powerjet carburators

[jensen]

Hi,

A few months ago while testing three sets of powerjet carburetors (CB72) on the dynojet, I noticed that seemingly identical carburetors performed very different.
A quick look didn’t reveal any differences other then the main jet, but that couldn’t explain the differences I measured.

There’s not much written about the powerjet carburetors , not on the forum, and in Bill’s Bible. The only thing I could find is that the powerjet system sucks fuel directly from the bowl and works in the higher rev regions.

I tested a early set of powerjet carbs (round float bowls) used in 1961 and 1962, a “early” late set of carbs (square float bowls ), used in 1962 and a “late” late set of carburators (square float bowls and a fuel tap screw underneath the bowl), used in 1963 and on (correct me when I’m wrong Ed).

The round bowl set gave a good performance, mixture a little lean in the low mid end (2500 to 3500 rpm), and a good higher region mixture (7500 to 10.000 rpm). The “early’ late set showed about the same results, but the “late” late set showed a very different graph. The overall mixture was a little to rich, but in the higher rpm regions it was on the lean side.

I compared all these carburetors closely and found out that one set CB72 carburetors with powerjet pipes have "closed" powerjet jets. The powerjet system fuel jet was closed and the powerjet system air jet was closed.
That means that everything is there, but it won't work. First I thought that the jets were clogged later I thought that someone closed the jets on purpose, but it seems to be a factory change.

It also appears that the carbs with a fully operational powerjet configuration have a 100 mainjet and that the carbs with the closed powerjet system has a 110 main jet.

All runs were done the same day, thus atmospheric differences were not the reason. All runs were made with identical mufflers (early stainless mufflers) on the same engine, on the same bike.

Can anyone confirm these findings ?

Jensen

[LOUD MOUSE]

Square bowl came along sometime in 1964.
The CB72 carbs were drilled and jets installed but I've yet to find a CL72 (same body but square bowl) with the passages drilled.
Same for the early 305 carb bodies.
During the 60's the Power Jet system was not appreciated and for some caused problems so many owners plugged the jets and increased the size of the main jet. ...............lm

Hi,

A few months ago while testing three sets of powerjet carburetors (CB72) on the dynojet, I noticed that seemingly identical carburetors performed very different.
A quick look didn’t reveal any differences other then the main jet, but that couldn’t explain the differences I measured.

There’s not much written about the powerjet carburetors , not on the forum, and in Bill’s Bible. The only thing I could find is that the powerjet system sucks fuel directly from the bowl and works in the higher rev regions.

I tested a early set of powerjet carbs (round float bowls) used in 1961 and 1962, a “early” late set of carbs (square float bowls ), used in 1962 and a “late” late set of carburators (square float bowls and a fuel tap screw underneath the bowl), used in 1963 and on (correct me when I’m wrong Ed).

The round bowl set gave a good performance, mixture a little lean in the low mid end (2500 to 3500 rpm), and a good higher region mixture (7500 to 10.000 rpm). The “early’ late set showed about the same results, but the “late” late set showed a very different graph. The overall mixture was a little to rich, but in the higher rpm regions it was on the lean side.

I compared all these carburetors closely and found out that one set CB72 carburetors with powerjet pipes have "closed" powerjet jets. The powerjet system fuel jet was closed and the powerjet system air jet was closed.
That means that everything is there, but it won't work. First I thought that the jets were clogged later I thought that someone closed the jets on purpose, but it seems to be a factory change.

It also appears that the carbs with a fully operational powerjet configuration have a 100 mainjet and that the carbs with the closed powerjet system has a 110 main jet.

All runs were done the same day, thus atmospheric differences were not the reason. All runs were made with identical mufflers (early stainless mufflers) on the same engine, on the same bike.

Can anyone confirm these findings ?

Jensen

[davomoto]

Jensen, I too have several sets of CL72 carbs, and the passages are not drilled in them either. All of the parts are there, just not functional.

[LOUD MOUSE]

Our CL72 wasn't issued with Power Jets. ..........lm

Jensen, I too have several sets of CL72 carbs, and the passages are not drilled in them either. All of the parts are there, just not functional.

[davomoto]

Ed, I'll have to post pics of the carbs when I get home. they have the brass plug over the jet, and the jet inside. They aren't functional.

[LOUD MOUSE]

Our CL72 carbs do have an aluminum threaded plug and under it was/is a main jet of a different size that the one being used.
I think that is what ya see. ......lm

Ed, I'll have to post pics of the carbs when I get home. they have the brass plug over the jet, and the jet inside. They aren't functional.

[jensen]

Hi,

During the 60's the Power Jet system was not appreciated and for some caused problems so many owners plugged the jets and increased the size of the main jet. ...............lm

Looking at the pictures I'm convinced that the power jet jets for the late square bowl powerjet carburetors are indeed factory closed., just as I suggested. I cannot find any info on this subject, so I think this is not written down somewhere.

and :

I looked at my collection of PJ carbs. (21 total)
I find that the Square Bowl issue (near as I know all CB72 with PJ carbs were Square Bowl) will have One Vent and it will be found on the Inside area (fuel inlet side) of the body and is a Brass Tube with a small hole.
The NOS/OEM Oval Bowl one I have has a small hole drilled in the same area but no tube. (I've never seen these on a CB72 engine) .....................lm

Ed, the things that you say about the powerjet carburetors, despite that you have 21 examples don't meet my observations (please Ed, I'm not trying to offend you here). The powerjet carburetors do show the brass pipe, but it's closed, not open.

I took a closer look to the carburetors tonight (as you suggest in these cases), and there's more going on. The CB72 powerjet square bowl carburetors don't have an air vent to the bowl (both sets, the "early" late and the "late" late set). The round bowl powerjet carburetors do have these air vents to the bowl.

The square type carburetors cannot "see" the atmospheric pressure directly, this is seen by through the power jet system. When you close the powerjet system, like in the "late" late type, there's no balance between the pressure in the bowl and the pressure outside.

A while ago I wrote about vapor lock, and on my 67 CB72 I have these "closed types" installed. These carburators don't have an air vent, so if the fuel evaporates, it cannot "balance" with the outside pressure anymore. This is of course a theory, but most likely the case.

The set of round bowl carburetors in the pictures say PW22 HOV.

Ed, especially for you I made some pictures, so you can see it yourself