650 triumph engine starts missing around 40 to 50mph

[Guest]

Hi guys,live down under built triumph unit 650 bobber with joe hunt magneto starts first second kick idles fine when riding misses pops every now again but when I get around 40 to50 mph really starts missing and farting when get over that range comes good again.Pretty sure bike is timed correctly have checked magneto,Spark plugs good color,etc.When riding to me sounds like intake problem?Any ideas much appreciated.

[TriNortchopz]

What year is it? Is this an issue that just started? Any recent changes to the bike? Is it also rpm related or just speed (45 mph) related?
Any change if engine cold, warm or hot?
What ya got on there for a carb?
Checked to make sure needle and jets all good and operating as they should? Loose slide? Still got the choke in the carb? No problem with fuel delivery to carb and float height right?
Checked for intake air leak?
Condenser in the mag good?

For further reference, here is a link to a typical Hunt mag parts diagram:

http://www.britcycle.com/Manuals/HuntDiagram.pdf

[Dougtheinternetannoyance123]

It SOUNDS fuel related, And where I would look first. Check you are not vapor locking and your tank is vented.
Had several people I know have that problem and one guy called me and said he cracked open the gas cap when it started acting up and it surged so much he almost crashed! (Bike suddenly started flowing fuel) check leaks around intake as mentioned
and internally carb function.

As to the Joe Hunt, I know some people like them but I dont. The newer versions with the super magnets are
the way to go if you persist on running them, But the biggest 2 issues with them are:

A) Low RPM spark energy is compromised. Mags are for higher RPMs,,, the faster they spin, the more spark,
low speed its feeble. (Thats why the new super magnets help) On some farm machinery they use whats called an
"impulse coupling" and it snaps the mags rotation speed at low RPMs as otherwise on some machines they are
impossible to start

B) No variable timing. Its fixed and I have not seen anyone with a mod that allows variable timing. Basic tech
says a motor needs variations to start, idle, off idle and mid, then high range. Each speed has its own timing
needs. If you look at a timing curve related to RPM, its a curve, not a straight line. So with a Joe Hunt Mag you have
one setting and thats it. So you gotta split the difference. give up top end power or sacrifice low end running
or starting. Go in the middle and it does nothing well. Look at a stock Triumph/Lucas adv mechanism. Its got
slots, weights and springs. You can get adv mechs in different ratios as well, IE: 2 deg, 4-6-8-12-14 deg So
different bikes use different specs but you can also mix and match. On preunits that DID run a Maggie.. they
did something the Joe hunt does not. Early ones had a manual adv. Cable actuated with a lever on the bars.
Later ones used a Auto adv on the maggie shaft. this again, has weights and springs.

Thats why you dont see many street riders with a Joe Hunt or ARD,, On a race bike they are timed and tuned
for specific power band, and thats why they have to start them on rollers as you would never get them to fire on
a kicker.

An electronic ignition-Boyer-sparks-wassell-trispark or others all do this electronically and some ignitions like
on my Buell or my sportster run a computer and is constantly adjusting the timing internally to the perfect
settings.

Now that we covered the basics,, Its POSSIBLE your coil inside the maggie has issues,, just enough spark
energy to fire it up and idle,, dies in mid range because its failing,, but as speed picks up, there is enough to clear
out,, But I am doubting that. The more speed the higher the energy so if its a coil failing I would think it would
falter from mid range on. They way to test this is to pull the mag and run it on a lathe or test rig spintron
with a variable electric motor to run it thru RPM ranges and visualize spark energy and measure it as well if you
have equipment to do so,
(Back in the day, many shops had a distributor and maggie test stand, not so much anymore)

one last tangent. Dont take this personal please,,, But a LOT of people are going with trends to look cool,
so, the Joe hunts and ARDs are popular for style points. But as I outlined above,, they dont work as good
as we might hope for real street riding. (Fantastic on a race bike at specific RPMS-like flat track & TT)
Also, the Maggies are not dependant on charging systems (And we all know Triumph-BSA-Nortons have issues with
charging) However, again, Its more of a race bike part. I AM seeing some stylish spark plug wires on the market as well
Some of them LOOK really cool, Cloth braided, or clear plastic with copper inside. VERY STYLISH! However MOST
work like crap. Look good-perform poorly.

Now,I have not tested all the products out there, but the ones I have tested performed very poorly. You need
ALL your spark energy going to the plug which has a very hard job and needs all the help it can get. A crappy
spark plug wire is a serious problem and if the wire looks pretty but doesnt pass full power you are choking your
ignition system. Its hard on the coil/mag and plugs struggle.

While its primarily written for auto applications, there is a ton of very valuable info in here for ANY engine with
an ignition system, Dr Jacobs has a LOT to say in many articles and several of his books on spark plug wires,
resistance caps and plugs,, To be a ignition guru and god, I highly recommend his books and excellent investment

This is one of his books. See: https://www.amazon.com/Doctors-Optim.../dp/B0050O8NBY

[Guest]

Thanks guys,fair bit to take in but all good ,will give carby tune up play around with jets see how it goes running 930 amal tr6 head on 72 cases,it's been couple years since build due for overhaul

[TriNortchopz]

Did you try a plug chop in the range it is missing and farting (40-50mph)? Guessin' that is mid-range...do you have a tachometer?

Those needles wear, so check what you got and you should get a replacement as part of the rebuild kit; see an example here:
Amal Carburetor Gasket, Float and Complete Rebuild Kit for 626 928 930 932 Concentric Carbs

https://www.theoringstore.com/index.php?main_page=index&cPath=367_2203_98