Idle mix turns vs Pilot Jet Circuit

jimdi · 650Rider Supporter Joined: Oct 27, 2005 Posts: 512

I know it has been posted - but I want to ask again -

A change of 1 pilot jet size is equal to about how many turns of the idle mix screw? (on a 34)

Somewhere in my mind I want to say 1 full turn of the mix screw = 1 jet size.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

It's more like a quarter turn.

kenb

Hi guys,

I'm not out to hijack this thread (I hope the OP doesn't mind), but I have a directly related question about the relationship between the pilot jet size and the setting of the mixture screws that may also be beneficial to the original poster.

I've taken xsjohn's advice and upgraded my pilot jet size to a 45 (don't know what the original was) and I'm unsure of how far in I can safely screw in the mixture screws as a result this change. The bike starts without being choked, suggesting a rich condition, but I'm still a bit hesitant to turn the mixture screws back in any further than the 2 1/4 turns I have them at now, because my gut instincts tell me that I'd rather run rich than end up with holes in the pistons.

I've read in related posts that 2 1/4 turns is in the ballpark for stock pilot jets, so would it be considered accurate to say that a setting of less than 2 turns would be in line with using a 45 pilot jet? If so, is there a practical limit that should be observed to prevent setting the mixture too lean?

Ken

xsjohn

Ride the bike kinda easy for 10 or 15 miles and check the plugs......when you aren't on the throttle you are on the pilots........xsjohn

jimdi · 650Rider Supporter Joined: Oct 27, 2005 Posts: 512

5twins - thanks for the quick response -

ken- that is exactly why I asked - is for John's needles.

I assure you with 45's (same as me) you will probably end up in the 1 3/4 turns out range. John runs his at 1 1/2 out.

And I can tell you at 1 3/4 out - I am still a tad rich.

With the stock 42.5's - stock needles and stock MJ 132.5 - My idle mixture was about 2 - 2.5 turns out.

Keep in mind there are 3 other holes feeding the idle circuit that are not effected by the idle mix screw - just the jet.

ine is - quick start - full choke - go to half choke in a minute - ride 3-4 blocks half choke - choke off - idle drops to 1k for about 1 minute or so - then slowly creeps up to 1.4K and stays there the rest of the day.
Sound - is deep and rich.
Plugs are currently being tested.
y guess is that I am slightly on the rich side.

still tuning

kenb

jimdi wrote:

ken- that is exactly why I asked - is for John's needles.
I assure you with 45's (same as me) you will probably end up in the 1 3/4 turns out range. John runs his at 1 1/2 out.

Thanks, that's the reassurance I needed before turning them in a bit more. Common sense was kind of pointing that way, but I'm new to the multiple carb scene so I'm treading carefully for a while.

xsjohn,
I'm at a bit of a dis-advantage for road testing at this point because the bike isn't legal for the road yet, but I'll definitely take that ride and look at the plugs when I get the paperwork cleared up. Thanks for the carburation tips you provided me with, the bike is starting to sound lots better these days.

Ken

xsjohn

Short ride they will be a bit dark........need a longer run for the engine to warm completely and the plugs to clean from the cold start.......hard to tune the way you described not being street legal......probably wait till you get it going to worry about that matter.........and either side may have a bit different setting to get the plug color the same or close.....xsjohn

xsjohn

Wondered lately why I was getting such good mileage on my 80...even jetted heavier and custom needles and larger pilots and still running great with good plug color....remembered lowering my floats 1MM lower (leaner) than factory recommended this spring.........to see what would happen I put them back to factory specs.....

Sure enough mileage went down to 47-48 from 53-54 and the plugs got noticably darker...........bet you can guess that I put it back to -1MM........

There could be a good reason for this........the factory had these so lean that the extra MM may have been necessary to make them run and -1MM may be right......hell who knows.......

Not suggesting to anyone to do this especially with stock carbs but some things are nice to know.....

Did notice something else lately.......I was getting some decel poping on one side........I screwed the mix screw in counting the turns then back out to where it was and the popping was gone.......would have to be magic...

xsjohn

jimdi · 650Rider Supporter Joined: Oct 27, 2005 Posts: 512

Wow - two things XSJohn -

One is my ride to work is 7 miles - which may not be enough to burn off the cold start carbon - (that is why I always have dark plugs!)

And 2nd - wow I can't believe 1 mm in the floats makes that big a difference - you get that when you hit a bump or bank in a turn... and the factory spec for accuracy on these is + or - 1 mm which could lead to an error of 2 mm that the factory says is ok???????????

xsjohn

THat's why I use a 134 MJ.........xsjohn

jimdi · 650Rider Supporter Joined: Oct 27, 2005 Posts: 512

how dow you measure jet size?

How big a hole is a 135 vs 134 or 42.5 vs45?

Is there meaning to the jet numbers?

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