77 carbs on a 72 motor

[yamchop]

i'm putting a set of carbs off a 77 motor on to my 72. i have pods and free flowin exhaust. do i jet the carbs for the year of the motor or year of the carbs? stock jets are considerable differnt size between the different years. i dont reall know where to start so any help would be great!!

 

[eric123]

Read your plugs. They will tell you where you need to be...Rule of thumb is up one size for free flowing exhaust and up one size for free flowing air filters...Those are just ball park though...

 

[yamchop]

that wasn't my question

 

[Sundie]

My advice: take whatever is stock for 78, whatever is stock for 72, and average them together as a starting point. It probably won't be right, but you might get lucky.

 

[inxs]

- might help to know, just to get an understanding of what could be happening in the cylinder, that
- the 72 was
8.1/1 CR
conrod length of 136mm,
inlet timing opens 47° BTDC, closes 67° ATDC...294°,
exhaust opens 60° BTDC, closes 41° ATDC...281°

- while the 77 was
8.4/1 CR
conrod length of 130mm
inlet timing opens 36° BTDC, closes 68° ATDC...284°,
exhaust opens 68° BTDC, closes 36° ATDC...284°

 

[Sundie]

Wow - lower compression with a longer con rod? Were the jugs taller on a 253 motor vs the 447?

 

[weekendrider]

yamchop, jet to the carbs. Not the specs of the carbs for a particular year model.

 

[Sundie]

WR: if you jet to the carbs for a 77, which had 8.4:1 comp, on a 8:1 comp engine, you're going to need less fuel - so the engine does play a bit into it. Sure, it will *run* with either and maybe the stock jetting for a '77 is bang on right. But i doubt it.

 

[inxs]

Wow - lower compression with a longer con rod? Were the jugs taller on a 253 motor vs the 447?

- piston pin positions were different

 

[weekendrider]

Sundie, While both parts combined make one unit, don't you need to consider the way the idividual components would work. The carbs have different needles, needle jets and while I don't and never have had a set of '72 in my hands I'm betting the slide cutaway is different. So considering the originals were set up to work with those components why would you throw a jet in there that was designed to work with different components?? Or to think of it in simpler terms if you put a set of '34s on there would you jet to the specs for a '72 or would you jet for the carbs?
Or another way. The '72s and '77s used different pilot jets. Jet to the carbs. Read your plugs for finer tuning.
__________________________________
After thinking about it more Sundie you are correct. He needs to jet for the motor.
I beginning to think it was one of those trick questions with multiple correct answers.
I'll defer to Eric123 since he has done it and I haven't.

 

[eric123]

that wasn't my question

My apologogies...I did this same mod in 2004 or 2005 except the carbs were 1978's, with the pods and exhaust...I was just trying to save you a few headaches is all...Anyway, sound like you are good to go, please keep us updated on your progress...We like pics too...Good luck...

 

[Sundie]

Sundie, While both parts combined make one unit, don't you need to consider the way the idividual components would work. The carbs have different needles, needle jets and while I don't and never have had a set of '72 in my hands I'm betting the slide cutaway is different. So considering the originals were set up to work with those components why would you throw a jet in there that was designed to work with different components??

I don't believe he's mixing carb parts together - i think (if i'm reading his original post correctly) he's just putting '77 carbs (the entire carb assembly) onto a '72 engine. So you should use '77 style jets, but possibly different sized ones for what fuel the '72 engine will require.

Unless i'm totally misunderstanding what you're saying above... Which is always possible.

After thinking about it more Sundie you are correct. He needs to jet for the motor.
I beginning to think it was one of those trick questions with multiple correct answers.
I'll defer to Eric123 since he has done it and I haven't.

Well, my gut tells me it's not going to be stock sizes for either the motor or the carb. Like i said, he's going to have to experiment.

 

[yamaman]

Well if it were mine, I'd (after giving the carbs a damn good clean) put the new carbs on how they are & begin the tuning process. Might not be too far out I think!

Of course pod filters & zaust will make a difference as well!

What filters are they?

 

[yamchop]

wow thanks guys for all the info. looks like i need to just put the carbs on and see where i'm at with the jets. i'm not interchanging parts just new carbs. the ones i had were screwed up just couldn't get em to work. hopefully get out to the garge today and get it goin. let you know how it turns out.