Carb fuel screw tuning tip...

[Bonz]

A fellow on the ZRX forum that I set up carburetors for just happened into a very logical way to set fuel screws. He has been chasing that elusive perfect response right off the bottom.

Once he got the carbs real close at idle and sync'd them, he used an infrared thermometer on each header with the bike up on the stand to adjust fuel screws until his temperatures at the same point on each tube near the top are the same or as close as they can be. Said it was the icing on the cake for the perfect carb set up.

I'm not sure if in an air cooled bike would act decidedly different in the short term while taking the temperature readings and adjusting versus the liquid cooled ZRX.

Seems it would be at least as good as the idle drop method or color tune?

Just passing on the info. Thoughts, if any?

[3Phase]

That's an interesting way set the carburetors without an EGA to sniff the exhaust or an EGT baseline for reference.

If it was on a stock engine, EGT will change with intake and exhaust mods, even a new air filter. How was the 'good' temperature range determined?

[Bonz]

I guess I'm comparing it to the idle drop method or a color tune which does not use EGT either but yields a pretty good idle setting. This seems like a creative and logical way to tune them.

I think it's fair to say most guys can get the fuel screws set pretty close "out of the box" when looking at data and experience from other riders using a given/known carb setup. I set these carbs up for him and shipped them to him in Pennsylvania based on my experience modifying the ZRX with cvk40 carbs from a ZX-11.

His lowest 3 header tube temperatures were within 25 degrees of each other for an average of 239 degrees (226,233,258). The 4th was running 300 degrees. He adjusted the hot one down to the average of the other three, and tweaked the high one of the three he used to average and brought that down a hair. He has them within 10 or 15 degrees of each other at a steady idle now.

I'm not saying it's scientific however his off idle response is perfect now using his words.

Regardless of what the air/fuel is or EGT readings, when the seat of the pants says the throttle response right off idle is as good as he can get it, it feels like a good tip to pass on.

Of course YMMV.

[Bonz]

Here's a pic of him doing a 1st gear 3000 RPM roll-on from light throttle, low speed. Look at his throttle hand, not much in it. Yeah, I put those carbs together for him! Kinda cool!

[3Phase]

Lofting the front wheel seems like it'd be a difficult way to take an EGT snapshot but it looks like a lot of fun!

[Bonz]

Lol... I think you know I'm not talking about an EGT snapshot. Just that from light throttle He can roll on into a wheelie that is very controllable and easy to modulate. After he has done the fuel screw settings by the infrared method.

Reality is, it's drivability on the street not what and air fuel mixture or EGT reads. If it feels better to any individual rider one way versus another, you go with what feels best. That's all I'm trying to say. He used this method and got the best drivability that he could find based upon working from known baseline settings. Every bike is rule into itself and nothing is cookie cutter. Guys will say Yamaha knows best or an EGT show this, but if the real world experience is better than what the empirical data showed, that's what most folks go with. Like if ATF feels better in my fork than 5 weight fork oil and feels just the same as 10 weight fork oil and has proven to work for millions of miles in forks just like mine, at 1/3 the cost, then I have gone with what works best for me and it may just work for someone else too. That's all I'm trying to say.

[3Phase]

If the rest of the engine's healthy and tuned, it sounds like a totally valid way to check and verify the idle circuits.

I'm going to try it tomorrow (today) when I find my IR thermometer and see how the engine's doing.

[Bonz]

After my ride on Sunday I check the header temp as soon as I came to a stop and it was very interesting that my stainless steel Muzzy header was consistent with readings in the 220's a minute or so after shutdown and riding buddy who is runnng a titanium Muzzy header had readings around 100 deg F. It must be the titanium is a thinner walled design and was transferring heat faster than the stainless.

[3Phase]

That's its nature, the SR-71 used titanium too.

[natemoen]

Would be hard if you have the stock XS pipes since they are double walled.

[skids]

Would be hard if you have the stock XS pipes since they are double walled.

I think it would take longer to reach equalibrium temperature after each tweek of the pilot screw.

[Bonz]

It doesn't take but a short time for stock header pipes to show heat.

To verify, I just went into my garage and fired up the SG. I felt heat in 30 seconds and still under a minute they were too hot to touch with the palm of my hand at idle with half enrichener.

Help me understand how double walled stock pipes would hurt the effort to get a comparable temperature across all four? It probably would take a bit longer for the temperatures to go up or down based on screw sitting, but I wouldn't think we're talking about a whole lot.

Scott, did you get a chance to try this with the XJ?

[3Phase]

I didn't get a chance yet but I did find the IR gun, I'll do it after dinner.

[BA80]

Stock pipes aren't a good guage, the air gap varies. Most particularly on 35+ year old exhaust.

[Bonz]

You have a point there Greg. You are saying at the same point on each header pipe coming off the engine, say at the first bend, air gap internally is not consistent from pipe to pipe? With that in mind, my thought would be the gap would be consistent at the origin of the pipe and would have the variation further away.

Like I said, it was just an idea and we don't need to spend anymore time on it than we have.