Tuning Carbs with an Infrared Thermometer

The basic forum. Want to show off your XJ or ask questions about it? Maybe you're not sure where to post? Put it here.
Post Reply
User avatar
radare
Site Founder
Site Founder
Posts: 10420
Joined: Mon Nov 07, 2011 12:42 pm
Location: Denver, CO
Contact:

This information is taken from 1Oldman's Yamaha Saga, page 1, post 9. I moved it here to make it easy to find for those looking for a cheap and reliable way to tune their carbs.

Before syncing carbs, it's imperative to adjust the valves. Valves that are out of spec will flow air differently than those in spec and will result in difficulty syncing carbs. Happy valves lead to happy carbs.
1Oldman wrote: With all the valves within spec it was now time to synch the carbs. I have an old carburetor synchronizer in my storage shed but the last time I saw it the mercury was gone so I broke down and ordered a new one. Paid about $50 for it from Saber Cycles.

http://www.saber-cycle.com/store/product790.html

At this time I pulled the carbs again and drilled out the pilot screw plugs and set the screws at 2 turns out. There was no consistency as to how they were previously set, any place from full closed to one and a half turns out. I talked to a race mechanic about this and he says that at the factory they are set according to gas analyzer readings taken at the intake plenums. That certainly doesn't help us in trying to tune these carbs after the fact. So I synced up the carbs and left the pilot screws at two turns out.

I ran it like that for a couple of weeks and still wasn't happy with the low end performance so I decided it was time to change the pilot jets. the pilot jets affect performance up to about ¼ throttle, and ordered a set of #20 and a set of #22.5 pilot jets.

While waiting for the pilot jets I got to thinking about how best to adjust those pilot screws. Not having access to an exhaust gas analyzer, I could measure the temperature of the exhaust gas, via the pipes. About this time I saw that Harbor Freight Tools was having a sale on a digital infrared thermometer for about $30. After comparing theirs to others on the market, I went right down and bought one.

At this time I would like to thank The Stig and Cedestech for their posts on 9/04/08 in, Changing Pilot Jets and a Carb Kit ,
specifically this link:

Changing Pilot Jets and installing a carb kit

And also thanks for this link;
Factory Pro Carb Tuning Procedures

The pilot jets arrived and I tore the bike back down. Previously, when I'd initially cleaned the carbs, I hadn't messed with the floats so now I set the floats at 7 mm (manual calls for 6.2 mm – 8.2 mm) which gave me a fuel level of about 5 mm below the line on the float bowl ( manual calls for 4 to 6 mm). The Float levels were measured with the stick end of the vernier calipers. The fuel level was a little hard to measure until I found a small 6 inch hem ruler in my sewing kit, hmm. Worked like a champ.

Image

With the #20 pilot jets installed I set the pilot jet screws at an initial setting of 1 & ½ turns out. This done I put the carbs back on, hooked up my auxiliary fuel tank and fired the bike up. If your using an auxiliary fuel tank remember to plug the vacuum line to the petcock. ('Embarrassed')

Image

After letting it warm up for awhile and idling well on no choke I broke out my new little tune up tool, the digital IR thermometer.

Image

The initial temperature readings varied significantly. Remember, I had set the pilot screws all the same, 1 & ½ turns out. I think the low temperature was about 250*f and the high was about 550*f.

The high temperature pipe, measured at the bend, meant that that cylinder was running lean so I opened up the pilot screw a half turn on that carburetor. The low temperature cylinder was running richer than the others so I turned it in somewhat, leaning out that cylinder. I kept at this until I had all pipes at approximately the same temperature. The temperature I ended up with was about 385*f average.

Finally, I broke out the synchronizer and refined the sync.

Image

Wow, what a difference! That little Seca was now purring like a kitten, except for that annoying clutch rattle. I need to mention here that all my readings had to be made with the air box on. I had to take the air box off to get at the pilot screws but the performance of the engine was vastly different with the air box off than on, so, air box on.

So, the conclusion of this procedure is:

“There is no magic number of turns to set the idle screw for all the carbs. They each must be adjusted individually.”

Remember the original factory settings set with a gas analyzer? They varied considerably. If you are into tuning your own bike and can afford to buy an IR thermometer ($30-$50), I highly recommend it. It's a lower tech approach to achieving the same results of the gas analyzer. Later, I discussed this approach with a local race mechanic and he readily agreed, saying that monitoring exhaust gas temp were so important in their land speed bike that they had temperature probes permanently installed in their exhausts.

I put the bike back together and have ridden it for several days now. The bottom end is greatly improved. And that buzz between 2500 and 3500 rpm is gone. Much progress, hallelujah.

This topic has 5 replies

You must be a registered member and logged in to view the replies in this topic.


Register Login
 
Post Reply