Another carbs tuning question

[Alf]

Hi:
My question is not strictly about FJs, but it is for a carburated bike, my CBX 750, so I think it could be useful

The bike runs great and sweet, but It has a hole in the power-band between 5000 to 7000 rpm. It is uncomfortable, much more after riding my FJ.

I think I have to raise the needles to give more fuel, but I´m not sure, because the bike runs the same hot or cold. I think it is due to the Mac exhaust design, but I can´t come back to the mild steel rotten OE exhausts and collectors

Settings: Dynojet kit as spec, except that running Keihin OE 115 main instead the DJ main (tested a lot of mains and this is the best main jet), Mac 4/1 exhaust (race) and KN filter

Ok, I know that I have only 2 possibilities (needles up or down), but I´m sure that the Murphy Law says that if I change anything I´m going to make the wrong way, so I would like to know your advices

Best regards
Alf

[yamaha fj rider]

My understanding of jetting is throttle opening, not rpm determine what circuit of the carb you are using. What % of throttle are you at when this happens? When was the last time the carbs were synced?

Kurt

[Alf]

Hola, Kurt

1/2 to full throttle. The effect is greater as faster you open the throttle. If you´re progressive with the opening the effect diminished.

And pasing that zone, from 7500 onwards, the acceleration is like a turbo is engaged (well, I´m exaggerating for a 70-80 hp air-cooled engine, but really is so fast there)

Sync is not the problem

Thanks

[andyb]

4-1 exhausts will always have a hole in the power someplace. 

Shove the needles around and you may be able to minimize it some, but it'll still exist.

[yamaha fj rider]

Try to raise the needles one notch, richer mixture. See if this helps.

Kurt

[Alf]

Thanks, Kurt

In fact that´s the movement that probably I will do, mainly for precaution (you know, if the bike runs too lean, it breaks), but only 1/2 notch with a shim, but It could bee too the bike is running rich between 5-7000 rpm 

[yamaha fj rider]

Thanks, Kurt

In fact that´s the movement that probably I will do, mainly for precaution (you know, if the bike runs too lean, it breaks), but only 1/2 notch with a shim, but It could bee too the bike is running rich between 5-7000 rpm 

Alf I think you are on the right path. Like Andy said you just half to try is and see what it happens. The fun of trial and error. Hope this helps.

Kurt 

[Pat Conlon]

Didn't DynoJet do something with the air jets to remove the flat spot in the FJ?

[Alf]

[/quote]Alf I think you are on the right path. Like Andy said you just half to try is and see what it happens. The fun of trial and error. Hope this helps.

Kurt 
[/quote]

Have you dismantled a CBX 750 carbs?. Well, the fun finish when you have to refit the airbox ducts to the carbs. The best mark that I´ve got (after boil the ducts in my kitchen and get sleep a night in my armchair after my wife discovered me) was 1 and half hours!! Horrible, really!

[The General]

4-1 exhausts will always have a hole in the power someplace. 

Shove the needles around and you may be able to minimize it some, but it'll still exist.

I`m really interested in this point. One of my three 93 models has 4-1 (stock air box) and it has never felt right in the throttle control area. Harvy once commented (when we had a bit of fun in the twisties with my stock 4-2) that mine seem to pull away quicker coming out of the corners. (nothing to do with gearing - and his is 4-1). So does 4-1 give away some noticeable mid range torque? (Cause it sure feels like it - but more stamina in the straights of course - but thought it could be my imajination as the whole bike has different ergos)
Just got a nice little parcel of goodies from Randy (excellent, convenient, fast, quality service as usual - thanks mate) that includes dual air filter system. So will have my first attempt at changing needle position half a slot using some sort of shim (will search to find out about shim item) next weekend.
(Off topic but- Also got my shiney shiney polished rims with new Roadsmart ll`s ready to fit. Left the wheel centres & spokes acid bathed, standard colour but when we gave the lot an experimental clear powder coat - came up a beautiful heaven gray in the unpolished area - looks great & saves a lot of maintenace polishing - experiment is to see if the clear powdercoat sticks with the shiney perimeter till at least MARCH! --- Heeee he hee  )

[Arnie]

Doug,

The shim washers you may need for your change to the Uni filters, you already have.   
They are just above the E-clip, on the needle, from the factory.  Very convenient for 1/2 groove adjustment.

Arnie

[SlowOldGuy]

Alf,
How do the plugs look?

If they are black-ish you might want to check the fuel level.

DavidR.

[The General]

Doug,

The shim washers you may need for your change to the Uni filters, you already have.   
They are just above the E-clip, on the needle, from the factory.  Very convenient for 1/2 groove adjustment.

Arnie

Thanks mate. I will attack it Sunday. 

[FJSpringy]

Doug,

The shim washers you may need for your change to the Uni filters, you already have.   
They are just above the E-clip, on the needle, from the factory.  Very convenient for 1/2 groove adjustment.

Arnie

Thanks mate. I will attack it Sunday. 

mate if you can wait I can send up my carb tuning kit, 4lb hammer, 20 inch shifter and a number 3 Phillips head screwdriver 

[The General]

Doug,

The shim washers you may need for your change to the Uni filters, you already have.   
They are just above the E-clip, on the needle, from the factory.  Very convenient for 1/2 groove adjustment.

Arnie

Okay, so the shiny washer on top, normally between E clip and little spring, I put under the E clip so lifting needle by the thickness of the washer. I measure the washer at 0.5mm. Then I measure between needle slots and it appears to also be 0.5mm. (or have I got that wrong cause Vernier calipers not that sharp). 
Therefore wouldn`t the affect mean that I may as well just move the needle up 1 slot? or should I buy a washer or shim  0.25mm?  Doug