Topic: 1976 CB750F - which engine do I have

[SigEpRider]

Finding some conflicting information as I dive deep into getting a 1976 CB750F running.

Here is some vital info:
VIN - CB750F-200XX35
Manufacture date - 5/75 (same month and year my older brother was manufactured!)
Engine VIN - CB750E-2525580
Cylinder head stampings - HM X 3
Carbs: 7A
Main jet: 105 (Keihin)
Pilot jet: 40 (Keihin)
Needle: 2720 clip on 3rd notch (Keihin)

Does this CB750F have an FO engine? Everything I’ve researched points to….Yes
Is that engine period correct? (I am obsessed with correctness since this is my older brother’s birthday bike)
Anything else I can do to verify if this is an FO or F1 engine?
Are the carbs correct for this engine?
Are the carbs period correct for a 1976 CB750F?

Any help in understanding what engine/carb combo I have or if I have a mix of parts would be appreciated.

[69cb750]

F1 22,317   Mar 1975 – Nov 1976   
CB750F-2000003 to CB750F-2022317    
CB750E-2515055 to CB750E-2537317

[grcamna2]

Following

[SigEpRider]

F1 22,317   Mar 1975 – Nov 1976   
CB750F-2000003 to CB750F-2022317    
CB750E-2515055 to CB750E-2537317

Thanks 69cb750.

Can you tell me if such an early production 76 cb750F would have an FO engine? Or should it be and F1 engine?

[69cb750]

Can you tell me if such an early production 76 cb750F would have an FO engine? Or should it be and F1 engine?

The frame and engine vin you posted are in the F1 range.
F1 production started in March 1975 so 5/75 is the third month of production.

[SigEpRider]

My research also indicates that the early CB750f1 used some F0 parts/engines.

My main concern is the carbs. I have 7A carbs that would be found on F0 engines. If I have and F1 engine then I should have 064A carbs. I think I can still use the 7A carbs, but I’m going for period correctness.

Hopefully others can chime in on early production F1s as to wether they had F0 engines.

[Ozzybud]

It is an F1 engine and correct for your Bike as it is higher Than the F1 Production start engine # of 2515094

Here is a chart That can be found a couple places online.

http://www.hondachopper.com/engine/engine_timeline/timeline.html

The above numbers do not look correct. per this page as well
http://www.sohc4.com/cb750/cb750f/
lots of info here

As my CB750F1 is original and unmolested has a frame number of 2020609 and engine number 2540685 with  a build date of 5/75

[SigEpRider]

As my CB750F1 is original and unmolested has a frame number of 2020609 and engine number 2540685 with  a build date of 5/75

Ozzy…. Looks like we have brother bikes.  My bike’s paint has been redone, but I can tell by the seat trim that it is supposed to be red also. I will be returning it back to the original pain and sticker scheme.

I’m feeling more confident then that the engine is an F1. Can you tell me about your carbs. Are they stamped 064A? Do you know the main jet, idle jet, and needle jet numbers?

[SigEpRider]

Here is a chart That can be found a couple places online.

http://www.hondachopper.com/engine/engine_timeline/timeline.html

Also..if you look at the timeline at the bottom of the charts on this page, it says the CB750F was produced from April 1975 to Okt 1975. That’s what has me confused. I know there will always be conflicting information, but is that timeline accurate?

[Ozzybud]

Yes! My Carburetors are stamped 064A

I had the carburetors off a couple years ago ad #3 was fouling the plug after running perfect for over 20 years. I bought the bike from a collector 20+ years ago with 5000 miles on it. It ended up being a cracked main jet Boss,and thanks to these forums and me not being the first this has happened to the fix was pressing a .45 caliber bullet casing over the boss.(perfect fit) so the main jets and slow jets were cleaned and put back in and they are the original numbers per the book.

As for the timeline I am not sure. I thought Honda made a ton of bikes for export in short bursts. And then shut down until the next production run.most of the F1's I have seen for sale have build dates between 3/75 and 08/75

[SigEpRider]

Thanks again for checking.

It appears that I have an early set of 1975 FO carbs if they are stamped 7A. 7A also show up on early K models. This is why I’m confused. My research leads me to believe that the 64a carbs should be the period correct carbs and they use 38 idle jets and a needle #27301. My understanding for the differences was for EPA idling emissions rules.

I still believe I’ll be okay leaving the 7As on. I just think someone might have swapped out the original 64as for carbs that had a larger idle jet and a needle jets that didn’t run lean also. Who knows. If anything….i just want to make sure I have the F1 engine as the F1 engine had several mods to increase horsepower.

What also confused me was the cylinder head having an X 3 stamp close to the 3rd cylinder. I thought an F1 engine should be stamped 392.

I hope more people chime in.

[SigEpRider]

Found this from Hondaman on another post as I was searching:

Honda swapped heads, cylinders, pistons and cams between the K6 and the F0. I have found them interchanged on virgin engines. The 'story' went like this: when the F0 was introduced, the market was underwhelmed and sales fell flat by February. The dealers were all crying out for more of the 4-pipers, and the K6 was born. This bike often has patched-together F0 wiring harnesses in it, confused wire colors with little jumpers plugging things together, F0 heads and cylinders and cams, and F0 bottom ends with K5 heads, pistons, and cams. The hotrod K6 has the F0 pistons inside of K5 cylinders and a K5 head & cam on top: this has compression ratios like 9.7:1 and compression test numbers like 150 PSI cold!

http://forums.sohc4.net/index.php?topic=175833.0

[Jerry Rxman Griffin aka MuthaF'er]

No, it is not a F0 engine.

The transition between F0 and F1 did in fact happen during early 4/75 as I have seen a late serial number F0 with a 4/75 build date.

From the 1975 75/76 Parts Book:
 75 Engine number 750F-2500001 to subsequent (which is in fact a misprint as the engine numbers are actually 750E-xxxxx)
 76 Engine number 750F-2515094 to subsequent (see above)

From a 1993 Parts book:
 75 Engine number CB750E-2500004 to 2515085
 76 Engine number CB750E-2515094 to 2563530
 
The 93 parts book shows carbs for both years are 064A

The engine is definitely a F1. The F0 started with CB750E-2500001 and ended at 15054. The F1 started where the F0 stopped at CB750E-2515xxx as 69cb750 states.

The engines were identical until the very end of the F1 run when they went to different thinner piston rings and valves with different keepers. The rings changed at 2551567. The earlier rings are standard CB750 rings, ie 1.5mm, 1.5mm, 2.5mm. I THINK the later rings were 1.25mm, 1.25mm, 2.5mm. Same thickness as the F2/F3. Can't find the valve page for the valve serial number changes.

Can't find my carb reference. They all have 40 slows from my experience. I still have my original owner bought new 75 CB750F0. 

Don't believe everything you read in HondaChoppers. I've seen mistakes. 
 

[seanbarney41]

+1 Jerry and Hondaman...really no telling what carb number is gonna be original in my opinion, seems like Honda was just using up whatever carbs they had sitting around.  One F bike oddity to look out for, is the tiny inlet screens on each float valve seat.  Have seen these on K6 as well, I think.  Never seen a 38 pilot installed in any roundtop keihin K or F out of dozens of bikes.

[grcamna2]

Which colors were the CB750F0 available with ?  just the orange ?

[Don R]

  The way I heard it was that Honda began 1976 production in April of 75. They lost the New York state class action lawsuit about re-titling left over bikes to the following model year. Honda then proved a point by going right into the next model year.   
 They had also planned to drop the K but the dealers were saying no, so there was a last-minute change that caused parts differences, shortages and swapping of parts between models.
 I'm sure some details are hearsay, but I got $39 for my re-titled "1973" CB500.

[jlh3rd]

Which colors were the CB750F0 available with ?  just the orange ?

here.

[grcamna2]

The candy sapphire blue must have looked nice when that bike was sitting in a showroom back in 75' 

[HondaMan]

Cylinder head stampings - HM X 3

That one is interesting: the F0/1 engines usually have a '392' cast onto the top center rear fin.

Carbs: 7A
Main jet: 105 (Keihin)
Pilot jet: 40 (Keihin)
Needle: 2720 clip on 3rd notch (Keihin)

That last number should have a "1" on its end, as '27201'. That is a needle from a K1-K5 carb calibration.
The "F" cams required a '271301' needle, with the last digit sometimes being something other than "1", like '2', '3', '4' or even '7'. This different needle is thicker at the top few mm of length to delay the richening of the mixture until the later cam's intake valve opened.

Are the carbs correct for this engine?
Are the carbs period correct for a 1976 CB750F?

In a word: probably. The F0 usually came with the '7A' carbs and the F1 with the '086a' carbs, but they also were modified by the areas where they were sold. For example, the '086a' carbs had little black limiter caps on their idle-mix screws to limit turning them more than 3/4 turn total from the OEM setting, for emission reasons, and were sold in California and (I have been told) New York. After about a year the caps broke and fell off anyway...but otherwise the 7A and 086a carbs mix identically.

[jlh3rd]

The candy sapphire blue must have looked nice when that bike was sitting in a showroom back in 75' 

I bought a brand new '76 ( supposedly) blue 550F in august of 1976. Knowing what I know now, I think they sold me a '75 because that paint was not metalflake like it was supposed to be. It was identical to what I now know as candy saphire blue. Candy saphire blue is a fine metallic.
It was really nice, sitting on that showroom floor, soon to be mine.

[grcamna2]

The candy sapphire blue must have looked nice when that bike was sitting in a showroom back in 75' 

I bought a brand new '76 ( supposedly) blue 550F in august of 1976. Knowing what I know now, I think they sold me a '75 because that paint was not metalflake like it was supposed to be. It was identical to what I now know as candy saphire blue. Candy saphire blue is a fine metallic.
It was really nice, sitting on that showroom floor, soon to be mine.

I purchased only one bike(slightly used)from a dealership(only time),the rest I bought privately.

[Jerry Rxman Griffin aka MuthaF'er]

I bought my new 1975 CB750F from Pug Vickers Honda, world's largest Honda dealer at the time, in Huntingdon, TN in 4/75. $1800. It is (still have it) a January 1975 build. Low serial number engine, CB750E-1001954. The carbs are 64A, not 7A or 086A, just as the book states. They also had those black caps with a stop added over the mixture screw. They had just hit our shores. My local Honda dealer had one and he wouldn't put it together for me for another week or two and wouldn't agree to the $1800. He had been goading me to buy a new bike to replace my 1968 CB350. I had cash in hand too. He said $1900 take it or leave it so I told him I'd leave it. As I rode past that afternoon saluting him he had that one displayed in the window. At Vicker's they were completing MANY bikes. A room full. I had my choice from maybe a dozen or so. More of the Flake Sunrise Orange than the Candy Sapphire Blue. The paint on the blue was smoother. The orange looked like orange peel, not smooth. I chose mine and rode off. OMG! It hit 70 QUICKLY. REALLY difficult to do so on the ~ 100 mile ride home. I was warned to keep it under that orange mark on the speedo for ? miles. Like I say, I still have it and will until I kick the bucket.   

[SigEpRider]

One F bike oddity to look out for, is the tiny inlet screens on each float valve seat.  Have seen these on K6 as well, I think.

 No screening on top of the float valves. In fact the PO installed Mikuni stamped float calves. Needles to say I order a set from CB750supply. They will have the screens now. A real Frankenstein this bike is

[SigEpRider]

That last number should have a "1" on its end, as '27201'. That is a needle from a K1-K5 carb calibration.
The "F" cams required a '271301' needle, with the last digit sometimes being something other than "1", like '2', '3', '4' or even '7'. This different needle is thicker at the top few mm of length to delay the richening of the mixture until the later cam's intake valve opened.

My bad. The needle number is 27201. I think I did t type correctly with my fat fingers.

In a word: probably. The F0 usually came with the '7A' carbs and the F1 with the '086a' carbs, but they also were modified by the areas where they were sold. For example, the '086a' carbs had little black limiter caps on their idle-mix screws to limit turning them more than 3/4 turn total from the OEM setting, for emission reasons, and were sold in California and (I have been told) New York. After about a year the caps broke and fell off anyway...but otherwise the 7A and 086a carbs mix identically.

As long as the 7A work also. That’s my biggest concern.  Thanks Honda man

[SigEpRider]

I posting a pic so everyone can see the casting number on my cylinder head.

I think the real question then is…..
What’s the difference between the F0 and F1 engines (if any at all) are the F0s just hopped up K6 engines? Did the new cams etc only come with the f1 update? Or are they both the same just different designations?

A real history lesson is needed here lol