Carb jetting

[Brigsy]

Just been out setting up the new gtt.

Std engine, 14lb boost at carb top, actuator minimum pressure :eek:

On first check with wideband on the afr's were 14's in mid range on wot/peak boost going to 12.5 at the top of the rev range (totally std jetting). Suprised it didn't blow up a long time ago to be fair. I always thought the std jetting was good for this amount of boost?

Ive increased the 2nd stg to 1.2mm as im aiming for maximum fuel economy. Afr's of high 12's in mid range going to 12.1 at 5k+ rpm.

I have a dilemma, will taking the 2nd stg out to 1.3 sort out my mid range fuelling? (don't want to ruin the carb body by going too big as its mint condition)

Fart about reducing the air corrector at this level of boost?

Or finally pikey the main up to 125mm, ive got one kicking about somewhere.

Thoughts all?

Also tuning came to an epic end tonight when me top rad hose decided to pop a hole in it ten mile from home. No tools, fone went flat & forgot to pack a torch. Only had half a litre of water in boot, really had to limp it home, thought these days had gone long ago:laugh:

[Scoff]

Try going a bit more on the 2nd stage, but it might make the higher rpm's a bit too rich. If it does, open the air corrector up 0.1mm and see how close that gets you.

And "those" days never go away!!!

[Mart]

I always thought the std jetting was good for this amount of boost?

It is. Mid 12's @ wot/high rpm is perfectly fine.

If it wasn't det'ing, and going on the fact you say it's been jetted like that for a while, I'd have left it well alone.

[Brigsy]

Afr on peak boost/wot with std jetting was high 13-14, Settling to 12.5 at higher rpm's (5k+). A recipe for disaster surely? Carb & fuel cct are in good working order.

Ill drill the 2nd out to 1.3 & see if that cures it scoff:agree:

[Scoff]

you'd be suprised how lean it can be at low boost like that before it knocks.

[Mart]

Afr on peak boost/wot with std jetting was high 13-14, Settling to 12.5 at higher rpm's (5k+). A recipe for disaster surely?

Not really mate. Mid 12's is what my old Raider used to run at wot, and that was running more boost, and I'd guess, more advanced ignition, than yours.

As said, if it's not showing signs of det'ing, there's no need for additional fuelling.

[Brigsy]

1.3mm 2nd stage has sorted it now.

Im using a sniffer clamp instead of sensor in the downpipe to monitor the afr, one thing ive noticed is that the afr doesn't snap round to 12.1 straight away on wot/peak boost.

Is this due to the sensor been further down the exhaust system, or has my 2nd stage gone pete tong?

[Spooky]

I had that too mate but I shoved the sensor in about a foot in total and it didn't seem to differ too much.

I think your going to get a slightly influenced reading because it is so close to open air :)

[Mart]

I've always reckoned on there being circa half an afr point difference between measuring at the tailpipe & at the elbow/downpipe.