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TOPIC: Help on diagnosing lean condition

Help on diagnosing lean condition 12 Jul 2019 05:30 #807417

  • dpivas7
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I have pod filters on my KZ750E engine with 1982 KZ750 LTD carbs (no overflow tube in float bowl). I also am running a Delkevic exhaust with the primary baffle removed. Originally, I had a DynoJet stage 3 kit installed (slide hole drilled, slide clip moved to groove 4, and mixture screw turned out 3.5 turns), but I also upped the Primary Main Jet to 70 and the Secondary Main Jet to 140, per suggestions on this site.

I'm confused as to why I'm running lean even though I've upped the jet sizes. The bike ran okay with the DynoJet kit, but struggled under load which is why I upped the jet size. Now the carbs pop when I give it gas and it is incredibly difficult to start. I have yet to check the valve lash BUT if that was a problem, why didn't I have any valve clearance symptoms when the jets were smaller?
Current: '82 KZ750 LTD, '71 XLCH
Past: '84 GPz 550

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Help on diagnosing lean condition 12 Jul 2019 05:39 #807418

  • 650ed
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Good luck getting those CV carbs to work properly with pods. Maybe the video in the link below will help. If you do get them working as well as the CV carbs work with an airbox be sure to post the info here for others who suffer the same problems. Ed

1977 KZ650-C1 Original Owner - Stock (with additional invisible FIAMM horn)

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Help on diagnosing lean condition 12 Jul 2019 07:10 #807421

  • TexasKZ
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Check the valve lash. Trying to adjust the carbs on an engine with badly out of adjustment valves is an exercise in futility.
A lean condition can cause popping in the exhaust system. Backfiring through the carbs is a different thing altogether. It is often the result of an intake valve not closing.
Hard starting can be due to clogged pilot system or a wildly rich condition from improperly set fuel level.
Also keep in mind that the different carburetor systems do not operate diacretely. The pilot system regulates idle mixture, but as soon as you open the throttle, the needle starts to function, then the primary main starts to function, and it is overlapped by the secondary main. Changing two at a time can create unexpected results down the line.
One of the preferred methods to get the jetting spot on in carbs with three circuits is to start with the main jet. It is primarily responsible for 3/4 to full throttle. Once throttle response there is as crisp as can be, then do roll-one in the mid range to get the needle right, then do low-rpm roll ons to get the pilot jet correct. Since these carbs have an extra circuit, you would need to find out what throttle opening it affects and add that to the procedure.
All of this is a waste if the valves are way out of adjustment.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough
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Help on diagnosing lean condition 12 Jul 2019 07:11 #807422

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Hi Ed,

That helped, but also didn't help lol!

I'm confused about how/when the air passes thru the main intake. It's something I've never understood - why does the jet needle retract when there is air pressure pushing it down? The throttle controls the amount of air passing thru the carb so how can the fuel push the needle up before the air pushes the needle down?

With pod filters, you have to restrict the amount of air passing thru the main intake somehow, so I think if I restrict it or redirect the air between the pod filter and the main air intake of the carb, I should be good... but we'll see how easy that is.
Current: '82 KZ750 LTD, '71 XLCH
Past: '84 GPz 550

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Help on diagnosing lean condition 12 Jul 2019 07:20 #807423

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Hey Texas,

My intuition was that the carburetor popping was due to valve lash, but my coworker who knows much more than I do convinced me that it was due to a lean condition. That was reinforced by the fact that I didn't have the popping when I had the smaller jets installed. My plan is to check the valve lash - but it's a bit of a lengthy job so I just haven't had the time lately.

I am also going to try to use stock airbox hoses in between the pod and the carb intake. I've read that that the offset shape of the hoses will help since it redirects the airflow by not giving it a direct path to the intake. The indirect path builds pressure slightly and speeds up the airflow which is what I need to make the pods work.
Current: '82 KZ750 LTD, '71 XLCH
Past: '84 GPz 550

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Help on diagnosing lean condition 12 Jul 2019 07:26 #807425

  • TexasKZ
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What?
The air entering the carb is forced under the slide. As the throttle is opened, more air enters, PUSHING the slide UP. At the same time, the low pressure behind the throttle plate creates vacuum that PULLS the slide UP.
Air always goes through the main bore of the carb, but at idle there is not enough velocity to pull fuel up through the main jet. At low throttle openings, the throttle plate creates a low pressure area at the pilot openings, thus pulling fuel through them. Once the throttle is opened enough, he incoming velocity is sufficient to begin raising the slide and pulling fuel from the main jet, which is regulated by the tapered needle. When the throttle is nearly wide open, the slide will be pushed all the way up, so the needle no longer regulates the main jet. At that point, the main jet is responsible for the mixture. Yes, fuel is still coming through the pilot circuit, but it is only a fraction of the overall mixture.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Help on diagnosing lean condition 12 Jul 2019 07:33 #807426

  • TexasKZ
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dpivas7 wrote: Hey Texas,

My intuition was that the carburetor popping was due to valve lash, but my coworker who knows much more than I do convinced me that it was due to a lean condition. That was reinforced by the fact that I didn't have the popping when I had the smaller jets installed.

This is exactly backwards. Smaller jet = leaner mixture.

I am also going to try to use stock airbox hoses in between the pod and the carb intake. I've read that that the offset shape of the hoses will help since it redirects the airflow by not giving it a direct path to the intake. The indirect path builds pressure slightly and speeds up the airflow which is what I need to make the pods work.

Backwards. Pods by themselves cause the air to make a 90 degree turn right at the carb inlet. Adding the air box tubes would straighten the air flow into the carb, but I have no idea how you could get the filters to stay on the tube and how you could possibly get all that to stay on the carbs without some Rube Goldberg support system.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Last edit: by TexasKZ.

Help on diagnosing lean condition 12 Jul 2019 07:35 #807427

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So you mean that air coming makes it's way into the float bowl? That's the only way the needle could get pushed up, isn't it?
Current: '82 KZ750 LTD, '71 XLCH
Past: '84 GPz 550
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Help on diagnosing lean condition 12 Jul 2019 07:38 #807428

  • TexasKZ
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No. The air goes under the slide. Between the slide and the bottom of the carb bore. The next time you have the carbs off, take a close look and you will see that the slide does not seal against the bore.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Help on diagnosing lean condition 12 Jul 2019 07:43 #807429

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This is exactly backwards. Smaller jet = leaner mixture.


Correct. I didn't think I had a lean mixture with the small jets. So when I increased the jet size and I spoke with my coworker, he convinced me that there was something else causing the lean mixture which is why I wrote "I'm confused as to why I'm running lean even though I've upped the jet sizes."
Now I believe that it's probably not lean.

Pods by themselves cause the air to make a 90 degree turn right at the carb inlet.


The pod filter is attached in line with the air intake on the carb body, so I don't understand how they need to take a right turn. This is what I was thinking of doing:
Current: '82 KZ750 LTD, '71 XLCH
Past: '84 GPz 550
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Help on diagnosing lean condition 12 Jul 2019 07:45 #807430

  • TexasKZ
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Also, if you have cheap filters, they may be covering the pilot air jet, which will make tuning the carbs impossible. APE, K&N, and Uni are the safe bet.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Help on diagnosing lean condition 12 Jul 2019 07:49 #807431

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dpivas7 wrote:

This is exactly backwards. Smaller jet = leaner mixture.


Correct. I didn't think I had a lean mixture with the small jets. So when I increased the jet size and I spoke with my coworker, he convinced me that there was something else causing the lean mixture which is why I wrote "I'm confused as to why I'm running lean even though I've upped the jet sizes."
Now I believe that it's probably not lean.

Pods by themselves cause the air to make a 90 degree turn right at the carb inlet.


The pod filter is attached in line with the air intake on the carb body, so I don't understand how they need to take a right turn. This is what I was thinking of doing:


The air enters the side of the filter, then has to turn 90 degrees to line up with the carb bore. Those filters already seem to have a tube of sorts between themselves and the carbs. Usually, there is nothing more than a small lip sticking out of the filter.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Help on diagnosing lean condition 12 Jul 2019 07:56 #807432

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PS You do know that the needle is connected to the slide, right?
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Help on diagnosing lean condition 12 Jul 2019 08:16 #807433

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When the throttle plate is opened, engine vacuum is applied to the carb throat. The bottom of the slide has a hole in it. When vacuum exists, air is pulled through that hole so that creates a vacuum above the slide diaphragm. This naturally pulls the slide upward.

In order to make that happen, the underside of the diaphragm is connected to outside air or airbox air through that wide crescent port at the carb inlet. The diaphragm has vacuum on top and intake air on bottom, so it moves up.

The diagram you posted doesn't show the hole at the bottom of the slide, but you mentioned you altered yours by drilling it so you know it's there.

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Help on diagnosing lean condition 12 Jul 2019 11:18 #807438

  • dpivas7
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Oooh I see, it's that kidney bean port is how the air is supplied to the diaphragm.

My crude calculations say that in order to maintain laminar flow at 75mph, you'd want an opening of about 0.4 inches in diameter so I'm going to try making a restrictor that fits inside the pod and see how that works out.

Every now and then I check eBay for an airbox and I got lucky today to find that there was one for sale that actually included the cap. I'm going to try to get the pod works but I will finally have a stock air box. Pretty bummed though, those things aren't cheap.
Current: '82 KZ750 LTD, '71 XLCH
Past: '84 GPz 550

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Help on diagnosing lean condition 12 Jul 2019 17:05 #807451

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YOU WON'T REGRET IT
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Opinions expressed by me do not reflect those of the the staff or members
kzrider.com/forum/11-projects/598262-kz-...-will-it-live#672882
kzrider.com/forum/2-engine/597654-poser?start=240#704229

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Help on diagnosing lean condition 12 Jul 2019 20:14 #807457

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TexasKZ wrote:


I did a similar thing to this with an XS400; clamped pods onto the stock intake runner tubes. Prior to doing this I couldn't get the bike to run for crap regardless of what jets I installed. On the XS the filters are behind the side covers, one per side. I couldn't afford the proper filters thus the pods. At any rate, I think it's a good approach assuming there is room.

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Help on diagnosing lean condition 13 Jul 2019 16:29 #807488

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Here are some visual representations of what I was trying to explain about the interrelatedness of tha various carb circuits.

www.kzrider.com/forum/3-carburetor/61106...rottle-stages#807484
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
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