TOPIC:

I will use this post to chronicle my success and failures with tuning my bike to run more smooth.
Here are my current symptoms:
1. Rough Idle - Small backfires
2. Larger backfires on deceleration
3. Near complete loss of pull (power) randomly.

Set up -
1976 KZ900
Stock Jets 117.5 mains and 15 pilots
PO maybe had work done to internals, not skilled enough to tell yet
Pod Filters (Would hope to replace pods and make bike run better, would prefer not to buy new stock airbox)
Kerker 4-1 Exhaust (No baffle, likely will be purchasing MAC/Kerker baffle from ebay)
VM26 stock Carbs

Mechanic had the bike for the past month and could not get a good tune on it. He stated the pods and the exhaust as being troublesome.

My first question/clarification is that with the pod filters and open exhaust, I should be looking at re-jetting to 120/20?
From what I've read, backfires on deceleration are caused by a lean environment. The fuel mixture being lean is leading to fuel in the exhaust where it is igniting and causing the backfire. My mechanic recommended that I buy carb rebuild kits since the gaskets are worn out (the bike previously sat for 10 years). The jets seem to be part numbers 04/042 Hex for the main and M28/1001 for pilot. Are those correct?

My main concern right now, reason why I am not riding, is due to the couple times I have lost power.
It happened twice on Saturday, which is the day I picked it back up from the mechanic.
Both times was after a harder pull 0-60 and both times I was going slightly uphill. A hard pull for me might only be getting to 1/2 throttle though since I am new rider. 1/8 to 1/4 throttle seems to get me anywhere I need to go in town.
Once I get into 4th gear or so I noticed both times that there was no response from the throttle. The engine was turning but the throttle could not rev it higher. I could not accelerate any further past 60 or so mph. Once I slowed down a bit, got back into 2nd gear, both times the power came back and I was able to continue.
My mechanic said that it could be that I was hitting the limit of the low jets and could not get any more fuel to accelerate. He used this as another reason I need to re-jet.

Another issue I noticed is that within 15 miles of picking the bike up I started to idle around 3200 RPM. The idle was ok for the first 5 miles and then started to increase. I pulled over and used the idle adjuster screw to back it down to 1500RPM. When I did this however it made the backfiring on decel more pronounced.

There are a few other issues that I hope to address but not sure if they are related to the carbs. One is that it seems the bolts that attach my header are not a proper fit against the engine. I will post pics to better explain. Please view the pics below. The first is what my exhaust bolts into on the right side of the engine. The next few look to be damaged where the bolt attaches the header to the engine. I have read that a incomplete seal at the exhaust gasket may also cause backfiring and think this may be contributing to my issues. Unfortunately I am not sure if that can be repaired for inexpensive.
I hope to hear some thoughts from the experts on if re-jetting with a carb rebuild may help me.
Thanks,

Go with 118 main jets and 17.5 pilot jets. Confirm the needle clips are in the middle slot. Also make sure the valve clearances are good.

So I was convince after reading google that I was lean. Backfiring on decel, blue pipes, etc...

Swest comes along to tell me that my carbs are NOT stock, but instead they are z1b carbs. That threw me for a loop so I decided to check my spark plugs. They definitely do not look lean to me. Pics below.

Here are pics of where my carb slides are. The first three look the same, the fourth is higher.

And here are the screws on top of the carbs. Again the fourth is WAY off...

Sure your bike is not a 75 Z1....someone could of swap the front end and by the damage on that exhaust bolt it may have been wreck.
120 mains 17.5 pilots with pods and 4-1

Also to note, my drain screws are pointing toward each other on each side. The example of the same carbs on wiresgeorge's website shows each side pointing outward. Is this a problem?

spdygon wrote: Sure your bike is not a 75 Z1....someone could of swap the front end and by the damage on that exhaust bolt it may have been wreck.
120 mains 17.5 pilots with pods and 4-1

I was thinking that as well but I am pretty sure it is a 76.

Frame VIN Z1F-505022
Engine VIN Z1E-085345

I know the VM26 carbs we would want to go up to 120 and 20, but will that hold the same for the VM28 carbs?
Also, take a look at the plugs posted above. Those do not look like they are running lean

baldy110 wrote: Go with 118 main jets and 17.5 pilot jets. Confirm the needle clips are in the middle slot. Also make sure the valve clearances are good.

Currently running 117.5 mains and 17.5 pilots. The stock z1 carbs came with 112.5 mains

Conspiracy wrote:

spdygon wrote: Sure your bike is not a 75 Z1....someone could of swap the front end and by the damage on that exhaust bolt it may have been wreck.
120 mains 17.5 pilots with pods and 4-1

I was thinking that as well but I am pretty sure it is a 76.

Frame VIN Z1F-505022
Engine VIN Z1E-085345

I know the VM26 carbs we would want to go up to 120 and 20, but will that hold the same for the VM28 carbs?
Also, take a look at the plugs posted above. Those do not look like they are running lean

Those are Z-1B numbers.

EDIT: Nope, wrong about that. If it was a Z-1 frame there should be a leading 0 in the frame #.
Looks like a KZ900 frame with a Z-1B motor.

Good Ridin'
slmjim & Z1BEBE

slmjim+Z1BEBE wrote:

Conspiracy wrote:

spdygon wrote: Sure your bike is not a 75 Z1....someone could of swap the front end and by the damage on that exhaust bolt it may have been wreck.
120 mains 17.5 pilots with pods and 4-1

Those are Z-1B numbers.

EDIT: Nope, wrong about that. If it was a Z-1 frame there should be a leading 0 in the frame #.
Looks like a KZ900 frame with a Z-1B motor.

Good Ridin'
slmjim & Z1BEBE

This is getting crazier by the minute. So I have a 76 kz900 frame with a z1b engine and carbs?

Knowing this, looking at the spark plug pics, is 117.5 main jet too big? The z1b had 112.5 main and the plugs look rich?

You will want your drain screws pointed toward the outside like shown in the picture. This way you can drain carbs bowls 1 & 2 from the left side of the bike, and 3 & 4 from the right side.

I have the same carbs on my bike with pods and a 4-1. I use 120 mains and 17.5 pilots.