delco1946 Posted July 13, 2020 Share Posted July 13, 2020 So I (finally) think I fixed an issue that manifested after getting my carb rebuilt, booster rebuilt, and carb and manifold gaskets replaced, all which greatly changed the vacuum leaks and air/fuel ratios. After getting all this done, it would take 10 seconds to start and just sorta "limped" to life. Two separate experienced mechanics/shops could not seem to address this problem, so its just weird that I spent two hours yesterday making a minor adjustment to the carb that seems to have fixed this (plate wasn't closing enough in the airhorn). Truck starts instantly now, cold or hot. With this issue out of the way and can focus on other things. I've slowly noticed (even before my adjustments) that this truck has a hard time accelerating up hills, even empty. I can floor it while cruising in 3rd and barely speed up. Heck with a full load I might not even speed up. I know 351Ms are gas pigs and not performance engines, but I would have assumed that a trailer pulling special would still be able to accelerate with a load up a hill. Is this normal???? If/when this occurs at 50 mph, it doesn't seem like i'd want to downshift to second to get better response or my rpms would skyrocket. I know 351Ms tend to have low compression, but cyclinders are in the right range and close (130 to 140) so unless the recent work could have changed that, That should still be the case. This may just be why people turn these into 400s?? I checked my coil, replaced the spark plugs and wires. The cap/distributor should get replaced at some point due to wear, but that probably isn't related to acceleration if it idles ok. Link to comment Share on other sites More sharing options...
old55pete Posted July 14, 2020 Share Posted July 14, 2020 From what you are describing, it sounds like it is out of time. Ether the ignition or the timing chain. there is a possability that the exhaust is plugged. If memorie serves, the 351M was a rather peppie engine when tuned properly. Another thing you can check is your fuel pump pressure and volume and the fuel filter. It sure sounds like it is starving for something. Link to comment Share on other sites More sharing options...
Gary Lewis Posted July 14, 2020 Share Posted July 14, 2020 From what you are describing, it sounds like it is out of time. Ether the ignition or the timing chain. there is a possability that the exhaust is plugged. If memorie serves, the 351M was a rather peppie engine when tuned properly. Another thing you can check is your fuel pump pressure and volume and the fuel filter. It sure sounds like it is starving for something. I agree. And I'm guessing it is an ignition timing problem. But if you think it is timed correctly I'd suspect a slipped harmonic balancer. I've seen many of them and if yours is slipped and you use it to set the timing you'll have very poor performance. The accurate way to check the balancer is via a piston stop. But you can get close enough with a straw or even a screwdriver to determine if your balancer has slipped. Pull the plugs and turn the engine over slowly by hand until you feel pressure coming up on #1 plug. Stop and put a straw or some such in down against the piston and then turn the crank slowly, stopping when the straw stops coming up. That should be really close to TDC and your balancer should show that. And there's also the timing chain, as Steve suggested. If it is really slack or, worse, has jumped time, you'll have poor performance. Link to comment Share on other sites More sharing options...
delco1946 Posted July 14, 2020 Author Share Posted July 14, 2020 I agree. And I'm guessing it is an ignition timing problem. But if you think it is timed correctly I'd suspect a slipped harmonic balancer. I've seen many of them and if yours is slipped and you use it to set the timing you'll have very poor performance. The accurate way to check the balancer is via a piston stop. But you can get close enough with a straw or even a screwdriver to determine if your balancer has slipped. Pull the plugs and turn the engine over slowly by hand until you feel pressure coming up on #1 plug. Stop and put a straw or some such in down against the piston and then turn the crank slowly, stopping when the straw stops coming up. That should be really close to TDC and your balancer should show that. And there's also the timing chain, as Steve suggested. If it is really slack or, worse, has jumped time, you'll have poor performance. well i will say that when all this rebuilding occurred, i had them install a second fuel filter which might restrict flow slightly, but I would also think/ hope they would have checked/noticed issues with this area? Is there an easy way to check for exhaust blockage? I have a cat so i'm guessing it would all need to come off upstream of that...? Also, if memory serves, the timing had been checked and was "spot on" (6 degrees i think). Records indicate the engine was rebuilt 50,000 miles ago but sometimes I wonder. They cleaned out lots of sludge in the valley pan apparently. If they didn't replace the harmonic balancer then, its likely the original (almost 40 years old with 153K miles). Even if they did in sure its from the rebuild (1993), so it sounds like thats my number one suspect. I did some quick research and i do have a slight lopsided vibration when at stoplights that my also indicate the balancer is no longer doing its job. Link to comment Share on other sites More sharing options...
Gary Lewis Posted July 14, 2020 Share Posted July 14, 2020 well i will say that when all this rebuilding occurred, i had them install a second fuel filter which might restrict flow slightly, but I would also think/ hope they would have checked/noticed issues with this area? Is there an easy way to check for exhaust blockage? I have a cat so i'm guessing it would all need to come off upstream of that...? Also, if memory serves, the timing had been checked and was "spot on" (6 degrees i think). Records indicate the engine was rebuilt 50,000 miles ago but sometimes I wonder. They cleaned out lots of sludge in the valley pan apparently. If they didn't replace the harmonic balancer then, its likely the original (almost 40 years old with 153K miles). Even if they did in sure its from the rebuild (1993), so it sounds like thats my number one suspect. I did some quick research and i do have a slight lopsided vibration when at stoplights that my also indicate the balancer is no longer doing its job. 351M's can usually handle a lot more than 6 degrees BTDC w/o pinging. But, they sometimes kick back on starting, so a DS-II module that provides that function would help prevent that. So once you find out if your damper is correct, I'd dial in at least 10 degrees if not 12 and test that. It will help the power. Link to comment Share on other sites More sharing options...
old55pete Posted July 14, 2020 Share Posted July 14, 2020 well i will say that when all this rebuilding occurred, i had them install a second fuel filter which might restrict flow slightly, but I would also think/ hope they would have checked/noticed issues with this area? Is there an easy way to check for exhaust blockage? I have a cat so i'm guessing it would all need to come off upstream of that...? Also, if memory serves, the timing had been checked and was "spot on" (6 degrees i think). Records indicate the engine was rebuilt 50,000 miles ago but sometimes I wonder. They cleaned out lots of sludge in the valley pan apparently. If they didn't replace the harmonic balancer then, its likely the original (almost 40 years old with 153K miles). Even if they did in sure its from the rebuild (1993), so it sounds like thats my number one suspect. I did some quick research and i do have a slight lopsided vibration when at stoplights that my also indicate the balancer is no longer doing its job. To check if the exhaust, unhook the Y pipe at the manifolds and drop it 3 or 4 inches and take it for a test drive and see if it changes the performance. Link to comment Share on other sites More sharing options...
1986F150Six Posted July 15, 2020 Share Posted July 15, 2020 Is there an easy way to check for exhaust blockage? I have a cat so i'm guessing it would all need to come off upstream of that...? https://www.bing.com/videos/search?q=checking+exhaust+blockage+with+cvacuum+gauge&docid=607993629045621911&mid=EF77EC6467A280BFD885EF77EC6467A280BFD885&view=detail&FORM=VIRE Link to comment Share on other sites More sharing options...
old55pete Posted July 15, 2020 Share Posted July 15, 2020 Is there an easy way to check for exhaust blockage? I have a cat so i'm guessing it would all need to come off upstream of that...? https://www.bing.com/videos/search?q=checking+exhaust+blockage+with+cvacuum+gauge&docid=607993629045621911&mid=EF77EC6467A280BFD885EF77EC6467A280BFD885&view=detail&FORM=VIRE Good find Dave, I had forgotten about doing it with a vacume guage Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted July 16, 2020 Share Posted July 16, 2020 Are you sure you're using the 335 firing order? 13726548 Link to comment Share on other sites More sharing options...
delco1946 Posted July 16, 2020 Author Share Posted July 16, 2020 Are you sure you're using the 335 firing order? 13726548 Thanks for all the suggestions! i'll try to look at these tomorrow or this weekend. Link to comment Share on other sites More sharing options...
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