It's running. Starts easy. Lousy Idle.

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Starts easy. Just got a new Power Arc installed. Same puff out of both peashooters. Doesn't burn oil. Valves set and checked. But closing the air screw down all the way gets increased rpm. And after I ride around for a while ( about 20 minutes) on a hot day idle may drop and I need to blip the throttle at a stop sign. Then the rpms shoot up and slowly come back down. 1st, what's the best spray to detect a leak. Carb cleaner? Ether?
 
Starts easy. Just got a new Power Arc installed. Same puff out of both peashooters. Doesn't burn oil. Valves set and checked. But closing the air screw down all the way gets increased rpm. And after I ride around for a while ( about 20 minutes) on a hot day idle may drop and I need to blip the throttle at a stop sign. Then the rpms shoot up and slowly come back down. 1st, what's the best spray to detect a leak. Carb cleaner? Ether?


If it's running right at 1/8 throttle or more, I wouldn't start by looking for an air leak. I would figure out what's wrong with the idle circuit. The air screws all the way in is very wrong - the engine should die long before you turn them all the way in. Low fuel levels in the bowls can cause trouble in many ways so I make sure they are right before anything else. Generally, an Amal carb in good shape with a completely clean idle circuit will need the air screws out 1-1/4 to 1-1/2 turn.

If they are standard carbs, you need to clean the hidden jet AND the passageway towards the rear of the carb from the hidden jet. If Premiers, then with the pilot and air screws you can clean straight across and then the right angle towards the rear of the carb and then the pilot jet itself.

If you don't have the insulators between the intake manifolds and head, especially if the engine is running overly hot, then you may be boiling the gas on a hot day causing the idle to change.
 
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More or less spray will do, for inlet joint leak detection, though ether may not stay wet long enough to block the gaps.

Poke a 16 thou wire or drill bit through the pilot jet bush, if it’s not a Premier carburettor.
 
I thought I had said I had a Mikuni VM34 with 30 pilot jet. But I didn't. Many apologies to both. I wrote about the bike, sorting this and that out so much before that I forgot the forum "talks" to different people all the time. But all the basic other things remain. Somewhere else here I'd read about "o" rings and I thought they were talking about on the pilot jet air screw and on the idle speed screw. Both mine feel sloppy loose even with the small tensioning springs in place, though I'm not familiar with it enough to know what it would feel like if it were new or right. If I read it correctly and there are o-rings that come into play here, do they stay on the screws if you take the adjustment screws all the way out? If that's a thing to check I'll look in the morning. But your replies did have good info about causes and what to be aware of, Amals or Mikuni. Thanks.
 
Be careful with ether when looking for air/vacuum leaks as it is very volatile and stays in the air for too long for accurate results as it can be pulled into the air filter. Carb cleaner works good but you need to be aware of any overspray can damage paint and plastics and some rubber.
What I find works good is propane, put a rubber hose on a small propane bottle and turn the valve on a little, then just put the hose where you suspect an air leak. You can follow gaskets and hoses along with the hose from the propane, If there is a vacuum leak your RPM's will go up. Harmless to paint .
 
Be careful with ether when looking for air/vacuum leaks as it is very volatile and stays in the air for too long for accurate results as it can be pulled into the air filter. Carb cleaner works good but you need to be aware of any overspray can damage paint and plastics and some rubber.
What I find works good is propane, put a rubber hose on a small propane bottle and turn the valve on a little, then just put the hose where you suspect an air leak. You can follow gaskets and hoses along with the hose from the propane, If there is a vacuum leak your RPM's will go up. Harmless to paint .
Using propane as you suggest sounds like a very good idea. Will try it today.
 
What size pilot jet is in it? Just read your post again and saw it's 30, if the screw is close to all the way in you may need a bigger pilot jet.
 
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Hi Joe, I also run a 34 Mikuni but I would switch to a 35 pilot jet from the 30 you have now.
I suspect you are too lean on idle circuit with the 30.
Then set both air screw adjustments around one and one quarter to one half turns out.
You want to set the air screws with the motor warm.
Also, check your plugs and replace with new if they look sooty and fouled.
If your ignition requires resister plugs then do you have them or resister plug wires?
 
I'll first look for air leaks, then try a 35 pilot. I bought the whole Power Arc package from Old Brits so I've got right wires, plugs, etc.
 
Until I get a #35 pilot jet (next week until it comes) I put the #40 back in to see what the effect was. Easy to change at least. It did tune a little better. Screwing the air adjustment screw for the pilot jet all the way in caused the engine to lose rpm. Unlike with the #30 pilot where the rpm's just increased slightly when I screwed it all the way in. But still the tuning was very sloppy. I would turn the air screw by half turns. There might be a slight change in rpm. But the rpm's were difficult to get settled down. And I noticed the rpms were susceptible to the position of the handle bars-- turn to the right and the rpms could increase a lot. I know I have to sort that out as well.
 
Loosen your cable adjustment. You'll never get the carbs right until you do.
I'm sure you're right about the cable. Toward the end of my fiddling around this afternoon, I noticed that there was absolutely no slack at all in the throttle cable. And when I tride to adjust the idle rpm screw, the rpm would only go down to 1000 or so. And no matter how much I backed off the screw that's where the rpm stayed. I'll work that issue first thing when I get to the bike tomorrow.
 
Loosen your cable adjustment. You'll never get the carbs right until you do.
That was it. With a little slack in the line, no matter how the handlebars are turned there is no effect on the rpm. When I close the throttle I can hear a tiny clunk of the slide dropping against the stop. And I've got the idle at 1000. Not much happens when closing down the air adjustment screw, just a fractional change in rpm until a certain point. Then a quarter turn more and the rpms begin to noticeably drop. I back off that, opening it up the quarter turn that caused the drop and an additional quarter to half turn more.

It now idles steady at 1000. No problems when I roll off the throttle. But it does sound like a real bucket of bolts sitting there idleing so smoothly and so sweetly

One last thing (how many times have I said that?). Looking at the key from the front, one click clockwise turns it on and one more click turns on the lights. And also turns off the ignition! One more thing to sort. The weather cooled down and I wore a jacket today. It's taken a while to get this thing on the road, but the longer it goes (now it's a heavy jacket) the safer I am. Until it snows.
 
Another "one last thing". Here's a question for Power Arc users. Sometimes it actually starts on first kick. And sometimes on first kick it stops my swing near the top and then on second swing it might kick back really hard. Might I have it a touch too advanced? I was very careful with making sure the light just next to the optical sensor (in my case, hard to see without bending way down to look just below the sensor module) came on at top dead center. Method I used was to first lightly tighten down the timing module in the middle of it's limited range of rotation. Then fully tighten the timing disk bolt, getting the notch as close to directly under the optical sensor housing I could without worrying about getting it exactly correct. Then, making sure of tdc, loosening and rotating the module to get it just right -- light coming on -- and tightening the module down again. Is this kick back due to not giving the sensor wheel a full rotation in the correct direction to get the correct count between firings? Could that be caused by lack of sufficient swing in the kick? I don't remember a large backward rotation on the first kick that would have increased the count and caused it to fire very advanced though that certainly would cause a kick-back. Also noticed often on first kick there is an "almost" kick-back where, after the slightest downward movement in the kick, my kick is stopped dead as if it has fired very early in the downward swing rotation. It often fires up next kick, but just as often kicks back violently at the bottom of that second swing. By the way, that's when my foot sliped off the kick-starter yesterday and the kick-starter banged back up. I got a good whack in the shin. That, I really want to prevent happening again. Is there, perhaps, a 'slight' backwards rotation where I percieve a dead stop in rotation that increases the count and is enough to cause kick-back? I have thought to kick it through with the key off, but if it's all turned off, what good would that do to the count? Also wondered if there is still power to the timing module if the red ignition cut-off button is pushed? Must say that once running it is now running very well. And would it do any good to install a cutout swich for power to the coil only so I could get a full rotation? Anyone else experiencing this -- or am I doing something wrong?
 
Not familiar with the Powerarc but are you supposed to check it with a strobe? Can't believe an ignition system would rely strickly on static timing. Those symptoms sure sound like it is over-advanced. How accurate is your detemination of TDC. Are you using a piston stop and degree wheel?
 
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