tricky jetting question-Eddie


I have a jetting question

Current setup (as you may remember)

DRZ400S 2003

stock motor and airbox

FMF Q and stock header

TMR flatslide 40-36 pumper(40mm bore and 36mm ID spigot.)

155 mj


2 turns on fuel screw

needle clip 5th from bottom (6 slots in total)

I researching various exhaust options, I plan to replace the stock header with an FMF powerbomb header for slight power gain , presumably without noise increase.

From what I've read on TT, an open exhaust does little for performance until the airbox is opened up so I am considering the 3x3 mod.

Tricky part (as you know) is there is no info on the TMR's re jetting changes and needle selection.(unless you can read japanese websites)

I went for a run today without the airbox side cover and bike still ran OK, not much difference in performance although the stock header would have been restricting flow

Anyway, given the lack of jetting/jet needle info for these carbs, should I avoid the 3x3 mod as too hard


Would 3x3 with possible changes to MJ and PJ only be a worthwhile endeavour.

I was thinking of ordering a 160mj with the header.

(please dont say get the yosh and FCR)

the major effect will be on main jet.

swapping mj's is easy enough although I have to order them with the header because not avail here.

do you think 160 would cover it or should I order a 165 as well.

depends on how close 155 was.i suspect 155 was rich,other wise taking the air box door off would have made the bike run worse.

depends on how close 155 was.i suspect 155 was rich,other wise taking the air box door off would have made the bike run wors

Yes, I was thinking that, although the stock header is quite narrow, much narrower than the FMF mid pipe.

Not sure, but perhaps Japan delivery S's have smaller headers than US versions ?

Will have to try the airbox door off again after Powerbomb is fitted

Probably safe to say I wont need bigger than 165mj ? and pj should be OK?

the pilot jet will stay the wont need any more than 165 main jet.

OK, thanks for that

Unfortunately TT only supply (large round) MJ's in packets of 4 per size so my idea to order a range of 4 sizes from 157.5 to 165 would have cost ~$40

So, prior to finalizing the Powerbomb header order, I cut the 3x3 in the airbox today to see what happens.

The bike ran OK through a familiar stretch of hilly highway.

The new exhaust tone from the 3x3 mod and the shortcomings of butt dynoing make performance comparisons a bit hard to judge, but I would say with confidence that it at least runs as good as before the 3x3 hole was cut

Top speed on the flat(15-40 gearing) was up to 145kmph pretty quick then wind out to 155-160kmph crouching down.

It seemed a bit torquier than before in that rolling on the throttle at low rpm in 5th at 85kmph (tall 15-40 gearing) would produce less bog than before

Anyway, the real test will be when I replace the restrictive (Japan delivery) header with the FMF powerbomb

I suspect the current stock header and Q muffler is mainly producing just noise atm rather than power increase over a stock muffler

Despite TT's policy of minimum order 4 per size for Main jets , I'm still wondering if I should order at least one size bigger MJ with the FMF header and if so, what size ?

Well, I fitted the powerbomb header and didnt notice any power difference over the stock header .

Tried 160 and 165 mj at wot but once again hard to notice any difference in power from the original 155 mj.

With the addition of the 3x3 and powerbomb header, my main concern is I dont want to be running too lean on any of the circuits.

Given I have no access to Dyno's and there is no avail info on jetting applications for Mikuni TMR flatslides, I tried the following test to eliminate the possibility of running too lean

Simply index the throttle for 1/4, 1/2, 3/4, full.

Remove airbox cover

Go to my favourite steep hill and run the bike uphill at 1/4, 1/2, and wot.

Bike ran ok, no surging/stumbling

So I am correct in assuming that rules out any circuits being jetted too lean ?

doesnt suprise me you didnt notice any gain.its just a single diameter tube.

if the main je tis will run noticeably bad.

Oh well, at least the stainless looks better than the rusty old oem header.

When I bought the bike, it was bone stock but I robbed the FMF can and mikuni TMR of another bike also for sale.

never knew about what the 3x3 mod was at the time, but given my bike still runs OK post 3x3 mod, I'm strongly suspecting the other bike had the 3x3 mod and the carb was jetted accordingly.

re exhausts, from what I'm reading on this forum, its seems that unless you go for a full unrestricted Yosh system and cop the noise increase, the other aftermarket exhausts dont offer much more power than what can be achieved with an E header and stock can, so your effectively forking out $400-500 for bling

that is pretty much it but they also weigh less.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now