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Discussion Starter · #1 ·
04 Raptor 660. All stock from what I know (not original owner). My son took out all the wadding and spark arrestor from the stock exhaust (to make it louder) and I did the parking brake and rev limiter delete. Cut the green/yellow wire at CDI and cut the green/white wire at CDI and ran a wire to the negative on battery (from green/white wire). Throttle stumbled starting at about 1/4 to 1/2 throttle and had loss of power. Took off carbs and clean them, adjusted clips from third from bottom to second from bottom on both. Seemed like it helped a little with the stumbling but still loss of power. I have done extensive research and most everyone says its the jetting since it runs (not the CDI) and not the parking brake and rev limiter deletes if done right. Would gutting the stock exhaust be enough to have to rejet the carbs?
 

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Master of the Electron
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Simply, yes... you probably need to move the right main jet (Mikuni 145) to the left carb and get a Mikuni 150 for the right carb.
 

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Master of the Electron
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No... the mains need to match the vacuum levels at the intake manifold. By opening up exhaust flow slightly that vacuum has decreased a bit so larger mains are needed to allow a bit more fuel to flow.

Same thing happens when the lid is removed - vacuum drops a bunch from inlet to outlet of carb... so main has to go way up to allow more fuel to get sucked up with less force.

The goal is to try to keep the air/fuel ratio delivered to the cylinder to stay around 13 parts air to 1 part fuel - this is the ratio where all the fuel and oxygen combine in the burn - called stochiometric in chemistry and it optimizes power generated.

Just upping the jets alone (adding more gasoline without also increasing airflow) causes the ratio to get too rich and the power to drop way off.

BTW, when opening or removing the lid you need to up the jets to 160 L and 165 R - so this gives you an idea of how much more fuel is needed and, therefore, how much more power can be generated just by letting more air in through the airbox (and upping the jets to match)!
 
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Discussion Starter · #5 ·
No... the mains need to match the vacuum levels at the intake manifold. By opening up exhaust flow slightly that vacuum has decreased a bit so larger mains are needed to allow a bit more fuel to flow.

Same thing happens when the lid is removed - vacuum drops a bunch from inlet to outlet of carb... so main has to go way up to allow more fuel to get sucked up with less force.

The goal is to try to keep the air/fuel ratio delivered to the cylinder to stay around 13 parts air to 1 part fuel - this is the ratio where all the fuel and oxygen combine in the burn - called stochiometric in chemistry and it optimizes power generated.

Just upping the jets alone (adding more gasoline without also increasing airflow) causes the ratio to get too rich and the power to drop way off.

BTW, when opening or removing the lid you need to up the jets to 160 L and 165 R - so this gives you an idea of how much more fuel is needed and, therefore, how much more power can be generated just by letting more air in through the airbox (and upping the jets to match)!
Thanks for the detailed explanation. It makes more sense now. I replaced the main jets as you suggested (left 145, right 150) today and it fixed the stumbling but still has loss of power. I checked the intake boots for a leak (carb cleaner while idling method) and no change in idle. Carb is clean and fuel screws are at 3 turns out. Floats seem to being at right level. I am stumped.

If it helps, I say loss of power because before this issue my son could easily do a wheelie (in first gear) with no problem at all. Now, he cant even get the front wheels off the ground.

We have our yearly Glamis trip in about 3 weeks so we are hoping to get this fixed. Thanks again and appreciate all the advice.
 

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Master of the Electron
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So it worked well 'before'... under what conditions? What has changed? How about some history?

This all began when the packing was removed... nothing else? Is your lid stock and intact? No change in elevation? Have you cleaned the carbs?

Describe your 'loss of power' in detail. What happens under what throttle position and other conditions? Be verbose!
 

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Discussion Starter · #7 ·
So it worked well 'before'... under what conditions? What has changed? How about some history?

This all began when the packing was removed... nothing else? Is your lid stock and intact? No change in elevation? Have you cleaned the carbs?

Describe your 'loss of power' in detail. What happens under what throttle position and other conditions? Be verbose!
I focused on what you said "What has changed". I remembered that my son and his friend put on the new clutch lever the end of last month. I checked it today and they didnt adjust it correctly. It wasnt disengaging the clutch all the way out. A little adjustment and it runs like a beast again!!!! Thanks QuadManiac for the advice.
 

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Master of the Electron
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Ah HAH! Troubleshooting 101 (What Changed?) to the rescue!

Congrats on your skills of deduction, Dr. Watson.
 
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