Sorting out possible pilot jet problems

L.A.B. said:

More likely a too large enrichment jet (a fixed jet in the float bowl) as the subject of the Mikuni starting circuit being very rich has been mentioned before.

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Can the enrichment jet be changed -- or, does "fixed" mean unchangeable? I've tried setting that lever part way, but that's not easy as it wont stay and not very repeatable. Maybe enrichment circuit on and throttle cracked open a bit is the answer.

Joe Schlaberdowski said:

Can the enrichment jet be changed -- or, does "fixed" mean unchangeable?

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The starter jet appears to be a press-fit in the bowl so not easily changed although I seem to remember somebody mentioned modifying it somehow?





Previous Mikuni hard to cold start thread

Joe Schlaberdowski said:

This is a reply to all actually. A question. I have probably tried every combination of throttle cracked, closed, enrichment circuit in, enrichment circuit off, that can be tried. With no seeming difference. What makes a difference (and only after kicking the hell out of it) is removing the plugs (now sooty and wet) and brushing them off. Then it starts right up. The plugs are dry and not sooty when it has been running and then shut off (of course). I will check results when shutting off, say overnight. And then after about two kicks. But an earlier question remains: could a wrong size pilot jet have this effect?

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I am running Mikunis on another bike (not my Norton) but have seen hard starting and wet plugs when starting with the enrichener circuit on. The way around this for my engine is before switching the ignition on, turn on enricheners and pump the kick starter against compression on each cylinder to create a bit of turbulence in the carbs. Switch off enricheners, and then start normally. The turbulence seems to introduce a temporary rich condition similar to tickling Amal carbs.

Another thing to check is your ignition system. In my experience of running worn out carbs, weak magnetos and points (Kettering) ignition systems is that corroded or just plan old connections (including worn or corroded ignition switch and/or kill switch) in the primary ignition circuit can introduce unwanted resistance. The result is a weaker spark which may not be able to overcome the gap of a wet or slightly fouled plug. An old battery can introduce its own internal resistance as well.

~998cc

Triton Thrasher said:

My theory is that it means the cold start jet is too big.

But- on my Amal Mk 1.5s, I found that a cold start jet that allowed easy starting by proper technique with closed throttle on a very cold morning was then too rich on a slightly cool morning.

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One size fits all Mikuni, with a crack in the magnificent facade. And here we are trying to fix what Amal and all the others fixed years ago. I haven't tried it yet, but opening (actually screwing in) the idle adjustment screw (not the pilot screw) on the Mikuni makes sense. The throttle is never completey closed, else how could it idle at 900 or 1000 rpm with hand throttle closed. So adjusting that slide height adjustment screw on the side of the Mikuni is a very precise way of adding air according to temperature conditions you may be facing. The hand throttle may make way too large and imprecise an adjustment for percieved hand motion. I'm going to try it at different settings and if that's a go, I'll experiment with different settings for different temperature conditions. I'll report as this proceeds. One thought is setting the idle for 900 (not 1000 rpm as I stated in my opening), as I did, might have resulted in an over rich condition with the starting circuit. So the starting jet may not be at fault, but where we have set the slide for our idle speed. I may have a part of this wrong so feel free to criticize and correct.

I have run a single vm34 on my ‘72 for more than 20yrs .... the enrichment circuit is either on or off no adjustment provision .... never ever a cold start issue .... “check your method“ .... for me , taps on enrichment on , key off , 2 complete kicks, then enrichment off , key on ,throttle touched as you give kick lever a good swing and fire every time .... couple of light twists on throttle to clear and idles nicely while I finish gearing up .... sometimes it is simple stuff not really very complicated .... I have idle set at just above 1000rpm .... if temps down to single digits same method only no throttle but leave enrichment on and turn key on for 3rd kick , then turn lever off as always fires , just doesn’t happen often as shed is heated ....for warm start, tap on , key on , touch throttle ,first kick and away ....

Joe Schlaberdowski said:

This is a reply to all actually. A question. I have probably tried every combination of throttle cracked, closed, enrichment circuit in, enrichment circuit off, that can be tried. With no seeming difference. What makes a difference (and only after kicking the hell out of it) is removing the plugs (now sooty and wet) and brushing them off. Then it starts right up. The plugs are dry and not sooty when it has been running and then shut off (of course). I will check results when shutting off, say overnight. And then after about two kicks. But an earlier question remains: could a wrong size pilot jet have this effect?

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As a quick “acid test”, before ATTEMPTING to start, spin in some NEW plugs.
Fires right up? Start looking for your fouling problem. (Oil? Fuel?)

Craig said:

I have run a single vm34 on my ‘72 for more than 20yrs .... the enrichment circuit is either on or off no adjustment provision .... never ever a cold start issue .... “check your method“ .... for me , taps on enrichment on , key off , 2 complete kicks, then enrichment off , key on ,throttle touched as you give kick lever a good swing and fire every time .... couple of light twists on throttle to clear and idles nicely while I finish gearing up .... sometimes it is simple stuff not really very complicated .... I have idle set at just above 1000rpm .... if temps down to single digits same method only no throttle but leave enrichment on and turn key on for 3rd kick , then turn lever off as always fires , just doesn’t happen often as shed is heated ....for warm start, tap on , key on , touch throttle ,first kick and away ....

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Thinking about it. Since the throttle is never completely closed ( you do set the slide for idle rpm) then where, exactly does the throttle slide opening cut off the enrichment circuit? I don't think it "cuts" it off. As we open the throttle slide we make the fuel enrichment circuit less effective -- to the point there is no enrichment. Being able to precisely control this via the throttle slide screw may be the key to the problems many (including me) are having. Thinking of it this way: the air slide is never completely closed (if it was there would be NO idle) so air of a certain amount must be getting in. The enrichment circuit being open (lever down) does NOT cut off this amount of air. After all, you do need some air for it to fire. The enrichment circuit just adds more fuel as air is pulled in. And, like the regular venturi effect it must work on the vacuum caused by air velocity past an open tube. As the throttle slide is opened a bit with no increase in piston speed (after all, you can only kick so hard) the venturi effect is lessened and so the mixture leans out a bit. In the reverse, you would want to close the slide just a bit more if it's really cold out (I did go to work on a 350 Kawasaki through a Chicago winter back in 1969). But using the method I outlined that would make for an idle below 900. That gets a little dicey because if that gets it running, you might have to open the throttle to keep it running.

Joe Schlaberdowski said:

The enrichment circuit being open (lever down) does NOT cut off this amount of air. After all, you do need some air for it to fire. The enrichment circuit just adds more fuel as air is pulled in.

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Not exactly, as the enrichment circuit has its own air supply (see my previous diagram, white arrows = Air, grey = Fuel, black = Mixture).

L.A.B. said:

Not exactly, as the enrichment circuit has its own air supply (see my previous diagram).

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But does this enrichment circuit cut off the air supply past the slightly open slide? Another question is does the open enrichment circuit shut off the fuel supply to the rest of the carb, or does that still operate? And then why does "cracking" the throttle negate the enrichment effects of the enrichment circuit? Say the air flow past the enrichment circuit venturi does not change as the throtyle slide is raised. Still then, the total air does change and precisely raising or lowering the slide would work to lean or enrichen the mixture. Anyway, I'll experiment with this and report the results.

Joe Schlaberdowski said:

But does this enrichment circuit cut off the air supply past the slightly open slide? Another question is does the open enrichment circuit shut off the fuel supply to the rest of the carb, or does that still operate? And then why does "cracking" the throttle negate the enrichment effects of the enrichment circuit? Say the air flow past the enrichment circuit venturi does not change as the throtyle slide is raised. Still then, the total air does change and precisely raising or lowering the slide would work to lean or enrichen the mixture. Anyway, I'll experiment with this and report the results.

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Meant to say above the air flow speed past the standard jets does change and for the low rpm of a foot driven spin, the velocity past the venturis changes and that mixture gets richer or leaner accordingly. One way or the other I think precisely opening or closing the throttle slide is a good way to modify a too rich mixture on the enrichment circuit.

Joe Schlaberdowski said:

But does this enrichment circuit cut off the air supply past the slightly open slide?

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No, but I'd guess it probably reduces its effectiveness due to the larger area of the enrichment air intake.

Joe Schlaberdowski said:

Another question is does the open enrichment circuit shut off the fuel supply to the rest of the carb,

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No.

Joe Schlaberdowski said:

or does that still operate?

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Yes, but when bypassed by the enrichment circuit then its effectiveness is probably reduced.

Joe Schlaberdowski said:

And then why does "cracking" the throttle negate the enrichment effects of the enrichment circuit?

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If the throttle is opened then more air will be drawn in through the main carb than through the enrichment circuit which almost certainly weakens the mixture.

I believe the enriching circuit is actually a very clumsy thing in comparison to all the other jetting. It is ON or OFF, no in-between. It is designed to deliver a LARGE amount of fuel just for cold starting and then a way to quickly cut it out of the circuit once the engine can sustain itself. The jet looks like a #400 main jet and will drown the plugs very quickly. When you start your car in the morning (when they were carbureted) it automatically goes on "FAST IDLE"
and choke, until warmed up. So I manually put the bike on "FAST IDLE" by cranking in the slide adj screw a couple of WRIST TURNS. As mentioned elsewhere in this thread, do this then kick the motor over a few times KEY OFF. Then KEY ON, it should start right away. What is very useful is while it is on fast idle and warming up you can be putting on your jacket, helmet and whatever. Just keep turning it down as it warms up. This is not a "PITA"...learn to use the throttle adj screw to your advantage. It only takes a second. Warning...this is almost impossible to do with dual carbs as you will quickly screw up your low speed carburetion. By the way...shut off the enricher AS SOON AS POSSIBLE.

L.A.B. said:

No, but I'd guess it probably reduces its effectiveness due to the larger area of the enrichment air intake.



No.



Yes, but when bypassed by the enrichment circuit then its effectiveness is probably reduced.



If the throttle is opened then more air will be drawn in through the main carb than through the enrichment circuit which almost certainly weakens the mixture.

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Exactly!

Just about every car carburettor has a: 'fast idle' cam built into the choke system that will open the throttle slightly during choke operation, no?
Had a Mikuni for a week but couldn't live with the on/off nature of the 'enrichment' device. Maybe fine 'on' for the initial start, and 'off' for normal running when warmed up, but a PITA in the grey areas between if ridden immediately. That was my experience, anyway....

seattle##gs said:

I use Mikunis and like them very much. I have noticed that when the motor has cooled a bit it becomes harder to start. What works is to give the idle adjuster one or two turns of the wrist and then it usually starts right up. Why this works when turning the throttle doesn't is one of those mysteries. Then turn it back to the idle you like. I have found that most people are very reluctant to turn this screw, I do it many times a day and consider it another one of the good features of a single carburetor.

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I think this is the key to it all!

baz said:

What does it indicate if it does start better?
I only ask as my t160 with mikunis will only start with full enrichener and a bit of throttle
If you don't open the throttle it will not fire at all

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I think you are on the right track. Read my later posts on this.

seattle##gs said:

I use Mikunis and like them very much. I have noticed that when the motor has cooled a bit it becomes harder to start. What works is to give the idle adjuster one or two turns of the wrist and then it usually starts right up. Why this works when turning the throttle doesn't is one of those mysteries. Then turn it back to the idle you like. I have found that most people are very reluctant to turn this screw, I do it many times a day and consider it another one of the good features of a single carburetor.

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Yes. It's the key

Triton Thrasher said:

My theory is that it means the cold start jet is too big.

But- on my Amal Mk 1.5s, I found that a cold start jet that allowed easy starting by proper technique with closed throttle on a very cold morning was then too rich on a slightly cool morning.

Click to expand...

I believe this is a key element to it all.

Of course spark plugs and a good electrical system are critical to starting. If it won't start quickly always check spark plugs, if they are fouled throw them out. No matter how well you clean them they will never work as well.

EstuaryBoy said:

Just about every car carburettor has a: 'fast idle' cam built into the choke system that will open the throttle slightly during choke operation, no?
Had a Mikuni for a week but couldn't live with the on/off nature of the 'enrichment' device. Maybe fine 'on' for the initial start, and 'off' for normal running when warmed up, but a PITA in the grey areas between if ridden immediately. That was my experience, anyway....

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Those cars also had accelerator pumps.

I can’t understand why this has to be discussed as if it rocket science .... many years ago it took me a couple or three go rounds when I initially got bike running (Mikuni came on it ) , once electrics and timing sorted and I realized what routine worked it has been an absolutely easy start bike .... think if OP takes a deep breath forgets all this theory and locks self in his shed until he figures out the starting routine for his particular bike he will be much happier .... if electrics, timing , carb all good , the ritual is there waiting for him to discover , have driven lots of old vehicles over the years that all demanded a different ritual to fire on a cold morning ..... spend some time and learn to start your bike .... honestly I just barely understand how the Mik functions , even I managed to learn what made my bike fire ..... no offence meant ....