Topic: Poor mans jetting kit

[Flying J]

From Cycle X
anybody try this? Thought i might give it a shot on my 550


"This extra gas circuit procedure has been done for years.
Carburetor jetting is an on going topic of discussion.
So, here is what we do from time to time.
Tap into the float bowl and insert pilot jets into the throat of the carburetor.
This procedure will give your bike a boost from  approx 1/4 to full throttle.
Also, it will allow you to check your float level at a glance and purge unwanted sediment from your bowl."

After thinking about it, i dont think its a good idea. Ill stick to the adjustments on the carbs that i know what range they work for.

[cb650]

 "purge unwanted sediment from your bowl"     right into the engine.     

[eideteker]

Call me crazy (or rookie, either works) but 'til you've done all that wouldn't it have been more efficient time- and money-wise to just get a set of bigger jets?  I'm all for bodgery if it's functional, but this seems like a waste.

[Flying J]

Yeah, its like adding another fuel circuit for the sake of adding it. Like tunning carbs isnt complicated enough with the few adjustments we already have.

[buffalogt750]

Nothing new. Lectron has been making power jet carbs for years. Used mostly on two strokes.

[Simpson]

I'd have to say, if the engine had the extra capacity from the start, I think Honda would have incorporated it into the existing carb. I'd have to assume they did some extensive testing with jetting back then.

[Terry in Australia]

Nothing new. Lectron has been making power jet carbs for years. Used mostly on two strokes.

Dead right mate, and nothing wrong with it either. There was a lot of stuff that Honda could have done to make our bikes better (oil coolers, twin discs with decent calipers, steering dampers, fork braces, better suspension, better ignitions, etc etc) but that would have made the bikes too expensive in the showroom, so they compromised a lot of things to keep costs down, hence the huge "aftermarket" industry that grew around the CB750 in the 1970's.

The "Power Jet" on lectron carbs works well, and the benefit that I can see straight away is that the little squirt of gas on accelleration will help reduce "throttle lag" when you open the taps. As far as tuning goes, just block off the hose if it's causing you problems, not that i think it will.

"Purge unwanted sediment from the float bowl right into your engine"? Can't see a problem here, it's already passed through your petcock filter and any additional filters that you've installed in line, passed through the fuel valve filters, so providing it'll get through your "power jet", (previously known as the pilot jet) it is so tiny it's measured in microns, so won't hurt your engine, or performance. Cheers, Terry.    

[wannabridin]

so terry,

would you do something like this to your bike?   

my concern would be for float bowl height?  would this affect the height you've set them to and allow the bowls to drain too far leading to a lean condition?  wait, wouldn't the lean condition be counter-acted by the fuel going back into the motor?  hmmm, seems like a bit of an infinite loop resulting in a stock carb condition.  at least to those that aren't carburetor saavy like some of the old timers??

 

[Cormac]

Looks like they are attempting to set this up like the fuel bars on a predator carb. Only problem I see is that the jet isn't "in" a venturi. The predators put theirs just after the venturi plates, basically the same place as the jet on bike carbs anyway. Would there be enough of a pressure drop there to actually draw fuel?

[edbikerii]

The only problem with the carbs on the 77 CB550K (yes, even with the stock airbox) is that when you whack the throttle open quickly, the engine gets too much air, not enough fuel, and stumbles slightly until the fuel flow catches up.  This problem was corrected later when CV carbs became the rule on motorcycles, and the slides would only rise to pass the air that the engine actually could consume.

The CB550s would have enjoyed great benefit from an accelerator pump like that on some of the later 750 carbs.

Ported vacuum is highest when the throttle is initially whacked open, thus pulling more fuel through these jets when needed.  Then the ported vacuum tapers off to equal manifold vacuum.

Seems to me these would actually work pretty well.

[Terry in Australia]

so terry,

would you do something like this to your bike?  

my concern would be for float bowl height?  would this affect the height you've set them to and allow the bowls to drain too far leading to a lean condition?  wait, wouldn't the lean condition be counter-acted by the fuel going back into the motor?  hmmm, seems like a bit of an infinite loop resulting in a stock carb condition.  at least to those that aren't carburetor saavy like some of the old timers??

 

Yeah mate, I've got a few sets of carbs here holding up my workbench, so I'd give it a shot, but I've got "Pumper" carbs on my F2, 29mm CR's on my K1, and I'm not gonna modify my K0's "unobtainium" cable carbs, and that's probably why I haven't done it already. I need to point out that this is common practise on some older dirt bikes (TS Suzuki's for instance) so is not "Snake Oil", it's just (obviously) something that a lot of folks here probably haven't seen before.

The amount of fuel passing through the pilot jets would be minimal (try finding a wire fine enough that will unclog one) and if the OEM fuel petcock will easily keep the fuel up to any carb ever fitted to a CB750, it'll have no problem keeping your float bowls filled. Cheers, Terry.

Looks like they are attempting to set this up like the fuel bars on a predator carb. Only problem I see is that the jet isn't "in" a venturi. The predators put theirs just after the venturi plates, basically the same place as the jet on bike carbs anyway. Would there be enough of a pressure drop there to actually draw fuel?

Put your hand over the back of your carb with the engine running mate, if it was small enough it'd be sucked in one end and spat out the other.  

[scunny]

keep those  rough hands of yours out of the discussion Terry

[Zaipai]

Ok I think if I can get my hands on a set of carbs I will give i a try.. I have seen this setup on other bikes but never asked how it worked. Seems it should work ok..

If I do it and it works I will update this post.. probably wont get to it till later in the spring tho.

.: Scott :.

[Flying J]

I like the idea that it would deliver fuel when i cracked the throttle open fast but i see two problems with that idea.

1) For that jet to deliver fuel it would have to suck it all the way from the float level, through the tube, up to the jet before it spat out any fuel.

2) the actual jets would be at a higher pressure differential and closer to the fuel so they would deliver fuel before that jet ever did.

Just my observation. I think it would work good to richen the mixture over a broad spectrum. For example, instead of raising the needle and changing the main jet you could just add this and accomplish both those task at once.

[edbikerii]

Yeah, fair enough.  I suppose that it is true that the fuel level will settle down in the tubes to the same as the float level, as there won't be constant vacuum at the jets (at idle, for instance).  The thing is, at WOT the vacuum will be pretty low, too.  Enough?  Maybe.  Ported vacuum gauge would tell the whole store.  Personally, I don't care enough to try it.  The slight hesitation is so minor that I think it is irrelevant.

Also, it really isn't that hard to tune the jets the normal way.  So, if it doesn't work like an accelerator pump, then it is a waste of time, in my opinion.

I like the idea that it would deliver fuel when i cracked the throttle open fast but i see two problems with that idea.

1) For that jet to deliver fuel it would have to suck it all the way from the float level, through the tube, up to the jet before it spat out any fuel.

2) the actual jets would be at a higher pressure differential and closer to the fuel so they would deliver fuel before that jet ever did.

Just my observation. I think it would work good to richen the mixture over a broad spectrum. For example, instead of raising the needle and changing the main jet you could just add this and accomplish both those task at once.

[Flying J]

I concur

[TwoTired]

A couple of arm chair comments, no actual experience with doing this.  I'm, just thinking about the physics of the mod.

The force drawing fuel through the added jet is the carb throat vacuum ahead of the main venturi.  It won't be variable with the air speed through the throat, unless the filter media ahead of it generates increased pressure loss with increased air volume passing through it.
It WILL be highly effected by the amount of restriction or pressure drop caused by the air filter media ahead of the jet.

From a physics viewpoint, I don't see the advantage of adding another jet in that location.  Strategically, you could make some fuel metering adjustments without removing the slides or main jet.  But, an added jet in that position is NOT better than having the carb's (slide needle, main air jet, emulsion tube holes, and Main jet size) internal fuel metering corrected/compensated for changes from stock,.  Anyway, that added jet won't see much of a pressure drop across its orifice until the slide is opened a bit.  Then, as it is only about a quarter of an inch away from atmospheric pressure near the carb throat, not much differential is going to be present.  Add an inlet runner and inlet restriction, and this is going to greatly effect how much that added jet flows.  I notice it is ahead of the choke.  So, it won't contribute during choke operation.
Seems like a compensation for carb re-jetting, without actually changing the internal jetting.  Getting the flow to balance right seems just as hard as going inside the carbs, but adding complexity.

It most certainly won't compensate for sudden open throttle wheeze/lean stumble.  Suddenly opening the throttle, presents near atmospheric pressure to all the jet orifices feeding the carb throat, wherever they may be.  Since all the jets flow at a rate related to the pressure differential between jet entry and jet exit, loss of differential causes fuel flow cessation.  This is why an accelerator pump, providing a mechanical push on the fuel rather than relying on air pressure differentials, alleviates sudden throttle opening stumble.

If you're interested...

[Buber]

So, I presume that if my carbs are the CV ones WITH accelerator pump, I have those worries solved..

I'd love to put some magic jet in my carbs to boost performance and reduce the fuel consumption, alas.... physics forbid that.... 

[Markcb750]

I wonder (as a few here have noted) how any significant negative pressure is applied to the jet orifice to allow atmospheric pressure on the gas in the bowl to be pumped through that set up. 

The other jets are within the venturi, an area of lower pressure due to the  acceleration/higher velocity of the air molecules. Additionally the existing venturis are on/near the surface of the venturi, where the highest negative pressure is. This jet appears to be at an area of the carburetors that I would expect little or no pressure drop, just air moving past the orifice.


As TT states the physics don't tell me this is a better method then re-jetting.


I do not see any problem with debris as the stand pipe for the drain tubes are in the top of the bowls, generaly above the fuel level.  This makes me wonder if the floats will need to be set higher to keep the tubes wet, or are the tubes shortened?  Lots of issues here including fuel running out the body of the carburetor because the drains are not functional. 

[wannabridin]

Mark,

you make a good point about the lack of a drain, because there's no simple way of getting it to drain without unplugging the hose to the inlet body.

TT,

with an increase in runner length, I would think that the pressure drops relative to atmospheric would be greater at the jets location in the inlet body.  what about if velocity stacks were added?  These increase the air speed due to their design, which would slightly aid in the ability of the jets to draw in more fuel, albeit is probably negligible.

My only concern with the set up is the fact that a high enough vacuum cannot be drawn that close to the inlet that will atomize the fuel well enough to cause a beneficial burn to the a/f mix.  it seems to me that even with a small idle jet that you would be nearly dripping small droplets or streams of fuel into the carbs, which would just burn slowly, even possibly retarding the flame because of wetting on the piston.  i don't know enough about carb operation and tuning to have any merit to my statements, just my ignorant thinking of the subject.

[Zaipai]

Well from what I get from this thread is that it probably wont work. That said, why would Cyclex claim it does, or do they.. Seems like they do. Are they looking for more sales of the parts needed to do this? Also seems like bigger jets would do the same thing, and wouldn't the motor just run richer with out really any gain to power?

I am confused at this point..

.: Scott :.

[edbikerii]

Again, we would do well to place a vacuum gauge at that location to know with any certainty what that jet is doing.

[edbikerii]

Perhaps cycle-x can show us (or actual potential buyers) the vacuum readings at that point in the intake at various throttle openings and rpm.

I'm not from Missouri, but I'd need them to "show me" before I laid out any cash for that mod.

Well from what I get from this thread is that it probably wont work. That said, why would Cyclex claim it does, or do they.. Seems like they do. Are they looking for more sales of the parts needed to do this? Also seems like bigger jets would do the same thing, and wouldn't the motor just run richer with out really any gain to power?

I am confused at this point..

.: Scott :.

[Markcb750]

Perhaps cycle-x can show us (or actual potential buyers) the vacuum readings at that point in the intake at various throttle openings and rpm.

I'm not from Missouri, but I'd need them to "show me" before I laid out any cash for that mod.

Well from what I get from this thread is that it probably wont work. That said, why would Cyclex claim it does, or do they.. Seems like they do. Are they looking for more sales of the parts needed to do this? Also seems like bigger jets would do the same thing, and wouldn't the motor just run richer with out really any gain to power?

I am confused at this point..

.: Scott :.

Attach a gauge to the drain connection point, and one to the set up port...Cycle x should be able to do that in just a few minutes.

Is that motor painted gold or is it anodized?  Just curious.

[Simpson]

As TT states the physics don't tell me this is a better method then re-jetting.

Not to beat a dead horse
+1 changing the main jet seems to accomplish the same outcome
I have confidence Honda sized it correctly.