Newbie jetting report

[V8Pinto]

Disclaimer: I have never jetted anything besides Holley's and Edelbrocks

Even though the jetting formulas are clearly defined by Pablo and others, I'm an Engineer so I have to do it myself

Baseline and mods:
1989 Nevada bike
Uni filter - unknown age
122.5's
37's
screws still had plugs over them

Changed to K&N pod filter RU-2970 and removed stock air box

Results:  WAY lean now.  Way way lean.  At full throttle the bike goes faster if I let off the throttle a bit.  Everything sucks now.  This filter moves a lot of air.

Jets were on order (the dealer seems to hire the 'challenged') so I threw in one washer so I know how to do it.  Make sure if you are going to buy jets from the dealer to say "I need Mikuni jets, large round".  The "large, round" part of that is how the Mikuni catalog identifies them.  There are a few different types and the "challenged" didn't know what to do....  I will include part numbers in later posts.

Results of 1 washer add:  None.  maybe a bit better in the midrange but could just be the placebo effect

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Mods 2:

Changed to 130 jets.  Drilled the plugs and put the screws at 3.5 turns out (they were at 4.25 and 3.75)

Results: MUCH better but still way off.  The top end is better.  The "back off a bit and go faster effect is gone now" but now it is just lean everywhere except at idle.  Improvement, I am going in the right direction.

Plug readings are showing first hint of color now (slight tan)

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Today I will put in the 135's and see.  Then I will go order some 140's, and 145's.

[scratch]

Good Job! And you're doing it yourself! I'm supprised you haven't changed the pilot jets to the 40's. Now my bike is stock and I'm using two .5mm washers under the jet needle circlip; originally I used one but I wasn't getting smooth roll-on on th freeway, but it depends on how it feels to you.

Congradulations on a good job!

[The Buddha]

I'll send you my reject pile...Its got nearly every size from 122.5 to 152.5, and even a pair of 160's. The only set its missing it what you need...150's...and 40 pilots...but do it your self...Id like independent confirmation...
Cool.
Srinath.

[bob]

...Make sure if you are going to buy jets from the dealer to say "I need Mikuni jets, large round".  The "large, round" part of that is how the Mikuni catalog identifies them.  There are a few different types and the "challenged" didn't know what to do....  

I had a long talk with a guy from Factory Pro Tuning a couple of weeks back & got quite an education (but that's for another post).  He told me that either the "large, round" or the "small, round" Mikuni jets will fit our carbs.  Acording to him, they are interchangeable unless the large ones are too big around to physically fit in the carb.  On our carbs, the flat at the bottom of the part where the jet screws into is wide enough to accomodate either the large or small round ones.  Other than the outside diameter ("large" or "small") the jets are identical, from a function standpoint.

Where I got tripped up is that I had a bunch of Dynojet jets mixed in with my pile of Mukuni parts & I didn't realize that Dynojet makes thiers with a different thread pitch than the genuine Mikunis.  Caused me all sorts of headache.  Stick with the Mikunis & let Dynojet stick with making dynos.  Use the OEM Mikunis.

[V8Pinto]

I hear ya.  On the second trip to the dealer (after they had given me hex jets) I got the small round jets.  I see where they would function the same but the thing that scared me was the mating surface.  Where the stocker is "large round" it gives about an 1/8" contact area at the jet/carb point.  The small round jet barely barely has a few thousandths.  I could see where someone with an overly zealous forearm could pull the jet down into the carb and possibly split the jet boss.  A washer under it would do the trick nicely and would probably serve the same purpose as adding a washer to the needle but I wasn't up for going that different of a route the first time around.

[V8Pinto]

I'll send you my reject pile...Its got nearly every size from 122.5 to 152.5, and even a pair of 160's. The only set its missing it what you need...150's...and 40 pilots...but do it your self...Id like independent confirmation...
Cool.
Srinath.

Srinath,

If you send me your reject pile, I will take them out to the track with me and do an "info" Saturday for everyone.  What I would like to do is take my bike (after I get the supertrapp on) and go through the jets one by one making drag runs in between.  No real reason other than the plot of jet size vs. ET would like nice in MSExcel.  I could post to the list here as an info thing.

Anyway, I'll do the racing if you donate the jets.  I like doing this sort of stuff and the bike is much easier to do it on than my car.  It seems a bike responds better to smaller changes.  Anyway, lemme know.

Shane
306 N2O Pinto

[V8Pinto]

Also, don't the pilots affect idle?  I wasn't worrying about changing from 37's because my bike is cool at idle.

[The Buddha]

Pilots are for idle and 0-1/8 throttle...and if the bike is fine there leave it...Typically people complain of unbelivably long warm up times, or gutless performance when starting from a light or some like that and 40's cures that...You dont want to turn out the mix screw more than ~5 turns...they may fall off... I also suggest 40's cos when the carbs come off best to change the stuff that needs carbs taken out to change...like mix screw brass cap and pilots. the rest can be done wiht the carbs on the bike really. Drag race with different jets...well there is a pointless excercise if there ever was one...the bike starts to improve till 150's and suddenly get bad with 152.5...I know that...and looks alike you are making a similar step by step trial and error thing...I'd say start with biggest and work down...cos too rich is much easier to recognize than too lean. The instant the bike gets good...leave it alone...
Cool.
Srinath.

[KevinC]

I don't think the policy of starting rich, and leaving it alone as soon as it "gets good" gets you anywhere near the optimum jetting. The performace keeps increasing, as well as the fuel efficiency and emmsions perfromance, right up to the "too-lean" point.

If you do some dyno testing, the hp will get better and better as you lean it out. The temperatures do not get too high until you get too lean, and then you need to go back one jet size or so.

150 is way too rich unless you have substantial engine mods.

[The Buddha]

Jetting is supposed to be done with an O2 sensor...Doing with a Dyno will leave you lean...or atleast on the leaner side...may be ok for race track but on the street the bike has to run fine before its fully warmed up, it has to run in rain, cold and hot weather, start up warm and hot and cold with ease...too many demands and people dont want to fiddle wiht it every weekend. Honestly err on the rich side...and leave it there...else on the coldest day of the year...it will be too lean to run right, will give you trouble strating and not run right. There is 2 ways to do it...1 is start lean and get richer 1 step at a time till you see a deterioration with a change...and go back 1 step. Or start rich and get leaner step by step till it becomes good with a change and leave it there. With stock needles, and a slip on or full pipe system and K&N pods in place of the airbox...150 is the only main that leaves you rich enough for canada in winter while still ok to run right in the mojave desert in summer (done it in both with an O2 sensor...well not exactly mojave but Central CA). 134 is what dynojet gives you for their stage 3 and their needle is very very much thinner than stock.
Cool.
Srinath.

[KevinC]

The needle doesn't effect the jetting at full throttle, so Dyno is providing a 134 in their stage 3? Obviously they don't think you need 150's.

Jetting should be done to get max performance and fuel economy, while not damaging the engine under the weather conditions it will encounter. Running a racing engine lean on the main jet is way worse than on a street engine, since they are at WOT a lot more, and running much hotter because of the hp demand.

The GS is very forgiving of bad jetting - it runs pretty good with the jetting off a long way in either direction.

Just going by hp on a dyno will usually put you right on the edge of lean (not always, depends on a lot of things). Most dyno operators have good oxygen sensors set up right to do jetting with. 132.5 was max hp on the dyno for my bike with an aftermarket exhaust, K&Ns, and some porting work, at 3,300 ASL and cool temps. Going down to  less than 1,000' ASL needed 135s. 140's were rich at +4C even with the head ported.

135 to 137.5 is lots rich with a full after market exhaust and pod filter. Any larger is a waste of fuel and hp.

150's will run like crap in hot weather, or at higher altitudes also.

[The Buddha]

Nope...Needle never comes out of the emulsion tube...even at full throttle...the needle always plays a part and the dyno needle is very very thin especially down low...In fact if we set a bike with DJ and another bike wiht the stock needle to be equally rich at 1/2 throttle the DJ will be very very much richer than the stock needle at full throttle even with 150 mains in the stock set ... The GS is forgiving of lean jetting...it reacts quite discernibly to rich and that is mainly because its air cooled.
Porting or any flow increases in reality will make the bike want smaller jets.
Lemme give you an example. The 38's that go in 750 GSXRs if you took it off exactly the way they were stock (lean as hell for the 750) and put it in the 1100, you will be too rich in that. 1 size smaller pilots (to 32.5) and 2 sizes smaller mains needed. More volume to fill = more vaccum = more venturi effect = more fuel getting sucked in = richer. So port the head and you effectively increase port velocity which = more venturi effect = more fuel getting sucked in. Putting in K&N's inplace of the airbox effectively reduces the vaccum, so for any rpm which is proportional to velocity you have less vaccum which is going to give you less venturi effect which will pull less fuel unless you make it easier to let it pull fuel off the bowl. Change the exhaust to a aftermarket and your jet change is minimal...(mostly from loss of air/fuel mix due to lack of back pressure keeping it in), lower the vaccum pre intake and that ripples through the whole system making you lean everywhere. Ported motors will behave different than stock, and if done right the increase in flow will make the bike need smaller jets. To verify this...put the stock crap back on. Pipe, air box, paper filter and all the stock jetting back on with the ported motor...and see how rich it is with the stock 122.5. Also has anyone else here wiht a stock motor had it show signs of being too rich on a hot day or at higher altitudes (we not talking Denver high...more like 1-2K ft high).
Cool.
Srinath.

[KevinC]

Srinath,

Well, your experience is very different from mine, or any of the jetting info I've ever seen. The needle has little to no effect at WOT. The restriction will only be the main jet, because the area of the top of the needle jet  minus any needle remaining is lots larger than the main orifice.

Before I had the porting work done, the engine was running great on 130's. I didn't do any dyno work on it then, but the plugs sure didn't look lean.

The more work I do on this engine, the bigger the main jet it needs. I'm at over 60 hp now.

http://204.71.0.3/motoprof/moto/mcycle/carb101/carb101.html
http://www.nightrider.com/biketech/carb_jet_ranges.htm
http://www.motorcyclecruiser.com/tech/jetkit/

[The Buddha]

OK I'll grant you that...head work/motor work etc dont neccesarily mean smaller jets...depends on what exactly has been done. You may suck longer but not as hard, or you may suck harder and longer or ...so That cant be decided that easy. But Lemme ask any one else ...
With Individual K&N's and stock carbs and needles...has anyone ever run right with 130 main jets...
Cool.
Srinath.

[V8Pinto]

Mine has the RU2970 K&N (stock air box delete), 130's with one washer, 37.5 pilots and 3 turns on the screws.  It ran consistent 14.80's at the strip Wednesday.  I'm changing to 135's and racing again this Wednesday.  Overall it runs fine, no hiccups or extended warm ups.  I just get on and go and I ride every day.  I can tell it is a bit lean though as the 14.80 is a bit "wanting".

As far as covering the jetting for all weather conditions.... ahhhh.  I fiddle with stuff to often to care about that.  I always wanted to be a mechanical Engineer and I don't mind changing jets once in a while to suit conditions if it get's me a few hundredths at the track.  I have to do that with the Nitrous on my Pinto or I'll blow it up real quick so I'm sorta used to the routine.

With nitrous I definitely tune fat to lean.  It's less expensive that way     You better start out really fat or you'll be sorry.  The rule for nitrous is "pill it till you kill it then rebuild it and back off one step".  With a non-adder motor there is much less danger of pyrotechnics and lean=fast so I tend to start there and go fat until I slow down.  Then I change something else and do it again.

I agree with what you say about an O2 sensor for tuning but I believe in the 1320 (twisties types or or circle track freaks are welcome to make fun of us straightline people).  I made an air/fuel ratio monitor using an LM3914 IC, a bar readout LED pack, and a toyota O2 sensor.  I use it to tune the Pinto and for fun on my F150.  I will put it on the GS once I find a bung to weld on but I don't want to mess up my pipe just yet.  I prefer to tune by ET.  

It's more fun that way and there's no "fishing stories" about how fast you and your bike are

"Throttle solves 95% of all problems.  The other 5%?  Well, it just gets rid of the suspense"

[V8Pinto]

132.5 was max hp on the dyno for my bike with an aftermarket exhaust, K&Ns, and some porting work

KevnC,

Did you do the porting yourself?  I've been wanting to take a die grinder to the GS head but looking through the intake tract, it looks to flow pretty well already (my porting experience is old ford iron though, not bikes).

Did you have to do more work on the exhaust port?

[KevinC]

I have 3 heads with various degrees of porting, all done by different people. Two of them have been basically cleaned up and smoothed out, the third one is more radical, with dual plugs and shim under bucket, etc. as well.

I'm probably going to be selling all my GS stuff because I think I'll race a SV next season.

[Piper5177]

How much do you want for the more radical head?