Topic: CB 750F '78 Top End Rebuild?

[markusarealius]

Okay fellows/gurus,
I have finally gathered the money/will power to tear into the love of my life.  I have very limited mechanical knowledge but I have a good array of tools and books.  Here is my situation: My bike (CB750F 1978) is difficult to start, but I think it has carb issues - and i will tackle that once I get the major problem addressed.  This leads us to the major problem.  Once it gets warmed up, it starts blowing blue smoke very bad.  My guess from reading all around on this site is it needs new guides or possibly new rings.

I have not tore anything apart yet, because I am afraid once I do, it may never run again (although I haven't been able to ride it in its current state anyways).  Here is my question(s).  I would like it to remain as close to stock as possible.  The bike has 16,898 miles on her.
1.) Can a novice rebuild the top-end without fancy high dollar machine tools?
2.) What should I replace since I have the top-end apart anyways?
3.) My budget is only $600 to get her running again, is this a feasible amount?
4.) What parts and gaskets will I need -- and where can I get them?
5.) If the guides need replacement can I do this myself?

This forum is fantastic, and I have learned a great deal -- thanks for any advice or suggestions, everything is welcome, you can't offend me.....i know i have a great deal to learn...i am grasshopper....you are master!!

[lucky]

First assumption is wrong.
Blue smoke on start up until warm can be worn valve guide seals.
If the seals are worn oil can leak into the combustion chamber.

NEXT...

If the valve guides are worn you can tell with a vacuum gauge test.
While it is idling the vacuum gauge needle will bounce all around.
With good valve guides the vacuum needle will stay steady.

If the cylinder head is removed and turned upside down and you pour  a teaspoon of gas into the combustion chamber and it leaks out of the intake or exhaust then the valve is not sealing. Also you need to measure the valve wobble with the valve in the
guide and it should have less than about .004 thousandths wobble. Check your workshop manual for maximum clearance and the position of the valve in the guide.
The valves rotate as they go up and down so a valve may seal at one spot but not in another spot if the valve seat is not concentric.


$600 for this?
Cutting it kind of close.
You need a valve seat cutter tool.
If you cannot get a factory tool that turns by hand and cuts all three angles at once then you will need three separate cutters to cut each angle.

Personally I think it would be best if you send the cylinder head to someone that knows what they are doing. Cycle Exchange in Wisconsin is one place and there may be others. Do NOT just take it to a car repair machine shop.
Many car machine shops may not have the tools ,knowledge or information to do your Honda cylinder head. Many car valves are just cut with ONE angle.

Also the cylinder head needs to be completely cleaned.
The springs need to be measured and checked also.

You would need these tools.:
A valve spring compressor.
Bead blaster.
Valve seat cutting tools and know how to use them.
Valve guide press and valve guide reaming tool.
Dial indicator to measure valve wobble in the guide.
Good quality gasket and o ring kit.
Head gasket.
Base cylinder gasket.

You said the engine has 16,898 miles on it. That is not many miles
but it is over 30 years old. I can tell you from experience that the valve guide seals are probably rock hard (old) and possibly cracked rubber.
 

[markusarealius]

thank you Lucky, very informative.....I will do the test you suggested while idling it -- if I can keep it running long enough....

[Old Scrambler]

To add to the Lucky advice............the hard starting is likely caused by the excess oil in the cumbustion chamber..........check your plugs.........probably very black.

Since you will have to pull the head............replace the guides and seals.  Yes, you may want to re-cut the valve seats and thoroughly clean the combustion chamber, valves, and the tops of the pistons...........Don't scrape anything..........Brake cleaner will do a lot..........bead blasting by a pro----maybe...........but do not touch the outer surfaces or you will have to repaint..............and then nothing will match your original finish.

Try to find a local motorcycle shop to replace the guides and seals...........you may want to wrap the head in tape and paper to prevent exterior damage............others not on this site tend to treat things like a side of beef.

You will need a top-end gasket and seal set from Cycle X...............They also have the guides, as to others.  While the head is being worked on.......take the carbs to a shop to have them sonic-cleaned......just detach the bowls..........if they were working before, they will work better when clean and minimal if any tuning is required.

Be careful to avoid scratching the black paint..............I use masking tape and old rugs to slide motors out of the frame.

When you reassembe the motor........torque the head and stop for 24 hrs............then retorque before assembling the cam-tower.  Remember to replace the exhaust-pipe gaskets in the head. Your budget is tight for a complete job.

[TwoTired]

If the valve guides are worn you can tell with a vacuum gauge test.
While it is idling the vacuum gauge needle will bounce all around.
With good valve guides the vacuum needle will stay steady.

I have to disagree with this statement. 
The engine only produces vacuum on the intake stroke which is one quarter of the time in a 4 stroke engine.  This very action causes the intake tract to pulse with vacuum pressure and is entirely normal.  Anyone who has vacuum synchronized the four SOHC4 carburetors with a sensitive gauge has seen this on the dials.  And, that is why they come with restrictors that dampen the peaks and valleys of the surges to smooth out the gauge reading.

It is possible to check valve guide wear and guide seal condition without removing the head.  You do need to remove the valve cover, though, which means removing the engine from the frame in a CB750, (unless the frame is cut - not recommended).

I don't believe the valve guides are the major issue for hard starting.  But, I would agree that the valve guides and seals are top suspects for blue smoke exhaust after warm up, particularly while idling.  As the engine warms up, the oil gets thinner and flows more freely in worn passageways, (via valve guides and cylinder walls).  If the intake valve guides/seals are worn, then oil deposits will show up on the spark plug tips.  If the exhaust guides are worn, the exhaust system will have oily deposits in it.  It's possible to have both symptoms.

However, with such low miles, the machine had to have sat for a good bit of time.  This can allow one or more cylinder walls to rust, and then running the engine can cake the rust into piston ring grooves and make them stick to ring lands instead of scrapping the cylinder wall, as they should.
This can be check by a compression test (for the skilled mechanic), or more directly and accurately with a leak down test.
Unless your cylinder walls have rusted, or there is/was a problem with the oil supply, the rings are very unlikely to be worn out at 17K miles.  The rings CAN free up with some run time, but only time will tell.

If you do have the cursed valve guide issue.  Then the head will have to be removed.  The valves will need refacing and the stem diameter measured for wear tolerance. If the stem tips have cratered (likely if the guides are bad) they will have to be ground, the adjusters will all have to replaced, and lash caps installed onto the stems to restore valve length and restore proper valve train geometry.  May also need to replace all the valves if they can't be restored to service limits.

The head will need the guides driven out and new ones installed (need driver tool), preferably in a heated head.  Then the seats are ground with at least three angles to an equal depth on all four positions.

I doubt you can properly fix this with $600 unless you already have access to the specialized tools and parts required.  And, there is ample opportunity to cause further damage if this is your first training exercise.  I would expect double that $600 estimate.
You should be able remove and take the head to the proper specialist to have the head itself restored, for you to reinstall.  I expect this is the best course of action for the novice.

Cheers,

[markusarealius]

Double the price - ouch.  I will leave it sit until i have saved the $1200 to do it right.  I am now depressed.....I was hoping to be able to replace some items and reassemble and be good to go.....blah!

[TwoTired]

You can still fix the starting issue, which is most likely carb, imo.  And, if it is just the rings needing re-seating to the cylinder walls, or getting free of the piston lands, running it for a while on the freeway 50-100 miles will either sort it or confirm it.

I have a CB550 that smoked like mad when I first got it.  Carb cleaning and synching reduced the smoke about 3/4.  It wouldn't smoke while riding around except at prolonged idling stops after warm up.  It cleared up while on the move and actually ran quite well.  I had hoped it was just valve guide seals.  But, while I was replacing those, I discovered all the exhaust valve guides wore worn.  I've not finished it yet, with the press of other activities taking priority. ...And I have 5 other bikes to ride, so there is little urgency.

Cheers,

[MRieck]

I don't believe the valve guides are the major issue for hard starting. 

Cheers,

  I agree in general but if the guides are very loose (which I've seen on the exhaust for F2's) parts of the valve face will not be making contact with the seat  (secondary to poor valve control and consequent wear). That kills compression (which is essential for cold starting). I have seen F2 exhaust valves in very, very poor condition.
 My advice is to replace the guides (you will need to bead blast the ports), seals and valves. You will need a valve job secondary to the guides being replaced as the valve center moves. I would also surface the head.

[lucky]

(SHORTENED to save space)

Cheers,

I agree basically, but motorcycle valves are never reground. New valves are always installed on Hondas.
Only the seats are cut on Hondas and they can only be recut twice safely.
Limits on depth of the seat cutting.
The seat cutter should only be turned 1.5 turns by hand.

Also about the vacuum gauge test on worn guides.
If you look at a good vacuum gauge it actually has a marked area of the gauge that indicates for worn valve guides. It is NOT like the vacuum gauge for syncing carbs. It is different.
 But when the head is apart all be revealed and no vacuum test would be needed.

BTW... You said this motorcycle is the love of your life. I would get it done correctly by a shop that has the tools ,experience, and parts and in the long run you will save money.
Summer is coming.....

[lucky]

For the younger people.... here is what a vacuum gauge can tell you.
It is a VERY good tool in addition to computers.

How a Vacuum Gauge Works Reading Diagnostics Virago Valve Adjustment

Below is a good design of gauge.

[TwoTired]

If the valve guides are worn you can tell with a vacuum gauge test.
While it is idling the vacuum gauge needle will bounce all around.
With good valve guides the vacuum needle will stay steady.

This is untrue because carburetted auto engines often have a common plenum between carb and cylinders where the intake pulses from each cylinder are averaged among all cylinders.  Vaccum readings taken from this test area have different interpretations and meanings than what we have for the SOHC4 which have each intake runner completely isolated from adjoining cylinders.

There are many specific knowledge points that can be transferred between multi-cylinder automotive examples and our Hondas.  But, not this particular vacuum generalization.  Gauges are like any other tool, in that you must select the proper one for the job, and interpret the readings by how it relates to the specific test application.

My sync set simply has 4 standard vacuum gauges screwed to a common plate.  I simply ignore the dial notations about ignition timing and valve issues, as they don't apply to the SOHC4's intake arrangement.

One must understand how any test tool operates in the test environment.  The gauge can't know what it is connected to.  It deals in absolutes.  All readings must be interpreted by a human, whether during the test real time or at the manufacturers test lab and printed on the dial or operator's handbook.

Anyone that has applied a needle type vacuum gauge to the SOHC4 Honda intake runner has observed the needle has wild variations at idle and the needle becomes more steady when the engine is revved up.  The gauge has a response time to actual conditions present.  It's response time is fast enough to more accurately track the slower events during idle speeds.  However, it can't keep up with the faster pulses that occur when the engine is revved up and therefore starts to average the pulses in the intake runner at the display needle.  The damping valve or restrictor used with a vacuum gauge masks most of the vacuum pulse variation at the slow idle speed.  Note that it is inserted between the intake runner being tested and the gauge itself.  It behaves as a damping resistor to the changing vacuum absolute pressures.

If I were able to suck on the gauge and make the needle flutter, or pinch the rubber coupling tube to make the needle jump regularly, would that mean I need new valve guides, too? 

Cheers,

[TwoTired]

I don't believe the valve guides are the major issue for hard starting. 

Cheers,

  I agree in general but if the guides are very loose (which I've seen on the exhaust for F2's) parts of the valve face will not be making contact with the seat  (secondary to poor valve control and consequent wear). That kills compression (which is essential for cold starting). I have seen F2 exhaust valves in very, very poor condition.
 My advice is to replace the guides (you will need to bead blast the ports), seals and valves. You will need a valve job secondary to the guides being replaced as the valve center moves. I would also surface the head.

No doubt sloppy valve seats CAN cause lost compression.  But, at 17K miles, I'd assumed it wouldn't be quite that severe...yet.  I have three F2's with more miles than that, and they still start up easily when cold.  It is possible that 3 examples out of the total population may not be representative.  But, it's the only real world indication I have.  Of course, none of us knows what specific abuse the OP's engine has suffered.

Cheers,