Carburetor Diagnosis - Spark Plug Test

[lennya12threh]

Well I finally went out today and did the throttle / spark plug test on the Victor. (tested at 25%, 50%, 75%, and 100% throttle for 20 seconds-ish, stopping the engine, pulling clutch and taking out the plug and looking/taking pictures)

Before the spark plug was much too white and the mechanic said it was because someone had already enlarged the carbo, but didn't change the jets, so he enlarged the pilot jet and the main jet. It seems to be running better ever since, but the only thing is that it seems the pilot jet may have been enlarged a tad too much because it seems to bog down a lot with low throttle when it isn't all warmed up, thought after it's warmed up there isn't much of an issue. Also, it still seems to be lacking a bit, and not smooth enough of a climb into rpms. I also already changed the ignition coil to a genuine one. The one thing the mech didn't change was the jet needle (and the slider valve of course). He only enlarged the pilot and main jets.

Here are some pictures. Note: I also tested 40% throttle, 65%, and 90%, and the plug looked about the same as every other interval, every time. The plug looked about the same throughout the test but here are some pics. It looks ok, but on the side where the tip attaches to the plug main body, it is still white.

The pics marked as "50%" and "100%" are as it says. The one where you can see my red gloves was 75% throttle, and the one that is alone with the patio behind it is 25% throttle.

Edited September 10, 2010 by lennya12threh

[lennya12threh]

The first pictures that were attached were way too big and when you click on them they fill up like 10 screens. I fixed them in this post...

[Pib]

It scares me when you say the mechanic enlarged the pilot jet and main jet. How did the mechanic do this? Hopefully you are talking a screw adjustment, and not by using a file or drill as there is no precision in that. Usually when wanting to increase the jet size they are changed with factory made jets of a larger size. But if you are talking screw adjustments just disregard this paragraph.

If the plugs were running really white there is the high probability the fuel mixture was lean; if they get black/sooty after while and you notice unburnt fuel coming from the tailpipe that would be an indication the fuel mixture is rich (assuming you are not burning any oil). Good luck.

[lennya12threh]

Thanks.

Whether he used a file or put in new screws or not, I think all the precision was gone when the previous owner had the carb enlarged. By this I mean boring out the carb manually with tools, which can be done well if the guy knows what he's doing, but there is no way to know what new factory jets to put in to accompany the manual carb enlarging. If we used factory jets we would still have to guess by spark plug condition and performance because this carb enlargement wasn't in the books.

Do the spark plug pics look ok? Before I was going to ask how to tell if it's rich, though I can tell if it's lean by the color and another picture given to me. But in my pics does it still look like it's more on the lean side? Because the engine still seems to get hot quickly when raced, and performs similar to how it did before the jets were slightly enlarged, though after enlarging them it did perform a bit better.

One thing I probably need is a slightly larger jet needle, which is not adjustable by filing, so would have to be replaced. It could be worth it just to get a smaller jet needle (allowing more fuel in) to test to see if it gets better as a part of the diagnosis. I think I'll see if it is even possible to order different sized jet needles for this old bike.

[VocalNeal]

Ah Lenny. You opened the box

Just curious in the beginning did you give the radiator a good flush. Just in case you are not loosing 10-15% of your cooling.

If the carb is a PKW they are cheap here but as you have now enlarged to ports etc ....

[lennya12threh]

Hi. Neal.

Well I'll wait a bit and double check the rad fluids (it's still hot now, just got back), but this should be fine because we just put the piston in not long ago and filled it right up again, and it's never had any leaking issues. I don't think we did anything more than just drain and refill though.

I'm not sure you mean as for the relationship between the enlarged ports and the PKW carb. On the side of my carb it says Keihin as the brand/make.

Also, to update, the jet needle was adjustable by a lock piece on the top of it, and when we checked today it was apparently too lean even for stock, so we adjusted it up (richer) a couple notches and when I drove home it seemed a bit better as well.

Edited September 11, 2010 by lennya12threh

[Pib]

For me on my laptop screen it kinda hard to see the real life color...that is, the pictures appear a little dark.

I would shot for a "light gray" color on the plugs. Now if you are checking the plugs everyday and they are still white, you need to give the plugs some time to display the correct lean/rich burn mixture. Now if you are checking the plugs very frequently/every day and they are turning black/fouled looking then that's pretty much a definite your mixture is still too rich (assuming no oil burn problem). Good luck and cheers.

[lennya12threh]

I was told by others that the desired color would be a light coffee brown no?

[LivinLOS]

For me on my laptop screen it kinda hard to see the real life color...that is, the pictures appear a little dark.

I would shot for a "light gray" color on the plugs. Now if you are checking the plugs everyday and they are still white, you need to give the plugs some time to display the correct lean/rich burn mixture. Now if you are checking the plugs very frequently/every day and they are turning black/fouled looking then that's pretty much a definite your mixture is still too rich (assuming no oil burn problem). Good luck and cheers.

Yeah I took it from the post hes like pulling the plug, putting it in, running it for minutes at a set throttle, and pulling it again..

I am no great shakes mechanic but I didnt think you could get a color reading in that time ??

[LivinLOS]

I was told by others that the desired color would be a light coffee brown no?

Light tan is I think ideal.. From greyish to tan..

Black / wet / yellowed / gunked / white powderish / etc are all (different) poor symptoms.

[Pib]

I was told by others that the desired color would be a light coffee brown no?

Light tan is I think ideal.. From greyish to tan..

Black / wet / yellowed / gunked / white powderish / etc are all (different) poor symptoms.

Agree....this indicates the plug is running clean/not too hot/not too cool and going through a slow, natural white color change/fade with little to no residue left behind.

But to me, the coffee color mentioned above is too dark of a color which probably indicates some oil burning/rich mixture...or the plugs have been burning clean but have been in the engine for a l....o.....n......g time as after gray/tan comes something like light coffee brown.

[lennya12threh]

Ok, so as is it should be ok then. Right now it is tan, and if you call it brown it is very light brown. The picture where you can see my red glove is probably the best lighting and I don't think it is too brown. It does run fairly well, and I don't think it runs hot anymore after adjusting the jet needle though I need to take it on a longer run to clarify.

Also, I assumed that the color would change quickly, because I was told if you keep it at a certain throttle opening for 20 seconds and then immediately stop the engine and check the plug, it should give an indication of how lean/rich the jet responsible for that opening is delivering the mixture.

Maybe this was inaccurate then.

Thanks for the insight guys.

[VocalNeal]

Ok, so as is it should be ok then. Right now it is tan, and if you call it brown it is very light brown. The picture where you can see my red glove is probably the best lighting and I don't think it is too brown. It does run fairly well, and I don't think it runs hot anymore after adjusting the jet needle though I need to take it on a longer run to clarify.

Also, I assumed that the color would change quickly, because I was told if you keep it at a certain throttle opening for 20 seconds and then immediately stop the engine and check the plug, it should give an indication of how lean/rich the jet responsible for that opening is delivering the mixture.

Maybe this was inaccurate then.

Thanks for the insight guys.

Erring on the side of rich is better than going the other way. Your "red glove" picture looks fine to me.

[Pib]

Ok, so as is it should be ok then. Right now it is tan, and if you call it brown it is very light brown. The picture where you can see my red glove is probably the best lighting and I don't think it is too brown. It does run fairly well, and I don't think it runs hot anymore after adjusting the jet needle though I need to take it on a longer run to clarify.

Also, I assumed that the color would change quickly, because I was told if you keep it at a certain throttle opening for 20 seconds and then immediately stop the engine and check the plug, it should give an indication of how lean/rich the jet responsible for that opening is delivering the mixture.

Maybe this was inaccurate then.

Thanks for the insight guys.

There would need to be some serious oil burning/rich mixture to make a change after only 20 seconds...it would be so serious you would probably notice very rough running/idle and probably black smoke.

Good to hear things appear about right. And remember, there can be a BIB around every corner.