Converting power jet to non power jet carbs

[bradv]

I have heard conflicting information about converting power jet carbs to non powerjet (by blocking the pipe from the bowl to the power jet). You will of course need to increase main jet size but some say that its not a good idea to do this as the power jet operates from around 50% throttle and it could cause issues (may cause it to run lean).

Any experts who can offer comment on this?

I converted my RZ250 to a 350 and am struggling a bit with jetting (using stock 250 PJ carbs) and thinking I will convert them to non PJ, just increase the main jet.

Thanks in advance.

[midlifecrisisrd]

Block off power jet tube with a ball bearing then add the power jet value to the main

185 plus 55 is 240

31k carbs were like that

Steve

[bradv]

Thanks Steve.

That was my thought but someone said that there can be issues as I mentioned in my post. Not sure if what I was informed is true as I don't know anyone who has done this.

Brad

[midlifecrisisrd]

In my opinion the pj carbs function was emissions related to lean it out at small throttle openings

You may have to mess about with the needle but should be workable

Can I ask why your considering it?

Is it because you are using pod filters?

Steve

[bradv]

Jan 17, 2018 8:41:07 GMT 1 midlifecrisisrd said:

In my opinion the pj carbs function was emissions related to lean it out at small throttle openings

You may have to mess about with the needle but should be workable

Can I ask why your considering it?

Is it because you are using pod filters?

Steve

I’m using the stock air box. It was a 250 but now a 350 and I can’t get the jetting 100%

I can’t seem to establish what size the power jets are in the carbs. Not sure if the power jet is the same size as what is in the 350 carbs?

Thinking it may be easier to just block the pj and then change main jet

I haven’t changed the needle yet. Not sure what to do there either (or where to source needle and emulsion tubes)

[midlifecrisisrd]

Would it not be easier fitting 31k carbs

Steve

[bradv]

Jan 17, 2018 14:56:11 GMT 1 midlifecrisisrd said:

Would it not be easier fitting 31k carbs

Steve

Would be but I can’t source any here in Australia.

[midlifecrisisrd]

Ahh, that explains it.

Take it that it's a 51L model then

Steve

[jon]

I’m quite interested to see how this pans out myself.

IMO powerjet carbs are only really necessary around 32-34mm oriface.

The needle jet (or emulsion tube to me) is positioned at the bottom of the bore. At WOT the airflow at the top of the oriface will have very little effect to draw out fuel due to the Bernoulli principals.

Therefore I agree with Steve that this was probably an attempt to improve immisions.

IMO a 26mm carb does not need a PJ and so blanking it off and adding to the main jet is OK.

I am building an F2 engine soon and am considering using 31K carbs (as I have them) but would consider blanking the PJ and upjetting.

Jon

[bradv]

Jan 17, 2018 20:30:52 GMT 1 jon said:

I’m quite interested to see how this pans out myself.

IMO powerjet carbs are only really necessary around 32-34mm oriface.

The needle jet (or emulsion tube to me) is positioned at the bottom of the bore. At WOT the airflow at the top of the oriface will have very little effect to draw out fuel due to the Bernoulli principals.

Therefore I agree with Steve that this was probably an attempt to improve immisions.

IMO a 26mm carb does not need a PJ and so blanking it off and adding to the main jet is OK.

I am building an F2 engine soon and am considering using 31K carbs (as I have them) but would consider blanking the PJ and upjetting.

Jon

Thanks Jon

As I mentioned in my post I have heard conflicting information but your explanation makes sense

Hoping someone who has actually blocked off the pj and has some experience will read this post and offer comment.

It’s a newly built motor and I don’t want to blow it up...yet

Brad