Topic: No power past half throttle

[uksparky]

Ongoing problem with 78 550 PD carbs ..i can rev it all day sitting in my garage,  but on the highway giving it throttle it will bog down past half to full then about ten seconds it kicks in like a rocket, backing off and on again same thing. It has to be fuel problem..I am about to pull the carbs, done everything
Float height ok
Advance ok
Jets cleaned ok
No vacuum leak
Idles ok at 1100 rpm
42 pilot
90 main
Mixture screws 1.5 turns
Valves checked.
Carb sync
So it feels like it's not getting fuel to the engine, not ignition, it's electric..past half throttle it just bogs down, easing off throttle power comes back, pulled plugs/tan ok

[flybox1]

Dont pull the carbs yet....Intake and exhaust type?
Are you able to do a road idle plug chop?
New plugs, find a section of road where you can hold the throttle recreating the stumble.  2-3 minutes if possible.  chop the engine and just pull #1&4.  have a look...

[lrutt]

gas tank venting ok?

[Duanob]

Petcock flowing OK? Clear tube method on the float height?

[Deltarider]

Any 'extra' inline fuel fliters that hinder fuel flow?
Maybe use a fuel additive that helps overcome sticky floatneedles?

[TwoTired]

If you changed away from stock exhaust, you will likely have to raise the slide needles a notch, and possibly go to a 95 or 100 main jet.

It is best to do a plug chop at WOT and 1/2 throttle position (with known clean plugs) to verify lean plug deposits, and also to note the difference before and after the carb changes.

Cheers,

[eigenvector]

I was thinking the same thing.

No power after half-throttle tells me that it's not getting enough fuel

[uksparky]

Stock exhaust and air filter, did the plug chop tan plugs..tank vent ok...fuel flow ok..did the clear tube level ok..in my previous post a member said I don't need in line filter, as the tank one is enough, thinking about it that's when my problem started. I know I have the in tank filter, so why did the P.O. fit the in line filter. I put straight line from petcock to carbs. It's not a spark plug stumble, it feels like the fuel was turned off...But if I hold it in the bogging down  it will clear out after 10 seconds once it gets over 6k rpm say 4th gear, backing of throttle and back on same problem am I making sence

[uksparky]

If it was fuel blockage ..my thoughts as well ....why would the carbs kick into full power after holding it there....But above 6k then it will rev to redline.. perfect no problems0

[BomberMann650]

Needle clip issue?

[eigenvector]

Well riddle us this...

Does it behave like this in 2nd gear, 3rd?  Only when you're in 4th or above?

I would ignore idle and neutral - you barely need any fuel to rev the engine. 

If it's only 4th or above to me that really sounds like a fuel supply issue.

What did you do to that poor thing anyway?

[uksparky]

Needle clip in bowl ok...pulled top of carbs slides and needle valve ok. Like I said when I took the in line filter off problem started....but ofcource it could be something else. Never tried it in first or second, going out of town shifting from second to third, winding throttle past halfway in third bogging, give full throttle more bogging, then snap of a finger hang on, the power is back on, does same thing in 4th/5th.. This engine has the kenny harmon DE cam and HC pistons...A week ago it ran perfect. I rebuilt the engine few weeks back.

[uksparky]

Only other thing I can try is drain carbs, fill with seafoam, crank over start few seconds, let it sit overnight...If that don't work I will pull the carbs, thinking about it fuel is sucked up past tapered needle valve by vacuum from air filter, what could stop that ...if it was fuel blockage, why would the power come back....scratching my head with this..Also I am getting a high idle after run out...does it work like this idle pilot jet....off idle main jet fuel...so coming back to idle the pilot does not kick in, hangs up on main jet..?

[calj737]

See if this helps you focus you're attention within the fuel metering:

[TwoTired]

How the engine recovers power tells you of fuel conditions that cause power loss.

If it burbles back to life before providing smooth power, the mixture was to rich to fire off, causing the power loss.

If it suddenly come back to life cleanly like the key switch was just turned on, Then the power loss was caused by a too lean mixture.

While tan plugs are a good sign in general, it is only a conclusive sign if the run time throttle position is known and fixed.  Otherwise deposits from other fuel metering circuits make or remove deposits indicating fuel mixture conditions.

Imagine if some metering circuits left red deposits and others left blue and white deposits.  When your plugs show purple, what ratio are the three metering circuits delivering?

Cheers,

[DaveBarbier]

Needle clip in bowl ok...pulled top of carbs slides and needle valve ok. Like I said when I took the in line filter off problem started....but ofcource it could be something else. Never tried it in first or second, going out of town shifting from second to third, winding throttle past halfway in third bogging, give full throttle more bogging, then snap of a finger hang on, the power is back on, does same thing in 4th/5th.. This engine has the kenny harmon DE cam and HC pistons...A week ago it ran perfect. I rebuilt the engine few weeks back.

Needle clip in bowl? What the hell is that?

Needle valve in the top of the carbs? You mean the slide needle? Saying it's "ok" doesn't mean anything. It can appear good, but if it's not set right you'll have issues. And since you removed a slide needle you'll have to resync. What clip position are your slide needles on? Are they all the same?

Since you have higher compression and a hotter cam you're likely going to be jetted differently than stock even with stock intake and exhaust.

[uksparky]

Sorry guys worded that wrong...the little clips that holds the float needle valve in place are ok....top of carbs i just removed the two screws and pulled the caps for sync, the needles looked like they were the same height, and move up and down no binding... but  i have not removed the slides...but remember a week ago the bike was perfect..stock jets Dave... 90 main 42 pilot, stock exhaust, i do have 100 mains that i was going to install...Yesterday i pulled the bowls and jets and sprayed up into the carb body with carb spray cleaner, jets were clean...road test today

[Lostboy Steve]

Sorry guys worded that wrong...the little clips that holds the float needle valve in place are ok....top of carbs i just removed the two screws and pulled the caps for sync, the needles looked like they were the same height, and move up and down no binding... but  i have not removed the slides...but remember a week ago the bike was perfect..stock jets Dave... 90 main 42 pilot, stock exhaust, i do have 100 mains that i was going to install...Yesterday i pulled the bowls and jets and sprayed up into the carb body with carb spray cleaner, jets were clean...road test today

100 mains with an air box and stock exhaust is going to be pretty rich.


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[BomberMann650]

Since it just started misbehaving.
I wonder
What fuel are you running currently?  Different than before?  Was the tank clean when you bypassed the filter?  You replaced hose completely or spliced it?

Aaannnddd, throttle technique.  Are you fast pinning the throttle or rolling on?

[uksparky]

Good news ..power is back through throttle range i pulled the pilot and main jets and used a full can of carb cleaner up into where the jets fit, and holding full throttle spraying into main jet hole. At least i dont need to pull the carbs.  Thanks for the help guys..Plus i dont have the high idle any more..

[DaveBarbier]

So all fixed? Just needed a carb clean? Good to hear.


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