BS34s and white plugs

[jpdevol]

Just gassed up madness, high compression 750 cammed, she's getting a bit tired, VM34 carbs, fairly high gearing, riding curvy state highways. Couple of hammer down runs to 75MPH or so, (testing purposes only)
112 miles 2.2 gallons, yup 50MPG.

Only 14gal to Mountain View then

 

[tommytwohands]

With jetting down the pilot air circuit and going up on the main jets…back from a sunny 1200 mile round trip to the beach with exactly 42 mpg

 

[5twins]

Did the jet changes yield any more color on your plugs?

 

[tommytwohands]

Did the jet changes yield any more color on your plugs?

Perhaps a very slight shade of tan while holding one up to the light. The threads of the plug showed some dark brown/black

 

[tommytwohands]

Dead cylinder method yielded roughly 2.5 turns out for each cylinder

 

[joebgd]

On either niche or jets are us, can you please post a link for the page with needle and needle jet? My needle jets on my bs34s, 1980, are out of round on the top. Or suggest what I may search for. I see in tech it's a y-o jet. I didn't have luck locating it. I would definitely prefer the genuine mechuni.
Thanks all.

 

[5twins]

I don't think originals are available any more so you'd have to get the re-pops. If I needed to replace some BS34 needles and needle jets, I think I would go with the "Canadian" ones. The needle (5IX11) has 5 clip slots so is adjustable, and there is a matching needle jet (336-Y-0) for it. This needle jet has more air bleed holes down the sides so is actually a little leaner than the original Y-0 needle jet. But, combined with the much richer 5IX11 needle, the result is still a richer midrange.

https://yamahaxs650.com/product-cat...arb-slide-needles-needle-jets-mixture-screws/

 

[AUTOMAN]

Not a WOT plug chop.
Typical part throttle lean emission plug. This happens on all lean setting part throttle and/or WOT emission carbs after time.
They always go leaner and leaner until no start.

cliff

 

[Paul Sutton]

Just an observation:

NGK plugs have a shiny metal plating to help release the threads when unscrewing. When heated the plating burns away as you commonly see on the ground electrode.

Photo taken from Entry #8: The plating is heat stained/burnt halfway up the threaded region. This is an indicator that the plug is on the hot side. That heated zone should only be about 1/4 to1/3 up the threaded section.

I recently, two weeks ago, ran my Virago using the recommended BP7ES and then colder BP8ES and BP9ES. In all cases the porcelain was White but the heat zone on the threads decreased significantly:

BP7ES: 1/2 like the photo above.

BP8ES: approx. 1/4

BP9ES: No heat zone showing on the threads. See photo below:

This BP9ES has had just over 100 miles running on a mixture of motorway, country and urban roads. In addition the idle carbon monoxide level for both carbs has been set to 4%. There is no heat zone at all on the threads as expected for going to a colder plug. What surprised me most was the porcelain is still white. Close inspection of the porcelain shows the surface to have a smooth satin appearance as when it was new. When a plug is running too hot the porcelain shows very fine white or silvery particles adhering to the white surface and that is the danger sign.

The next photo is a closeup on the ground electrode of this same BP9ES:

Notice how the metal plating on the ground electrode has been burnt back halfway. We would expect the electrode plating to be burnt completely off and a heat line in the bare metal at about 2/3 to 3/4. Also a carbon ring at the base of the insulator is barely visible. Clearly this BP9ES is not running hot yet displays white porcelain.

It is important to consider more than just the porcelain color. If you see white then treat it as an alert then look more closely for over heating signs e.g. heat zone on threads and ground electrode plus fine particles adhering to the porcelain, especially the latter.

 

[AUTOMAN]

Perfect
So far

Look at the porcelain below the shell(threads)
Side electrode will burn-back about 1/2
Part throttle will not indicate WOT jetting

 

[The Standard]

It was mentioned earlier in this post to only run one resistance source within the spark circuit- either plugs or caps but NOT both.

How do you tell if you have resistor caps? I am running BR8ES plugs as cannot seem to find the B8ES plug for the ‘72 model locally. I install with the threaded aluminum cap off the plugs.

Here are some pictures of my plug caps. Not sure what I am looking for.

 

[The Standard]

Forgot to mention. I am running electronic ignition.

 

[5twins]

Going by the size and shape of those plug caps, I'd say they're the resistor type. You can confirm by measuring through them with a multimeter. The NGK non-resistor caps are much smaller .....

 

[The Standard]

Thank you @5twins.

Last confirmation on this as new to this subject matter. The bike has either a Boyer Bransden or Pertronix type ignition. Does anyone know if that type of setup would call for the extra resistance from the caps and resistor plugs? For the most part the bike runs great but every so often it will die at idle, like while at a stop light. Starts right back up with one kick and off and running fine. No backfiring or coughing ever noticed.

The optimism is for a possible seat of the pants lift in performance in switching to the non-resistor plug wire caps.

 

[5twins]

Even with the electronic ignition, I think you still only want one instance of resistance in each plug wire. So yes, change the cap or change the plug to a non-resistor type.

What do you have your idle speed set at? The spec is 1200 RPMs and this bike really needs that. Set it lower and the bike will randomly just up and stall at idle sometimes.

 

[Jim]

The bike has either a Boyer Bransden or Pertronix type ignition. Does anyone know if that type of setup would call for the extra resistance from the caps and resistor plugs?

For future reference, the ignition system used has no bearing on HT resistance. All the ignition does is allow the coil primary to charge the secondary, then open the primary so the secondary field collapses into a spark. That's true for points... and any electronic ignition you can think of. Resistance used is strictly between the coil, HT lead, cap and the plug... and not what triggers it.

If you had too much resistance in the HT side of things, you might notice a bit easier starting when you fix that. As far as performance... maybe at very high revs you'll see a tad more power, but doubtful for normal riding.

 

[The Standard]

Just getting to installing the tach for the first time. Will be able confirm speed although I presently set the idle somewhat high as I think the motor sounds better at stoplights but maybe not close to 1200 rpm.

Thanks for insights on the Ignition system / HT lead independence. Good to know going forward.

 

[5twins]

You don't want the idle speed much above 1200 because the ignition can start advancing. When that happens, the engine speed increases, which causes more ignition advance, which causes more engine speed increase, etc. The bike can run up to like 2000 RPMs without you touching the throttle.

 

[The Standard]

Funny you mentioned this. I noticed a little bit of this, thinking it was a surge due to tune not being fully locked in.

Didn’t go that high but definitely an increase and then would come back down some rpm. I’ll get a better gauge on this once tach is installed, pun intended.

I don’t think the engine was loading up.

By the way, upon initial observation the bike felt strong and pulled clean with those jetting changes. Need to resolve a minor exhaust leak at the head and then with tach installed will see how it pulls right up to redline.

I did end up dialing back the mixture screws as was getting a little white smoke puff blipping throttle off idle at 3 turns out. Went away after.

 

[The Standard]

Feeling over ambitious now wanting to squeeze more performance out of those BS34’s in enlarging the slide lift holes to 3mm to quicken carb responsiveness.

Any experience with that endeavor?