Re-jetting! Yessssss!!!

[awaffa2003]

Hey guys, just got back on the road for our 60 degree day here in Illinois and I was reminded of my bikes lagging performance ever since i got my K&N's and footlong pipes. What size jets should i get, where can i get them from, and how much will they cost? Everything's still stock in the carb dept. right now cept for justin's nifty flat screw kit

[steve hawkins]

You are going to have to supply some more info here!

Are you pipes more or less restrictive?

I assume that your airfilter set-up is less restrictive?  Do you have pod filters or do you still have your airbox.  Pod filters will make you lose top end as they can be restrictive at high speed - don't ask me how I know

Check your plugs, and do some homework.  Be prepared to go lean (i.e smaller jets and dropping the needle), rather than the other way round.  Does the engine have a flat spot at 70 odd mph, if so then it is too rich at that point - try dropping the needle.

I assume you have removed the extra air pipes gubbins that you boys in the good old US of A are often saddled with.

Here is a picture of mine:

[aussie]

Geeze Steve - the more I see of your bike, the more I like it. And the Davida - I'd really like one - but they just ain't legal in Australia yet.   I saw a picture of a guy on Ducati 750ss with the Classic in green and I just drooled over both. (Sad ain't it?)

[steve hawkins]

yeah, it don't look bad - but if I had started with a post 81 it would be a lot lighter!

This picture was taken before I lost the second disk on the front.  If you go and add/removed large weights on the front wheel - be prepared to balance it again.  It makes a difference!

Its sad that the Davida matches the paint  If I change the colour of the bike I would have to change the colour of my helmet!

Steve H

[awaffa2003]

yea i do have pods and way less restrictive pipes, the baffle is about 8 inches long if that. and i do have a flat spot at around 70. if i'm goin dawn hill i can probably get up to 80. 90 if its a big one lol. and i did chuck the extra plumbing. so now what do i do lol?

[steve hawkins]

For the flat spot - Amazingly enough you will probably find you are running too rich.  Try dropping the needles down a click on the Bings and see if that improves things.

What colour are you spark plugs?  Are they still dark and sooty after a long run. Just as a general indicator.  You want a medium brown to beige colour.

You should be able to lose the flat spot.  But 90 odd will probably be all you can get......

No great shakes for me, if caught doing over 100mph, its an instant driving ban.  At least I will get a second chance to wise up, otherwise.

As a general thing, hold your revs and use all the rev range through the gears when accelerating.  If you short shift into 5th, you will never get beyond 80mph...

[fastcataz]

That is one good looking bike!

[not-so-fast-ed]

How do you keep it so clean, Steve?

Really sweet ride!

[Rob Valdez 79 R65]

For the flat spot - Amazingly enough you will probably find you are running too rich.  Try dropping the needles down a click on the Bings and see if that improves things.

What colour are you spark plugs?  Are they still dark and sooty after a long run. Just as a general indicator.  You want a medium brown to beige colour.

I believe the best way to do the spark plug test, is to start with a new set of plugs, run it up to speed, and shut off the ignition, coast to the side of the road, and pull the plugs for the check.


Watch your traffic!

[Ed Miller]

snip
You should be able to lose the flat spot.  But 90 odd will probably be all you can get......

No great shakes for me, if caught doing over 100mph, its an instant driving ban.  At least I will get a second chance to wise up, otherwise.

snip

Why just 90?  Mine will red line in top gear, which I calculate must be around 105 or so.  I weigh about 180 pounds.  No ban for us, but you go to jail and get a big fine.  If I'm going to jail I want to set a speeding record or something, not just do the ton.

[steve hawkins]

Perhaps I just stopped trying hard enough.  I have had over 100mph out of the old bird some years ago, but that was when I was running the standard Ally airbox with a K&N filter and more or less standard (looking) silencers.  But I still had the flat spot at that time.

Some of the short noisy pipes and pod filters are actually restrictive at high speed/rpm

I must try harder and see where she tops out at - and yes I am in 5th when I finally top out, but I was in fourth and third and second a loooong time on the way up.  I have a self calibrated electronic speedo and no rev counter - I just change up when she stops pulling in the gear that I am in.

[Justin B.]

Harrison may have discovered the reason why a lot of the "experts" seem to agree the stock BMW intake/exhaust is pretty well optimized for all around driveability and is best left alone.  As with most  powerplants when you do something to improve one part of the RPM range it is usually to the detriment of another part ...

[fastcataz]

I'm thinking if you do get the jetting correct for your set up you will have a very narrow powerband. Most of it up in the higher revs... if it were mine I would consider a larger pipe, a bit of back pressure in the pipes is a good thing at lower r's. Some length in the intake will also help down low, perhaps a few inches of pipe between the pods and carbs.

[nhmaf]

One of my theories about the pods is that they are pointed the wrong way to help with proper breathing at higher speeds - an open hole pointing backwards tends to be more of a vacuum port than an intake port, whereas the post-80s plastic airbox with front facing snorkels works more like the higher performance air intakes on modern sports bikes - more air gets crammed in the faster you go.   I think this helps explain the careful balance of carb performance on these engines, and the tendency for engines with pods to run richer at higher road speeds.

[steve hawkins]

I saw a BMW last year with pod filters on the elbows that normally go into the airbox.  The owner claimed it helped.  But it sure did look odd!

Anyways - you start changing things and you will need to make adjustments.  

I would never suggest that change is not a good thing - but one change always leads onto another, and you gotta be prepared for it.

I am aware of the plug chop technique, but I was talking in general terms - and we are just trying to rid ourselves of the flat spot.  My old nail is running standard jets, but with the needle down a notch - which leans things out at 2/3 throttle.

The Haines manual has a good explanation about what you need to be looking at and where in the rev range it is.  Mains should be adjusted for 2/3 ro 3/4 throttle onwards, I think, and needle position a little lower down the rev range.  Under that we are talking about other parts of the carb.  I have not got a haynes manual with me so I am just generalising again

Steve H