Another question for Chuck

exupturbo · Post by **exupturbo** » Thu Jan 02, 2003 5:04 pm

I'm running stage 3 dynojet, 160 main jets and the needles as low as they will go. My problem is bad overfueling up to around 4500rpm making the bike splutter in warm weather untill I reach the above revs. Will fitting the stock needles or going back to stage 2 help? I need the big main jets for when I'm on boost and was under the impression that main jets do not affect low rpm running but this does not seem to be the case. Thanks for any help. Mark

ChuckD · Post by **ChuckD** » Thu Jan 09, 2003 1:46 am

Dear Turbo, I wish I could be of more help, but the turbo world is one I have never touched. You probaly know more about it by far than me. I worked for Yamaha and for tune shops, that's where my expertise comes from, but for turbos, I really am at a loss. I can try and dig up some ideas on your dilema, but can't really say that I have tried it though. Chuck D.

exupturbo · Post by **exupturbo** » Thu Jan 09, 2003 3:18 pm

The carbs are the original 38mm CVs from an RU Exup and work great when under boost but are giving me a headache trying to set up low rpm fueling. If for arguments sake I had no turbo do you think that 160 main jets would have an effect on carburation up to say 4500rpm? as there is no boost at all up untill this point. As in one of the other posts, thanks for your input on the site. You are the only person here that does this stuff for a living and are far more clued up than us weekend tuners. Mark

flyingcircus68 · Post by **flyingcircus68** » Fri Jan 10, 2003 12:00 pm

Mark, Where are your bowls being vented to? I assume that they are connect to the plenum just before the carbs. Take into consideration that as demand for air increases, the velocity of the air increases past your vent point and causes a drop in pressure in your bowls. If you jet to make up for this phenomenon, then it will run too rich when velocity is low. You could rig up a manometer to see how pressure changes in your bowls under various conditions. Ideally you would want the same pressure in the bowls as that which is in the plenum. You might have to tap into another part of the plenum which doesn't see high air velocity. I would try tapping on the side opposite from the duct delivery the air to the plenum. You might also want to play around with the main air jet. The purpose of the air jet is to linearize the fuel curve. The relationship between air pressure and velocity is exponential, so with the air jet, the carb would run increasingly rich with higher air demand. The air jet acts similarly to a pitot tube where air velocity creates positive pressure. The summing of the positive pressure from the airjet and negative pressure from the needle jet is balance by the size of the air jet. You might have a case where the air being introduced into the carb is biased toward the lower half of the carb inlet. This would enhance the positive pressure being introduced into the main jet circuit and cause lean out. To answer your question about the main jet - yes. It has an affect on mixture at any throttle position, except idle. Its the dominant circuit at full throttle, but plays only a minor role in the lower rev range.

exupturbo · Post by **exupturbo** » Fri Jan 10, 2003 1:41 pm

Sounds very complicated to me. I have not got as much experience with these carbs as I would like. I do not think the positioning of the vents is causing the problem, here's a pic of my half built plenum/intercooler. <img src="http://www.digitalpose.com/mbr/1/9324/p ...  the carbs and all the boost signals take their air from the same face, and the plenum is plenty big enough to prevent really bad turbulence. What would be the best thing to alter on the carbs to lean off the jetting at revs up to 4500? Thanks Mark

flyingcircus68 · Post by **flyingcircus68** » Fri Jan 10, 2003 7:24 pm

Mark, my interpertation of your system is as follows: air is introduced to the plenum directly from the intercooler. the air flows out of the intercooler, heading toward the back end of the bike. It has to do a sharp bend downward to enter the carb throat. The airflow in your carbs is be biased to where the majority of flow will be in the zone occupied by your air jet. It looks like the pressure taps are in the zone between the carb throats and intercooler ( ahead of the carbs). If this is the case, then I would move the pressure taps off the face and onto the cover you removed. The best place to put it is the zone on the plenum that would get hit with air flow if it headed straight out of the intercooler. Try that first to see how it runs. I would expect this to make your bike run richer. You will then have to go down in the main jet and air jet. You will need a smaller air jet because the bias of the air flow at the carb inlet accentuates the action of the air jet and causes the main circuit to go lean with higher air demand. Think back to the stock airbox. It had individual stacks attached to each carb with a bell shaped entrance and a slight reduction in area from inlet to carb attachment point. This insured smooth air that centered the flow into the carbs. This situation is gone; your airflow is not centered. Playing with the airjets should help with this problem. From looking at your picture, it looks as if the carbs protrude above the face shown. That would create a situation where the lip protruding would create turbulance at the carb entrance. It must be flush, or below. If your curious and you have some time on your hands, you can your intercooler, plenum and carbs to a home made test rig and map the velocity gradiant at the carb throat. Hook up one of the center carbs to a shop vac. Block off the part of the intercooler that doesn't directly face the carb and make a plexiglass plenum cover. Cut a hole over the carb to be tested and glue a finger from a rubber glove into it. put a hole in the end of it and glue in a small diameter tube. Brass tubes from hobby shops are nice. the tube diameter should be small; somewhere around .05 in.This rubber finger will allow you to manipulate the tube without causing a leak. You could hook up this tube to a water manometer. The manometer would simply be an clear acrylic tube similar to what you find in carb sync tool. The end can be dunked in a test tube full of colored water, or some other type of reservoir. The other end of the tube can be connected to the brass tube mentioned above with vinyl or rubber tubing. Tilt this acrylic tube at like 15 deg. from horizontal to make it more sensitive to pressure changes. Turn on the shop vac and position the end of the brass tube at various points around the carb inlet to see determine where the majority of the flow is occuring. The point that cuases the highest reading with your manometer has the highest flow. If you want to accurately map this flow path, you could put a grid on cardboard, or an index card, and stand it edgewise on the carb inlet. You can then move the brass tube around to take your measurements and plot position against inches of water on graph paper. Assingn colors to different pressures and go to town on your graph with color pencils. Areas that have high pressure indicate static air, low velocity, or turbulance. This excercise won't give you jetting numbers, but will allow you to visualize whats going on. If all this is too much banter, let me know to tone it down. Paul

flyingcircus68 · Post by **flyingcircus68** » Fri Jan 10, 2003 8:09 pm

Mark, in my previous post I failed to mention that in your situation where the carbs are between the turbo and engine, they see a varying volume of air and variation in density. Carbs are designed to change the amount of fues being delivered based on the volume of air. Your carbs don't recognize changes in air density, and therefore don't adjust the volume of fuel being delivered to match boost pressure. With that in mind, you will always have a mixture that is richer than it would need to be at low boost conditions. Having a smaller main air jet can help by creating a fuel curve that spikes under highest demand conditions. The ideal solution would a separate circuit that delivers extra fuel under boost. You never mentioned anything about your fuel pump. You know that the stock pump delivers fuel at a low pressure and under boost, the float bowl pressure approaches, and can exceed, the pressure of the pump. If that happens, there would be little, or no, incentive for the fuel to leave your tank and refill the bowl. At the very least, your fuel level would drop in the bowl and cause lean out. Paul

exupturbo · Post by **exupturbo** » Sat Jan 11, 2003 4:16 am

Thanks again, It seems like the air jet may be the key. My turbo is off being fitted with a bigger compressor wheel at the moment so I can not try out any carburation mods at the moment. Unfortunatly the plenum/ intercooler is all welded up now and without choping it up I can not get to the inside. The top of the carbs are flush with the floor of the plenum chamber allthough it is not very clear in the pic. Fuel system consists of a 30psi rollervane turbocarb fuel pump and a Malpasi boost referenced fuel pressure regulator from a Lotus Esprit Turbo HC which are fitted with carbs. Fuel pressure is set 2.5 psi above boost. The strange thing is when I first did the conversion it seemed to run OK at low rpm, and I can not remember at what point I changed over to the stage 3 dynojet kit which does block up some air holes on the front face of the carbs. I really do not think that it is a problem with the way the air enters the carbs as th one thing that has stayed constant is the design of the plenum. I have not got a manual with me at the moment and am unsure as to the location of the air jet, can I assume that it is inside the float chamber along with the main jet? and are different sizes easily obtainable? When I get the turbo refitted I will try removing the stage 3 bungs from the carbs and try it out. The only problem being that it is winter here and the cold air makes the jetting problem a lot less severe, in hot weather it is a major pain. Mark

flyingcircus68 · Post by **flyingcircus68** » Sun Jan 12, 2003 6:42 pm

The air jets are visible in your picture. If you look at the bottom of the carb inlets ( top in picture), you will see the recessed holes with screw in brass inserts. One of those is the main air jet. Air enters through the air jet and proceeds to the emulsion tube that surrounds the needle jet. The emulsion tube is where the summing of the pressure from the needle jet and air jet takes place.

Guest · Post by **Guest** » Wed Feb 19, 2003 11:16 pm

Who put the turbo on your bike,who built the motor,and what type on turbo is it. I ask this because I have a turbo setup for my Bimota YB-11 (Yamaha YZF 1000 motor) that I need so info on. I am real good on normal asperated bikes, but this turbo thing is new. Any info will help. Ernest at 1-785-883-4399

exupturbo · Post by **exupturbo** » Sat Feb 22, 2003 4:57 pm

I scratch built the conversion myself. I used a IHI RHB52 turbo American part no. 5T503 English part no. VC19 as recomended in Joe Hailes book on turbocharging bikes. The compressor side of this turbo is a bit on the small side and I have recently fitted a bigger compressor wheel. The Exup engine is VERY tight for space when fitting a blow thru turbo and I had to make do with a simple log type exhaust manfold to keep the turbo within the fairing and high enough to allow oil drain back to the sump. I also had to machine a billet ali. oil filter re-location bracket that moves the filter thru 90 degrees so that it points downwards to allow enough room to fit it all in. Mail me at toolmaking@yahoo.co.uk and I can take some pics of any area you are interested in and answer any questions that I can. I am not a bike tuner, only an engineer who gets bored too easily and have made up my conversion as I went along. Mark

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Another question for Chuck

Another question for Chuck

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Re: Carbs

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