Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: petermigreen on October 01, 2017, 05:57:45 PM
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Good evening Lady and Gentlemen,
I've been offered first dibs from a collection of a 750F3 in black. I've been on ride outs with the owner on it (him riding not me) and I think it's a solid bike, definitely a rider not a show'er. A price hasn't been discussed yet as it's not an imminent prospect.
Do any owners past and present have any for or against comments.
I actually prefer the earlier bikes (and those cornstars! Urgh!) but might consider ownership if it was worth it.
Thanks in advance, Peter
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Major drawback....PD Carbs, don't know if Honda sorted the valve/guide problems for the F3, or whether the motor had the same issues as the F2, just that the yanks liked a "New" bike each year.
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As Hairy says, the valves do like to drop and the tank was waffar thin metal.
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If it don't smoke and does more than 350 miles per pint of oil the guides have been done or it is so low a mileage they havent worn out yet. PD carbs need regular use(more than once a week)
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The F3 was USA only and was just a cosmetic update of the F2. I gave up on the PD carbs on my F2 and sent them off to a carb man that knows what he's doing. Matt Harper at Harpers Ultrasonic who does carbs for all the classic racing boys and girls. Since that it's run great and not a moments problem.
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I've always liked the F2 and it was the last of these motors produced. I know there's a general dislike of cleaning and looking after the PD carbs but they do work well when clean and setup I think.
It's the right way to run an engine too, generally leaner when not accelerating helps to avoid unburnt excess fuel in both the oil and exhausts, it's just better for engine health.
I may be wrong but thought the valve guide wear was related to increased cam lift that moves the rockers through more of an arc which puts more sideways load onto the valve stem. If that's true, a K7 cam could possibly be used to avoid that long term.
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any pic from a F3? I have never seen a F3 before................ ??? ???
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They fitted bigger valves into the F2 so had to offset the rocker mechanism to get them in. The geometry change forces the valves into the outside of the guide causing wear so a K7 cam wouldn't help. They fixed the problem with harder valve guides. Failing that use the bronze guides from Kibblewhite.
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Royhall i sent mine to harpers too had know bother with them at all .
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Piki, the F3 I don't think you'd tell it apart from a F2 as they're essentially their same bike just moved the name along one notch, did the same with the K7 to K8 as well. Think USA only.
Roy, I know it's recorded some places ( I've found it mentioned in contemporary road tests) that the F2 used different valve angle but I can't find anything official that says that, don't know if anything exists.
I've seen the two head's apart but never next to each other to make a comparison directly. The F2 has bigger fins as far as I know but it just doesn't seem to me that the valve angle changed, after all its only 3mm on exhaust and 2mm on inlet. I'm not certain of it but certainly intrigued if they did change it.
Also, more time ago than I want to remember, a friend ran what I thought was K2 cam gear on an F2 head to replace a damaged set, or at least that's how I recall it. I don't know how true that is but would obviously not be possible if the valve were moved.
They also seem to be in the same place inside the tappet adjust caps with the rocker covers being the same part it seems.
It don't know if anyone else on here has for example the rocker part numbers to compare perhaps.
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It's a while ago now, but I think that information came from Hondaman on the US site.
Had a look and here it is http://forums.sohc4.net/index.php?topic=10540.0 (http://forums.sohc4.net/index.php?topic=10540.0) It was a Hondaman thread but sixth reply down by "OldFart" has the details. Makes for very interesting reading about the Delta changes.
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Piki, the F3 I don't think you'd tell it apart from a F2 as they're essentially their same bike just moved the name along one notch, did the same with the K7 to K8 as well. Think USA only.
Roy, I know it's recorded some places ( I've found it mentioned in contemporary road tests) that the F2 used different valve angle but I can't find anything official that says that, don't know if anything exists.
I've seen the two head's apart but never next to each other to make a comparison directly. The F2 has bigger fins as far as I know but it just doesn't seem to me that the valve angle changed, after all its only 3mm on exhaust and 2mm on inlet. I'm not certain of it but certainly intrigued if they did change it.
Also, more time ago than I want to remember, a friend ran what I thought was K2 cam gear on an F2 head to replace a damaged set, or at least that's how I recall it. I don't know how true that is but would obviously not be possible if the valve were moved.
They also seem to be in the same place inside the tappet adjust caps with the rocker covers being the same part it seems.
It don't know if anyone else on here has for example the rocker part numbers to compare perhaps.
I believe the valve angle would have had to be wider due to the deeper combustion chamber in order for the valves to fit the shape of it. Also this would gain full advantage of the cross flow effect. In a way it shows that the original CB750 SOHC design was spot on and later attempts to "improve" stretched it to the point where DOHC were the only way forward.
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Rushed updates to keep the 750 current against newer competition. Not enough testing time initially as the 'delta' part later fixed the problem.
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I guess it's interesting for those working on them as to what may fit considering how few parts may be left.
I understand the reasons for moving valves but just from an straight update engineering point of view it just doesn't look like the valve springs moved or have any space to move from original architecture. As before, I'm not certain at all what happened it just looks like they didn't move.
As you say Roy, they must have already been developing the next engine, the twin cam 750/900 while they did a last iteration of the SOHC motor for its last year.
You can see the thinking in the endurance bike they first ran in 1976 I think. Called the RCB, it was a 750 SOHC bottom end with a narrower angle 16valve twin cam head on it. They even did one with a gear primary drive modded into those cases as it developed. Then there racer switched to the 900 dohc road bike design as a complete update.
Interestingly the road test I've got at the time puts the F1 as faster in time at 13.2 secs over the 1/4 mile as opposed to the F2 at 13.5, they speculate it's because the F2 gained over 25lb in weight. So all that development and risk to go slower.
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The location of springs and the top of the valve stems would have stayed the same the slight change in angle would affect the position of the valve head within the deeper combustion chamber, moving the head slightly higher to allow the bigger valves room to overlap. Hence the exhaust valve is slightly more in a horizontal position, (too slight to see by eye I would imagine). This in effect threw out the geometry so the stem is pushing to one side in the guide and so the excessive wear. Honda's original cure was to do away with a couple of oil drains which I imagined they hoped would cure the issue.
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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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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.
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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.
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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.
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well i never knew that
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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.
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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?
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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.
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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.
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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.
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What us „PD“ (carb) ?
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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.