Author Topic: CB750F3  (Read 2510 times)

Offline Rozabikes Tim

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Re: CB750F3
« Reply #15 on: October 03, 2017, 12:33:39 PM »
Rushed updates to keep the 750 current against newer competition. Not enough testing time initially as the 'delta' part later fixed the problem.
What's a "delta" part Roy?
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Re: CB750F3
« Reply #16 on: October 04, 2017, 09:02:48 AM »
I went to have a look at an F2 which a friend has got parked next to a K6 to see if there's anything obvious (it's fine to point out that I should get out more).

What they seem to have done is kind of a combination of quite a few bits in the posts above,  I'll say "seem" as I can only look at them in situ and take valve caps off.
To create space in the combustion chamber along with the volume it looks like they've just moved the valves,  and seats, back into the head but with no change in angle. As they are splayed from bore axis this moves them apart, didn't think of it like that before. This appears to move the valve top up but still under the same rockers so that this interface is in a different position where it intersects the rocker arc. It's not by much, but is there.
I'm wrong about the K7 cam as it seems to have the same lift as the F2 example,  they just changed the timing,  duration and overlap to get the engine to rev and not go flat at about 9000 rpm. This shifted peak power from 8500 to 9500.
They say it's safe up to 10500 rpm and to get this they've changed the spring rates from 154 to 202lb. That's a massive jump and must be one of the biggest contributors to imposing a sideways load on the valve. If they've taken the tappet to valve angle away from optimal and wacked that much extra spring rate on it, then it's not surprising that the tappet screw it reluctant to slide across the top of the valve.
They appear to try and contain this by pairing the oil pockets (which are separated on non F2 heads) around the valve springs and then reducing the exit route, it looks like an attempt to get that valve top covered in oil to try and keep the tappet joint covered.
On American V8s when they get into this realm of revs and valve gear loading they use roller tipped rockers to stop the valves pushing sideways. Looks like they're at those margins here.
As has already been said,  the original was fairly close to perfect and trying to push some areas like this illustrate why it was replaced.

It's funny that it's also a bit of an illusion regarding power also. BHP is kind of slightly fictional (I know it makes comparison easier) as it takes real torque and uses rpm as a gain factor in it's calculation. So, if you get the same torque just moved up the rpm scale, it just makes the BHP look higher. This makes more difference certainly at the gain seen in this engine for marketing purposes really.
As I think Roy said, it just shows its a stop gap to try and hold market position by making it look newer.

I still like them though.

Offline neilg

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Re: CB750F3
« Reply #17 on: October 04, 2017, 05:17:50 PM »
For info, a Delta Part is one that is an updated modified part but still carries the original part number. Or in other words it's a modification done quietly by the manufacturer. So in reality if you buy a NOS part unless it's an obvious mod you might get either an updated piece or an old inferior piece with no way of telling just by the part number.

Offline royhall

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Re: CB750F3
« Reply #18 on: October 04, 2017, 06:11:37 PM »
For info, a Delta Part is one that is an updated modified part but still carries the original part number. Or in other words it's a modification done quietly by the manufacturer. So in reality if you buy a NOS part unless it's an obvious mod you might get either an updated piece or an old inferior piece with no way of telling just by the part number.
Thats correct. When I did my guides I had to get them one at a time from all over the place. I ended up with three the same and an odd one all in sealed Honda bags with the same number. The odd one was an original part so I had to source one more Delta item to do the job. Sneeky way of avoiding warranty claims.
Current bikes:
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Honda CB750F2 in Candy Apple Red
Triumph Trident 660 in Black/White
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Offline Woodside

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Re: CB750F3
« Reply #19 on: October 04, 2017, 07:10:11 PM »
well i never knew that

Offline Bryanj

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Re: CB750F3
« Reply #20 on: October 05, 2017, 08:27:56 AM »
The only one I did many years ago came in as a part ex for something and was in a poor cosmetic state but cheap in order to put right with new genuine bits, should have guessed mechanics would be as bad!. I borrowed it to go to the TT as is and noticed in Liverpool it had used some oil, about 350mpp but came home early having gassed an IOM plod on mad Sunday (Max 99mph and 150mpp oil). I stripped it expecting bad pistons and rings but, eventually, found very bad exhaust guides and 2 stem seals broken with lots of oil in exhaust ports(inlet OK) ordered and fitted Honda parts from Fowlers(no idea if same or delta) borrowed a reamer from the local dealer, grinding paste on valves(lucky they came out OK) after re building, along with many cycle parts put on showroom floor sold quickly and never seen again.

Offline neilg

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Re: CB750F3
« Reply #21 on: October 05, 2017, 12:11:25 PM »
For info, a Delta Part is one that is an updated modified part but still carries the original part number. Or in other words it's a modification done quietly by the manufacturer. So in reality if you buy a NOS part unless it's an obvious mod you might get either an updated piece or an old inferior piece with no way of telling just by the part number.
Thats correct. When I did my guides I had to get them one at a time from all over the place. I ended up with three the same and an odd one all in sealed Honda bags with the same number. The odd one was an original part so I had to source one more Delta item to do the job. Sneeky way of avoiding warranty claims.

Can I ask what was the difference and how were you able to distinguish which is which?

Offline royhall

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Re: CB750F3
« Reply #22 on: October 05, 2017, 04:23:40 PM »
Nothing you can really tell visually. I could fit the valve stem through the later ones (tight), but not on the older ones. They still required reaming after fitting but virtually nothing was removed (I think they were made tighter tolerance as the material was harder and therefore tougher to ream). Looked like the only difference to me. It all went back together nicely.
Current bikes:
TriBsa CCM 350 Twin
Honda CB350F in Candy Bacchus Olive
Honda CB750F2 in Candy Apple Red
Triumph Trident 660 in Black/White
Triumph T100C
Suzuki GS1000HC
Honda CB450K0 Black Bomber
Honda CB750K5 in Planet Blue Metallic (Current Project)

Offline Yoshi823

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Re: CB750F3
« Reply #23 on: October 18, 2017, 08:10:15 PM »
I fitted S&W valve springs & machined up some new valve guides out of aluminium bronze for my 1978 CB750F2 back in '82. I had a custom reamer made for me by a local place. 6.6mm from memory & I still have it somewhere.
Hadleigh Custom down in south east Essex used to have a page in their catalogue about the differences between the F2 & the other 750 engines & how it was suggested that the valve collets should be replaced whenever they were removed. The catalogue also showed the difference in piston crown height with the F2 piston showing the substantial increase as the combustion chamber was increased in size to accomodate the bigger valves. The OEM head gasket showed that the combustion chamber was also elongated to a degree to get the bigger valves in.
I never had a problem with oil consumption except for when the bores were increased to take the bigger Yoshi 823 pistons which were 65mm if my memory serves me correctly. But when the barrels were bored out the operator of the boring bar didn't hone the bores to clearance, so the rough bores chewed the new piston rings up. Once home from Assen after attending the Dutch TT in '82 then I took the head & barrels off & had the bores honed correctly & installed new piston rings (I could do this easily as i'd modified the frame so that I could remove the top end of the engine without having to take the whole engine out).
I had played with fitting a Yoshi Road/Track cam but I also needed to change the carbs to more race ready items, which effectively held the development of more power back quite a bit. So I found a good condition standard F2 cam & had that ground to my own specification (one mm off the base circle, which increased lift & duration). The exhaust was more easily modified for better flow.
I installed Andrews high output coils, Boyer Bransden electronic ignition, NGK Gold paladium plugs. I also installed a Derale oil cooler, a Lockhart/Phillips oil cooler thermostat, an increased poundage oil pressure relief valve spring, a GL1000 oil filter housing, main jets from a CB750K7 & K&N individual air filters.

With modified gearing (one tooth less off each sprocket) it was faster than my brothers GS1000S, not only because of an increase in power but also because I modified the bike with parts that were a lot lighter than standard.
« Last Edit: October 18, 2017, 08:14:42 PM by Yoshi823 »
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Re: CB750F3
« Reply #24 on: April 04, 2018, 09:31:14 PM »
https://ls1tech.com/forums/generation-iv-internal-engine/1506256-rocker-arm-bearings-theory.html

Some good illustrations that shows what I think is the effect of rocker geometry on valve guides if it gets outside realistic limits,  relevant to previous discussion.

It's that rocker tip moving across the valve stem that loads the valve sideways.

Offline Sgt.Pinback

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Re: CB750F3
« Reply #25 on: April 04, 2018, 10:15:27 PM »
What us „PD“ (carb) ?
cheers, Uli

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Re: CB750F3
« Reply #26 on: April 05, 2018, 10:34:53 AM »
They we're fitted to the later F2 and K7 bikes,  amongst others. They use leaner / more optimum, jetting on the 750 and supplement the acceleration phase with a "accelerator pump" to make it richer at that point.

People generally don't like the type of push in jets they have in some places as they seem hard to clean if left for a long time.

Not sure of the spec on 550K3 which uses a smaller version but still designated as PD.

Nigel.

 

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