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Surging at Full Throttle 22 Jul 2018 10:13 #787400

  • cg94
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Hi everyone,

I had a question about how my bike has been performing. I've noticed when I open the throttle all the way the bike starts to surge and feel starved of fuel. It performs great at all other speeds and pulls well. I noticed it on the highway with the throttle wide open in 5th and the bike started to lurch at around 95 mph.

Could this be a symptom of my float levels being off? I recently cleaned my carbs, but did not set the levels at the time. My spark plugs seem to be reading well so I think the mixture is on point and I have synched the carbs.

Thanks!
1978 KZ 650 SR

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Surging at Full Throttle 22 Jul 2018 11:32 #787405

  • TexasKZ
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Do you mean the bike suddenly accelerates, or do you mean it suddenly loses power?
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Surging at Full Throttle 22 Jul 2018 13:17 #787424

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The bike will decelerate suddenly for a second and then come back up to speed. It will do this over and over as long as I have it at full throttle, but will ride fine if I close the throttle a little bit. It feels similar to when my bike has been running on the last bit of gas in my tank.
1978 KZ 650 SR

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Surging at Full Throttle 22 Jul 2018 13:26 #787425

  • SWest
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I'd try going up one step on the main jets.
Steve

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Surging at Full Throttle 22 Jul 2018 14:46 #787433

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Definitely get the fuel levels correct. Use the clear tube method as described in the manual. Setting float heights will only get them in the ballpark.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Surging at Full Throttle 22 Jul 2018 16:22 #787451

  • loudhvx
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Do a flow test from the petcock.  Do you have an inline filter?  The see-through type with paper element can sometimes restrict flow too much.  They are meant for fuel pumps.

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Surging at Full Throttle 22 Jul 2018 21:52 #787463

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Had it  happen to me after a float level setting once. But above 60-80 mph highway cursing speed. No where near full throttle. I also have a single feed fuel petcock instead of the double feed design of the original OEM petcock. Probably a contributing factor in the fuel delivery rate. I adjusted the level using the clear tube method at fixed my issue. I still plan on changing out my petcock in order to maintain a better flow.
Current project 76 KZ900 (This was a Vetter model)
76 KZ900
81 XJ550H SECA (Current Project)
82 XJ550R SECA
Past:
86 FJ1200
74 Z1900
72 CB450

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Surging at Full Throttle 23 Jul 2018 00:09 #787465

  • Z1Driver
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But above 60-80 mph highway cursing speed 


I have a cursing speed too. I believe when my 900 was running better at almost WOT than at WOT, I adjusted pilot air to fatten it up a bit. I was getting a hesitation at WOT and if I backed the throttle off a bit it would smooth put and run better.  
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Last edit: by Z1Driver.

Surging at Full Throttle 23 Jul 2018 09:27 #787492

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Thank you for the suggestions! I'm planning to set the float heights this week and will try going up a size on the main jets while I have the carbs off the bike.

I had been having issues with my pilot screws so I wonder if that could be playing a part. My bike was dying at red lights and I leaned the mixture out a bit, which solved that issue. But maybe that could be affecting performance at the top end if I'm running too lean with the mixture screw?
1978 KZ 650 SR

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Last edit: by cg94.

Surging at Full Throttle 23 Jul 2018 09:47 #787494

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Z1Driver wrote: But above 60-80 mph highway cursing speed 


I have a cursing speed too. I believe when my 900 was running better at almost WOT than at WOT, I adjusted pilot air to fatten it up a bit. I was getting a hesitation at WOT and if I backed the throttle off a bit it would smooth put and run better.  

 
There should really never be much change in mixture from 3/4 throttle to WOT since there will be no sudden changes in the needle and it's predominantly main jet and needle that control things there.  A fuel flow issue might cause it, but seems unlikely since just off WOT runs fine.

One thing that can cause this, and often is a problem on 550's is the throttle stop screw.  On the 550's with mechanical carbs, the throttle stop screw is often set to stop the throttle at a point beyond where the slide is 100% open.  This means that after the slide reaches fully open, which is where the slide is flush with the roof of the carb throat, you can still pull it open a little farther.  For that little extra pull, you are adding more fuel since the needle is raising and letting in more fuel, but you cannot add any more air because the slide is already past fully open.  This means the mixture takes a sudden nose dive towards rich and you will lose power.

On a stock 550, it's not so drastic.  When you hold WOT for awhile and the bike comes up to speed, if you start to roll the throttle off very slowly, you will get a sensation of accelerating.  Only after you roll off a little more will it start to slow down.

I'm not sure how the 900 carbs are setup, but if there is a throttle stop adjustment, you may want to make sure the slides are not opening beyond full open.

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Last edit: by loudhvx.

Surging at Full Throttle 23 Jul 2018 09:53 #787495

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Yes they may not be clean enough. After I moved out here in the 70's the bike would act like it was running out of gas at high speeds while climbing a long grade on the way to town. I wound up changing the float needle seats from 1.5 to 2.0 allowing more fuel at WOT. In the city or the FWY it wasn't a issue but out here with lots of long roads and higher speeds it became an issue. I'd also check the needle position. Too lean could cause that as well.
Steve 

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Last edit: by SWest.

Surging at Full Throttle 02 Oct 2018 12:03 #791708

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Hey guys I know this thread is pretty old but I'm getting ready to try and up my main jets to see if that helps with my top end power.

I noticed the previous owner had changed the needle position down 1.5 clips using a washer. My question is why would the bike need to be re-jetted even though I am running all stock (with the air box and original exhaust)? Is this indicative of an issue elsewhere on the bike? Thanks!
1978 KZ 650 SR

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