Bolt-on Adjustable Powerjet Tube Length

[Adam_Winstone]

Hi All,

Whilst this is a topic that is being covered on another thread (http://scooterotica.org/forum/viewtopic.php?f=7&t=18415), the thread's TMX35 title might put a few people off from looking at it, hence I'm posting a link under a different/generic title as the subjects being covered are applicable to all carbs using these ever more common bolt-on PJs.

I particularly need some input/feedback on brass throttle length and position, based on experience rather than just reiterating what may be quoted on the limited info published elsewhere. The whole other thread is worth a read but the bit that relates to all carbs is....



I've fitted a Camlam add-on PJ and positioned it to reach the centre of the Venturi, which brings in the enrichment way too early, making things too rich at low throttle positions, which is killing performance and progression. Out of interest, I looked at the bellmouth and brass tube length of my old Mk2 Amal and that only reaches half way to the centre of the Venturi, functioning well to enriched the top end only. The difference being that the Amal tube comes in from the top (vertical) and reading up would suggest that this is important as it relates to the air intake being restricted at the top of the Venturi until the slide raises above the brass tube at 3/4 and above. If this is the case then a tube coming in on an angle will influence Powerjet function.

It seems as though I will need to employ a combination of winding out the Powerjet and cutting it shorter, however, I don't want to cut it too short and regret it. What have the rest of you done and have you managed to find any good info/websites?

https://www.ooracing.com/store/carb-bod ... r-jet.html

This link should show another pj carb body with a short brass pj tube, which I understand will bring in enrichment later, but this confuses me slightly as surely the purpose of a pj is to kick in later in the progression range, so why produce a pj with such a long tube? That said the Lectron pj carbs have a tube that comes down to the centre of the Venturi.

Certainly my current tube length, to centre, is causing enrichment to happen way too early on my motor and I'm having to close up the adjustment completely, otherwise it just makes it boggy from less than half throttle upwards!

Adam

Edit: I note other 2-stroke fans are asking the same question and it appears as though the vertical nature of the tube is pretty important, which is something not really being addressed in the instructions available. Tube length also very important, with uncovered length relating to enrichment, which would make the middle of the bore position silly if you want the PJ to flow from 3/4 throttle up and support Amal's position of the tube end only 1/4 down into the bore (3/4 throttle position). I realise that there are 2 elements to flow; uncover position and suction rate in venturi but there is little clear info.

[Adam_Winstone]

Too late, I've chopped half of the tube off and now need to take it for a spin. I'll report back for those of you that are thinking of buying/fitting one of these.

Adam

[Adam_Winstone]

Woohoo, now we're getting somewhere... a PJ that kicks in at higher revs/air flows, rather than kicking it at half throttle or less!

Plenty more testing and adjustment to be done but I am now able to have the PJ on, without ruthless flooding at low throttle positions, and can start to feel it functioning as it should, with the ability to adjust through a workable range of too little to too much.

Adam

PS - I must stop writing threads to myself!

[Donnie]

LOL, it's piqued my interest i have to admit as I have NO clue about power jets at all, what they do how they work etc.

[Adam_Winstone]

Hey Donnie,

I used to run a PJ 34mm MKII Amal on my iron 238 motor and went through the whole testing and development from the non-PJ 34 that I initially took off my TS1. It became evident how a PJ should function and of the benefits that it could deliver.

In hindsight, the good performance of the Amal's PJ bellmouth was in no small part a result of the PJ tube being in the right place and of the right length, which are 2 elements that it is easy to get wrong with the bolt-on PJs. It is good that the brass tube/jet of the bolt-on is longer than needed as this then allows it to be cut down to different lengths to suit different types and different sizes of carb (too long can be cut, too short and you're stuffed!).

The orientation of the PJ tube is controlled/dictated (certainly on my VHSH) by the shape of the carb body that it is being fitted to. As the VHS_ ranges have a web at the 12 o'clock position, it is impossible to mount the PJ at the optimum vertical position (optimum as it is then in line with the movement of throttle slide and is uncovered as the slide goes up) but efforts should be made to keep the PJ as near to vertical as possible. Given this task again, I would even have a go at reducing the lower half of the PJ's hexagonal body so that it could fit closer to vertical.

So, 2 things to consider if any of you decide to give one of these a go (in order of addressing):

1. Common PJ carbs from manufacturers have the PJ at the 12 o'clock orientation to venturi, for good reason, and you should try to fit a bolt-on PJ as close to vertical as possible (they will work at an angle though, just not as efficiently for the intended purpose).

2. The length of PJ tube is directly related to the throttle progression stage that it starts working (air flow and throttle movement [point 1]), with a shorter longer tube flowing extra fuel earlier and a shorter tube flowing extra fuel later. As the point of fitting a PJ is to deliver additional fuel at higher throttle positions and revs, getting the right length is very important.

FYI - My Amal PJ bellmouth tube is only 6mm long in a 34mm venturi, meaning that it protrudes less than 1/4 of the way (8.5mm) into the bore, which kind of supports why my initial bolt on position, which was into the middle of the bore, provided too much too soon, regardless of trying to adjust it by the screw (off or too much), whereas the current shortened tube allows me to adjust it as intended.

Adam

PS - This is not my Amal but does show that the Amal PJs used a common length of tube... http://i.ebayimg.com/00/s/NjgyWDEwMjQ=/ ... G/$_86.JPG

[rosscla]

Interesting work.

I think the old formula of main + power = main without power jet shows where the PJ is meant to have it effect - full on the main.

Did you cut the tube at an angle or straight?

[Adam_Winstone]

Hi Rosscla,

For now I have cut the tube 'square cut' as I'm not sure what length I'll end up with yet and, as the PJ screws into place (threaded), I didn't want to 45 degree cut it until I'm 100% happy with where it is (depth and position).

Yep, the PJ + MJ = Previous MJ is what I worked to with my Amal but the fully adjustable PJ does not make it quite so easy to know exactly how it would relate to jet sizes. There are some loose guide figures about but they seem like they might have been plucked out of the air, they may have been determined on a different carb size and certainly would have been determined on a different motor so I don't think they can be deemed too accurate.

I'm still working on it but have horrible grey plug colour, which I can range from weak to rich, through all shades of grey, but I can't seem to convince the grey to turn to brown (?).

Cheers for now.

Adam

[Knowledge]

A good thread Adam. Thanks for starting it.

Just to continue the thread as a source of information, I attach the following (sorry for any "teaching Granny to suck eggs")

I guess the PJ plus MJ theory needs a bit of thought. Amal jets are flow rated, so the sums are easy. I have never used a Mikuni, so I don't know how their jets are measured, but the Dell'ortos are measured by the size of the hole. Therefore, you would need to think about the cross sectional area of each jet. For example, if you replaced a Dell'orto 170 main jet with a 120 main jet and a 50 PJ, you might have a problem. Remember pi R squared?

A 170 MJ has a hole 2.27sqmm in size.
A 120 jet has a hole 1.13sqmm
A 50 jet has a hole 0.2sqmm.

Therefore, you might think about replacing a 170 MJ with a 140 MJ and a 96 PJ.

A 140 Jet has a hole of 1.54sqmm
A 96 jet has a hole of 0.72 sqmm.

Am I correct in my thoughts here?

[Darrell Taylor]

good post adam,,id ask does it need a power jet,some carb types do a great job of fuel enrichment thru increased operating rpm and some engines manage without ok when are equipped with retarding ignitions ,ive found piston port motors mostly dont need them and stay rich past peak often enrichening further but generally reed valve motors can benefit as they lean off past peak forcing the use of richer main jets to provide adequate fuelling into the overrev ,this then limits hp production,and decreases fuel economy ,the hp being beneficial in race applications ,problem for road users is there is an area in the transition where you can sit on the leaner main prior to full flow delivery from the power jet and find yourself in trouble but then richen the power jet a little more and your just fueling as heavily as you could have been with just a fat main jet which you know will deliver without being as throttle position dependant ,,the various clone pwk carbs have different power jet depths and found working with some it best to just blank them off and do without as is often done with the mikuni 30 too ,,,id not want to set up without a good yard stick ,be it dyno, egt or cht to evaluate

[Adam_Winstone]

^... thanks for the replies guys.

Funnily enough, although I'm not there yet, I had decided to turn off the PJ function (simply screw home the adjuster screw all the way) and rejet before going to a local bike night last night. My reason for doing this was that I've been going around in circles attempting to get a decent colour at the plug, which I've so far been unable to do with the smaller main and PJ operating. I can get it to ride well but still end up with the plug suggesting it is lean, even when adjusting so that it rides rich. I can't tell you what the plug is like yet (haven't had it out yet) but the bike rode really well.

Does it need a PJ?.... I'm not yet sure. This motor is running a reed and is also running an advance/retard ignition so the info given is really good food for thought, thanks.

Adam