Author Topic: 1978 Cb750F2 - carb work  (Read 785 times)

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Offline blackeagles

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1978 Cb750F2 - carb work
« on: July 26, 2019, 05:26:42 AM »
Hi.  I have just bought a 1978 CB750F2 which had something of a partial rebuild some 7/8 years ago and not run since.  I can start it but its well rough, ‘hunting’ and when reved the carb pistons drop without sticking and the choke seems to be adjusted ok yet the revs stay high and return slowly and then it dies so…I have a few questions/need advice before I start taking it apart. 

I have a Haynes (better then now’t) and at Section 8 it says that the F2 is.. main jet 105, slow running jet 35, main air jet 120, pilot air jet 150, jet needle setting F2051F-2 and pilot jet setting  13/4 turns out.  Does this sound correct?

Also, there is a warning “Do NOT remove the slow running jet as it is a tight pressure fit” and needless to say the exploded diagramme does not show it so anyone know its location and how it is supposed to be checked/cleaned? 

Any help gratefully accepted – I will no doubt need some more soon!!

Offline Bankerdanny

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Re: 1978 Cb750F2 - carb work
« Reply #1 on: July 26, 2019, 07:54:15 AM »
When you remove the bowls the main jet is the larger one that is in the center of the carb, it is a 2 piece assembly with the jet itself screwing into a holder that also serves as the needle jet. The slow jet is the tall narrow brass jet that sticks out of the carb body below (or above depending on how you put the rack on your bench) the main jet assembly.

The slow jet CAN be removed using pliers with a gentle twisting and pulling motion. A small amount of heat from a propane torch might help too. Just make sure to pull UP, not from side to side so that you don't crack the tube that the jet goes into. It is a tight fit and you should be able to press it back in after you have cleaned it.

There is a short video on YouTube that is helpful, search Honda PD42A carb jet cleaning.
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline Bankerdanny

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Re: 1978 Cb750F2 - carb work
« Reply #2 on: July 26, 2019, 07:56:46 AM »
Also, when cleaning jets, especially slow jets, a small piece of metal guitar string (e string) can be helpful. If the jet is completely gummed up the string will help poke a hole in the gunk so that carb cleaner can be sprayed through to get the rest.
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki K10 80, '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline blackeagles

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Re: 1978 Cb750F2 - carb work
« Reply #3 on: July 28, 2019, 04:04:47 AM »
Hi, thanks for this info...I didn't even know the carb model but now I know its PD42A there is lots of info out there.