Topic: New Problem with 1972 CB750

[Arrico Wanders]

I am a beginner at this.  I bought a 1972 CB750 about a month ago, I've been learning about simple maintenance and repair.  The members here have been very helpful on several issues I've run into.  Last week, I removed the front tire, replaced it, and rebuilt the front brake.  Meanwhile, I cleaned everything in the front end.  That went well.  I followed the instructions regarding rebuilding the brakes in the FAQ.  So far so good.  Finished that Saturday night.  Rode it Sunday for several short trips, none more than about 10 miles.  It ran great, brakes worked well, no problems, good power.

This morning, I noticed that there was a puddle of oil under the bike.  Importantly, as background info, I have drained and replaced the oil and oil filter, replaced the fuel and included a fuel treatment, replaced the spark plugs, replaced the battery.

Today, I decided to take it for a spin, and it ran really poorly.   Previously, it was requiring no choke, starting on first try, idling very smoothly without problems.  Today, it would not idle without using the choke at first (wide open, then gradually closing it).  I could barely keep it started!  I was able to get moving, and I had no power!  It was running, but just barely.  The exhaust note was different than normal, it sounded like a deeper rumble than normal.  The bike would stall out any time I slowed down, or let off of the throttle.  I went around the block, then limped it back home.  Upon arriving home, I looked under the bike.  There are definitely signs of a leak around the bolt at the base of the oil filter assembly.  I could be mistaken, but I don't think I messed with this bolt previously.  Can't say for sure, so I included a picture of it.

I did a search on here, and found a mention of testing to see if the cylinders are firing by licking your finger, and touching each pipe as it exits the engine.  I did this, and the pipes coming from cylinders #1 and #2 are not warm.  #3 and #4 are hot. 

Hopefully unrelated, but might be important:  when I replaced the spark plugs, the spark plug wires for #2 and #3 were old and sort of frayed.  I trimmed the ends of these wires, screwed in the spark plug caps and reinstalled.  Didn't seem like a big deal, and it still seems to feel tight and sturdy.

Any advice would be greatly appreciated.

AW

[Arrico Wanders]

Forgot the picture...

[Duke McDukiedook]

Where was this puddle at in reference to your bike? Looks like you have an oil leak somewhere, if it is not too significant (a quart every couple of weeks that is not too bad).

Your gas treatment could have displaced some dirt or particles into your slow jet circuit and gummed up the works, looks like you will get to learn about your carburetors real soon. This wil probably involve a teardown and cleaning of the carbs.

You will also want to check your spark from the points or CDI to the plugs, testing to see if you get a decent, reliable spark.

[Arrico Wanders]

So, you think this is 2 separate issues?

[Duke McDukiedook]

Check for fuel delivery to carbs and spark, could be a combination, might not.

You don't know until you check them off the list, I would recommend performing a complete tune up on this bike in cluding looking at the ignition and then attacking the carbs LAST.

[sangyo soichiro]

Do a compression check if you have a compression tester.

[Arrico Wanders]

OK, potentially stupid sounding question:

What's a tune up?

[flybox1]

got a clymer manual and or downloaded the shop manual?
honda recommends certain maintenance(tune-ups) be done at specific mileage intervals.
the 3000mi maintenance procedures will cover all areas of a well tuned Four...
start there.

[Johnie]

Regarding that oil leak by the filter housing. Did you use a Honda filter or aftermarket? And did you replace the small O ring behind the oil filter bolt?

[Arrico Wanders]

I bought the filter at the Honda dealer.  I think it was Honda equipment.  I'm not positive about that specific O ring, but I'm thinking I need to check.

Isn't some of the stuff I'be already done part of a "tune-up"?

[Arrico Wanders]

Weird.

I just went out and started it again, and it was back to normal power.  Sounded better, all four pipes getting hot now.

Still leaking some, but something's better.  I didn't do anything.  Wonder what happened there?

AW

[Johnie]

Just to clarify, that O ring actually goes on the oil filter bolt in a groove. If it is missing or torn you could have a leak like yours.

[Arrico Wanders]

Thanks, will definitely take a look at that.

What do you guys think happened?   It seems to be back to normal, but it's sort of disturbing.  What if I get out somewhere on the bike and this happens again?!?!?   I'm definitely not confident about the reliability right now.  Guess I need to go through some tune up steps before I get too far away from home...

[77cafe750]

sounds like maybe something got dislodeged by that fuel cleaner and maybe cloged a jet then fell back to the bottom of the carb bowl i would at least pull the bowls and check
peace crash

[Roach Carver]

As everyone else said, time to go thru the carbs. You need to know how they work anyway so you can fix them if they do strand you somewhere. Not that difficult of a job and when they are really clean it will amaze you how smooth it will run.