Cb750f1 1978 carb over revving

[Stevew63]

I have a recently bought a1978 f1 uk specification. I have the haynes manual and workshop Manual and the carbs fitted to my bike are different.i believe they are a Kiehin carb but mine have a different top than in the Haynes manual and different than the one shown in the workshop Manual.
They have the choke leaver on the left hand side carb and they adjuster nuts at the top of the carb under a rubber cap that I believe changes the needle length?
I’ve gone though cleaning the carbs and need to change one as it had broken float mount.
When I started it it ran first time but revved to 6k rpm and would tick over slower.
Does anyone have a schematic of this carb and know what I need to check please? Thank you .

 

[dirtdigger]

Sounds like it has early model K carbs on it but Im not familiar with uk versions...not sure what is different. We didnt have a f1 in 78 ours would have been an f2 or K8 and would have been keyhole carbs. If yours are the round tops then they are older K carbs and the adjusting nut doesnt change needle it changes the slide position and is used to sync the carbs.

 

[Stevew63]

Sounds like it has early model K carbs on it but Im not familiar with uk versions...not sure what is different. We didnt have a f1 in 78 ours would have been an f2 or K8 and would have been keyhole carbs. If yours are the round tops then they are older K carbs and the adjusting nut doesnt change needle it changes the slide position and is used to sync the carbs.

Thank you for the explanation I’m going to balance the carbs once my gauges arrive. I’m hoping checking throttle cables and adjusting throttle stop screw. Will get it to tick over. The pilot adjusting screws confuse me I believe they adjust the air. I presume screwing in reduces the air?

 

[hinksy]

Steve , check the " technichal references " in this forum , there is a chart with engine numbers that will tell you what you have . ( there is no 78F1 ) Yes with the pilot screw , they adjust airflow which draws up fuel from the floatbowl , more air more fuel . Like the digger said , if you have these type of carbs , the lock nuts just set the carb slides height . Cables will effect the idle if to tight , but it may have overevved if there is an air leak in one or more of the carb boots . Do you have this type of carb in the pic ?

 

[Stevew63]

Steve , check the " technichal references " in this forum , there is a chart with engine numbers that will tell you what you have . ( there is no 78F1 ) Yes with the pilot screw , they adjust airflow which draws up fuel from the floatbowl , more air more fuel . Like the digger said , if you have these type of carbs , the lock nuts just set the carb slides height . Cables will effect the idle if to tight , but it may have overevved if there is an air leak in one or more of the carb boots . Do you have this type of carb in the pic ?

Thank you Yes it is this type of carb.
My bike is defo a 1978 registered F1. It has the different tank than the k series. A 4:1 exhaust and the disc rear brake etc. il
Check the engine number later this morning and post back.

 

[Stevew63]

Steve , check the " technichal references " in this forum , there is a chart with engine numbers that will tell you what you have . ( there is no 78F1 ) Yes with the pilot screw , they adjust airflow which draws up fuel from the floatbowl , more air more fuel . Like the digger said , if you have these type of carbs , the lock nuts just set the carb slides height . Cables will effect the idle if to tight , but it may have overevved if there is an air leak in one or more of the carb boots . Do you have this type of carb in the pic ?

The engine number is cb750E 2560011

 

[dirtdigger]

Not sure if serial numbers are done different for non US bikes but that would be a 76 K motor here. F engines use a cb750FE- designation and the K uses cb750E- You frame should have a cb750F- designation if it is an F model. I suspect it may have had a engine swap to a non F engine.
Again not sure on non US spec bikes but we didnt have a 78F1 we had 76F1, 77F2, 78F3

 

[Stevew63]

Not sure if serial numbers are done different for non US bikes but that would be a 76 K motor here. F engines use a cb750FE- designation and the K uses cb750E- You frame should have a cb750F- designation if it is an F model. I suspect it may have had a engine swap to a non F engine.
Again not sure on non US spec bikes but we didnt have a 78F1 we had 76F1, 77F2, 78F3

I suspect it is a k series engine fitted in a f1 frame. I’ve seen a few like this is the uk around this age. Probably the end of the k series engine bikes fitted in a f1 frame, registered in 1978 although prob originally built in 1976-77.
Do you know if I acre out regality screw. Does this add more air? Running leaner?
If when balancing the carbs I raise the sliders what affect does this have?
Thanks

 

[Loki]

The engine number is cb750E 2560011

Hi Steve I have a late F1 registered in 1977 and my engine number is 2558650, have you checked the frame plate riveted to the frame tube? it should show the year of manufacture, check the frame number against the many frame/engine number tables on line, by 78 we were well into the F2, Comstar wheels and twin front discs or perhaps yours was unregistered for some reason until 78.
Those carbs look the same as mine btw.

Loki

 

[Loki]

Sorry Steve just checked my frame plate and year not shown, must be just the K's that show it.
My engine number is prefixed CB750E no F and original to the bike and all as per the V5.

Loki

 

[Stevew63]

Hi my chassis plate says it’s a 750f no year on the plate. It has wire wheels and single front and rear disc brake.

 

[Loki]

Hi my chassis plate says it’s a 750f no year on the plate. It has wire wheels and single front and rear disc brake.

Hi Steve, same like mine but mines all in bits!

Loki

 

[Stevew63]

Mine must be a late registered 1977 model registered in 1978.

 

[hinksy]

This chart says K6 motor , it is in german I think

 

[Stevew63]

Thank you it looks as if my bike is a 1976 specification not road registered in the uk until 1978. Probably lickly a bike that sat a while. It’s only done 8200 miles from new.
Thanks