Spark Plug reading

I think this is too lean, but would like a second opinion before I start drilling out jets or whatever it is I have to do.





Its a 76 KZ750 Twin. Pod filters, 2 in to ! with a glasspack. I was hoping maybe I could get away without rejetting as I live at 5300 feet, but I dont think I got that lucky.

Before anyone jumps in with an opinion, let me be the first to give you the bad news. There is no way for anyone to tell anything much from those plugs unless we know a whole lot more about the engine and how the plug chop was done. If it was a WOT full bore clean plug chop then we need to see down inside the plug where the ceramic meets the steel shell. That's the only place to read plugs.

What we can see is a yellow tint and light colored deposits on the side electrodes. The color comes from additives in the fuel or it may be burning some oil. If you pulled the plugs out after a normal average sort of run around, they appear to be running too hot. That may be mixture or it could be timing. If the timing is spot on and the motor is in perfect health, then the hot clean tips are a sign that the plugs are too hot for just running around.

You will probably need larger main jets but they only color the plugs at WOT and full bore. Wide open plugs should look like that but slower running should leave them with a gray to brown to black coat.

They look perfect to me.

teazer said:

Before anyone jumps in with an opinion, let me be the first to give you the bad news. There is no way for anyone to tell anything much from those plugs unless we know a whole lot more about the engine and how the plug chop was done. If it was a WOT full bore clean plug chop then we need to see down inside the plug where the ceramic meets the steel shell. That's the only place to read plugs.

What we can see is a yellow tint and light colored deposits on the side electrodes. The color comes from additives in the fuel or it may be burning some oil. If you pulled the plugs out after a normal average sort of run around, they appear to be running too hot. That may be mixture or it could be timing. If the timing is spot on and the motor is in perfect health, then the hot clean tips are a sign that the plugs are too hot for just running around.

You will probably need larger main jets but they only color the plugs at WOT and full bore. Wide open plugs should look like that but slower running should leave them with a gray to brown to black coat.

Click to expand...

It was not at all that exact. The plugs have been ran about 100 miles both around town and on the highway. The bike is having an intermittent issue while/after highway/high rpm riding that got me thinking that it was leaning out.

Looks OK to me, slightly lean at top end?
Go up about 2.5 on mainjet (i.e. from 110 to 112.5, etc)
Check ignition timing is spot on and advancing properly (needs timing light)
The side electrode shouldn't be the same colour all the way to body, it means it's getting too hot.
Do a search and see if you can find the pics I posted showing how to 'read' spark plug.
(I have no idea where I put it : )

crazypj said:

Looks OK to me, slightly lean at top end?
Go up about 2.5 on mainjet (i.e. from 110 to 112.5, etc)
Check ignition timing is spot on and advancing properly (needs timing light)
The side electrode shouldn't be the same colour all the way to body, it means it's getting too hot.
Do a search and see if you can find the pics I posted showing how to 'read' spark plug.
(I have no idea where I put it : )

Click to expand...

Ok, I will try to figure something out. As gas as I can tell larger jets dont exist for these carbs.

Would raising the needle/ running a colder plug help?

Jets for older bikes are readily available from many sources.

Here is a good source with lots of useful tech info:
http://www.pjmotorsports.com/jetting-tuning.html

As was pointed out, a plug reading is only valid for the throttle/load that you were running the bike before pulling the plugs. Running around town will basically just give you an idea of your midrange, which is largely controlled by the slide needle.

Changing the heat range on the plug does not compensate for wrong jetting. Your goal is not to get a certain plug color. Your goal is to get the mixture right. If you change plug heat range, you invalidate plug reading. Start by taking a reading for full-throttle, and get the main jet right. A light tan color like you are showing is just fine. Brown would be too rich. (Don't forget, we don't have lead in the fuel anymore, so reading plugs is not the same as 40 years ago.) Modern fuels will leave plugs with little more color than a bit off-white to gray in a lot of cases. You will have to get out on the highway where you can hold it wide open for a stretch, then pull in the clutch and kill the engine. Inspect the plugs at the side of the road. (You may have to let the engine cool a bit. Make sure you have anti-seize on the plug threads, because installing plugs in a hot engine can cause problems getting them out later.

Once you have your main jet dialed in, you can work on the midrange. That's where the needle comes in.

Pod filters will generally cause a bike to be lean enough that it will run poorly, even cause engine damage, if the jets are left stock. The higher the throttle opening, the greater the effect of free-breathing filters. Jets with a screwdriver slot come in increments of 2.5. Pod filters generally require you to go at least 4 jets richer. That would be 10 points in jet size.

Don't even think about "drilling out jets." You will just ruin them. Having said that, I have actually reamed out jets using a tapered needle file, but I know what I'm doing. <G>

What carbs do you have?
Unless they are Hitachi or something even more obscure, jets should be available
Jets-R-Us has most jets, either OEM or aftermarket (there are a few they don't stock, which is why I make air jets for CB360)
Fitting pods doesn't affect low rpm operation (the stock filters don't restrict airflow at low rpm)
Carbs will get slightly rich in mid-range then progressively lean out as airflow exceeds jet flow capabilities

I see the charts on PJ Motorsports follow conventional 'wrong' thinking
Pilot jet has a minor effect all the way to full throttle, (maybe close to 'pissing in the ocean' but it does have an effect) plus, main-jet has an effect at much lower throttle positions (If you don't believe me, try drilling a main-jet to about 200~250 and see how well bike runs at 2~3,000 rpm, fixed throttle)

I Believe the carbs are Mikuni BS38s with a 125 main and a 40 pilot. I'll try to find jets after work, but from what I've read on kz sites the 125 is the only size available.

Do I need to go up on both the mains and the pilots, or just the mains?

Start with the mains.

Pilot jets can usually be compensated enough with the air screw.

BS 38 are on 1977 XS650. Try Mikes XS for jets
They are standard Mikuni large round head
I don't even remember how many different sized jets I have for them

ADC, They are pretty weird carbs, mains don't have any effect until about 4,500 rpm when slides start to lift
You may be better off joining XS650 garage, if only for the carb section (lot of XS650's running around with BS38's)
http://xs650temp.proboards.com/index.cgi?

PJ is correct. Regardless of what Kawasaki list in the parts books jets are available from any Mikuni stockist. Be careful with the pilot jets. There are three designs that all look similar at a quick glance but they are very different.

http://pinkpossum.com/GT750/carb/BS40carbs.htm

Some use the same BS30/96 as the larger carbs and some use a normal Mikuni VM22/210. Check to see which your have before ordering jets.

The same with main jets. Some versions seem to use Small round and some use large round. The set I have for a GT750 uses small round main jets. Measure the head of the jet and we can confirm what you have in there. They don't flow teh same from one series to another, so it's important to get the right jets.

Thanks, crazypj.
I happen to have an XS650 that needs carb work that I haven't tackled yet. It is stone stock at the moment, but I will eventually be putting pod filters on it and have to jet accordingly.

You will have fun jetting it, every single one is just a bit different.
The carb info is damn good though, whatever combintion you use (except for the 800~840 conversions I've been doing, just too many changes )

XS650 BS38 all use small round main jets (N102/221) and BS34 use Large round (N100/604) but the KZ appears to use N100/606.

I checked out one on ebay and it's the N100.606 style.

If the OP would remove a main jet and measure the round head and post a pic, that would be most helpful

Typical Kawasaki, be different just because they can :
They do the same with a lot of components, make things half or one millimeter different so you can't use generic parts
I've got both types of XS650 carbs, got mixed up over jets

crazypj said:

BS 38 are on 1977 XS650. Try Mikes XS for jets
They are standard Mikuni large round head
I don't even remember how many different sized jets I have for them

ADC, They are pretty weird carbs, mains don't have any effect until about 4,500 rpm when slides start to lift
You may be better off joining XS650 garage, if only for the carb section (lot of XS650's running around with BS38's)
http://xs650temp.proboards.com/index.cgi?

Click to expand...

I finally got around to taking your advice.
Very good PDF on fixing and tuning these carbs. I am in your debt!

Have to get some parts from MikesXS. The place where I have been getting jets is more expensive, and they don't carry the floats. Once I got all the varnish soaked off of the parts, I found that the floats were corroded and had barely visible cracking that let fluid inside of them. I was amazed that I could get two floats from Mikes XS for only $27.

I finally got around to arranging my inventory of jets in small parts organizers. I have small round jets, but none of the large round jets that this bike needs for main jet. I have to put together a good order of jets from Mikes to round out my collection. I have a couple GS850's out there that need jetting.

Found some pics
First one I pulled off the interwebs, can't remember site

This is what mine looked like after 200+ miles

Your plug looks a tad on the rich side for street use, though it is probably jetted about right for maximum horsepower. Emissions and fuel consumption could be better, though. Just my opinion based on my tree hugger tendencies. <G>

Plug is out of my CB360 (378cc)
Performance is relative, it needs all it can get ;D