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Offline OApb

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carb help needed
« on: April 24, 2010, 05:42:57 PM »
Hey guys, Need some insight on some carb problems Im having. The bike runs rich at idle, will foul plugs if it idles for too long or catch some lights. It is also losing power above about 5500 rpms, will kind of sputter and stop pulling. The bike a a 78 750f with keihin pd42a carbs. Running pod filters and some sort of 4-1 exhaust with silencer but hollowed out. Any help is appreciated. thanks

Nick
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Offline Spanner 1

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Re: carb help needed
« Reply #1 on: April 24, 2010, 07:23:57 PM »
What size main jet ?.....and could it be drilled-out to some unknown size??
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline OApb

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Re: carb help needed
« Reply #2 on: April 24, 2010, 07:27:08 PM »
main is a 105, no signs of drilling. stock needles so I cant move the clip at all
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Offline Spanner 1

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Re: carb help needed
« Reply #3 on: April 24, 2010, 07:35:45 PM »
Any carb work done that changed anything, or did this prob. just suddenly start happening?
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline OApb

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Re: carb help needed
« Reply #4 on: April 24, 2010, 07:42:06 PM »
Other than a cleaning nothing has been done. It started when I took her out of storage
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Offline Spanner 1

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Re: carb help needed
« Reply #5 on: April 24, 2010, 07:47:11 PM »
Gas turned to varnish in the idle/slow circuit. Prolly the slow jets ( at least) need to come-out and be cleaned. Usually the 'emulsion' holes will crud-over..... my 2c. Have you pulled them out before ?
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline OApb

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Re: carb help needed
« Reply #6 on: April 24, 2010, 08:06:28 PM »
yes, carbs were taken apart and soaked/cleaned over the winter. Since the cleaning only new gas has been used
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Offline markac

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Re: carb help needed
« Reply #7 on: April 24, 2010, 08:24:53 PM »
105 mains sound small for pods/4-1 setup.
Not sure about the stock size for your year, but I usually find that 4-1 and pod setup requires a size or 2 up in main jets to compensate for the changes. Please correct me if i'm wrong.

the loss of power at 5000 rpm is not as important as to find out what position of your throttle the power loss happens at. If its 1/2 way or greater I'd suspect the main jets.
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Offline Spanner 1

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Re: carb help needed
« Reply #8 on: April 24, 2010, 08:29:16 PM »
With respect, you didn't say that you removed the slow jets ( yet).?..or any jets  and cleaned them.
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline OApb

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Re: carb help needed
« Reply #9 on: April 25, 2010, 09:59:45 AM »
Sorry, the carbs were taken apart, all jets cleaned and inspected, emulsion tubes cleaned and everything soaked in simple green and rinsed after. I'm pretty sure 105 is stock so I'm already looking into larger jets. The loss of power comes around 1/2 throttle
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Offline BobbyR

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Re: carb help needed
« Reply #10 on: April 25, 2010, 10:15:00 AM »
Quote from: OApb on April 24, 2010, 05:42:57 PM
Hey guys, Need some insight on some carb problems Im having. The bike runs rich at idle, will foul plugs if it idles for too long or catch some lights. It is also losing power above about 5500 rpms, will kind of sputter and stop pulling. The bike a a 78 750f with keihin pd42a carbs. Running pod filters and some sort of 4-1 exhaust with silencer but hollowed out. Any help is appreciated. thanks

Nick
You seem to have two conflicting conditions going on. At idle you would seem to be overly rich and when you are on it, she sees to be starving for gas. If you pulled the slows and cleaned them, did you push a wire through them, I ask since I was too aggressive with them and punched them too open. With open exhaust and Pod you could be running lean, or ignition could be breaking up, point bounce, wire ends etc. I have a K8 and I am running 120s with no fouling.
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Offline OApb

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Re: carb help needed
« Reply #11 on: April 25, 2010, 10:24:41 AM »
Yea something funky is going in. I was gentle when cleaning them so I don't think I opened them up to far. I thought maybe the idle mixture was off but its at the stock 1 3/4 turns out so I'm stumped
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Offline BobbyR

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Re: carb help needed
« Reply #12 on: April 25, 2010, 03:07:02 PM »
Quote from: OApb on April 25, 2010, 10:24:41 AM
Yea something funky is going in. I was gentle when cleaning them so I don't think I opened them up to far. I thought maybe the idle mixture was off but its at the stock 1 3/4 turns out so I'm stumped
When you are dealing with 30 year old carbs the stock 1 3/4 turns is kinda out the window. I would tune them to sound or feel, as lean as you can get them with a good idle. Those jets would not be causing your high speed problem, have you checked the float heights, and also look at the ignition system.
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Dedicated to Sgt. Howard Bruckner 1950 - 1969. KIA LONG KHANH.

But we were boys, and boys will be boys, and so they will. To us, everything was dangerous, but what of that? Had we not been made to live forever?

Offline OApb

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Re: carb help needed
« Reply #13 on: April 25, 2010, 04:08:32 PM »
alright ill retune the idle mixture. float heights were set when the carbs were put back together. the coils and points plate are both new with less than 100 miles on them. points were set to stock settings(cantremember off hand) and timing was set
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Offline BobbyR

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Re: carb help needed
« Reply #14 on: April 25, 2010, 04:30:37 PM »
Quote from: OApb on April 25, 2010, 04:08:32 PM
alright ill retune the idle mixture. float heights were set when the carbs were put back together. the coils and points plate are both new with less than 100 miles on them. points were set to stock settings(cantremember off hand) and timing was set
Don't get discouraged, with these old bikes it is a marathon not a sprint, they take some time. Just go over things point by point and also use the search function on this site. 
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Dedicated to Sgt. Howard Bruckner 1950 - 1969. KIA LONG KHANH.

But we were boys, and boys will be boys, and so they will. To us, everything was dangerous, but what of that? Had we not been made to live forever?

Offline Spanner 1

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Re: carb help needed
« Reply #15 on: April 25, 2010, 05:18:39 PM »
Seems your good with the jet cleaning...... the 41A carbs have a float height of 12.5mm, but the 42A ( your carbs ) have a float height of 14.5mm. Any chance your at 12.5mm?.... that would make the bike much richer across the board with a higher level of fuel in the jets.....
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline OApb

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Re: carb help needed
« Reply #16 on: April 25, 2010, 07:09:55 PM »
nope 14.5mm. I made sure to double check because That was one problem the PO mentioned he had
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Offline eurban

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Re: carb help needed
« Reply #17 on: April 26, 2010, 05:24:04 AM »
I would figure out what is causing my plugs to foul at idle and then sort out my main jet size and main jet needle position. Are you using resistor plugs with caps that already have resistors built in?  Are your pod filters way over oiled?  Has a critter made a nest in the exhaust pipe?  Has your battery / charging system become impaired over the storage period?  Low voltage a idle will not be able to fire the plugs properly. . . .Stock jetting and needle position will likely be too lean for the pods and exhaust.  You can shim the main jet needles with small washers to richen the mix in the mid throttle positions.  Again find out whats causing the plugs to foul at idle first!
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Offline OApb

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Re: carb help needed
« Reply #18 on: April 26, 2010, 03:04:32 PM »
the caps are OEM caps, plugs are NGK D8EA plugs with no resistors. The pods dont seem over oiled to me but its possible. There is no critter in my exhaust, it was taken off the bike and cleaned up and repainted over the winter. The battery, coils, points plate are all new with less than 100 miles on them, The rest is original so maybe something is starting to go bad. I got my hands on a stock airbox so will be switching back to that once it arrives. hopefully that will help a bit
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Offline OApb

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Re: carb help needed
« Reply #19 on: April 29, 2010, 07:21:28 PM »
anybody else have any insight? Im off for the next 2 days so Id like to get things squared away with this so I can ride
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Offline OApb

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Re: carb help needed
« Reply #20 on: May 01, 2010, 06:36:08 PM »
I just bought some plug caps, mine were originals. They tested ok but only cost $20 so I figured it wouldnt hurt. Ive also got a stock airbox on the way so hopefully these help out a bit
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