rough starting point for jetting

Visegrippe

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i'm pretty sure i'm going to have to go up several sizes in the jets, but i don't feel like getting 5 different ones just to find out i'm still way out.

the engine's been bored to 750 (or 780 actually, if you believe wiseco's datasheet), will have xs performance pods and a relatively free flowing exhaust. and the stock bs38 carbs.

i don't expect the exact jets, just what experience would expect :)

oh, and do you think it would even start with the stock jetting?

Thanks,
 
If you don't want to spend the cash for a whole bunch of jets you can do what i do quite often.
Your engine should start with the stock jetting.What i've done and been quite successfull is take some numbered and lettered drill bits,find the one that fits your existing jet then find the next biggest size bit and drill out your jets then try it out and take it from there.
I've done this on XS 400's and XS650's with pod air filter ,some with straight pipes and i've always managed to get them to run real nice.
The CV carbs are a lot more forgiving on jetting usually a small increas in jet size and maybe one notch up on the needle and you're in the ballpark.
Bonne chance. Alain
 
- it should at least start with the stock jetting-if you look at the specs for the various models there is a wide range of stock combinations
- what year carbs are you running at the moment?
 
Thanks, all,

i hadn't realized that they could be drilled out and still work (they seemed to have been profiled)

as far as i can tell, they are the stock '79s.

Thanks
 
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Each increase in main jet size is only 0.002"
Dont use a hand held drill & if you go this route, good luck in getting both jets drilled to the same size :thumbsup:
think Id rather splash out $6 a pair for a few sets :bike:
 
- i agree, rather keep your jets intact

- stock 79 carbs were fitted with a leanish needle jet, a Z-2 with a 502 needle clipped at the # 3 position, a 135 main and a 27.5 pilot..vm22-210 type, 48-0500 series

- a 135 main is already quite rich, maybe you could richen it up with a Z8 needle jet, maybe try a #30 pilot, 48-0500 series.or.vm22-210

- i cant find a needle taper chart for these needles so am not sure of the compatibility
 
Vise, as KevC stated, getting them the same size take's some skill at doing that. We used to do it on the drag car's, but they are allot more forgiving than small displacement engine's.
Your right in waiting until you can ride 'er. Spark plug reading's are needed for true rejeting. XS John has allot of know how on these machine's, & He's writen that too much on the main's, even pilot's, will 'wash' the cylinder's.So I guess what I am saying is wait until you can get at least 15-20 mile's for a good plug color.
On the subject of XS John, I think he is making needle's for those carb's too. PM him later on, he is allway's glad to help.
 
well, i need to go a lot richer, it seems to keep this thing fed. very lean, and won't rev past 4k in gear, any guesses as to how many sizes i'll have to jump?

thanks
 
So where are you at now? Most 650s, no matter what the mods, don't need mains any bigger than the mid 140s. I tried all the way up to 150s in mine. It ran with those but the midrange was all mucked up and I couldn't clean it up. Reducing the main jet size was the only cure. I run 140s now.
 
i'm at 137.5 on the mains, #35 on the pilot, needle still in the middle slot, i'm thinking of starting from 145 and working from there.

considering it's still lean with the enricher on, i'm guessing it's a big jump.
 
You have to realize something about the '78-'79 carb sets. They came with a 135 main, the largest ever put in a 650 from the factory. To make that work, they leaned the midrange by using Z-2 needle jets (previous models ran 6s and 8s). Still, this puts the midrange right on the verge of being too rich. Any main jet size increase in this carb set, even just one or two, will probably make the upper midrange too rich and cause stumbles under heavy throttle applications in the 4 to 5K range. You'll need to lean the needles a step. By the time you get all the way up to 145s, you may need to lean the needles a second step. That upper midrange richness is pretty much the limiting factor on main jet size with the stock CVs and is more pronounced in the '78-'79 sets than earlier ones.

Main jets obviously effect mostly the main circuit but can influence the entire RPM range, especially if you go up lots of sizes. What I'm getting at is your 35 pilots may prove too rich with the bigger mains. Mikuni now makes a 32.5 size which may be just the ticket for you.

If you have trouble tuning the upper midrange break-up out, I recommend switching to UNI pods. They're less prone to causing that and easier to tune with.
 
since the bike is a 780cc, not 650, would the proportional increase in main jet size end up just maintaining this balance? the bike is very lean throughout the whole range and especially so on the main circuit.

i have a feeling i will just have to do this "the hard way", but that information will certainly help make the compromises i'll need to make.
it'll be good experience.
 
Actually, there really isn't much difference, if any, in the way a 650 and a 750 are jetted. The old TX750 Yamaha made in '73 and '74 ran the same jet sizes as the 650s from those years. If it does make a difference, it's not going to be a huge one, maybe just a size or two.

There's usually a range of several main jet sizes in succession that will work pretty well. Once you find that range, further tuning and testing will allow you to determine which size is best for overall performance. You'll want to find the size that gives best operation throughout the entire RPM range. Re-jetting for mods is a trade-off many times. You may need to fore go best performance near redline (slightly smaller main) in order to get the midrange cleaned up. You may need to lean the midrange more than you'd like to clean up the pilot circuit (that's why I shim needles).

If you're just outside that range of mains that will work (which you may be with a 137.5, only one size larger than stock), going up one size could make a world of difference. A 140 or 142.5 could make a big difference.
 
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Just to throw this out there, when I did Mike's 750 kit I expected to need jetting to make it run right so I started swapping parts around. After a week of fooling around I was right back where I started.
The 750 won't draw enough extra air to need extra fuel.
On Harleys the Sportster 883, 1200 and the 1450 twin cam use the same carb. The 883 use a 170 main and a 42 pilot. The 1200 a 175 main 44 pilot. the 1450 uses a 190 main, 46 pilot.
You can see the jump from the 883 to 1200 is only one size for over 300 cc. And only a couple up for the next increase.
So the 100+ cc jump to a 750/780 would only need about a third of that.
What 1/3 of one up? 1/3 up jets might be hard to find.
If the carbs you have ran well on a 650 engine then they will be very close on the 780 engine. Might just be an airscrew adjustment.
Leo
 
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