Engine Seems To Misfire Around 4500+ RPM’s


Local time
Today, 09:23
Jul 5, 2013
My Ride
Road Glide Special
Riding Since
I’ve asked this question on another format and got some good answers, but haven’t found the solution as of yet. I have a 79 T140V. It’ is not original as far as the electrics go, but the PO did an extremely clean job with them. It has a Boyer ignition, and Mikuni carbs. The bike misses and will not rev over 4500-5000 RPM’s. It did the misfire trick with the Amal MkII’s, and I thought I was chasing a carb issue. I’ve changed jetting, plugs. I’ve looked but haven’t found a loose ground wire. The wires coming out of the Boyer were bunched together, I’ve separated them thinking that there could be an RF issue. Nothing has helped. I don’t know if the coil is original, it has NGK plug wires. I’m leaning towards plug wires and/or coil issues. What say the experts??
The bike cannot be this, Triumph did not make T140V after the 78 model year, then the company made T140E; while it made T140V before the 79 model year, they were not sold in the US after early in the 1978 calendar year.

Please be clear about the bike you have - all T140E (78 and 79) had Amal Mark 2 carburettors originally, all T140V had Amal Mark 1 carburettors originally. Please also be clear about the original ground - 79 and later T140E had negative ground electrics originally, T140V and US only 78 T140E had positive ground electrics originally.

It has a Boyer ignition
Which model - black Transistor Box MicroMarkIII or IV, red Box MicroDigital or blue Box MicroPower?

Depending on the electrical ground now, the Transistor Box is connected differently.

I don’t know if the coil is original,
If the bike has only a single coil, with two HT leads, it is not original; Triumph never fitted dual HT lead coils.

Nothing has helped.
Mark the twistgrip housing close to the twistgrip, where you can see the mark riding the bike.

Mark the twistgrip itself with four marks beside the mark on the housing, corresponding to 1/4-, 1/2-, 3/4- and wide-open throttle.

When riding, if the misfire occurs, note the twistgrip position and approximate rpm, change gear, if the misfire occurs again, again note the twistgrip position and approximate rpm; repeat for several different, or all, gears:-

. if the misfire occurs at the same twistgrip position irrespective of gear and rpm, the problem is most-likely carburation;

. if the misfire occurs at the same rpm irrespective of gear and twistgrip position, the problem is most-likely electrical.
Right you are, T140E. I have a T150V and got my letters jumbled.
It's a 79, when I got it it had the spayed carb head and MK 2 carbs. Thinking the misfire was carb related, after messing with the Amal's, I put on the Mikuni's. I got the kit from MAP Cycle.
Ground is negative.
I got a good picture of the coil, and this is the first time I've noticed the label. It's a Boyer coil to be used with the blue MicroPower
I'm attaching photos. I'll do another jetting change to the carbs and try it again tomorrow.


  • 674A668B-09BB-4742-B091-847DD86C39C9.jpeg
    151.6 KB · Views: 2
  • 6F16C772-034F-4BA9-8FEE-17179CD39692.jpeg
    122.8 KB · Views: 2
  • FDB86856-E7CA-4510-8F3A-1C2E4A104F93.jpeg
    75 KB · Views: 2
79, when I got it it had the spayed carb head and MK 2 carbs.
As one of your photos is of the exhausts, nearly all Triumph twin engines have splayed exhaust tracts (the few that do not are from the 1940s and 1950s).

The significance of the term is the inlet tracts - pre T140E originally have splayed inlet tracts and, after mid 67, Mk.1 Concentrics; all T140E originally had parallel inlet tracts and Mk.2 Concentrics.

Ground is negative.
I advise checking the Boyer is wired properly (I have deliberately not typed "correctly" :():-

. Transistor Box White wire should be connected directly to the battery negative terminal, not anywhere else (e.g. not Boyer's "Negative frame ground" or other electrical nonsense).

. Box Black wire is connected to the coil's negative terminal.

. Box Red wire should be connected to the coil's positive terminal.

. The coil's positive terminal should be connected to the handlebar kill switch. This was originally a White/Yellow wire, the switch wire connected to the main harness wire inside the headlight shell, the main harness wire ran from there to the original coils' mounting area (where it also connected to the original Lucas Rita e.i. amplifier's positive wire).

. All electrical connections should be properly crimped terminals, not soldered nor the useless red (or blue or yellow) insulated hardware store junk Boyer supplies.

. As the MicroPower is digital electronics, each HT connection from coil to cylinder head must have 5,000 Ohm resistance but only 5,000 Ohms:-

.. a "connection" is the HT lead, the plug cap and the spark plug;

.. the spark plug can have the resistance - specifically Champion RN5 or NGK BR6ES with a parallel port T140 cylinder head;

.. if the spark plug code does not have the "R" in it, either the HT lead or the plug cap in each "connection" must measure 5,000 Ohms with an Ohmmeter or multimeter set to Ohms;

.. if using resistive plug caps (NGK?), these connect to the end of the HT lead with a self-tapping screw; be aware these always come loose - first test after a misfire is turn the plug cap clockwise gently; if it never stops turning, the cap must be unscrewed and the short damaged length of HT lead cut off; thereafter (until the next time :rolleyes:), you should be able to refit the plug cap in the lead until gentle clockwise pressure cannot turn it further;

.. be aware resistive HT lead and resistors built into plugs and caps can fail and do regularly; :( I prefer to avoid resistive HT leads and caps, using only resistive plugs and having spares on hand.

I'll do another jetting change
You did not fix the problem changing from Amals to Mikunis.

The Boyer MicroPower is notoriously unreliable.

Therefore, would you not be wiser spending the time checking the e.i. is wired as I have advised then determining whether the problem is carburation or electrical? Although you have muddied the waters for yourself by changing the carburettors before knowing for certain they were the problem, which has introduced the possibility the real fault could be masked by incorrect Mikuni jetting. Nonetheless, fiddling with the carburettors will not fix an electrical fault.
Thank you for the leads in following the wiring. After a quick look today, it appears that this Boyer is wired correctly as you have outlined above. I checked the plug wire caps, and both of them I was able to turn almost two turns gently before they became "snug" using gentle pressure. A test ride is in the making for tomorrow!

As for the head, the intake is splayed as well as the exhaust. Consensus of others is it's not the stock head. There are a lot of custom, aftermarket and hand made parts on this bike, it's almost a "Frankenbike."

This little lady is only my second Triumph, and my first Bonny. The learning curve can be quite steep what with all the changes to the models over the years.
intake is splayed as well as the exhaust. Consensus of others is it's not the stock head.
Correct. In this case, plugs should be Champion N3 (resistor RN3) or NGK B8ES (resistor BR8ES).
And those are the plugs I am running. I have a pair of both, Champion and NGK non-resistor. I am going to do an ohms test on the leads/caps today as well
. As the MicroPower is digital electronics, each HT connection from coil to cylinder head must have 5,000 Ohm resistance but only 5,000 Ohms:-

I did the test, left cylinder has 4960 Ohms and the right has 5010 Ohms. Close enough I'd say.
Well well well. How little things can cause issues. Mr. Rudie, thank you for your input and educating me on some of the finer points of electronic ignition. I simply snugged the plug caps on the wires as noted in an earlier post. Problem SOLVED! She runs very strong now, and rev's right up. I didn't take it over 6500 RPM's, but she was still pulling. Thank you again fine sir!!!
snugged the plug caps on the wires as noted in an earlier post. Problem SOLVED
(y) Just a long time owner of Japanese bikes too, the Japanese fitted them to all their bikes for years, constant pita. :mad:

Otoh, all my Triumphs have the plug leads Triumph fitted in the 1970's, with both the coil terminal and the plug terminal crimped to the lead, the "plug cap" is just a plastic cover. Ime, they never give trouble - some of my Triumphs, the leads are the ones the factory fitted ... :cool: if a bike has to have HT resistance for digital electronics, I use resistor plugs.

Support TriumphTalk by becoming a Premium Member.

 What You Get





Top Bottom