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Discussion Starter #1
I have a 2013 V Star 650. About 4 months old. I put Roadburner Exhaust, K&N Filter and used a dynojet kit main jet and jet needle. I turned the screw out 3 turns. I carb synced and adjusted the idle. AIS is removed.

The engine sounds good, but I must be running rich. I get loud decel pop when I downshift or roll off the throttle. When I cold start with the choke pulled all the way out it doesn't start easily.

I live in Charleston, SC so the air is not thin.

The whole reason I broke down and bought a Dynojet kit is because the nearest dealers didn't stock Mikuni jets and Roadburner recommended it. Though I think Roadburner just recommends Dynojet so if you have problems you call Dynojet customer service to complain instead of Roadburner customer service.

Should I be able to lean it out with just the air mixture screw, or should I ditch the dynojet parts and start trying different sized Mikuni Jets?

I guess I could also try the smaller sized main jet that came with the kit.

I definitely need to do something, because I want to get the smartpartz quiet baffles and if anything that will make it richer.
 

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Just a small word of caution.....if you're popping on decel...you are lean on the idle circuit, not rich.
 

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Discussion Starter #3
Just a small word of caution.....if you're popping on decel...you are lean on the idle circuit, not rich.
The Dynojet troubleshooting guide says for decel pop says to turn air screws out more. Which I believe would make it leaner.
 

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OK... I'm not sure what an "air screw" on a Mikuni carb is........ but if you mean pilot mixture screws...turning out=more fuel=richer mixture.

Directly from the Mikuni tuning guide...

"Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.

3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.

4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires."
 

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Discussion Starter #5
I'm talking about the screws you unplug on the bottom right side of the carbs. The side with the brass caps that you remove to adjust the needle.



I looked at the crude Dynojet installation guide and it actually calls them "fuel mixture screws." 650ccnd.com calls them "pre-mixture screws." Cruiser Customizing calls it the "air/fuel mixture screw" in the V Star 650 re-jet how to video. Other people are calling it "air mixture screw" online.

My confusion is that all the diagrams for Mikuni carbs I find are oriented a lot differently.
 

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Discussion Starter #6 (Edited)
When I cold started it today, it idled great with the choke pushed in, but the idle would get worse and worse as I pulled the choke out. All the way out, the engine would just stall.
 

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think the way it goes if the screws are in the front of the carb they are fuel mix. screws so turning them out would allow more fuel to go in thus enrichening. air screws are opposite turning them in decreases the amount of air thus enrichening the mix. although you may find in reading that the fuel screws are in the back and the air screws are in the front confusing no.
 

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Mikuni calls them "Pilot Mixture Screws".....and that is exactly what they are. The pilot jets control the fuel flow up to about 1/3 throttle and the PMS allows a small amount of fuel to bypass the slide so that you get fuel into the engine with the throttle closed. So basically they control the throttle closed fuel mixture through the pilot jets.



Opening them gives you a richer mixture at idle, closing them makes it leaner. If you are getting backfire through the exhaust on decel...you are too lean. I have mine dialed in to just barely backfire on occasion.....it just takes some riding and tuning to get it right. One of the best investments I've made is s pair of PMS thumbscrews from Metric Magic.....quick and easy PMS adjustment when you need it.

 

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Discussion Starter #9
Do you have a V Star 1100 or 650? Because all the aftermarket PMS screws I can find for Star Motorcycles are V Star 1100 and Road Star 1600 and 1700. Not for V Star 650.

I played around last night trying to get to the screws without removing the gas tank. No such luck. :mad: The rubber piece of the airbox joint that goes on the side of the airbox silencer is in the way. I took the tank off last night. I'll try adjusting it and putting the tank back on this evening. If I could find a comically short screwdriver maybe I could do it.

Mikuni calls them "Pilot Mixture Screws".....and that is exactly what they are. The pilot jets control the fuel flow up to about 1/3 throttle and the PMS allows a small amount of fuel to bypass the slide so that you get fuel into the engine with the throttle closed. So basically they control the throttle closed fuel mixture through the pilot jets.



Opening them gives you a richer mixture at idle, closing them makes it leaner. If you are getting backfire through the exhaust on decel...you are too lean. I have mine dialed in to just barely backfire on occasion.....it just takes some riding and tuning to get it right. One of the best investments I've made is s pair of PMS thumbscrews from Metric Magic.....quick and easy PMS adjustment when you need it.

 

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Discussion Starter #10
I just picked up some allen wrench style screwdrivers, so I won't have to take the whole gas tank off next time.
 

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Problem with those is you can only swing them so far. I have heard folks speak of a right angle ratcheting screwdriver but don't remember the source for it.
 

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Do you have a V Star 1100 or 650? Because all the aftermarket PMS screws I can find for Star Motorcycles are V Star 1100 and Road Star 1600 and 1700. Not for V Star 650.

I played around last night trying to get to the screws without removing the gas tank. No such luck. :mad: The rubber piece of the airbox joint that goes on the side of the airbox silencer is in the way. I took the tank off last night. I'll try adjusting it and putting the tank back on this evening. If I could find a comically short screwdriver maybe I could do it.
If I remember correctly.....the 650 and 1100 use the exact same thumbscrews from Metric Magic.
 

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Discussion Starter #13
Problem with those is you can only swing them so far. I have heard folks speak of a right angle ratcheting screwdriver but don't remember the source for it.
My other idea was to use one of those interchangeable screw bits.
 

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If I remember correctly.....the 650 and 1100 use the exact same thumbscrews from Metric Magic.
Verified....they use the same PMS Thumbscrews.
 

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Discussion Starter #15
I took the tank off and turned the PMS screw back in to 2.75.

Decel popping went way down, but the choke still indicated that the idle circuit was too rich. Took the tank off again and restored it to the factory 2.5 setting.

At that point the idle speed went way down. So I had to turn the idle screw in some. This definitely gave me a leaner idle circuit since I had now decreased gas flow and increased air flow.

I was just testing it. Decel popping is totally gone. It did cold start with the choke all the way out but idled super rough and got better as I pushed the idle in click by click.

Right now I am glueing smartpartz quiet baffles into the end caps. This should increase back pressure. I'll have to wait and see what it does to the idle circuit.

Other than problems with the idle circuit, I have a massive power increase. I used to never go over 20 in 1st gear because acceleration would get so slow. I just went 36 in 1st gear and engaged the rev limiter.

Acceleration is excellent all across the board.
 

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Discussion Starter #17 (Edited)
if you went from 2.5 to 2.75 you are turning the screw out making it richer,making it richer will cause the popping to go away.
what do you mean the choke indicated it was to rich?
No. I had originally put it at 3.0, as per Dynojet's advice. Then I went to 2.75, then all the way back to a factory 2.5. At 3.0 and 2.75 I could not even start the bike with the choke pulled out all the way. The popping when away when I turned the screw in and made the mix leaner.

According to the tuning guide on the Mikuni website, decel pop does not necessarily mean it is a lean mixture. It also says decel pop can be exacerbated by aftermarket pipes with a large exit hole. The Roadburner 2.5 I used may have the biggest exit hole of any aftermarket pipe for v star 650. The exit hole is about to get smaller though. I'm adding smartpartz quiet baffles. The silicone gasket maker is curing now. After I add the quiet baffles, there should be more back pressure, so I'm sure I'll have to change the settings more.
 

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Is it so cold where you live that you even need to use the choke?..... which by the way is really an "enrichener" not a choke ...and it just drops raw fuel into the intake....like turning on a garden hose. You may be just dumping a ton of fuel into the system for no reason.

Just because the bike won't start on a fairly warm day with the choke on 100% most likely will not have a lot to do with your AFR at idle and IMO is really not a valid test to determine whether or not you are too rich on that circuit. Testing the AFR is a valid test.....anything else is just a wild guess and is probably not going to be very accurate.

Yes...Mikuni does say that larger pipes can cause a backfire on decel...and that the cure for that is to enrichen the idle circuit slightly. So IMO.....the right way to tune it is to turn the PMS in until you just begin to get a back fire....then back out about 1/8 - 1/4 turn. That is how you find the happy medium for the idle circuit based on jetting, intake and exhaust set up.
 

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Discussion Starter #19
For me, reducing the gas on the idle circuit (returning the PMS screw back to the 2.5 stock setting) made the popping go away.

I understand why you say this is a lean condition, because a lean condition gives unexploded gas plus oxygen. The amount of oxygen falls as the mix gets richer. However, the tail portion of these pipes are so big, I think that outside air can just sit in them.

I simply used the choke as a diagnostic tool. Maybe that is not what it's for, but I just compared how it started to how it used to start before

One thing I would like to try is add slack to the choke cable just in case it isn't closing all the way. I had to remove the choke cable from the carbs to re-jet. It looks like when I pull out the choke one click, the plungers move slightly. Each click after that they move a larger amount. The plungers appear to be closing, but I'd like to make extra sure.

Do you know how to add slack (free play) to the choke cable.

Is it so cold where you live that you even need to use the choke?..... which by the way is really an "enrichener" not a choke ...and it just drops raw fuel into the intake....like turning on a garden hose. You may be just dumping a ton of fuel into the system for no reason.

Just because the bike won't start on a fairly warm day with the choke on 100% most likely will not have a lot to do with your AFR at idle and IMO is really not a valid test to determine whether or not you are too rich on that circuit. Testing the AFR is a valid test.....anything else is just a wild guess and is probably not going to be very accurate.

Yes...Mikuni does say that larger pipes can cause a backfire on decel...and that the cure for that is to enrichen the idle circuit slightly. So IMO.....the right way to tune it is to turn the PMS in until you just begin to get a back fire....then back out about 1/8 - 1/4 turn. That is how you find the happy medium for the idle circuit based on jetting, intake and exhaust set up.
 

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