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TOPIC: KZ750 B2 Slight Runing Issues, Engine or Carb?

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528582

  • 750steve
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Hopefully these are simple fixes;

Firstly;
float levels are correct
carbs have been bench sync'd & then fine sync'd on the bike
Air screws are 2 turns out
Valve clearances have been checked
Running an ELECTRONIC ignition
Fuel is Super Unleaded (97 Octane here, ordinary stuff is 95 octane)
Carbs are fully cleaned (Ultrasonically & vapour (wet) blasted)
All airways are clear & so are the jets
#50 pilot & #135 mains (running Pod filters & baffled but short end cans)
Compression is good & well within spec
Plug gap set to 0.72mm (0.7-0.8mm for spec)


Now you may be wondering why i mention so much about the carbs in an engine thread, it may be related to both??
So i got her fired up to balance the carbs on the bike & i have a few issues;

1/ Backfires when coming down the revs on the right cylinder

2/ Left cylinder running hotter than the right. I have heatwrap on the exhausts & you can grab the right header when the bike has been running about 5 mins, you can touch the left header but you wouldnt want to hold onto it.

3/ Right cylinder makes a kinda phut-phut-phut noise when warmed, i wouldnt call it popping unless maybe its very quiet kinda popping

4/ Takes a while to return to idle (1000RPM), when i rev itthe RPM comes down quickly enough to 1300 RPM but then takes about 20 secs to return to idle speed


Does this sound like i need to check the timing?
Or would the valves on the right cylinder be 'coked up' & stopping shutting properly?
OR maybe valve stem seals?


Here are the plugs after about 10 mins of running at idle & a few light revs. Pics are taken with & without flash but its hard to show a true picture. The RIGHT cylinder was more sooty than the left to the naked eye


Right Plug (bit of a bad 1st pic) & may have gotten the non flash pics mixed up but you get an idea of the sooty deposits





Left Plug

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1
Last Edit: by 750steve.

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528588

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Have you tested the carb holders for leaks? Ed

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

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528593

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Yes Ed, well i did in the only way i know how. Sprayed some carb cleaner around them & there was no increase in RPM.

The holders got a fine bead of high temp sealent applied to them between them & the cylinder head. The carbs are also supported by way of a 1mm thick aluminium support to a welded tab on the main frame tube under the tank, no support at the other end as im using pods, the airbox just wouldnt look right on it & cant go back on now anyway.

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528599

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Were those New Plugs before the 10 minutes of idle and few revs ???
Reading your text first I was thinking lean, but plugs look way fat. Before I went any further I would get fresh plugs.

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KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528600

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Is the timing advancer working correctly? There is a specific advance curve it has to track.

Those sooty plugs look rich, but the backfiring sounds lean. It's possible that with pods, the needle taper just doesn't work right.

Those also look like the wrong type of plug: the twins use the NGK B6ES plugs or the ND equivalent. They don't have the tiny electrodes shown on those plugs.

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1979 KZ-750 Twin

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528602

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ADD: sooty plugs can also be an electrical problem. How good is the coil? If it is an old coil, chuck it and get the four Ohm Emgo from Z1 for $36. Great coil comes with new wires.

You changed to electronic ignition? Which kind?

Are you sure it is giving good spark and tracking the spark advance curve correctly?

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1979 KZ-750 Twin

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528603

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1/ Backfires when coming down the revs on the right cylinder

2/ Left cylinder running hotter than the right. I have heatwrap on the exhausts & you can grab the right header when the bike has been running about 5 mins, you can touch the left header but you wouldnt want to hold onto it.

3/ Right cylinder makes a kinda phut-phut-phut noise when warmed, i wouldnt call it popping unless maybe its very quiet kinda popping


This really sounds like the exhaust valve on the right cylinder is not sealing well. Sure the compression is good? What readings did you get?

For a 750 twin, stock is 152 PSI.

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1979 KZ-750 Twin

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528615

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WOW, i've alot to answer here, hope i make sense!

Street Fighter LTD wrote: Were those New Plugs before the 10 minutes of idle and few revs ???
Reading your text first I was thinking lean, but plugs look way fat. Before I went any further I would get fresh plugs.

Yes, new plugs, NGK Iridium BR8E IX

bountyhunter wrote: Is the timing advancer working correctly? There is a specific advance curve it has to track.

Those sooty plugs look rich, but the backfiring sounds lean. It's possible that with pods, the needle taper just doesn't work right.

Those also look like the wrong type of plug: the twins use the NGK B6ES plugs or the ND equivalent. They don't have the tiny electrodes shown on those plugs.

I'm running an optical electronic ignition, so no mechanical advancer is present
Will the needle taper matter at idle? Or are you referring to backfiring coming down the RPM? I dont know much about plugs, are the BR6's a hotter plug? I took BR8's out of the bike so thats what i replaced them with only i used the Iridium plugs

bountyhunter wrote: ADD: sooty plugs can also be an electrical problem. How good is the coil? If it is an old coil, chuck it and get the four Ohm Emgo from Z1 for $36. Great coil comes with new wires.

You changed to electronic ignition? Which kind?

Are you sure it is giving good spark and tracking the spark advance curve correctly?

Coil is brand new & came with the ignition so it's all matched ok, the whole bike has been carefully rewired so there should be no issue with power to the coil either.
The <Ignition> is KZ Jim's off the twins forum, no reported issues with any of his other customers.
I'm not sure how to check if the spark is good or how to trace it but the gap is set correctly & i have cleaned the plugs a couple of times with a light wire brush as i didnt want to put them back in looking like that to diagnose something

bountyhunter wrote:

1/ Backfires when coming down the revs on the right cylinder

2/ Left cylinder running hotter than the right. I have heatwrap on the exhausts & you can grab the right header when the bike has been running about 5 mins, you can touch the left header but you wouldnt want to hold onto it.

3/ Right cylinder makes a kinda phut-phut-phut noise when warmed, i wouldnt call it popping unless maybe its very quiet kinda popping


This really sounds like the exhaust valve on the right cylinder is not sealing well. Sure the compression is good? What readings did you get?

For a 750 twin, stock is 152 PSI.

Compression was taken cold as at the time i had a dead battery so used a car jump pack, compression was 122 on the left cylinder & 132 on the right. I was told to add about 10PSI for hot readings & the guy who told me is as knowledgable as you're gonna get! (Steell)

I stuck a post on the twins forum too & Steell kindly replied with;

My best guess is:
(1) air leak on right exhaust
(2) Vacume leak on right intake
(3) Idle jets are a bit to large, maybe 45's are needed.

Could be totally wrong though.


The guy does know his stuf!! I said i'd try smaller pilots (im running 1 size over stock) to see if that helps the fouling issue. I know you guys say alot about better running with a size up but maybe the UK climate does different things? YOur air will be warmer & therefore thinner so maybe you guys really do need bigger jets? In saying that i always thought that denser air like we probably have here in the UK would need more fuel for the heavier air.

The bike hasn't moved from my garage either (so still air), would things change outside in circulating air? Its absolutely pissing doun outside over here so i cant just take it out at the minute ,its near 1AM here too.

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1
Last Edit: by 750steve.

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528628

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Bounty Hunter is right, those *'s are to cold, the 6's are whats for the twin. Those 8's won't self clean, especially at running in the garage. Get the correct plugs, the popping could be from the plug fouling, there is a lot of carbon there.

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1982 KZ1000LTD K2 Vance & Hines 4-1 ACCEL COILS Added Vetter fairing & Bags. FOX Racing rear Shocks, Braced Swing-arm, Fork Brace, Progressive Fork Springs RT Gold Emulators, APE Valve Springs, 1166 Big Bore kit, RS34's, GPZ cams.
1980 KZ550LTD C1 Stock SOLD Miss it
1979 MAZDA RX7 in the works, 13B...

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528665

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750steve wrote: I said i'd try smaller pilots (im running 1 size over stock) to see if that helps the fouling issue.

It won't. I always found 50 pilots run best in my 750 twin even with stock pipes and airbox. Going back to 45 will make it pop even more. The stock pilots were on the lean side for emission reduction.

Definitely need to use the hotter B6ES plugs it was designed to run.

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1979 KZ-750 Twin
Last Edit: by bountyhunter.

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528669

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750steve wrote: Coil is brand new & came with the ignition so it's all matched ok,

Is it still using the stock advancer? Those cause all kinds of problems.

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1979 KZ-750 Twin

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528768

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Ok guys. I'll try to get some B6ES's today & try them tonight. I'm still wondering why they would create a hot or cold cylinder?

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528769

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Okay, got some W20ESU's sourced, they're the Nippon Denso equivalent. Are they as good as the NGK's? Looks like NGK's would take a couple of days to get here & im impatient :laugh: , if they're much better i'll order them too.

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528788

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Here is a Wiki write up on Heat Range from this link, en.wikipedia.org/wiki/Spark_plug#Sealing_to_the_cylinder_head





Those Denso plugs are good, NGK is also good. Once you get it running, take it out and build some heat. Then check the plugs.
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1982 KZ1000LTD K2 Vance & Hines 4-1 ACCEL COILS Added Vetter fairing & Bags. FOX Racing rear Shocks, Braced Swing-arm, Fork Brace, Progressive Fork Springs RT Gold Emulators, APE Valve Springs, 1166 Big Bore kit, RS34's, GPZ cams.
1980 KZ550LTD C1 Stock SOLD Miss it
1979 MAZDA RX7 in the works, 13B...

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528798

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Thanks Motor Head, that has helped my lack of spark plug knowledge.

I paid particular attention to the 2nd last paragraph where it states that the plug hotness or coldness will not affect the cylinder temp. Maybe my excessively sooty deposits are making things run badly.

I'll be going home with the new plugs & im going to change the high octane unleaded fuel to ordinary stuff incase it is burning too fast or per igniting. There in only 2 RON between the 2 types of fuel we have here, super UL at 97 & ordinary UL at 95, i think i also have the option of no ethanol in some places.

I'll change the fuel & plugs to see how that goes & report back later

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528800

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Higher Octane fuel burns slower than a lower octane. If it burns to fast, then you have problems. These engines need a higher octane at sea level, but with altitude can handle less octane as cylinder pressures decline. You service manual, and the sticker under your seat will suggest a minimum octane rating. 87 I think, but can't remember which rating type that is

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1982 KZ1000LTD K2 Vance & Hines 4-1 ACCEL COILS Added Vetter fairing & Bags. FOX Racing rear Shocks, Braced Swing-arm, Fork Brace, Progressive Fork Springs RT Gold Emulators, APE Valve Springs, 1166 Big Bore kit, RS34's, GPZ cams.
1980 KZ550LTD C1 Stock SOLD Miss it
1979 MAZDA RX7 in the works, 13B...

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528815

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Motor Head wrote: Higher Octane fuel burns slower than a lower octane. If it burns to fast, then you have problems. These engines need a higher octane at sea level, but with altitude can handle less octane as cylinder pressures decline. You service manual, and the sticker under your seat will suggest a minimum octane rating. 87 I think, but can't remember which rating type that is


Yeah sorry i did get that ass about face! Flamin eck i knew high octane burned more slowly!! :laugh:

Anyway it wont hurt to try changing the fuel & the plugs to see if that makes a difference for all it'll cost

where i live the atmospheric pressure is about 1000 - 1010mb & about 200ft above sea level.

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1
Last Edit: by 750steve.

KZ750 B2 Slight Runing Issues, Engine or Carb? 4 years 8 months ago #528883

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Guys i gotta thank you so so much, i'd honestly never have thought that 2 seemingly small differences would have such a big effect on things!

I replaced the plugs & fuel & she seems to run pretty good now. Settles back down to idle nicely, infact nearly too nicely, & revs clean with both cylinders at about the same temperature. There is no unburnt fuel stinging my eyes in the garage either now.

I dunno if you can hear it in the video but she gargles a bit when setteling down in RPM. Im just putting that down to the small baffled cans, i can live with it & somehow i find it pleasing as its a bit 'raw'



Plug after running this time

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07 ZX6R Race Bike
1977 Z750 B2 Twin
1976 Z650 B1

KZ750 B2 Slight Runing Issues, Engine or Carb? 3 years 3 months ago #612944

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After 1.5 year a big thank from me!
I have a z750 1984 completely rebuilt.
For weeks trying to solve the problem, the engine is not started without choke.
idle only when the engine was hot
Check again valves, camshaft, cylinder compression, pulled the carburetor is controlled again, check out the air intake boots, everything was okay., The plugs ( NGK BR9ES ) are new only few hours working.

Finally read this Post and try the same solution.
Change petrol and new spark plugs.
And there was miracle!
Now works perfectly, I'm getting ahead easily and without choke.
So simple was the solution!
The last time buys plugs ngk, short-circuit very easily already spoiled 3 sets.
Will try next time DENSO W24ESR-U

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KZ750 B2 Slight Runing Issues, Engine or Carb? 3 years 3 months ago #612950

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Did anyone notice that the first Iridium plusg he was running were "R" BR8EIX resistor plugs, instead of the "non-R" B6ES? That would cause excessive resistance if the wires he has, have the built in resistors in them also, no?

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82 KZ1000-K2 LTD
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