Jetting help....

1985 Honda XL350R. Stock engine, stock air filter, air intake snorkel pulled and only performance is a super trapp muffler. The 85 XL has a dual carb, dual intake single cylinder 350cc engine.

I believe I am currently running rich. The bike runs decently, low end is a little weak, but that could be normal 4 stroke eng, but my super trapp muffler disk and the whole end has a moderate amount of soot on it. Sometimes I have some idle issues and some sluggish accel issues from low throttle opening...but other than those two issues, the bike does run good. Starts good when cold...but it is hard to start when hot. 3-5 kicks is not uncommon before it fires, sometimes more.

My valves are in spec, clean air filter, gaped plug, fresh fuel.
Bike currently has a 112 main and 42 slow in the primary carb, and a 110 main in the secondary carb. (secondary carb has no slow jet, main only.) The tuning instructions in my jet kit are to run a 120 main in primary and 115 in secondary. They are also suggesting I raise the needle.

By going to a larger main, and by raising the needle won't it cause the eng to run richer? I'd rather run slightly rich than lean. but to rich isn't good either..

Its a pain to strip the bike, pull the carbs out, make changes, re assemble, over and over again....hopefully, with some learned input, I can get it right in one or two attempts.

It will certainly run richer all things being equal. Raising the needle will richen between idle and full throttle.

Not sure why you would go to a bigger needle if you think it is running rich?

I haven't made any changes yet...trying to gain some knowledge before I start playing with the jetting so I can minimize the amount of times I have to pull the carbs. Lots of near 30yr old rubber pieces that are hard to find involved with carb R&R. Less stuff I break the better.

I believe I am rich, but it could be lean also...bike is running stock jetting. Open the air box a bit, performance muffler...that can cause it to run lean correct?

It's hard to set jets if you can't listen and feel it yourself. I will give you some things to think about, though.

It's not uncommon for a Super Trapp to get sooty. A regular exhaust will hide it in the pipe and whatever makes it to the end gets flung out and away. The ST baffles are at the end and they kick back exhaust in all directions.

More importantly is how many discs the ST has on it. Is it set up to be close to stock breathing or less restrictive. If it's less restrictive, then you'd be running lean.

The 112/110 mains are stock. Stock jets with free flowing pipe will be lean.

Here's what i'd do:
1) Count discs on ST and look up what it's set for
2) Take out carbs and check typical stuff, e.g. no garbage in bowl, free flowing passages, float condition and height, no degraded rubber bits. Be double sure your enrichment valve (aka choke) isn't leaking.
3) Put in the larger jets and shim the primary needle.
4) Set thottle plate and idle adjustment screw
5) Put carbs back in, start bike, adjust idle and idle mixture.
6) Go for a ride.

Normally, i'd suggest using the stock jets first, but the bike was known to be lean stock. Not as bad as previous years, but still lean. Also, rarely does anyone put an ST on their bike and resist the temptation to add an extra disc or two.

If your bike was running rich at all, i'd suspect it would be a leaking "choke", float needle or float height.

There's a clumsy way to determine if your bike is lean or rich from a start. Use your choke, to make it richer. Start with it half on. If it starts up quicker, you're probably lean. Turn off the fuel and crank the throttle wide open to make it leaner. If it starts, it should rev up, run for a bit and die if it was rich. Again, this only good for starting.

For all other times, you can use the choke to enrichen the mixture. So, if you're cruising along at 55 at a steady throttle and the bike starts surging, you can start slowly adding some choke. If the bike starts running better, you're lean. If it starts to slowly bog down, you're too rich.

Again, it's clumsy, but usually gives a good indicator which way you need to go. There's a more precise way of figuring out jetting, but you need to mount a vacuum gauge, some knowledge, and some experience.

What's the spark plug telling you? Before I would assume its one way or the
other I would check there first

To get the correct plug reading, I run the bike at WOT, say in 3rd, then cut the eng., coast to a stop and pull the plug right there correct? Ideal is a light brown color, black is rich, white is lean. Correct?

yes that is a good plug chop method. Start with a clean fresh plug not the one that is in there already

MaxxTraxx said:

To get the correct plug reading, I run the bike at WOT, say in 3rd, then cut the eng., coast to a stop and pull the plug right there correct? Ideal is a light brown color, black is rich, white is lean. Correct?

Click to expand...

Yes, so long as your using fresh fuel from 1985 or so

The additives and make up of current gas make plug reading a lost cause.

The "color" is no longer useful.
Oily plugs, fuel soaked, carbon crusted, or physically damaged of course will still tell you something... but doing a plug chop... will not.

The post above suggesting to see what you have NOW, as a starting point is a great plan.

The Twin carb of that year,, left Carb open at all throttle positions, right carb mechanically linked and at higher throttle positions (more than 1/2) opens for more fuel..worked great when they worked... and less great to not at all when they get old.

The air screw (left carb) from the OEM has a tab that limits adjustment to about 3/4 turn,,, it was common to cut or otherwise remove that tab for a wider range of adjustment..

BUT before you start adjusting anything, ..

Go through the carbs, verify each passage is free and clear.. no jets plugged...figure out what you have for the disk set up on the ST.. there is a specified number of disks that will replicate a stock exhaust as far as jetting goes.. might start there.

Ill pull and count the disk tonight.

Changed main and pilot, raised needles 2 shims per instructions. Set my air screw 1.25 turns out. Took it for a spin around the block, seemed ok for about 20sec then wouldn't run well much past 1/4 throttle and it now burbles and pops, where before the exhaust was smooth and consistent. I think I need to pull a shim off of my needle. Sound like the right direction to y'all?

Yes, that sounds right. I'm surprised they suggest 2 shims unless they're very thin.

You should check at WOT as well before tearing it down again. Work your way to 3/4+ throttle and see how it runs. Use one gear higher (or two if needed) than you normally would to keep it from revving up too fast. This will tell you if your mains are close since you'll be "off the needle". You want a smooth consistent pull.

Well, after teardown, I found the main jet had worked itself loose and fallen out, as it was sitting in the bottom of the float bowl. Going to reassemble and try again. That explains why the bike ran ok until about 20 sec into my initial test ride...then started running badly....

Btw the ST muffler has 8 cone disk and one solid disk at the end cap, total of 9. ST says 9 is the correct amount.