Kerker and jets

[oldsbastel]

Does anybody know, which jets are necessary for a 4-1 Kerker Chrome Megaphone?

[chevy45412001]

4-1 kerker

Im not sure on your bike but my xj11 uses 137.5 mains .Stock were 112.5,I also have pod type air-filters though.You might not have to with a stock airbox?The guy at the parts counter told me to go up or down in5's whole numbers till you get close then fine tune. It must have worked i went from 13.5's to 13.1's in the 1/4 mile timesand gained3-4 miles an hour

[TopCatGr58]

Here's the link to the Jetting Guide for modifications to your bike, ie., 4-1 pipes, Indy filters, etc.!

Jetting Guide Link

Your profile doesn't mention what year your standard and specials are. The early models (78-79) have totally different sized main jets than the later (80-81).

I have an 81SH, put 4-1 pips on it and drove it stock without rejetting for many years, never burned up the cylinders and did some long distance runs with it!!

The guide states only 1 size up is needed for just 4-1 pipes with stock air filter box, this is after subtracting the 1 size due to mixed components.

After my top end rebuild, and adding Indy filters to my system, I followed the guide, 4 sizes, minus 1 =3, at 2.5 increments, went from 110's to 117.5's, have nice tan color, runs great, only decent mileage due to heavy wrist factor!!!! HTH.

[buffalo]

My experience with my bikes, for what it is worth. Tuned Kerker pipes seem to flow more than your basic 4-1 system, especially on top end. I would go 1 size larger than the jetting recommendations say for your setup if you are running a Kerker. Seems to work best on both of my '78 standards. May even like more if you are running an open meg or high altitude.

[oldsbastel]

We are talking about a '78 standard (XS 1100 E ?? / 2H7) with 137.5 mains. I bought the bike, when I lived in Seattle in 1997. The previous owner installed the Kerker 4-1.

After I went back, I rebuilt the whole bike on a 2H9 frame. I did that for less hassle with the German authorities. At that point, I also installed a single K&N inside the original airbox (I didn't drill holes into the airbox).

Using 137.5 mains, the bike ist definitly running lean (even with a stock filter in a stock airbox). I also opened a set of older jets up to 143 by using a reamer, but it is still running lean.

So, which mains do I need? 145? 147.5?

[TopCatGr58]

Well, a 142.5(reamed 143) would then be the right size per the guide and Buffalo's recommendation of 1 larger size than the guide. However, with the K&N filter, it may be breathing much better than the stock box, so 145's couldn't/shouldn't hurt. Run it for a while, and do the drive on a hill/ throttle chop and pull the plugs on the road to see how they look! IF they are still bleach white, you'll have to go up even higher.

However, I disagree with the higher altitude requiring more jet!! The air is thinner, and so you actually have less air to mix with the available fuel, so you actually run richer with the same size jets at higher altitude! Good luck!
T.C.

[buffalo]

Da-oh! You are right TC, smaller jets at higher elevation (I knew that)...
I have run into problems with drilling/reaming jets (in race cars) actually flowing less fuel than factory jets of the same size due to changing the shape of the hole through the jet from a somewhat venturi shape to a cylindrical shape. I am not sure how Mikuni jets are designed as to whether this holds true in this case (it does in Holley's) but I think 145's would not be too large.
I currently have a 78 XS11E and am running a Kerker with a drilled airbox and factory filter and 145's are just about right. It is a little hesitant in cold weather (30's to 50's) but runs good when warmer which tells me it could even go a little richer.
You should also double check your float heights when you have the carbs apart to make sure they are not too low, this will also make it run lean.

[pgg]

Mikunis aren't rated on diameter - drilling with a 1.5mm drill wont equal a 150 mikuni. (150 mls per minute) A 150 dynojet is damn close to a 137.5 mikuni. Dynojets are rated by diameter - a 1.5mm drill slops around in a 142.5 mikuni - god only knows what other aftermarket jets are rated on. A mikuni 137.5 drilled to 1.5 mm runs rich as hell. I'm confused. Funneled holes seem to make a difference. A standard XS11 mikuni 140 air jet has a tiny hole. To make matters worse I heard mikuni were changing to a new goddamned system.

[skids]

I agree with you on the elevation comment, TC.

Originally posted by TopCatGr58

However, I disagree with the higher altitude requiring more jet!! The air is thinner, and so you actually have less air to mix with the available fuel, so you actually run richer with the same size jets at higher altitude! Good luck!
T.C.