carb help

[chris myers]

ok, working on a '78 750E
I went to a Kerker 4 into 1 and k&N pods this is where I'm at
120 mains
needle in 5th pos w/ spacer
17.5 pilot
pilot screw out 1 1/4 turns
May still be lean at idle though

does this sound right ?
Thanks for any help
Chris

[chris myers]

bump to the top

[chef1366]

Is it running? Did you do a throttle chop? That is the way to jet you bike.

[wiredgeorge]

That doesn't sound right... First, your bike came with #100 main jets originally and jumping to #120s will provide tooooo much gas on the top. Try some #107.5 mains. #110s might be OK after doing the previously recommended "plug chop" since the Suzuki VM26 is really lean to begin with... Reclip the 5F21 jet needle back to the 3rd position if you are using a #17.5 set of pilot jets. That size pilot jet is a good size for your set up and the 1.5 slide cut out. The trick is mucking around with the pilot fuel mixture screw and air screw to get it to idle correctly and transition off idle. Try pilot fuel mixture screw out about 1 3/4 turns and air screws about 1 1/2 turns initially. USE NEW RUBBER ORINGS ON BOTH!!! That should get you into the ballpark. If you need further help rejetting, you can contact me through the carb forum on my web site which I check in on every day. I don't look at this board all that often. Cheers!

BTW: The plug chop idea isn't a bad one but ONLY works for wide open throttle. It helps set the size of the MAIN JET and main jet only. It doesn't help tune the idle circuit or mid range. Most people use main jets that are too large thinking this somehow adds power. Kind of a "more is better" philosophy... well what it actually does is: 1. waste gas 2. ruin performance. You are looking for the best performance which won't come from running either too rich or too lean.

[SqDancerLynn1]

You need to take plug readings

[KEITH KRAUSE]

Those initial jets sound like a good start to me.
Remember to remove the two floatbowl vent lines too.
1 1/2 full sizes up on the mains? For a Kerker and K&N's? Check DJ's jet kit. I believe it includes a 142 main jet or close, not sure of the other size. Their kit is specifically designed for use with K&N's and a good pipe such as your Kerker. A DJ 142 main is about the same size as a Mikuni 132.5, they just number/size their jets by different methods. Check first. No reason your 750 can't pull something close to that size main jet. At least 125's.
Your stock jet needle has the e-clip in the 3rd position (factory setting). If you put it back there, as suggested, you'll run very lean. Only chance the stock needle has is in the 5th position, with or without a .022" jetting spacer on top the clip. Be sure the two plastic jet needle spacers were put back in correct order. Thicker above the clip, thinner below.
I've never had a problem getting accurate plug reads at 1/3 throttle for the jet needles, full throttle for the mains. The plugs have to burn/color up at the throttle position/jet circuit being used.
17.5 pilot jets sound good. Try 1 1/4 to 1 1/2 turns out on the pilot fuel screws (underneath) initially.
Set the side air screws using the highest rpm method.
Be sure the carbs are vacuum synched with a gauge before any reads are taken.
I've had much better success with DJ and K&N jet kits, than trying to get the stock jet needles to work. But you can try first.

[chris myers]

What circuit do the pilot screws affect(the screw under the bowl?)just the idle circuit or all of them?
What order do I do this in?
pilot screws 1 1/4 to 1 1/2 turns out
air screws 3/4 turns out?
then sych?
I'm trying to get this thing going for the June ride. Working on the carbs now the master cylinder quits. One thing after another.


Thanks for all the help,
Chris

[KEITH KRAUSE]

What circuit do the pilot screws affect(the screw under the bowl?)just the idle circuit or all of them?
What order do I do this in?
pilot screws 1 1/4 to 1 1/2 turns out
air screws 3/4 turns out?
then sych?

Thanks for all the help,
Chris

The pilot fuel screw regulates a second passage for fuel. It's the fine tuning for each cylinder. It's part of the pilot circuit but there's some overlap effect with the jet needle at smaller throttle positions. They are sensitive to adjustment and fine tuning is usually needed. The carbs should be vacuum synched for accurate testing/performance.
With your 17.5 pilot jets, about 1 1/4 turns out is a good starting point. Plug reads after minimal throttle position testing are hard to get with any real consistancy but you can certainly get the plugs to run a grayish color which is acceptable. You want the bike to start up without "spitting" out the filters, serious decel' popping, slow warm up, etc.
Make adjustments and write down where each screw is at. They sometimes end up set a little differently from each other which is OK since they allow for differences in each cylinder. For example, cylinder 1= 1 3/8 out...2= 1 1/4, 3= 1 3/8...they'll end up fairly close to each other.
What I find strange is your side air screw setting. 3/4 turn out on the VM26 carbs with a 17.5 pilot jet isn't right. Adjusting them by using the highest rpm method usually brings them about 1 1/2 to 2 turns out. Set them using 1,000 rpm as a base, not higher. After setting them for best idle, THEN you synch with a vacuum tool.

[chris myers]

that helped, thanks Keith, I was wondering about the air screw. Just guessing about the amount of turns. I'm used to lawnmower carbs which don't have as many mysteries. Do the pilot screws, sych, then adjust to highest idle with the air screw then take readings and adjust till plugs are brown at every range for every cyllinder with no serious spitting back?
Chris
Chris

[KEITH KRAUSE]

that helped, thanks Keith, I was wondering about the air screw. Do the pilot screws, sych, then adjust to highest idle with the air screw then take readings and adjust till plugs are brown at every range for every cyllinder with no serious spitting back?
Chris
Chris

Ha! You make it sound so easy! But maybe it will be.
Actually, you're almost right.
Adjust the pilot fuel screws (underneath)...
Set the side air screws at about 1 1/2 to 1 3/4 turns out...
Be sure the ignition timing is correct...
Now warm up the bike fully (on the centerstand) and adjust the side air screws for best idle...
The bike should idle at 1,000 to 1,100 max rpm...
Then vacuum synch.
If you end up moving the pilot fuel screws more than about 1/4 turn from initial settings, it's best to re-check the side air screws for best idle. Sometimes this is necessary, other times you won't notice any change.
"Spitting" out the filters is an obvious lean condition, though poorly adjusted valve clearances can contribute to it too. A little decel' pop is normal in my opinion and easy to live with, especially if trying to richen and eliminate decel' popping just brings on worse problems.
Good luck on the throttle tests and I hope the bike runs well.

[chris myers]

That clears it all up for me, I think.
Can't thank you enough,
Chris