Jump to content
Sign in to follow this  

How do I know if I am running lean

Recommended Posts

I have just finished some mods on a 98 xr400. It had a yosh pipe and header before, a uni filter and the airbox opened up. The yosh pipe was wide open at the end and had lots of leaks in it. Now I just put what I believe is the stock header(slight ceramic color, guard, good fit) on and a super trapp muffler with 7 discs visable from the outside. I still use the uni-filter and the box is still open on top. The engine used to run perfect. It now seems to be too lean because it seems very hot. I do notice the popping on decel. Where should I start, I dont mind having to get it rejetted at a shop, but I dont know where to start. Any advice on the correct set up. Also, would it not seem that to go more restrictive on the exhaust would richen the mixture up a bit?

Share this post


Link to post
Share on other sites

poping is a sign, but not always the case... Ride it for a while then pull your plug... Golden/dark brown is what you want... Black is too rich and white is too lean...

Share this post


Link to post
Share on other sites
Ride it for a while then pull your plug... Golden/bark brown is what you want... Black is too rich and white is too lean...

What he said, take the bike for a short ride and check the plug.

Share this post


Link to post
Share on other sites

Thanks guys, I will try that as soon as I can find a plug wrench that will fit down in the hole. My bike did not come with a tool kit.

Share this post


Link to post
Share on other sites
Now I just put what I believe is the stock header(slight ceramic color, guard, good fit) on and a super trapp muffler with 7 discs visable from the outside. Any advice on the correct set up. Also, would it not seem that to go more restrictive on the exhaust would richen the mixture up a bit?

I've also got the stock header on mine and also with a SuperTrapp. I believe I am also using only 7 of the 10 discs. Mine is the L model. Which means it has the pumper carb. Mine was running real lean too. The header pipe was glowing orangish-red (seen only a night ride) and very hot engine temps. I've since added a larger main jet and adjusted the air-fuel mix screw.

I think I'm gonna trying adding the other few discs to see what that'll do for it. :thumbsup:

Good luck with yours..

Share this post


Link to post
Share on other sites
poping is a sign, but not always the case... Ride it for a while then pull your plug... Golden/dark brown is what you want... Black is too rich and white is too lean...

I second that. Dark brown rusty color is good. White or black no good. checking your plug is the golden standard to determine your jetting.

Share this post


Link to post
Share on other sites
...checking your plug is the golden standard to determine your jetting...

Unfortunately it is also widely misunderstood. The only way to get accurate, precise data is by doing a "plug chop". The procedure goes like this:

1. Warmed up the bike to operating temperature.

2. Install a NEW plug. Using a new plug very important.

3. Accelerate in 4th gear holding the throttle wide open (to test the main) or 1/2 throttle (for the needle circuit). 5-10 seconds per pass is fine. Don't bang off the rev limiter--that could skew the results.

4. Hit the kill switch while pulling the clutch in and holding the throttle constant.

5. Examine the plug.

Spend the absolute minimum amount of time possible at any throttle position other than the one you are testing for. One pass should be sufficent to properly color the plug. A second or third pass might make the reading the plug a little easier, but again--don't spend any time idling or putting around between passes. The pilot circuit is best checked using methods that don't involve plug reading.

You've got to look WAY down inside the plug at the base of the ceramic insulator. It can be quite hard (sometimes impossible with the naked eye) to get a good look at this area without cutting the threads away, like this:

plug_section_16.gif

Or these (linked because they're big images). These plugs are 2 jet sizes different:

Plug w/ threads removed

Plug w/ threads removed

An alternative is to use plug reading tools which are a lot like the thing your doctor uses to look inside your ear:

180px-2spark_plug_viewers.jpg

spk1.jpg

Forget about plug reading charts that have pictures like these:

Plgnorm1_small.jpgPlgoilf1_small.jpgPlgcrbn1_small.jpgoil.gif

That might be useful for figuring out why your lawn mower won't run but when you're trying to decide between a 150 or a 160 main jet, those guides are useless. Most of the "common knowledge" about reading spark plugs is for commercial mechanics trying to diagnose engine problems--NOT performance enthusiasts trying to fine tune their carburetion.

If your engine is missing badly, you pull the plug out, and a quick glance reveals it's covered in sooty deposites--well, obviously the jetting is too rich. You don't need to cut up a plug to tell that. But it doesn't tell you which jetting circuit is too rich. Frankly, anyone who is considering doing plug chops to fine tune their jetting shouldn't need to look at the plug at all to tell if the jetting is THAT far off. Engine sound and seat of the pants feel will make large swings in jetting blatantly obvious. When one starts testing different jetting to fine tune a carburator, that's when it becomes important to know how to read plugs from a performance tuning perspective.

Just to be clear, I'm not saying that pulling a plug to take a somewhat causual looksy doesn't provide some information. What that gives you is a very rough idea if the jetting is even close. This can be helpful for jetting beginners that are trying to figure out why their engine runs like crap. But don't think that pulling out an old plug and taking a look at it's general condition is really telling you very much--it isn't. An old plug has been run at all sorts of loads and throttle postions. That confuses the coloring and doesn't tell you what you need to know.

In theory you could get good coloring on the end of the insulator of a new plug but you'd have to ride at WOT for a very long time to get the coloring to climb that far up. It's completely impractical. Don't make the mistake of doing a plug reading on a new plug and interpret it as way too lean because the visible part of the insulator is still white.

Different parts of a plug gives clues as to what might be happening inside your engine (correct/incorrect plug heat range, abnormal combustion, ignition timing). But if you're looking at your plug to evaluate air/fuel ratios you've got to look in the right place and you've got to use a brand new plug. It only takes a few seconds for a test plug to begin showing meaningful data (that is, after the engine has been warmed on a non-test plug and everything is set to begin the actual plug chop).

Hope that info helps.

Share this post


Link to post
Share on other sites

I got a 96 XR400, had it shipped from Chicago to Cali. When it arrived the main was too rich, with a 160. Dropped it down to a 155 and it still "garbled" at wot. dropped it again to a 150 and it pulls smooth, with consistent power at all stages of the throttle. Not popping on decel/accel either. Am I to assume that the jetting is "spot on?" Plug looks good, but who knows how accurate that is or what I'm exactly reading!! My fear is going to lean...is a drop in 10 that drastic of a change? (Warm chicago I'm assuming sea level or near to sea level 90+ cali heat).

The bike feels great, but I'm new to this whole jetting thing!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×