4 to 1 exhaust jetting

Hey guys. I just put a Mac 4 to one exhaust on a 81 csr 650. It's running a bit lean over 2000 rpms. I was wondering if anyone else has already done this and if so where to start when rejetting.

Thank you!

Jet questions and a few answers

Usually a 4-1 exhaust will make it run leaner so you'd expect to change jets. It's virtually impossible to advise anyone else as to what jets they need exactly with all the variables in bike setup and altitude On my KZ650 stock jets work with my V&H because I'm at 5.000 feet. Just an exhaust doesn't make a huge difference, I'd try going up one size on the main jets and then checking you plugs. What was you mixture like before the changes to the exhaust, if it was ok then, one size bigger is likely all you need. Some bikes come weak from the factory so even at stock you could probably use on size bigger, in which case you might try 2 sizes bigger.

You say you are weak at 2,000 rpm what are the symptoms and how much throttle are you applying, it could be your pilot jets are too small too.

weak at 2,000 rpm what are the symptoms and how much throttle are you applying

Yes, it is load and speed.

Try lifting the needles a notch.

Is the stock airbox still in place? Typically, only minor jetting changes are needed when a header is installed by itself. Pods on the other hand, whether by themselves and/or in conjunction with exhaust system changes, require more extensive jetting correction. Best bet is to purchase a jetting kit from either Sigma or Dynojet, particularly if you are using pods.

Thank you all for the info. I will give one of the jet kits a try. Yes, it still has the original air box. It was always a tad bit lean running.

If you are running the original air box then I would suggest you would only need to go up one size on the mains, the pilot should be fine. I have a V&H 4-1 on my 77 KZ650B1 and run stock jets and it works fine, but I am at altitude so normally would be jetted down one size. I have the VM24 carbs, i expect yours are the later CV carbs. Is everything clean in your carbs if it was running a bit lean any way? although a lot of the later bikes were set up pretty lean because of the emissions standards of the day, which were not an issue in 1977.

I'm personally a little wary of some of the jet kits, because they are rather one size fits all. I tend to double check what's in there and then just buy the individual jets one size up, or whatever I guess will work and go from there, sometimes there's some trial and error, but most times if it's running pretty good already only small changes are needed.