Lockup on start

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   1976 351M wants to lock up when I try to start it with foot off the gas pedal.. It's like a cylinder is trying to fire and won't let the engine turnover.. To actually start it, I have to depress the gas pedal and it spins over and starts normally.  Doesn't matter, hot or cold. Anyone come across this issue before?  I've retarded the ignition timing to the point where there's a loss of power.  New timing gears and chain 0 to 0... 2150 carb, FMX transmission.. 76,000 miles.
Thanks

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piper62j Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 21-July-2018 Location: 34613 Status: Offline Points: 131	Post Options Post Reply Quote piper62j Report Post Thanks(0) Quote Reply Topic: Lockup on start Posted: 13-February-2024 at 1:52PM
	1976 351M wants to lock up when I try to start it with foot off the gas pedal.. It's like a cylinder is trying to fire and won't let the engine turnover.. To actually start it, I have to depress the gas pedal and it spins over and starts normally. Doesn't matter, hot or cold. Anyone come across this issue before? I've retarded the ignition timing to the point where there's a loss of power. New timing gears and chain 0 to 0... 2150 carb, FMX transmission.. 76,000 miles. Thanks

Inkara1 Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17-November-2021 Location: Ridgecrest, CA Status: Offline Points: 515	Post Options Post Reply Quote Inkara1 Report Post Thanks(0) Quote Reply Posted: 13-February-2024 at 4:33PM
	Just a shot in the dark here, but is it somehow able to make such a good seal with the throttle blades that it somehow creates too much vacuum and keeps things from moving right? I can't imagine how else it wouldn't crank at all unless the throttle is open. Like maybe the transfer slots are full of gunk or something. Maybe unplugging a vacuum connection to see if it'll at least turn over would be a way to check that. It seems unlikely, though. Only other thing I can think of is a leaking power valve or accelerator pump dumping the contents of the fuel bowl into the intake when it's off, then it running into a cylinder with an open intake valve, but no one cylinder should get enough fuel into it to hydrolock, especially since opening the throttle doesn't change that but just introduces air and lets it crank. What condition is the starter in? Also, does it immediately lock up, or does it crank a little bit and then lock up?
	1972 Gran Torino station wagon, light blue, 351C 2V, prior owner upgraded to 4V (2V heads), originally had a C4 but prior owner changed to C6 for some reason.

peter.jenerette Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 08-February-2023 Location: Nevada, TX Status: Offline Points: 1054	Post Options Post Reply Quote peter.jenerette Report Post Thanks(0) Quote Reply Posted: 14-February-2024 at 1:54AM
	"new chain and gears" are you sure its dead on per spec for your cam? You may have a late opening valve causing early compression. Check your push rods and rockers, see if there is something off there. Years ago, (like 1990), i had Q code that would do a partial spin, seemingly lock, hold the key and it would 'lump over' the spot and then fire. Turned out my cam was partially wiped delaying the valve opening...just a thought. Good luck!
	Pete Jenerette 1972 Gran Torino (H-Code - 4R70W) 2022 F250 XLT 7.3 2003 Thunderbird

piper62j Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 21-July-2018 Location: 34613 Status: Offline Points: 131	Post Options Post Reply Quote piper62j Report Post Thanks(0) Quote Reply Posted: 15-February-2024 at 7:32AM
	Great suggestions..Starter is rebuilt and did the same thing the old starter did.. I removed the carb and found gas in the power valve bowl. I replaced it and have the same issue.. I will pull it down again to make sure the valve is working and not defective.. Compression is all in the 160 to 170 range and I installed an oem rebuilt distributor.. Will update sortly..

72 RS 351 Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 04-September-2014 Location: Knoxville TN Status: Online Points: 3445	Post Options Post Reply Quote 72 RS 351 Report Post Thanks(0) Quote Reply Posted: 15-February-2024 at 12:15PM
	I'd adjust the timing a good bit and see if it affects the cranking effort, see if the timing could be way off. Unlikely but at least it runs, and thus it is easier to work on.
	*Don* 73 Ranchero "Sport 72 front end", floor shift/console, planning EFI 7000 rpm 351 stroker 73 Ranchero GT 351C-4V &4R70W for sale later. 92 Lincoln Mark VII SE GTC, OBDII 347/4R70W

78FordLtd2 Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 19-April-2021 Location: Canada Status: Offline Points: 184	Post Options Post Reply Quote 78FordLtd2 Report Post Thanks(0) Quote Reply Posted: 15-February-2024 at 2:35PM
	I'd go back to the basics and static time your engine.When putting in your distributor, you may not have been at TDC on #1 and you might be a tooth off on the distributor. Make sure you're at TDC on #1 and verify with the marks on the harmonic balancer. Alsi verify #1 cylinder with the rotor before re wiring your distributor. Try the easy things first and hopefully it fixes your problem.

piper62j Members Profile Send Private Message Find Members Posts Add to Buddy List Member Joined: 21-July-2018 Location: 34613 Status: Offline Points: 131	Post Options Post Reply Quote piper62j Report Post Thanks(0) Quote Reply Posted: 03-March-2024 at 12:43AM
	Here's what I've found so far.. The power valve is leaking fuel into the engine. There is always ignition available when starting and what fuel is in any cylinder, it will ignite causing a fireup in that cylinder. I've gone thru 4 power valves so far and they are all leaking.. Don't know the cause yet and the response from Mike's carburetor is that there is something in the fuel eating away the power valve diaphram. More research is coming and I will update soon.

aquartlow Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 19-December-2011 Location: Summerfield, Fl Status: Offline Points: 2341	Post Options Post Reply Quote aquartlow Report Post Thanks(0) Quote Reply Posted: 03-March-2024 at 2:54AM
	IIRC, the Motorcraft 2150 carb uses the same type of power valve that a Holley uses, if so you may want to try a power valve plug that is usually used on the secondary side of a Holley 4150 to verify if it is in deed a diaphragm issue or a cracked carb body issue. This is the Holley PV plug I am referring to. Hope this helps. Quick Fuel 25-1QFT Power Valve Plug \| eBay
	www.supermotors.net/22468 Yeah, It's a Hybrid...It burns gas and tires. No matter how good she looks, somebody, somewhere, is tired of her sh*t. Beauty is skin deep, ugliness goes clear to the bone.