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Timing and Pre Ignition Issues 351M

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78FordLtd2 View Drop Down
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    Posted: 18-July-2021 at 8:27PM
I've been having some pre ignition problems for a little while now. I first thought it was because of some bad or stale gas in the tank or most likely a timing issue. I suspected a vacuum leak as well as the engine had a slight surge at speed along with a rough idle. Engine is bone stock, btw.

I found that my vacuum leak was coming from the stock intake,  egr and carb base gaskets. I replaced the valley pan gasket, egr gasket and the gaskets under the egr spacer and carb. After getting it running, I tested for leaks and replaced a few suspect vacuum hoses. No leaks were found but I still have a rough idle and some pinging under light load.

Timing and idle setting as per the tag at 600 (N) and 14*BTD.

According to my meter, I can only drop the idle to about 650. I can almost back the idle screw right out and the idle will not drop much more than 650 rpm. Setting the timing to 14* with vac advance hose off and plugged. This hose goes to a ported vac switch on the water pump. There should be no vacuum at idle, but there is. Unhooked the hose from ported vacuum at the base of carb (port on driver side of carb slightly above throttle plates), strong vacuum coming from this port at idle. There shouldn't be any vacuum from this line until the throttle plates open. All vacuum hoses appear to be correctly hooked up according to the vacuum diagrams here on this forum and on others as well. 

Setting the base timing to 14* is fine until I replace the vac advance hose back on the distributor where then the idle increases (750-800 rpm). I'm going to assume that the increase in idle speed also advances the timing at idle giving full advance at a much lower engine speed which might be the  cause of the pinging. Cruising at moderate speed (40-50) surging is noticeable.

Could this be a carb problem? Brake cleaner spray around carb base and around the throttle shaft does not affect idle quality.

Vacuum leak at modulator valve? Transmission fluid is not being sucked into the manifold and tranny shifts normally.

Vacuum leak at power brake booster? Brakes work excellent and pedal feels normal.

Going out on a limb on this one: Burnt valve? Compression test has not been done. Car is low mileage (85k) but that might not mean anything. Plugs are normal in color and correct heat range (engine tag).

Usually burn mid grade quality fuel (Chevron) but no difference in pinging on 87 or 93 octane fuel.

I'm stumped!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 1:13AM
Lower the initial timing as needed to get it to run better under any current conditions. The initial timing should be the last fine adjustment after modifications, or problems that you have to work out. The distributor is the first and huge timing control, they vary by applications and over the year models. I haven't dealt with many older Ford engines, but I learned that long ago in 1980 and the early 80's.

Later Fords called for 10* initial, but most people modifying cranked that to 14-16* always. I never got far from stock with my two 1985's, 86, 90, two 91's, or my 92 302's. A couple of extra degrees can make one a little more frisky, but detonation is close as you go up.

I suggest dropping the 14* initial back to 12*, that should help it idle lower and reduce the detonation. Keep working on the vacuum, and get coolant temps down, higher heat for any reason is bad. Run a 180* thermostat and be sure the radiator and fan are in top condition.
Don
73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000+ rpm 351-4V &4R70W
73 Ranchero GT 351C-4V &4R70W for sale later.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pete rad Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 2:51AM
Originally posted by 78FordLtd2 78FordLtd2 wrote:

According to my meter, I can only drop the idle to about 650. I can almost back the idle screw right out and the idle will not drop much more than 650 rpm.
Maybe the throttle plates on your carburetor are not fully closing. The best way to fix this is to remove the carb and spray carb cleaner into the throttle bores and move the throttle open and closed and it might make the throttle plates close all of the way shut.

You might be able to do this with the carb still installed on the engine. Just spray carb cleaner into the carb throttle bores and move the throttle shaft open and closed. But every time that you do this with the carb installed on the intake manifold you will be spraying gas from the accelerator pump into the intake manifold. Plus the carb cleaner will get into which ever cylinder has an open intake valve. So it might be better to do with the engine running.

The picture below shows the blue arrows as the throttle plates and the black arrows as the throttle bores.


Edited by pete rad - 19-July-2021 at 10:37PM
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Edited by pete rad - 19-July-2021 at 10:32PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 3:15AM
The carb may need to be rebuilt, is that original and untouched? The throttle blades are supposed to be open at idle enough to allow air past which begins to pull fuel out. On a Holley, those all have a slot just above the blade closed position, which is the transition slot. That slot must be visible from below when it's at warm idle. That contributes to fuel intake, and creates a smooth increase as the throttles are opened. When that is upset, not set properly at idle, that creates a stumble if the blades are shut too far, and a bog if the holes are exposed too much.

Try some carb spray with the engine running, down into the throats of the carb, clean that as much as you can.
Don
73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000+ rpm 351-4V &4R70W
73 Ranchero GT 351C-4V &4R70W for sale later.
92 Lincoln Mark VII SE GTC, OBDII 347/4R70W
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pete rad Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 3:21AM
Be careful of turning some carburetors upside down, some of them have a steel ball for the accelerator pump. When you press the gas pedal the accelerator pump sprays gas into the carb venturis and the steel ball is there so no fuel goes back into the fuel bowl. If the steel ball becomes dislocated in the carb it can block one of the accelerator pump sprayer nozzles and you might get some hesitation on acceleration, until the steel ball goes back into its correct place.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 11:38AM
Originally posted by 72 RS 351 72 RS 351 wrote:

Lower the initial timing as needed to get it to run better under any current conditions. The initial timing should be the last fine adjustment after modifications, or problems that you have to work out. The distributor is the first and huge timing control, they vary by applications and over the year models. I haven't dealt with many older Ford engines, but I learned that long ago in 1980 and the early 80's.

Later Fords called for 10* initial, but most people modifying cranked that to 14-16* always. I never got far from stock with my two 1985's, 86, 90, two 91's, or my 92 302's. A couple of extra degrees can make one a little more frisky, but detonation is close as you go up.

I suggest dropping the 14* initial back to 12*, that should help it idle lower and reduce the detonation. Keep working on the vacuum, and get coolant temps down, higher heat for any reason is bad. Run a 180* thermostat and be sure the radiator and fan are in top condition.

The 14* timing setting is what is stated on the factory emissions tag that is on the valve cover. That is what I've set the base timing at. I've tried other setting listed in other manuals for 1978 that range from 8-16* with no success. I went back to the emissions tag setting instead. My timing light is just a basic unit that I've had for years. Nothing fancy.

You're probably right...the carb might need to be gone through. No. It's not the original but a reman Autolite 2150. It might be completely out of adjustment. It's been a long time since I've "set" one up (lol!). 

I'll give it a shot ;)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 11:54AM
make sure you're not sitting on the choke idle speed screw.

if the throttle blades are closing far enough you will pull vacuum from the ported source

and you might want to verify your harmonic damper tdc with a piston stop tool,
the outer ring may be slipped and your scale is not where you think it is

time for a timing chain, set a double row set in the + key position Wink
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 4:08PM
Originally posted by Rockatansky Rockatansky wrote:

make sure you're not sitting on the choke idle speed screw.

if the throttle blades are closing far enough you will pull vacuum from the ported source

and you might want to verify your harmonic damper tdc with a piston stop tool,
the outer ring may be slipped and your scale is not where you think it is

time for a timing chain, set a double row set in the + key position Wink

No, only adjusting mixture screws and the idle. Idles a bit better and smoother...still pulling vaccum though.

Yes, I have thought about the balancer. I re-timed the engine when I dropped the distributor back in. I did not use a screw driver or a piston locator tool when I did this. Thumb over #1 cylinder and lined up the timing marks. Fired right up no problem. It might be out though...planning to check that out. The balancer that is.

Timing chain is on the list as there is a slight bit of slop. More of a preventative measure because of the mileage. 

There is a ported vacuum switch that is on the water pump that the distributor is hooked up to. The vacuum hose comes off the drivers side port and branches into 2 lines that connect to this switch and then the vac advance hose comes off of that. Could this switch be the problem? Is there constant vacuum going to this switch and when it reaches a certain temperature it opens and allows vacuum to the advance at idle? 

Thinking out of the box...


Edited by 78FordLtd2 - 19-July-2021 at 4:13PM
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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 19-July-2021 at 5:35PM
   Do you need all the emission crap hooked up for a sticker? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 20-July-2021 at 12:21PM
Originally posted by 78FordLtd2 78FordLtd2 wrote:

Originally posted by Rockatansky Rockatansky wrote:

make sure you're not sitting on the choke idle speed screw.

if the throttle blades are closing far enough you will pull vacuum from the ported source

No, only adjusting mixture screws and the idle. Idles a bit better and smoother...still pulling vacuum though.


i'm not sure if you're missing my point? if you're turning the curb idle speed screw (driver side) out to reduce idle rpm and it'll only drop to 650 or so, the throttle shaft may be sitting on the high speed idle screw in the choke assy on the other side of the carb. possible the high speed cam is sticking in it's travel or the screw itself is turned in too far leaving the curb idle speed screw ineffective below 650 rpm. turning the curb idle speed screw all the way out should allow the throttle blades to close completely enough that the engine shuts off
 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 21-July-2021 at 9:53AM
Originally posted by Rockatansky Rockatansky wrote:

Originally posted by 78FordLtd2 78FordLtd2 wrote:

[QUOTE=Rockatansky]
make sure you're not sitting on the choke idle speed screw.

if the throttle blades are closing far enough you will pull vacuum from the ported source

No, only adjusting mixture screws and the idle. Idles a bit better and smoother...still pulling vacuum though.


i'm not sure if you're missing my point? if you're turning the curb idle speed screw (driver side) out to reduce idle rpm and it'll only drop to 650 or so, the throttle shaft may be sitting on the high speed idle screw in the choke assy on the other side of the carb. possible the high speed cam is sticking in it's travel or the screw itself is turned in too far leaving the curb idle speed screw ineffective below 650 rpm. turning the curb idle speed screw all the way out should allow the throttle blades to close completely enough that the engine shuts off
 

[/QUOTE
Ahhh! Yes, I guess I completely missed your point! Homer Simpson moment, lol!

That makes complete sense. It might be hanging up or binding. Will check it.

This is a completely unmolested car with all of its plumbing intact. Most people would have ripped this crap off long ago and plumbed it the way they think is right. I want to work with what I've got first, then make a few changes. That carb might need attention. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote handsofstone Quote  Post ReplyReply Direct Link To This Post Posted: 21-July-2021 at 10:02AM
  More pictures of both sides of carb would help.  Your choke may be the culprit.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 21-July-2021 at 1:52PM
Originally posted by handsofstone handsofstone wrote:

  More pictures of both sides of carb would help.  Your choke may be the culprit.

Some pics of my carb...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 21-July-2021 at 1:56PM
Another. Forgive me if it's sideways ;)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 21-July-2021 at 1:58PM
TVS on water pump
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 21-July-2021 at 2:13PM
Hose routing

S port (choke side)
To air cleaner, charcoal canister, TVS (blue), EGR

E port (driver side)
Single line branching into 2 lines to TVS (grey) on water pump, 1 line from TVS (grey) to vac advance. 

At idle, no vacuum on S port.
At idle, vacuum present on E port.

Question, am I suppose to have vacuum on E port IF it is going to the vacuum switch on the water pump? If there is suppose to be vacuum, I'm guessing that grey switch is bad. Am I to understand that this is a switch designed to advance timing and increase idle when the engine gets too warm (overheat)?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Big Bird Quote  Post ReplyReply Direct Link To This Post Posted: 22-July-2021 at 1:36AM
There was a vacuum scheme where the distributor got vacuum advance or retard with a goofy version of the vacuum advance canister and thermal switches. Unsure of years/engines at this point as everyone just bypassed it all 40+ years ago. Vacuum advance would have multiple hoses going to it.
351M/400 were notorious for preignition/pinging and running hot.
Emissions equipment was poorly conceived and poorly executed. Destroking the 400 was also a half-assed endeavor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 22-July-2021 at 1:43AM
Originally posted by Big Bird Big Bird wrote:

There was a vacuum scheme where the distributor got vacuum advance or retard with a goofy version of the vacuum advance canister and thermal switches. Unsure of years/engines at this point as everyone just bypassed it all 40+ years ago. Vacuum advance would have multiple hoses going to it.
351M/400 were notorious for preignition/pinging and running hot.
Emissions equipment was poorly conceived and poorly executed. Destroking the 400 was also a half-assed endeavor.


I rearranged mine also in the 70's, that vacuum advance system. I did like one thing about those, and I kept one of the vacuum temp switches. Applying full vacuum will raise the idle, I used one of those switches for possible over heating, I think it was about a 228* version. Now decades later, it's best to just remove all of those and manage the timing better, and keep the engine from over heating to start with.

Later EFI vehicles do also raise the idle when the coolant temp gets too high. I had it happen one time for a 99 work truck I have. That didn't help to cool the engine(I have an electric fan and no mechanical OEM fan). But I noticed the higher idle, the truck wanted to drive away on its own at idle. So I caught it, though it did hit 270*, with no harm, losing about a quart of coolant. The fan power relay had tripped, a marine version which washing the engine bay should not have hurt, but it did.
Don
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73 Ranchero GT 351C-4V &4R70W for sale later.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 78FordLtd2 Quote  Post ReplyReply Direct Link To This Post Posted: 22-July-2021 at 5:36AM
I.think I'm just going to bypass that switch on the water pump and tap into one of the other hoses on the choke side of the carb and plug that E port. That just might be the ticket and it might save me from needlessly rebuilding the carb.
Thanks! 
Will keep you posted :)
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