Main Jet Causing Rough Idle?
Outstanding HDF Member
Joined: Oct 2009
Posts: 2,488
Likes: 30
From: Maryland near DC
Hey guys, was wondering if anyone could help with some some carb issues that I am having.
Once I hit about 60 mph in 4th gear my acceleration goes flat, and the same thing in 5th gear. I figured I needed to lower my main jet, as it seems to be flooding a bit at top end speeds.
what I hear is at wide open throttle its flat...you need more gas or timeing...maybe the slide is sticking or throttle not opening all the way
Right now the bike has a 200 main jet, 48 slow jet, and is shimmed to the third clip position from the bottom
Modification wise I have an aftermarket exhaust and airbox.
what is the exhaust????
I tried a 185 jet and a 190 main jet, as this is what was recommended to me with these mods in the Buell forums, and for some reason the bike will idle rough. Finally I put a 195 jet in and had some lean popping, but for the time being i advanced the timing to 3° past stock to fix lean conditions.
Anyone have any idea why the bike would idle rough and backfire through the carb after putting a smaller main jet on the bike? It doesn't make sense to me, since at idle the fuel should be moving through the slow jet, and then past the idle mixture screw.
once again, sounds like the slide is messed up and not closing all the way,or the needle is too high or the taper is wrong
The 195 main jet seems to alleviate my top end flatness issues a bit. I would like to run a 185 or 190 jet, as they feel very comfortable at 3/4 to WOT, but the bike keeps dying at idle.
check your throttle cables for proper adjustment
I found these two websites, which seems to conflict with that I am seeing
Carb Chart
Carb Exploded View
Anybody have any ideas? I posted in another forum but they seemed to be stumped too.
Once I hit about 60 mph in 4th gear my acceleration goes flat, and the same thing in 5th gear. I figured I needed to lower my main jet, as it seems to be flooding a bit at top end speeds.
what I hear is at wide open throttle its flat...you need more gas or timeing...maybe the slide is sticking or throttle not opening all the way
Right now the bike has a 200 main jet, 48 slow jet, and is shimmed to the third clip position from the bottom
Modification wise I have an aftermarket exhaust and airbox.
what is the exhaust????
I tried a 185 jet and a 190 main jet, as this is what was recommended to me with these mods in the Buell forums, and for some reason the bike will idle rough. Finally I put a 195 jet in and had some lean popping, but for the time being i advanced the timing to 3° past stock to fix lean conditions.
Anyone have any idea why the bike would idle rough and backfire through the carb after putting a smaller main jet on the bike? It doesn't make sense to me, since at idle the fuel should be moving through the slow jet, and then past the idle mixture screw.
once again, sounds like the slide is messed up and not closing all the way,or the needle is too high or the taper is wrong
The 195 main jet seems to alleviate my top end flatness issues a bit. I would like to run a 185 or 190 jet, as they feel very comfortable at 3/4 to WOT, but the bike keeps dying at idle.
check your throttle cables for proper adjustment
I found these two websites, which seems to conflict with that I am seeing
Carb Chart
Carb Exploded View
Anybody have any ideas? I posted in another forum but they seemed to be stumped too.
Club Member
Joined: Feb 2007
Posts: 16,320
Likes: 306
From: Boynton Beach, FloriDuh
1 - I'm not sure what kind of carb SteveO is using and referring to, but your bike should have a CV carb which does NOT have a Pilot Air Jet that's removable or adjustable.
2 - Dynojet kits suck IMO. I'd start by putting the stock parts back in the carb, then install a #45 pilot jet and a 185 main jet. You should increase the pilot to a #48 only if the idle mixture adjustment requires you to turn the mixture screw out more than 3 turns. There's no way your motor can handle or need a 200 main jet unless you've done stage 3 with headwork and cams.
3 - The main jet does very little until you reach about 3/4 throttle. Based on you description, your problem is with the pilot jet and/or the idle mixture screw setting.
4 - Make sure you have a good seal between the carb and intake manifold. The manifold gaskets are known to develop a split in them that's hard to see unless you take it off and inspect it closely. A vacuum leak here will screw up everything.
2 - Dynojet kits suck IMO. I'd start by putting the stock parts back in the carb, then install a #45 pilot jet and a 185 main jet. You should increase the pilot to a #48 only if the idle mixture adjustment requires you to turn the mixture screw out more than 3 turns. There's no way your motor can handle or need a 200 main jet unless you've done stage 3 with headwork and cams.
3 - The main jet does very little until you reach about 3/4 throttle. Based on you description, your problem is with the pilot jet and/or the idle mixture screw setting.
4 - Make sure you have a good seal between the carb and intake manifold. The manifold gaskets are known to develop a split in them that's hard to see unless you take it off and inspect it closely. A vacuum leak here will screw up everything.
Last edited by cHarley; Apr 28, 2012 at 06:35 PM.
Thread Starter
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6th Gear
Joined: Nov 2011
Posts: 11
Likes: 0
From: Wisconsin
I was finally able to figure out what the problem was. I was confused as to why the main jet was effecting my idle. I took the carb off and gave the whole thing a once over. Not quite sure how it happened, but the throttle plate had a gouge in it that was preventing it from closing all the way. This must have allowed extra fuel to flow through the slow jet circuit.
Anyways, I threw a new throttle plate on it and it runs like a charm with 185 main jets now. Only thing I had to tweak was to advance the timing 3 notches.
Is one notch equal to one degree?
Anyways, I threw a new throttle plate on it and it runs like a charm with 185 main jets now. Only thing I had to tweak was to advance the timing 3 notches.
Is one notch equal to one degree?
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