Rough idle when getting hot

higher pressure of what? the video shows a great vacuum signal and a great running engine.

Thats an interesting gauge. Looks vintage!

I think he meant vacuum.

By the way, I don't understand the 'adjusting idle to passing emissions'. Sorry I don't live in a state where emissions testing is needed on such an old vehicle...

I thought there was only 1 way to adjust idle

-- adjust both screws to achieve best vacuum
-- keep adjusting back and forth until vacuum reading and idle quality is optimal
-- back out the screw just a hair on both sides to keep things on the side of rich (this will allow the needle valve to allow more fuel)

Yeah, I always back out 1/8 or a little more.
Idle affects mixture right up to the limits of the mains.

that is a great basic way of adjusting the idle.
when you open the throttle, the first thing added is air. it helps if afr is actually wanting air more than fuel. sure, the carb has mechanical means to address this. one is transition slot to allow a richening as the throttle passes by it. this fuel is emulsified as the idle fuel is. then there is the accelerator pump but it takes reaction time, and it is raw fuel so it still is not vapor and must mix with the air. next is the power valve which reacts to an excessive vacuum drop and again it is raw fuel. having the idle fuel just slightly on the rich side makes all of these mechanisms work better. as to the emissions test. is the cat still in place? is the air pump? these are the ways ford met the standards.  

Sorry y’all. I resorted to inaccurate shorthands

Increase in pressure = increase in absolute pressure ( just how my mind was thinking since these are consistently negative pressures (aka vacuum).

I may have also been saying idle and meaning mixture screws or vice versa. Bad habit of mine to be less than accurate. I have to adjust the mixture screws to get the carb running as lean as possible, just before dying in order to pass emissions. My truck won’t even idle on less than 2 to 2.5 turns (manual says start w 1.5!). Not sure why she runs so rich compared to what the manual suggests, but I score high on hydrocarbons as a result. In Oregon, only vehicles older than 1975 are exempt. My Datsun also has challenges passing. Doesn’t help that my renewal date is February so it’s hard to really get the engine roaring hot.

I’m a nut - I like all things vintage hence a pretty sweet vintage Sun tune up set which includes a timing gun ( don’t know how to really use this yet), but it has a cylinder compression gauge (never used), dwell/ rpm meter, vacuum gauge, remote starter and the original instructions and such. Pretty neat find from an estate sale. My truck has a New catalytic converter, egr, etc prolly all from around 2019ish. Doesn’t make it easier to pass emissions tho! The air dump system only operates at rpm’s above idle (deq only tests that after you failed the idle emissions so it’s a moot point), egr only operates above idle etc.

You will need to get the timing situation figured out. Maybe your initial ignition timing is at TDC (or worse ATDC), so the mixture is not fully burnt (causing it to fail emissions unless you lean the idle mixture). Initial timing should be ~8 to 10 BTDC. Put that timing light to use!

Wouldn't the truck be completely gutless and running incredibly hot if it was timed at TDC?  

I've seen those Sun tune-up kits. Having a dwell meter is super convenient when you have points.  👍

Generally cam timing was retarded to hold the exhaust valves closed a few more degrees, allowing combustion to continue before dumping out the ports.
And let's admit, Fords exhaust ports aren't optimal to begin with.

I’ve been searching and not finding what I’d like to see - are there any videos of timing for a 351m? I’m not sure I fully understand what bolt to loosen before I start turning the distributer.

I’ll have to check on a different day, but is there an obvious pointer? Not sure I’ve ever noticed one.

After some googling it sounds like I’ll need to sand my balancer to find the old timing marks? And how many are there - ie does it go high enough to timing total timing (so 30 to 40?). Thanks!

There's nothing unique to a 351M.
The 400, my 460 and the 429 all use the exact same distributor.

Down where the distributor goes into the block there should be a bolt going through a hold-down clamp that fits up against the 'neck' of the distributor.



If the balancer is really rusty you may have to sand it off.
If it's not so bad you can get by with rubbing chalk into the embossed numbers and lines.

Timing marks should go from 0°=TDC, there should be a highlighted mark at static timing (probably 8° on your truck) then longer lines at every 10, with the #'s flanking the line.  Up to 30, at least.



The 335 series don't have a separate cast timing case, but there should definitely be a pointer.
Fords are typically at about 10:00 if looking at the front of the crankshaft, but some (like the 5.0's) put it on the driver's side.

Hope this helps!

Thanks Jim!

I love how none of these parts are easily visible or accessible. Sigh.

Is this the distributor bolt to loosen?





Balancer isn’t even visible from the top 🫠

Ope! Looks like someone marked something!



I’ll look underside to see if I can find other marks!

yes, that is the hold down and bolt. it is 1/2" bolt. this is precisely what the dist offset wrench was designed for. however, I always use a box wrench. only loosen enough to turn the dist. if needed back out the bolt til the hold down is loose and move the dist back and forth then "snug" the bolt down to give it enough drag to stay where you put it. of course, mark it in some way beforehand so you will see exactly where you started from.
visibility to the damper and timing pointer should be visible through a very narrow view looking between the water pump pulley and the alternator. it is crowded for sure, but the hardest part is shining the timing light through the same line of sight that you need to look through.