full throttle problem
Thread Starter
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1st Gear
Joined: Mar 2021
Posts: 5
Likes: 0
From: montreal
full throttle problem
got a custom bike don't know exactly what seems to be a 1200 sportster engine around 2000 with carburetor. bike starts great and run pretty good, on choke at start up I can sometimes hear little back fire and when I got out of the drive way, when I shift in second gear it began to run only on the front cylinder, unplug spark plug wire and plug again, after several time it run on both cylinder. when I go 3/4 to full throttle and the rpm begin to be high it is bucking jerking like it have to much gas. Wondering if a coil could do such issue? Just putting as much info I have, thanks for helping if possible 
Seasoned HDF Member
Joined: Sep 2009
Posts: 25,310
Likes: 2,446
From: Eastern NE
Cool story...nice 1st post. >.<
What's the back-story of this bike...is it a Sportster (truly), and any mods? How long has it been sitting? Fresh fuel/filter? Carb work?
All I can say is...maybe kit the carb, drain/fill fuel & see if it shakes out of the ill motor condition.
What's the back-story of this bike...is it a Sportster (truly), and any mods? How long has it been sitting? Fresh fuel/filter? Carb work?
All I can say is...maybe kit the carb, drain/fill fuel & see if it shakes out of the ill motor condition.
Thread Starter
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1st Gear
Joined: Mar 2021
Posts: 5
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From: montreal
I'm new to harley bike and don't know much of the bike, it's a friend bike I stored for the winter and he ask me to do oil change and maintenace. what I know is the bike sat for 4 years and the engine is 1200cc with oil tank under the seat and has same oil for engine and tranny thats look familiar to sportster engine and it seems to be straight pipe or so. the bike runs great and pulls good except at high rpm full throttle. the guy put new fuel in it before winter, I was suspecting coil due to the misfire I had
Road Warrior
Joined: Feb 2017
Posts: 1,528
Likes: 646
From: Saint Charles, Missouri
Over the winter is long enough for the fuel to be suspect. Also could have caused issues inside the carb. Drain the old gas, put in new along with some Sea Foam to clean the carb some.
Seasoned HDF Member
Joined: Aug 2008
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there could be several issues. the 4 years set up could cause valve issues because it it highly probable that it sat with an open valve. even putting in fresh fuel before storage is not a guarantee the fuel will last, it is better to treat the fuel with an inhibitor, run through the system and then drain out and store it dry. could be electrical due to corrosion but i opt not (unless highly humid conditions). rubber hoses degrade and the hose from the fuel petcock and voes could be bad. the carb probably needs cleaning and while products like sea foam may help, remember the carb is a dirty port devise, aka, has air passages which sea foam cannot reach. also, spark plugs can foul up and run under light load but barf when loaded up.
Road Master
Joined: Jan 2019
Posts: 1,123
Likes: 909
From: Texas
got a custom bike don't know exactly what seems to be a 1200 sportster engine around 2000 with carburetor. bike starts great and run pretty good, on choke at start up I can sometimes hear little back fire and when I got out of the drive way, when I shift in second gear it began to run only on the front cylinder, unplug spark plug wire and plug again, after several time it run on both cylinder. when I go 3/4 to full throttle and the rpm begin to be high it is bucking jerking like it have to much gas. Wondering if a coil could do such issue? Just putting as much info I have, thanks for helping if possible
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Thread Starter
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1st Gear
Joined: Mar 2021
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From: montreal
hoses have been changed lol
here some pics! didn't see any fuel filter, test resistance of coil, seems good, plugs too, I don't want to put all new parts cause it's not my bike but it'll be fun to fix it lol
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Thread Starter
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1st Gear
Joined: Mar 2021
Posts: 5
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From: montreal
ok today I've put brand new plug and clean the little wire connection on the coil. start the bike and get out of the drive way and again the rear cylinder wasn't firing up, got back to the garage and try to switch little signal back to front and same for spark plug wire to see if 1 of the 2 coil in the coil was bad but the problem didn't change place. rear cylinder is firing but seems weak cause when I unplug spark plug wire from front cylinder the bike die shortly but can run forever on the front cylinder. So at this point I'm wondering how it work behind the right engine cover cause the wire from the coil are going there, is a magneto giving signal to coil when to fire each cylinder? any test I could do at this time? can I remove this cover or it's a pain? ok made some search and if I'm right I only have to get this skull face out and see what ignition module (cam sensor ) I have there! does it sounds right that this module could be my problem du to intermittent misfire on rear cylinder?
Thanks again for any advise
Thanks again for any advise
Last edited by Mick85; Apr 5, 2021 at 07:39 PM.