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carb tuning help after 2-1 install

  • Thread starter Thread starter donimo
  • Start date Start date
D

donimo

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Hi all. I have had a bit of trouble finding out how much my carbs are off (if any).

The exhaust that came on the bike was the stock headers with aftermarket JC whitney style megaphones. The joints leaked about 30% of the exhausr out before it ever got to the cans so I am sure the tuning was off before.

I got a deal on a set of MAC 2-1's and finally got the sound where I want it (stock it sounds pretty horrible) and now the bike feels different powerband wise.

It used to be pretty linear, power increasing with RPM, decent low end and decent top end, my first bike so forgive me that I don't know if it felt "off" anywhere. Best I can do is say it was linear.

With the 2-1 it seems to have lost a small amount off idle, as soon as it hits 3500-4k it seems to take off quite rapidly with a surge of power that then seems to level off around 6500-7k and then seems to die off a bit.

Now for the tricky bit. Is the bottom and top end really less than it was? or is it just "less" compared to a hightened "power band" from the new pipe? I have no idea.

I still have to do "plug chops" but as it stands the plugs still look the same REALLY light tan/grey with no pitting etc, according to what I can find on NGK's site it is running just right, perhaps a touc to the lean.

So does this all sound normal or can you point me to what part of the carb needs tuning?

thanks!
 
nothing eh? OK I will just leave it for now I guess, plugs dont show anything dangerous...
 
so really? nothing? hmm.

I wasnt looking so much for a step by step, I can search elsewhere if that's what's holding you guys up, heh.

I just noticed a change in the bike since the 2-1, but since the original system was leaky as all get out I have no "baseline"...
 
With you new pipe, it will breathe better, so an increase in main jet will probably be needed, maybe why you say it drops off at higher rpms. Find out what size main you have and go up one or two sizes. After you get the main right adjust the needle for best midrange. Did you get your carbs synched?
 
gearhead13 said:
With you new pipe, it will breathe better, so an increase in main jet will probably be needed, maybe why you say it drops off at higher rpms. Find out what size main you have and go up one or two sizes. After you get the main right adjust the needle for best midrange. Did you get your carbs synched?
Click to expand...

No, I "had" to got o a buddies stag this weekend in Vancouver, man I knew I was getting older, but I've never been so hungover off so little booze, ah well I don't drink any more anyways, nice to have a reminder why ha ha.

I did try making a fuel tank out of a pop bottle, leaked though, not cool. I guess I will try again. Need that before I start, gas tank lines are too short...

Hardest part I am having is deciding wheter it is "dropping off" at high rpm, or if the pipe has given me a boost at mid range?

I will try to track down the jet info. thanks!
 
aftermarket 2 into 1's will usually have to conform to emission/ sound regulations & will restrict performance. what size are your original headers on 400's. i have a 250 version of this bike & am having some stainless steel ones made
 
see that's where I get confused, how can a stock system that was 2x 1.5" pipes into 2 seperate mufflers (with a crossover) be MORE restrictive than a system that is 2-1 2.25" pipe into a muffler with a 1" pipe ID throug it?

but it DOES run differently, seems that its worse to me, lost the off idle to 3500 completely, then it seems to take off nicely, then just plateaus where I would expect a linear increase in power...

So the only part that like the new pipe is whatever part is inbetween 3500-5500 rpm...
 
big bore doesn't nesseserilly mean more power,thats why i've gone for 32mm as opposed to 38mm when i found i couldn' t get the (standard 250) 35mm headers bent.
 
ok so I took out all the stainless packing except for at the ends, and then wrapped a peice of steam pipe insulation (pre formed in a cylinder, compressed to hold its shape) around the middle part. Bike seems to ride MUCH better.

I still plan to do plug chops weekend after next (wifes 30th birthday this weekend, have to keep her busy ha ha). But I think I went too far in my packing, strange how it affected parts of the powerband differently.

thanks for the suggestions guys!
 
so after riding for a bit I am even more confused...

It has a dead spot off idle, say until 1500-2k, then takes off REALLY well, great torque, but then it really seems to level off and almost "lose" power until about 6-7k then it takes off again with good power.

If it was the main jet wouldnt the problem get worse as RPM went up, not worse then better?
 
It should be reference to the position of the throttle rather than rpm. 3/4 to full throttle is the main jet 1/4 to past 3/4 is needle needle jet no matter the rpm. Use the search function you may have to weed through a few pages but all the information you can ever need about carbs is there. I have an 81 850 pods and pipe and it loves fuel.
 
I will test again with that in mind, but it definitly has a "shelf" in the power curve at WOT, the bottom end could be me not getting on the throttle fast enough but it is definitly flat and "down" on power, then picks up again at approx 7k.

I will order some jets.

am I right in that I could swap jets without pulling the carbs?

and I HAVE searched, the search on this site gives me more hits than I have time...
 
Can you swap jets without dropping carbs? Are you a contortionist?

I dropped the bowl on mine yesterday and it was doable, but a bit awkward. I had to make a really short screwdriver out of a ratchet adapter, and use a mirror to find the screws, but it is possible. Some carbs have the jets in the bowls (like mine) which made things much easier. Otherwise you'll have more upside-down unscrewing to do.
 
yeah mine are in the bowls too, seems like it should be pretty easy, but I have been proven wrong on everything on this bike so far ha ha.
 
OK, so it seems that the first power drop IS from going to part-to-full throttle so it must meant that the needle is ok but the main is off, but I am still a bit confused over why it would suddenly pick up again after 7000rpm? Seems it should stay the same or get worse...

If I were to buy jets how many sizes should I go up, I was thinking 2, but really dont have time to get it wrong too many times...

Part throttle seems great, really takes off then falls off at WOT. plugs really dont seem that bad colour wise...
 
OK I ordered some 120's, from the stock 117.5's, hopefully that is enough, we'll see I guess. Barring any helpful info, ;)
 
Also I ordered the "small round" style, I assume this is correct for my BS34SS carbs?

Anyone know if going from a 117.5 to a 120 is enough, I really dont want to waste my time?

It never sputters or backfires, just seems to lose power like you are dragging the brakes a bit, sounds fine, still smooth. and only at WOT
 
gah, I knew that would happen, the only site I saw said that the BS carbs took the small round, why is this so hard to figure out on a bike as common as a GS with BS mikuni cv carbs on it?
 
I changed my order to 125's see how that goes, guy at the shop thinks that is too much, everything I find here says it won't be enough.
 
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