Topic: CB750 F2 SS 1978

[Ace]

Hi all.  I've rebuilt a '78 CB750 F2 SS.  The project was a lot of fun and took a lot of time.  Not all went as well as I'd hoped.  I bought the bike which had a K2 bottom, with an F2 head.   It wouldn't kick over.  I decided to drain the oil only to find about nearly 1/4 of a gallon of water mixed with the oil.  After that, it kicked over.  Wouldn't start as it needed a lot of work.  Hopefully I've uploaded the pictures as well.

When I was rebuilding it, I sourced parts from all over the world, and I have some spares left over.  When it was time to fire up the bike, it fired first time and sounded like a beast with only the headers on.  I put a wiseco 900cc piston kit in which I had to change the sleeves in the barrels.  Also got a port and polish.

Unfortunately after about 300 miles it developed an oil leak out of the head between # 3 and # 4.  I found this out when I put it on the side stand and noticed oil slightly dripping from the head fins.  It looked like it had only just started to leak.  Once again I had to pull the engine out (3rd time).  While apart, there was also a drop of oil on the head of the piston on 2 cylinders.  Two intake valves were open so I'm thinking valve guide seals didn't quite work like they should have.  Simple fix, replace them.  With the big bore kits usually 1000cc my mechanic usually dowels off a few passageways to stop the potential leaks.  He was going to do it on the one but decided against it.  Costs me more time off the road.  Haven't go the engine back yet.  When I did ride it, it went ok but not as I was expecting.  I put a good street cam in it.  I think I have to play around with the jetting.  Mains I'm using 112 and on the 3rd notch.  Stock 35 pilot jet.  I've got a few other jets: 115, 120, 123 mains so I'll try them and might go up to 37.5 pilot jet.  Bike doesn't seem to accelerate as quickly as my K1.  Might be just me.  Any thoughts about carby idea would be appreciated.

Another problem I'm yet to fix is an electrical issue.  While riding to work I suddenly lost all electrics and bike stopped.  Took off the cover to the fuse panel and noticed all 3 fuses still ok.  Had power from the battery.  On the main fuse I did notice some solder or something looking like it and the plastic holder itself at the back was melted.  I changed the main fuse and it all worked again.  Strange as I couldn't see that the fuse was blown.  Might clean all the connectors again around the fuse area.  Anyone got an ideas?  This is the second main fuse to fail.  The first one did blow.  I had the bike idling for about 10 mins with a fan blowing on it to keep it cool and it stopped.  I thought it may have been out of fuel as I didn't have the tank on just my sync tank.  When I looked it was the main fuse.  Interesting problem developing.  At least I carry spare fuses for just such an occasion.

Apart from the rebuild of the engine, I put it all together, painted the engine but not the other parts such as frame etc.  This is my second rebuild of a CB750.  My first was a '71 K1.  I'm also putting together another engine.  Difference now is that I'm starting from the engine cases and working my way up.  I've managed to find a K4 case that hasn't had a hole through the back of it.

If anyone has any hints or suggestions, let me know.  Thanks for looking.

[750goes]

Ace,

looks like a decent job you have done there... you riding this everyday to work?

[Ace]

I was when I had the engine in it.  I decided to give my K1 a well earned break so I built the F2.  I'm getting another tank, duck tail and side covers repainted at the moment the same colour, Candy Presto Red.  Timed it nicely with the engine being away.  When I get it back, it will be the every day ride and the K1 for the weekend trip.  Thanks.

[eurban]

For the fuse block clean all the clips carefully, then bend the tangs closer together to grip the fuses tighter.  Tarnx works well as a cleaning dip BTW.  You could also locate a replacement fuse block with automotive type blade fuses.  In general, it is wise on our old machines to go through every wire / connector, inspecting  and cleaning.  Resistance generates heat, heat damages wires /connectors, and creates more resistance, etc etc.  Use electrical contact grease on your cleaned connections.  The wires that connect up to the alternator components (underneath the left side engine covers) have a particularly hard life. . . .Looks like you are using the stock carbs for the 78SS?  Make sure the accelerator pump is working.  Airbox removed, whack open the throttle and look for a squirt of fuel from each nozzle (bottom of carb bore next to the choke plates) down the throat of the engine.  Search (from the Tech Forum or SOHC4 bikes page) the subjects of "accelerator pump", "pressed in pilot jets",  "78 carbs" etc etc.  I would advise you to make sure your carbs are tip top before you spend too much time fiddling with main jets and needle position selections. . . .Kind of strange to use the earlier bottom end with the F2/F3 top.  The valve angle on the F2/F3 heads causes premature valve guide wear.  Most choose the robust F bottom with the the K top. . . Anyways there are a number of threads discussing oil leaks, engine rebuild tips, issues with F models in the SOHC4 bikes section.  Search there and read away.

[Ace]

Thanks eurban.  I'll give most of the connectors a clean and give them a bit of electrical grease and hopefully fix the problem.  I haven't been able to pull apart the wires from the alternator.  The plastic is dark and hard, I don't want to break them all so I just left them.  Could this be part of the problem?

The carbys are in good condition.  The accelerator pump is working shooting a good straight shot of fuel out the little holes.  I replaced the diaphragm when putting them together.  If I had the money I'd go to something like the CR smooth bores and see how that goes.  I'll get it working just takes some time.

The engine is all F2.  I didn't make that clear sorry.  When I first got the bike it had the K2 bottom in it.  I restored it as much as I could back to original condition, you'll notice that the rear shocks are after market.  The frame and engine numbers match up and I did some research and found out about the possible problems with mixing K and F parts.  Unfortunately I found out about this forum after I built the bike.  There are some good tips here.

If anyone else has and F2 with big bore kits in them I'd be interested in what carb setup you are using.  Cheers.

[eurban]

Big bores in F2/F3 engines??  An important thing to realize is that the F2/F3 (I am assuming that the Australian market Fs got the same black engine with larger intake valves that the US market got?) have a larger combustion chamber than the K series bikes.  The same piston installed in a F engine will produce a lower compression ratio than when installed in the K engine.  For instance, a Wiseco 836 10.25:1 piston is designed to produce the 10.25 to 1 ratio in the K engines and if used in an F2/F3 motor will yield a CR around 9:1 or even less.  I believe that it is Dynoman that sells a specific big bore kit for the F2/F3 engines, as the regular Wiseco 836 does not yield acceptable results.  If your 900cc pistons were not designed for the F engine (not too many of those pistons out there) then there is a good chance that your motor doesn't have a high enough CR for proper performance.  Hopefully this not the case . . .1 day later. . . .Here's the Dynoman link with the custom f2/f3 big bore pistons and some discussion on the subject:
http://www.dynoman.net/bikepages/F2/pistons.html

[Ace]

Same engine here in Oz as you have over there.  I was under the impression that the Wiseco 900cc kit that I put in was for the F2 and had a compression ratio of about 10.2.  The kit was quite a few years old.  When I get the engine back I'll check it out.