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Discussion Starter · #1 ·
Hey guys an gals,

I've done some searching, and have found some topics close to what I need, but not specific enough - so I am reaching out for some help.

While in the process of getting my bobbed 2002 Volusia painted, I decided to replace the football sized air filter with a Thunder MFG air filter. The one I got is the simple round setup, not the hypercharger with the butterfly flaps and all. We also removed the PAIR Valve using a kit I got from Moccasin Mike.

My carb had been rejetted to match up with the pair of Hard Krome Drag Star Staggered pipes I installed during the bob process, but we figured we might have to rejet to match up with the new air handler.

After the air filter install and no rejetting yet, the bike would fire up and go, but die of gas starvation with almost any turn of the throttle.

We did find one small leak in the vacuum hose coming from the vacuum fuel pump and we replaced that hose.

The jet was set at 45 before we installed air filter, and after the starvation issue, we reset it to 50. The problem got a little better, but we still cannot throttle passed about half way. If we go full throttle, she starves for gas again.

Our thoughts are going in two directions, and this is where I am hoping y'all can help us out.

Thought #1 - We're thinking about taking the jet to 65 and see if we can overload the carb, then back it down to a good working flow. Our thoughts were that if we went to a 65 and we're still starving for gas, we'll pretty much know the vacuum fuel pump has gone bad.

Thought #2 - If the vacuum pump or diaphragm in the vacuum pump is bad, were thinking that rather than replace that thing, we want to put a low pressure electric pump with a regulator on it and just do away with the entire POS vacuum system.

Do you think we're headed in the right direction?

And do you think we should just go straight to replacing the stock vacuum fuel pump with an electric pump and regulator?

Any thoughts?

As always, thanks in advance!
 

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I don't get where you are saying the jet was set at 45 then reset it to 50? What??? Do you mean to say you installed a larger main jet? Because I thought that would be about a 145-150. Are you sure you are replacing the main jet and not the pilot jet?
 

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Discussion Starter · #3 ·
Thanks for the reply 03volusiarider - I know that if anybody can help me, it is you.

My buddy is the carb guy, so I may have misunderstood what he told me. All I know is that he asked me to pick up a 65 to 70 jet today.

He has never worked with a vacuum system like this, but he knows his carbs. Texting him.....

Ok- he just replied to my text message on the cell.

He said he rejetted the pilot from 45 to 50. I've asked him why he is rejetting the pilot and not the main.

Can you expand on why you pointed this out? I don't know much about carbs, but I'm learning real fast.
 

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Discussion Starter · #4 ·
He said that he feels like the main jet is set right, and that the pilot jet is the one that needs adjusting. He must have gotten busy and could not explain to me why this is what he feels.

Again, I don't know much about what each jet does, but common sense seems to make me think that the pilot is for idol and the main is for throttle up.

I do know that when he pulled the fuel line, turned the key on, he said the stream of fuel looked like it was less than gravity feed fuel coming out of the line. But I don't know if pulling that hose changes the vacuum pressure or not.

Obviously putting in a larger jet would cost less, but if its the vacuum fuel pump, we might be wasting our time on a larger jet. The more we mess with it, the more we are leaning towards the electric fuel pump with regulator.

Thoughts welcomed.
 

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Discussion Starter · #6 ·
Ok. I just took a quick lesson on jets.

Closed to 1/8 throttle—air screw and pilot/slow jet
1/8 to 1/4 throttle—air-screw, pilot/slow jet, and throttle slide
1/4 to 1/2 throttle—throttle slide and jet needle
1/2 to full open—jet needle, spray-bar/needle jet, main jet, and air jet
 

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I think your problem is the jets, if it was running fine before you took it apart, and the vac lines are hooked up right. I have uni filter and de-pared. My settings are 138 main,45 pilot, needle on 4th notch,3 turns on adjustment screw. Runs great!
 

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Discussion Starter · #8 · (Edited)
Thanks Skrapiron.

I sent him the Aftermarket or stock pipes and Hypercharger airkit specs in a text before we started all of this and he says the main jet is set right.

I am wondering if my Thunder MFG airkit is more like a K&N airbox or carb mount setup.

The Thunder MFG airkit I got is simply a round K&N Airfilter fitted between a machined back and chrome plate on the front. It pretty much looks like one of those single carb airkits you put on top multiple carbs on a hotrod car. You can see all the way around the air filter where it sucks in the air.

It is not the Thunder MFG Hypercharger that has the two holes facing forward with the butterfly flaps that open and close based on vacuum pressure.

This the airkit I have -

 

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Discussion Starter · #9 ·
Updated information.

I just got off the phone with my buddy.

The symptom is this.

Bike fires up perfectly fine.

If you turn the throttle slowly, the bike will run right on up to whatever speed you want to go. You can eventually get to full throttle if you could stay on it, but it takes a long time to get there.

If you fire it up, release the clutch and immediately go 1/2 to full throttle, it dies with no fuel.

So in his experience, he is saying that the pilot jet is what should be keeping the carb bowls filled up when you go to full throttle.

That is why he wants to increase the pilot jet size.

The main jet has been set to around 142 and the pilot is at 50.
 

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Discussion Starter · #10 ·
He said if going to 65 on the pilot did not fix it, then it is probably the Fuel Pump Vacuum.

If it is the vacuum, would you guys recommend converting to electric fuel pump with a regulator or just get another pump (or rebuild kit) and stick with the stock setup?
 

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Aftermarket or stock pipes and Hypercharger airkit:
142 main jet, 45 pilot jet, dyno spring, dyno needle (for Thunder kit) on 3rd notch (Thunder says 2nd notch, but no one has had this work well), mix screw at 2.5 - 3 turns, plugged air jet. Adjust idle knob as needed to reach 1100 rpm.

Have him do THIS setup before he does ANYTHING else to the bike. A larger pilot jet will only make things worse....
 
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Discussion Starter · #12 ·
Can you explain why the larger pilot jet will make it worse?

Also, can someone tell me how much fuel flow there should be if the vacuum fuel pump is working properly. We get what looks like less fuel than if we just use gravity.
 

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Because going bigger is going to flood out the carb..... As it is, he's got it too rich and it is going to only get worse with the bigger jets in place.
 

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Discussion Starter · #14 ·
Humor me, because I don't really understand much of this.

But if we are not getting enough fuel when you go full throttle, why wouldn't you go with a bigger jet to get more fuel?
 

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You are not giving the engine a chance to get to full throttle, by going with too large a pilot jet, you are choking the engine out before you even get there. that's why it will start and idle, but if you add any fuel whatsoever it dies, it is already too rich in the pilot circuit, which as you know by now controls everything up to 1/4 throttle. go back to a 42 pilot and it should run fine. And leave the fuel pump alone. you wont get any fuel with this system unless you have vacuum from the engine. It is a safety issue that shuts the fuel off if the engine dies. leave it alone.
 

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Discussion Starter · #17 ·
You are the man 03volusiarider. You have confirmed what others have said, both on this forum and a couple locals I spoke with. I'll post the results of what it took to get her back on shape.
 

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Also, can someone tell me how much fuel flow there should be if the vacuum fuel pump is working properly. We get what looks like less fuel than if we just use gravity.
How the hell are you seeing any flow from the fuel pump?!?!?
The engine has to be running and the airbox and vacuum lines need to be in place for the pump to work...if the fuel line isn't attached to the carb it ain't going to run for long before the bowls are empty.
 

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Discussion Starter · #19 ·
How the hell are you seeing any flow from the fuel pump?!?!?
The engine has to be running and the airbox and vacuum lines need to be in place for the pump to work...if the fuel line isn't attached to the carb it ain't going to run for long before the bowls are empty.
You got me. The more I hear from you guys, the more I'm not sure my guy knows what he's doing.

I assume that he pulled the fuel line off of the carb and turned the ignition on to see how much flow there was when the pump primes up. I also thought that the vac lines had to be in place or it to work.

Right now my plan is to get the pilot jet set back to 42 as 03volusiarider recommended.
 

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You got me. The more I hear from you guys, the more I'm not sure my guy knows what he's doing.

I assume that he pulled the fuel line off of the carb and turned the ignition on to see how much flow there was when the pump primes up. I also thought that the vac lines had to be in place or it to work.


This bike doesn't have an electric fuel pump. Unhooking the fuel line and turning the ignition on will do NOTHING. It is a Vacuum operated system. Which means, there has to be vacuum created from a turning engine to open the petcock to allow fuel to be drawn into the system, not fed into as a electric fuel pump would do.
 
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