Topic: my plan to un-freeze a 1974 cb750 motor

[Hondawggie]

Can you pros check my approach and method?
1974 cb750, 25k miles, bone stock, hm341 pipes in great shape, stock airbox, yada yada.

- ISSUE:  kick starter will not budge.  Gearbox is in neutral when attempting to kick it over.

- clutch lever works, ie. I can pull it in and it feels like the clutch is disengaging (has normal amount of resistance when I pull in the clutch lever)

- bike has some very minimal surface rust.  Has been inside the past 10 years except for the past 10 months. 

- it shifts fine.  When I put it into neutral, it rolls fine (front brake not stuck; rear brake not stuck; chain is fine if a bit dry).  It rolls fine, all day. 

- no battery in her and no key (yet).


MY PLAN to find out if only the kick starter lever is stuck or if the pistons are stuck in the bores
1) all 4 spark plugs: remove

2) spray some Amazing Blaster penetrating oil into each bore through the spark plug holes (it's been sitting 10 years, even if the pistons aren't froze, the bores are dry)

3) bike up on the main centerstand to get the back wheel off the ground

4) click it into 5th gear

5) see if I can rotate the back wheel, have my buddy listen/feel the spark plug holes, he will hold a paper towel in front of the plug hole in case the penetrating oil spews out

5.5) check if my hunch that the clutch lever's actually working -- pull in the clutch and see if the back wheel suddenly spins freely

6) if I can't rotate the back wheel, drop it off the centerstand and use the mass of the entire bike, see if she'll roll in 5th gear

7) if I can't get the back wheel moving, I guess the kick starter's not the only thing stuck


IF IN FACT I FIND THE PISTONS ARE STUCK, MY PLAN TO UN-STICK THEM
1) Marvel Mystery Oil in the cylinders for a few days

2) follow steps 1 through 6 above


My question then is this:

- Even if I SUCCEED in un-sticking the pistons -- is it then safe to run the bike?


One thing I'm not taking into consideration here is something wrong with the starter motor, ie. it got stuck in the 'engaged' mode, it is burned out and stuck, and that's preventing the motor from turning over.  I think that is a super longshot, so I'm not considering it (yet).
I will consider it when/if I get to the point where I'm cursing the cb750 designers for requiring engine removal to pull the top end.

[MoMo]

Try Seafoam, I have successfully unseized 3 Honda four cylinder bikes that are still running to this day without tearing them apart.  Take the point cover off and try to turn the motor over using the large 23mm nut. If it does not turn the pistons are stuck, my guess is they are. My "technique" with Seafoam is to put an ounce or so down each plug hole, wait several days and try to turn the motor over either with the kick, starter or crankshaft nut.  If it does not turn , I add a bit more Seafoam and wait several days and try to turn it over. I had one bike bread loose within a week, another took several weeks. Put a towel over the plug holes when attempting to start as the Seafoam will fly all over when the motor breaks loose-that is from experience.

I also had two that were so badly seized that I could not break loose without motor tear down.

I have heard that some people use a torch to heat the cylinders.  When younger and dumber we would wrap a rope around the handlebars, tow the bike to 40 or 50 mph and pop the clutch. Worked once, if the motor didn't  break loose you were in trouble.  A very undesirable method!

[Hondawggie]

Okay, I already have on the to-do list for tomorrow to buy a 23mm socket and box wrench, I will buy some Sea-Foam at that time.   I think you're right, that the pistons are probably stuck in the bore.

This afternoon as I tried to explain to the seller this issue the conversation went:

"The kickstarter is locked, so this bike probably has a stuck motor."

"Stuck?  What do you mean, stuck, it was running when it was parked." (tries the kickstarter)

"How long ago was it parked?"

"Ten years."

I explained that moisture creeps in through open valves etc. and that 10 years is a long time even if it was parked inside most of the time (and it looks like it was, just some surface rust, and none at all on the stock pipes.)

So I suspect I'll need to un-freeze it the patient way as you have outlined.   My only issue is -- with the bores oxidized enough to freeze the pistons, truth be told the top end should be pulled and cylinders honed and maybe new rings.  And maybe the valves lapped and oxidation removed from any affected valve stems.  Not that I intend on doing that unless absolutely un-avoidable.

[Grnrngr]

You should be twisting the nut on the alternator side rather than the points side as it is more capable of handling the torque, break the nut off the points shaft, or strip it, and you have a bigger project. PBBlaster or WD40 or something similar is a good start, my 72 750 was froze, got it broke loose and I've put an easy 20k miles on it without a teardown. On the other hand, I have a 74 motor that was so stuck, we had to pull the head and use a block of wood and a BFH to drive the piston out of the cylinder, even after soaking for several days.

[dave500]

if it does free up the rings will be stuck in their grooves and what not,imagine how dry the gudeons are,ide free it up then take it apart,sure you can get things going again but its not really the done thing,itll only wear fast.

[Hondawggie]

Yep this was my big point there, even if I do free it up, it's curing only a symptom of a possible problem that is un-knowable without a visual inspection, such as:  how bad is the whatever is locking up the motor.  

I can't really take what the prior owner told me.   The bottom end could have filled up with water for all I know.   However, freeing the motor without pulling it for inspection could well work here.  I'm inclined to believe it would as the bike clearly has not been outside much during its 10-year sleep.



Lots of bikes I've worked on allow pulling the head and cylinder without yanking the motor (kz1000, 1977-78 comes to mind).

I've never yanked a cb750 motor.  So I would ask, for a newb,
1) can I do it myself and if so, how?  The thought of removing all attached stuff (carbs etc) then putting on the garage floor a big soft sleeping bag and leaning the bike over onto the sleeping bag and wrestling the motor out of the frame comes to mind, as a prospective 'one-man-removal' method.

2) how much time to remove then reinstall the engine should I expect?  Not talking about pulling the head etc., just the amount of time to get her out of there then back in the frame when I'm done -- I'm thinking 2-4 hours max.

[ekpent]

Also note that if the motor has been sitting for 10 years than the clutch plates will surely be stuck together and the clutch will act like it does not "work" when you pull in the lever.Usually on unstuck engines its as simple as warming them up good and circulating fresh oil into that area to free them up. Good Luck on it.

[Duke McDukiedook]

50/50 ratio of ATF and acetone is the ultimate unsticker. Try and see if you can rotate the bolt on the points side first, don't hammer on it too much though. If it doesn't budge on that side pour in the 50/50 mixture and let it work it's magic for a day or two then repeat.
 

[eastoak]

Yep this was my big point there, even if I do free it up, it's curing only a symptom of a possible problem that is un-knowable without a visual inspection, such as:  how bad is the whatever is locking up the motor.  

I can't really take what the prior owner told me.   The bottom end could have filled up with water for all I know.   However, freeing the motor without pulling it for inspection could well work here.  I'm inclined to believe it would as the bike clearly has not been outside much during its 10-year sleep.



Lots of bikes I've worked on allow pulling the head and cylinder without yanking the motor (kz1000, 1977-78 comes to mind).

I've never yanked a cb750 motor.  So I would ask, for a newb,
1) can I do it myself and if so, how?  The thought of removing all attached stuff (carbs etc) then putting on the garage floor a big soft sleeping bag and leaning the bike over onto the sleeping bag and wrestling the motor out of the frame comes to mind, as a prospective 'one-man-removal' method.

2) how much time to remove then reinstall the engine should I expect?  Not talking about pulling the head etc., just the amount of time to get her out of there then back in the frame when I'm done -- I'm thinking 2-4 hours max.

i pulled mine out alone but had to take the wheels off to wiggle the frame off after laying it on it's side.
but i was taking the whole thing apart anyway.

[Hondawggie]

50/50 ratio of ATF and acetone is the ultimate unsticker. Try and see if you can rotate the bolt on the points side first, don't hammer on it too much though. If it doesn't budge on that side pour in the 50/50 mixture and let it work it's magic for a day or two then repeat.
 

automatic tranny fluid and acetone, whew that's a brew.  Never would have come to it on my own.

I bought my marvel mystery oil today but with your recommendation I'm gonna buy some atf tomorrow (already have the acetone) and put that in instead.  I have tried to unstick a stuck motor before (a cbx) and after 3 weeks of pentrating oil and a breaker bar on the right side of the engine, I got it moving but only about 20% (approx.) of piston travel so this time I want something a little more aggressive  -- acetone and atf sounds like it means business.

I got caught up and could not work on anything today but will pick it up tomorrow and thanks for the tip, and thanks to all for helping out here.

The cbx is in still in the yard under a canopy, if this new brew works that's my next stop.

[MoMo]

If you get it unstuck, either with Seafoam, acetone or Marvel and get it running you may not have to pull the motor.  The bikes I unseized are still running with good compression and no smoking, I think the Seafoam must have also loosened the stuck rings.  After I got the motors free, compression was low so I kept adding Seafoam and eventually compression came up to the 120 range and the motors then started.  After running to full operating temp compression was up to the 150 range. I hope you experience the same luck.

Get someone to help if you have to pull the motor, in all the years I worked at various shops it always took two men to pull or install a750 motor.  I can handle a 400 four, 63 years old and 160 lbs but anything bigger requires two men.

[Grnrngr]

As soon as I got mine unstuck a yr ago June (it had only been leaning against an outside garage wall for 2 or 3 yrs), put some Seafoam in the trans, pumped it around with the kickstarter a buncha times, drained the oil and pulled the pan. Had a nice little pile (actually, a perfect little cone) of rust sitting in there from the rings being rusted in the cylinder, maybe 2 or 3 ccs..a square inch worth? anyway, cleaned it out and put new oil and filter in, ran it 500 miles and changed it again. Also had to pull the clutch plates and pop them apart with a screw driver. Scuffed them a bit with some 220 sandpaper and put it back together, no clutch issues. Put a 1500 mile trip on it about a month later, hit 100+ a couple times, ran like a champ and still does. The chain rusted and had to be replaced as it ate the front sprocket on the trip, and the rear wheel bearings powdered themselves about a month later, but nothing wrong with the internals of the motor. I've put about 12,000 miles on it, it does have a little ring noise in #4 cylinder at 3600-3800rpm, but nothing to really worry about, at least not until I get my other motor rebuilt. I'm gonna run it till it starts running bad, should have the spare done by then, then it'll be time to pull it apart and see what it actually looks like inside. I suspect I may have to make an 836 out of it...  

[Stev-o]

Just bought a '77 750K and the motor is stuck. I was able to rotate the gears on the kickstart about 20 degrees but then it stopped.
Just put some 2 stroke oil in the cyls [didn't have ATF] and will try various methods to get it moving tomorrow.

[CrankyOldGuy]

Where were you guys years ago ... I manhandled the engine in and out twice in two weeks by myself with the bike standing upright .. maybe that's why my back hurts now    No scratches on the frame though.

Thanks for the info for unsticking stuck pistons .. this should work for most small engines (i.e snowblower, lawn mower, etc.) don't you think?

Harry O.

[Grnrngr]

I would think so, with the reminder that the smaller they are, the easier they are to break if you go beatin on them with a hammer like I did....

[import_tuner13]

i was told some pb blaster and patience.  broke the key on the points side for the adjuster today.  i have 2 frozen 78 cb750 engines.  lol.  you have it easy. my bike sat OUTSIDE for 10 years.  pulled the motor a week ago, took 2 people and 3 hours to strip my bike from fully dressed to frame.  to give you an idea.

[zzpete]

I've freed up many a cb750 motor in the following manner. First remove spark plugs and blast some PB blaster in each cylinder then ad some marvel mystery oil to each. I usually wait a week then I put the bike in 5th gear and rock it back and forth. It's always worked for me. After its free I spray a little more blaster in there and let it sit for a couple more days. If you have a hand vac you can suck out all the marvel and PB Blaster. Or you can have CB750 Olympics. I forgot to take the excess oil out once so when I kicked it over, the oil shot 14 feet 1 inch! 

[GammaFlat]

Excellent splooge distance!

I just freed a motor moments ago.  I have been trying to operate the kick starter for months now with atf and acetone in the cylinders (head off).  I feel like such an idiot.  I just hooked a battery up to the engine case and starter cable and it turned over.  I'm wondering how long ago that would have worked.... and maybe before I took the head off.  Admittedly, I have been tapping on the pistons with wood but I'm thinking this engine would have turned over when I first got it in "locked" condition.  I may have to attack another tonight with the sweet smell of success still in the air (acetone and atf).  

[dave500]

in the mid 80s my nieghbour gave me a little hb torana(car) for free,it had been out doors for years under a tree and was siezed,i pulled the head,plenty of crc/wd40 and a big hammer on a block of wood got it turning within half an hour,reassembled with the same head gasket and fired it up,pretty smokey,quick wet cut the tree sap off the paint and got 200 bucks for it.,these young kids had to drive it way across town home,with a james bond smoke screen behind them.no rego no insurance,those were the days.

[The_Crippler]

So, Hondawggie - did it work?

[Hondawggie]

So, Hondawggie - did it work?

I forgot to check today!  Goes to show you how life can get.  You have to do all the important stuff first.  Then the cool stuff gets forgotten. 

Reminds of growing up 'no you can't go hang out with your friends until you're done with homework and cleaned your room'.     Good 'ole Mom and Dad and discipline. 

I'm gonna check it tomorrow.  I did check it 2 days after filling all 4 bores full of 1/2atf + 1/2acetone -- I put it in 5th gear and moved it back and forth and it just dragged the back tire.  I put it in neutral and it rolls easily.  Now it's been sitting for many days and tomorrow I'm gonna put air in the tires and rock it back-and-forth.  I may also hook up my battery from my other cb750 (the running one) and see if what Gamma was able to do will work in my case.  I'm willing to be patient -- to a point.  If I have to pull the motor I will.  But I'll try everything I've read about on the forum here before that radical step.

I'll post tomorrow with the results, and thanks for reminding me.

[dna_level_c7]

ATF has always worked for me on seized car engines. Just pour a little in each cylinder and wait about a week then hit it with the breaker bar. Pops loose and then it's really smokey!  

If you're lucky it'll run fine!

[Hondawggie]

So today I got a chance to check out my '74 Honda cb750 whose engine was stuck, stuck solid too, no movement at all in the kickstarter, etc..

I have let the beast sit over a week with cylinder barrels full of 1/2 Auto Transmission fluid and the other 1/2 of the mixture was Acetone.

I had it up on the centerstand with a bike cover on it for a week -- today I took off the cover and then dropped her down from the center stand, my plan was to get it out in the driveway and rock it back and forth.  Dropped it off the sidestand, start rolling it backward to clear the cars in the parking lot, it's rolling back, "Good it's in neutral, I'll just push it back past the cars here..."

Then 'gurgle - THWOOOP!'

I'm still rolling it backward, and again I hear a 'gurgle-THWOOP'

"Hmm now just what could that be"

I put the sidestand down and went in the direction of that weird noise,
to the front of the bike -- and there is atf+acetone shot out, it's all over the front fender and fork legs and dribbling down the front of the engine, it is several feet in front of the bike on the asphalt driveway.

"Oh, I get it, it WASN'T in neutral -- I must have left it in 5th gear from the last time I tried to rock her loose......she's free!!!!!"  And the oil scavenge pump is working, because with oil tank currently removed, oil started coming back out of the engine via the scavenge pump and out the open oil-return hose.  

The kickstarter now works fine too!!!!


THE ATF+ACETONE has a date with  my CBX stuck engine next.  That brew works.
Thanks dudes, this '74 has a new lease on life now.


I now have a problem -- the huge spots on the asphalt -- besides re-sealing the asphalt,  in all seriousness, how do I clean up those spots?  It's a big mess.  I'd rather not have to apply asphalt sealer to cover it up.

[Stev-o]

Try kitty litter on the oil spots [or oil absorbant].

I'm going to try the ATF/Acetone trick this week on my 750K.

[Grnrngr]

Chuck Norris could scare those spots white.....