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GPZ500s Carburetor - Power jerking when riding

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Blockheads121
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PostPosted: 11:42 - 22 Sep 2018    Post subject: GPZ500s Carburetor - Power jerking when riding Reply with quote

Hi guy's, just wondering if you could help.

I bought a neglected GPZ500s with a genuine 9000 miles (I suspect this is true as the cam cover has never been off), changed the plugs, set the valve clearances - Cleaned the carb - Set mixture screws and balanced the carb using a home made kit (Two milk bottles and some fish tank hose - Works great) - The bike now revs up great from Idol, but when actually riding the bike, the bike jerks with intermittent power (Fuel starvation) - The power is there for a few seconds, then it dies off before coming again - The Diaphragms seem ok in the carb, so just wondering if I have set the floats too high.

They both originally measured 12mm, so I set them to 17mm as stated on forums - The problem possibly being, I have been reading American posts.

This is a 2002 English Model.

Any help would be much appreciated.
Thanks
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Blockheads121
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PostPosted: 13:21 - 22 Sep 2018    Post subject: Reply with quote

Think I might of sussed it - Float height is 17mm with needle valve spring not compressed - Anyway, see if it works.
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jaffa90
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PostPosted: 21:52 - 22 Sep 2018    Post subject: Reply with quote

The carbs without valve springs you can turn upsidedown and check float height,
carbs with valve springs i just blow through a connected pipe whilst lifting the float with a finger and usually shuts off with a level float.
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Blockheads121
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PostPosted: 22:16 - 22 Sep 2018    Post subject: Reply with quote

Cheers, having a nightmare with it.
Set them at 17mm spring compressed - To low - Bogging.
Set them at 17mm spring not compressed = Flooding.
Then smashed synch bottles on the floor.
Anyway, bought another bottle of pasteurized milk, £1.29 from Raj's - Robin git, and repaired the set up.
I think I've got to swap the valves over tomorrow - I'm guessing 16 year old needle valves are going to be bedded in.
Then balancing and mixture tuning.
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Blockheads121
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PostPosted: 20:31 - 23 Sep 2018    Post subject: Reply with quote

Just an update, just in case anyones interested.
Smashed another synch bottle taking the carb off a few more times - Left pot wasn't firing - Turned out to be one of the new plugs - Put the old ones back in and it was fine.
Anyway, set the floats spot on - Valves closed at 17mm after blowing down fuel intake. Synced the carbs (Allthough, this may not be super accurate - Tuned the mixture screws in and out to attain strongest idle, and I still have the same problem - This time it's trying to kick in but no longer kicks in and drops off as it did when I accidentally set the floats to high - Meaning they shut the fuel off way too low in the float chamber.
Diaphrams seem to look good and nearly new, yet there's little in the way of vacuum suction by them - They do drop down pretty quick - So ile have to put it down to weak diaphrams - Slides seem to have play in them too.
Think ile tighten the clips on the carb to inlet manifold too - Could be that.
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pompousporcup...
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PostPosted: 09:42 - 24 Sep 2018    Post subject: Reply with quote

did you ultrasonic the carbs or just spray carb cleaner?

Does the bike have an aftermarket exhaust ?
ive rebuilt a couple of these bikes now. They're simple enough but can get upset by what seem like very minor changes.
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Blockheads121
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PostPosted: 09:51 - 24 Sep 2018    Post subject: Reply with quote

Hi, no - I've just blown through pipes and cleared the jets n nipped em back in - Just bought some clear hosing for another synch and aluminium tape for the inlet - I think it might be pulling in air which won't open the diaphrams. See what happens !
Cheers!

Oh, and standard exhausts.
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temeluchus
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PostPosted: 17:25 - 24 Sep 2018    Post subject: Reply with quote

If the needle valves are original I'd get new ones.

The floats should move freely and be adjusted with the carbs upside down and without you touching them.

Check if the inlet manifolds have any cracks. If they have vacuum take off ports/spigots make sure they are blocked off. Also, if i recall the GPZ500 motor really doesnt like running without the airbox fitted and in good condition.
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Blockheads121
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PostPosted: 17:49 - 24 Sep 2018    Post subject: Reply with quote

Hi yeah, cheers - All seems fine with what you mentioned and tested - It revs up pretty well from idle now, but still lacks power - Runs a little boggy still - Air filters a bit manky, and i'm still not sure of the correct mixture screw settings (They both work, 3 & 1/2 either way reduces revs) - Going to clean air filter.
Cheers!
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jaffa90
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PostPosted: 18:01 - 24 Sep 2018    Post subject: Reply with quote

Blockheads121 wrote:
Hi yeah, cheers - All seems fine with what you mentioned and tested - It revs up pretty well from idle now, but still lacks power - Runs a little boggy still - Air filters a bit manky, and i'm still not sure of the correct mixture screw settings (They both work, 3 & 1/2 either way reduces revs) - Going to clean air filter.
Cheers!

It will need more fuel under load.
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Blockheads121
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PostPosted: 20:37 - 24 Sep 2018    Post subject: Reply with quote

Ile have to eat my words - Right manifold still sucking in air.
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pompousporcup...
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PostPosted: 08:44 - 25 Sep 2018    Post subject: Reply with quote

Blockheads121 wrote:
Hi yeah, cheers - All seems fine with what you mentioned and tested - It revs up pretty well from idle now, but still lacks power - Runs a little boggy still - Air filters a bit manky, and i'm still not sure of the correct mixture screw settings (They both work, 3 & 1/2 either way reduces revs) - Going to clean air filter.
Cheers!


set it to 2.25 turns out and go from there 3.5 may well cause bogging especially if the air filter is knackered.

As a test, just remove the air filter and go for a ride.
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Blockheads121
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PostPosted: 21:22 - 25 Sep 2018    Post subject: Reply with quote

Thanks - I've taken your advice.
It's one for Greek mythology this.

Roughly a week ago as posted above, I unknowingly refit the carb without one of those copper needle guides after also setting the floats wrong (Too high which sat too low - Setting with needle valve spring compressed, dugh!) - It ran well but only intermittently - This was because the synch gear I made (Immediately after destroying) - Worked very well.

Today - 10 carb removals later, and botched synch attempts, I found the copper needle guide under my bed, dugh!

(I did previously suspect this, but upon checking close up google images of GPZ500s carbs, I only saw one of those copper guides - Thus assuming this was correct) - Must of just seen what I was looking for - Double checked today and same image showing two - Which prompted the search for the sacred copper guide.

Anyway, put it all back together while the new synch gear I constructed (Hose and gravity fed oil), wasn't working.
I know this because of erratic idol, popping and only heat from one pot) - So I've decided to go by ear and feel, while listening for a good sounding idle and heat from the ends of both exhaust - Sounds better already and revs up well.

Thanks for the 2 &1/4 turn advice as this seemed to help too.
Ile get the synch as close as I can tomoz then tune the mixtures. - Currently experiencing constricted wind pipe, a dodgy chest and downing with a can of Speckled Hen watching Back to the Future.
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Blockheads121
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PostPosted: 14:52 - 26 Sep 2018    Post subject: Reply with quote

Seems to be ok now - Thanks guy's.
Just been struggling with tuning due to inlet boots being over tightened and synch - Just been experienting.
Think I will leave the mixture screws at 2.25 - feels and sounds about right - Cheers!
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pompousporcup...
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PostPosted: 15:12 - 26 Sep 2018    Post subject: Reply with quote

good work Thumbs Up

carbs can be a right pita sometimes
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