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351M wonít rev past 1,000

  #1  
Old 09-05-2018, 07:34 PM
jtagge86
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351M wonít rev past 1,000

looking for some suggestions. Just tried to fire my 351M (79 bronco) after a complete rebuild, wonít go past 1,000 rpms, then if letting off gas engine shuts down completely. Iíve installed: new MSD coil, edelbrock 1406 carb (used but told it is in working condition), comp cams camshaft, new cap and rotor plus new plugs and wires, fresh gas. Not sure where to go? Any help is appreciated!!!

 
  #2  
Old 09-06-2018, 12:49 AM
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Did you get new valve springs to match the cam?
Are the throttle plates actually moving? Will it idle?
Did you go through the carb, make sure no crud, clean inside, assembled correctly?
If it idles, does it idle smoothly, no miss fires or back fires?
 
  #3  
Old 09-06-2018, 08:52 AM
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I actually purchased a complete kit from comp which included new springs. Plates open correctly, I didnít do a full cleaning of the carb though, I did take off the top to check accelerator pump operation and that does push fuel to the squirters. Floats seem correct. It does appear that the choke plate isnít opening all the way, but Iím not exactly sure the electric choke works, I have not tested that. Do you think it sounds more like fuel delivery? Iím tempted to just buy a new carb but I thought I could get away with a used one.
 
  #4  
Old 09-06-2018, 02:35 PM
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timing set correctly, vacuum advance working? is there a cat converter on that thing? if so it may be clogged.
 
  #5  
Old 09-06-2018, 03:52 PM
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Have you checked over the engine for vacuum leaks? Maybe a big one, like you left one of the big PCV / brake booster ports on the intake or the carb without a vacuum cap?
 
  #6  
Old 09-06-2018, 07:02 PM
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All ports are covered, I think after reading everyoneís suggestions Iím going to dig into timing a little further. I set initial timing to 10 degrees BTDC but now Iím wondering if I possibly didnít set the rotor position correctly. Iíll post back this weekend, thanks again for all the advice!
 
  #7  
Old 09-07-2018, 12:55 PM
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and the vacuum advance is hooked up correctly?
 
  #8  
Old 09-07-2018, 01:06 PM
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Also, don't overlook the off-chance things. On my fresh rebuild, I really struggled to get it to run right. Anything more than 1/4 throttle and it would pop and backfire. All the while, it ran rough and sputtered. I was really worried about something being done wrong and readjusted valves, checked timing, swapped the ignition out for another, tried another carburetor I had, etc. etc.

Finally, it turned out that it was one of my brand new spark plugs. I just got a bum one from O'Reilly.
Swapped them out with a fresh set and I had a whole new engine.
 
  #9  
Old 09-07-2018, 03:22 PM
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how did you adjust the valves or are they not adjustable. only way to set timing is with a timing light, anything else is just close maybe. electric choke will open if supplied with 12 volts and it takes 30-90 seconds to open fully. the timing marks lined up on the cam sprockets right.
 
  #10  
Old 09-08-2018, 09:34 AM
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Consider putting a vacuum gauge on the manifold? Thereís good guides out there correlating vacuum level to various issues.
 
  #11  
Old 10-09-2018, 08:41 PM
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Thanks to everyone for the insight. Since my last post I have: installed a brand new carb, new Duraspark ignition box, new fuel pump, checked all vac ports. I still only get about 1,000rpms then it pops and backfires through the carb and essentially doesn't run. It will return to idle and seems fairly smooth. I won't really let it idle because I have a brand new cam installed that I need to break in.

Stupid question.... if I mistakenly set valve lash when the piston was on top dead center after the exhaust stroke and not the compression stroke could that cause the engine to run this way? I'm just trying to think of all options.

I have new motorcraft plugs ordered as per AbandonedBronco's recommendation. That would be great if that solved it!

Thanks!
 
  #12  
Old 10-10-2018, 11:34 AM
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when setting the valves you have to do it on the compression stroke like you mentioned and yes it could makes problems for you. if there not set right you will find it real quick. you need to go back through and check each cylinder to make sure.

have you rechecked the firing order.
 
  #13  
Old 10-10-2018, 11:58 AM
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If you're so inclined, you can adjust the valves while the engine is running. It can just be VERY messy by splattering oil.

I made an adjustment cover/shield for mine by taking an old valve cover (I only have 1) and cutting off the top.



While running, loosen each one until it starts to clack, and then tighten it until the sound barely goes away. Then give it an 1/8th to 1/4 turn to set the lash. Go through all 16. If they're hard to hear because they're all out of adjustment, do it twice. The first will get them close, the second will hone it in.
 
  #14  
Old 10-10-2018, 12:12 PM
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Thanks for the replies. Iíve checked and re checked the firing order multiple times so I feel confident there. I think Iím going to have to chech valve lash ad you guys mentioned.

I have a brand brand new set of plug wires on but theyíre a little long and making contact along various points. Iíve read where signals can jump across wires but I wouldnít think this would impact brand new wires.

Ill start with valve lash. Thanks again. Iíll keep you posted.
 
  #15  
Old 10-10-2018, 01:51 PM
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Last time I had RPM can't go over 3000 problem, it's found to be melted down cat converters: the engine can't exhaust/breath.
 

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