Mikuni 45 idle problem
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Road Captain
Joined: Feb 2005
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Mikuni 45 idle problem
Short question. What slow/pilot jet comes in a Mikuni 45? Long question. Hears the basic build as far as fuel circuit would be affected. 04FXST with the old SE Preformance heads worked by Scott Hillside at about 10:2. S&S .570 gears, Man O War 4 3/8 Crank, full bore to 107 cubs with the Mikuni 45. Now the problem. The bike will lite off with stock Mikuni choke perfect. Nice warm up idle at 1500 no problems. Run it down the road half mile and take off the choke and sets into a nice steady 1200 idle with a classic worked HD lope. Run it down the road 2 or 3 miles and stop at a red-light where the problem starts. As soon as you pull the clutch it falls to a 1500 idle for about 5 seconds then drops to 1200 lope every time. Still no real problem but unusual. Now ride it till the bikes at full temperature and pull up to a stoplight. Bike falls to 1500 for a few seconds then drops to about 800 and stalls out. Try and speed up the idle and as long as you keep it at or above 1500 it will idle but of course that's back to choke speed. Try and adjust down to 1200ish and it falls on its face. Build has only 600 miles on it and I'm old. Long, slow brake in on my stuff so never hammered on it but all the other circuits seem great. No stumble, surg, ect. Plugs look good. Did notice carb bowel was touching front fin so loosened carb up and filed to both till cylinders have good gap. Tried adjusting air/fuel today after it got good and hot exhibiting the low RPM stall. Had to set idle at about 1500 to keep running but at base 2 turns out start it would fall on its face at 1 turn in. Really couldn't make any change up to 3.5 turns out from base. Initial thoughts are to try a smaller pilot jet. Any other suggestions? I'll put it on a Dyno North of 1K but that will be a spell b4 I have time. Also anyone have a reputable Dyno guy that know Mikuni carbs, live in S. Arkansas. Thanks for the read.
Tourer
Joined: May 2011
Posts: 469
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From: Alberta
It sounds like you may have a vacuum leak at the intake. It is always a good idea to change the intake gaskets out when you change or reinstall the carb on an older bike. Idle problems are almost always related to a vacuum leak as the carb will overcome that once off idle. A hanging idle is an indication of a lean condition.
I run a Mikuni 42 on my bike and the setup that comes with that carb is a 25 pilot, 160 main and 70 accelerator nozzle, 98 needle but I'm sure you could do a search and see what the 45 carb comes with. I took mine apart and checked and recorded all the pieces so I would know what I had for a start. Also you have to adjust the idle on a fully warmed up engine. If you adjust it before that you will have to readjust the idle to the 1000-1100 rpm range.
On my bike when I installed the Mikuni 42 on a totally stock bike, exhaust and intake, it was just a plug and play and the only adjustment I had to do was adjust the idle once the bike was fully warmed up. I get about 53 mpg ( Canadian Gallon ) if I stay around 60 mph and if I travel at 70 mph or into a wind, it drops to about 48 mpg ( Canadian Gallon). I've checked the front plug and it has a grey, not white colour so the jetting is pretty close for my altitude which is approx 3000 feet. It likely will be a bit rich in the heat of the summer.
So I guess after my babble, I'm saying that the stock jetting is likely more than fine, as these carbs are known for being jetted on the rich side right out of the box, I'd check for a vacuum leak first before I started playing with jets.
I run a Mikuni 42 on my bike and the setup that comes with that carb is a 25 pilot, 160 main and 70 accelerator nozzle, 98 needle but I'm sure you could do a search and see what the 45 carb comes with. I took mine apart and checked and recorded all the pieces so I would know what I had for a start. Also you have to adjust the idle on a fully warmed up engine. If you adjust it before that you will have to readjust the idle to the 1000-1100 rpm range.
On my bike when I installed the Mikuni 42 on a totally stock bike, exhaust and intake, it was just a plug and play and the only adjustment I had to do was adjust the idle once the bike was fully warmed up. I get about 53 mpg ( Canadian Gallon ) if I stay around 60 mph and if I travel at 70 mph or into a wind, it drops to about 48 mpg ( Canadian Gallon). I've checked the front plug and it has a grey, not white colour so the jetting is pretty close for my altitude which is approx 3000 feet. It likely will be a bit rich in the heat of the summer.
So I guess after my babble, I'm saying that the stock jetting is likely more than fine, as these carbs are known for being jetted on the rich side right out of the box, I'd check for a vacuum leak first before I started playing with jets.
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Road Captain
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Well I think I have found my problem. Temperatures hear in S. Arkansas are all ready in the low 90s. Went looking for leaks as suggested and noticed the bowel of carb was up against the fin of the front cylinder. Took a file to both cylinders to make sure I have plenty of clearance and tighten everything back down. Got engine good and hot and readjust the pilot jet. At 3.5 turns out its still not stumbling so ordered a smaller size. Sprayed WD 40 on both sides of both intake flanges and zero change in idle. New gaskets on install. However after gaping the bowel from the fin and readjusting it is running great. No more falling on its face when hot. I'm wondering if it was flashing the gas in the bowel from direct contact with the cylinder. Do find it strange needing a smaller pilot with my set up but it needs what it needs. At least no longer worried about it stalling out at intersections anymore!
Last edited by JLOCKHART29; May 16, 2023 at 11:45 PM.
Tourer
Joined: May 2011
Posts: 469
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From: Alberta
The contact with the cylinder would be stopping the carb from seating properly in the rubber intake gasket. I know on the 42's the stock setting for the idle air jet is 2 turns out, that is a factory setting and is found to be right in most cases. I run that setting in my bike. You might have to adjust this slightly in either direction to get a good idle, but the amount you mention seems like a lot.
On a Mikuni the idle mixture screw is adjusting the air, not the fuel. If the adjuster is on the front of the carb, it adjusts air flow and if it is close to the intake it adjusts fuel. So I'm trying to get my head around your situation, but if you are turning that screw out then you are adding more air to the mixture, so whatever pilot jet is in the carb is too big. Something isn't adding up.
You likely live a lot closer to sea level than I do, so I would think you'd be leaner if everything else was the same. The hotter temps also make things run a bit richer.
On a Mikuni the idle mixture screw is adjusting the air, not the fuel. If the adjuster is on the front of the carb, it adjusts air flow and if it is close to the intake it adjusts fuel. So I'm trying to get my head around your situation, but if you are turning that screw out then you are adding more air to the mixture, so whatever pilot jet is in the carb is too big. Something isn't adding up.
You likely live a lot closer to sea level than I do, so I would think you'd be leaner if everything else was the same. The hotter temps also make things run a bit richer.
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