Evo Stumbles at WOT
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Club Member
Joined: Jan 2007
Posts: 3,554
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From: NE Florida
Evo Stumbles at WOT
Issue: If I slam the throttle wide open, the engine will flutter from ~3400 rpm and up but if I back off a bit, it smooths out. If I roll on the throttle somewhat quickly, the engine runs smooth to redline.
Specs: 80" Evo, SE Heads, W6 cam, Mikuni 42, CompuFire Single fire ignition, Single fire coil, VH Straight shots. (Done by my buddy who is the previous owner)
What I've done: Rebuilt the Mikuni and set it up with the jets and needles as they told me to when I call their support line. The bike was running rich and the slow speed screw adjustment did nothing. After the rebuild with the recommended setup, I was able to set the slow speed screw correctly. I also went through the CompuFire timing procedure. Stumble still happens.
The issue rarely presents itself as I don't crack the throttle wide open much, but I should be able to without the stumbling. Winter boredom is prompting me to fix it.
Thoughts?
Specs: 80" Evo, SE Heads, W6 cam, Mikuni 42, CompuFire Single fire ignition, Single fire coil, VH Straight shots. (Done by my buddy who is the previous owner)
What I've done: Rebuilt the Mikuni and set it up with the jets and needles as they told me to when I call their support line. The bike was running rich and the slow speed screw adjustment did nothing. After the rebuild with the recommended setup, I was able to set the slow speed screw correctly. I also went through the CompuFire timing procedure. Stumble still happens.
The issue rarely presents itself as I don't crack the throttle wide open much, but I should be able to without the stumbling. Winter boredom is prompting me to fix it.
Thoughts?
Seasoned HDF Member
Joined: Aug 2008
Posts: 5,080
Likes: 2,152
From: Ohio
Issue: If I slam the throttle wide open, the engine will flutter from ~3400 rpm and up but if I back off a bit, it smooths out. If I roll on the throttle somewhat quickly, the engine runs smooth to redline.
Specs: 80" Evo, SE Heads, W6 cam, Mikuni 42, CompuFire Single fire ignition, Single fire coil, VH Straight shots. (Done by my buddy who is the previous owner)
What I've done: Rebuilt the Mikuni and set it up with the jets and needles as they told me to when I call their support line. The bike was running rich and the slow speed screw adjustment did nothing. After the rebuild with the recommended setup, I was able to set the slow speed screw correctly. I also went through the CompuFire timing procedure. Stumble still happens.
The issue rarely presents itself as I don't crack the throttle wide open much, but I should be able to without the stumbling. Winter boredom is prompting me to fix it.
Thoughts?
Specs: 80" Evo, SE Heads, W6 cam, Mikuni 42, CompuFire Single fire ignition, Single fire coil, VH Straight shots. (Done by my buddy who is the previous owner)
What I've done: Rebuilt the Mikuni and set it up with the jets and needles as they told me to when I call their support line. The bike was running rich and the slow speed screw adjustment did nothing. After the rebuild with the recommended setup, I was able to set the slow speed screw correctly. I also went through the CompuFire timing procedure. Stumble still happens.
The issue rarely presents itself as I don't crack the throttle wide open much, but I should be able to without the stumbling. Winter boredom is prompting me to fix it.
Thoughts?
Throttle whacks can be rich or lean, rich will black smoke out the exhaust and the engine is lazy setting down, lean will pop out the carb when the engine stumbles. Winter time, read up and become familiar with the Mikuni, very rewarding.
Personally ran the Joe Minton set up on a 89" final tuning with equipment, it worked well, Mikuni tech isn't going to tell you this setup because they speak the manual that Joe wrote, not his recommendations.
https://www.mikuni.com/pdf/hsr_tuningmanual_021003.pdf
https://eviltyrant.com/wordpress/201...omment-page-1/
Outstanding HDF Member
Joined: Dec 2008
Posts: 2,007
Likes: 353
From: Ohio
That seems likely to be the problem. Had a similar situation with an S&S. A rebuild of the pump fixed it.
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Club Member
Joined: Jan 2007
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From: NE Florida
Thanks for the help.
I rebuilt the carb and the accelerator pump shoots like a **** star. LOL.
While I'm keeping the Mikuni in mind, it's odd that I made a few changes to the way the carb was setup and the problem is identical. According to the guy at Mikuni support, the needle, low jet, and the nozzle were all too rich and I installed the correct components exactly how he instructed - even the number of turns on the pump screw. The bike starts easier and even when ice cold, idles with the choke on without having to blip the throttle. If it weren't for this small trouble area I'd say the bike runs as good as it could. That makes me think it could in the ignition.
I think I'm going to start from the beginning with the ignition and pull the timing cover to make 100% sure the cam is timed properly. Then I'm going to install the Compu-Fire back by the manual so I know for sure it was done 100% correctly. I've already checked the wiring and it was all connected the way it should be. Once I've ruled out the ignition, I'll go back to the carb and fuel. I have the stock CV and might even give it a quick rebuild and reinstall it and see if that makes a difference. I've sprayed carb cleaner around the intake and can't find any vacuum leaks but I might start from the beginning there too. Ain't nothing but something to do, right.
If that doesn't work, there is always an El Bruto 113" option!
I rebuilt the carb and the accelerator pump shoots like a **** star. LOL.
While I'm keeping the Mikuni in mind, it's odd that I made a few changes to the way the carb was setup and the problem is identical. According to the guy at Mikuni support, the needle, low jet, and the nozzle were all too rich and I installed the correct components exactly how he instructed - even the number of turns on the pump screw. The bike starts easier and even when ice cold, idles with the choke on without having to blip the throttle. If it weren't for this small trouble area I'd say the bike runs as good as it could. That makes me think it could in the ignition.
I think I'm going to start from the beginning with the ignition and pull the timing cover to make 100% sure the cam is timed properly. Then I'm going to install the Compu-Fire back by the manual so I know for sure it was done 100% correctly. I've already checked the wiring and it was all connected the way it should be. Once I've ruled out the ignition, I'll go back to the carb and fuel. I have the stock CV and might even give it a quick rebuild and reinstall it and see if that makes a difference. I've sprayed carb cleaner around the intake and can't find any vacuum leaks but I might start from the beginning there too. Ain't nothing but something to do, right.
If that doesn't work, there is always an El Bruto 113" option!
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Seasoned HDF Member
Joined: Apr 2011
Posts: 6,185
Likes: 2,134
From: W. Texas
Not gonna say about a HRS 42 I don't know them.
Your symptom mirrors a problem I had some time ago.
It turned out I had the float level a bit low. The bike did everything great Except when on the hwy
Hard acceleration and at hwy speed was running the bowl barley dry.
I will ad when it stumbled it was a long bad stumble.
Motor would die on occasions.
Best of luck
WP
Your symptom mirrors a problem I had some time ago.
It turned out I had the float level a bit low. The bike did everything great Except when on the hwy
Hard acceleration and at hwy speed was running the bowl barley dry.
I will ad when it stumbled it was a long bad stumble.
Motor would die on occasions.
Best of luck
WP
Seasoned HDF Member
Joined: Aug 2007
Posts: 8,198
Likes: 3,087
From: NW AR
Not a Mikuni guy, but... Sounds to me like the accelerator pump is just fine, but whatever that thing uses for a main jet is wrong on top, and likely too lean. I think you screw the needle in or out on those, right? Anyway, however you change the "up top everything it's got" setting, go one fatter and see how it does.