Topic: Cb750f

[calikid]

What's up guys, have a 78' cb750f. Bought it for $1,000 and been slowly tinkering and riding. She starts and rode when I picked her up. Welded an new backend.

[jontc]

Whats next?

[calikid]

Going for the cafe look. She does leak so I'm gonna rip the engine out. Detab the frame and cut off the center stand. Oil tank is located in the seat along with electronics

[calikid]

I think the leak is a common top end problem I've been reading. Any direction to a guide to fix? Also anything else I should go over while the bike is apart??

[calikid]

Also don't really know much about bikes so I'm learning as i go! This is my first bike, although my dad has a bunch in the garage! Any info helps. Gone through the FAQs and have been talking to a bunch of people like Carpy in OC. I live in SoCal

[Tugboat]

Depends on how much it leaks, and if it has any other problems. If it's just a little oil leak, I'd leave it alone. A lot of work for little gain.

A wise man once told me: if it leaks less than a quart a week, you're good.

[calikid]

It does leak less than that.. My left leg of my jeans is a darker oily color though haha

[Tugboat]

Where's the leak from? Nothing you can do about the tach seal.. but if it's the head-->cylinder gasket you can always shove some rope between the fins as an oil tampon.. That's what I'm running

[calikid]

That's what I do right now! I figure if im going to rip the engine out to detab and cut the frame i should fix it right??

[brandEn]

Good looking F. Those top end leaks can be a #$%*. It could be from a few different places. Valve cover gasket, head gasket, or the "rubber nickels" located under the camshaft holders are the most common sources. The best way to find a leak is to completely clean the engine and ride it a bit and try to track it down. Easier said than done. Prepare to open a can of worms if you decide to bust into the top end. Most likely you will find parts that need replacing. Just something to keep in mind.

[Geeto67]

is that a basanni pipe?

why did you lower the bike? you know it ruins the handling, right? Plus with that basanni you are gonna scrape the heck out of it everywhere.

it looks like someone spent some time making that a proper street racer circa 1980s, maybe you should take some stock of existing mods before you jump in and do all the douchebag mods. I can think of about 90 things which improve the bike's riding expirence and "detabbing" and "cutting" the frame are not any of them.

[madmtnmotors]

Those top end leaks can be a #$%*. It could be from a few different places. Valve cover gasket, head gasket, or the "rubber nickels" located under the camshaft holders are the most common sources. The best way to find a leak is to completely clean the engine and ride it a bit and try to track it down. Easier said than done. Prepare to open a can of worms if you decide to bust into the top end. Most likely you will find parts that need replacing. Just something to keep in mind.

After washing off the oil use some aerosol foot powder on the engine before you ride and stop to check frequently during the ride to narrow down the leak(s). The 77/78 750F models are unique and must be properly assembled to be leak-free. There are no less than FORTY-TWO FORTY-FIVE potential leak points (from top to bottom):

[8] Valve inspection cap o-rings
[1] Tach seal
[1] Valve cover gasket
[4] Of the head nuts are acorn nuts to prevent oil from seeping down the four cylinder studs at the DELETED oil return path locations (77/78 F model ONLY). These particular cylinder studs are exposed to the atmosphere on the 77/78 750F models.
[4] Copper washers at these four acorn nut locations (77/78 F model ONLY)
[4] Cam bearing studs are "bottomless" holes and must be assembled with thread sealer if they are removed from the head
[6] "Coin" or "Nickel" seals under the cam towers that seal the head nut pockets below
[1] Head gasket
[2] O-rings on top of cylinders around the two cylinder studs in the oil FEED path
[4] Rubber "collars" with steel dowels on top of cylinders at the four cylinder studs in the oil RETURN path (K models as well as 75/76 F models use eight of these)
[2] O-rings at the BASE of the cylinders around the two cylinder studs in the oil FEED path
[4] FAT LITTLE O-RINGS at the base of the cylinders around the four cylinder studs at the DELETED oil return path locations (77/78 F model ONLY) THESE ARE NOT INCLUDED IN ANY GASKET SET! They are also no longer distributed by Honda for the CB750F but are still available for certain Honda outboard and lawn mower engines.
[1] Cylinder base gasket
[1] Crankcase breather cover gasket (top of valve cover)
[1] Breather hose connection to valve cover
[1] Timing chain tensioner gasket


I won't count the four cylinder sleeve o-rings (at base of cylinders) since many have run leak free without them even in place (especilly when overbore prevents their use).

[calikid]

is that a basanni pipe?

why did you lower the bike? you know it ruins the handling, right? Plus with that basanni you are gonna scrape the heck out of it everywhere.

it looks like someone spent some time making that a proper street racer circa 1980s, maybe you should take some stock of existing mods before you jump in and do all the douchebag mods. I can think of about 90 things which improve the bike's riding expirence and "detabbing" and "cutting" the frame are not any of them.

Bike isn't lowered. Thanks for giving no input on the "90" things which improve the bikes riding experience though.

[calikid]

Those top end leaks can be a #$%*. It could be from a few different places. Valve cover gasket, head gasket, or the "rubber nickels" located under the camshaft holders are the most common sources. The best way to find a leak is to completely clean the engine and ride it a bit and try to track it down. Easier said than done. Prepare to open a can of worms if you decide to bust into the top end. Most likely you will find parts that need replacing. Just something to keep in mind.

After washing off the oil use some aerosol foot powder on the engine before you ride and stop to check frequently during the ride to narrow down the leak(s). The 77/78 750F models are unique and must be properly assembled to be leak-free. There are no less than FORTY-TWO FORTY-FIVE potential leak points (from top to bottom):

[8] Valve inspection cap o-rings
[1] Tach seal
[1] Valve cover gasket
[4] Of the head nuts are acorn nuts to prevent oil from seeping down the four cylinder studs at the DELETED oil return path locations (77/78 F model ONLY). These particular cylinder studs are exposed to the atmosphere on the 77/78 750F models.
[4] Copper washers at these four acorn nut locations (77/78 F model ONLY)
[4] Cam bearing studs are "bottomless" holes and must be assembled with thread sealer if they are removed from the head
[6] "Coin" or "Nickel" seals under the cam towers that seal the head nut pockets below
[1] Head gasket
[2] O-rings on top of cylinders around the two cylinder studs in the oil FEED path
[4] Rubber "collars" with steel dowels on top of cylinders at the four cylinder studs in the oil RETURN path (K models as well as 75/76 F models use eight of these)
[2] O-rings at the BASE of the cylinders around the two cylinder studs in the oil FEED path
[4] FAT LITTLE O-RINGS at the base of the cylinders around the four cylinder studs at the DELETED oil return path locations (77/78 F model ONLY) THESE ARE NOT INCLUDED IN ANY GASKET SET! They are also no longer distributed by Honda for the CB750F but are still available for certain Honda outboard and lawn mower engines.
[1] Cylinder base gasket
[1] Crankcase breather cover gasket (top of valve cover)
[1] Breather hose connection to valve cover
[1] Timing chain tensioner gasket


I won't count the four cylinder sleeve o-rings (at base of cylinders) since many have run leak free without them even in place (especilly when overbore prevents their use). My current project (also a 78 CB750F) had an EXTRA leak point that was introduced by a goober of a P.O. that I like to never have found. Pulled the engine FOUR TIMES chasing top end leaks. I will be documenting the trials and tribulations of this build in a forthcoming build thread.

Geez that's a lot! Thanks for your help!i'll run through all those things and let you know what did the trick!

[736cc]

I recently read a post here where a nagging oil leak on a 750F was fixed by adding an extra breather by drilling a hole in a valve adjustment cap and JB welding an air hose fitting into the hole then running a simple hose dump to it. Its kinda crazy-sounding in its simplicity; I'm thinking of trying the same thing 'cause my F has a moderate oil leak upstairs somewhere and this is an ez mod. Big bore kits can cause more huffing and puffing, additional breathing needed.

[calikid]

I recently read a post here where a nagging oil leak on a 750F was fixed by adding an extra breather by drilling a hole in a valve adjustment cap and JB welding an air hose fitting into the hole then running a simple hose dump to it. Its kinda crazy-sounding in its simplicity; I'm thinking of trying the same thing 'cause my F has a moderate oil leak upstairs somewhere and this is an ez mod. Big bore kits can cause more huffing and puffing, additional breathing needed.

Sounds interesting! So it gives the excess oil a place to go? I saw that of the 750f thread.. Is there a specifc link to the guy who did it??

[calikid]

So i took apart the top and this is what my pistons look like. I know thats a lot of carbon, but is anything going wrong? I also am going to get a new gasket set, any recommendations or vendors?

The walls seem fine and smooth. What else should i inspect in depth?

[brandEn]

Pistons look normal to me.

For gaskets you should get Cometic brand base, head, and cam cover gaskets. Dynoman has the best prices. Unfortunately I don't think you can get a complete gasket kit that has Cometic gaskets in it so you will have to order them separately.

Cometic is listed as "CFM-20"
email Buzz aka Dynoman and tell him what you need. He is VERY helpful and will set ya straight!
http://www.dynoman.net/bikepages/sohc/gaskets-head.html

[goldarrow]

Wow you're quick getting down there. Make sure you follow manual when reassembling.  Since you're already down there, might as well remove pistons and do full cleaning especially the 18 oil holes in each.  remove the rings and clean the grooves, be very careful not to break the rings.  Or just get new set of rings.  Check condition of cam chain related components. Be patient, don't rush. 

[Henning]

Oooh'er. This thread raises a couple of red flags for me.
1) "Detab the frame and cut off the center stand." You're just farking it up for the Future Owner. You're young, I can tell by the use of the deadly word "Dad". You won't understand what I mean until you're older and the value of an unmolested frame.
2) - Major ambitious Café project - . Just get it running and have some danm fun on the bike. Annoy the neighbors, scrape the pegs, get a few speeding tickets, go to the beach. Hell, these bikes where made for fun, and how interesting is it to get elbow deep in grease rather than ride it. When and if you have had the bike for a few years think about a personalized project.

Take my comments in a well meant manner! 

[Thousandson]

I am also rebuilding a Cb750F 78'. Please take a look through some guys have dropped some good notes, read up. I'll be watching your posts to.

http://forums.sohc4.net/index.php?topic=113672.0

[Geeto67]

is that a basanni pipe?

why did you lower the bike? you know it ruins the handling, right? Plus with that basanni you are gonna scrape the heck out of it everywhere.

it looks like someone spent some time making that a proper street racer circa 1980s, maybe you should take some stock of existing mods before you jump in and do all the douchebag mods. I can think of about 90 things which improve the bike's riding expirence and "detabbing" and "cutting" the frame are not any of them.

Bike isn't lowered. Thanks for giving no input on the "90" things which improve the bikes riding experience though.

I suppose you want me to wipe your ass for you too. here are the 4 places I would start: rearsets, shocks, forks, brakes. I'll do you one further - there is a bolt on caliper upgrade for two piston calipers, search for it. Forks? cartridge emulators and springs. Shocks? get good ones setup for your weight. now there are 86 things you can figure out for your self. You don't seem to have any fear pulling the bike apart so it should be easy with some thought.

here is the first thing I would do if I were you: Take the rear shock, remove the spring (if it is the type that is removeable), then compress the shock and reinstall it. Then check to see what the rear wheel hits at full compression. your rear tail section looks like it doesn't offer much tire clearance in the rear but then again your pics are terrible.

what is detabbing the frame going to get you? not much, skipping lunch will save you more weight and it is premature if you plan to make more modifications. if you feel you HAVE to do it then it should be the last thing you do after you have figured out all the other USEFUL mods you should make, cosmetics come last.

BTW, what happened to your oil tank? if you replaced it with an aftermarket one, did you use one with appropriate sized lines? Harley/triumph lines are 3/8ths or 5/16s and cb750 lines are much larger (1/2" roughly). running too small lines will starve your top end over time. 

[shinyribs]

Geeto67,why are you so rude? Have some manners,please.

[Geeto67]

Geeto67,why are you so rude? Have some manners,please.

you are totally right....I forgot motorcycles are totally the touchy feeley let's all hug each other hobby.   

[RSchaefer]

I think it was reasonable for Geeto67 to get a little spicy.  This guy Calikid has posted on our forum 28 times, 10 of which are in this thread.  He gets a little abrasive by thanking Geeto67 for not providing him the 90 suggestions he mentioned.  Maybe being a smartass in forums is typical for the younger generation!

I for one am not a newbie, but not yet a major contributor and when I entered this forum I gave the senior guys a lot of respect.  I may not have agreed with everything, but I gave them respect because they probable knew more about the subject matter then I, which was typically the case.

I think it would be reasonable for Geeto67 to give the senior guys a little respect.  While this forum is obviously for guys who have decided to cut their frames, etc. we older guys tend to respect what the SOHC's were and not try and make them into something they are not, regardless, each to their own!!