Engine turns free only without plugs...

[BradH71]

New guy here. I've recently acquired my great grandfather's shop truck. a '68 short box stepside 250 I6 with a 3 speed Saginaw. It was last registered for the road in '90 and "the truck ran great" when parked in my family's barn...23 years ago... Anyways, I got it towed to my garage and changed oil, plugs, filters, emptied and cleaned the fuel tank and put fresh fuel in. While the spark plugs are out I can spin the motor easily and the starter will turn it with no issues. The problem surfaces when I put the plugs back in. The inline 6 will turn to a point and then it not go further, not by the starter, not by hand. I've checked rocker movement with the plugs out and they seem to be in normal operation. I've since then ran out of ideas and my online search fell short with very little to no information. Help?

[RodnRudy]

Are you sure you have the correct plugs for the engine? If the plugs are extended reach the piston can be hitting the plug and not let the engine turn over any further. Check the plug length against one of the originals you took out.

[RodnRudy]

Oh, and Welcome to the board!

[Steelheader]

I'd have to say as long as your engine cycles completely and you have the correct spark plugs installed then your issue is most likely bad starter , weak battery or bad ground . With your plugs out you have no compression so it wouldn't take much force to turn over .
Good luck and welcome to the site , you should post some pictures of the truck .

[demian5]

Originally Posted by Steelheader

I'd have to say as long as your engine cycles completely and you have the correct spark plugs installed then your issue is most likely bad starter , weak battery or bad ground . With your plugs out you have no compression so it wouldn't take much force to turn over .
Good luck and welcome to the site , you should post some pictures of the truck .

He tried it by hand too, so not the starter.

[BradH71]

Thanks for the welcomes! I'll post up some photos as soon as I can get some taken. The old spark plugs yield the same results unfortunately, I'll go through all the starting grounds I can find and clean them up good, and try to bench test the starter. The battery is newer but I'll test that also, thanks for the quick responses guys!

[demian5]

Originally Posted by BradH71

Thanks for the welcomes! I'll post up some photos as soon as I can get some taken. The old spark plugs yield the same results unfortunately, I'll go through all the starting grounds I can find and clean them up good, and try to bench test the starter. The battery is newer but I'll test that also, thanks for the quick responses guys!

Did you put a ratchet and socket on the front of the crank and try turning it over using that?

[1968k20]

Hydro Locking?

[BradH71]

I turned it over using the fan, not the best way I know but I didn't crank on it too much before realizing the pistons weren't moving anymore. Is there a quick way of knowing if it has been hydrolocked? It was stored under a roof and the cylinders didn't push any water out with the plugs removed, and the oil didn't show any signs of water or coolant.

[Killer Bee]

assuming correct length plugs since you would probably hear metal contact if they were too long and you matched length to ones you removed..

baring engine over without plugs installed is easier due to lack of compression, naturally..

baring engine over with plugs installed will turn smoothly and tighten up as you come up on compression for each cylinder and let off as you pass top dead center..

good compression is a good sign and very possible starter/battery/connections are weak and you're not creating enough force where you're attempting to turn it over by hand..

the fan btw, being a bad choice if needing excessive force as you could bend or distort it out of balance..

a couple of small squirts of clean motor oil down each plug hole before baring over may help.. not too much to flood cylinder but just enough to lightly coat cylinder walls..

the crank bolt is limited on strength but generally acceptable for barring healthy engine over..

you'll want a snug fitting socket, extension if needed to pull straight, and a long handled ratchet or breaker bar..

should be smooth as it turns without abrupt mechanical sounds or stopping and tighten and loosen as you pass each compression cycle for each cylinder..

two full rotations of the crank will ensure one full cycle of all six cylinders and should be safe to fire..

you mentioned 'it turns to a point'.. same point each time or stops at different positions?

you can also try baring it over at the flywheel with a pry bar if you have a good angle against block or bell housing..

if you try this, be careful to anchor pivot point and engage teeth firmly..

sure sounds like everything is in order, please share your results and good luck!

[chainsawman]

Just wondering if you poured any oil down the spark plug holes before you put the new plugs in. Than can be causing hydro locking. Fluids can't be compressed.

[Steeveedee]

As has been mentioned, you may be running up a compression part of the cycle on some cylinder, and in any kind of healthy engine, it is going to resist quite a bit. Please tell us what the number of the spark plug is.

[demian5]

Originally Posted by BradH71

I turned it over using the fan, not the best way I know

You need to put a socket and ratchet on the front of the motor on the crank bolt and verify it can or cant be turned. As stated elsewhere, a healthy motor will be pretty hard to turn but it can be done.

Make sure that battery is charged and in good condition.

Maybe squirt some oil in the cylinders or a spray of wd40 in each one to give it some lube in case it has some crap in there from sitting.

Im thinking you may be pretty green on motors. Don't overthink it, like when you pulled the valve cover off before putting a tool to the crank to try and turn it. Pretty soon someone else thats green (or a youtube master tech) is going to chime in and say its the ignition switch, next thing you know, you got the damn dash apart.

[Grumpy old man]

It's a 250 L6 there is no crank bolt to use a socket on and if it won't turn WITHOUT spark plugs there is no reason to try with spark plugs ...

[demian5]

Originally Posted by Grumpy old man

It's a 250 L6 there is no crank bolt to use a socket on and if it won't turn WITHOUT spark plugs there is no reason to try with spark plugs ...

I quote:
"While the spark plugs are out I can spin the motor easily and the starter will turn it with no issues. "

[VetteVet]

Ihad the same thing happen to me with an electric generator with a single cylinder Briggs engine. I could turn it over and it would stop. Then turn it backwards and it would stop.
With the spark plug in or out.

Turns out there was some built up carbon and junk on the top of the piston that had hardened up over the years of non use. I ended up pulling the engine all apart and cleaning it all up and putting it back together and it started up on the second or third pull.

If you pull the distributor cap off and turn the engine you can tell which piston is up when it locks. Then turn it backwards until it locks again and you can tell if more than one piston is locking up. You may be able to pour some carbon dissolver in the cylinder(s) and waiting for a few days and try to turn it through with the plugs out.

[BradH71]

Thanks all the replies, the "tough spots" are when cylinder 4 and 5 come close to TDC. I didn't oil the cylinders before hand but I used a small amount of marvel mystery oil in those and the rest of the cylinders today and let it soak a bit. That seemed to work a little as I can now complete a full cycle by the crank only, with the plugs in. I'm starting to think I have a weak starter/cruddy bore combination. Would it be a safe bet to remove the head and see what I'm working against or would that be a waste of time and gaskets? Any suggestions for carbon solvents? Oh and I'm using Auto lite Copper 65's for plugs. Thanks again everyone for your help, I'm new to any engine that's 25+ years older than I am!

[vics stuff]

I would venture to say by the description that you have the incorrect spark plugs in it.
Vic

[Grumpy old man]

Pull the plugs and look into each cylinder with a flash light to make sure you see each piston come up to the top ,You may find after sitting for 24 years that there is a mouse nest inside that won't allow the engine to turn thru a complete rotation of the engine , if nothing found use the starter to turn it over WITHOUT the plugs in it using some marvel mystery oil in each cylinder .

[davepl]

Turn the motor until it won't go any further. Now wait 5 minutes and try again. If it's compression it will have long since bled off and the engine will turn.

If it's mechanical interlock, as I suspect, it will still be locked and will not turn after 5 min.

Almost gotta be the wrong reach on the plug I think.

[sleepertruck72]

https://www.summitracing.com/search/...nding&N=401131

Spark plugs seem correct...If it turns over easy enough by hand, maybe just a weak starting system.

[Alex V.]

I've seen an engine with gunk built up in the cylinder (Briggs) that wouldn't let it hit TDC, but plugs in/out wouldn't change that. Does it feel like it hits something solid and defined, or just gradually gets harder (building up compression) before it refuses to turn any more?

[68is4me]

Are you trying to start the engine? As in do you have spark? Maybe timing is off or wires switched? Just throwing some ideas out.