philfingers wrote:isn't std Japanese unleaded a higher octane than std (at that time) UK unleaded, so in effect you'd need smaller jets for the same calorific value, if that's the right (weight watchers!) term!

No quite the oposite, i believe its much lower than our standard unleaded :S

My JDM Celica GT4 has a sticker in the petrol cap - 100 octane only..lol

That is a '92..

The Jap market bikes were all 91 octane IIRC - but we were rolling on leaded fuel back then..were we??

philfingers wrote:isn't std Japanese unleaded a higher octane than std (at that time) UK unleaded, so in effect you'd need smaller jets for the same calorific value, if that's the right (weight watchers!) term!

NO NO NO NO NO.

Octane is NOT a measure of power in fuel, higher octane has no more power than lower octane unless the engine can use it.

The bikes were designed to run on 91+ octane (read the owners manual, in English or Japanese)

A higher octane fuel that 91 wont make anymore power in the VFR.


The higher the octane of a fuel, the more it resists spontanous combustion (detonation) which is bad for an engine.. the higher the comp of the engine and other factors that effect its state of tune require a minimum grade of fuel before it'll run safely.

Anything higher isn't making and difference in this engine (and most actually).

Some modern cars like BMWs have 'knock' sensors and when on higher octane fuel will make more power as they alter ignition timing etc to compress fuel more and generate more power.


Ordinary unleaded – 95 RON
Super unleaded – 98 RON
Leaded Four Star – 98 RON

Many mfgs add 'detergents' and cleaners to their 'super' fuels to try and jusitfy the extra cost.

In terms of power, you will make no more and not need to adjust any jetting so long as it is at least 91ron..

Japanese / uk jetting may be different due to a combination of two issues.

1. UK altitude
2. UK bike making slightly more power.

Yes the Ignition units are different, as woudl the loom be.. (no speedo restriction)

Neo's right. Unless you have quite seriously raised compression and/or advanced the ignition, you won't see any gain from higher octane petrol.

This is quite interesting, I thought:



I think it's fair to say that the 1980's nc's have more in common with the Clio in terms of electronic sophistication than they do with the Subaru. So you would literally be chucking your money away, unless as said you had already tuned the bike for the higher octane fuel. At that point, you would have to use it, as it would probably pre-ignite (pink, detonate) on lesser gas.

Another thing people confuse is the RON number and 'PON' which is what US gas pumps use.

Approximatly.

RON PON
90 - 87
92 - 89
95 - 91
96 - 92
98 - 94
100 - 96
105 - 100

So American 'high-oc' 94 PON like 'Ultra94' and I can't remember any others off the top of my head are the same as UK Super Unleaded.

The most common grades of gas that used to be available are (might no longer be the case anymore...)

Regulr 87 PON - UK 90 RON
Plus 89 PON - UK 92 RON
Premium 91 PON - UK 95 RON (aka premium / regular)
Ultra - 94 PON - UK 98 RON (aka Super)

RIGHT!

I have been out for a spin & as suspected & suggested the bike feels lean, it goes well enough but does have aa couple of 'hesitations' on it's way to the top.

I'll try some more mains from Rick

Phil

phil x wrote:RIGHT!

I have been out for a spin & as suspected & suggested the bike feels lean, it goes well enough but does have aa couple of 'hesitations' on it's way to the top.

I'll try some more mains from Rick

Phil

Where abouts are the hesitations.

Different parts of the rev range are affected by different things.

5-8000 are needle transition phase.. changing the needle height affects this area.

8k+ and your on main jets.

Idle your on about pilot circuit

Part throttle cruise an ability to take full throttle are more to do with float heights.

Common problem is a mid-range flat-spot on the 30 which is why shimming the needles (which raises the height slightly) richens up this area (has no effect on wide open throttle / full revs) and overcomes this area

In the 5-7k mark the engine/exhaust note sounds 'muted' - this is where the power/torque dip is, is the ignition retarded for noise or emissions purposes in this area?

The stuttering was from 8k+ at WOT, so purely on main jets, maybe just coming off the needle.
Trouble is, what is different about the K models that made Honda fit the same sized jets F/R rather than the following models' 115/118 & does it matter??

Do I go for 4 same No'd jets or unequal ones F/R???
Thinking of maybe RO 112/115 or (RO) 4x112?

Phil

phil x wrote: The stuttering was from 8k+ at WOT, so purely on main jets, maybe just coming off the needle.

Sounds exactly like lean main jets.

phil x wrote:Trouble is, what is different about the K models that made Honda fit the same sized jets F/R rather than the following models' 115/118 & does it matter??

I think honda got the jetting wrong (or too right, whichever way you see it!) on the k models. It may be fine for most riding conditions, but when fitted with even just an aftermarket muffler it runs too lean, hesitating up top. Hot weather wouldn't help it with the standard jetting either, just no room for variable operating conditions with the 110 jets.

The later models run larger jets which gives them more margin for variable weather conditions (or if owners stick on a loud pipe etc). The larger jets in the rear cylinders might help to keep them cooler as they get very hot in slow riding in hot weather. The nc30 cooling system is barely adequate in these conditions, so I guess every bit helps once you open the throttle.

It's debatable whether you need to keep larger jets in the rear cylinders, the rvf had the same sized jets all round.

phil x wrote:In the 5-7k mark the engine/exhaust note sounds 'muted' - this is where the power/torque dip is, is the ignition retarded for noise or emissions purposes in this area?

The stuttering was from 8k+ at WOT, so purely on main jets, maybe just coming off the needle.
Trouble is, what is different about the K models that made Honda fit the same sized jets F/R rather than the following models' 115/118 & does it matter??

Do I go for 4 same No'd jets or unequal ones F/R???
Thinking of maybe RO 112/115 or (RO) 4x112?

Phil

Only thing changed carb wise was the front emulsion tubes, everything else in the carbs is completely the same apart from larger jets.

Why not just order kehin jets.. you can buy them off ebay for around 2gbp each, there known to work.

115/118 kehin jets drop straight into K model carbs without changing the emulsion tube and it runs great.

The jetting was a touch on the lean side from the factory... you had a bit of stumbling at full throttle wtih a standard bike with standard can, made worse by aftermarket can or system.

The mid range 'flat-spot' might be aliviated by shimming the needles an extra .5mm to richen this area up.

I drilled the hole on the throttle slide to 2.5mm and didn't need the shim after this, i'm guessing they raise quicker now but it doesn't have any flat spots