Carb Tuning with Commando Style Muffler

[wideload]

I've been though a lot of threads and the carb guide; I believe I have a fair grasp of the concepts, but not much real-world experience. I hope to get a bit of feedback and maybe some creative ideas about how to proceed after making a series of air/fuel changes.

This is a stock '76 that I have been riding a lot this spring and know that it has been running well. I replaced the original equipment mufflers with the Commando style. The new ones are clamped to the original head pipes (not sawed off). The stock main jets were 122.5 and the rest of carb was in its original settings as well. I hoped that the muffler swap would not require carb modification, but this was not the case.

My first test run, after muffler replacement only, produced moderate lean fueling symptoms at 6,200 to near red line at WOT.

I installed 125 Mikuni main jets (one step larger) and tested again. Testing this time indicated rich condition break up between 4,200 & 5,200 at WOT. The bike ran ok outside this RPM range, but the spark plugs showed fluffy black deposits on the body and insulator.

Next, I removed the muffler plug from the Commando baffle, hoping to lean it out a bit. No change at all.

I dropped the needle 1/2 step with a shim. this time rich break up occured between 4,500 & 5,500 RPM. Spark plug still indicating rich, but better than the previous trial.

At this point I tried removing the air filters, shooting for a leaner condition. That was the worst test running too lean at 5,000 and up.

Replaced the filters and dropped the needles one full step from the original position. Rich break up moved higher in the RPM range - 5,000 to 6,000.

Seems like I am getting closer on correct tune, but I am not sure if dropping the needle any further would be a good idea - seems like Yamaha engineers liked the middle range of the needle. Any Ideas?

 

[CDNTX650]

Try dropping the needle a step leaner with your bigger main...if that don't work...

Go back to stock mains and try moving the needle clip down a step or shim half a step up with small washers.

 

[wideload]

Thanks for the feed back CDN.

Try dropping the needle a step leaner with your bigger main...if that don't work...

Go back to stock mains and try moving the needle clip down a step or shim half a step up with small washers.

I guess dropping the needle another step would follow the traditional recipe. My concern is that would affect the pilot jet circuit (which up to now has been ok) & run into a whole new set trial and error adjustments there - I sure would like to avoid that.

I could consider returning to the stock 122.5 main jet. Do you think raising the needle would have an effect when I reach the lean spot at 6,200 RPM - or is that speed the exclusive domain of the main jet. If I am running slightly lean at high RPM & WOT, do I risk piston damage?

 

[5twins]

I don't think you'll hurt anything as long as you're not running high RPM and WOT constantly. I ran mine a bit lean on the mains for a few years. I felt I needed another step up on the mains but my carbs just wouldn't take it without causing upper midrange break-up I couldn't tune out with the needle. The problem turned out to be my K&N pods, they were causing the premature break-up. I switched to UNIs and was able to bump the main another size no problem.

Your carbs don't seem to be responding as they should. Due to that very small stock main (122.5), your carb set is usually in need of a few sizes up on the mains for mods. You might want to inspect your needle jets and the o-ring that seals them into the carb body. If the jet isn't sealing properly into the body, that may be thwarting your tuning efforts. I would also check the float levels. The spec for your set is 25mm. That's 1mm leaner than the 24mm spec used in some of the other BS38 carb sets. If you're running at 24mm, that added richness may be part of the problem.

 

[wideload]

Thanks 5T. The needle jet o-rings were replaced two seasons ago when I treated the carbs to a good cleaning & the float levels were checked with a sighting tube. I'm glad to hear that a lean condition at high RPM wont poke a hole through the pistons.

I am going to try one more adjustment. With one size up on the main jet, I'll drop the needle 2 notches. Will go back to the original settings if that doesn't cure the upper midrange break up or if it induces pilot jet symptoms.