1979 CB750 tach oil seal

[LukeM]

Well, I was mistaken.  The oil leak is not from the gasket where the cylinder head cover meets the tach drive.  It looks like the seal has gone bad where the cable meets the drive.  I spent about 3 hours trying to get the seal out, with no success.  The seal was cheap enough: $3.50 from the dealer.

Has anyone else done this replacement on their motor?  I'd like to avoid a trip to the dealer if I can.

One posting on the Internet said the drive had to be removed to get the seal out.  I'd like to avoid that as well, but I'm willing to do that as well.

Thanks in advance.
Luke M

[Re-run]

Sorry I dont know the dohc better. On the sohc, the seal just sits in the hole and you use a screw the thread it and pull it out.

[LukeM]

Yeah, I was trying everything I could think of to get that seal out.  Tried a bent paperclip to slide under the center, and pry it out.  No go.  Even tried cutting the seal: still no go.

I may pull the cover, and pull the drive and take it to a shop and see if they can get it out.  If I read the parts diagrams right, there's a thrust washer that goes in first, then the seal, then the tach cable end, secured by the screw from the top.  I'm not sure if the Previous Owner put the seal in first, then the washer. I'll get it out somehow.  I like the screw idea, but with it in place, it's tough to get any kind of leverage: too close to the frame.

Thanks for the advice, Re-Run.  Your knowledge on these bikes is amazing!
Luke M

[LukeM]

After a trip to Autozone, I got the old oil seal out. It took a right angle pick and some force to remove it. It was very brittle. I looked inside there for a thrust washer, but couldn't see one. It could be that the previous mechanic forgot it, or left it out on purpose. So, after the holiday, I'll head for the dealerman to get a 7mm thrust washer. It's possible that the local hardware store would have it, but chances are there's something special about it.

Interesting thing about the seal: there's a little spring inside the seal that goes around the drive shaft for the tach. Pretty sophisticated, but not surprising

The good news is that it should be fixed in a day or so.  Updates to follow.
Luke M

[LukeM]

I bought 2 of the oil seals, and 2 of the thrust washers, just in case.  After checking to assure there were no left over bits of crud or seal in the cavity, I put a dab of grease on the "open" end of the seal (towards the engine) and then stuck the thrust washer to it, centering it on the seal.  I then took the seal, centered it on the drive thingie, and pushed it home with a 3/8" socket.  I made sure it fit inside the cavity, and covered as much of the seal as possible.  

Then I put the tach cable back on, only to find it was still about 1/4" short.  I took it out and reseated the seal.  It didn't budge.  Turns out, I didn't mate up the tach cable end with the drive.  Once I did that, it went home nicely.  The securing screw went home, and job was done.

While the tank was off, I got a 1/4" inline fuel filter from NAPA and put it in the gas line.  Anything is better than nothing, right?

I gassed up at the local Shell station (89 octane this time, and no Seafoam) and took it for some street and freeway runs.  Accelerates about the same.  Got it home before the big thunderstorms hit.  Close examination shows there's no oil leaking at the tach drive.  Success!  Also, 172 miles on 4.1 gallons = approximately 42 mpg.  Not bad.  Almost Ford Focus-like.

Total cost of the seals and washers: $11.  Fixing a 31 year old motorbike...again...priceless.

Next step: pull the carbs and do the cleaning.  Maybe/hopefully that'll fix the gas problem with cylinder #4.
I've heard that Pine-Sol works good for carb bodies.  Then again, there's a place here called Rice Paddy that will ultrasonically clean the carbs for $60.  No disassembly, no nothing.  Decisions, decisions.

Luke M

[LukeM]

Since I had some time and leftover money, I decided to better wrap up the wiring harness going to the front forks.  The previous owner took off most of the tape and armoring, and ran it outside the frame.  Don't ask me why.  Anyhow, I pulled the headlight, and disconnected all the multipin connectors, and went to the car parts place for some plastic cable routing stuff.  I slid the wires inside, then followed up with a wrap of electrical tape from the connectors down to past the 'armor' under the tank.  

It looks a WHOLE lot better.

While the tank was off, I pulled the spark plugs to check them for wear and color.  Cylinders 1, 2, and 3 look absolutely perfect.  Very light brown color, no oil or carbon, textbook condition.  Cylinder #4 looks a bit black and a little shiny.  Re-run thought it was gas fouled, and it may be.  I swapped plug #4 with a new one, and will monitor it over the next little bit.  My travel schedule starts up again, which means not as much bike time as the last month.  My son has volunteered to take the Avenger out 1-2x/week to keep it fresh.  What a guy...

Also, the tach oil seal is holding nicely, and there are no drips there.  Just a little black stuff at the middle of exhaust pipe for cylinder #4.  Not sure why that is: maybe just gas going through the system?  Dunno.

Have a great week, and I welcome your comments.
Luke M

[vern401]

Still sounds like the #4 carb has something in one of the passageways. I'd do another good cleaning on it

[LukeM]

I haven't had the opportunity to pull the carbs since I got the bike in mid April.  As soon as I get some time at home, I'll pull all 4 and give them a good cleaning.  Re-run thought the idle jet or low speed jet may be clogged.  We shall see.

Thanks for your input on this and the oil situation as well.  For now, I'm running Quaker State until I get time to change the oil.  The crash bars on the bike have a tube/pipe that goes directly across the oil filter cover, meaning I have to take the bars off to change the oil.  I'm debating on whether to take the bars off : I haven't been riding this bike long enough, and I'm still a bit leery of laying it down and messing up the nice factory pipes.

Luke M

 

 

-----Original Message-----
From: vern401 [via Honda CB750'S] <[e-mail]>
To: LukeM <[e-mail]>
Sent: Sun, Jun 6, 2010 10:11 pm
Subject: Re: 1979 CB750 tach oil seal


Still sounds like the #4 carb has something in one of the passageways. I'd do another good cleaning on it




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[vern401]

 I agree with Re-Run. you would not think that a plugged port would make it run rich but it will. I am going to do an experament with a cx500 Custom and see what it does if I remove the air cutoff valve. It feeds extra ags durind deceleration to keep the extreme lean mix from igniting in the exhaust system. Re-Run has a post about it so for my fellow 750 riders I'll sacrafise the 500. My 750 it a 78 and does not have CV carbs on it.