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What are all possible causes of a lean condition?

Jethro

Jethro

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Past Site Supporter
TGSR Superstar
Charter Member
Cannot believe that I still have stark white plugs after installing Mikuni 160 main jets and shimming my needles about 5 mm below the lowest notch on my DJ needles. How could this ever happen? Bike runs like crap like that, but I'm dying to see at least a little bit of tan or soot on the insulator! No such luck, white as snow.

Anyway, I'm 100% convinced that it's not the carbs now. Can anyone suggest a possible cause for this? Here is what I have thought of for possible lean running casues:

1. Air intake leak
2. Exhaust leak
3. Float heights
4. Poor timing
5. Gas cap vent
6. Vaccum line broken or not hooked up
7. bad petcock
8. poor valve clearances
9. undersize jets
10. needle too high

Can anyone suggest other possible causes? Or does my bike just run so freakin' clean that my plugs don't get any soot on them whatsoever? They look like they are right out of the box, except the electrode end has a off white glaze on them. The insulator is clean as a whistle.

The bike also seems to run pretty cool. Maybe my oil temp gauge isn't working right, but it rarely gets over 160. Yesterday, although pretty cool out (50F) I rode for an hour and a half and could put my hand on the outside of the block no problem. Very strange.

People must be sick of my constant b!tching about this.
 
carbs

carbs

one thing a found on my 1000 with vm carbs was that the petcock has a small vacum one way valve in it that stops the fuel cut of diagram, from pulsuing with the engine vacuum pulse, it was bad and was straving the engine for fuel, checked ok with a vacuum pump for fuel flow, but when hooked to the engine the pulsing that is normal, combine with the bad one way valve in the petcock was only allowing enough fuel for engine operation but could not keep the bowls at proper fuel height, and my plugs kept coming up white, you may also have the wrong needle jet holders? if you have the needle up 5mm and not .5mm then you need to start over from the beginning. or your doing some thing wrong with your assm. of the units, also use NGK plugs,
 
Yup, got NGK plugs BR8ES as called for stock.

I'm pretty sure that I have the jet needle right- I have (from the bottom of the needle) a small spring then a small flat washer, then the e-clip, then another flat washer, then the small delrin or plastic spacer that comes with the stock needle on top. Is that all correct?
 
Jethro said:
Yup, got NGK plugs BR8ES as called for stock.

I'm pretty sure that I have the jet needle right- I have (from the bottom of the needle) a small spring then a small flat washer, then the e-clip, then another flat washer, then the small delrin or plastic spacer that comes with the stock needle on top. Is that all correct?
Click to expand...

Here is the page from my CD parts manual. Hope it helps
jethro%20carb%20picture.JPG
 
Jethro, do the carbs have an air screw setting like the VM carbs? I know some will argue it's used only for idle, but it could affect things.

You raised the needle and the main jet which is 40% throttle and above for the most part...but does it run lean at idle too? I think that will help you answer how 'big' the problem is, and maybe which direction to go.

On your idea of tracking an air leak, your idle would be increased if you had a leak, and a quick check can be made by spraying some WD40 (or if you're nuts, propane) around the intake boots. Any change in idle speed means you have a leak, careful not to get it in the filters.

~Adam
 
Jethro, do the carbs have an air screw setting like the VM carbs?
Click to expand...

I do have an air screw on the CV carbs, and at idle the burn seems ok. But whenever I take a plug reading at 3500 or above, I'm crazy lean, no matter what I do.

The is no air leak at all. I went so far as to spray carb cleaner on the boots 8O just to ensure there isn't an air leak. I was convinced that was the problem, but no change in idle at all.

I'm confused, and am gonna have to pony up the dough to have it dyno tuned. I can't stand having someone else wrench on my bike :evil:
 
Re: What are all possible causes of a lean condition?

Which bike is it? You running pods and a V & H 4 into 1? The plugs for my 83 1100 are NGKD9EA. You're running hotter plugs than I am if you're using 8's.

Earl
 
Have you tried running the bike with the petcock in the prime position? Any difference?
 
It is highly unlikely that there is an air leak in all four cylinders. The problem is something that all four have in common if, that is, you are finding the same problem in all four plugs. A gas station may be able to put an exhaust gas analyser up the pipe and give you more info to chew over but its hard to do that under load which brings you back to the dyno.

Don
 
I just remembered someone saying a test you can do is put something over the air filter, like a towel...something restrictive, but not choking, and run the engine up over 3500 and see what happens.

~Adam
 
Have you checked that the float bowl vent tubes are open to atomsphere?

Earl
 
A couple thoughts:

1) Good list of possible lean causes. I haven't been following this saga too closely, but are you sure the top end of the motor is sound?

2) Is there something other than a lean condition that could be causing your plugs to come out white? After all, I think that if your engine is running better with the smaller jets, and any size jet gives you white plugs, there is something we're not thinking of.

3) Have you thought about getting a hold of a Colortune kit? I really want one of those things, they look very useful for situations like this.
 
Only two reasons for a lean condition//// Too much air or too little fuel
 
2 things that i used to modify the needle setting are small washers above or below the whatever to push the needle down and i believe that i once filed the plastic spacer to allow more travel down((i had a rich condition)). the suppliers like murdoch usually sell asjustable needles so you dont have to buy a jet kit. I woild supose that grinding a few thousands off the needle tips would do the job.
just wild guess that i have tried from time to time.
 
Which bike is it? You running pods and a V & H 4 into 1? The plugs for my 83 1100 are NGKD9EA.
Click to expand...

It's the 83, I'm running pod filters and one of the last MAC true 4-2-1 performance exhausts. The book calls for NGKBR8ES plugs, so that's what I got. Maybe I need to try a colder plug?

Have you tried running the bike with the petcock in the prime position? Any difference?
Click to expand...

I did try the bike in the prime position too with no difference. I am now convinced it isn't a fuel supply problem with either the petcock or the floats, because once it stumbles past the 6-7k rpm mark for a few moments, the power kicks in again and it pulls hard to redline.

but are you sure the top end of the motor is sound?
Click to expand...

I'm as sure as I can be. I just finished a valve adjustment a few weeks ago, and a compression test recently showed a consistant 180 for all four cyls. I haven't done a leak down test, but I don't think I need to with those numbers. While I was in the head I pulled a few of the cam seats for inspection. Everything looks real healthy. The motor has 16k miles on it.

Is there something other than a lean condition that could be causing your plugs to come out white? After all, I think that if your engine is running better with the smaller jets, and any size jet gives you white plugs, there is something we're not thinking of.
Click to expand...

That's what I am trying to figure out. I agree, it must be something I am not thinking about.

The bike runs nice. I have no hesitation off the line and it pulls hard with no flat spots until I hit around 6 or 7k rpms. Then I have a major flat spot that almost feels like fuel starvation, but it kicks back in hard after it stumbles for 5 seconds or so. That tells me that fuel is fine, becasue once I get to redline, it's pulling hard again. There is no popping anymore on deceleration. Basically runs nice, but plugs read way lean.

I'm mostly scared about burning a piston. I can live with a flat spot up top, but it sucks. After all, before I hit that flat spot at 6k I can be going 90.
 
Well Jethro, it looks like you and the others have covered everything I could ever think to check, and then some!

The only thing I would suggest is that you make sure it's running lean, because from what you say it doesn't sound like it's running lean. Are you running unleaded fuel? (Here in Oz we can't get leaded any more.) The reason I ask is because I've always found that 'reading' plugs with unleaded fuel is a lot more ambiguous. So it just might be that things aren't as dire as you fear.

Sooo... I'd suggest getting some exhaust gas analysis done, maybe at a dyno centre to check what's happening to your air-fuel ratio at different RPM and engine loads, etc. That way you will KNOW whether it's actually running lean or not, and if it is, then exactly how lean.

All the best,
Mike.
 
That's good advice, tfb. I have an appointment for some dyno time at a local shop. He can't fit me in for a few weeks though, and I can't stand not riding my bike until then, but really don't want to burn a piston. Maybe I'm not really running lean? I am using unleaded, can't get the leaded stuff over here any more.
 
Did you try riding with the choke partially oin and then reading your plugs???
 
once you're at, say a third throttle, does pulling the choke still affect anything or will the open throttle plates just ignore it?

I'm following this as closely as I can because I think mine is lean on idle, but not at running speeds. if I pull the choke while running I get no extra push... but that may mean nothing?


y'know, I don't think I've hit WOT on my bike yet... :oops:
 
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