Topic: 750 'F" owners: some questions for you...

[HondaMan]

OK, I've never owned the 'F', so I don't have these details. But, I'm playing Sherlock again, wondering how those hotrods got tamed down, between the F1 and the F3 models. Honda did this pretty often: introduce a hot new model, then back off on the power in subsequent years, probably for warranty cost reasons. But, some of the little things they did were reversible, and restoring the missing power was pretty simple.

So...this investigation begins with the fuel system. I already know this changed a lot on the 'K' bikes over the years, and it appears this is where some of the missing hiway power came into being on the K5 and K6 bikes. But, that's going to be a different topic: don't post the 'K' stuff here, please.

What does the stock fuel petcock and fuel line routing look like on the F1, F2, and F3 models? do any of you have such a (stock-configured) bike and can supply a picture or two?

[eurban]

Honda didn't tame down the F series.  For the F2 and F3 they (among other things) used the larger intake valve head with stiffer valve springs and more aggressive cam profiles.  Carbs were the accelerator pump style and are essentially the same as the carbs on the k7/k8.  Fuel line routing and petcocks were also essentially the same as the K7/K8s. I doubt you will find much to support a taming down theory with the petcocks and fuel line routing. In fact, the F2/F3 made more peak HP than any of its predecessors.  Also, the performance improvements made to the first generation F engines (F,F1) were (again among other things) carried into the K7/K8 engines . . .I don't have a picture for you but go to any of the online parts diagram sites and pull up the fiche pic for the fuel tank and you can see a drawing of the fuel hose and petcock.   Compare the pics from the 75,76 F/F1s with the 77/78 F2/F3s

[Grumpol]

Theres an F/F1 parts fiche here mate

http://members.lycos.nl/hondaf1/cb750f1_partslist.htm

 shows you most of what your looking for, along with wiring routing etc.
I have a pdf version if you want a copy

[Jerry Rxman Griffin aka MuthaF'er]

Hondaman,

I have a 75/76 Honda Parts Catalogue 1st Edition with diagrams and parts lists should you need me to scan and fax or email any pages. 

[TwoTired]

I have examples of the 77 and 78 Cb750F models.  I also have the parts catalog for the 77 and 78 F models (second edition).

The petcock has a single outlet feeding a 5.3mm fuel line about 160mm long.  The PD 41 and 42 carbs have a single inlet nipple, with a log type interconnection between the carbs acting as a fuel rail for gas delivery to each carb.

How badly do you need pictures?  I'm not sure I can get a clear picture with the tank on.  And, can't leave it connected with the tank off (duh).

I though the F3 was the highest HP 750F they produced.     (Had the worst longevity to go with it, too.)

Cheers,

[Bido]

I have carbs from 77 and 78 F's and K the only difference I can see is on the 78's there is a vent tube running between each carb with an outlet that goes into the air box. Also the needles on the 78 are not adjustable. As far as the petcock goes I sure whish someone could sort out the sizes I am looking for a petcock to fit my 78'F3. Whish I could give you some pics but my camera is down but the bike runs great.

Bido

[GroovieGhoulie]

I though the F3 was the highest HP 750F they produced.     (Had the worst longevity to go with it, too.)

Cheers,

That's the word.  According to period literature, the F0 and F1 brought the 750 back to the original 67-68hp of the Sandcast and K0, but with the smooth running and "idle-all-day-around-town" user friendliness of the K5-K6.

From the same sources, the F2-F3 upped the hp to 72, to keep pace with the Suzuki GS750, and did this with larger valves. 

According to the forum, the problem is, this was accomplished on the same size bore, so Honda had to increase the included valve angle to allow for the larger valve faces to fit the head. This put the valve stems under a lot of stress with the new rocker-arm angle, and this causes extreme wear to the exhaust valve guides, wearing them out in less than 30,000 miles.  Honda was aware of it and I heard of a warranty repair, but don't know what it is.  There is an improved F2-F3 exhaust valve guide out there that corrects the issue, but I forget who makes it.

On the upshot, the F2-F3 bottom end is supposedly the strongest bottom end with the beefiest big ends and rod bolts of all stock 750s.

Also, the F2-F3 combustion chamber is resultingly about 5cc larger with the steeper-angled valves, making for reduced compression with K pistons.

[Bido]

I found my camera so I thought I would send a few 78 SS carb picks I hope this will help. I just finished rebuilding this set. Also I am testing to see if I can figure out how to send pics I hope this works.

Bido

[Flying J]

Any F rebuild parts can be bought at: http://www.dynoman.net/bikepages/F2/index.html
The guy that runs the place knows everything about theses bikes. He also knows that they are prone to failure and has all the parts to correct the problem as well as make your bike faster and can make you think you need it all. It would be nice to have a bullet proof F but the $$$$ is the missing link for me.

[eldar]

I think the k7/8 used the same bottom as the f2/3. I think the main differences were the combustion chambers and valves and of course the hotter (maybe?) cam. The carbs were almost the same except the 78F had 105 mains and the 78K has 110.

[HondaMan]

I found my camera so I thought I would send a few 78 SS carb picks I hope this will help. I just finished rebuilding this set. Also I am testing to see if I can figure out how to send pics I hope this works.

Bido

Great shots, Bido!
I see now why the 'F' does not suffer from a single-line feed like the K6 has: it uses a fuel rail design, which uses the carb's vacuum to help pull the fuel down from above. That's an improvement over the earlier ones.

In the early carbs, they use a little bit of vacuum, in the form of a tiny passage that goes up to the slide area, to help pull a slight negative pressure in the bowl. This ensures a better flow than gravity alone. But, the vacuum is a pulse-type pressure, not terribly efficient, and these passages often grow smaller with time from corrosion on the brass in the little holes. This causes irregular fuel levels in the 4 carbs. An extreme cleaning will clear this problem up, but mostly this is overlooked. In those 'F' carbs, the vacuum feed is going to be nearly constant, like in a manifolded engine, because the bleed ports are joined in all 4 carbs. A nice improvement!

One thing we used to do (and I still do now) is: bore some 3/16" holes into the airbox, just below the parting line for the lower half of the box, and insert the vacuum balance lines from the carbs into the box. At this point in the airbox, there is a more even negative pressure (on mine there is an additional baffle inside there that I added) to 'pull' on the float bowls a bit better. This causes a higher average fuel level in the bowls during large-throttle runs, keeping things richer overall. It makes a noticeable difference in near-ton riding applications, and it makes mine run cooler on the highway from the not-so-lean mixtures.

If we get something besides snow this weekend, I'll try to take a pix of mine to explain what I mean.

[medic09]

Not to sound perverse, but Bido - you can clean my carbs anytime!    I've rebuilt two sets of K8 carbs, but they just didn't look that good!

Mark, your analysis of the carb improvement is one of the reasons I hang out here!  I never would have know that, if you hadn't offered up that info and given us some insight into what you had been looking to learn.

Thanks to all of you for making the forums a worthwhile place for folks who want to learn!   

[754]

77 carbs, needles adjust, metal connector bar is gold(zinc plate)

78, no adjust, bar is silver

[eurban]

Somewhere I saw specs that the air jets (brass pressed in orifices in the choke plate area) were of different sizes in the PD carbs depending on the K or F application and even perhaps the year.  Couldn't find it again in a brief search but I will look some more.  754, I do believe that all the stay plates were zinc plated but depending on the condition will appear more or less so.

[Bouncer]

Sorry to tag along here. But if the needles are non adjustable, how does this effect tuning for little things like pods and exhaust? 

Does anyone offer a solid after market rebuild kit to replace these non-adjustable needles?

[HondaMan]

Bouncer:

Sorry to tag along here. But if the needles are non adjustable, how does this effect tuning for little things like pods and exhaust? 

Does anyone offer a solid after market rebuild kit to replace these non-adjustable needles?

For exhaust headers that actually work, the mainjet change is sufficient tuning. For pods, IMO, the best thing to do there is to get ones that also have the velocity stack inside, as this will ease the flat middle spot problems for these pulse-type carbs. There was a thread about 2 years ago over this situation.

Others: thanks for the great info and pictures! Right now, all I see from the existing 'F' parts is an improvement for the fuel bowl levels. If the vent line(s) are routed to a moderate vacuum source, like the airbox, then this will help raise the float bowls in prolonged high-RPM situations. The 'K' likes this change.

I'm also now curious about the F3/K7-8 heads: did they share the same guide wear situation? According to the info I have for the 'F',(Honda's published shop notes, which were not always 100% accurate), the cam on the F1-F3 was the same as the cam on the K7/8. It has a bit more duration, and is a little later, than the 'K' cams. I have one K6 engine in my parts collection, and it also has the F1 "late" cam, so I suspect it sneaked into production before it was published as being there.

Next question: the early 'K' engines all sported a "ventilated cam sprocket" for lightness, which went away in the K3 to become a cheaper solid sprocket. Yosh still carried the lightened ones, and I ran mine until 2004 that way. I would like to get another one. Has anyone tried using (and measuring) the holey 'F' sprocket on the 'K' cam, and knows if it's close, or if it requires slotting, to get closer to 'K' specs? (Or, if a new one from Honda is still holey?) I'm interested in keeping with an advanced 'K' profile (3-4 degrees earlier than stock) in my next build.

[Flying J]

you could just bye an adjustable one. http://www.dynoman.net/bikepages/F2/camchain.html#camSprkt
they are pretty cheap
The f3 and k8 head are different. The valves on the f3 are bigger and at a different angle which causes the early valve wear, im told.

[GroovieGhoulie]

I'm also now curious about the F3/K7-8 heads: did they share the same guide wear situation? According to the info I have for the 'F',(Honda's published shop notes, which were not always 100% accurate), the cam on the F1-F3 was the same as the cam on the K7/8. It has a bit more duration, and is a little later, than the 'K' cams. I have one K6 engine in my parts collection, and it also has the F1 "late" cam, so I suspect it sneaked into production before it was published as being there.

Not to my knowledge.  The F0 and F1 heads had the same sized valves as the K-series, but the cam was more aggressive and tuned to work with the 4-into-1 exhaust and its resulting higher-RPM powerband.  Since a 4-into-1 moves the powerband up the rev scale, the cam was adjusted to fit that "character".  The 4-into-4 is meaty all through the rev range and cammed accordingly.

A note on the redline, the F0 and F1 brought back the higher redlines of the K0.  The K0 had the 8500-9300 red zone.  The K1 and later had the 8000-9300 red zone.  The F0 and F1 were 8500-9600.  According to my "Seat o' the Pants Dyno", my F1 has power higher up the rev scale and really doesn't "come on" until revved, while my Sandcast feels like it has more "grunt" all over.

Redline comparo.  Note they start at the same 8500, but the F1 "discovered" 300 more RPM at the end.
F1 tach:



Sandcast:



The K7/K8 engines are the F0 and F1 engines, but with different carbs and 4-into-4s and the internal gearing is similar to the K models, since the F had a shorter primary ratio (one less tooth on the countershaft gear) and shorter 4th and 5th, along with 17/48 sprockets from the factory.

Here are the gearing differences between the K series and F0-F1 bikes:

Gearbox Differences:

4th:

K - 1.097:1
F - 1.133:1

5th:

K - 0.939:1
F - 0.969:1

Gears 1-3 were identical.

Combined with the overall gearing changes (sprockets and primary) from the K, the overall drive ratios are:

K:

1st - 11.38
2nd - 7.78
3rd - 6.07
4th - 4.99
5th - 4.27

F:

1st - 14.01
2nd - 9.57
3rd - 7.47
4th - 6.35
5th - 5.44

Source: Cycle Guide Road Tests of 1975 K5 and 1976 F1.

[Jerry Rxman Griffin aka MuthaF'er]

Mark,

The heads for the F0 and F1 are identical to the K7 & K8. All use the -392 head. The 392 identifier is found as a casting on the top fin intake side between #2 & #3. The only difference in any of these years was the K8 used different stronger springs, keepers and thus valves due to the keepers. The valves are standard size. So the only similarities these heads exhibit to the F2 & F3 would be found in the K8 keepers and springs. Also I might add that my K8 head has been hand stamped "K8" next to the "392" presumably to differentiate the valve hardware differences.

My 75F has the lightened "holey" cam chain sprocket as does my K8 engine.   

The F0 & F1 cams are different from the K7 cam which is different from the K8 cam which all are different from the F2/F3 cam which were identical.

More pixs headed your way. I have the carb spread and parts numbers for the 75/76. Notice the jet parts numbers as -392 as well as the screws being -392. The carbs themselves are also -392 which means even though the bodies are basically identical to all earlier 750s they have played with internals.

We need to find a way to get together for a day so you can check out my bike, the disassembled K8 engine, the 75/76 F parts catalogue and the Honda Shop manual. 

[Bouncer]

Bouncer:

Sorry to tag along here. But if the needles are non adjustable, how does this effect tuning for little things like pods and exhaust? 

Does anyone offer a solid after market rebuild kit to replace these non-adjustable needles?

For exhaust headers that actually work, the mainjet change is sufficient tuning. For pods, IMO, the best thing to do there is to get ones that also have the velocity stack inside, as this will ease the flat middle spot problems for these pulse-type carbs. There was a thread about 2 years ago over this situation.

Others: thanks for the great info and pictures! Right now, all I see from the existing 'F' parts is an improvement for the fuel bowl levels. If the vent line(s) are routed to a moderate vacuum source, like the airbox, then this will help raise the float bowls in prolonged high-RPM situations. The 'K' likes this change.

I'm also now curious about the F3/K7-8 heads: did they share the same guide wear situation? According to the info I have for the 'F',(Honda's published shop notes, which were not always 100% accurate), the cam on the F1-F3 was the same as the cam on the K7/8. It has a bit more duration, and is a little later, than the 'K' cams. I have one K6 engine in my parts collection, and it also has the F1 "late" cam, so I suspect it sneaked into production before it was published as being there.

Next question: the early 'K' engines all sported a "ventilated cam sprocket" for lightness, which went away in the K3 to become a cheaper solid sprocket. Yosh still carried the lightened ones, and I ran mine until 2004 that way. I would like to get another one. Has anyone tried using (and measuring) the holey 'F' sprocket on the 'K' cam, and knows if it's close, or if it requires slotting, to get closer to 'K' specs? (Or, if a new one from Honda is still holey?) I'm interested in keeping with an advanced 'K' profile (3-4 degrees earlier than stock) in my next build.

Hmm well as for the headers that "actually work" I am planning on a set of drag pipes. Which alot of people will frown upon but that is what i want. Now knowing that the needle is non-adjustable I am concerned with the tuning issues with combined pods/drag pipes. I wonder how many times I am going to have to change my mains to get the tuning at least close to running well. This is the first time I will ever be working on anything but a dirtbike carb. I am going to have to read up ALOT, however this site is an incredible reference for every issue I have come across thus far.


-Derek

[eurban]

You can use small washers as shims to lift the needle and thereby enrichen the mix.  Finding washers that are all the same thickness can be a bit of a challenge tho.   The adjustable needles from the 77s work fine in all the carbs and they should still be available from Honda.  I ordered an adjustable set from David Silver Spares a few years back.

[MRieck]

 I don't see why the needles can't be shimmed with .020 washers. The Suzuki part # is 13382-44030

[Bido]

I find that using washers on the needles you can get a finer adjustment if need be. I am running 122.5 mains with the 2 washers on my F3. Kerker headers with 2.5" baffle,Dyna-s, Dyna coils 3 OHM, 1 OHM resistor and stock air box with K&N filter.

Bido

[eldar]

The thing I worry about is the washers slipping down and jamming the needle. I suppose if you can get some that fit snug, then it should be ok. As for changing jets, exhaust will require less changes than intake does.

[eurban]

The thing I worry about is the washers slipping down and jamming the needle. I suppose if you can get some that fit snug, then it should be ok. As for changing jets, exhaust will require less changes than intake does.

Eldar, I don't see how the shimming washers could "slip down"?  The shim goes below the circlip (mounted in the needles single slot) and rests the top of the slide.  The needle and circlip are then pushed down by the spring on the linkage arm so that the washer is sandwiched in place.  I guess it is possible that when the throttle is closed abruptly that the spring gives and a small gap appears but I doubt it.