pre mix jet size

[fattaffracingco]

hi all,i know i'm probably entering witchcraft territory here but i'm using a 31k as a track day tool on pre mix(synth) and I understand I have gotta increase main jet size can anyone recommend by how many sizes
i'm gonna be running roughly 40 ish to 1(to be decided),thanks chris

[ianr4863]

Im running 31K carbs on my 4LO on premix .
Had to go up 1 size on both pilot and main jet when I changed from pump to premix

[fattaffracingco]

did u purchase new ? if so from where ? have u used allens performance ? or did u drill them out ?

[flames]

I went up one size on my lc on premix also.didnt change pilots.

[phil38]

40:1 means 2.5% less fuel in the mix, so logically 2.5% bigger jets, which is about one size. Not sure what's standard on a 31K, but probabaly ~200, so a 205 would be 2.5%.

[rich]

Standard 31k mains are 240, I would go up to 250 to be safe. Some reckon 40:1 is a bit lean so as it's a track bike, it may be safer to run 33:1

[fattaffracingco]

Aug 16, 2013 14:23:48 GMT 1 rich said:

Standard 31k mains are 240, I would go up to 250 to be safe. Some reckon 40:1 is a bit lean so as it's a track bike, it may be safer to run 33:1

I ran a race mz on 33/1 but with Castrol r40 (min) with zero blow-ups !!! on 180 jet lol
was told back then that running good quality synthetic u cud lean it out a bit ?? but I will run @ 33/1 as per post and see wot plugs looklike ? will also go up one size on jetting(in my head I was going two)
can anyone recommend an outlet for jets
thanks for advice all-this forum is the mutts nuts,, ta chris

[billy]

I have 260 mains in my LC (YPVS engine with LC carbs), a bit on the rich side I think, but I'd rather run too rich than too lean.

[Deleted]

Norbs does jets give him a ring

[phil38]

Aug 16, 2013 14:23:48 GMT 1 rich said:

Standard 31k mains are 240, I would go up to 250 to be safe. Some reckon 40:1 is a bit lean so as it's a track bike, it may be safer to run 33:1

250 jet would be sensible for 33:1 or 40:1. Can't comment on right premix ratio, but I'd guess the 250 should be rich enough for either ( assuming exhaust and inlet I'd all standard.

I always put 98RON in my bikes just to make it a little bit safer.

[higgsy]

We run 30-1 on our TZR's and my YPVS, with Gulf racing 2T which is really a cart oil, but the engines come apart with no noticeable wear and it burns very well with no carb sticking issues in the wet. Tesco super unleaded (club card points doncha know!) because the local drift boys use it so it gets new stock pretty regularly.

I think 40-1 is a little lean to be honest, there are a couple of articles on the net showing power gains using more oil, it's all to do with frictional losses etc. 30-1 seems to be about right.
Conventional thinking is that going up on a jet size to account for the oil in the petrol is about right, if it's a track bike I'd go up one size on the mains and pilots to start anyway, the engine will be working harder than on the road. Also you might find the stock cooling system a bit marginal if the bike is spending more time flat out, grafting on a bigger rad helps.

H

[tobster]

you wont go wrong if you follow everything that Higgsy says, well at least with his last post. I can't vouch for everything he says....

[higgsy]

you wont go wrong if you follow everything that Higgsy says, well at least with his last post. I can't vouch for everything he says....

Lol, I've learn't over the last thirty years of messing around with strokers that nothing is set in stone!

But I've discovered the hard way with racing them that setting up rich first pays massive dividends, keep the temperature constant, my valve seems to like 65 degrees rather than the the 55 that is considered the holy grail. Use a decent 2t oil and get a good amount in the engine. You can play with the jetting once you have a good base setting where the bike runs cleanly, if it's a track bike rather than a racer where you are after the slightest marginal gain, then running it rich will prolong engine life. There is a point where it's too rich i think and you are in danger of washing the bores with fuel and coking everything up. Run it on a dyno with an A/F meter to be sure, it's very difficult to get a decent plug reading these days with the sheeps piss fuel we are stuck with :-(

[fattaffracingco]

cheers chaps ,its all good stuff
incidently can u swap main jets with another type,ones in carbs have 8mm nut shape , I have 250 jets from unknown which do fit ?? can I inter swap em ? or not ?

[m1ke]

Are they 8mm hex? If not then I'd get 8mm hex...