Topic: F3 - Up from the realms of darkness

[Patrick]

So I picked up a new project today. It's a 1978 CB750 F3. It's complete, but it has issues. It has a good tank in good condition, both sides covers, and came with a spare front end, that being from a 1979 CB750 DOHC. I'm told that front end has better calipers and it is in much nicer condition. A robin's song bears a greater cost than what this bike set me back.

This is my question (s). I've been reading up in the archives about F3s and it appears they have some known weaknesses. Primary among those appears to be the valves. Seems the springs are too stiff for the original valves and there is a wear issue. I don't know the condition of the valves in the bike. It has only 4700 original miles (the accompanying paperwork weaves a trail all the way back to the dealer), but it sat inside for a long time and the engine is stuck. I don't yet know why it is stuck.

Obviously I have to fix whatever made the engine stick but I was wondering what y'all might be inclined to do about the valves. I am assuming at this point that I will find the valves to be in good condition. That may not prove to be so, but if it is should I go ahead and switch them out anyway? I saw that Dynoman sells SS valves. Would changing the valves alone address the valve issue? Would I need to also replace the springs and keepers? I also saw a post about Mr. Rierk fabricating some type of valve cap to help with the valve wear problem. Did those work out? Will they help me? I've also seen posts about switching the block and head out with an F1. Would that fix the problems without sacrificing performance.

I want this to be a reliable bike, but I'm not going to enter it in any parades or anything, so I'd rather the bank remained unbroken.

Suggestions, anyone?

Patrick

[upperlake04]

I also saw a post about Mr. Rierk fabricating some type of valve cap to help with the valve wear problem. Did those work out?

 Good deal Patrick   Mike helped solve the top end problems on my F3 with the lash caps. They are small puck shaped pieces that fit onto the (trimmed) end of the valve stem. The tips had been mangled from excessive slop in the valve guides which I have been told was a result of the revised F2-3 valve stem orientation in the head. The larger valves and different combustion chamber of these models coupled with the same rocker assembly from the previous engines caused the rockers to push slightly sideways on the valve tips causing the accelerated wear. With the apparent low mileage on your machine, this might not be a problem and even if it is, the valve stems could very well be fine, as mine were, and  only guides and lash caps would be needed. 
  This was my one-time experience so hopefully a real expert will give you some pointers.   Get it running first     Good luck.

[Patrick]

First I have to get it apart. It has never been apart. I have the carbs and airbox. They have been in a separate box for a long time.

[jjwaller]

BTW, That tail piece is not off a 78.  Maybe a DOHC?
Depending on when the bike was built, you might not have a
valve seal problem.  I'm on my second F now and have never
had a valve seal problem with either bike.  30000+ on both
bikes.  I read somewhere, that in mid production, Honda
changed the valve seal material so there wouldn't be a problem.
Someone correct me if I'm having a senior moment.  In any case,
Enjoy the best and fastest CB750 ever made.  If you are not having
a case of blowing oil out the seals, just ride it and enjoy!

John

[Patrick]

You're right, John. The tailpiece is off a '79 DOHC CB750, like the replacement front end. The original seat and tailpiece came with the bike, but this seat is in much better condition. If I can switch out the seats I'll probably go with the original tailpiece.

Patrick

[mazingerzeca]

I was at the same dilemma when I decided to rebuild my 750 F2 engine. I saw many threads about the F2 head and the problem with valve guides wear. I thought about swap the head by a K head, but it was not easy for me to find a K head. Also, I didn't know if my F2 camshaft would work on a K head. Due to that, I decided to buy new guides for the F2 head, new valves and to keep the stock F2 head.
Buzz, at Dynoman, told me about the problem of the lash on the valve stem, and that installing titanium spring retainers instead of the normal retainers would prevent the valves to lash and, consequently, they wouldn't wear the valve guide. Also, I installed Kibblewhite guides. But i think that it only would help with new valves, because, as Buzz told me and as I saw looking at the valve stem, the top of it had some scores due to lash that happened on the past.

[scondon]

  Nice find Patrick   If the bike was stored with the carbs off as in your pic I would expect that the rings are stuck to the cylinders. I left an engine outside under a tarp for a year and the rust in the cylinders was an inch thick.

   As has already been pointed out, the trouble is with the guides and valve stems wearing. You can inspect the valve stem tips for wear by removing the lash adusters. Should be able to see some of the tip and see whether it is mushroomed or worn at an angle. Same with the lash adjusters as they can wear at an angle too. I didn't think to check the adjusters and re-used them on a new head with Kibblewhite parts installed and within 18,000 miles had turned a perfectly good Black Diamond valve to #$%*e.

[Patrick]

That the rings are stuck and rusty is my hope and expectation, Scondon. Considering the low mileage on the motor I expect it unlikely that the PO blew it up. The person I got the bike from is an old friend. He got it from the original owner. I have the original bill of sale and maintenance records from the late 70s and early 80s. My suspicion is that the original owner for some reason lost interest in the bike (new bride and spawn, perhaps?) and parked it until it stuck. It hasn't been registered since '82, so I believe that's how long it was parked. It was my friend who pulled the carbs several years ago in a very half-hearted attempt to unstick the motor.

I have started projects with mush less and with bikes in much worse shape. Each has turned out better than the last. I want this one to be a ride-around-the-world-without-tools bike, so I'll do what I need to do with the valves.

Should I be looking at valves guides as a bottom line, and then whatever else I need based on condition? Are the valves and lash adaptors inherently bad and in need of upgrade? If all parts are still in good condition, would be valve stems caps Mike Rierk designed solve the design problem and allow me to upgrade valves, keepers etc as I choose instead of based on need? How about the issue with the too stiff original valve springs?

I don't want to buy a bunch of new parts just to find out a design problem is going to cause them to wear prematurely as did the originals.

Patrick

[sprayn50]

Patrick, I just got done rebuilding the top end of my '77 750f2, with 16K miles,and it was costly. My valves were beat up, a PO had used a bench grinder to the end of them, guides were worn only on the exhaust side, and the rocker adjusting screws were mushroomed over. I wanted to do the job right the first time so I contacted Buzz at Dynoman and he made it sound like this engine was always going to eat up the valves. I ended up putting his valves in, his guides, his springs, and his titanium retainers. I went with his retainers because he advised me that the stock ones move around on the keepers and break, causing a dropped valve. I also asked him about installing a cam since I had the engine apart but he advised I would run into piston to valve issues unless I notched the pistons or went to an 836 kit. -T.J.

[scondon]

   Buzz is a good man and has certainly put his knowledge to use in making/providing engine parts specifically for the F2/3 models. He can definitely help put together a strong motor that can be ridden hard

   As I understand it, and has been referred to already here, Honda changed the valve angle on the F2 head in order to keep piston to valve clearance while using larger valves (34mm/31mm vs 31mm/28mm in older F and K models). The head chamber got enlarged to fit these valves and domed pistons were added to help boost the resulting loss of compression ratio.

    It's this change in the valve geometry that accelerates the wear. The lash adjusters contact the valve stem at an angle that is even greater than other SOHC 750 engines(which are pretty angled themselves IMHO ). The force the rocker arm applies to the valve is not a "straight" force until the valve is opened somewhat. The initial strike is quite off and the angled force gets transferred to the valve guides and stem tips.

   Sounds like buzz has worked a way to decrease this angle by using valves/ retainers that would lower the valve tip so that the initial stike is not at such an angle.

     Honda put stronger guide material in the F's that hit the stores during the F3 run and replaced old guides still under warranty. It was a bandaid on the larger problem but for some it was enough.

     I don't know what type of riding you do Patrick, or how much money you have to spend, but pull that motor apart and see how it looks. If you decide to keep it stock and need to replace the cylinders due to rust let me know. I've still got some servicable F3 top end stuff

[MRieck]

  Same with the lash adjusters as they can wear at an angle too. I didn't think to check the adjusters and re-used them on a new head with Kibblewhite parts installed and within 18,000 miles had turned a perfectly good Black Diamond valve to #$%*e.

Sure did. New adjusters are short money and well worth it.

[MRieck]

One of the big problems with that head is the valve angle combined with the weight of the valve. Those pieces are HEAVY. If you throw 5mm stem pieces in their really no problems.

[MRieck]

  Same with the lash adjusters as they can wear at an angle too. I didn't think to check the adjusters and re-used them on a new head with Kibblewhite parts installed and within 18,000 miles had turned a perfectly good Black Diamond valve to #$%*e.

Sure did. New adjusters are short money and well worth it.

That's OF Sean...I'll grind it and put a lash cap on it. Those Kibblewhite valves are about 45.00  each. No need to discard it.

[scondon]

  Same with the lash adjusters as they can wear at an angle too. I didn't think to check the adjusters and re-used them on a new head with Kibblewhite parts installed and within 18,000 miles had turned a perfectly good Black Diamond valve to #$%*e.

Sure did. New adjusters are short money and well worth it.

That's OF Sean...I'll grind it and put a lash cap on it. Those Kibblewhite valves are about 45.00  each. No need to discard it.

    I don't even think about it anymore, if I have a head that needs attention it goes straight in the box and off to MR in MA. Always know it will get nothing less than the best attention and I'll never get sold something I don't need.  You rock, Mike

[mazingerzeca]

Patrick, I just got done rebuilding the top end of my '77 750f2, with 16K miles,and it was costly. My valves were beat up, a PO had used a bench grinder to the end of them, guides were worn only on the exhaust side, and the rocker adjusting screws were mushroomed over. I wanted to do the job right the first time so I contacted Buzz at Dynoman and he made it sound like this engine was always going to eat up the valves. I ended up putting his valves in, his guides, his springs, and his titanium retainers. I went with his retainers because he advised me that the stock ones move around on the keepers and break, causing a dropped valve. I also asked him about installing a cam since I had the engine apart but he advised I would run into piston to valve issues unless I notched the pistons or went to an 836 kit. -T.J.

I did the same rebuild in my F2 head, but I didn't installed it yet. Is yours running? and if so, did you experience the same guide wear? I ask this because I want to be sure that I've done all I could to prevent guide wear. Any other improvement suggestions?