Topic: A cb750 for a song and dance

[nubie]

I recently purchased a 1975 CB750 from my neighbor.  I bought it for about $150.  It had been sitting for about 10years.  The speedo said 39K and it appears to have very minor modifications.  My friend and I are somewhat mechanically inclined but do not have user specific tools for certain engine requirements.  We changed the oil, checked and tested the electrical and changed the spark plugs, rebuilt the carbs (all 4) with a quality kit, emptied out the old gas and used new gas.  After jumpstarting it with my truck(12v) we were able to start the motor.

2 Problems: 

1.  It burns oil......we checked the plugs (new and old) and they are not oil fouled (maybe a valve sticking?).While the bike was running we sprayed carb cleaner through the carbs and put Marvel mystery oil in the oil tank.  Still same affect.  It will burn clean for about a minute (no smoke) then it will start to smoke. 

2.  The sprocket that the chain attaches to......I believe it is the housing for the transmission?  Anyways....the previous owner disclosed to me that a rag got caught in the chain and put a hole in the transmission housing as the rag passed over the sprocket.  He patched the hole with JB Weld but it still leaks oil.

My questions:  What are our options with the above two scenarios without having to take it to a mechanic.  We love to get advice on troubleshooting.

I just did some cross referencing and it appears that the frame number is 2524822 and falls under 1975.  The engine number falls cb750e-2406422 falls under 1974.  I am going to naturally assume that it is not original and if an engine swap is in order...what years should I be looking for?

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[bryanj]

easy fix is another used engine

[GoatBaSS]

And get me the # to the used engine store as I need one also!!!!!!!

[Geeto67]

Don't know where to start, except to say that if you own a digital camera, take a pic of where the rag (WTF?!?!) made a hole in the case (then explain to me how a soft rag makes a hole in a hard engine case).  It may be a small hole in the cover, or you might need a whole new bottom case. Without pics we can't tell ya what needs replacing.

As far as the burning oil thing, let's start in reverse. Clan the pipes out. 10 years of crap collecting in the pipe and then the pipe getting hot can cause all sorts of smoke. For all you know you could be barbecuing a dead mouse in each pipe, and let me tell you the fur smokes good. Stop mixing oil into the gas, marvel mystery oil is a good product for unsticking rings, etc, but it is not 2 stroke oil and this bike is not a two stroke, oh and by the way use fresh gasoline. If you are not seeing oil fouled plugs, chances are you are burning it else where.

If there is anything mechanical wrong causing the smoking my money is either on valve seals, or blow by. To check blow by do a leak down test (you will need compressed air or a bicycle pump to do this). After the pipes are cleaned and it still smokes you might as well pull the motor out and check the top end. Before you have to ask, the valve cover does not come off with the motor in the frame so yes you will have to pull the motor. 39K is a decent amount of miles so you probably want to check the valve train tolerances.

Clean the carbs and rebuild them, Use only the gaskets from a rebuild kit, keep the original hardware. Get the bike running under it's own power, 1st.

as for an engine swap, don't put the cart before the horse. At one point this bike was running fine, and then it got put away, try and get it back to it's pre-storage condition before you decide it needs to be replaced.

oh and fill out your bio and let us know your location, if there is someone close to you who can help out, that is 100 times better than words on a screen.

[dusterdude]

geeto fyi,mmo is also a good carb/combustion chamber cleaner too.

[Geeto67]

geeto fyi,mmo is also a good carb/combustion chamber cleaner too.

yes it is but it smokes when you burn it. at least the mmo I put in my buick 455 did when I ran it.

[nteek754]

Hey there as far as the  hole in the case I would take all of the  j b weld off clean area rough up area The end of a file will do fine then take som,e aluminum tape make a little patch out of it then mix up some new j b weld I DID THIS 25 YEARS AGO WORKED FINE AND i HAD A HOLE  ABOUT 2 INCHES BY 2 INCHES 


ps keep after the j b weld cause it will want to sag and droop

[TwoTired]

1.  It burns oil......we checked the plugs (new and old) and they are not oil fouled (maybe a valve sticking?).While the bike was running we sprayed carb cleaner through the carbs and put Marvel mystery oil in the oil tank.  Still same affect.  It will burn clean for about a minute (no smoke) then it will start to smoke. 

 A valve sticking will make the engine idle badly, at least.  It's not, generally, a cause of oil burning.  I'm assuming your oil burning diagnosis stems from exhaust smoke with a blue color?
Valve guides, valve guide seals or piston rings leaking will burn oil.  A dry compression test, engine hot, followed by a wet test, will isolate the problem between rings and valve guides (or guide seals).

The following horror senarios may or may not apply:
A) The bike sits and some valves are open.  Humid air rusts the cylinder walls, rust is abrasive and soon grinds the rings down to where the they can't seal.  And, if the cylinder walls have pitted, rings will never seal.
B) The rings can also glue themselves into the piston grooves and not move out to seal against the cylinders.  They sometimes free themselves after operating for a while. Sometimes not. MMO should help, but, it may take some miles to do so.  You could try direct application of MMO to the cylinders through the spark plug holes.  Let them sit and soak for a day or two like you would penetrating oil.  Then do some high power high load runs to blast the rings loose.  If the follow up dry to wet compression test results don't come closer together, you're going to have to go in and touch the rings and examine the cylinder walls.

2.  .......He patched the hole with JB Weld but it still leaks oil.

JB weld patches can work well if applied properly.  I'd try and clean off the old patch and do it over.  Or, you can replace the bottom case.  Or, have the one you have welded up.

A rag, huh.  More likely the chain broke, me thinks.

I just did some cross referencing and it appears that the frame number is 2524822 and falls under 1975.  The engine number falls cb750e-2406422 falls under 1974. 

According to the Honda Motorcycle Identification Guide, both your Engine and frame are 1975 models.

Cheers,

[Bob Wessner]

Or, you can replace the bottom case.

I thought I had read someplace that the upper and lower case halves were a matched set for fit at the seam? 

[TwoTired]

Or, you can replace the bottom case.

I thought I had read someplace that the upper and lower case halves were a matched set for fit at the seam? 

I can easily believe that for the sand cast models.  But, after production tooling was instantiated, it seems the cases would be very uniform in dimension.  It would be pretty time consuming and expensive to mill each case half set for precision fit back in the 70's.
But, I don't know for certain.  You may well be right.
Cheers,

[Bob Wessner]

I caneasily  believe that for the sand cast models.  But, after production tooling was intantiated, it seems the cases would be very uniform in dimension.

Makes sense, thanks.

[dusterdude]

geeto fyi,mmo is also a good carb/combustion chamber cleaner too.

yes it is but it smokes when you burn it. at least the mmo I put in my buick 455 did when I ran it.

  yes it do but aint it purty.

[nubie]

I talked to my local mechanic who does nothing but work on old Honda's.  He has an engine from a 1978 that had low miles on it.  We did a compression test and the readings came out at 125-130 for all of them.  He sold me the engine for $350.  All the covers are in excellent condition and there is very little oil leakage.  Since we do not have all of the tools that would require an engine rebuild.  We figured it would be a simple fix to swap engines ourselves.  If we decided to stick with the original engine, we would have to replace the transmission housing for the hole.  Engine out....Engine in....Wax on....Wax off.....As much as I want to use JB Weld to solve my problems...If it starts to leak and I am not on top of it, I could be bumming at a later date......Maybe when I am driving.  I will keep everyone posted on my results.

[nubie]

Well, it appears that the engine we swapped for may have been a good idea.  The original engine (when we dropped the oil pan) appeared to have a lot of metal, glass, jb weld (etc) on the bottom of it.  There was an area on the bottom of the engine which I believe is an oil pump filter?...was coated with debris essentially preventing normal oil flow from entering the pump area.  What it really looked like is a lint screen that you clean between dryer loads.

Now.....The new engine is in and we tried to fire it up to no avail....we both feel it is the points becasue we used the old timing and contact plate from the original motor(1975 750) and put it on the new motor (1978 750).  My mechanic said to do a static timing test with a tester to get it properly timed.  Buy a 12v tester....hook up the positive to the contacts and ground the other and make sure you time it to "F"....

Any input ? .....

[SteveD CB500F]

You need to use the scientific method:

1. Spark
2. Fuel
3. Air

Without any of these it won't fire.

So,  check the spark first

then check if the plugs are wet

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