New Exhaust

[jack02]

Well I've finally got my 89' back on the road after a very long lay-up. I'd forgotten how fast the ol' girl is after being restricted to a car for all that time - I need to recalibrate,and quickly. Oh,about sixteen months without being touched and the engine fired up on the very first push,almost as soon as I touched the button. Amazing. Anyways,part of the reason for taking it off the road in the first place was the corroded state of the (then) N-eta exhaust. I've replaced it with stainless steel collector and four into one can. This combo was advertised as being suitable for XJR 1200 and 1300's. I contacted the vendor to ask if it suited the FJ but he said he didn't know. Well I already knew that the collector would fit my downpipes as they're the same items fitted to the XJR series, so took a chance as the price (for a new item) was too good to miss. At worst I figured on new bracketry,which wouldn't be a problem as I'm a steel fabricator. In the event I didn't have to fabricate anything - it went straight on,no messing around and looks/sounds great. The guy must be missing loads of sales by not advertising it for the FJ too...

Anyway,getting to the point! It runs great,really good except for a stuttering between 2000-3000rpm. After that the power hits very abruptly and runs cleanly thereafter. While it was off the road I dismantled the carbs and cleaned everything meticulously - no problem there. Clearly there's something in the jetting/mixture settings that doesn't quite align with this exhaust. Anyone got any idea where I ought to start tweaking? All jets are standard,mixture screws 3 turns out and the airbox/filter is standard and new. In fact the whole bike is standard bar the exhaust! Thanks.

[Dan Filetti]

Sure sounds like a jetting issue.  My guess: the old girl can breathe better with the new exhaust so she needs re-jetting.  There are folks on  this list that have forgotten more about carbs and jetting than I'll ever know, so I'll defer to them as to the which, how much, where to get, and what to do while you're in there of jetting...

Good luck.

Dan

[SlowOldGuy]

How do the plugs look?  If they are black, then check the float/fuel level.  It may be too high.

DavidR.

[jack02]

Thanks. I've checked the plugs and they are indeed sooty. Will look at float heights as time allows,but I'm still curious as to how it's fine throughout the range except for that window between 2000-3000? Does the stuttering mean the mixture is too rich or lean at that point? I would have thought leanness but hey what do I know ?

[andyb]

Is it running really poorly from idle to 3k, or only between 2-3k?  The former sounds like fuel height, the latter seems to be the needles coming on before or after they ought to. 

If you've done a plug check and used the choke in the past few minutes of running, they'll be sooty, so be careful on your diagnosis without a plug chop.

My opinion is that you're running what I take to be a free-ish flowing aftermarket exhaust with an airbox, on stock jets?  I'd likely start looking at rejetting the lot, probably a size or two up on the mains, moving the needles around (up .020-.060 someplace), and then looking for pilots that work for you (might need a 40, might not).

[jack02]

andy,thanks. It idles beautifully,and runs seamlessly from 3K to 6K - beyond that I haven't ventured sorry to say! It's just that very narrow window between 2-3K. What I have noticed though is that it requires only a whiff of choke to start,and it can be dispensed with immediately. I'll take on board your advice and act as time allows (something I seem to be very short on these days). I should add that I've changed nothing from when it had the old exhaust fitted....

[andyb]

Jack, just keep your eyes open when you're reading up about people with a similar setup and what their jetting is.  The key is to watch which needles they're using, as the transitions at either end of the needle circuit are going to require totally different jets. 

Expect a mild flat spot around 6k, but if you had a 4-1 before, you'll be used to it.

If the plugs are sooty, and you're not using the choke much, then either the choke isn't working (stuck open/leaking on) or the idle mix is too rich.  Maybe it's going from a little too rich to wayyy too rich when the needles start moving.  Still, start at the main and work down.  There's a lovely instruction guide out there somewhere, or you could bite the bullet and buy a jet kit.

[jack02]

Excellent info in that link andy,lots of food for thought and I'm sure I'll find the solution within. Cheers.

[SlowOldGuy]

Stop using RPM as a diagnostic reference and tell me how much throttle you're giving it when this happens.  
The carb circuits don't care about or respond to RPM, they are only a function of the throttle position.

Black plugs mean rich.  My plugs never look black.  Well, they did once and it was due to bad needle seat o-rings.

Still sounds like fuel level (which could include inproper float setting, bad float, bad needle, or bad o-rings)

DavidR.

[jack02]

David - happens just as I'm pulling away,or if I'm changing up in that rev range,ie dialling the power in gently,nothing too heavy.I did install new float needles about three years back,but admit to having never looked at the float height. Never felt the need to,as this phenomenon has only appeared with the new exhaust.... I'm not too excited at the prospect of mutiple carb removal,dismantling and refitting sessions - I find it an absolute pig of a job with the standard airbox but hey-ho,needs must.

[racerman_27410]

i had a flat spot after installing my supertrapp 4-1 exhaust that was not there with the stock exhaust.... a switch to the 155 air jets fixed it.


KOokaloo!


Frank

[SlowOldGuy]

Frank, I think you meant to say the 144 air pilots?

Jack;
Unfortunately lean and rich sometimes show up as the same symptom.  That sounds like a lean stumble to me.  But since the plugs are black, you could have more than one problem.  You could still have too much fuel.

Is it like a switch when you get past the stumble?  Stutters bad then pulls hard once you're through it?

DavidR.

[racerman_27410]

Frank, I think you meant to say the 144 air pilots?

Jack;
Unfortunately lean and rich sometimes show up as the same symptom.  That sounds like a lean stumble to me.  But since the plugs are black, you could have more than one problem.  You could still have too much fuel.

Is it like a switch when you get past the stumble?  Stutters bad then pulls hard once you're through it?

DavidR.

oopz.... yes i replaced the 155's with the 144's  :-)

[jack02]

Is it like a switch when you get past the stumble?  Stutters bad then pulls hard once you're through it?

DavidR.

That's exactly what it does. Going to be another week or so before I've time to put some of the suggestions on here to the test,but it's just occured to me that seeing as it runs so good either side of 2-3K - could the problem not be carb related??

[racerman_27410]

That's exactly what it does. Going to be another week or so before I've time to put some of the suggestions on here to the test,but it's just occured to me that seeing as it runs so good either side of 2-3K - could the problem not be carb related??

the 4 into 1 exhaust pipe is affecting the carbs


Kookaloo!

Frank