TOPIC:

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

Have you tested the carb holders for leaks? Ed

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.

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.

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.

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?

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.

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.

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.

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.

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.

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?