Intermittent miss under low load
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6th Gear
Joined: Feb 2015
Posts: 10
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From: Louisiana
Intermittent miss under low load
- Ive got a 99 FXDX that got an overhaul/performanceupgrade @ 100K, At about 105K I noticed the misses and started trying toaddress the issue but I have reached the point that I now have it at an Indy . Hehasnt/cant duplicate the issue; Im looking for some suggestions as my hopethat a second set of eyes would do the trick is fading fast.
Facts: My bike is a 99 FXDX, 103, headwork Comp Releases,10to1, S&S crank, 211s, SE 7K ignition, 42mm Mikuni and Thunderheader. Iveput another 15K miles on it since it started to miss, and during that time theparts changer in me has replaced the following: plugs&wires, coil, ignitionmodule, crank pos sensor. It doesnt throw codes and is real strong under acceleration,but it aint quite right just running between 2000-2500 sometimes.
Interesting observations that may/may not be related: 1. Itseems that its taking more RPMs to smooth it out as time goes by. 2. I have occasionally started it and it soundedlike its only hitting one one cyl. 3. Itcoughed through the carb once even though it was at running temp.
I hesitate to open the cam chest to replacethe CPS because if there was something mechanical out of whack I would likelymiss it, hence the Indy, but he is waiting to ride test / duplicate before hedoes anything.
Anyone have some nuggets of wisdom? Should I look at theHSR42? (The miss feels electrical if that makes any sense) I got concernedthat it may be mechanical and my previous thought that eventually somethingwill fail and I will replace it may not be such a good idea if that something goesboom inside the motor.
I want my bike right.
Facts: My bike is a 99 FXDX, 103, headwork Comp Releases,10to1, S&S crank, 211s, SE 7K ignition, 42mm Mikuni and Thunderheader. Iveput another 15K miles on it since it started to miss, and during that time theparts changer in me has replaced the following: plugs&wires, coil, ignitionmodule, crank pos sensor. It doesnt throw codes and is real strong under acceleration,but it aint quite right just running between 2000-2500 sometimes.
Interesting observations that may/may not be related: 1. Itseems that its taking more RPMs to smooth it out as time goes by. 2. I have occasionally started it and it soundedlike its only hitting one one cyl. 3. Itcoughed through the carb once even though it was at running temp.
I hesitate to open the cam chest to replacethe CPS because if there was something mechanical out of whack I would likelymiss it, hence the Indy, but he is waiting to ride test / duplicate before hedoes anything.
Anyone have some nuggets of wisdom? Should I look at theHSR42? (The miss feels electrical if that makes any sense) I got concernedthat it may be mechanical and my previous thought that eventually somethingwill fail and I will replace it may not be such a good idea if that something goesboom inside the motor.
I want my bike right.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,116
Likes: 7,639
From: Poolville
Sounds like you changed a lot of parts, stabbing at the possibilities. Did you ever drop your float bowl to check for impurities. Almost sounds like you have problems picking up fuel on your low jet or possibly the main jet. I would first check for vacuum leaks at carb to intake and intake to heads. to confirm no leaks. Not sure but do you think a 42 mm carb is proper for that 103. And actually it all sounds more like a jetting problem especially since its all present since the upgrade to 103. find a good tuner or go to the carb section for more help. Are your pipes blueing? If so very well could be a lean condition.they also have a sticky in the mechanical section on how to read plugs. Are they white or tan or black. Tells a lot.. good luck man.....
Last edited by 98hotrodfatboy; Feb 24, 2015 at 07:19 PM.
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6th Gear
Joined: Feb 2015
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From: Louisiana
Smoke stop
#1 parts changer here, NOT a mechanic.
I did drop the bowl and take a peak and shot some carbcleaner in there, thinking it is new gave me no reason to suspect itafter that. The main jet is flowin fuel, when I twist away we go. As far as leaks goI didn't consider that as a possibility due to the intermittent nature of themiss. Didn't spring for bling, the Thunderheader is LOUD and BLACK. Plugs are anice tan front and rear. It didn't miss for the first 5K after the overhaul, otherwise I wouldhave brought it back to my mechanic. Imarked the throttle grip with dots at closed, 1/2 and WOT, the throttle is barelycracked when the miss develops and disappears as soon as I give her atwist. (and generally starts up again when the acceleration levels back off) I trust that the mechanic thatrebuilt my motor matched the carb to the rest of the mods, I imagine shemakes about 100hp and 100 pounds.
My Bike and my buds:
Tourer
Joined: Apr 2006
Posts: 463
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From: Arizona
Did this just start when the outside temps started to get cold? If so it could be that going one size richer on your pilot jet will take care of your miss. As others mentioned you may be a a little lean at cruise.
As far as sounding like your only running on one cyl. at start-up that is another issue. I had a DTT ignition that had one of the adjustment ***** become loose & while I was riding the **** would vibrate between settings. This caused one cyl. to not fire correctly & I would only be running on one cyl. intermittently for extended periods of time. I doubt this is your problem because you absolutely know there was something really wrong.
I occassionally get a little miss while riding at hiway speeds, nothing major, just kind of a hiccup while riding. I've had this happen with a stock CV, a 44mm CV & my 45mm Mikuni. I have come to the conclusion after checking for intake & any other leaks that it is just the nature of the beast & nothing to worry about.
As far as sounding like your only running on one cyl. at start-up that is another issue. I had a DTT ignition that had one of the adjustment ***** become loose & while I was riding the **** would vibrate between settings. This caused one cyl. to not fire correctly & I would only be running on one cyl. intermittently for extended periods of time. I doubt this is your problem because you absolutely know there was something really wrong.
I occassionally get a little miss while riding at hiway speeds, nothing major, just kind of a hiccup while riding. I've had this happen with a stock CV, a 44mm CV & my 45mm Mikuni. I have come to the conclusion after checking for intake & any other leaks that it is just the nature of the beast & nothing to worry about.
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,925
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From: poway
There are a number if things to look at..
First, I doubt it problem is caused by the cam sensor.. While the older bikes lack good diagnostics, they were pretty good determining that the cam sensor and crank sensor were in sync..
Second. The pilot jet on a Mikuni carb really only effects idle and the slightest bit above idle as there are no transfer ports like in a CV or butterfly carb where cruise and rollon are effected.. At idle the pilot and main nozzle share the fueling requirements so the transfer the needle happens quickly.. One of the reasons Mikuni used needle sizes to control off idle fueling.
Back to the bike.. The first thing you should do is pull the plugs and look at them.. They could be fouling especially if they are colder than stock or you have an intake valve seal leak.. If suspect replace the plugs..
If the plug wires have not been changed in the last 5 years, I'd replace them to eliminate them as an issue. Also make sure that the plug ends are on the spark plug nice and tight. They can comes loose and create a fine dust that will cause flash over on the plug..
While the plugs are out, if you can check compression..
The map sensor may have failed in such a way that puts out a voltage that says the manifold pressure is low. This will cause the module to think that the throttle is wide open and it will use the slowest timing curve.. The modules typically only look at the Map voltage from a diagnostic perspective at first ignition switch on and only to see that the voltage is roughly 4.5 volts.
Look for manifold leaks.
Also look at fuel flow to the carb and carb mounting.. Mikunis don't have emulsion tubes so they are susceptible to low fuel level or vibrations that can allow air into the mainjet..
First, I doubt it problem is caused by the cam sensor.. While the older bikes lack good diagnostics, they were pretty good determining that the cam sensor and crank sensor were in sync..
Second. The pilot jet on a Mikuni carb really only effects idle and the slightest bit above idle as there are no transfer ports like in a CV or butterfly carb where cruise and rollon are effected.. At idle the pilot and main nozzle share the fueling requirements so the transfer the needle happens quickly.. One of the reasons Mikuni used needle sizes to control off idle fueling.
Back to the bike.. The first thing you should do is pull the plugs and look at them.. They could be fouling especially if they are colder than stock or you have an intake valve seal leak.. If suspect replace the plugs..
If the plug wires have not been changed in the last 5 years, I'd replace them to eliminate them as an issue. Also make sure that the plug ends are on the spark plug nice and tight. They can comes loose and create a fine dust that will cause flash over on the plug..
While the plugs are out, if you can check compression..
The map sensor may have failed in such a way that puts out a voltage that says the manifold pressure is low. This will cause the module to think that the throttle is wide open and it will use the slowest timing curve.. The modules typically only look at the Map voltage from a diagnostic perspective at first ignition switch on and only to see that the voltage is roughly 4.5 volts.
Look for manifold leaks.
Also look at fuel flow to the carb and carb mounting.. Mikunis don't have emulsion tubes so they are susceptible to low fuel level or vibrations that can allow air into the mainjet..
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6th Gear
Joined: Feb 2015
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From: Louisiana
Follow up, it’s 2017 now and Black Betty’s issue hasreturned. The issue, an air leak on the intake manifold caused by cracked rubber was resolved in 2015. Beingthe parts changer that I am I’m headed there first this time.
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