New(ish) to carb tuning. Need advice/guidance.

Okay, let me get the lore out of the way. I've done carb balancing/mixture adjustments in the past but never rejetting, so I'm a little confused where to start.

I own a 1983 Yamaha XS400 SECA. This is the DOHC two-cylinder model, so I know it differs from many of the bikes on this sub. It uses the same carbs as the Maxim 400 but with different stock jetting (Left 127.5, Right 117.5, Pilots for both are 42.5). I ASSUME that my bike is equipped with the stock main jets, only because the numbers have long since worn off.

A few weeks ago I had a successful FB marketplace score. A listing with a 1982 model popped up one morning, and this bike was equipped with the aftermarket MAC 2-1 exhaust. You can still buy them new, but I'm a canuck and after conversion rates, taxes, and border fees this kit would cost me over $800 CAD new (yikes!). So I offered the guy $100 bucks just for the pipes and after a four hour round trip now own the coveted exhaust.

Sort of... the muffler that came on it wasn't the original and was ill-fitting. So I bought a generic slip-on muffler off Amazon (with a pre-installed baffle).

Before I continue I just want to state that I DID NOT buy this pipe because I have any delusions about it making my bike faster/more powerful. I also own a 90's four-banger, if I want to go fast I can. I just really, REALLY like the look of the 2-1 pipe. Plus it makes the bike sound deep and throaty (although a slight bit too loud for my liking... and my neighbor's liking, probably).

I don't want a faster bike (it will never be fast), I want a properly running bike with well-adjusted fuel system that will last me a long time.

First test with the new pipes and NO changes to stock carb settings.

At idle:
- idle speed was low, 1000 RPM instead of the 1200-1250 suggested by my manual.
- It's loud as h*ll (to be expected, I don't run it after 8 PM because I don't want to be a nuisance).

Short ride (end of my road and back, a couple hundred meters altogether):
- I have definitely lost some low end power. The bike feels a little sluggish until about 3000 RPM where it picks up again.
- I did not hear/detect ANY lean popping, either on acceleration or deacceleration/engine braking.

The next day (as I'm writing right now) I pulled the spark plugs for a quick inspection.

- Left cylinder plug has a slight sheen of unburned gas on it. It could be caused by the low idle/poor scavenging or a combination of both.
- Right cylinder plug was visibly sooty.

For context, my carbs were tuned almost perfectly for the stock exhaust. Both were a nice tan colour with the stock pipes (I checked before I pulled them).

So now my bike is running richer. I really did not expect this, I expected a lean condition.

To compensate, I have turned both mixture screws 1/4 clockwise to lean out the mixtures. I think I probably turned it too much, but we'll see, as the bike is still cooling at time of writing. I also adjusted the idle speed to roughly 1200 RPM, as recommended by the manual.

Took it for another test ride before sitting down to write this. Throttle feels a little more responsive than before. I got it up to 80 KPH (49MPH)/ in fourth gear/ 8k RPM on the stretch of road I have to work with. Once again, no lean popping at all either on acceleration/deacceleration. I do admit to being deceived by the change in noise, it sounds like I'm at 9k when a quick check of the tach informs me that I'm actually only at 6k. This will take some getting used too.

I'll post an update once the bike cools down enough for me to inspect the plugs again.

Now, as for the reason I'm posting any of this. I've never done an exhaust swap before. I've done carb tunes/cleans/mixture adjustments but never anything that would potentially require me to rejet the carbs.

Has anyone here ever tuned/rejetted the carbs for this model of bike/equipped exhaust before? What were your experiences? Did you change your jet sizes?

Lmk. Thanks for your time.

 

I have never personally rejetted an XJ, but I know the prevailing wisdom from the “church of clean” guide is that you shouldn’t need to rejet for an exhaust swap. There should be enough range in the mixture screws to compensate, as long as you’re still running the stock airbox.

For what it’s worth, a colortune spark plug makes adjusting the mixture way easier than doing things the old fashioned way. You’ll be able to see the color of the fire in the cylinder and adjust until you get a Bunsen-burner-blue flame at idle. Once you’re there, the mixture should be in a good place with the throttle open too.

 

I'm still running the stock box/air filter. I have no intention of changing those. I've dealt with pods in the past - enough to know that they're more trouble than they're worth.

I reinspected the spark plugs once cool. The left cylinder plug is a nice tan brown with no trace of unburned gas. The right plug is a little on the sooty side, so a minute adjustment is still needed.

That being said, I'm starting to think a rejet might not be necessary. The bike doesn't lag anymore after I adjusted the idle speed to the proper 1200 rpm. No lean popping and the the power delivery feels pretty linear.

I'd love to test it on the road again. But the weather forecast says that my neck of Ontario is about to get dumped on with snow. Drat.

I'll look into a color tune system, sounds pretty neat. I've never heard of them before. Could come in handy.