speedzephyr Posted January 28, 2009 Report Share Posted January 28, 2009 Back on the old site I had a turbo 550 zephyr I built, it ran (pretty quick) but it was never really right. The A/f is ok at and slightly off idle, then it begins to lean out toward midrange ultimately leaning to 17:1 right before the turbo spools. Once it gets about 2lb boost A/f goes rich and often goes out of range on the gauge which is less than 10:1. Tuning to get this far was a PITA, and have found the problem to be the intake pilot tube (feeding more than Manifold press to the carb bowls), it has such a pressure difference between carb bowls an throttle bore I had to put smaller than stock mains it to lean out the upper rev, and they're small enough they cause it to go lean in midrange when the engine isent under boost. Ok that looks pretty confusing, hopefully you, the reader, have turbo experience and can understand that! To summarize, the motor is way lean off boost but way rich on boost, problem being too effective of a pilot tube. But the question is WTF is wrong with my pilot tube? It is made to match all the others I could find (mostly on Old skool suzuki)----> I would ask on the OSS forum but they get fired up pretty easy over there especially if I was asking about a Kawak... almost guaranteed to get flamed my pilot tube is in the intake tubing about 100mm from the turbo, it is about 6mm ID mounted at 45deg. and slash cut toward the turbo centered in the tube. The tube I made is from aluminum and threads into a mounting boss so I can take it out fook with it and reinstall, just looking for some insight before I go hacking on it. I'm thinking about shortening it to get it out of the tube center an putting a straight cut on it. OH It runs on on E85 only, (85% alcohol 15% Gasoline) so while the lean is bad, it has yet to hole a piston I dont have a pic of the tube (its on the bike) but here's one of it in the intake doesn't help much though Link to comment Share on other sites More sharing options...
blow_away Posted January 28, 2009 Report Share Posted January 28, 2009 That is a great bike, good to see you have not given up on it Contact Tim Blakemore, he knows loads about building turbo bikes, and a Kawasaki man as well! http://www.timblakemoreracing.co.uk/ Link to comment Share on other sites More sharing options...
spacemonkey Posted January 28, 2009 Report Share Posted January 28, 2009 I've not dealt with blow through turbos but recall seeing this problem solved by fitting a piloted (piloted, as in switched by a pressure from a separate feed line) 2 way valve in the pilot intake. The piloted switching feed is taken from the turbo and the inlet feeds into the valve are split between open atmosphere and the turbo with the outlet supply into the carb bowl pilot, and in simple terms means that when the turbo is not boosting the carb bowl is pressurised by atmospheric pressure but as soon as the turbo starts boosting the pilot feed pushes the valve over and it feeds the carb bowl from turbo boost pressure. In theory this should mean that the carb bowl is pressurised in direct relation to inlet manifold pressure. Am I missing something? Surely you just connect the plenum to the float bowl vents and make sure the pressure in the float bowls is the same as the pressure being fed through the carbs? This way the jets are simply operating off the pressure difference caused by the venturi. Link to comment Share on other sites More sharing options...
speedzephyr Posted January 29, 2009 Author Report Share Posted January 29, 2009 That is a great bike, good to see you have not given up on it Thanks! But isn't the plenum at a pressure lower than atmsopheric pressure off boost and higher than atmospheric pressure on boost. My understanding was that a carb bowl has to be at a pressure higher than the inlet pressure in order for fuel to be drawn out of the jets? I know from fitting FCR's inside an air box that the float bowl that you have to be carefull where the float bowl pilot feeds are vented to and that this must be outside the airbox to get the preesure differential between inlet and ambient air pressure. Though having said that I still don't fully understand the nuances of carbs and would have injection any day... Actually on a normal engine ( not boosted) the bowl pressure/airbox/throttle bore could be all the same pressure and operate normally, fuel is drawn up through the jets by the difference between the bowl pressure and the low pressure area right around the venturi, not the vacuum from the motor. Hope I explained that right, or another way to say it, if you were to remove a carb and blow sufficient air into the inlet it would operate the same as if the engine was pulling the air through it as the movement of air is creating a vacuum around/above the venturi. Whew! I don't know hope people write books about this stuff! Am I missing something? Surely you just connect the plenum to the float bowl vents and make sure the pressure in the float bowls is the same as the pressure being fed through the carbs? This way the jets are simply operating off the pressure difference caused by the venturi. That would work on a non boosted engine, but boosted engines running blow through must compensate for the changes in air density as manifold press changes. It would be like jetting an engine to run properly on the top of a mountain then bringing it down to sea level, at such a lower altitude the air density would be much higher (# of molecules in a given space) but carbs meter fuel based on volume, not density, so 1 liter of air at low altitude has much more oxygen than 1L at high altitude and requires much more fuel. Ultimately carbs cannot adjust to this as they are just dumb pieces of metal.....So in a boosted engine a pilot tube located in the turbo piping (connected to carb vents) operates as a sort of tiny ram air inlet causing a slightly higher pressure (than the air its in) to be sent to the bowls, the difference between the bowls and manifold/ throttle bore causes the Usually mediocre venturi vacuum to go into high gear and draw up much more fuel. Thus correcting the A/F ratio at the higher density. So what I'm thinking now it that the pilot tube being so close the turbo is already in a much higher pressure area than the manifold and the pilot tube is amplifying that, so it may be possible to get the pressure compensation with out a pilot tube, given its location. I have some time to get it sorted, its -12deg C and snowing here! and I bought a supermoto today so i'll have somthing to ride Though with the weather I wont be able to test out the changes for a bit. Link to comment Share on other sites More sharing options...
dodgy_tom Posted January 29, 2009 Report Share Posted January 29, 2009 I'm sure you've read this book by Joe Haile? On the subject of blow through applications, he mentions running a pressurised float bowl by using a fuel pump and regulator to ensure even supply of fuel to boost pressure. That's if my memory is working right! I'll look more later. Link to comment Share on other sites More sharing options...
spacemonkey Posted January 29, 2009 Report Share Posted January 29, 2009 But isn't the plenum at a pressure lower than atmsopheric pressure off boost and higher than atmospheric pressure on boost. My understanding was that a carb bowl has to be at a pressure higher than the inlet pressure in order for fuel to be drawn out of the jets? I know from fitting FCR's inside an air box that the float bowl that you have to be carefull where the float bowl pilot feeds are vented to and that this must be outside the airbox to get the preesure differential between inlet and ambient air pressure. Nah, the float bowls need to be given the same pressure as the air that is feeding the venturis. Carbs rely on a pressure difference to push fuel up the jets from the float bowl into the venturi. This pressure difference needs to be caused by the air speeding up to go through the narrow venturi. Normally the float bowls are vented to atmosphere but if the carbs are breathing through a pressurised airbox then compensation can be achieved by making sure that the vents are plumbed into the pressurised airbox. If not, the fueling would go very lean as the pressure in the airbox increased. Thanks! Actually on a normal engine ( not boosted) the bowl pressure/airbox/throttle bore could be all the same pressure and operate normally, fuel is drawn up through the jets by the difference between the bowl pressure and the low pressure area right around the venturi, not the vacuum from the motor. Hope I explained that right, or another way to say it, if you were to remove a carb and blow sufficient air into the inlet it would operate the same as if the engine was pulling the air through it as the movement of air is creating a vacuum around/above the venturi. Whew! I don't know hope people write books about this stuff! ......Wot he said..... Thanks! That would work on a non boosted engine, but boosted engines running blow through must compensate for the changes in air density as manifold press changes. It would be like jetting an engine to run properly on the top of a mountain then bringing it down to sea level, at such a lower altitude the air density would be much higher (# of molecules in a given space) but carbs meter fuel based on volume, not density, so 1 liter of air at low altitude has much more oxygen than 1L at high altitude and requires much more fuel. Ultimately carbs cannot adjust to this as they are just dumb pieces of metal.....So in a boosted engine a pilot tube located in the turbo piping (connected to carb vents) operates as a sort of tiny ram air inlet causing a slightly higher pressure (than the air its in) to be sent to the bowls, the difference between the bowls and manifold/ throttle bore causes the Usually mediocre venturi vacuum to go into high gear and draw up much more fuel. Thus correcting the A/F ratio at the higher density. So what I'm thinking now it that the pilot tube being so close the turbo is already in a much higher pressure area than the manifold and the pilot tube is amplifying that, so it may be possible to get the pressure compensation with out a pilot tube, given its location. Ah, I see what you mean... That makes perfect sense... How does the 'pilot tube' thing work then? I don't quite see how it sends higher pressure than boost pressure to the float bowls.... Presumbably this has to be referenced to your fuel pressure regulator too? I can think of other ideas involving auxiliary jets controlled by tapered needles, diaphrams/pistons, springs and such.. Or electronically controlled solenoids, but ufortunately nothing simples comes to mind! ....Another thought occurs... What did they do with Spitfires and Hurricanes? I think the Merlin engines were blown engines fitted with carbs. I'm not sure used a blow through or draw through setup though. Maybe something can be learned there? I'm sure you've read this book by Joe Haile? On the subject of blow through applications, he mentions running a pressurised float bowl by using a fuel pump and regulator to ensure even supply of fuel to boost pressure. That's if my memory is working right! I'll look more later. This kind of regulator is just intended to make sure that fuel can still flow into the float bowl despite the higer air pressure in there. It's nothing to do with the change in air density flowing through the carb and it's effect on the quantity of fuel that needs to flow through the jets. The variable air density issue is the problem Speedzepher faces. Link to comment Share on other sites More sharing options...
gregtrx Posted January 30, 2009 Report Share Posted January 30, 2009 ....Another thought occurs... What did they do with Spitfires and Hurricanes? I think the Merlin engines were blown engines fitted with carbs. I'm not sure used a blow through or draw through setup though. Maybe something can be learned there? If you wanted to know more on this point, Haynes do a Spitfire manual! Link to comment Share on other sites More sharing options...
turbor1steve Posted February 3, 2009 Report Share Posted February 3, 2009 hi m8. i have a turbo on my 2000 r1. we use a pigot pipe on the planum inlet to make the pressure a little higher than the boost pressure. and also a fuel pessuser regulater. its a bit hard to put it all in to words but if you want to chat about it. send me a pm with ya phone number and maybe we can sort it for ya. once you get it running as it should you will love it. Link to comment Share on other sites More sharing options...
turbor1steve Posted February 3, 2009 Report Share Posted February 3, 2009 sorry m8 just seen where your from phoneing not a good idea. will have a look to see if i have any of the old links that helped me when i was doing my r1 Link to comment Share on other sites More sharing options...
Tennis Posted February 5, 2009 Report Share Posted February 5, 2009 My pitot tube is 6" from the plenum. Its a 10mm dia tube inserted across the middle of the charge pipe from the turbo with a 7mm dia hole in the middle facing into the air flow. the ends of the 10mm tube conect to the float bowls. I assume your using a rising rate fuel pressure reg, ie for every psi of boost you get a psi of fuel pressure, and that your base fuel pressure is between 1-2psi at idle. needle position and size is the major player from 1/4 untill 3/4 throttle, if your rich at full throttle change your mains if its rich all the way to full throttle drop your needles, if its still rich, try dropping your floats a bit. This graph may help it shows whats playing at certain throttle positions Link to comment Share on other sites More sharing options...
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