Author Topic: Motor Rebuild? Who's with me? (Read 6604 times)

Ephesmax · « **on:** December 06, 2011, 11:23:28 PM »

OK, So this cb750 has been giving me hell since I started with it. Coming down an off ramp the motor quit on me and now it wont turn over i think its locked up?
So my question to you bike gods is this. I've seen other rebuild threads before would you knowledgeable vets be willing to help me?

ekpent · « **Reply #1 on:** December 07, 2011, 05:13:45 AM »

Well did it run out of oil by chance ?. Try dropping the oil pan and filter and look for some fresh metal in there and maybe pop off the tappet covers and have a peek in there also to see if the top is dry and maybe siezed.
Could be a pretty long drawn out thread and may be better over in the Project/Shop catagory.

octagon · « **Reply #2 on:** December 07, 2011, 10:21:55 AM »

another option is a used motor - worth thinking through anyway - lots of parts bikes out there with locked motors from the cylinders rusting slightly - easier and cheaper to deal with that compared to a frozen crankshaft, if that's what you have.

Duke McDukiedook · « **Reply #3 on:** December 07, 2011, 10:40:26 AM »

Yeah, run the normal tests to see if your motor is seized- top gear see if you can push the bike and the motor turns.

If the motor is not seized run a compression test and if that is OK then find out about fuel and spark.

Ephesmax · « **Reply #4 on:** December 13, 2011, 04:40:30 PM »

OK, Sorry its been a while ive been working alot lately. Switched the bike into 4th gear and she wont roll the back tire even starts to skid a bit. Pull in the clutch while its in gear rolls fine. Pulled the left and right spark plugs and they looked ok. I dont think that i floated a valve. Pulled the dip stick out of the tank, Not even any oil on the stick. Pulled the oil filter and it seemed to drain off maybe half a quart? There were def some sandy filling on the filter but no huge chunks Pulled One of the Tappet covers off and it looked pretty dry in there could see some residue but no pools of standing oil.This leads me to believe it is indeed locked up. Question at the cylinders or the crank? Whats next fellas

Btw if a mod feels he needs to move my thread its fine with me

dragracer · « **Reply #5 on:** December 13, 2011, 05:01:16 PM »

If it has a kick starter, can you get the motor to turn??? If not, I would take the valve cover off next if you can remove it in the bike. See if the cam isn't broken or the cam chain. Inspect all the valve springs and retainers for breakage or something out of place. Put the rear of the bike up on jack stands as high as you can get it. Elevate the front wheel enough to level the bike. Take the oil pan off and see what you find there. If you come up empty then. its time to pull the head off along with the block so you can see whats going on with the rods and crank.

octagon · « **Reply #6 on:** December 13, 2011, 05:46:15 PM »

another thing you could try - take off the points cover and get somebody to watch while you rock the bike forward and backwards while it's in gear. see if you have any movement at the crank.
hope some others weigh in here, but with a bad crank or rod bearing you often have some play that would show up when you rock the bike back and forth - if a piston is seized you won't see any movement. that's what I would try anyway.

wrenchmuch · « **Reply #7 on:** December 14, 2011, 02:33:51 PM »

You didn't mention whether you drained the tank and sump as well . A bad back flow valve in the oil pump will allow oil to drain from the tank to the sump . This might be the reason you show no oil on the stick . Anything you find in the filter housing has been through the high pressure side of the pump . This will reduce the pumps efficiency . Were the revs low when it seized ? Oil might have been thin after a run on the highway . Highway speeds keep the pressure up . Then revs drop and the first thing to loose pressure is the head . Cam seizure in 750's is not uncommon . It happened to my K1 . I've counted 4 this year that I noticed . I would do as Octagon says but after rolling it the first time back the can chain tensioner off and try again . Go gently back and forth to avoid having the cam chain hop a tooth . If the crank moves now its cam related . If not its pistons or crank . I think its going to need to come out of the frame though .
Good luck
Mike

Ephesmax · « **Reply #8 on:** June 07, 2012, 12:50:31 PM »

Ok fellas so it's been a while but I'm going to pull the motor from the frame and start tearing it down. I'm thinking about doing a full rebuild pending the problem. I've got a full set of tools and some inspection equipment. I also got my hands on a factory Manuel. Look fwd to some pics bc I'm counting on ur knowledge to help me through the process

xsmooth69x · « **Reply #9 on:** June 07, 2012, 12:54:09 PM »

big bore 863 kit might as well!!

be a boss and get it done

harisuluv · « **Reply #10 on:** June 07, 2012, 12:54:58 PM »

Complete motor teardown/rebuild sounds almost fun at first, but I bet you could get another running motor for a decent price and save you a lot of time, money, and headache.

xsmooth69x · « **Reply #11 on:** June 07, 2012, 01:04:18 PM »

Quote from: harisuluv on June 07, 2012, 12:54:58 PM

Complete motor teardown/rebuild sounds almost fun at first, but I bet you could get another running motor for a decent price and save you a lot of time, money, and headache.

time money and headaches are the best part!

harisuluv · « **Reply #12 on:** June 07, 2012, 01:10:26 PM »

Fair enough!

wrenchmuch · « **Reply #13 on:** June 07, 2012, 03:00:07 PM »

I like re builds too but it is riding season. If you look for an engine look it over very carefully. You don't want two lumps of metal. If your existing engine just had cam problems it won't be too expensive to fix. Even if it is just a seized cam you would be wise to disassemble the whole engine and clean it out.

Ephesmax · « **Reply #14 on:** July 01, 2012, 02:34:35 PM »

Ok so i got the whole motor tore down to the source of the issue. I reread back through some of the old motor posts they were very helpful on how to keep everything organized.

The con rod im sure is shot from all the scouring as well as the crank, The scratches in the crank seem to be .005in deep as far as i can tell by eye. We will see when i take the block in for inspection. There is no discoloration of the aluminum around the journals or is there any pealing or cracks that i was able to spot so hopefully the block is still good. What do you guys think about turning the crank down ? Can i get oversize bearings? If not what issues would I encounter with a used crank if i try to replace it?

Now just to get out the stupid roll pin for the kick gear!

wrenchmuch · « **Reply #15 on:** July 01, 2012, 03:28:11 PM »

Yikes! What does the screen on the pump look like?

octagon · « **Reply #16 on:** July 01, 2012, 05:35:12 PM »

what caused that? what's the oil pump look like, or were there signs of a leak?
i've got a fairly low mileage crank - 20,000 miles - with the rods still on - pm me if you're interested.

Ephesmax · « **Reply #17 on:** July 02, 2012, 12:27:32 PM »

This is the pickup on the oil pump. It had a slow drip. I told my buddy to check his oil and he neglected it so thats what he gets i guess. I might be interested in that crank, once i get the kick gear out im taking it to get inspected i duno if they can turn this one down or not so we will see.

lucky · « **Reply #18 on:** July 02, 2012, 01:33:51 PM »

What was the milage when this happened?

Did it run out of oil?

Ephesmax · « **Reply #19 on:** July 02, 2012, 09:04:48 PM »

Quote from: lucky on July 02, 2012, 01:33:51 PM

What was the milage when this happened?

Did it run out of oil?

Milage is unkown bike doesnt have the factory speedo best guess is about 12k. Yea he ran it low bike had an oil leak.

Ephesmax · « **Reply #20 on:** July 05, 2012, 11:28:24 AM »

Aside from debris on the screen is there anything im looking for visually on the pump

wrenchmuch · « **Reply #21 on:** July 06, 2012, 08:07:42 PM »

Take the rotor assembly apart and check for rotor damage due to lack of lubrication. Look for ecsessive pitting on the rotor tips. Also check rotor and pump body clearances as per the manual. Flush the oil tank out. I would disassemble the engine and clean everything. Pay particular attention to the oil pathways to the head including the oil paths in the cam holders. Replace the pump cover o rings. The oem manual has sizes for them in the beginning of the engine section. There is an oil seal on the pump shaft between
The scavenge and high pressure sides of the pump. This is available as a 82 goldwing part. Look for it on the goldwing pump assembly.

vaughnsmith · « **Reply #22 on:** July 07, 2012, 02:29:52 AM »

I hate to be the one that sz this, but dude that motor is toast. this is not just an upper end job that could be done in a cost effective matter- we are talking a lot of very expensive and hard to find parts. It would be cheaper to buy another bike. At very least buy a new (to you) motor out of a wrecked bike.

To put the time and money into that motor does not make sense. plus you could have so many things wrong with it that you could get it back together only to stick it again when some parts you didn't replace fail.

Bodi · « **Reply #23 on:** July 07, 2012, 07:16:32 AM »

That is what we call a "parts motor".

Used motors are chancy but you have to be able to find a good one cheaper than what fixing that would cost.

Ephesmax · « **Reply #24 on:** July 07, 2012, 12:33:39 PM »

while i appreciate the input fellas, short of the block being toast im prolly still going to rebuild this. I like the challenge, i have other bikes to ride and have some experienced friends when i get to the hairy stuff. I like to learn new things and enjoy working on this motor in my free time.

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Author Topic: Motor Rebuild? Who's with me? (Read 6604 times)