Filter pods, carbs, and rejetting...

[wkmpt]

Hey all.
'77 750. K&N-style filter pods. Stock carbs, though the main jet is allegedly much bigger than stock--#102 vs. (previous owner tells me) #115. Stock exhaust.

Due to the larger main jets, I figure one of the PO's was trying to jet to match the pods, but the bike runs rich currently, and I'd like to fix that.

I figure I should probably rebuild the carbs anyway, so I was looking for some basic jetting advice to put me in the approximate ballpark of where I need to be to fine tune the pods with the carbs.

Advice? Thanks in advance.

 

[Steve]

This one sounds like it's right up CafeKid's alley. He might check it at work tomorrow, then he will be at Brown County all weekend, so you might not get a response from him until Monday.

One thing that can make a BIG difference, though, ... are they true K&N filters or only K&N-style filters? Some of the cheaper pods, like EMGO, will require different jetting than K&N, so it's important to know which they are.

.

 

[wkmpt]

One thing that can make a BIG difference, though, ... are they true K&N filters or only K&N-style filters?

I'm about 98.7% sure they're knockoffs. In the pics I've seen of the K&N's, they're slightly oblong, the ones on my bike are definitely symmetrical. Also, the filter material in my pods is blue, not K&N red.

 

[tkent02]

K&Ns are white, it's the oil that has the color.

 

[TheCafeKid]

First, ditch the Emgo crap filters if you can all afford it in favor of real K&Ns, the Emgos dont filter sh!t and they dont move any air either. Second, before you go about setting your jetting up, you need to assure that the carbs are CLEAN. And by clean i mean completely disassembled, dipped, and have NEW ORINGS in the propper places. THEN, you need to set your float hight (its been a lil while but I wanna say 24mm from the gasket mating surface is just about propper) If you dont know how to do this, read the tutorial here on the site. FLOAT HIGHT IS JUST AS IF NOT MORE INPORTANT THAN YOUR JET SIZES. If your float hight is not accurate, its going to cause you frustration down the road. My 77 750 had the same #115 main jet that yours has with pods and a pipe. It was too LEAN. So your running rich is a problem elsewhere. So, once you have the float hight set, you want to set your AIR screws (these are the ones on the sides of the carbs) at TWO turns out as a baseline setting. Set your FUEL screws (the ones on the bottom of the carbs) at 1.5 turns out as a baseline setting. Your main jet size may vary a couple of sizes, depending on what pipe you are running, elevation, filtration, etc etc the list of variables goes on and on. However, for a DECENT ballpark, I would suggest you purchase from www.Z1Enterprises.com main jets in the #122.5 size, the #125 size, and the #127.5 size. I think i ended up at 127.5 but could have easily gone 125 with some more playing with the needle...there are so many factors involved that its pretty hard to pigeon hole it to one size with out being there to see whats going on. The jets are cheap enough (couple bucks a piece maybe) that I ordered like that, so i wouldnt have to wait if i was wrong. I can ALMOST guarantee that 122.5 is going to be too small still, but not with enough certainty that I would tell you NOT to order.

Once your jets come in, i would try the 125 FIRST, with the slide needle clips in the 2nd slot from the bottom. Note how the bike runs COLD, and note if there is a change in how it runs when its WARM. THIS is very key in letting you know a number of different things, but mainly if your jetting is close, things wont change much when the bike warms up vs when its cold. If the bike runs better WARM, you know your jetting is too lean. If it runs better COLD, you know its too rich. Then you can work from there. Plug chops will tell you everything you need to know from that point on. Read about how to do them on the board here. Its a little time consuming, but its just about the only thing short of an exhaust gas analyzer or a Dynomometer thats going to tell you WHERE you are rich, lean, or perfect.

Try these things, if you're still having problems, i will try to help out as best i can once i get back from the BC rally this weekend...

Hope this helps


TCK

 

[rustybronco]

Hey all.
'77 750. K&N-style filter pods. Stock carbs, though the main jet is allegedly much bigger than stock--#102 vs. (previous owner tells me) #115. Stock exhaust.

Advice? Thanks in advance.

IIRC you have the stock 4-into-2 exhaust, is that correct?

***MORE***You will need to ensure that the valves are in proper adjustment and that the o-rings between the carb adapters to the head are replaced, unless you are sure they are in good condition. TCK suggested you go through the carbs, while you have them apart, you can check the jet sizes that were installed and what the float levels are set at.

what you can do before you get into the carbs, is pull the spark plugs, i.d. the cylinders that they are in and post up some good pictures of them.
when you find out what jets are currently installed, the plug coloration may help to give a better idea on a starting point for the correct jet selection, or it may help identify any other problems.

 

[trippivot]

make jetting changes in 10% increments as a rule.

 

[skidMarkNZ]

I'd try lift your needle a clip one or two before mucking around with jetting... (lifting clip lowers needle... making it leaner)

Worth a shot...

 

[KEITH KRAUSE]

I doubt the 115 mains are causing it to run rich. Even though the exhaust is stock you may still be lean at 3/4 to full throttle. An increase of 12.5 on the mains is not generally too rich for pods.
If this is jetting related, then it's more likely a PO raised the jet needles too rich and/or may have richened the pilot circuit too. Do a plug/chop test at 1/3 to 1/2 throttle and read the plugs. Now immediately do a full throttle test and read the plugs. Follow that with a minimal throttle test and read again. You should see how each jetting circuit is mixing.
Let us know the results.

 

[wkmpt]

A brief follow-up

A brief follow-up

Okay. I know it's been a minute or two, but I have more information on the carbs. I'm in the process of dipping/cleaning. The carbs that were on the bike (that are now on my workbench) appear to be totally stock.

These are the numbers I've pulled off parts so far:
Main jet 100
jet needle 5F21-3
pilot jet 15
float needle seat 2.3

All of these numbers match what my Haynes manual says is stock for my engine number (27949).

So I'm going to clean the bejeezes out of everything, make sure all the adjustable stuff is set to factory spec, and go from there. I'll keep you all posted.