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74 z1 - insane hot day - started hesitating WOT highway. Why?? 20 Jul 2020 05:56 #831041

  • emy111
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Question about hesitation at wide open throttle.
My buddy and I have basically almost identical KZ's with almost identical mods.
Both bikes have been running great. Strong and smooth.

Specs below:
- mine is 74 z1
- buddies is 78 kz1000
- z1 is 1015 weisco / kz is 1075 weisco
- z1 cams .400":256˚ / kz cams .365":246˚
- z1 has mild LarryC head port
- both have Keihin CRS29 carbs / 112/62/yy7 middle
- both have UNI pod filters
- both have same Kerker exhaust with 2" baffle
- both have Dyna coils and Dyna S ignition
- both have APE valvetrain springs / guides etc
- both have same Barnett full clutch kit
- both of us only run 93 octane gas

Here is my question...
My buddy and I did a long ass ride around the middle of Pennsylvania the entire weekend.
It was insane hot outside. Mostly around 95-100˚ entire weekend.
Sunday on our way home, we hopped on the turnpike highway for a solid 2 hours.
z1 has a Lockhart oil cooler and an oil temp gauge which sits in the oil fill cap area. It never went above 200˚ so I was pretty happy about that.

At one point on the highway we decided to race.
4th gear around 65mph, hit the gas full throttle, go into 5th gear, and probably hit 110mph.
It was pretty much dead even.
I'd say my buddy has a little more low end grunt, and I might possibly pull a little harder at redline? It's hard to tell, they're basically the same.

Anyway, after another 30 mins, we stopped for gas and talked about the race.
I said during race, I felt some weird minor but consistent hesitation when at WOT around 6-9k rpm which i've never felt before.
He said he felt the exact same thing. wtf...
My question is.... why?
Bike has never hesitated once, and i've had it 8 years.
Was it just insane hot outside?
Entire weekend bike didn't hesitate at any point other than the race.
My gut tells my Dyna ignition products aren't the best and maybe the coils got hot as shit and spark was crapping out at full load?
Or maybe detonation of some sort occurred?

Looking for opinions about what hesitation could have been caused by, and how it could be avoided.
Thanks.

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74 z1 - insane hot day - started hesitating WOT highway. Why?? 20 Jul 2020 09:39 #831055

  • loudhvx
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Dyna S and Dyna coils are the most ubiquitous aftermarket ignitions used on Kz's and are about as solid as anything you will find. That doesn't mean they don't fail, but they have proven to be reliable. Hesitation like that could be spark failure, but if it was, it would happen in all gears and during hard acceleration as well, and you will usually notice it sooner and more often, rather than just at the top of a high speed run. Any hard acceleration to redline at WOT would have the hesitation.

Since you've had the bike 8 years, you will want to make sure the spark plug gaps are close to stock. It wears bigger with time and will cause hesitation at WOT high-RPM use.

It's hard to say what happened based on one single WOT pull. It could be heat. I think Pa gets pretty humid too. Both factors reduce the available air. Maybe you are a tad rich at WOT and it only shows up on the hottest days.

Here are a few things to check to determine if it's lean or rich.

I'm not familiar with your carbs, but if it's possible to apply the choke slightly, and then perform another WOT pull, the hesitation will be worse if it's due to being overly rich.

However, usually, pronounced hesitation like that is associated with being lean. So if it's jetting, you will want to find out if it's lean or rich. Just having someone behind you will usually tell. If it's rich at a sustained WOT, the smoke will be pretty black.

If it only happens on long pulls to top speed in top gear, then you will want to look at fuel delivery. Do you guys have inline fuel filters like the see-through type with paper elements (designed for cars)? They can sometimes interfere with flow. I've had issues with those as well. And of course you will want to make sure the gas cap vents are free to let air into the tank.

When you hold a throttle at a fixed position and accelerate, the mixture swings from richer to leaner as the RPMs increase. So if it's lean, the acceleration may start off smooth and as the RPMs increase, hesitation will increase and get worse with increased RPMs.

Another thing, though, when you were at WOT, did the bike seem to smooth out or even go faster just as you started to close the throttle? If so, it's possible the slide is opening beyond fully open. For that tiny bit of throttle movement near WOT, the slide will move upward beyond the roof of the throttle bore. This does not allow anymore air, but still lifts the needle thus adding more gas. Many stock Kz's with manual slide carbs are set up this way at the factory. I think it's a way to ensure a rich mix at WOT as a safety factor. You will want to check your carbs to see if this is the case.

For the various possible problems, there are of course a lot of overlap in the symptoms, so one single symptom is usually not enough to diagnose with any certainty.
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Last edit: by loudhvx.

74 z1 - insane hot day - started hesitating WOT highway. Why?? 20 Jul 2020 10:59 #831059

  • hardrockminer
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I'm a strong advocate of Dyna equipment on Z1's. I have their pickups on both my Z1's and pickups and coils on my KZ1000 B4. Never had an issue.

There could be many reasons for hesitation and loudhvx has touched on many of them. The only way to really know is to put together a list and begin checking each item on the list under similar conditions. As you eliminate items you will eventually work your way down to the cause.

I don't know those carbs at all so I can't comment on whether that was the source of your hesitation. I would look at the connections in the ignition system...what voltage are the coils receiving? Are the plug wires in good condition? Same for spark plugs.

You say the two bikes are almost identical, but there are a few differences. I think the KZ1000 has 2 ohm coils with a ballast resistor (Not positive on the '78's but my B4 was set up that way until I switched.) while the 900 has 3 ohm coils with no resistor. The KZ1000 also has a heavier crank, which may explain your point about better low end grunt. It also has a 33 tooth rear sprocket vs 35 for the 900, although that wouldn't be a cause of hesitation.

Lastly, when the throttle is fully open the needles are retracted from the needle jets and fuel supply is totally governed by the main jets. You might want to change main jets to see if that eliminates your hesitation at full throttle.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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Last edit: by hardrockminer.

74 z1 - insane hot day - started hesitating WOT highway. Why?? 20 Jul 2020 17:13 #831080

  • VTEC
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Man that sounds like vapor lock. When an engine sits after a hot run, it gets hotter when the cooling air stops. If it's hot enough to boil the fuel in the fuel lines or carb bowl -- that will cause a hesitation on first initial run until the air again cools down the engine and carbs,

Winter fuel blends (more ethanol) often extend at the pump into the hot summer months. Ethanol has a lower boiling point than gasoline, and will promote vapor lock.

Did you stop briefly before that run? Maybe stuck at a light before WOT? Nothing hotter than a KZ at a red light!
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Last edit: by VTEC.

74 z1 - insane hot day - started hesitating WOT highway. Why?? 20 Jul 2020 19:32 #831088

  • Nessism
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In addition to what's already been said tight valves can cause hot running issues. How long since the last valve clearance check/adjustment?

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74 z1 - insane hot day - started hesitating WOT highway. Why?? 20 Jul 2020 20:16 #831091

  • stone fruit
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I had a similar issue with a Dyna S and it was the ignition and spark plug combo. I was using NGK Iridiums and they would break down under load at full operating temperature. Same problem with 7's & 8 heat range. I switched back to copper electrodes and the issue went away. Now I'm using a Dyna 2000 and I've been using those same Iridium plugs with no issues. It seems the Iridium plugs didn't like the longer dwell.

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74 z1 - insane hot day - started hesitating WOT highway. Why?? 21 Jul 2020 06:00 #831107

  • Rick H.
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IMHO I wouldn't worry too much about one isolated incident in a situation as described. The bikes were run for quite a while in very hot conditions and then at some point you took them to the extreme and felt some hesitation. No doubt those fuel bowls were pretty warm if not hot so the gasoline was also warm to hot. Let's just say the situation was less than ideal for a race at that point. Couple that with some really humid air which the bikes aren't necessarily prepped for and I am surprised a bit of hesitation at high speed was the only problem you had. I certainly wouldn't go messing around looking for a problem in a one-off situation like this. It's almost as if you setup the bikes for failure at that point. Once temps return to more normal ranges and humidity drops if there is still a problem then worry about it. I know I can feel a difference in the way my KZ-1000 runs when it is very warm and humid versus cooler and less humid. I think anyone with a carbureted engine will admit to the same thing. One shouldn't expect perfect engine performance in extreme temps and humidity especially from an already hot engine that's been run for several hours. The question I have is how do the bikes run in cooler less humid conditions? And having both bikes experience the same exact problem just confirms my feelings regarding ambient weather conditions. You may be chasing a ghost on an air cooled, not liquid cooled, non-fuel injected motorcycle.
Rick H.
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Last edit: by Rick H..

74 z1 - insane hot day - started hesitating WOT highway. Why?? 21 Jul 2020 06:36 #831108

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Rick H. wrote: IMHO I wouldn't worry too much about one isolated incident in a situation as described............. You may be chasing a ghost on an air cooled, not liquid cooled, non-fuel injected motorcycle.
Rick H.


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

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74 z1 - insane hot day - started hesitating WOT highway. Why?? 21 Jul 2020 09:46 #831121

  • emy111
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Yea, I like Rich H answer.
For reference, I did a test run yesterday, which it was also 95˚ outside.
30 miles of highway.
During that 30 miles I did four different semi quick 4th to 5th gear redline pulls.
No hesitation at any point.
It was strong and smooth through entire rpm range.
Oil temp gauge eventually got up to 180 - 190˚ ish.

So, I agree with Rich H.
Yea, maybe over the weekend it was just insane hot on an insane hot road of a ton of highway... and everything including the bike / gas in the bowls / coils .... everything was just at its limit with heat.
Okay, I won't overthink it anymore than that.
Thanks for the input.

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74 z1 - insane hot day - started hesitating WOT highway. Why?? 21 Jul 2020 11:47 #831136

  • Mikaw
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Yeah...

Rick H, you rule!!!
1976 KZ 900 A4 kzrider.com/forum/11-projects/613548-1976-kz-900-a4
1976 KZ 900 B1 LTD
1978 KZ 1000 B2 LTD
1980 KZ 750 E1
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74 z1 - insane hot day - started hesitating WOT highway. Why?? 21 Jul 2020 15:45 #831162

  • Rick H.
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Thanks Mikaw! Now I am embarrassed.....I must be more of a wuss than I thought I was. I think the most RPM I have spun up on my KZ-1000 since the rebuild is about 6,000. More than that would cause a heart attack especially if something broke!

Rick H.
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