Free flowing mufflers, stock intake, (rejetting)

Last night I rode over (with open header) to get my Cherry Bombs put on my '82 750 LTD (4cyl). It's standard 4 into 2 header with a Cherry Bomb glass pack on each side. 2inch diameter muffler, with a 1/2" piece of all thread through the middle of the input for some resistance/ turbulance. It sounds pretty mean. I'm assuming I'll need to rejet. I still have the stock airbox.

I had a float cause some overflowing into my airbox earlier this year, so I need to get into them anyway to settle that. I was figuring on buying rebuild kits for all 4, and figuring out the jetting while I'm in there.

I wanted to read up on this and get as much info as I can. I read through some of the longer threads last night and this morning and now I want to light my bike on fire and say never mind. I've had them off only once before to clean them up when I first got the bike and it was a horrible act of contortionism to get them in place between the airbox and the motor. After reading about all the trial and error and minor changes to get it right, the thought of pulling the carb rack a dozen times to tweak them and reinstalling has me going insane.

So I've got some questions to hopefully get me started off right.

1. First thing, am I shooting myself in the foot trying to tune when it's 40 degrees or colder like it is in PA right now? I'm thinking I'll get it real good when it's 40, then it will run horribly when it's 95+.

2. I've seen charts or lists for various carbs (usually single carb units) that say something to the tune of: For each of the following increase one jet size, then back down one. Then it lists like 5 different mods that you would need to jet for. The idea being if you did 3 mods, you'd only increase by two sizes. 4 mods is 3 sizes. 1 mod is no change but tweak the needle or something. It was for a single carb v-twin, so I don't put much stock in those exact numbers for a 4 carb, inline 4. But I was hoping at least for a starting point and maybe some pointers on fine tuning. I guess I basically have an almost free flowing exhaust and more or less stock air box. I do have the air box hole exposed (KZ COP seat, set back a bit), so it's no longer pulling air under the seat. When I first did that, I noticed a roar from the intake, similar to when I put a cone filter on my car back in the day. So I don't know if that makes it flow better on the intake side or not.

3. I'd rather not buy a kit with every jet under the sun. I don't ever plan to go to pods. Breadbox MAYBE someday, but not pods. I was hoping to buy what I think I need, and maybe a size or two around thoses sizes for fine tuning.

4. Not the original owner, so I don't know that whats in there is stock. The mufflers I am replacing aren't KZ originals, but they are the JCWitney OEM replacements and just as restrictive and quiet as factory.

2" diameter, 8" body length, 12" OAL. By my calculations, the 1/2" all thread blocks 1 sq. in. of the 3.14 sq. in. area of the pipe, leaving 2.14 sq.in. Which is still much more area for the exhaust to move than the previous mufflers. I don't know how this is releavent, but I had my 10 year old figure it out as an example of a practical use of the math he has to learn.

I doubt it will change much by just changing the mufflers as those stock headers aren't that free flowing. If you went to a proper header I think you would definitely need to rejet. If anything you might need to adjust the pilots and maybe raise the needles one position...but I'm sure other will chime in with more.

I think going to pods makes a lot more need to jet than just the pipes. My guess would be up the jets one size, maybe two if you modded the air box pretty good. Probably no needle changes. Just a guess on my part is all.

The carbs mix air and fuel. When you add more air, you need to add more fuel and without a computer to control this, that means manual changes in jetting. I completely understand why pods or similar intake mods need jet changes. In theory, the exhaust pipes are isoalted from the carbs by the valves, which is why I have no clue why changing pipes requires carb changes, but everything I've ever read online says it does. And they can't put anything on the internet that isn't true. That's why I am asking.

I plan to get the motor nice and hot Friday and see how it runs. I'm assuming the fact that it ran like crap was the 30 seconds of choke I gave it before riding the 3 or 4 blocks home. But since everybody always talks jetting, I wanted to get some background on it ahead of time so I could look at it some on Friday when I try not to freeze my tail off riding it. I'll also see if I can get my son to take some video for an audio comparison. I did take a before video.

Had an old 750-4 Honda (1976 K6) that I bought from the original owner's grandson. I think when they put the 4 into 1 header on it with stock airbox they jetted up and moved the needles also. Bike had a big flat spot right at cruising speed that bugged the heck out of me. I ended up putting the needles back to stock location and that cured it. I even put the stock jets back in it after that and it was just fine without any jetting needed. Don't know how they could stand to ride it like that and I believe it had been like that for a long time.

djturnz wrote: I completely understand why pods or similar intake mods need jet changes. In theory, the exhaust pipes are isoalted from the carbs by the valves, which is why I have no clue why changing pipes requires carb changes, but everything I've ever read online says it does.

Because the jets, needles and needle taper are all designed to exactly match and track the airflow as the RPM's rise and the carb slide raises. Changing the backpressure on the exhaust changes the flow characteristics of the whole system. I have no idea what, if any, changes will be caused by the new mufflers but you will probably be able to know if you ride it. A lean condition will make the plugs turn white and make it balky at throttle opening, it will feel weak on acceleration.

I'm doing the same thing on my cop bike , and I'm interested in your findings.




I'm looking at turndowns on the ends, and I might lollipop it like yours.

How much will the cold air now effect my "testing"? That's a big concern for me. I'd like to get this dialed over winter so when the riding season is back,I can just ride.

The air is denser in winter so by summer, when the air gets warmer and thins out, it'll make the bike run richer than how it was tuned in the winter.

Stazi wrote: The air is denser in winter so by summer, when the air gets warmer and thins out, it'll make the bike run richer than how it was tuned in the winter.

Yeap low humidity is thinner air and not so heavy or soupy. More air in and more fuel needed. Just tune it for the winter and let it eat all year round if you want to.

djturnz wrote: How much will the cold air now effect my "testing"? That's a big concern for me. I'd like to get this dialed over winter so when the riding season is back,I can just ride.

Bike will make better power with the cold air also. Mine is loving it.

guitargeek wrote:
I'm looking at turndowns on the ends, and I might lollipop it like yours.

I was going to use 2" copper 45° elbows as turn outs and polish them up like brass. I decided not to, but feel free if you think it would look right on yours.