351M timing issue
Thread Starter
|
Senior User
Joined: Jan 2013
Posts: 303
Likes: 0
351M timing issue
My initial timing using a light at idel with the vacuum advance disconnected is 12 degrees TDC, but with it connected it stays at the BTC mark at idel, I am new to timing an engine and could use some advice on what to do, its got a 1406 edlebroke and 4 barrel intake, not sure if it has a aftermarket cam or not as I bought it not running it was supposed to have been rebuilt. Thanks
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
The timing shouldn't (necessarily) move at idle spec with vac advance connected to a ported vacuum source. If you were to rev the motor up you'd see the timing advance from both the mechanical and vacuum advance mechanisms doing their thing.
Postmaster
Joined: Jan 2013
Posts: 3,512
Likes: 3
From: Panama City
Are you saying when you reconnect vacuum line to advance it doesnt change? It should not change at idle if you are hooked up to ported vacuum. When you give it throttle then the vacuum advance should do what it is supposed to do and advance the timing. If it is not doing this then you either have a bad vacuum advance diaphragm, no vacuum at your port or possibly a vacuum leak.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Yes, if you have a pump you can watch the breaker plate move with increasing vacuum. Or use a piece of clean hose and draw on it like a straw. If the idle is set real high, the vacuum advance might start to come in. Normally, at idle spec there is little to no vacuum from the ported outlet. Disconnecting and plugging the vacuum advance hose is just good practice and eliminates this a source of bogus numbers. It depends on whether we are talking a sewing machine at 500 RPM or a fire breather that won't "idle" below 1100. Most people want to use ported "spark" vacuum source.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Well, my advice would be to follow the procedure in the manual. I bet it's online somewhere if you dig around. It takes all that into account. For that matter engine base timing doesn't change unless somebody changes it, and generally speaking no reason to change, unless one is certain what they are trying to do. Base timing changes means carburetor idle mixture settings need to be reset.
Set the base timing with the vacuum advance hose disconnected and plugged and at the specified idle, this is important. Make sure the vacuum advance diaphragm isn't leaking, and reconnect. Then follow the carburetor adjustment procedure and you should be good to go.
Set the base timing with the vacuum advance hose disconnected and plugged and at the specified idle, this is important. Make sure the vacuum advance diaphragm isn't leaking, and reconnect. Then follow the carburetor adjustment procedure and you should be good to go.
Senior User
Joined: May 2014
Posts: 212
Likes: 0
From: auburn, ca
if youre running the stock DII dizzy, you can adjust it with a very small allen wrench going thru the vacuum port on the dizzy. maybe someone cranked it up not to engage at all.
Trending Topics
Thread Starter
|
Senior User
Joined: Jan 2013
Posts: 303
Likes: 0
My vacuum advance line is connected to the drivers side front port on the 1406 and it draws vacuum at idle, I switched it over to the passenger side which does not draw vacuum and it idled slower and the timing stayed at 12 degrees, what's all this mean? And which port should I be running my advance on? Thanks.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
10 Most Surprising 2026 Ford Truck Features!
Joe Kucinski
Top 10 Ford Trucks Coming to Mecum Indy 2026
Brett Foote
5 Best / 5 Worst Ford Truck Wheels of All Time
Joe Kucinski
Ford Super Duty: 5 Things Owners LOVE, 5 Things They LOATHE!
Joe Kucinski
Every 2026 Ford Truck Engine RANKED from WORST to FIRST!
Michael S. Palmer
The Best F-150 Deal of Every Trim Level (XL through Raptor)
Joe Kucinski
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Heh. "What oil is best"? Usually ported vacuum is utilized to keep a steady idle. Manifold or constant, vacuum source is also used by things like air conditioning, A/T, and other vacuum operated solenoids and adversely affects idle. That's actually why ported vacuum was first utilized, steady idling. You'll hear people dismiss it as a "smog" era atrocity but it's been a part of carb design prior to 1950.
Lumpy cams in highly modified engines usually require using manifold vacuum because they won't idle smooth at a low RPM. Using manifold vacuum can allow for a smoother idle if there aren't other engine vacuum devices competing for it, and cooler engine temps. It kind of depends on your application. At anything above idle RPM it isn't going to matter much and the advance will be the same.
Lumpy cams in highly modified engines usually require using manifold vacuum because they won't idle smooth at a low RPM. Using manifold vacuum can allow for a smoother idle if there aren't other engine vacuum devices competing for it, and cooler engine temps. It kind of depends on your application. At anything above idle RPM it isn't going to matter much and the advance will be the same.
Postmaster
Joined: Jan 2013
Posts: 3,512
Likes: 3
From: Panama City
Normally ported vacuum is recommended for vacuum advance.This means it does not draw vacuum until you start opening the throttle. Which means your base timing is set up with no vacuum, and it only starts to advance as needed with throttle.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
One thing it took me a while to figure out is how misunderstood vacuum advance is. It usually gets unhooked at the first sign of trouble. That's not a bad plan for troubleshooting, in order to remove a possible source of vacuum leak. But unless you're a WOT all the time drag racer, you want vacuum advance, and you want it to work right.
Hotshot
Joined: Feb 2004
Posts: 10,726
Likes: 2,584
From: The Shenandoah Valley
As to timing a 351m, mine has Edelbrock Performer Intake and 1405 Carb (with Holley 12-803 FPR at 5.5 psi), old Crane Fireball cam (near same specs as popular "torque" cams I notice) and a OEM distributor fitted with a quick advance kit since 1986. I let her idle at about 900.
To time it, I disconnect the vacuum line at the distributor and close it off (plug) and loosen distributor and speed up motor to where timing no longer advances with more RPM (which is pretty quick with lighter advance springs) and set timing at 34-35 degrees BTC total advance (I have my balancer marked up to 50 degrees BTC). That's the most it'll see under load when no vacuum is available to pull the vacuum advance more. It might stand more but it don't ping at that setting.
Then I "unplug" the vacuum line and reconnect it to distr. and check just to make sure it's working, not unusual to see over 50 degrees at higher engine speed when closing throttle and vacuum is higher, but then the engine is not loaded so no ping no problem.
A good vacuum advance is absolutely essential if you care about gas mileage. Currently mine get's it's signal off ported port on carb intended for distributor vacuum suignal .... but I have used straight manifold vacuum with some setups, but the idle screw must be backed off and some setups don't like that as well.
To time it, I disconnect the vacuum line at the distributor and close it off (plug) and loosen distributor and speed up motor to where timing no longer advances with more RPM (which is pretty quick with lighter advance springs) and set timing at 34-35 degrees BTC total advance (I have my balancer marked up to 50 degrees BTC). That's the most it'll see under load when no vacuum is available to pull the vacuum advance more. It might stand more but it don't ping at that setting.
Then I "unplug" the vacuum line and reconnect it to distr. and check just to make sure it's working, not unusual to see over 50 degrees at higher engine speed when closing throttle and vacuum is higher, but then the engine is not loaded so no ping no problem.
A good vacuum advance is absolutely essential if you care about gas mileage. Currently mine get's it's signal off ported port on carb intended for distributor vacuum suignal .... but I have used straight manifold vacuum with some setups, but the idle screw must be backed off and some setups don't like that as well.
Thread
Thread Starter
Forum
Replies
Last Post
EDMist
1973 - 1979 F-100 & Larger F-Series Trucks
7
Oct 28, 2009 01:02 PM
steeldust
335 Series- 5.8/351M, 6.6/400, 351 Cleveland
5
May 17, 2004 05:40 PM
spikedog
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
4
Oct 25, 2003 12:50 PM