Author Topic: spark plug gap effect rich/lean mixture? (Read 15270 times)

SquiglyUpright · « **on:** July 27, 2008, 04:58:42 PM »

I was wondering real quick if the spark plug gap will change the air fuel mixture. I recently just set the correct gap on my spark plugs, and now the bike is running rich. Pulled the plugs and they were black and sooty. Was wondering if changing the spark plug gap could be the reason.

Deltarider · « **Reply #1 on:** July 28, 2008, 03:16:40 AM »

What bike? Which type spark plug?

SquiglyUpright · « **Reply #2 on:** July 28, 2008, 09:36:27 AM »

I have a 78 550k. the carbs are off of an older model, and the exhaust is a kerker 4 into 1. the spark plugs are D7EA's. I took them off and cleaned them, and noticed the gap was off, not big enough, so i adjusted it to .028" and now it is running pretty rich.

mlinder · « **Reply #3 on:** July 28, 2008, 09:43:26 AM »

Gap will not change AF ratio. AF ratio is a function of air intake system and fuel system. Namely, airbox, filter, carbs and their jetting.

Gap can effect, slightly, the way the AF mixture is combusted.

Hows your air filter look?

SquiglyUpright · « **Reply #4 on:** July 28, 2008, 09:48:00 AM »

air filter is clean, only a month and a half old.

mlinder · « **Reply #5 on:** July 28, 2008, 09:50:08 AM »

Change anything else recently?

SquiglyUpright · « **Reply #6 on:** July 28, 2008, 10:07:50 AM »

nothing else changed. I drove her into work and she ran beautifully, and i had some free time at work so i pulled the plugs to check the mixture, and they looked good. two of them looked good, like number 14 at this website. and the other two were a little black with some soot.

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

So i cleaned them off with some light sandpaper, adjusted gap, and put them back in. then on my way home from work she was running way rich at 1/8 throttle and less, so i turned out the air screws, to lean up mixture, which helped a little but it is still running too rich, and air screws are pretty far out.

eurban · « **Reply #7 on:** July 28, 2008, 10:16:38 AM »

Do your idle mix screws control fuel or air? I don't know much about 550s but was under impression that the late year bikes has Idle Mixture Screws which control fuel rather than air. If so then counter clockwise (out) on the screws will richen the mix . . . .

mlinder · « **Reply #8 on:** July 28, 2008, 10:18:12 AM »

Yeah, screws on airbox side or head side of carbs?

Deltarider · « **Reply #9 on:** July 28, 2008, 10:20:02 AM »

Since you've only worked on the spark plugs, I would suspect something went wrong there. Are you sure plugs, leads and plug caps (and connections) are undamaged and still in good working order? If plugs fire intermittent, they'll show soot too. I would not suspect carbs and airfilter as you haven't change anything there.

SquiglyUpright · « **Reply #10 on:** July 28, 2008, 10:28:55 AM »

Idle screws are on airbox side. I'm almost positive turning them counter clockwise leans the mixture up. but i guess i could be wrong.

Do you think i could have messed up the plugs when i cleaned them? all the cables and caps are in good condition.

SquiglyUpright · « **Reply #11 on:** July 28, 2008, 10:40:37 AM »

i just pulled my plugs, and on this chart here
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
they look like number 3. A light coat of soot, and black underneath.
my manual said the set the gap between .024"-.028"
but I'm thinking I might have set it a little higher. could that be the problem? too much gap?

markjenn · « **Reply #12 on:** July 28, 2008, 10:47:53 AM »

It's possible that too much gap might cause enough misfire to have them tend to foul. Wider gaps and leaner mixtures are harder to fire than narrower gaps and richer mixtures, unless very rich.

Anyway, not much work regapping lower and seeing what happens.

That being said, I don't spend much time pulling plugs and worrying about perfect plug readings on a 35+ yo bike. If it runs well and doesn't foul plugs, I'm happy. Screwing in a new set of plugs every few thousand miles is so easy and cheap, I view it as normal maintenance.

- Mark

Deltarider · « **Reply #13 on:** July 28, 2008, 10:53:32 AM »

Couldn't agree more. Personally, I never clean plugs, let alone with sandpaper. It looks like by opening the gap a bit to wide, firing has become weaker. Treat your bike with a new set of plugs and oh, yeah reposition the idle screws as before.

eurban · « **Reply #14 on:** July 28, 2008, 11:01:25 AM »

Quote from: SquiglyUpright on July 28, 2008, 10:28:55 AM

Idle screws are on airbox side. I'm almost positive turning them counter clockwise leans the mixture up. but i guess i could be wrong.

I missed the part in your post where you said you have older style carbs on the bike. Idle screws on air box side would be air screws and you are correct that counter clockwise leans the mix.

TwoTired · « **Reply #15 on:** July 28, 2008, 11:03:20 AM »

Quote from: SquiglyUpright on July 28, 2008, 09:36:27 AM

I have a 78 550k. the carbs are off of an older model, and the exhaust is a kerker 4 into 1. the spark plugs are D7EA's. I took them off and cleaned them, and noticed the gap was off, not big enough, so i adjusted it to .028" and now it is running pretty rich.

When you cleaned the plugs, what did you clean off? Soot? If so, the gap is inconsequential. And the condition existed before your spark plug exercise. Look toward carb issues, or Air filter.
Did the air filter get wet? Do the plugs still soot up if you temporarily remove the filter?

Cheers,

mlinder · « **Reply #16 on:** July 28, 2008, 11:35:05 AM »

Forget to turn off the choke....?

eldar · « **Reply #17 on:** July 28, 2008, 11:52:02 AM »

What brand of filter do you have?
Just this weekend I pulled off mine and this morning, rode to work with no filter.
not a common practice, actually the first time I have done so. my plugs look about like 17/18 on that chart.
I have fuel screws and they are turned quite a ways and it still run rich. I am leaning to the filter being too restrictive.

It is an emgo filter. Good build quality but I have been running rich ever since I put it in.

SquiglyUpright · « **Reply #18 on:** July 28, 2008, 02:51:36 PM »

I have a uni foam filter, it is very clean. I took it off and it did ride better, i kept it about 1/8 throttle, then i pulled the plugs and they were still dark, except for plug #3 which had a little white/gray on it.my air mixture screws are turned out (counter clockwise) almost all the way, and my needle clip is on the top notch, lowered all the way down. So im guessing the only other real option is to put in smaller slow jets. Is that right? Smaller would lean out the mixture?

TwoTired · « **Reply #19 on:** July 28, 2008, 04:01:49 PM »

How confident are you that the carbs are clean internally?
In particular, the main emulsion tubes.
I'd also check the main jet orings.
Check, maybe lower, the float height.

If you drain out the float bowls, is the drainage clean?

What are the markings on your carbs? And, do they have original parts inside them or from a rebuild "kit"?

I've never known a 550 Foam filter, in the stock filter box, to cause "richness" unless it was over oiled.

Cheers,

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Author Topic: spark plug gap effect rich/lean mixture? (Read 15270 times)

eldar