The vacuum signature pulls fuel from the largest orifice first (metering rod). It will not pull fuel from the powerjet until the vacuum signature gets really high. This is because the powerjet orifice is very small and at the end of a rather long tube closest to the vacuum source.

The powerjet is for the top end. The metering rod is for the bottom end.

This is why you can do even more precise tuning with a dual powerjet. If you make one a 75 and the other a 25, the 75 will start drawing first because it requires less vacuum signature to pull fuel through a larger hole. Once the vacuum signature reaches the point that it will start drawing on the 25 jet, the 75 will be about maxed out.

This in effect adds another tuning circuit.



OMR is exactly right about checking the slide. Make sure that thing is up and out of the throat at WOT.

Also, as you lean the metering rod, it will allow you to run a bigger powerjet without running rich in the upper midrange. But you can only do this to a point. Once there is a slight hesitation just off idle, richen the metering rod back up a 1/4 or 1/2 turn. At this point, start increasing the size of your power jet until you have the best curve. This is usually how it goes because usually they are setup rich when they come to you.


Your situation sounds different though. In your case, i would try richening your needle a 1/2 turn and see if this allows you to run a little smaller powerjet for better numbers.

I ran a 90 powerjet on my 730 but your porting and exhaust might be different than mine. Eric can get you any powerjet you need.


The leaner you run the metering rod, the bigger of a powerjet it will want and vice versa. This is all assuming you are making adjustments within the ballpark of where you should be.

 

Thanks Dsnut

Appreciate the advice

 

<div class="FTQUOTE"><begin quote>Originally posted by: DSNUT

The vacuum signature pulls fuel from the largest orifice first (metering rod). It will not pull fuel from the powerjet until the vacuum signature gets really high. This is because the powerjet orifice is very small and at the end of a rather long tube closest to the vacuum source.



The powerjet is for the top end. The metering rod is for the bottom end.

</end quote></div>

This is how I have always understood the metering rod and power jet to operate and my hill testing has also verified. OMR's description of the power jet being for bottom/mid had thrown me for a loop[img]i/expressions/face-icon-small-confused.gif[/img] Thanks for verifying my belief.

 

<div class="FTQUOTE"><begin quote>Originally posted by: DSNUT


OMR is exactly right about checking the slide. Make sure that thing is up and out of the throat at WOT.

</end quote></div>

This is one thing I have always struggled with on my Lectrons. If I adjust the cable to bring the slide all the way to the very top it is too tight on bottom and I loose any slack that is required in the cable to hold idle. It looks like I can get a little more throw out of the Moose thumb throttle if I grind down the one side of the lever arm that butts up against the inside of the case at full throttle.

 

That is a good idea. I got one cable that i wasen't able to adjust. I sent it back and got a new one and it worked fine. Have no idea what was wrong with it....

 

<div class="FTQUOTE"><begin quote>Originally posted by: DSengineer

<div class="FTQUOTE"><begin quote>Originally posted by: DSNUT





OMR is exactly right about checking the slide. Make sure that thing is up and out of the throat at WOT.



</end quote></div>



This is one thing I have always struggled with on my Lectrons. If I adjust the cable to bring the slide all the way to the very top it is too tight on bottom and I loose any slack that is required in the cable to hold idle. It looks like I can get a little more throw out of the Moose thumb throttle if I grind down the one side of the lever arm that butts up against the inside of the case at full throttle.</end quote></div>



Have you tried adjusting the cable guide nut at the top of the cable ?

I know what you mean, you have to take all the slack out of the cable to get the slide up that far.

 

http://www.h2madness.com/carbtuning.html


Attached is an old chart showing the numbering system for Lectron needles.

History:
When the Lectron carb first came out it did not have the powerjet option.
Because they could atomize fuel in cold weather, they became popular for snowmobiles. Dirt bikes were also very popular, and they like snowmobiles, were two stroke motors. Thus the powerjet was added.

As four stroke motors became popular, these carbs found there way into high performance applications. They became the drag bike carb of choice. The use of a powerjet was used only for special applications. Even the Pro Bikes did not start to use them till very recent.

The powerjet was mainly added to enrich the mid-range. Like every good engine builder will tell you, if the motor wants more fuel, give it more then. With better understanding of velocity and new design motors with 4 valves and dual plugs, we have motors that can take more fuel in mid range.
Please note that the Lectron needles only have 4 size ajustments for mid and 10 for top. So to get more fuel into todays high performance motors we are using powerjets to add the extra needed fuel, because the needle has become exhausted by design. The needles thus become leaner sized for top to balance the total fuel requirements for top end.
So when the carb as been initally setup and has the correct needle, then any fine tuning for top is done by rejetting the powerjet. But it only applies 10% of the top end fuel requirements.

Powerjets need to become activated around 3000rpm or even lower is better. To accomplish this, during inital setup, the sizing of the jet for powerjet has to be larger to help get the powerjet to activate sooner, if it is starting at a higher rpm.
Powerjet activation rpm and top end fuel requirements, have to be balanced together.

If you have a dual powerjet, tune them identical. If you have #10 jets installed and activation is right off idle, you may want to turn off one powerjet and just use one. If still activates early, check float height and retests. If still activates too early and top end is rich, you probaly need a different needle.

OMR

 

<div class="FTQUOTE"><begin quote>Originally posted by: MRDS650

<div class="FTQUOTE"><begin quote>Originally posted by: DSengineer



<div class="FTQUOTE"><begin quote>Originally posted by: DSNUT











OMR is exactly right about checking the slide. Make sure that thing is up and out of the throat at WOT.







</end quote></div>







This is one thing I have always struggled with on my Lectrons. If I adjust the cable to bring the slide all the way to the very top it is too tight on bottom and I loose any slack that is required in the cable to hold idle. It looks like I can get a little more throw out of the Moose thumb throttle if I grind down the one side of the lever arm that butts up against the inside of the case at full throttle.</end quote></div>







Have you tried adjusting the cable guide nut at the top of the cable ?



I know what you mean, you have to take all the slack out of the cable to get the slide up that far.</end quote></div>


Yes I have tried adjusting both at the top of the carb and at the throttle assembly. Both essentially do the same thing just at different ends. Either way you are trading one for the other (full-pull vs. slack needed at idle). I think the only way to get around the issue is to allow a little more pull at the throttle assembly. Griding the lever arm down slightly looks like it will do the trick.[img]i/expressions/face-icon-small-happy.gif[/img]

 

If I understand things correctly, don't the 730 Big Bore require the 48mm carb. This guy has a 46mm. If I am correct on that, my guess is you are starving the motor at this point.

 

I was told that I would be ok with the HV46 (By Eric) but I would lose a couple or horse on the top end. I am ok with that. The problem I am having is I have less horsepower at the wheels after I did the head and bigger valves. It was dynoed before and after the head job on the same dyno with the same tires.