4500rpm power hits!

[BGJohnson]

Just curious. I've been riding my new to me xs1100 for a few hundred miles. It has Mac mufflers, pods with home made velocity stacks. 45 pilot, 142.5 main jets. Starts right up, idles great, throttle cracks crisply.

Bike runs pretty darn good. Just noticed it's almost like I hit a nitrous switch at 4500rpm. Why is that?

[donebysunday]

Just curious. I've been riding my new to me xs1100 for a few hundred miles. It has Mac mufflers, pods with home made velocity stacks. 45 pilot, 142.5 main jets. Starts right up, idles great, throttle cracks crisply.

Bike runs pretty darn good. Just noticed it's almost like I hit a nitrous switch at 4500rpm. Why is that?

How's it feel going from idle to 4500 rpm. ?

[BGJohnson]

It's kind of "soft" between 3500-4300. Super torquey under 3500rpm though.

Is this the flat spot people talk about?

[BA80]

Just curious. I've been riding my new to me xs1100 for a few hundred miles. It has Mac mufflers, pods with home made velocity stacks. 45 pilot, 142.5 main jets. Starts right up, idles great, throttle cracks crisply.

Bike runs pretty darn good. Just noticed it's almost like I hit a nitrous switch at 4500rpm. Why is that?

That's the power curve in the early cams. When I put this 79 engine in my 80 the first time I hit that hot spot it pulled the grips out of my hands. Then I got used to it.

With what I'm running (see signature) I went up one size on the pilots and the mains and it moved the hot spot down to about the 4000 - 4200 range. It's still just a touch lean and probably could have gone 2 sizes at least on the mains but I really don't want to kill my MPG on the highway.

[BGJohnson]

Ok, that's what I wanted to hear.

My mileage is not the best, around 30mpg. So she's a little fat, and runs much better fully warm.

Wonder if I could go down in jet size and gain some power/mpg?

[Bonz]

Much better fully warm means she's running lean typically, for what its worth.

A choke richens the too lean mixture when starting the bike cold. Choke is removed when it warms up and the leanness goes away because the warm engine allows the fuel to atomize better. More fuel mixed in the air is the result. When cold, the fuel can't mix and there is an over abundance of air for what fuel does mix with the air, thus a choke helps with lean.

So, if it improves how it runs as it gets warmer, it isn't rich. Rich would only start to run worse as the fuel mixes more completely with the incoming air.

MPG can be in the crapper even when lean if the carbs are out of sorts and need going through.

[BGJohnson]

Think I should step up another jet size?

I'm already up 3 sizes from factory. 137 to a 145

[jayel]

it's also the rpm range where the carb slides lift

[Bonz]

Pods flow a crap ton of air, up 3 sizes on the 78-79 carbs is not that much of an increase percentage wise.

My ZRX1200 went from 95 mains to 130 mains going from airbox to pods with a full aftermarket exhaust system, maybe that offers some relative perspective.

What does the collective say on how much increase in mains on the early carbs when going to pods?

You may well be ok up top with your mains, if it's running well at WOT and pulling hard from 4000-4500 and up.

What are your needles like? Are they adjustable with grooves so you can set the height? The needles control the mid range you are dealing with.

What is your float height?

Ultimately you get the main that pulls best above 5-6k rpm, then set needle height to control the mid range, and finally you work float height for the under 3k range. Once those are done in that order, then fine tune the idle.

Your mains are probably close, maybe up another size but I want to hear from the collective on jet size with pods and a full system on the early carbs. That would give you the hard pull and I am going to assume when you say It runs better as it warms up that means it pulls better up top as it warms up.

The low end is all pilot and float height, which sounds good from your "torquey" description. But that spot that seems flat/soft is all about the needles as it transitions from low speed circuits of pilots and float height up to the mains on the high end. Needles are like a traffic cop, they keep things going from one place (circuit) to another flowing smooth.

At least this is what I have learned in a carb project on my ZRX 1200 where i have transplanted carbs from a ZX11 and gone through all this in the past 3 weeks.

But the results will be worth it when you get them dialed in, so stick with it!

[TADracer]

If you want to be sure you have the best tune in all RPM ranges tune from the top down like explained here - www.factorypro.com/tech/carbtun.html use the high RPM version.

As I have stated a hundred times over the past several years, tuning like this is a pain in the butt however, the result is solid. Also, you might just find that you tune out that soft spot caused by your pipes and pods. Also, only make one change at a time and record your result. Before you start do a complete tune-up and make sure you have the correct spark plugs installed. All this must be done before you dive into the carbs. Where you end up is now your new tuning baseline for when you make a change.

[Pat Kelly]

My E did that. Discovered that the pilot jet on 1 carb was blocked. Ran nicely on 3 cylinders until 4200 rpm then carbs transition to main jets, now suddenly 4 cylinders. Moved pilot jet to a different carb, problem followed. New pilot jet, problem solved.

[BGJohnson]

Well, I've been riding it more. Found a pretty bad lean spot right at 4k. Which is where it cruises at 60mph. Pulled plugs, they were white as a sheet on the porcelain except for #1 which was super fat. I might pull these carbs apart and space the float needle up a click and check sync again.

[mack]

Float heights

Set up a bowl with a hose attached to the drain plug thread. Check the float height. Should be 3 mm down from the lowest part of the mixing chamber flange. Get all four at the 3mm height and set your idle screws to 1.5 turns out from lightly seated. Sync your carbs to #3 at 1000 rpm, then ride it for a couple hundred miles and check your plugs.
If this doesn't work then clean the carbs.
If that doesn't work then I'd play with the jetting.

[BGJohnson]

I should have mentioned the carbs have been fully rebuilt, fully synced by me.

Spaced the needle up one, the spot moved down to 3500rpm.

[BA80]

Is it kind of a tug, not really a miss but you can feel it kind of a little uneven power feeling?