whats normal acceleration for a 351M?

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.

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.

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.

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.

delco1946 wrote

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

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.

So I’m not sure what you meant by a 335 firing order - I looked up the firing order for a 351 and my engine seems to match that. I then hooked up a vacuum gauge based on the cat being suspect or a clog of some sort. Ive watched several YouTube videos and I guess I’m still not sure how to interpret my results.

At idle I measured approx 22 psi to 24ish. When I accelerated and raced the engine and steadied it ( no tach) the vacuum dropped to 17-18 ish and the needle was bouncing a lot within a narrow range, but if I held it steady it would stay in that general range. I can’t tell from the vids if it’s supposed to keep dropping or if the bouncing needle is more important. Either way it seems like a maybe have a clog.

Adding to this, starting late last summer, when I come to a stop, with the fan blowing, I can sometimes smell a stink. Never have been able to figure it out. Doesn’t smell like what comes out of my tailpipe ( after cat and muffler) but when I stick my head under the hood the closest I’ve ever been able to pin point it is somewhere near the passenger exhaust manifold. A blockage would make a lot of sense in that the back pressure is causing exhaust to escape into the bay and get sucked up by the blower motor.

I crawled under the truck and I don’t see any obvious easy way to disconnect the cat. Saw a couple of u bolts that looked liked they’ll be a pain to get off.

Christopher, can't you loosen the head pipe to manifold connections?

I’ll take a look Jim. I’ve never done that but I guess there’s a first time for everything.

Do my symptoms and read outs sound like a clog to someone more familiar with this than me?

David,
This is the same guy Reamer (Ron) referenced when he swore up and down that he didn't have an exhaust blockage.

We all know that turned into a two month, $2,000 adventure when my advice in the third post of that thread was to disconnect the head pipe.

Of course YMMV, but I'm not listening to Eric's poorly thought out internet advice -at all- anymore.

Thank you, Jim.

David,
I don't mean to pee in anyone's Cornflakes.

But I will go grab a link to that thread, and we can relive all the twists and turns (including, but not limited to: a new timing set, carburetor, distributor and Ignition box)


Edit: ironically it was almost exactly a year ago...   http://forum.garysgaragemahal.com/Hmmm-loss-of-power-won-t-hold-speed-tp33697.html

Then this thread spawned "My timing chain adventure begins"
Which did net a replacement for Ron's sloppy timing chain, but that thread went on for 12 pages still didn't fix the problem..  

Does it smell like rotten eggs? With those readings, I would spray down the nuts that hold the Y pipe to the mianfolds with some PB blaster and let it soak, spray it down several times, then hit the nuts with some heat from a propain torch. Do what ever you have to do to NOT break the studs as they are a pain to drill out. Then take it for a test drive and see if the performance returns. If it does, eather the cat is plugged or the whole system is plugged.....If not, it's back to square one.

Hey Steve - the smell ive gotten isnt quite rotten eggs, but is certainly unpleasant. I guess if i had to pick "a smell" it reminds me more of burning dirty electrical stuff, old crud/rubber etc. But i've never been able to see anything being damaged. I seems to be focused at the back of the passenger manifold and near the heater.

I've removed the heater, cleaned it, and vaccumed all the crud out so nothing related the dropping resistor or heater should be "burning" etc. I wonder if something has plugged up the pipe between the manifold and cat that may just be slowly burning. I dont recall having this acceleration problem when i first got the truck (i didn't have the smell either )

I'm not really following some of the last posts to "David" but regardless I think this has been very helpful. Confirming a plugged exhaust is at the top of that list with installing a new harmonic balancer shortly after!

delco1946 wrote

I'm not really following some of the last posts to "David" but regardless I think this has been very helpful. Confirming a plugged exhaust is at the top of that list.....

Go read for yourself what a clogged cat seems like, Christopher.

Given the smell you say you have I'd consider dragging brakes, because we don't have much sulphur in any fuels anymore.
The acid rain was a big problem back in the '70's and early '80's before sulphur was regulated.

delco1946 wrote

Hey Steve - the smell ive gotten isnt quite rotten eggs, but is certainly unpleasant. I guess if i had to pick "a smell" it reminds me more of burning dirty electrical stuff, old crud/rubber etc. But i've never been able to see anything being damaged. I seems to be focused at the back of the passenger manifold and near the heater.

I am likely way off base, but could the engine be running very lean? This condition has a distinctive odor and would explain difficult starts and being low on power.