Dynojetting...

....worthwhile or not?

I've seen a new Dynojet kit for the GSX on eBay - £40, assuming no one
else bids.

The bike has a full Gazelle stainless exhaust but nothing to suggest
it's been jetted to match (1) so was wondering if this would be worth
doing.

I take it the fuel consumption would go to shit if this was fitted as
well - at the mo I'm getting rather more to the gallon out of it than
I expected, as it's doing between 35 to 38mpg.

(1) And yes, appreciate this might not necessarily match either

 

IWHT you'd be better off doing a bit of measurement first, rolling
road/mixture measurement etc. Then you know where you are starting from
rather than re-jetting by guesswork.

 

Ok - shame the local rolling road is out of action at the mo.

The kit on eBay is being sold as a 'Stage One' kit - IIRC, they're
normally sold as a 'bolt this onto your stock bike and see
improvements' kind of thing, aren't they?

If so, just wondered what others had experienced with these.

 

Does it normally *need* rejetting?

Not necessarily. It might run more efficiently. But that's only if it's
needed, of course.

Stick it on a dyno first. It'll cost £60-70. Then decide.

 

Most bikes they recommend rejetting if you go down the aftermarket
pipe route, don't they?

 

Not afaik. "It depends"

 

Ok, well... I suspect newer stuff with injection (and therefore more
sensors etc to help the ECU to determine any changes to fuelling which
are necessary as a result of a different pipe etc), it's less likely
you'll need to modify the system - before anyone starts guffawing and
saying 'hey stoopid, they don't have jets in injected system', I mean
you don't need to change the fuelling in terms of mapping.

Older stuff... jets are what regulate the fuelling... and are set in
stone - something changes elsewhere in the system, the jets are
ensuring the carbs are still only fuelling for the old set up,
regardless of the needs of the new one.

*All* the carb fed 600s I've had... if they've had a aftermarket can
or whatever, they've benefitted from rejetting in terms of power
delivery etc.

The VFRs, of which there have been many, none have really felt like
they needed jetting when I've fitted an aftermarket can etc.

 

A DynoJet kit is not just jets, but new needles and springs as well.

The whole point of the kit is that it is making the fuel richer by
giving you bigger jets at the size they have found makes a power
difference to the stock bike.

However, what's good for one stock bike might not be good for
another. The only real way to tell is to put it on a dyno and
see what the power curve is like, then decide on any adjustments
if necessary.

Personally, I'd spend my money putting it through a dyno run and
get some guidance from the technician there. A few jets won't
cost you much but will give a much better seat-of-the-pants result
with the dyno run than just fitting the kit alone. It'll probably
work out about the same money anyway.

Is it snake oil? Well no, but you're giving the bike a tune-up
without measuring the affects that it's having, so you've no
idea whether it's working or not.

 

In communiqué <1j9lmh8.1fnb7oy18g0a9cN%>,

Indeed. You can get close by guesswork and plug chops, but if going
beyond the normal envelope of exhaust system and carb setup a dyno is
the only way to get jetting spot on, particularly mid-range.

Had the 604 Nordwest on a dyno and found it was pretty near perfect
(though asking other nordie owners with Keihin smooth bore conversions
helped). Just needed a tad bigger mains.

The 375 Morini with race cam and 28mm carbs was tricky to get right.
Just couldn't get clean mid-range despite trying different needles,
atomisers and even slides. Got it much better after a rolling road
session. Never perfect though - the L5 cam is renowned for giving a flat
spot around 5000 rpm. Not a problem unless riding on the road though ;-)


--
+-------------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Yamaha WR250Z/Supermoto "Old Gimmer's Hillclimber" |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
+-------------------------------------------------------------------+

 

Best answer so far.

Cheers.

Think I'll buy it.

The local rolling road isn't at the mo, as they're looking for new
premises - given I'm pally with them and tend to trade the odd favour
with them, it'll be worth waiting til that's back up and running
before I start fecking around the carbs on this.

 

I dynojetted my Blackbird a decade ago. There was a flat spot around
4-5k before it then took off aound 7k. I was told that they had
designed it that way to meet noise requirements, whether that is true
or not I don't know. But the bike definitely went better after the
dynojet kit was installed. I had no access to a dyno, but it certainly
felt better. The bike still had stock pipes.

Next bike was a Hayabusa with Yoshis on it and the only way I could
get that to run properly was to Power Commander it. The pipes made a
hell of a difference that the standard ECU could not handle.

 

Actually, many FI systems, well, certainly open-loop ones, don't
measure the air mass flow; they infer it from the map and other
sensors. They rely on the pumping efficiency of the engine being the
same as it left the factory (I imagine they allow for wear).

Closed-loop stuff with an exhaust sensor might have enough adjustment
to compensate but that's generally limited to engines with a cat.

I can only recall one FI vehicle I've ever owned that had an air flow
sensor and that wasn't even a mass flow sensor, it was an air speed
sensor.

 

As a counter POV. After fitting a K&N and a Skorpion endcan to my
fazer 600 I figured it should really need rejetting. A dyno run with a
probe inserted <fnar> showed the carburation was pretty well spot on.
Guess it must have been running rich as standard.

 

First Fazer I had came with a Wolf can and was dynojetted to suit
before I owned it.

Seemed to have more top end than the second one I had which was
standard... and it also seemed to drink a lot more fuel.

I had a Micron 'not for road use' can for the second one - when I
tried it with that, it seemed to make a lot more noise at the expense
of power, so the conclusion was it would have almost certainly have
benefitted from being rejetted.

 

My 106 GTi relied on a MAP sensor.

Most VAG / BMW stuff relies on AFMs, as do Pug HDIs.

And a lot more stuff come to think of it.. but those are the ones I'm
more familar with.

*spit*

Never again - Mk1 Vectra, that is.

 

Not necessarily, no.

 

Conversely, I'd have another Mk3 Cavalier - even a late one saddled
with the Ecotec lump.

 

£40 at Banjax.

£10 at Bob Farnham's place in Hildenborough, though that was kinda
mate's rates in 1994.

 

Deep ends, dunnit? If there's a flat spot, or the bike isn't pulling
properly at any point, then possibly. If your plugs (or encan exit) are
very pale or sooty black, then probably. If none of the above, then
probably not.

About the going rate - that's what I paid for a Bandit kit a couple of
years ago.

Not necc: AIUI, most Jap bikes (and carbed Suzukis in particular) are
set up from the factory to run richer rather than leaner. This means
that you can fit either a race can OR an aftermarket filter (K&N, eg)
without taking the mixture out of the optimal range. Fit both and
you're looking at leaning out at higher rpm.

Never heard of a Gazelle exhaust - is it a freeflow/race type? A
properly set up Dynojet wouldn't neccessarily lead to high fuel
consumption, it should produce optimum mixture throughout the rev range.
The fuel consumption may increase as a result of the rider using the fun
handle more vigorously as the response has been improved.

Having said all that, my B12 was the thirstiest bike I'd ever want to
ride: enormous fun at anything over 3k rpm, huge flat spot below 3k, big
hole in wallet due to tank range dropping to less than 60 miles on
occasion. That was after having it "set up" with a Stage Two/Three
Dynojet kit, race can and K&N. Previous performance was poor, with
lackadaisical grunt and poor starting - no go, no poke and 25mpg, that's
why I thought a jet and setup was called for.

 

There is something "odd" about that B12 though. The fuel consumption was
insane.

I don't really see the point in having carbs 'jetted without a dyno session,
for all it costs. It's only a stab in the dark otherwise and may just
complicate matters.