carb question

[Joe Sull]

My bike is running really good. She purrs. For lack of nothing to do I pulled the plugs to see and there white or white/ gray. I want them brown. Before I pulled the plugs, I took a 20 mile ride at no faster that 45 mph around town.
This is what I got;
I have disconnected the vacuum to the TCI a month ago because somethings not right with it.
#40 pilot jets
Stock main jets #112.5
Stock needles are shimmed.
My mixture screws are 4 turns out.
Uni-pods and F1 cans

I've been reading and I'm keeping in mind what Andy said about getting some #42.5 pilot jets. Do I need buy adjustable needles or will larger mains take care of it.

The bike almost were I want it. If I could get it a little richer, I'll be there.

[simi_ed]

Are you kidding?  You're complaining about white/gray plugs?  So what?  If they're (the plugs) a touch lean, but it's happy, leave it alone. Does it surge?  Is the throttle slow to respond? You might be on the edge of needing 42.5s, since you're at 4T on the mixture scews.  But, my answer is "If it feels good, don't mess with it!"  I tried 42.5s in my FJ and found it wouldn't idle above 5,000 ft.  It would load up & die in seconds!

Larger mains and/or adjustable needles won't help small throttle leanness anyway.  You're adjusting nearly 1/2 to full throttle with needles & mains.

JMHO!

[Fj.itis]

Your running nearly the exact same setup as me, and i couldn't be more happier with the response. It starts perfect, revs crisp and good top end. Maybe we could go up one on the mains but now we are just getting picky.

My plugs are the same colour as yours after highway cruise for an hour or so. I say if you got the money up the mains if not leave it alone and ride the hell out of it.

[movenon]

"but it's happy, leave it alone."

George

[Joe Sull]

All-righty then . I just didn't want it to run to hot is all but if gray/ white's good then I'm good.
I'm running out of things to do on the bike. Were putting the boat in the water the end of this week coming.
Good lobster money coming soon and I can buy some stuff. Mirrors, red hand grips, Hydraulic hoses,
rebuild m/c's and a bunch of little things. I want to contribute to the site and hopefully buy randys headers
this fall.

Thanks guys

[Arnie]

One other thing not mentioned by your respondents is that the FJ (and most other I4 bikes) uses a "wasted spark" ignition.  The plugs fire EVERY time the piston approaches TDC, not just on the compression stroke.
This 'extra' spark will tend to burn off any unburnt fuel in the exhaust mixture as its exiting.
This 'extra' burn will help clean the plug insulator and reading the plug will then appear to be running leaner.

If you have no other indications of the engine running lean and therefore hot, leave it alone.

[andyb]

+1

If you're using it as a means for artistically modifying the color of the plugs, jet for the color of the plugs.  If you're riding it, go with what makes it ride best!  The FJ's plugs look really lean when it's running right, if you're comparing against an old smallblock or something.  I usually recommend checking the plugs because it gives you a good idea of where you're starting at if you're troubleshooting something, making it a great first step.  Once they start getting close, I just try to get it as smooth as possible at cruise and on transitions... and of course as fast as possible at the track.

I had to go with 42.5 pilots because of a surge that just would not go away, it gave me a smooth 3krpm cruise in the lower gears in town.  The 40's surged slightly, the stockers surged significantly.

If you want a real laugh, start experimenting with fuels.  You can get all sorts of neat colors on the plugs then

However:

The bike almost were I want it. If I could get it a little richer, I'll be there.

If you want richer because it's flat on the top, or jerky at the bottom, buying some brass is likely the way to get there.  If both, start with the mains and work your way down.  It's annoying to do it that way, because often it seems like you get the mains right, the needles close, the pilots right, and then have to go back and screw with the needles to get it perfect.  Too small of a main for me meant that the bike nosed over somewhere on the big end; when it's right, it'll pull hard well into the redline (~10k on the tach).

If it's too rich, it'll mostly be happy, but on hot, humid days you'll notice that it splutters slightly when it transitions onto the main jet.   For my bike, that was someplace at or just after 6krpm, but that was with a full exhaust.

I think most riders would have a difficult time telling the difference give or take a size on their mains--it's very difficult to accurately butt dyno at those speeds.  A racetrack is a great tool for getting it right, as is a dyno.  The other, more common tool that people have access to is a video camera mounted to the tank, which you can then import into whatever video-editing software you like (Windows Movie Maker is probably on your computer already, for example), and calculate the rate of change of rpm for different main jet sizes.  Remember to do your testing on one set of sprockets, and the usual disclaimers and warnings apply about riding too fast on the street, instant death, dismemberment, and so on.  Another thing to watch is the DA during your testing.   On a dyno, it's just called the correction factor, and ideally you want it to be pretty consistent between the runs.  At a dragstrip, it's common for the mph to get worse through the day as the air temperature changes from a cool morning to a warm afternoon.  If a cold front comes through, you can watch the number of breakout runs in bracket classes increase because the opposite happens!  On a video run, it's handy just to make a note of the time, then look up what the weather was (including wind!) at the time of the run.

[Joe Sull]

Thanks for the write up Andy. Yesterday I tried to wind her up past 8k but the roads suck so bad I was skipping across the bumps. I think I did make it though. It felt like what you called "flat". 8000 rpm should be right in the middle of the power band and it didn't really pull any better than at  4 or 5k. Another thing was I took a ride last week on some good road. I was at 55- 60 mph steady. I thought I was getting surging but passed it off as the windy conditions. On the way back though, the wind was on my back and I felt it again. It's right around 3000 rpm.
These are the only things I am aware of.

 

[Burns]

quick question: where do the mains start kicking in?

I'm guessing something like 6 grand with more than 3/4 throttle.  In other words  almost never in the under 45 mph loops that gave the lean plug readings. Please feel free to correct me here, this is just intuitive, not evidence based, but could you be getting a little air leaking past the emulsion tubes?

[Joe Sull]

Hey Burns, This has been an ongoing learning thing for me. I've read everything in the carb files at least twice and some things 3 or 4 times. If I had the bread this spring, I would have changed every jet out to the prescribed sized for my modifications. These Guys have helped me learn how carbs work and so only spend money on exactly what I need.
I just learned something else; Plug color is not a good indication even if the TCI is disconnected because, as Arnie said, The FJ fires twice.

So I got these conditions;
1. Light surge in the mid range, 3000 rpm
2. Flat on the top end, 8000 rpm

I can change my needles to adjustable and set them to the first notch up, or I can buy some #115 mains.
I don't use WOT so my original question was about needles but Andy's convinced me to start with the mains and I will do that.

Thanks everyone.

[Fj.itis]

Don't you already have adjustable needles? Mine have about 6 or so notches, i have it set at 3.5 and no surge or top end flat spot with 112.5 mains.

[Burns]

Hey Burns, This has been an ongoing learning thing for me. I've read everything in the carb files at least twice and some things 3 or 4 times. If I had the bread this spring, I would have changed every jet out to the prescribed sized for my modifications. These Guys have helped me learn how carbs work and so only spend money on exactly what I need.
I just learned something else; Plug color is not a good indication even if the TCI is disconnected because, as Arnie said, The FJ fires twice.

So I got these conditions;
1. Light surge in the mid range, 3000 rpm
2. Flat on the top end, 8000 rpm

I can change my needles to adjustable and set them to the first notch up, or I can buy some #115 mains.
I don't use WOT so my original question was about needles but Andy's convinced me to start with the mains and I will do that.

Thanks everyone.v

I am certainly not holding myself out as a carb expert of any kind.  I spent a Summer trying to get a pair of Mikuni  VM 34's sorted out only to discover my problem wasn't the carbs at all - the lip of the  K&N filters was blocking an air inlet.

I decided then that carb work was for folks who do carb work for a living.

My FJ pots have a date with RPM




Best of luck.