Topic: CB750 K and CB750 F Differences

[RRRToolSolutions]

I did not know anything about the CB750's until about 5 months ago. I just noticed the F model has a redline of 9,500 rpm. My K4 has 8,000 rpm. What is different about the F and K models that allow a higher rev range?

I've enjoyed looking at Sparty's dyno sheet. It shows exactly what I felt. The bike is just getting happy at 8,000 rpm. I have not gone over the 8,000 in fear of the connecting rods failing. I was told they are the weak link in regard to rpm. What does the F model have that my 836 does not? I ask this question because I've got the 836cc Wiseco, the Megacycles 836 cam, titanium retainers, ported stage II heads, stainless valves, and the best dual springs I could get. The bike is strong, but the dyno sheet on Sparty's is no surprise. The ZRX1100 has 97 horsepower and 79 ft/lbs torque and feels considerably stronger than the 836 in every rpm, so the 2 numbers "fit" the bikes. I'd love to see the dyno pull up to 9,500 rpm. I don't believe torqe would fall that quick.

Thanks,
Gordon

[Jerry Rxman Griffin aka MuthaF'er]

75/76F redline at 8500. 77/78F at 9500. 75/76F has stronger cam than 72/73 (not sure when it got milder) to 76K and 9.2 vs 9.0 comp ratio. 77/78F has entirely upgraded top end. Larger valves, stronger springs, different cam & retainers.

[GroovieGhoulie]

However, the 77/78F had bum exhaust valve guides, which would often go bad with around 30K or less on the bike.  Honda issued a fix, but the correct parts are hard to come by.  As far as the Fs go, I like the 75/76 the best because while the engine is a bit milder than the later F engines, it's a bit hotter than the Ks, finally coming back to the power levels of the original K0 AND it's got the K's dead-reliability.

[Lumbee]

http://hondachopper.com/modifications.html

CB750FE-2500004 F1
          new camshaft, carbs w/additional idle air. New pistons, compression ratio 9.2/1. New case w/o primary chain oiler. Final drive gear ratio 43/50 i/o 48/50. 4th gear on main shaft 31 i/o 30, 5th gear on countershaft 31 i/o 32

CB750E-2600014 and CB750GE-1000014 F2
          new cylinder head with bigger valves (34/31mm i/o 32/28mm), larger combustion chamber, larger carb mount rubbers, stronger cam chain, new camshaft, harder valve springs, new retainers, cotters, new pistons compression ratio 9/1. New rod big end bolts and bearings, harder clutch springs, additional fins on crankcase, larger fins on oil pan, "oil cooler" - finned plate between oil filter case and engine. Final drive 15/43 or 14/43.

[Hockers Choppers]

Anyone know the specs on the f3 cam? Will it work in an pre 77 engine? Or with the bigger valves does it have a lower lift?

[Hockers Choppers]

However, the 77/78F had bum exhaust valve guides, which would often go bad with around 30K or less on the bike.  Honda issued a fix, but the correct parts are hard to come by.  As far as the Fs go, I like the 75/76 the best because while the engine is a bit milder than the later F engines, it's a bit hotter than the Ks, finally coming back to the power levels of the original K0 AND it's got the K's dead-reliability.

My f3 has 13,000 miles and the exhaust guides are shot! I'm putting an older k head back on it with 836, webcam 63a, etc. should be a good combo.

[Lumbee]

the exhaust guides are shot!

...how do know?  Just wondering...I hear about this alot but havn't experienced it.  Had a 78F with 30K and now currenly have a 77F daily rider with 17K...what should I look for?

[Hockers Choppers]

Bike ran fine, I took apart for the mods. I'm going to use the f3 springs, and when I took off the springs and removed the valves the exhaust could be moved back and forth probably .020-.030 !! they should fit pretty tight like .003. But like I said the bike ran fine ( 56.4 hp ) Smoked a little.

[Lumbee]

...yea, thats what I was wondering, if you found it after dissassebly, or it it was exhibiting signs of running crappy.  I've rebuilt the topend on the 78F I had, and on the current 77 and the valve guides on both "looked " OK...

[rruser]

I'm thinking about buying a CB750 very soon. Is there anyway to tell if the one i'm looking at would have the poor ex valve design?  I don't know the year but it has the larger gas tank with the locking door/flap... and the dash lights are up near the tach/speedo vs on handlebars. I'm kind of thinking about backing out if it has the poor head design.

[scondon]

I'm thinking about buying a CB750 very soon. Is there anyway to tell if the one i'm looking at would have the poor ex valve design? I don't know the year but it has the larger gas tank with the locking door/flap... and the dash lights are up near the tach/speedo vs on handlebars. I'm kind of thinking about backing out if it has the poor head design.

   The motor in question is on the 77-78 750F SS model. The motor is painted black. When the valve guides start to go it is most evident when you bring the rpm's above 6,500(or so) and quickly let off the throttle. Do this while riding(load) not parked on the centerstand(no load) and you  will see a trail of smoke out the exhaust. As Hocker says, it doesn't affect the performance much until they get really bad.

  The 77-78 750K engine does not have this problem."larger gas tank with the locking door/flap" makes me think you may be looking at this model.

However, the 77/78F had bum exhaust valve guides, which would often go bad with around 30K or less on the bike.  Honda issued a fix, but the correct parts are hard to come by.  As far as the Fs go, I like the 75/76 the best because while the engine is a bit milder than the later F engines, it's a bit hotter than the Ks, finally coming back to the power levels of the original K0 AND it's got the K's dead-reliability.

My f3 has 13,000 miles and the exhaust guides are shot! I'm putting an older k head back on it with 836, webcam 63a, etc. should be a good combo.

 Use a 75-78 K head. Definitely a good combo

Anyone know the specs on the f3 cam? Will it work in an pre 77 engine? Or with the bigger valves does it have a lower lift?

   Haven't answered this 'cause I honestly don't know.In the 77-78 750F engine the combustion chamber was made a bit bigger to accomodate the larger intake valves and the pistons were "domed" to make up for some of the resulting loss of compression. The valve angle was changed so the larger valves wouldn't knock into the pistons( wild-ass guess). It's the angle of the valves and not the guides themselves that cause premature wear.

   Anyway, the 77-78 cam has more lift but because the valve angle has changed(more from the side rather than straight down) it is not bringing the valve head down as close to the piston as it would if you were to put the cam in any other motor. It might still have enough clearance if you were to put it in a older motor, but the lobe profile might not work so good. Just dunno

I'm going to use the f3 springs

  I know that the F3 retainers and keepers are not compatible with earlier valves. Maybe double check that the F3 springs are compatible with the retainers on whatever head you end up using(just a thought). Also, the springs have a "top" and a "bottom" end so make sure they get installed correctly. You probably already know this, but I didn't and installed mine improperly. Good thing I sent the head to get worked on by someone who knows these things BEFORE I put it on the motor

[Hockers Choppers]

I didn't know about the f3 springs until I went to the machine shop my buddy has and he told me about it. We checked the springs in a scale and the f3 springs are a lot heavier than the stock k4's (almost double) probably because of the larger valve and the higher rpm. They are wider so you have to use the f3 cups. I still don't know about the cam. I guess I'll just run the 63a webcam I have. Anyone recommend anyone for head porting or can I do it myself?

[jaknight]

Howdy Hocker,------------>

     Your question about heads, porting, etc......

     Contact  MRieck, here on this board.  He is Super good on valves, heads, and many other such intricacies........

     ~ ~ ~ jaknight ~ ~ ~

[736cc]

1969 sandcast redline was 8500 rpm. They had "wild" cams, carbs had 120 mains, airbox had bigger inlets, plus they had really low gearing. Quarter mile times at 12.9 @ 105 mph w/ top speed of 125 if rider is prone. Sandcast method of crankcase production was much slower than die-casting and allowed time for a little more care in assembly (blueprinting). Later K models ride nice but don't pull nearly as hard on top in comparison. Last F models were hot-rodded and may have been the quickest of iall but had problem w/ weak valves.

[goodmachine]

Hello, I have a 1976 CB750. engine was destroyed. Got an engine from a '79. put that in the frame and all is well, however wondering if I can use the carbs from the '76? They "seem" the same but looks can be deceiving. I have them on the bike now and it runs, but not as well as I'd like. Any suggestions?
Thanks

[Jerry Rxman Griffin aka MuthaF'er]

That's cool. Show us how you put a DOHC engine in a SOHC bike please. 

[Bankerdanny]

Hello, I have a 1976 CB750. engine was destroyed. Got an engine from a '79. put that in the frame and all is well, however wondering if I can use the carbs from the '76? They "seem" the same but looks can be deceiving. I have them on the bike now and it runs, but not as well as I'd like. Any suggestions?
Thanks

I could be wrong, but I am pretty sure a '79 DOHC 750 would have come with CV carbs, not the conventional mechanical slide units on a '76 SOHC 750. The difference in appearance should be pretty obvious.

Pictures of your replacement engine and the carbs are required for anyone to provide useful advice. If your new engine has a similar looking top end as your old one, then it isn't a '79.

[Lumbee]

That's cool. Show us how you put a DOHC engine in a SOHC bike please. 

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