Ok here it is. I have a '77 f250 trailer special with a 400 in it. I pulled the motor and rebuilt it. I used summit racing parts to keep it close to stock. It has new pistons/rings, lifters, summit 5200 cam, summit G6505 timing gear and chain set, new gaskets and seals. I got rid of all the emission stuff. I also put on a edelbrock performer intake and a Holley 600 carb with a 1" spacer between. It also has a reman Cardone stock replacement distributor.
So here is the issue I am having. I set the timing to 12deg btdc and it really lugs and with the vacuum tester hooked up the needle bounces around 15in of vac. As I turn the timing up (advance) it smoothes out in idle and the needle settles down. I wind up with about 20in of vac around 30deg of timing in idle with no pinging. Then I drive it and its fine in slower speeds. When I get it down the highway it has a lot of vibration in the gas pedal. I know having the timing too advance is bad. I recently set it about 18deg btdc and it gets around 17in of vac with about a quarter in of needle flutter. It has better power when I have the timing cranked up. How should I set the timing to get optimum running power and fuel mileage? By the way the balancer is correct I checked it. and also I checked the timing set. It is not a tooth off. Thanks in advance for any help.
Is your vacuum advance pipe to the dizzy connected to full manifold vacuum or ported vacuum at the carb ?
Check for vacuum leaks at the carb/manifold, manifold/block, and all vacuum pipes (brake booster etc)
On that engine setup you should be seeing a steady needle at +/-20hg vacuum at +/-800rpms.
A 12* idle and 36* total, with no vac adv, will be a good starting point (and very close to optimal). 16* of vac advance will be a good start, in addition to the 36* total, and the total timing of 36* being reached at +/- your max torque rpms will ensure an optimal timing curve.
When I do the timing I unhook the vac advance line and plug it. When I do hook it up it gets connected to the ported vacuum.
I can only get the 20in of vac when I turn the timing way up (advance). When I set it around the 12deg the manifold vac drops way down.
I only replaced the distributor and plugs as far as the ignition system goes. Is it possible the coil and/or box are going out on me causing a weak spark? Or something to that effect?
The initial setting as specified by Ford, be it 4, 6, 8 degrees or whatever is only so good, and that was near 35 years ago when the distributor's mechanical advance was new and the springs unmolested.
Both my 351m's in the '77 (and the 400's on the stand) balancer have timing marks from the factory. I made sure what was there was really clear by lightly deepening or defining the marks with a sharp file. They're factory marked in 2 degree segments between the tens to 30 or 34 degrees (I maybe added the 32 and 24 marks as well). I know that I also did add a mark half way between the factory marked 0 & 10 and 10 & 20 and 20 & 30 and then that far above the 30 for 15, 25, and 35 degrees. These _5 degree marks are between the factory _4 and _6 degree marks.
To set my timing with my light on #1 I pull my vacume tube and plug it, then speed motor up while watching the marks with the light until centrifugal advance stops advancing. I then set the timing at 34 degrees by adjusting the distributor and rechecking with my light so that total mechanical advance is at 34 degrees.
By all acounts of what I've read, these engines like 34 degrees total under load ..... mine certainly seems happy at 34 anyway. It pinged and burnt some valves a few years back when I tried higher (much lower and it's sluggish ... but where it's at there is no ping .... yet it's responsive). Then I hook the vacume hose back up and check to see vacum diaphram is working (light on marks as I slip the hose on nipple), done.
With vacume hooked up it'll go up into the 40s under really light load and coasting at speed for good mileage but when you mash down on the go peddle, vacume dies, and timing drops back for load. My distributor has a advance kit in it which let's mechanical advance come in sooner so the stock low idle timing specs result in what is actually retarded timing since the advance has already started then.
Did you get the crank balanced ? If not that probably is your vibration. Did you check the heads and verify that all the valve tip heights were equal ? That is very important and should always be checked when you get your heads back from the machinest. Did you check for push rod length when you were setting up the valve train ? Any aftermarket cam will have a different base circle and with the machine work done to it I'd doubt the stock length would even be in the ball park, you need a min .020-.060 max preload in the lifters. A bouncing vac needle can mean a valve train problem. Did you take the new dizzy apart and put the mechanical advance stop in the short slot ? Did you change the advance springs ? You should be running 2 light ones.
Tbear- I set the total mech timing at about 36deg and it seems to have really mellowed out once its warm. Having a little trouble getting it to warm up properly but once it does it runs like a champ.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
Mark a. - I tried a different vac gauge and it didn't have the slight shake in it like it did before. Its getting about 19" of vac now at idle since I adjusted it for 36deg at full mech advance. I didn't have much machine work done to the motor. Nothing was done to the heads. The cam isn't a very large cam that I would need head work or anything. When I put everything together I checked all the clearances. They were fine. <o></o>
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