TOPIC:

Setup:

1980 KZ650. 12" ebay shorty mufflers, stock 4-into-2 headers, K&N pods. 4500-5000' elevation. Current temps around 80 deg F. Previous owner put in big idle jets, 107.5 main jets and 2 shims on the needles.

Background:

I finished a year-long custom job (see avatar photo) and am super excited to ride this thing. It actually runs surprisingly well after 6 months of sitting. Carbs were all cleaned out a month or two ago. So this week I finished the job and was able to start the thing up. At first, with the air screw where it happened to be, it started up really easily. The problem was that any time you let off the gas and coasted it would backfire every second or two. Even giving a little gas would eliminate the backfiring. I then just spent 30 minutes just listening to the response of the RPM as I messed with the air screws. Once I hit what I considered the "best" running engine I stopped. The air screws, 1 to 4, turned out to be set at 3/6/5.5/3 turns out. I know that's a lot. I had to adjust the idle screw a couple times during this process to slow the engine down which I interpreted as getting it to its happy place.

I decided to drive to work for the first time on the new bike yesterday and to get it registered. Now, instead of firing right up without any choke when cold, I had to start with full choke, which is fine. It fires right up on full choke and a minute later it's off the choke and idling great. Now, if I just hit the gas at this point it bogs until you can get past the initial flat spot. Once it's been running for about 5 minutes that bog is gone and takes off without a problem. Another plus is the backfiring is basically all gone! (You're welcome, neighbors) I rode it 4 different times (morning, noon, 1pm, 10pm) and it worked the same every time.

Today I'm thinking I'm a hot shot and that figured out the issue on the first try. I tell my neighbor this morning I fixed it and to take it for a spin as he's been very interested in the project. He drove it around and noted the initial bog down until it warmed up. However, I get a call from him while he's out testing it (yes, my heart sank when I saw his name pop up on my phone as I'm standing on my driveway waiting for him to return). He was going up this long-ish steep hill nearby and right at the top it just sputtered out and died. I went and picked him up and was able to hobble home on the bike on full choke ONLY. It didn't ever get better during the approximately 3/4 mile flat road home. I threw it up on its center stand and tinkered with the air screws again. I also added some gas (it was pretty low, not sure HOW low though). We pulled the plugs and they were ok, 1 and 4 were a bit white. We ended up putting the air screws down to 1.5,1,1,1.5 turns out and it runs like it did before, backfiring returned and all.

Here's where I stand on this: I have zero problem getting this to start up. Depending on the air screw settings it'll start up with no choke when cold or with full choke and will quickly warm up to run fine with no choke. I much preferred the air screw settings that eliminated the backfiring even if they were way out. I think the pilot jet is too big, hence the issues there. When I chop and check the plugs we'll almost always get one that's pretty white. My neighbor and I think the 107.5 mains are too small. We are thinking somewhere in the 110-115 range would be better, even at our altitude. I wonder if that is causing us to see some white plugs as well. I also wonder if the cause of the bike dying on the side of the road was nothing more than low gas in the tank and/or an issue with the float settings. Seems a bit too simple but who knows. I think I need smaller idle jets. Lastly, I wonder if I should be looking at the mains or perhaps the needle heights on this.

Any discussion to help me arrive at a solution would be fantastic!!

Sorry if this is long - I'm a guy that thinks context and background are vital to really getting to the bottom of a complex issue. Also sorry, I don't know exactly the size of the idle jets or needles.

When running pods you need air corrector jets and leave the pilots stock .
If the motor is stock apart from the mufflers then up a size or two on the mains .
Mixture screws should be around 1 -1/4 to 1-3/4 turns out on average.
All other settings and needle position should be left stock.
Be wary of cheap pods as they can be restrictive and cause rich running so use only good quality K&N etc.
Read the attached thread to the end.
www.kzrider.com/forum/23-for-sale/611992...-round-slide-mikunis

Are you running a fuel filter? When it ran low on gas I suppose it could a got a slug of dirt or rust.

I was going to suggest the earlier dying out problem was unrelated to tuning, but I think you already suspect that.

Thanks Zed!

@KZB2: No, just the filter build into the petcock

So you would suggest leaving the pilots at 15 and moving the mains up to what...110? 112.5? 115? Would you recommend messing with the mains before the needle position?

Re-read my last post - it's all there.
You should put all settings to stock.
Leave the needle position stock and everything else stock..
Go up 2 sizes from stock on the mains and fit air corrector jets.
Simply put -
The corrector jets take care of the low end flat spot .
The bigger mains takes care of the extra air from the pods.
If there's nothing else wrong that should be all you need.

That all sounds good but I will admit that I've never heard of air corrector jets... care to educate an ignorant n00b on this?

jekyllman wrote: That all sounds good but I will admit that I've never heard of air corrector jets... care to educate an ignorant n00b on this?

Did you not read through the thread i gave the link to ???
www.kzrider.com/forum/23-for-sale/611992...lide-mikunis?start=0
It's all there and feedback from members who have used them.
Here's a portion of the first post... You should go read the thread to the end.

"They are for use with pods, open carbs and velocity stacks … NOT to be used with airbox ( unless it has been cut open and offers virtually no restriction ).
They simply screw in the air bleed to needle jet tube in the mouth of the carb ( You tap this m5 at 8mm deep . No drilling required )..
( They do NOT go in the pilot air bleed )..
Fit and forget then make adjustments as normal to the main to suit engine size/tune..
Keep the jet needle in original position..
If the jet needle has been lifted as usual in an attempt to mask the issue it MUST be returned to the stock position.
They compensate for the extra air flow and higher venturi pressure at low speeds that results when removing the airbox which then weakens the off idle mix and causes the flat spot and bogging at around 2k rpms..
Helps improve running throughout the rev range and gives an average increase of 2 to 6 HP overall …"
These jets are like the missing piece of the puzzle when removing the airbox.
They cure the symptoms that are a result of the extra air flow and negate the need for complicated adjustments to the pilot and needle that don't really work anyway .