacuum advance and this is where I ran into issues with the Hooley 4180 carburetor. Just a plug for Scotty – great to deal with!
On my RV vacuum advance from the factory is manifold vacuum and it is restricted when the engine is at operating temperature. The RV has 21 inches of manifold vacuum at idle and when the engine is at operating temperature the vacuum to the distributor is decreased to 14 inches. I observed it happen, running 21 inches vacuum to the distributor and when the coolant temperature reached the trigger point the vacuum was cut to 14 inches. There is a port on the carburetor for ported vacuum but the signal is weak and the engine RPMs need to be quite high to reach 15 inches. Presently I am using unrestricted manifold vacuum for the advance.
Am I missing something on the ported vacuum?
I can say there is quite a difference with the recurve distributor and cam timing change at part throttle acceleration but I did not notice large difference at WOT throttle acceleration until today when test driving unit. I felt the secondaries open – when that happened the 9,000 plus pounds stated to move, so I suspect my secondaries are not operating has they should. I
Any relation to the low ported vacuum and my secondaries not opening?

Your distributor was set up with ported vacuum in mind. Use it and eliminate the vacuum break.

Excessive initial timing at idle can cause missing or an erratic idle and increase HC emissions in conjunction with a performance curve.

There are exceptions when cam duration is obnoxious and or static c/r is less than ideal.

The need for vacuum advance when the engine is throttled is due to the slower flame front propagation rate under conditions where cylinder filling is sub optimal, a/f ratios are near stoic and EGR if utilized.


SJ
used 2b RHP


:D
 

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Discussion Starter #5
Thanks for the feedback and thanks Scotty for your input. The problem is the vacuum from the ported tube on the 4180 Holley is very weak. Since the previous owner rebuilt the carburetor, I am not sure if the proper kit was used, I have read the metering blocks are unique on the 4180 verses other Holley carburetors. Since I need to pull carburetor to plug the EGR hole in the manifold and replace the EGR spacer with a one-inch carburetor spacer, at that time I will check the carburetor for proper gaskets.
 

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Thanks for the feedback and thanks Scotty for your input. The problem is the vacuum from the ported tube on the 4180 Holley is very weak. Since the previous owner rebuilt the carburetor, I am not sure if the proper kit was used, I have read the metering blocks are unique on the 4180 verses other Holley carburetors. Since I need to pull carburetor to plug the EGR hole in the manifold and replace the EGR spacer with a one-inch carburetor spacer, at that time I will check the carburetor for proper gaskets.

Under loaded / dynamic operation the ported vacuum signal will increase.


SJ
 

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Discussion Starter #8
Update for those who are interested.
I took the carburetor off to troubleshoot the secondary not opening and found the signal port to the secondary diaphragm was partially clogged. This is a Holley 4180 and there are 2 external ports on the carburetor base below the throttle plate, one large tube and one small tube and I had thought both were manifold vacuum due to their location. It turns out the small tube is ported vacuum and now I have a strong ported vacuum signal.
After driving the unit today I can say it was definitely worth changing the cam timing to straight up and getting a distributor from Parkland performance with a custom spark advance curve for my application.
 

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Discussion Starter #9
Just returned from my first RV trip after installing the straight up cam timing set and installation of a recurved distributor from the Mad Porter with EGR delete and I can say without a doubt it is worth the change on the power side. I am quite confident along with the improved power I picked up half to three quarters in the MPG department. One thing I did notice is the exhaust pipe is darker in color than before the change and on start up the engine loads up with the choke on, I can address this by increasing the vacuum pull off on the choke.
 
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