90% of Carby Issues Are... Clutch???

Lately, the old girl has picked up the nasty habit of crapping-out at the end of my favorite morning commute side-road blast. Assumed ignition-related as one cylinder would drop out, but would clear out after returning to a "normal" speed (65 mph). "Assumed" ignition, since we're running a single Mikuni, which shouldn't cause only one cylinder to die. However, switching the petcocks to "Reserve" seemed to help.
Then, the dropped cylinder transitioned to an annoying off-on-off-on as if the kill switch was being repeatedly pushed and released. Again, it would clear up after 15-20 seconds of steady cruise. Pulled the fuel tank. Pushed and pulled all the associated wiring and connectors; checked the points; messed around with the ballast, hoping to get it to fail; tugged on the points lead-in wire while the engine idled, hoping to have it die, to no avail.
A couple days later, we we're even able to maintain a constant speed, forcing us of the side of the road. Gave it a quick once-over; nothing out of the ordinary. No dripping fuel, plug wires tight on both ends. Fired up, and headed on into work.
The final straw was having the engine run so rough that it wouldn't idle at the stop-light near work, then died while on the last stretch. I had to push her into work (yes, I had an audience).

The trip home:


Pulled the float bowl off to find it almost empty. Hm-m-m... Pulling the float arm revealed a pretty healthy dimple where it contacts the spring-loaded, non-Viton-tipped needle. That, and the seat's through-hole looked to be awfully small. Guess what? I'm running a used carb that appears to have been originally off of a two-stroke, meaning snowmobile, meaning fuel pump, meaning a 1.5mm hole. In other words, about the same size as the main jet (240/1.4mm). Let me guess; gravity won't feed fuel as fast a a fuel pump. Ya think? Digging through the Mikuni manual, pressure-fed carbs use 1.5mm seat, while a gravity-fed carb runs 3.0mm.

Seat next to the main jet:


After "massaging":


As for why it wasn't refilling the float bowl, I can only surmise it was from the notch that has since been smoothed out. This last weekend's ride seemed to prove that out, as she never skipped a beat over the 225 mile ride.

Now, (if you're still awake) you're probably wondering what the hell this has to do with the clutch? Up until swapping over to the Barnett clutch plates clutch-slippage-part-enter-your-number-here-t22376.html, I've never been able to stay on the loud pedal long enough for this to happen. 'Funny how one thing can lead to another. What next?

Nathan

Thanks for expanding our awareness of mystery issues at your expense. Last pm finally got SuVee going and took 100 mile test run and swore not to go very fast but decided I would repeatedly test my brake bravery to have it sputter dead to road side wondering how this would work out. I had spendtcouple wks on mystery electric issue killing starter and ignition but nothing else. Turned out a small hidden fuse no one every pointed at on days of online searching, just happened to flick at a box box on loom end over battery to discover a hollow sound, hum. I had SuVee 15 yr and never knew about these fuses only the main power one. I discovered I had hit the kill toggle playing with different brake grips, whew. - cheap cell phone got bounced off on ride to visit my buddy. Now what will be our next show stopper stump-er?

One thing I always do on any new bike is check fuel flow...detach the fuel lines from the petcock and turn it on to see how good the fuel flow is.

Second, Mikuni makes a few different float needle/seat combos and I usually install the biggest.

I had that dimple occur in my float loooong ago. A thin (0.003" or so) brass shim bonded onto the float has been the cure for 40 or so years. For reference, The standard orifice in the seat for an Atlas is 0.125".

Slick

I had a similar situation happen to me at the INOA rally. I would have sworn on a stack of bibles that it was the carbs but it turned out to be one bad coil which ran but didn't until it finally crapped out. boy I was glad that it finally did as I was chasing a tail. When you think its carbs....think electrical. When everything looks OK on the carbs, then go to the electrical side. I had my Amals apart 3 times trying to figure out a problem that they didn't have.
Cheers,
Thomas
CNN

seattle##gs said:

One thing I always do on any new bike is check fuel flow... Mikuni makes a few different float needle/seat combos and I usually install the biggest.

Click to expand...

Well, early on, I did pull the main jet access plug and let it run and it seemed like a lot of fuel ran out. Now, with the 3.00, it pours!
As originally posted, this didn't show up for a long time, due to the slipping clutch. I can only assume the original owner of the carb never let it run out for an extended period of time. That, or their clutch was as bad as mine! I falsely assumed the needle and seat were up to the job, as this came off of a running 850.

texasSlick said:

I had that dimple occur in my float loooong ago. A thin (0.003" or so) brass shim bonded onto the float has been the cure for 40 or so years.
Slick

Click to expand...

'Turns out that it wasn't so much an issue with the notch in the arm. Instead, what I was feeling was the spring-loaded needle assembly starting to come apart. Most of the time it would work normally and set the float height just so, sometimes it would hyper-extend. This caused the spring to randomly push against the float arm long after the floats had dropped to their proper running position, essentially keeping the needle shoved up in the seat much too long, running the bowl dry. To paraphrase a line from "I, Robot", the Norton might say "My vibration is undeniable!"

ugh it takes a determined mechanics village to raise a Commando & over whelms my mechanics mind to keep all these issues in mind. Peel weird one mimicking this current post, intermittent short but good then bad to dead, popped cap in shed in case of vent blockage + thots of lighted match - sloshed it noticing a dark thing > big moth floating free till settled over fuel drains, as one wing separated from body+wing. Classic innocent prank is small rag in a tank, classic nasty trick, sweeten well with sugar. One other just in case, Peel gradually got more and more intermitent mis fires with ritual endless exams no joy til it began mis firing on each bump across pature and died. Past any logical mature mechanic wits point - I thot about Sherlock homes eliminating the possibles so thought what is the most insane stupid place I could fart with that did not require much risk or tools, tail light, so opened in instead of tank with match... to see earth bare braid strap had frayed and touching power on bulb holder shorting enough not to blow fuse just bayah brains.

Trixie caught me and Wes out once with everything checking when assembled, fire then dead, power got past glass fuse with intack blade and nice tight ends and fit in holder but cig foil test got us spark - tested fuse to find it a dud internally somehow. Used spent .22 LR case in pocket and some wire coil to finish ride and later replace w modern plastic blade fuse.

Peel once stumped half a dozen world famous Nortonneers with six things all causing the same thing, but 4 slugs each of Arkansas KickaPoo juice each one ..> in turn found a fault like magic till acres and acres of horns and rev ups and shouting bad things ~ 11 pm shut open header Peel down with lasting bad hobot annoyance reputation i was just a prepexed finally comfortable numb bystander. sheeze. The last fault was a plug fouling so only found a old non R plug which turned Peel LED signals and brake into timing light strobes while blipping at camp sites dodging thrown up finger signs and items getting back to mine. Peel and me more welcome I found out at Harley only crazy parties, ugh no sleeping around them.

I and a few fighting enemies now fast friends have left a few forums during Peels mysterious reports and hand holding help d/t nasty remarks about being able to do the correct normal diganostics and corrections instead of just scratching asses on what is being missed no one else had or can imagine.