Needle jets?

I'm running way rich, gotta swap out the plugs every 500 miles- they are black as night.

I believe the problem is with the slides and needle jet- she runs perfect at 5k and above, when the main jets are working for their money. Low in the RPMs; however, it idles like crap and usually if I'm cruising along at around 3k then dip into the throttle hard it will sputter, backfire and generally fall flat on its face. I've already dropped the needles as far as they go.

Any suggestions?

mtbspeedfreak wrote: I'm running way rich, gotta swap out the plugs every 500 miles- they are black as night.

I believe the problem is with the slides and needle jet- she runs perfect at 5k and above, when the main jets are working for their money. Low in the RPMs; however, it idles like crap and usually if I'm cruising along at around 3k then dip into the throttle hard it will sputter, backfire and generally fall flat on its face. I've already dropped the needles as far as they go.

Any suggestions?

Would:
Adjust valve clearances;
Compression test;
Visually observe for fat blue spark;
Assure correct ignition timing;
Assure air filter is fresh, clean and unobstructed;
Perform clear tube test for fuel level;
Assure correct adjustment of pilot screws;
Leave jet needle clip in stock position (probably middle).

Good Fortune!

Thanks Patton, I am systematically working my way though that list. Two questions, how can I observe a fat blue spark? Do I pull the plug and ground it on the cylinder head? Second: pilot screws, are these also known as the air screws? I adjusted these last night, the air screw on the cylinder (#4 if it matters) that was running perfectly was turned out significantly more than the first three cylinders, which were black, black black!

I need to run the clear tube test soon.

And I'm still looking for needle jets, they are not included in the rebuild kits. I'm tearing down my carbs very soon and would like to replace them while the carbs are apart.

Anyone have a rack of donor carbs perhaps?

I'd try to work with the air screws and existing jets before rejetting anything. Set the other air screws similarly to the one that isn't turning black and see if that helps.

True, but air screws should be on the bottom of the tuning list- you're just skirting around the problem, not fixing it.

I don't plan on rejetting, just replacing the stock sized needle jets. The slides vibrate and sometimes elongate the jet, no? This is probably a factor in my problem. But before I do anything, I'll work through the rest of Patton's list. I should also add that I'm running NGK B6ES's, which are hotter than what's called for- another work-around to an existing problem. :blush:



Just another thought- there is a fuel filter inside the fuel tank, right? Do I have to remove the petcock to access and replace this? Once the carbs are sorted out, I'll be adding inline fuel filters to the duel fuel feeds to the carbs.

mtbspeedfreak wrote: ... how can I observe a fat blue spark? Do I pull the plug and ground it on the cylinder head? Second: pilot screws, are these also known as the air screws? I adjusted these last night, the air screw on the cylinder (#4 if it matters) that was running perfectly was turned out significantly more than the first three cylinders, which were black, black black!....

Yes, I prefer removing all four plugs (may rubber band or tape the plug pair together) before holding them against the engine head while spinning over the engine.

Yes, air screw allows more air into the pilot circuit when turned out counter-clockwise.

running NGK B6ES's -- this is a No-No for KZ900 where "7" spark plug is the hottest permissible plug.

Good Fortune!

Patton wrote:
running NGK B6ES's -- this is a No-No for KZ900 where "7" spark plug is the hottest permissible plug.

Good Fortune!

The service manual says 7s and 8s are OK. The manual doesn't mention 6s, obviously. The manual also doesn't mention 4-into-1 exhaust, pods and rejetting the carbs.

Is a single heat range really a problem? (other than what I said earlier about hotter plugs being a workaround for another problem) I was told I could run 5s during the right circumstances. I mean, I've put twenty thousand miles on 6s without any problems- the only time it overheated was cruising down the Vegas Strip, stop and go and stop and go for 60 minutes in 95 degree weather at 10 PM on the Sunday before Labor Day last year. Once I got back on 15 and air was flowing around the jugs, she was fine. I run top shelf Amsoil 10W-40.

mtbspeedfreak wrote:

Patton wrote:
running NGK B6ES's -- this is a No-No for KZ900 where "7" spark plug is the hottest permissible plug.

Good Fortune!

The service manual says 7s and 8s are OK. The manual doesn't mention 6s, obviously. The manual also doesn't mention 4-into-1 exhaust, pods and rejetting the carbs.

Is a single heat range really a problem? (other than what I said earlier about hotter plugs being a workaround for another problem) I was told I could run 5s during the right circumstances. I mean, I've put twenty thousand miles on 6s without any problems- the only time it overheated was cruising down the Vegas Strip, stop and go and stop and go for 60 minutes in 95 degree weather at 10 PM on the Sunday before Labor Day last year. Once I got back on 15 and air was flowing around the jugs, she was fine. I run top shelf Amsoil 10W-40.

Per FSM:



Enhanced engine performance may sometimes benefit from a colder running "9" spark plug (NGKB9ES) in racing or extended high speed applications.

I would concur with the assessment of "hotter plugs being a workaround for another problem," but also thinking that standard NGKB8ES plugs may prevent onset of more serious problems.

Good Fortune!

Ugh... Patton you're a blessing and a curse. Mostly a blessing though.

mtbspeedfreak wrote: ... Just another thought- there is a fuel filter inside the fuel tank, right? Do I have to remove the petcock to access and replace this? Once the carbs are sorted out, I'll be adding inline fuel filters to the duel fuel feeds to the carbs.

The stock KZ900 has a manual petcock with a removeable sediment bowl (that screws off for cleaning). There's a round screen strainer between the sediment bowl and body of the petcock, which screen is also removeable for cleaning.

The petcock must be removed from the fuel tank to access the petcock's tubular screen(s) inside the tank (which tubular screen(s) are delicate and easily broken off).

As known, the stock petcock screens do indeed strain out the larger boulders, but allow grit (likely rust particles) into the fuel hose, where the inline filters are supposed to catch the grit.

Would go ahead with installation of the two aftermarket inline fuel filters, and not allow any fuel to ever reach the carbs which hasn't first gone through the in-line filters.

Many folks use this style fuel filter, also available at Z1E.



Good Fortune!

P.S. OMR curses me a lot. :lol:

mtbspeedfreak wrote: I'm running way rich, gotta swap out the plugs every 500 miles- they are black as night.

I believe the problem is with the slides and needle jet- she runs perfect at 5k and above, when the main jets are working for their money. Low in the RPMs; however, it idles like crap and usually if I'm cruising along at around 3k then dip into the throttle hard it will sputter, backfire and generally fall flat on its face. I've already dropped the needles as far as they go.

Any suggestions?

u said " she runs perfect at 5k and above, when the main jets are working for their money. "
this is an often miss understood area. just because u are running at 5000 or 9000rpms, does not mean that u are running on the mains. the only time u are running on the mains is when the throttle (or more rightly said, the slides)is wide open, regardless of rpms.

u also said, "Low in the RPMs; however, it idles like crap ".

this is also determined buy slide height or throttle opening, not rpm.
idle to 1/4th slide openings , are controlled by the pilot system, 1/4 to 3/4 slide height is controlled by the needle and jet. then the main controls 3/4's to full opening.
it is often easiest to determine which fuel circuit u are riding on by adding tape to the throttle grip, and marking off increments that tell u when u are at 1/4, 1/2, and 3/4 throttle openings.
This will help u determine more accurately, where the problem area is in relation to the carbs circuits, rather than a particular rpm.
I hope this is of some help.
Mark