Are E and SF carbs the same???

[Mr. Schweer]

As it says, I need to know if they are the same. Carb numbers, size, etc.
I have a good running SF and a project E. It will be a while before I need carbs for the E and I need to rejet the SF, so I am curious if they are the same carbs so that I can rebuild and up the jet size in the E carbs and just r & r the carbs on the SF.
Hope this makes sense. LOL

[dbeardslee]

The 78/79 carbs are the same, and the 80/81 are the same. Jetting on the 80/81's was different depending on year, but between an 'E' and an 'SF' it's the same.

[Mr. Schweer]

Cool. I know it's not quite the same as one of my old muscle cars as far as 'matching numbers', but I'd like to keep the SF as close to original as I can. Everything I've changed from original I still have and can put back on in an afternoon.

Now, I've read that the E was the most powerful of this bikes. Does that translate to different stock jet sizes?

[dbeardslee]

The stock jets are bigger than the 80/81's (137.5 mains on the early ones, anywhere from 110 to 125 on the later models), but the 78 and 79's also had a higher compression ratio than the 80/81's. 9:1 on the 80/81 and 9.2:1 on the 78/79's. The later models also had bigger jets on 2 & 3, and I've heard conflicting reports on why. Some say it's for cooling as the fatter jets make the inner cylinders run closer to the same temp as the outers. I've also heard it was done for emission requirements, which I think is more likely - or maybe a little of both. The 80/81's have bigger valves than the 78/79's but the earlier models have more lift and duration on the cams. I've heard that the hot ticket is the later model heads with the earlier model cams. I haven't gotten around to that mod yet, but as soon as I get an 80/81 head I plan to.

[DGXSER]

Doug has it correct as I know it about the cams being hotter on the 78 than any other year.

The cause of the jets being larger on the 78-79 carbs was a change in how they feed the pilot jets. on the 78-79 (and some early 80 model) carbs, there is a tunnel from the main jet or emulsion tube section to the inlet side of the pilot jet so that the pilot jet gets its fuel after it has passed through the main jet. They also have a screw or rubber plug over the pilot jet opening as no fuel passes through there.

On the later model carbs, the pilot jet is fed straight form the bowl and does not need the larger main jets, thus the reason for smaller mains stock on the newer carbs.

So, yeah they are interchangable all the way from 78 to 81 and even the XJ carbs can work but the choke is a bit different mechanism IIRC. Just remember to jet your carbs based upon the year they were made and not on the year the bike was made as to stock or starting point.

[Mr. Schweer]

Good info....the SF runs well now and I have read so many horror stories about dialing in the carbs that I thought it would be a good idea to build a 'spare' set so that if something goes wrong, I can throw on the originals and not be stuck without a ride.

[XJOK2PLAY]

Mr. Schweer,

Wildkat sent this chart to me the other day when I had a carb question...

Maybe it'll help you as well w/ your questions...
Bob



http://www.xs11.com/forum/showthread.php?t=73

[T99Ford]

"I have read so many horror stories about dialing in the carbs"

Not if you did your research first, and know what you are trying to accomplish. The carbs are the same thing, over and over and over over again!

[yamahansolo]

i thought the cams from the 79 were the same as the 78. the only true difference was supposed to be the ignition advancer. one is a little more advanced than the other.

[D. P. Larmee]

I think I remember someone mentioning that the emulsion tubes in the SF were different than the E and F. If I recall correctly, E and F tubes had 16 holes and SF had 14 for the inner two carbs and 16 on the outer two.

[tarzan]

Ive read here that some '79 carbs were older style and some were newer style.
I have not seen it personally, but you may want to check to see if you change over.Although it shouldnt matter as long as you dont mix the parts between the two types.

[dbeardslee]

i thought the cams from the 79 were the same as the 78.

They are. The fiche verifies it - same part numbers for 78 and 79. Speaking of which, the yamaha parts fiche can answer a lot of these questions - just takes a little digging. Also, the exploded parts diagrams come in handy if you disassemble a part and then forget how to put it back together.

[Mr. Schweer]

This is an uninformed question I'm sure, but if the carbs are jetted according to this chart

Year &
Model Main
Jet Pilot
Jet Start
Jet Main
Air Pilot
Air Throttle
Valve Carburetor
1978 E 137.5 45.0 40 140 180 135 Mikuni BS34-II

1979 F/SF 137.5 42.5 32.5 140 180 135 Mikuni BS34-II

If I just clean and slap a gasket kit in the E carbs, will the larger pilot jets be enough for what I need?? All else being correct that is.

[dbeardslee]

Before I put the pods on Betsy ('79F), with a 4/1, drilled airbox, and a high flow filter, the jetting that was working for me was 145 mains with 45 pilots. You can get them at mikes xs - $6 a pair. You don't have to mess with the air jets. You will need to adjust the carbs and sync them. With your setup if you don't bump the mains on the 'E' carbs you're probably going to be running lean.