Author Topic: CB750F3  (Read 2511 times)

Offline petermigreen

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CB750F3
« on: October 01, 2017, 05:57:45 PM »
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

Offline hairygit

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Re: CB750F3
« Reply #1 on: October 01, 2017, 06:06:44 PM »
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.
If it's got tits or wheels, it's hassle, if it's got both, RUN!!!

Offline Trigger

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Re: CB750F3
« Reply #2 on: October 01, 2017, 06:23:45 PM »
As Hairy says, the valves do like to drop and the tank was waffar thin metal.

Offline Bryanj

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Re: CB750F3
« Reply #3 on: October 02, 2017, 07:19:33 AM »
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)

Offline royhall

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Re: CB750F3
« Reply #4 on: October 02, 2017, 07:24:34 AM »
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.
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 K2-K6

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Re: CB750F3
« Reply #5 on: October 02, 2017, 09:10:27 AM »
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.

Offline Piki

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Re: CB750F3
« Reply #6 on: October 02, 2017, 09:20:56 AM »
any pic from a F3? I have never seen a F3 before................ ??? ???

Offline royhall

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Re: CB750F3
« Reply #7 on: October 02, 2017, 09:23:36 AM »
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.
« Last Edit: October 02, 2017, 04:40:39 PM by royhall »
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 yozzer74

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Re: CB750F3
« Reply #8 on: October 02, 2017, 07:14:40 PM »
Royhall i sent mine to harpers too had know bother with them at all .

Offline K2-K6

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Re: CB750F3
« Reply #9 on: October 02, 2017, 07:17:30 PM »
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.

Offline royhall

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Re: CB750F3
« Reply #10 on: October 02, 2017, 07:26:57 PM »
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  It was a Hondaman thread but sixth reply down by "OldFart" has the details. Makes for very interesting reading about the Delta changes.
« Last Edit: October 02, 2017, 08:21:16 PM by royhall »
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 neilg

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Re: CB750F3
« Reply #11 on: October 03, 2017, 10:05:03 AM »
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. 

Offline royhall

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Re: CB750F3
« Reply #12 on: October 03, 2017, 10:08:57 AM »
Rushed updates to keep the 750 current against newer competition. Not enough testing time initially as the 'delta' part later fixed the problem.
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 K2-K6

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Re: CB750F3
« Reply #13 on: October 03, 2017, 10:24:37 AM »
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.

Offline neilg

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Re: CB750F3
« Reply #14 on: October 03, 2017, 10:38:23 AM »
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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