MKIII build complete but trouble starting

Hi all,

I have just finished my 6 month refurbish on my Commando and I'm having a heck of a time getting it to start. Here's what I have:

-New Trispark electronic ignition with matching Trispark high output coils, static timed and appears to be getting spark to the plugs
-I'm reading about 90 psi compression per cylinder cold (should be enough to start)
-Ignition on, kill switch set to 'Run'
-Mikuni VM34 carb, freshly rebuilt with air screw 1.5 turns out and needle in middle position
-Fresh oil, fresh spark plugs
-Engine case drained prior to starting to prevent wet sump condition
-Mikuni choke on with no throttle and choke off with open throttle
-Charged (albeit somewhat old) battery

I have gotten an isolated occasional pop once or twice, but other than that nothing. I've tried a little ether down the carb--nothing. Is there anything I'm missing?

Thanks guys,
Anthony

Get a fresh battery.

What do the plugs look like?

You sure the timing is right? Too advanced makes them hard to start. Static should be no more than 28 degrees if you want to make it easier to start. When you had the head off did you check to see when the pistons actually were at TDC, then correct the timing scale and marks on the stator if they were off?

Amals do make cold starts easier because of the tickler. The Mikuni choke is a mystery wrapped in an enigma.

Resistor plugs do not require a resistor cap on the end of the spark plug wire.

Did you try a search? Could be a lot of reasons why it won't start. Most of them have been covered in the past.

Pull the spark plugs, put a teaspoon of gas in each side, spin the plugs in quick, and try again. Report back please.

What do you mean by "appear to be getting spark to the plugs"? Either you get sparks or you don't. With plugs out, it's easily verified.
If an engine doesn't start, it's usually carbs or ignition (or both).

- Knut

90 psi cold compression seems low to me. I get 120-150 psi on my MK2 Commando.

Verify a good spark at each plug with them out and grounded to head.
Check your new coils with Ohm meter to confirm low tension and high tension sides are at spec from Trispark.
Does your engine have direct ground to battery? I beleive I read some notes from Greg Marsh's blog site where he recommends running a ground line from trispark ign to battery since ground through timing cover can be less reliable.

Anvill said:

Hi all,

I have just finished my 6 month refurbish on my Commando and I'm having a heck of a time getting it to start. Here's what I have:

-New Trispark electronic ignition with matching Trispark high output coils, static timed and appears to be getting spark to the plugs
-I'm reading about 90 psi compression per cylinder cold (should be enough to start)
-Ignition on, kill switch set to 'Run'
-Mikuni VM34 carb, freshly rebuilt with air screw 1.5 turns out and needle in middle position
-Fresh oil, fresh spark plugs
-Engine case drained prior to starting to prevent wet sump condition
-Mikuni choke on with no throttle and choke off with open throttle
-Charged (albeit somewhat old) battery

I have gotten an isolated occasional pop once or twice, but other than that nothing. I've tried a little ether down the carb--nothing. Is there anything I'm missing?

Thanks guys,
Anthony

Click to expand...

Most likely you used the wrong timing mark on the rotor!

Do both Tri-Spark tests run correctly and when you try to start does the red light flash once each engine revolution?

Sounds like ignition - either no or weak spark or timing. Can't be fuel since you've tried starting fluid and compression is adequate...though low.

FWIW, a method that works well for starting after a rebuild is to set the static timing at TDC - makes for very easy starting. Then, after you've got it running, do whatever adjustments/checks, set the timing to the proper value. We did that with every engine we ever built - it's especially useful anytime a camshaft is replaced in order to get the engine started literally instantly and get RPM immediately up to cam break-in specs - 2500-3000RPM.

Think how easy this is all going to be after we are all forced to ditch these ICE vehicles for E-vehicles!

Are you kicking it or using the button? If your battery is at all weak, it will drag the voltage down so much that it will not spark.
Try kicking it or putting a jumper on it.
Jaydee

My guess would be ignition timing.

Trispark's are pretty easy to set up, but definitely not fool proof. You definitely want to make sure that you are coming up on TDC (easily checked with a screw driver) and use the associated timing mark on the rotor. Mark it with a red sharpy for consistent reference.

When I installed mine, I made the discovery that my cam chain was woefully out of adjustment, and had to get it back into spec before my bike would run with reliable ignition timing.

With respect to your compression test, was the carb at WOT during this test? Fresh rebore? Shoot a bit of oil into the cylinders? Valve clearances are good? 90 psi is kinda low. I would have expected something above 120 psi on a Mk 3.

Either timing or open/leaking valve on compression.
I purposely marked my degree wheel in RED on the ATDC side. A simple reminder to myself - "wrong side dummy" - for when I get discombobulated in my thinking regarding TDC.
I put a green mark on the BTDC side as well.

Lineslinger said:

Either timing or open/leaking valve on compression.
I purposely marked my degree wheel in RED on the ATDC side. A simple reminder to myself - "wrong side dummy" - for when I get discombobulated in my thinking regarding TDC.
I put a green mark on the BTDC side as well.

Click to expand...

I put YELLOW paint marker on the one I WANT to use...... lol

Thanks everyone for your suggestions. Many of you were spot on with your assessments, it was (at least largely) a timing issue. I don't know what happened...I thought I had static timed it correctly but when I pulled it apart to double check it was about 90 degrees off . She fired right up once that was fixed. Thank you again for all your suggestions.

Its currently idling quite high (even higher with the choke off) but I'm hoping that one more tear down and cleaning of the Mikuni will help. Currently the air and idle screws don't seem to do a whole lot and I'm getting a little fuel in the carb's lower overflow tube. Hopefully there's just a little dirt in there that's facilitating too much fuel.

The lower PSI is because while I rebuilt the electrical, gearbox, brakes, switch clusters, fork, wheels, primary, etc, I haven't yet done any major work to the motor itself. That was the most intimidating part of the refurbish (I don't really have any experience rebuilding motors) and I wanted to get the bike to a running baseline before I tore down the motor. But hey its helped me learn my bike in a digestible manner, and plenty more to do but after 20+ years the bike is once again alive. It's a great feeling!

Anthony

Mikunis, like all modern carbs really, are super sensitive to dirt in the fuel. I mean dirt that’s so fine you can’t see it, and so fine it’ll pass through the ’strainers’ fitted to most fuel taps.

In line filters are the answer.

"Its currently idling quite high (even higher with the choke off) but I'm hoping that one more tear down and cleaning of the Mikuni will help. Currently the air and idle screws don't seem to do a whole lot and I'm getting a little fuel in the carb's lower overflow tube. Hopefully there's just a little dirt in there that's facilitating too much fuel."

Those symptoms are indicative of a clogged idle circuit in the Mikuni carb, and it's actually running on the needle jet. Improper storage (deposits from evaporated fuel) is the leading cause.

Anvill said:

I don't know what happened...I thought I had static timed it correctly but when I pulled it apart to double check it was about 90 degrees off .

Click to expand...

If it was the Tri-Spark rotor "90 degrees off" then it had been timed to the wrong (180 degrees out) alternator rotor mark.

Fast Eddie said:

Mikunis, like all modern carbs really, are super sensitive to dirt in the fuel. I mean dirt that’s so fine you can’t see it, and so fine it’ll pass through the ’strainers’ fitted to most fuel taps.

In line filters are the answer.

Click to expand...

Unless a person rides mostly on THE gravel, that "dirt" is usually iron oxide particulate from inside the tank. Those fines are usually settled in the bowl or drain plug of the carb. I run the see-through fine nylon screen inline filters, and never is there anything to see. My point is: that fine "dust" generally won't clog an idle circuit. The most common ailment I've seen is "varnish deposits" back when we had real gas, and now corrosion left behind when corn-gas evaporates while the bike is unused for a while.
JMWO


Insist on GENUINE Visu-Filter

Agreed, the ‘dust‘ wont clog the idle circuit, what it can so, and has for me, is prevent the float needle from seating properly.

concours said:

"Its currently idling quite high (even higher with the choke off) but I'm hoping that one more tear down and cleaning of the Mikuni will help. Currently the air and idle screws don't seem to do a whole lot and I'm getting a little fuel in the carb's lower overflow tube. Hopefully there's just a little dirt in there that's facilitating too much fuel."

Those symptoms are indicative of a clogged idle circuit in the Mikuni carb, and it's actually running on the needle jet. Improper storage (deposits from evaporated fuel) is the leading cause.

Click to expand...

Throttle stop doing little makes me think that the slide i not being allowed down by the cable. If the slide is too high, then the pilot circuit is not fully involved so the air screw won't matter much. You need to check the fuel height in the bowl as well. Doesn't sound like a clogged idle circuit to me.

Fast Eddie said:

Agreed, the ‘dust‘ wont clog the idle circuit, what it can so, and has for me, is prevent the float needle from seating properly.

Click to expand...

My experience has been the brass seat has corrosion/varnish deposits, (that keep the needle from sealing) to cure I use Mother's Mag (fine abrasive metal polish on the end of a wooden toothpick, or wooden match. A small pencil eraser ( like from a mechanical pencil) sanded to a point, will do the trick also to polish that seat.