Before someone points this out, I know what my Dad owned is not an F100 or an F150. It's a '78 thunderbird with a 351M that has challenged just about every bit of knowledge I have on this family of engines, starting with the 302 I am most familiar with (over a 7 year build period) and now this 351M.

To Explain:

I built a 1962 Falcon Futura in the late 1960's (some might have called it modified production), with a modified 1968 302 V-8 (hipo exhaust manfolds, medium highrise 4V / 600 CFM holley and the 289 - 271 HP cobra cam and hydraulic lifters, complimented by a dual point mechanical advance distributor). Car weighed 2750 lbs and the engine was probably pushing 275 HP although it was never dyno'ed. (The magic 10 lbs / HP) Would almost lift the front wheels on the 1-2 power shift.

My point is I know how to set up a carb to run well, I can do it in my sleep on my '78 Dodge. My '69 Tbird had a Ford style flat top 4100 4V in place of the factory Autolite 4V and immediately improved from 12 mpg to 19 mpg on the highway without any low end bog like the Autolite provided. Would pass everything but a gas station!! Not bragging, just trying to say I know which end of a screwdriver to hold onto around a Ford.

The '78 Tbird might as well be a fullsized truck anyway, - everything on it is huge and you practically have to lay on top of the engine to reach parts of it. Brake components are huge too. Anyway, the vehicle was in storage for about 10 years until we pulled it out and began to make it road worthy again. Then it went back into storage for another 3 years until I brought it home to live here.

Here's the story:

The carb has been off it four times since moved from storage. It developed an idle and flooding issue about 6 years ago that was traced to a power valve (7.5) that had the gasket leak, which ruined the diaphragm. It had two power valves before I realized that sealing the edges of the gasket helped the valve to survive. I also chased a flooding issue that was the result of a mismatched needle and seat that caused the needle to lodge in such a way as not to seal properly once the float said 'enough'.

I replaced a defective dual port EGR, replaced the cap, rotor, wires, plugs and DuraSpark unit; - all of which significantly changed the way it ran, but I still had an issue with off idle throttle response where it would stumble and then clear it's throat and, well go ... sort of. I discovered that the accelerator pump was set at the max stroke (4th position away from the throttle plate) and through trail and error moved it down to the 2nd position where it is now. It seems to tell me that in some way it's still getting too much fuel at idle and the accelerator pump just makes it a little more rich too soon.

The vehicle will start on its own every time even without setting the choke, albeit with a lumpy curb idle (around 400 RPM) that slowly goes away as it warms up, or it will step up onto the choke if you touch the pedal just enough to set the choke. So clearly, parts of this carb's design are working properly.

I've fiddled with the air bleeds too many times but it doesn't seem to smooth out a noticeable idle roughness issue (which some have told me is inherent in the 351M's design). Yes, I could use a vacuum gauge to set the curb idle mixture, but I like doing it by ear. It's definitely running on 8 and I've rechecked for vacuum leaks several times, but no dice and no vacuum leaks, while using new gaskets to seat the carb on the intake spacer when it has been removed.

I searched the forum for info on metering jet sizes (not sure what size are in the carb now) thinking that maybe the wrong jets might be in the carb now, but my Dad's meat nuggets are all over this vehicle (he was a mechanic, and a good one) and everything you touch has an indication that 'someone's been here before!' I know he tried replacing the 2150 with a small Holley two barrel ('cause we found it in a box of bits when my parent's home was sold), but I'm guessing he didn't get the result he was looking for and returned to the 2150.

What I do know:

Almost 100% certain the power valve is fine, that there are no mismatched vacuum hoses connected to the wrong ports, and the electric / mechanical choke is not leaking. I'm chasing a gremlin, but not sure what to look at next.

Does this sound familiar to anyone here on the forum?

Robert

 

Moved to engine forum.

Welcome to FTE.

Have you fiddled with timing along with the carb idle adjustment? I had a stumble like this on a Mustang and was never able to really clear it up until I changed carbs. I improved it a little with a carb rebuild but it sounds like you've been through this one already.

 

Hi, and thanks for the reply and the welcome.

No, that is the only thing I haven't touched myself, but I need to pull out my timing light and see where Dad set it I guess. It's running on Premium fuel right now to avoid all the gunk the eco fuels create when the fuel sits for a while. Every once in a while when I do a hot start it has this high compression 'knock' that engines in the 50's and 60's used to have, but I know the compression ratio is not that high in this engine.

Pulling out the old school timing light and Tach and Dwell meter (dwell is useless due to the electronic ignition), but at least I can peg the idle RPM better than the sport tach it came with. I'll disconnect the vacuum advance on the distributor (plugging the vac line) and see what I can read.

He bought a set of Ford manuals for the 1978 model year, but frankly there is a lot missing in the engineering drawings and platform specific details I would like. He didn't buy the electrical or vacuum diagram supplements however, so that poses a problem trying to figure out vacuum hose routing and which hose goes where and why. Since we didn't remove any vacuum lines (and they are color coded) to do carb work it's safe to say everything is the way Dad left it.

On the hunt ...

Dad's answer to, 'What's the torque reference on 'such and such'? - One half turn passed stripped!

Have a great one!

Robert

 

I'm curious if the choke isn't drifting shut a little at idle. Maybe wire it to stay full open as a test?

Also curious what vacuum looks like at idle and just off idle. Does it do anything erratic?

I agree on checking the timing.

 

The timing check and the vacuum readings would be good diagnositcs, I agree.

A little vehicle history:

The 'Bird has 158,000 Kms (just shy of 100,000 miles) and is all original. it was a daily driver until 2005 when it was placed in storage until we removed it in 2015. At the time the battery was bone dead, the fuel pump was clogged and it had one Yokahoma tire with the sidewall ripped out of it (the owner of the building where it was stored moved it brutally by sliding it on the flattened tire!) Ouch! We had it flatbedded via AAA / CAA, replaced the battery, the fuel filter and the fuel pump, oil and filter change and it came to life. But it wasn't running quite right, so we pulled the carb and did a re-build (my late brother who was also a mechanic did the re-build in 2015). Then it went back into storage locally until 2018.

Current situation:

The choke does pull off at start nicely when first set, but it does this thing where it will run up on the fast idle and then 'hunt' for a good RPM to idle at on choke, then when I kick it down from 1500 to 1100, it either continues to run or slowly snuffs itself. On restart it's fine, comes up to 1100 and sits there. I know the ignition plays with the timing during the cold start and I figured, without an example to compare it to, that this hunting would be normal. The choke vacuum pull-off isn't leaking, but the diaphragm in it is 44 years old.

What's done:

Plugs, rotor, cap, wires, new DuraSpark unit (the unit on it was not original to the vehicle - evidence and meat nuggets!) Dual port EGR replaced (diaphragm bunned)
Two new power valves and copus tinkering with the idle and driveability issues since.

Plan moving forward:

I may have to pull the carb off again and go over it. (Getting pretty good at removing fasteners and yanking it off after three re and re's) I have one more intake / carb gasket left. Will check that the power valve hasn't died again due to the gasket leaking (which on this particular carb has been an issue three times now). I searched and found a thread somewhere that suggested sealing the edges of the gasket with fuel resistent permatex (left to fully cure for 24 hours) and that this usually prevents fuel from seeping passed the bottom of the bowl and the gasket, so I'll be surprised if the valve has traces of gas in it and the diaphragm is compromised again! Can also check and report the metering jet sizes as these may not be factory original (remember the meat nuggets evidence?).

I'll check and re-adjust float level to spec, re-adjust the idle air bleeds to an initial 1-1/2 turns, re-confirm choke pull-off adjustment and check the power valve. We're roughly 400' above sea level here in Niagara Falls so not a mountain setting for the accelerator pump (The situation with the accelerator pump is mentioned below and suggestions for a starting point are welcome)

I'll take the vacuum test a step further and disconnect and plug every vacuum hose connection to the carb with the exception of the vacuum advance and the choke and see what kind of idle readings I can get. I know the diaphragm in the vacuum advance holds because I checked it when I replaced the rotor and cap a year or so ago. Will disconnect the brake booster and associated vacuum tree (most of these vacuum lines are for the ATC inside which works fine) to isolate just intake vacuum.

A couple of notes:

I removed the purge solenoid at the carb two years ago because the leads broke off and it was stuck open. Sourcing, I learned the solenoid is now unobtainum. The line to the solenoid / bowl vent is plugged while the cannister is still under vacuum and capturing fumes from the tank. With the vent on the bowl capped I figured the two air vents in the top side of the carb would provide sufficient venting anyway (old school before emissions approach).

Second, I noticed a little trace of stray voltage from the electric choke present in the linkage that connects to the choke plate. You could see this 'aura' of trace voltage as the engine ran and the linkage jiggled about, so I ran a ground strap from one screw on the choke adjustment clamp to a carb stud and that solved that phantom issue. I know this isn't factory, but at least the choke was finding a good path to ground. I thought perhaps it might be induced voltage from the spark plug wires running close to the base of the carb, but I don't know and am open to opinions on that one!

Something I don't know (well let's just say there are plenty of things with this engine I don't know!) is the specific position for the accelerator pump. There are 4 holes that the rod can go in on the throttle shaft end. Shop manuals are hopeless. Uuugh!

PS - If I'm writing too much, let me know. I figured the more I could share the better view someone might have of the situation.

 

I didn't read all that, way too much typing. Did you say what carb you have? All the manufactures during that era had some pretty crappy carbs. With way too much emission junk on them that barely worked when it was new. Maybe you just need a new Holley without all the stuff on it.

 

Thanks for the reply.

 

Pull the carb back off again? Pull the top off maybe
You only need a few things to have a carb work
The float level needs to be correct (within the big range) (altitude and engine size mainly being the difference)
The choke must close cold and then pulloff the correct amount when started (absolutely necessary)
The throttle shaft needs to be tight (carb base plate not worn out) or all the above will not work
The little ball and weight must be in there for the accelerator pump circuit
The gaskets must all be correct and match the holes
Good luck

 

Thanks for the reply.

 

Trying to follow up on the suggestions received has proved problematic.

Put a timing light on the 351M this morning, and the timing is 6 or 8 degrees advanced beyond spec. So I can't reliably check vacuum until I get it to curb idle at factory advance spec. I fumed at how one would get the hold down loose to adjust the distributor, forgot there was such a tool as a distributor wrench but I know my Dad had one and now I know why.

And then I found GlennB's post below: https://www.ford-trucks.com/forums/1...loosen-it.html

I think I muttered something like 'Gawd, just kill me now, really!' (laughing about it now after reading GlennB's post)

Gave up for the day and will check my Dad's tool boxes (all 8 of them) looking for a distributor wrench.
Who knew?

 

Carb + todays fuel = off idle issue
Start with the basics like Timing
Noticed altitude at Niagara Ontario is only 198'
So timing at 4 degrees and watch for it to jump all over the place if the breaker plate in the distributor is loose
Check all that
Buy some better alky free fuel from a local oil company who sells racing fuel
Test with the good fuel. You do not need high octane, you just need alky free fuel for testing
Good luck

 

Thanks for the reply.

Couldn't agree with you more. The Premium at Costco is Ethanol free, unlike the Regualr fuel which is minimum 10% Ethanol, and that is why I chose the Premium. Not looking for a performance gain, just trying to avoid Ethanol, - period, and this is the least costly way to do it!

I had to check with a Geo map for the elevation you mentioned. Niagara Falls, Ontario is 603 ft above sea level. Kinda makes sense too since all the water in Lake Ontario and the St Lawrence River still has to flow 1200 miles to the east. Not enough elevation to give one a nose bleed and probably not too noticeable to the carb either. Either way, not adversely affecting factory settings or necessitating richer jets either. (still need to report back on jets in the carb now).

I'm still interested in knowing what the accelerator pump rod position should be relative to the throttle shaft as there are 4 positions possible.

 

The carb kit uses your tag number and your altitude to come up with the correct accelerator pump position
I think you should drive it in all 4 positions then decide which way it runs best

 

Any Luck yet?

If you havent found the problem I suggest you remove the carb and egr spacer and verify you are using the correct gaskets and the correct spacer for your manifold.
Most people do not know there are two styles of intake manifolds for the 351M/400 motors. Maybe more?
You have to make sure the egr/carb spacer matches the year of intake you have
I found this twice in the last few years, that someone replaced a 1978/9 manifold with a 1975 manifold but tried to use the 1978/9 egr spacer
And in one case the lower gasket blocked off the egr completly

Just a thought

 

Hi 427Mike, and thanks for the reply.

Wow, I forgot I had posted this but I am glad you found it and took the time to reply.

The EGR on this 351/M400 is a dual port I believe. I replaced the gaskets with the same ones it had on it when I removed the old one a year or two ago. It was factory original and the diaphram was shot.

Nothing, and I repeat, nothing I have done thus far has made it run any better off idle. At one point it seemed to run better, but I think the combination of the right position for the accelerator pump rod (relative to the throttle shaft) and a leaking / not leaking power valve may have contributed to the Tbird's off idle issues.

I have replaced the power valve three times, once because the fuel leaked passed the fibre seal and ruined the diaphram, again because it leaked passed the fibre seal until I finally replaced the valve again and sealed the edges of the gasket with fuel resistent gasket sealer where the seal fits between the valve and the bottom of the fuel bowl. I think I've finally fixed that right now, but will remove the carb yet again to have a better look at it.

I'm beginning to wonder of my Dad or my younger brother (both licensed mechanics) may have monkeyed with the metering jets and at this writing I have not pulled the carb to examine it on the bench and verify the size of jets. I know my Dad tried to run a 2 barrel Holley on it, but I'm not sure what demons he was chasing when he did that back in the mid to late 1990's. In any event the Holley didn't live there very long and he re-installed the 2150.

I find this particular engine exasperating to work on. I've considered whether the intake manifold gaskets may be leaking, but haven't gotten around to re-torquing them to see if that helps at all. I've considered blocking all the vacuum ports in the carb to see if I'm dealing with a vacuum leak. I reset the timing to factory specs and noted that their meat nuggets were on the distributor too, - the grease stick markings on the damper pulley on the crank. The car is 45 years old now, and meat nuggets have been all over it. I've corrected so many 'why did they do that' little things on it now (like ground straps, or lack of them, etc) that I'm beginning to better understand how this engine is supposed to run.

I had a '62 Ford Falcon Futura straight six that went from a 260 V-8 swap to a 302 HiPo swap over 7 years in the late 1960's and early 1970's. The older kids in the neighbourhood considered it a gasser, and I guess maybe it was. Would almost lift the front end on a 1-2 powershift. So I'm familiar with the original 221 through 289 family and 302 V-8's. Carbs and adjustments were never as finicky as this 351/M400.

And, It has an emissions air pump which may contribute to some performance issues, but I doubt it affects off idle issues. That's my story and I'm sticking to it!