sputtering

Hi All,

My 74 850 starts and runs great; however, it sputters ever so slightly when it is fully warmed up and I am cruising along at a steady rate. I just put a single Mikuni on. Could the carburetor just need some fine tuning?

Welcome aboard radiofun1,

Can you give us some more details? For example, is this a problem that's developed since you fitted the Mikuni? Does the bike run ok for the first 5 miles or so, that is when it's cold? It does sound like the carb needs a bit o fine tuning. Plenty of chaps can advise on the finer art of tuning a Mikuni, but just to be sure- is this a probem that occurs once warm, at a specific throttle opening? In other words, you're cruissing along at 50mph/80kph on a constant throttle and it just starts missing?

Radios are fun ,ever tryed a crystal radio? First thing I would do is pull out a spark plug and observe the colour. You are aiming for a colour around that electrode tip that is a light brown ideally. So if it looks sooty and black then you are getting too much fuel (rich) into the mixture . On the other end of the scale is too light a colour (lean). Weak running or gagging when cracking the throttle open are typical. Err on the side of rich when making changes as too lean can result in engine damage. Get back to us on the colour.

It was happening after about 20 minutes so the bike was at operating temperature. I never noticed it before fitting the Mikuni, but then again the bike never ran consistently well with the Amals. When I got the Mikuni the main jet was a 230 and the plugs were getting fouled, so I put in a 220 (the smallest one they sent with the kit) and the bike ran a lot better. After my last ride I let the bike idle for a few minutes (amazed at how well it was idling) and then removed the plugs. They had some soot on the insulator, but not nearly what they had with the 230. Perhaps I need to try a 210 jet. I live at 4800 feet.

That's high up. Thin air. Gasping. So try unscrewing the top , lifting out the slide and moving that needle clip to the next highest position on the needle. Drive , pull a plug and get back to us on colour .

The range at which you speak has nothing to do with the main. I agree with the suggestion to lower needle but I would also get the main to 240 to 260 or you'll fall flat when you get up top. I realize that it does not seem logical to lean up the mid range and richen the mains but that's how they work with Nortons.

Yep that's right, spluttering or surging is definately too rich. The engine could handle the extra fuel while it was still
cold, but not after it warmed up. Put the bigger main back in and lean off the needle..It should clean up and get
better further up the rev range, untill way up on throtle opening, and revs, when you might need to then tune the Main jet.
Make the engine work when you are doing these tests.... With a passenger or uphill or both.
AC.

I didn't think about lowering the needle. I'll do that and then should I put back in the 230 (which came in the carb) or go right to 240?

radiofun said:

I didn't think about lowering the needle. I'll do that and then should I put back in the 230 (which came in the carb) or go right to 240?

Click to expand...

Go right to 240 if you have one. I have never heard of success below 240 main. I ran a 300.

G'day radiofun1, This might help you understand if you'r not quite getting it.
When you changed the main from 230-220 and things improved, what actually happened was ,..
You restricted the fuel flow enough to not allow the needle and needle jet to work fully and so
it did lean off your Mid range a bit, however if you could have got the bike to rev it would have been very lean up high.
That's why you need to refit a bigger main, to let plenty of fuel through, as needed, and start playing with the needle.
260 + Main I would think. Dont worry about the Main being too big at this stage, you won't even use the Main
Jet untill you are revving hard constantly. You can probably run without a Main, up to 3/4 throtle.
If you end up at one extreme, that is, top or bottom notch on the needle, you might need more coaching.
Hopefully the Needle Jet is big enough. # P4 or P6 I think... Others will know.
Write down what sizes you have got... Of everything inside the carb. Look in the back as well.
AC.

Yes your small main is restricting any accurate needle and jet operating properly. Change main up ( more total gas flow), lower needle (less gas where it matters), give us numbers . And new plugs or cleaned up with soft brass brush . Plug colour after a good ride please.

Thanks for all your good advice. Plan to make adjustments on Friday (day off) and will get back to you.