250 crossflow flat spot

[Topsy11]

Hey all got my old 250 crossflow running. New fuel pump
, filter, leads, plugs, cap, new carby and set.the timing to factory specs at 4degress btdc. It has a big flat spot when u throttle to much. Backfires and blew my muffler to pieces. Any ideas on what the cause could be. Had a mechanic look at it and he was stumped. Pcv valve is missing. Redone all the hoses as per manual. All vacuum on egr advanced timing and what not hold vacuum. Can feel it when u suck on the hoses. Not sure what is a main cause for it. 1978 250 crossflow.

[Terror_Tree]

What sort of carby? The jets might be too big.

[Topsy11]

Stromberge single barrel. Reco factory carby

[Terror_Tree]

Mate, I had a 78 XC Fairmont with 4.1 Litre, a long time ago. I could never get the Stromberg to work, I put a 350 Holley on it and it drove so much better. The only other thing I can think of is a vacuum leak.

[Topsy11]

Easy to fit a 350 holley ?

[yearby]

If your engine is backfiring and damaging the muffler, means that fuel has been accumulating in the exhaust system, so I would re check your ignition timing.

I suspect the distributor has been fitted with it being a tooth or 2 out of phase with the lead/spark plug.

Peter

[Topsy11]

My timing I got number 1 cylinder at tdc. Compression stroke. Marked the dizzy on the side. Rotor obviously points at number 1. Put the cap on and start number lead in order. Then when started advance the timing to 4degress btdc as written. Even if I move the timing up and down it still runs the same. Ickes well. Holds high revs well. Not backfiring as much if I punch it when the revs are up. As the timing advances at higher rpm so should I advance the timing more. Was at 14 btdc when I got it.
I dont understand what out of phase cap means. Does it mean they are not lined up to where the button should be when firing?

[yearby]

Originally Posted by Topsy11

Does it mean they are not lined up to where the button should be when firing?

Yes.

Okay this is going to be difficult.

I presume when you say the PCV is missing, that it has been blocked off or has been connect up to the crankcase, and not leaning out the fuel mixture?

The PCV is there to prevent backfiring from igniting the gas/fumes inside the crankcase.

You are going to need to check everything and not presume anything.
It could still be a carburetor induced flat spot, but my gut feeling is it is timing.

You will need to go back and put the no 1 cylinder on TDC after it's compression stroke.

The correct way to do this involves removing the rocker cover. Then turn the engine over clockwise by hand until the valve rockers on the number 6 cylinder are on the rock. On the rock means that the exhaust valve rocker is just closing and the inlet valve rocker is just opening. Then check and make sure that the harmonic balancer timing mark is lined up with TDC. If the timing mark is within a couple of degrees, rotate the engine until the timing marks are lined up. It is now on Number 1 compression stroke at TDC.

If your harmonic balancer timing marks are out by too much it will mean the harmonic balancer has slipped on it's rubber. For an engine this old it is a high probability.It is also where many a mechanic have come undone, they have presumed the timing marks where correct.

Now go to your distributor and with the cap off, see if the rotor lines up with the number one spark plug lead.

If the rotor lines up correctly, continue, if not get back to me with your results so far.

Now disconnect the coil lead from the distributor cap and earth it to the engine block such that there is a small gap between the lead and the engine block, You will need to see it when it sparks.

Back off the retaining bolt that holds down the clamp holding the distributor housing. You will need it loose enough to be able to rotate the distributor by hand.

Turn the ignition key to the run position, don't crank the engine.

Now rotate the distributor left and/or right, make sure you can make the coil lead spark.
If you get a spark, rotate the distributor housing as far as you can in the direction of rotation of the rotor.
Now rotate the distributor housing in the opposite direction to rotor rotation until there is a spark at the coil lead.
Clamp down the distributor housing at the point where you get the spark.
The rotor should be pointing to the number one spark plug lead position in the distributor cap. If not then something is wrong and may require the distributor to come out and be repositioned.

Your engine should now be statically timed.

Put the engine back together, fit timing light, start engine and reset timing to the desired 4 Degrees advance as specified.

Get back to me with how you went.

It maybe simpler in the above operation to remove all the spark plugs to make the engine easier to turn by hand, it's up to you.

Don't turn the engine over by hand with the ignition switch in the on position, it may start, and it may result in blood loss.

The engine may have been at the 14 degrees BTDC, because the previous owner could not time the engine correctly.

Peter

[Tassie f100]

Originally Posted by Topsy11

My timing I got number 1 cylinder at tdc. Compression stroke. Marked the dizzy on the side. Rotor obviously points at number 1. Put the cap on and start number lead in order. Then when started advance the timing to 4degress btdc as written. Even if I move the timing up and down it still runs the same. Ickes well. Holds high revs well. Not backfiring as much if I punch it when the revs are up. As the timing advances at higher rpm so should I advance the timing more. Was at 14 btdc when I got it.
I dont understand what out of phase cap means. Does it mean they are not lined up to where the button should be when firing?

If the engine is running properly and doesn,t backfire under normal,accelleration I would discount a timing issue. You have either an ignition supply problem in either coil or leads or spark plugs. You say the carb is a reco,but is the accelerater pump giving a good squirt of fuel when you open the throttle. Easy to check,just look down the carb while you operate throttle manually without engine running.

[jpd80]

Make sure you float level is not too low, that used to be the prime cause for a lot of flat spots in early stromberg Falcons...also check accelerator pump functions properly.

[Topsy11]

Thanks Pete,

Bit more involved in checking my number 1 lead is actually pointing at number on the rotor than I thought. Your instructions make perfect sense. After I find true top dead center the rotor should be pointing at the number 1 lead. As i did check through the spark plug hole and had it at top dead center and used.the plug that the rotor was pointing at as number 1.
So no matter what after finding tdc the rotor will always point at number 1 lead.if it's not the harmonic balancer may have moved and not be in the correct position and checking with it sparking the way you suggest is truly the perfect way. I will do just this. To make sure it fires at the right time as well and get back to you.

As for the carby it puts a big squirt of fuel down its throat when I push the throttle. The.only part electrically I have not changed is the coil. Pcv is on it's way which I will connect as well but the hose is just sitting in the crank case hole for.the minute.i will get onto this as soon as I have time in the next day or 3

[RANGEREST]

Vacuum leak?

[Tassie f100]

Topsy, because the engine runs perfectly except for the back fire when you try to accelerate hard,I,m pretty certain that you don,t have an initial timing problem. If the dizzy advance is working either your coil, plugs or leads are faulty.As you say you have a healthy squirt of fuel from the throttle pump so that eliminates that problem.

[The Yeti]

Back to basics

Is the vacuum advance working? And are you getting vacuum to the advance?
You say your plug leads are new? But just check the resistance (should be between 4,000 - 6,000 ohms per foot (30cm)

Silly question but when you threw the plugs in did you check the gap? Or use the factory set Gap? From memory I think they are fairly close at about 0.8 but you’ll need to check as the last time I worked on a cross flow six the year started with 199

Also check your point gap I won’t even Hazard a guess on that as I used to do them by eye

[solarite_guy]

Originally Posted by The Yeti

Also check your point gap I won’t even Hazard a guess on that as I used to do them by eye

We had a rule of thumb for the '50s, 60s and 70s cars when no doco was available:

8 cyl .016"

6 cyl .018"

Not always dead nut perfect, but more often than not was and never failed.

[Craig@AUDI]

A 351 will solve all your problems!

[solarite_guy]

Originally Posted by Craig@CumberlandFord

A 351 will solve all your problems!

It only LOOKS like a 351...

[Topsy11]

Originally Posted by yearby

Yes.

Okay this is going to be difficult.

I presume when you say the PCV is missing, that it has been blocked off or has been connect up to the crankcase, and not leaning out the fuel mixture?

The PCV is there to prevent backfiring from igniting the gas/fumes inside the crankcase.

You are going to need to check everything and not presume anything.
It could still be a carburetor induced flat spot, but my gut feeling is it is timing.

You will need to go back and put the no 1 cylinder on TDC after it's compression stroke.

The correct way to do this involves removing the rocker cover. Then turn the engine over clockwise by hand until the valve rockers on the number 6 cylinder are on the rock. On the rock means that the exhaust valve rocker is just closing and the inlet valve rocker is just opening. Then check and make sure that the harmonic balancer timing mark is lined up with TDC. If the timing mark is within a couple of degrees, rotate the engine until the timing marks are lined up. It is now on Number 1 compression stroke at TDC.

If your harmonic balancer timing marks are out by too much it will mean the harmonic balancer has slipped on it's rubber. For an engine this old it is a high probability.It is also where many a mechanic have come undone, they have presumed the timing marks where correct.

Now go to your distributor and with the cap off, see if the rotor lines up with the number one spark plug lead.

If the rotor lines up correctly, continue, if not get back to me with your results so far.

Now disconnect the coil lead from the distributor cap and earth it to the engine block such that there is a small gap between the lead and the engine block, You will need to see it when it sparks.

Back off the retaining bolt that holds down the clamp holding the distributor housing. You will need it loose enough to be able to rotate the distributor by hand.

Turn the ignition key to the run position, don't crank the engine.

Now rotate the distributor left and/or right, make sure you can make the coil lead spark.
If you get a spark, rotate the distributor housing as far as you can in the direction of rotation of the rotor.
Now rotate the distributor housing in the opposite direction to rotor rotation until there is a spark at the coil lead.
Clamp down the distributor housing at the point where you get the spark.
The rotor should be pointing to the number one spark plug lead position in the distributor cap. If not then something is wrong and may require the distributor to come out and be repositioned.

Your engine should now be statically timed.

Put the engine back together, fit timing light, start engine and reset timing to the desired 4 Degrees advance as specified.

Get back to me with how you went.

It maybe simpler in the above operation to remove all the spark plugs to make the engine easier to turn by hand, it's up to you.

Don't turn the engine over by hand with the ignition switch in the on position, it may start, and it may result in blood loss.

The engine may have been at the 14 degrees BTDC, because the previous owner could not time the engine correctly.

Peter

So I have the engine at tdc. Same spot it was last 3 times I did this. The harmonic balancer mark I'd dead on zero. Rotor button points towards number 1 lead. I assume its number 1 lead because the leads were mixed up when I bought it. It has been running using this lead as number 1. When I tried different spots it would not start and backfire even worse. This spot starts instantly. So I assummmmmme this is number 1 as the cap has no numbers. Other caps online have numbers and the number 1 is in a different spot. I'll put photos up. I turned my ignition on got spark. Rotated clockwise as far as it would in the direction of the rotor movement. Then turned anticlockwise slowly until it sparked. Locked it in Eddie. This would be static times is it not. Now I'll put it back together and start. On another note my egr from the exhaust manifold has been blocked. I dont believe this would cause an issue as it only sucks exhaust gasses for emission control and clog up my intake. First 2 photos was where it sat at tdc. The second 2 is after it sparked. I dont believe I messed this up. Timing has to be correct unless the number 1 lead actually is not suppose to be where it is which makes no sense. The rotor button however can be moved about 5mm moving the shaft in the dissy back and forth if I wiggle it. Not sure if that's normal.

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[Topsy11]

Originally Posted by Craig@CumberlandFord

A 351 will solve all your problems!

I wish haha

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[Topsy11]

Originally Posted by The Yeti

Back to basics

Is the vacuum advance working? And are you getting vacuum to the advance?
You say your plug leads are new? But just check the resistance (should be between 4,000 - 6,000 ohms per foot (30cm)

Silly question but when you threw the plugs in did you check the gap? Or use the factory set Gap? From memory I think they are fairly close at about 0.8 but you’ll need to check as the last time I worked on a cross flow six the year started with 199

Also check your point gap I won’t even Hazard a guess on that as I used to do them by eye

Gaps are at 0.8 I have no idea how to check the resistance using my bolts gauge thing

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[Topsy11]

Originally Posted by The Yeti

Back to basics

Is the vacuum advance working? And are you getting vacuum to the advance?
You say your plug leads are new? But just check the resistance (should be between 4,000 - 6,000 ohms per foot (30cm)

Silly question but when you threw the plugs in did you check the gap? Or use the factory set Gap? From memory I think they are fairly close at about 0.8 but you’ll need to check as the last time I worked on a cross flow six the year started with 199

Also check your point gap I won’t even Hazard a guess on that as I used to do them by eye

Vacuum advanced is getting vacuum from the manifold via the 3 port valve. The vacuum advance holds air when sucked on or blown In.nit sure hiw else to check it

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[Topsy11]

So after all that retiming. It's the same. So I'm guessing not timing after all. When I reply the vacuum advance on the dissy after setting timing the idle goes up and so does the timing to around 10 btdc. Besides that I'm guessing I may just put an electric dissy in and go from there.

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[Topsy11]

https://www.youtube.com/watch?v=V1fN...&feature=share

https://www.youtube.com/watch?v=3smv...&feature=share

Here is some vids. Of the timing and the flat spot. I dont throttle.to hard or it goes bang or dies on its butt

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[Topsy11]

However after I timed this to 0 then started it was running I initially at maybe 6 degrees atdc was running like crap until I advanced it. Is that nornal

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[The Yeti]

Originally Posted by solarite_guy

We had a rule of thumb for the '50s, 60s and 70s cars when no doco was available:

8 cyl .016"

6 cyl .018"

Not always dead nut perfect, but more often than not was and never failed.

Sounds fair, like I said I always eyeballed them

[solarite_guy]

Originally Posted by Topsy11

However after I timed this to 0 then started it was running I initially at maybe 6 degrees atdc was running like crap until I advanced it. Is that nornal

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Yes it will run like crap if running atdc. Under load can burn the exhaust valves too.

The very least amout of advance should be what is stated in the manual. Those settings were very conservative and matched with the low cr and retarded valve timing of the day made for lethargic engines. I am assuming you have a stock camshaft.

I know a lot of things are being presented. List them and try 1 thing at a time and cross things off the list.

Pretty much everything that has been listed are possibilities.

I have my own order of things to try, but I want to avoid the "too many cooks in the kitchen" scenario.

After you have gone through the list of tests here and if the problem persists, then I'll add my 2 cents if there is anything still to test.

Hang in there.

Make sure Peter has a chance to review your results vs tests.

[Topsy11]

Originally Posted by solarite_guy

Yes it will run like crap if running atdc. Under load can burn the exhaust valves too.

The very least amout of advance should be what is stated in the manual. Those settings were very conservative and matched with the low cr and retarded valve timing of the day made for lethargic engines. I am assuming you have a stock camshaft.

I know a lot of things are being presented. List them and try 1 thing at a time and cross things off the list.

Pretty much everything that has been listed are possibilities.

I have my own order of things to try, but I want to avoid the "too many cooks in the kitchen" scenario.

After you have gone through the list of tests here and if the problem persists, then I'll add my 2 cents if there is anything still to test.

Hang in there.

Make sure Peter has a chance to review your results vs tests.

Thank you. After my pcv valve shows up I'll try again. Timing is not the issue I strongly believe. Carby I believe not be new but I'll have to have it tuned by a carby guy in case. Coil might buy a new one. Theres some extra wires down near the coil that have nowhere to plug into. One of the wires coming off the coil goes to nothing. Maybe they are no good. Put an electric dissy to get eliminate vacuum leak problems that I may have. Everything else to my knowledge is good.

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[yearby]

Originally Posted by Topsy11

So I have the engine at tdc. Same spot it was last 3 times I did this. The harmonic balancer mark I'd dead on zero. Rotor button points towards number 1 lead. I assume its number 1 lead because the leads were mixed up when I bought it. It has been running using this lead as number 1. When I tried different spots it would not start and backfire even worse. This spot starts instantly. So I assummmmmme this is number 1 as the cap has no numbers. Other caps online have numbers and the number 1 is in a different spot. I'll put photos up.

Originally caps were numbered, but when someone removes the dissy from they block, they don't alway put it back in the same position, so yes yours is good.

I turned my ignition on got spark. Rotated clockwise as far as it would in the direction of the rotor movement. Then turned anticlockwise slowly until it sparked. Locked it in Eddie. This would be static times is it not.

Yes

Now I'll put it back together and start. On another note my egr from the exhaust manifold has been blocked. I don't believe this would cause an issue as it only sucks exhaust gasses for emission control and clog up my intake.

should not really affect the it as the EGR is only to cool down combustion temperatures when at cruise mode.

First 2 photos was where it sat at tdc. The second 2 is after it sparked. I dont believe I messed this up. Timing has to be correct unless the number 1 lead actually is not suppose to be where it is which makes no sense.

Yes good

The rotor button however can be moved about 5mm moving the shaft in the dissy back and forth if I wiggle it. Not sure if that's normal.

Yes this is normal, you are just moving the advance mechanism. You should be able to advance it with your fingers and let go and it will return to it normal position. if you can retard it (rotor) with your fingers , then there is something wrong with the advance mechanism.

Peter ....

[yearby]

Originally Posted by Topsy11

Vacuum advanced is getting vacuum from the manifold via the 3 port valve. The vacuum advance holds air when sucked on or blown In.nit sure hiw else to check it

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All you need to know is it holds vacuum and that it moves the points contact mechanism when vacuum is applied.

[Topsy11]

Originally Posted by yearby

Peter ....

Thanks peter. The rotor only moves the direction of the rotor movement and flicks back. So we all now know the timing is correct. Time to move on. Waiting for my pcv to show up.

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