question for a carb Guru

[john650]

Hi, I'm pleased to say that my 1975/6? engine/frame no. 447-120904 is a joy to ride.
She starts easily on the choke and when after 20 seconds or so, the choke is turned off, she settles down to a steady idle.
Only done about 150 miles since new barrels/pistons,camshaft..... so haven't taken the engine to more than 4000 rpm yet.
She picks up cleanly, runs smoothly, and there is no popping or spitting on the over-run.
I checked the plugs colour, because I was unsure what effect the Heiden high-performance camshaft would have on the fuelling. The plugs are both very light grey, where I would be more comfortable with a light tan colour.

I have checked the numbers on the pilot jets (45) and main jets (126) and the needle clip position is 4 from the top.
I was a bit puzzled at 126, because the sizes available from Mikesxs/ Heiden seem to go up in 2.5 increments. The other puzzle is that the mixture screws did not seem to have much effect during initial set-up, so I left them at 0.75 turns out.
My question is what to do to get the engine running a tad richer? Do I need to go back and get the pilots right (when it is running so well anyway), and what is their overall effect on the plug colour?..................Thanks for any help.

 

[grizld1]

I don't know what to tell you about those main jets except that 126 is not and never was a Mikuni number. Re. buying jets: if you're smart you'll spend a little more for brand name Mikuni jets and stay away from the Chinese knockoffs that Mike's XS and Heiden peddle.
Re. plug reading, go here and follow the menu: www.strappe.com . Re. fuel screw settings, read the XS650 Garage Carb Guide.

 

[5twins]

You're going to need to positively I.D. the year of your carb set in order to know what jets should be in there. Once you determine what should be stock, you can start tuning from there. In your case, this is particularly important because there were large jetting differences between the '75 and '76 carbs. The type of pilot jet used also changed from the BS30/96 style in '75 to the VM22/210 style in '76.

If your carbs are separate units using two throttle cables, one run to each carb, they are '75 or older carbs. If the carbs are bolted together as a "bank" and use only one throttle cable run to the left carb, they would be a '76 or newer set. The jetting you mentioned seems to indicate they are the '75 or older carbs. A 45 pilot and 127.5 main, along with #4 needle clip slot settings, were stock for the '75 set.

 

[john650]

They are definitely 1975 then, as they are only connected together with fuel and choke tubes.

So the jetting isn't so far off stock?

What is the best way to move from light grey to tan?

 

[john650]

That Strappe article is interesting, but perhaps a little difficult for me to be able to read plugs like that. I suppose I am just going on the received wisdom of light grey =bad, whereas tan=good ?

 

[5twins]

With today's new unleaded fuels, you may never see tan but instead, shades of grey. So, grey is considered OK now. Still, I would experiment with 130 mains, maybe even try 132.5s, just to make sure you're not running too lean. We can't tell you exactly what's best, your bike has to tell you.

 

[john650]

5twins: Thanks for that. Hmmm...... fuel aint what it used to be. I suppose some lead-additive might help the valve seats a little too.
There was an excellent article in our (UK) Classic Motorcycle Mechanics magazine recently, about how modern fuels are ruining seals, plastic floats and tank-liners to name but a few, and especially in older machines. As for the colour of spark plug deposits, I guess you are right. Perhaps there is not so much petrol in petrol any more.

I will get some new main jets, in the sizes that you suggest. The pilots are pretty well 'graunched up' by the previous owner anyway, so I'll get some new ones of those too. Oh and those tiny O rings that fit over the needle jets. (I found a couple of ordinary O rings in my garage that fit, but are probably not fuel-proof) ..........get it? fool-proof?, oh dear.......

Anyway, thanks again for your help.

 

[5twins]

Yes, you'll want Buna-N or even better, Viton rubber (more heat and chemical resistant). The actual size for those o-rings was a rather odd 1.1mm thick x 3.1mm I.D. Good luck finding that, lol. What does work is 1mm thick x 4mm I.D., a much more common size. Stretching the smaller 3.1mm I.D. o-ring probably makes it's thickness closer to 1mm and is why the 1mm x 4mm makes a good substitution.

 

[john650]

Yes, I think the ones I used were a standard 4mm id by 1mm thick. They came in a box of mixed (i.e nobody wants these any longer) sizes from Beaulieu Auto Jumble. To be honest, I never thought that I would find a use for a single one of them.
To make matters worse, if you drop one on the floor it disappears ,only to be found days later when you dont need it any more.
Viton eh? never heard of that before. I'll look around, but I have the sneaking suspicion that not many vendors would go to the trouble of selling me a couple of those bad boys.


Blimey........... I just found 3.5mm id by 1mm and 4mm id by 1mm viton o-rings online ! .............in a packs of 2............at simplybearings.co.uk how weird is that?

 

[5twins]

You don't absolutely need Viton for internal carb o-rings. CycleO-rings.com, a large specialty motorcycle carb o-ring supplier, sells mostly Buna-N. The only exceptions are o-rings for between the intake manifolds and motor. They sell Viton o-rings for there to withstand the higher heat.