750 Tripple carb tuning help

GuitarGuy1996

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Hey guys! I've got a 1979 xs750 triple who's carbs are being a little....cryptic as to what they want. Let me start by saying, the spark plugs are BLACK like it's running super rich. Compression is fine and no noises so it's not oil in the cylinder. However, it won't start without choke and the idle speed screw has to be dang near all the way in. If it's warmed up a bit, you can turn the choke off but the choke always helps it run better. However, the main jet is a 145 (which my manual says is stock) and between the plugs all being black and everyone else seeming to run 130-140s seems like it's running way to rich after all. Also, a 145 is the biggest my re-jet kits all came with. Everything has been cleaned and the floats are all in spec. Which direction do yall think I should go here? Keep digging for bigger jets or lean it back down a bit? I'm running pod filters and an aftermarket exhaust (cafe build.....shocker I know). Thank yall for your help.
 
Couple of points first: 1) the plugs will be black unless operated at operating temperature and highway speeds for a bit, regardless of jetting and 2) the starting is not affected much (if any) by the main jet size.

IIRC, the 750's have BS34's, so the starting is controlled by the pilot circuit - pilot jet, fuel screws and connected passages. Have you verified those passages are clear? Increased pilot jet size? Adjusted fuel screws out (probably spec at 1.5 turns)?

The first quarter throttle response is also controlled mostly by that circuit, then the jet needle.
 
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Yeah I went through them pretty thoroughly and everything is clean. I haven't messed with the pilot jet, I don't think I have the right style of jet for it. I've got the 1t5 carbs. And yeah the fuel screws are at 2.25 out as per the Haynes manual I got for it.
 
If it were me(if stock 42.5 pilot jets ?), I'd get 45 & 47.5 Mikuni BS30/96 pilot jets and try 'em - along with blowing contact cleaner through the passage between there and the fuel screw. I'd also maybe try going down on the main jet to 137.5 (stock 130 ?)
 
Gotcha. I'll give it a shot. Also, I'm pretty sure I need to adjust my valve clearances. Once it's warm, it doesn't like to start up again. I'm guessing clearances are tight. Kinda doubt that's effecting how it acts cold but yeah. I already had to push it a mile because of it 🤦‍♂️
 
I agree with JP, your plugs are black from starting and running with the choke on. But also, if your carbs are original '79 ones, your 145 mains are too big. Yes, the 750 came with 145 mains but only in '77, the first year, and that carb set differs greatly from the '78-'79 set you most likely have. But it's best you confirm what carb set you have because all the jetting differed between the two sets, and you base your re-jetting on what was stock in your year carb set, not the year of the bike. Here's a couple of links to carb parts drawings for the two different sets. In particular, look at how the needle mounts in the slide and where the main and pilot jets mount (into the float bowl or the bottom of the carb body). Those are the big differences between the sets and should tell you which set you have. The '78 drawing also gives a nice stock jetting list .....

https://www.partzilla.com/catalog/yamaha/motorcycle/1977/xs750d/carburetor

https://www.partzilla.com/catalog/yamaha/motorcycle/1978/xs750e/carburetor

So, if you do have an actual '79 carb set, the stock main was only a 130. For mods like yours, a couple sizes up is all you should need at most, so say around 135's. 145's are 6 sizes up and that's way too much. It's easy to over-jet these CV carbs and they'll still run somewhat OK. That's because they work on engine demand. The slide only lifts as much as the engine can use. With mains that are too big, the slide won't lift all the way and that too large jet won't flow fully.

As JP also mentioned, to burn those black plugs clean, you'll need to run down the road at least a few miles. If you can't do that then it's best to have some extra clean plugs you can swap in. Start the bike with the choke and warm it up enough so it will re-start without the choke. Shut it down, pull the black plugs out, swap in the clean ones and re-start without the choke.
 
Something else you should check is if the needle is assembled correctly. The thick plastic spacer ring should go on top of the e-clip, thin metal washer below it, and the little spring under all that stuff .....

'78-'79 Slide.jpg


5O2 Needle Washers.jpg


Mix up the plastic spacer placement, putting it under the e-clip instead of on top of it, will make the needle sit too high and the carb will run really rich.
 
in '77, the first year, and that carb set differs greatly
Wow!; that '77 (1J7) carb set appears to be a BS38 - I thought they were all BS34's - big difference and could explain a lot of confusion......
 
No, they're BS34's, a smaller version of the '76-'77 BS38's mounting their needles the same way. XS360 carbs were very similar, BS34's with that style of needle mount.
 
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Holly crap thanks guys! Sorry it took so long to get back to yall, these replies went to my spam email for some reason. I'll drop the jets to a 135 or so and see how that does. Thank you all for the info! I'll dig into it more probably Tuesday or Wednesday.
 
So I've got...sort of an update. I just checked the valves on this thing and all but 1 (middle exhaust valve) are way too tight. Just ordered a shim kit. I'll do the carbs after that. Probably explains why it was shutting off once warm and running weird. I'll adjust the valves once the shims come in and go from there.
 
So I've got...sort of an update. I just checked the valves on this thing and all but 1 (middle exhaust valve) are way too tight. Just ordered a shim kit. I'll do the carbs after that. Probably explains why it was shutting off once warm and running weird. I'll adjust the valves once the shims come in and go from there.
Good catch.....
 
Any update? Having the same kind of issue on my end but also I’m having flames coming out of the exhaust and an excess amount of fuel left in the carbs with pods. Runs and everything just overly rich.
 
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