4-1 Exhaust and Implications - Discussion

1982 XJ650RJ Seca. I replaced the badly rusted stock exhaust years ago for a 4-1 setup (recommended by the bike shop) not knowing any better. After reading about the necessity to rejet after changing the exhaust I wonder if that is really beneficial?

So here is the question. If you replace the stock exhaust with 4-1 and synch the carbs and colourtune the engine, are you not balancing the air-fuel mixture properly for the new exhaust? Is it really a benefit to rejet at that point? Admittedly, I lack some technical knowledge here.

I am fortunate enought that I have no real concerns with power, bogging etc. but I do find that the bike runs hot - as is evident with the blueing of the pipes out of the cylinders. But I am always looking to have the bike run better!

I am curious as to what others have done in a similar situation.

 

Mixture adjustment using the mixture screws will really only set the mixture for low rpm (idle and off idle - about 1-3k). Your bike will likely still run lean at wider open throttle settings, as the mixture at these settings is controlled by main jet size and needle height. Check out the factorypro website for jetting tips if you decide to go that way. If your bike is too lean at higher throttle settings you may be looking at top end work in the near future...
Lots of info here regarding jetting, air intake, and exhaust - good luck!

 

Yes, mixture screws regulate the low speed stuff, jets for the higher speed. Probably part of the reason you're running hot is because you're leaning out on the top end, and lean = hot, especially on an air-cooled motor.

 

Okay, I did some serious searching of the site and others about jetting and what is involved. I tried to look into some prelim pricing but I couldn't find a jet kit for an 82 XJ650RJ (Seca).

Anyone know where I can order one online?
Rob

 

There's no such thing as a jet kit that would have the jets you need since they have to be determined by trial and error for your bike. You can get some jets from motorcyclecarbs.com. Others you have to make yourself such as drilling out ones that are too small to get the size you need.
I installed a stainless Supertrap 4-1 and pod filters on the 750 Seca and pod filters will tend to make it too lean but the 4-1 will tend to make it too rich, but not at the same times. They tend to cancel each other out at some RPMs but not at others. To get the correct mixture over the whole RPM range I had to make the pilot jets a size larger, the main jets larger but lower the tapered needles because the midrange was too rich. It takes a bit of experimentation and a Colortune and exhaust gas analyzer are a necessity if you don't have access to a dyno.

 

Steve, I think I need to hang around you... sounds like you have quite a bit of knowlegefloating around up in that head of yours.

 

Thanks Steve, I check out that site. I am aware of the need for trial and error but I thought that these jet kits came with several different sized jets (main jet for example) and you tried out each size for the best performance.

Please correct me if I am wrong.
Thanks.
Rob

 

I'm not sure what the Dynojet kits come with, never had one. I'm basically doing by own "jet kit" the same way as ArizonaSteve said. I went with some starting suggestions from a member here (srinath, excellent carb man!), and will do some fine-tuning from there. I have pods and a Mac 4-2 on my 650, I'll be starting with the stock 40 pilots, 135 mains, a #4 washer underneath the needle and seat, and 3 turns out on the mixture screws.

 

I would be inclined to purchase a range of jets since once you drill the existing jets, they are improperly marked forever and you can't go back. Unfortunately, this means a fair bit of expense ($3-5 a whack aren't they?). I am just thinking of down the road, should you ever wish to put it back to stock, you might wish to hold onto the original jets.

 

Definitely I would not want to drill these jets out. Dynojet offers a jetting kit but they don't ship to Canada. Guess I'll have to do it the hard way. I see on motorcyclecarbs.com that they have a whole range of different jet styles. Any way I can tell what stock is without pulling my carbs? I know stock sizes but I have no idea about the style?

 

There are lots of photos of the jets in the service manuals. They are basically just little jets with big flat heads.
BTW, the jet kits usually come with a small assortment of jets to allow you to tune for more power when you install an aftermarket muffler and usually just include the main jet. Sometimes they are bigger than stock but not always, the ones for a Suzuki Bandit were smaller so you never know what you're going to get and if they will really be a size you can use or not. In my opinion it's just a big waste of money to buy some unknown jets for a high price and they don't usually include the pilot jet but you will need that too.

 

Thanks Steve, I have the xj cd so I'll check out those service manuals for some pics. Any recommendations as to how many jets to pick up to get me started? The only mod is a 4-1 exhaust. I currently run 112 main and 43 pilot.

 

Fraps, why don't you see if you can get one of our northernly brothers to scoop the kit for you and post it from their home. If worst comes to worst, I guess I could offer my services.

 

Thanks Robert. I'm in the US quite frequently for work and could get it on my next trip but now I'm a bit concerned about what I'm going to get in this DynoJet kit as Steve suggested. I'm going to look into it some more. I'm hoping someone here can provide a suggestion for a starting point though regarding sizes.

 

>The only mod is a 4-1 exhaust. I currently run 112 main and 43 pilot.
That's no help since 4-1 exhausts differ widely as well as temperature, atmospheric pressure and humidity.
You need to check the mixture at just off idle, midrange and wide open throttle on YOUR BIKE under the conditions where you normally ride then adjust accordingly.