Dynojet kit messes with choke?

[CatatonicBug]

I was going to put this in another thread, but I decided against hijacking that one and start my own.

let it sit that way a minute or two until the RPMS jump or it may even start boggin down

Funny you should mention that... I used to be able to have the choke on, and RPMs would gradually rise, and keep rising until I felt like the bike was going to explode (4k+)! Then, I installed a Dynojet kit, and suddenly, even after 3 additional cleanings, and checking all sorts of settings, the choke raises the RPMs to about 2k, then starts to drop off again. I push the choke in halfway, and the RPMs rise again, but only to about 2200, unless I hit the gas, then it climbs pretty good. This change was immediate after the Dynojet kit installation, so I know it was directly related.

What might cause the change? The dynojet kit doesn't touch the choke circuit, except maybe the change in the air jet... I just don't get how the choke could cause anything but a climb in RPMs as the engine warms up.

[DGXSER]

My only thought is this, the choke is actually an enricher. Choke implies cutting off the air flow to make the mixture richer, on these carbs, it does nothing to air flow, but increases the fuel being put into the mix.

So, the air fuel mix is rich and it will start and run that way, and may JUST need more than half "choke" to start (which is how mine acts). But as it warms up initially, it actually gets a bit to rich and will bog out a bit.

that is what I see happening anyway. Could be your Dynojets richened things up enough to put you in that mode.

[clcorbin]

I just don't get how the choke could cause anything but a climb in RPMs as the engine warms up.

It's actually fairly simple. The fuel enrichment on these bikes adds a bit of extra air and a lot of extra fuel. When a bike is cold, the fuel does not full atomize, so it can't burn. That's the whole reason you need extra fuel anyway is that the "normal" amount of fuel doesn't atomize fully, so the mixture is actually way to lean to run correctly.

But what happens as the engine warms up? The extra fuel DOES start to atomize more an more, so the engine starts to run richer and richer. Eventually, it gets to a point were the mixture is so rich the engine can't run properly, so the idle speed starts to drop (even with the extra air) and eventually the engine can die from too rich a mixture.

This is the reason it goes from fuel choke when cold (and the bike STILL doesn't want to run), then down to 1/2 choke as it warms up and finally to no choke with things are all nice and warm and the fuel is atomizing fully. It's also why you need more choke when it is COLD versus a cold start on a "cool" morning.

[XS1100_OEM4ME]

Fuel pump?

I was going to put this in another thread, but I decided against hijacking that one and start my own.



Funny you should mention that... I used to be able to have the choke on, and RPMs would gradually rise, and keep rising until I felt like the bike was going to explode (4k+)! Then, I installed a Dynojet kit, and suddenly, even after 3 additional cleanings, and checking all sorts of settings, the choke raises the RPMs to about 2k, then starts to drop off again. I push the choke in halfway, and the RPMs rise again, but only to about 2200, unless I hit the gas, then it climbs pretty good. This change was immediate after the Dynojet kit installation, so I know it was directly related.

What might cause the change? The dynojet kit doesn't touch the choke circuit, except maybe the change in the air jet... I just don't get how the choke could cause anything but a climb in RPMs as the engine warms up.

These bikes do not have fuel pumps. That is why the choke, is not realy a "choke" at all. A "real choke" does just what its name says, it "chokes" the air intake and thus add's gas to the mix. Not so on these bikes, the miss named "choke" add's more fuel by opening up flow to the carbs, not by restricting air flow. If you have plenty of fuel flow with your new kit, the "choke" will work dif. as the rpm will respound to the air/fuel mix wich is already rich and not needing the extra rich mix provided by the "choke"

[CatatonicBug]

These bikes do not have fuel pumps. That is why the choke, is not realy a "choke" at all. A "real choke" does just what its name says, it "chokes" the air intake and thus add's gas to the mix. Not so on these bikes, the miss named "choke" add's more fuel by opening up flow to the carbs, not by restricting air flow. If you have plenty of fuel flow with your new kit, the "choke" will work dif. as the rpm will respound to the air/fuel mix wich is already rich and not needing the extra rich mix provided by the "choke"

I am fully aware of the fact that the choke is actually an enricher, but my main question is, how does the Dynojet kit affect the carbs in such a way that the enricher reacts differently? The Dynojet kit does not replace the pilot jets or the jets in the float bowls, and the main jets are not even used at idle, and neither are the slide needles (according to the diagram in the Dynojet kit instructions). The only thing that leaves is the replaced air jets. Where do they work in this equation? Could that make any difference?

[clcorbin]

The enrichment circuit doesn't add pure fuel. It adds a very rich air/fuel mix. I would assume (but ONLY an assumption!) that the air is metered through the air jet and picks up fuel through the pilot jet (maybe not). I do know it is a super rich air/fuel emulsion that comes out of the enrichment circuit.

[XS1100_OEM4ME]

In trouble

I am fully aware of the fact that the choke is actually an enricher, but my main question is, how does the Dynojet kit affect the carbs in such a way that the enricher reacts differently? The Dynojet kit does not replace the pilot jets or the jets in the float bowls, and the main jets are not even used at idle, and neither are the slide needles (according to the diagram in the Dynojet kit instructions). The only thing that leaves is the replaced air jets. Where do they work in this equation? Could that make any difference?

Ok, not trying to make anyone less of an exbert Just trying to say what I have learned over the years The carbs on these bikes enrich the fuel when choaked, the butterflys are not effected (so no air flow is effected) nor is the die. level changed, still no change to air flow The only real change to "choke: is the gas flow from normal to rich to very rich I could be wrong, but I am not

[clcorbin]

Ok, not trying to make anyone less of an exbert Just trying to say what I have learned over the years The carbs on these bikes enrich the fuel when choaked, the butterflys are not effected (so no air flow is effected) nor is the die. level changed, still no change to air flow The only real change to "choke: is the gas flow from normal to rich to very rich I could be wrong, but I am not

I'm not saying the butterflys are affected (they are not, to the best of my knowledge). What I am saying is the enrichment circuit does not allow in only pure fuel. It is an extremely rich emulsion of fuel and air. That is the ONLY way to INCREASE the idle speed with this circuit.

If you have set your idle mixture screws, you have already optimized the idle A/F for each cylinder by richening/leaning the warm engine with each mixture screw. Given that you have already optimized the idle speed via mixture, how could adding extra fuel alone do anything other than decrease the idle speed from the "hot" idle speed? That's just like if you kept turning the mixture screws (in or out) past the optimum point.

The extra air for the high idle is not coming through the throttle plates. It is coming through the enrichment circuit itself along with the extra fuel.

[CatatonicBug]

Right. The air that comes through the air jets does not get fed through the butterflies. Without the air jets, there would be no way to get fuel to "suck" into the engine without moving the throttle. If you spray carb cleaner through the air jets, where does it come out? I honestly can't remember...

[petejw]

Hi CB,
If you were to squirt fluid in the air pilot jet (back of carb bell)
you would have fluid coming out the fuel pilot jet, block this jet
with your finger, fluid will come out of the mixture screw and the
3 small holes that sit near the butterfly.

The air jets on the dj kit are slightly leaner than the stock air jets,
190 compared to 185, dyno jet then compensates this by richening
the mixture screws (4 turns out).

The pilot jets either air or fuel can change how the enricher works,
as does the float level. Did you change the float levels?

other than the changes with the choke, is the bike running well?

[79XS11F]

I was going to put this in another thread, but I decided against hijacking that one and start my own.



Funny you should mention that... I used to be able to have the choke on, and RPMs would gradually rise, and keep rising until I felt like the bike was going to explode (4k+)! Then, I installed a Dynojet kit, and suddenly, even after 3 additional cleanings, and checking all sorts of settings, the choke raises the RPMs to about 2k, then starts to drop off again. I push the choke in halfway, and the RPMs rise again, but only to about 2200, unless I hit the gas, then it climbs pretty good. This change was immediate after the Dynojet kit installation, so I know it was directly related.

What might cause the change? The dynojet kit doesn't touch the choke circuit, except maybe the change in the air jet... I just don't get how the choke could cause anything but a climb in RPMs as the engine warms up.

The chock adds fuel to the system based on a stock setup. You have now changed the jetting so the chock is basically wrong for the jetting you now have. The chock fuel delivery is a fixed setup and can not be changed but you may be able to adjust its setup points by drilling new set points in the slide bar for the chock. This will work for you if you can confirm a proper chock setting by holding the chock somewhere between the factory settings and have the engine warm up as intended. My engine also needs a chock fix because it is not consistent since i added the 79 extra CCs, porting, headers and carb jetting etc.
Rob

[XS1100_OEM4ME]

Imho

I'm not saying the butterflys are affected (they are not, to the best of my knowledge). What I am saying is the enrichment circuit does not allow in only pure fuel. It is an extremely rich emulsion of fuel and air. That is the ONLY way to INCREASE the idle speed with this circuit.

If you have set your idle mixture screws, you have already optimized the idle A/F for each cylinder by richening/leaning the warm engine with each mixture screw. Given that you have already optimized the idle speed via mixture, how could adding extra fuel alone do anything other than decrease the idle speed from the "hot" idle speed? That's just like if you kept turning the mixture screws (in or out) past the optimum point.

The extra air for the high idle is not coming through the throttle plates. It is coming through the enrichment circuit itself along with the extra fuel.

IMHO, there is no increase in "air" to the mix, in fact, a real choke cuts back on Air flow to "increase RPM" when cold. Less air, more fuel is what makes it work. On these bikes, they increase the fuel and keep the "air" the same, same effect as a "real choke" rich mixture. Don't know who told you that the "choke settings" on thease bikes somehow add "air" to the mix, but, I think they were wrong, can't see it in how the carbs are made or how the fuel flows on choke

[clcorbin]

The chock adds fuel to the system based on a stock setup. You have now changed the jetting so the chock is basically wrong for the jetting you now have. The chock fuel delivery is a fixed setup and can not be changed but you may be able to adjust its setup points by drilling new set points in the slide bar for the chock. This will work for you if you can confirm a proper chock setting by holding the chock somewhere between the factory settings and have the engine warm up as intended. My engine also needs a chock fix because it is not consistent since i added the 79 extra CCs, porting, headers and carb jetting etc.
Rob

Rob, sorry bud, but you are wrong. Let's go over a real choke on something like a Holly 4150 4 barrel carb first. When the choke is activated (via a thermal bimetallic strip or mechanically with a choke cable) it does TWO things:

• Closes the "choke" plate above the air horn on the carb almost completely closed.
• Rotates a fast idle came to hold the throttle plates in a more open position.

The choke plate causes an artificially high vacuum signal to be seen at the ventures (normally, the vacuum signal is ONLY caused by air velocity so is proportional to airflow through the carb) due to the inlet restriction ABOVE the venturies, which causes extra fuel to be introduced by the carb. This is what actually richens the mixture by "choking" the airflow on the carb. This is what provides the extra rich mixture that will burn properly even through the engine can't fully vaporize the "normal" A/F mixture while it is cold.

The fast idle cam is what raises the idle speed. And it does it simply by opening the THROTTLE plates more than normal (aka: it holds it off the throttle set screw). Understand this: you can NOT increase the engine speed WITHOUT extra air! Period. (Yes, this is assuming the "normal" mixture is close to correct!)

The engine MUST have air and fuel in the correct ratios to make power. Try to get your engine to idle by holding the throttle wide open. It won't work if the gas tank is empty, no matter how wide you hold the throttle. It takes air AND fuel to make power. Adding extra fuel can NOT make more power in a properly tuned engine, all it can do is get a cold engine to make it's normal amount of power by over coming the poor atomization of the fuel in the cold engine.

When you first start your 350/351 on a cold day, the idle is a bit higher, but not super high. Even with the excess fuel being added by the choke, it is still running lean. But with a lean EFFECTIVE mixture and the higher air/fuel flow into the engine due to the fast idle cam, the engine is still idling faster than it normally would as it making slightly more power than it normally would.

As the engine warms up and the fuel starts to atomize better, the mixture starts to risen up to normal and the engine is making MORE power on the excess A/F that is going into it due to the choke and fast idle cam, so the idle speed starts to increase. If nothing changed in the carb (aka: choke is opened up and fast idle cam is pulled back), eventually the idle speed would peak out in the 1800-2000 rpm range and then start to drop off again.

The reason it drops off is the same reason it picked up speed: the engine is getting warmed up and it is more and more effective at atomizing the fuel in the mixture. Unfortunately, just like an engine can't make maximum power on a lean mixture, it can't make maximum power on a rich mixture either. So, with a fixed throttle setting (set by the high idle cam) and the effective mixture going from lean (slower idle) to stoichiometric (very fast idle) to rich, the idle speed HAS to drop because the engine is now making less and less power, so the idle speed drops to compensate (friction decreases as the engine speed decreases and visa-versa so the equilibrium engine speed is dependent upon how much power the engine is making and the external loan upon it).

Now if you have the "automatic choke" version of this Holley carb, it uses electricity to heat the bimetallic strip the instant you start the vehicle. The strip starts to uncoil slowly and it slowly opens the choke plate and slowly rotates the lock pin on the high idle cam out of the way so the engine can eventually drop back down to it's normal idle screw position. When things are working well, it all works quite well and you as the driver don't have to do anything other than drive the vehicle and understand that it won't respond as quickly as normal until it warms up.

So, back to our bikes. They do NOT have a "choke". There are no butterflies ahead of the venturies that "choke" the carb and cause it to draw extra fuel into the mixture. Nor do they have a high idle cam that causes the throttle plates to be held open and additional amount while cold. That hardware simply doesn't exist on these bikes.

What it DOES have is an enrichment system. And that system does the exact same thing as the Holley choke system: it provides more air and more fuel to a cold engine to enable it to run and idle when it is cold and can't atomize fuel properly.

But it does it in a completely differently way. Instead of the choke and cam, the enrichment circuit (when open), allows the carbs to draw in a very rich mixture of fuel AND air (aka: an emulsion) into the normal A/F flow. The emulsion mixes with the standard A/F flow and causes the mixture entering the cylinder to be both richer AND have higher air flow. This is what increases the power produced by the engine and increases the idle speed.

And just like our Holley, the idle speed increases to some max, then starts to decrease. Usually at this point the rider will push the enricher lever in one notch, which causes the idle speed to drop and the engine to "clean up". Why?

Let's look at the enrichment plunger. Basically, you have a passage that pulls an A/F mixture through the pilot jet a air jet. This mixture enters the engine in the venturi through the enrichment hole. It would just flow and flow all the time if we let it. But look at what happens with the enrichment lever is all the way in: That brass plunger on the side of each carb is pushed in and BLOCKS this hole into the venturi, which stops the A/F mixture from entering the venturi and engine, so your engine idles normally. But how do we get a two stage "choke"???

Take a close look at the enrichment plunger: It has a little brass rod in the center of it. When the lever is pulled all the way out "full choke", the plunger opens the enrichment passage AND the little rod is pulled all the way out of the passage, allowing full and unrestricted flow of our rich A/F mixture into the carb venturi (ok, it IS restricted by the diameter of the passage). When you push the lever into the "low choke" position, the plunger is pushed further in, but it still does not block the enrichment passage. BUT the little brass rod DOES enter into the passage. This decreases the effective area of the passage and decreases the amount of enrichment mixture that can flow into the venturi, thus reducing both how rich the engine's mixture is AND how much air is flowing into it. These decreases the idle speed and cleans up the engine by leaning the mixture out once more.

When the engine warms up more, you push the lever all the way home and the plunger seats against the enrichment passage and blocks it off completely, forcing the engine to run on just the air and fuel that would normally enter it through the venturi and jets.

Sorry for such a long post, but there is a lot of misinformation on how the fast idle / fuel enrichment system works on these bikes. It's not rocket science, just a little bit of physics. But it DOES add extra fuel AND extra air to work.

[CatatonicBug]

Thanks for the detailed description Clint! The fact that we can't see the actual flow of air and fuel while running seems to make these carbs a bit mysterious sometimes. The more times I take them apart, the more I slowly begin to learn how the physics actualy functions inside them.

other than the changes with the choke, is the bike running well?

Yes. Once the engine is warm, which does not take an unusually long time, and I push the choke lever in, everything runs perfectly.

I had not messed wth the float levels since I got the bike, since I never seemed to need to. Plug color was always really good, and sync and colortune (at idle) always worked great. However, when I pulled the choke lever, the RPMs would always slowly climb over 4K. I'd push he lever in the 1/2 choke, and the RPMs would drop, then slowly climb to 4K again.

Now, after the Dynojet kit was installed, I noticed my plug color was super WHITE! That, along with the strange change to the way the engine responds to the choke made me start wondering what was going on. So, I decided that I should check the float levels. They were set REALLY lean, based on the manual for the '80 carbs (I have an '81, so generally fuel, not foat, levels are measured with the carbs ON the bike). I richened the float levels, and set them to the exact setting that the manual calls for on the 'SG. Of course, this didn't affect the choke response (I'm learning while I'm doing this).

I just read this in another thread:

OK, just me again, but i have read many who say their bikes rev to 4000 rpm with the choke on full I am saying, from starting many of these bikes back in 1979-80, that is not right (mine did the same when I got it with Mac exaust and PO's handy work. After re-build of carbs and install of OEM exaust, it starts on full chock, rev's to 2000 or so and kicks down to 1300 rpm on 2nd choke, this all takes about 1 min. From there it is off to the races ALL the XS bikes I have rode (and ther have been some) worked this way then and mine works this way now

After instaling the Dynojet kit, this describes exactly what is happening. I guess my main concern is that, since I ride all winter, in temps below freezing, I am worried that the "new" choke respose will affect my ability to start on cold mornings. However, if the new choke response is "normal" then I can simply ignore it and wait for the winter temps. As mentioned earlier, everything else is running perfectly. Should I just smile and be thankful that the Dynojet kit put my choke response back to were it is "supposed" to be?

[clcorbin]

Personally, I would say smile and ride!