Landracing Forum
Tech Information => Technical Discussion => Topic started by: dresda on July 09, 2015, 12:23:48 AM
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Hi guys, I'm running a 55 Triumph 650 with a supercharger on Methanol, the recommended jet through a 1 3/4" SU carb would be 11/64 at sea level should I bother leaning it out for Bonneville, each jet size I change I have to make a needle and jet to match because I have no idle circuit so the needle would be .005" clearance in the main jet.
Thanks.
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Can't speak to methanol or a supercharged application, but the beauty of SUs are that they're relatively self adjusting, so it's likely you have more latitude than you would with many other carbs.
Despite that, I'd bring along a set of jets and needles either side of your tune.
The strategy I would use would be to start with what you have, and when you get to Bonneville, warm it up and do the piston lift trick where you lift the piston a touch at idle. If it speeds up, then returns to normal, run it - you're good. If it stays high and doesn't return to idle, go leaner. If it bogs, you're too lean.
I'd bet your tune will be fine, but I'd rather file needles at home than on the salt.
All bets are off if you're blowing through it.
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The DA at Bonneville can go above 8k in the heat of the day, you will need to compensate for that. If you're not setting idle on the SU, you can lean it out by lowering the needle. I would advise adding an EGT if you don't already have one & get a base line where you are now with a tune you know is good to assure your readings are accurate.
Sid.
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Are you running the carb as blow through or suck through?
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My blower is suck and I can't lower the needle it's fixed. Not running a float bowl just pumping the methanol through the carb, no idle circuit and have to make each needle fit the main jet so at the top of the needle would have a small parallel section with about a .005" clearance this almost acts as a fuel shut off, just don't want to make too many jets/needles that's why I would like to know if I should drop the main jet/needle like 10-20% or so.
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Well you definitely have a weird setup going there if you don't have a constant fuel level in the jet tube from a float bowl. I've never seen an SU that didn't mount the needle with a set screw in the piston so I'm sorry I don't understand what you have.
All I can tell you it's a given that you'll be chasing jetting at the salt.
Sid.
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I do mostly car carbs, and I have never seen an SU without a jet adjustment. What model carb is it?
If you know the jetting is correct for a given Air density, an air density calculation for expected conditions @ B'ville, gets you "in the ballpark". The problem is, as others have stated, the conditions @ B'ville can vary wildly.
Like other forms of racing, the best plan is to prepare with everything you think you might need. Make up a set of needles that goes in some sort of steps, up to the leanest you would dare to run, @ the hottest temp with the lowest air density.
I can pretty much guarantee that if you don't bring something along, you WILL need it . . . . . . . . . Just one of Murphy's laws . . . . . . . .
And given your "one off" setup, it is not likely someone else will be able to loan you what you need. The chances of finding SU parts in Wendover is likely zero. I think even in SLC it would be a major "snipe hunt".
Be prepared or :dhorse:
:cheers:
Fordboy
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In British cars (like MG's) at a 4000' rise in sea level you raise the jet 1/6th turn. I'm not sure what that comes out to in inches of jet rise, but its not much. If you have the option of raising fuel pressure, that could be a very small, easy adjustment.
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This set up is very common on drag bikes in the UK, yes I do have a pinch screw to hold the needle but as I said no float bowl because SU carbs can't put enough Methanol through the needle valve, I was working on a Holley float bowl mod but for now I'm just plumbing into the bottom of the carb and have modified the inside to take Holly Methanol jets. The setup works well with one jet size on most drag strips around sea level, I'll just wing it and make 3 or needles and jets but it's a pain making a needle on a 16" swing lathe.
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This set up is very common on drag bikes in the UK, yes I do have a pinch screw to hold the needle but as I said no float bowl because SU carbs can't put enough Methanol through the needle valve, I was working on a Holley float bowl mod but for now I'm just plumbing into the bottom of the carb and have modified the inside to take Holly Methanol jets. The setup works well with one jet size on most drag strips around sea level, I'll just wing it and make 3 or needles and jets but it's a pain making a needle on a 16" swing lathe.
Yeah, that's not what would be my first choice either!!
In an attempt to be helpful here:
What size jet do you use?
Will a .100" or .125" jet flow enough methanol with a "standard" needle that might be available?
Without an idle circuit, you could use say .090" needles in .100" jets which would be waaaay richer than a gasoline setup, and might be close to the flow you need for methanol . . . . . . ?
Or maybe .125" needles with a custom diameter jet . . . . . ?
Can you raise the fuel pressure and use a return line with a bypass pill to alter the flow rate through the needle and jet?
Sorry, just tilting my head here and letting the thoughts flow out. :-)
I'm unsure if any of this is useful. My main line of thinking here is to try to find a way for a "standard", available, needle group or sub-group, to fill your need so that you don't have to fabricate custom needles.
:cheers:
Fordboy
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jet size at sea level .170" no electrics on the bike so no fuel pump, the bike will be fine just wondering how many jet sizes to go down, I did read one jet size down for every 2000ft so Holley main jet for Methanol go up in .002" as long as I have a baseline to work with, Methanol doesn't mind running rich anyway.
Thanks.
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Do away with the SU carb.....go on E-bay and get a S&S carb......works fine on suck blower set-up, all the jet sizes you will ever need...................
Been there....done that.................
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Made a new needle with a little mod and it runs great even a nice fast idle. The SU carb is a great carb been around for ever and I don't want to re invent the wheel, every 60's blown Triumph drag bike uses an SU but thanks for the advise. I did think about the S&S set up but then I would have to start all over again. The great John Hobbs in the UK has been a great help to me, the only thing he couldn't help me with running at 5000ft altitude and on salt and I wanted to use old shit even my blower is over 50 years old, hey if it doesn't work you can be the first to say "told you so" then I'll try the S&S, still working on the Holley float bowl mod but don't think I have enough time.
Thanks again for all the comments.
Ray.
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Decades ago I helped a guy with a setup like yours using an Amal carb with no float bowl. The fuel pressure was based on gravity and the fuel pressure was governed by the vertical distance between the upper surface of the fuel and the carb inlet. The greater the distance the more the pressure. It was on a drag bike so the fuel level ony dropped a small amount during the race and this was also true for the fuel pressure.
Salt races are long and bikes burn a lot of methanol. The fuel level, and the resultant fuel pressure at the carb, will drop significantly in the tank. It might be a good idea to run the shortest distance possible to get a time slip. That way, there is less of a drop in fuel pressure.
This might not be the optimal setup for long races.
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I am only running a mile, my tank hold 1.5 gal and feeding the with 7/16" I/D hose based on 3 miles per gal I hope I'm ok, was thinking about strapping another tank on if I have to.
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Growing up in a place where SU's were pretty much everywhere we used to hang them on everything, I even had 8 on a flathead at one time but never without a float bowl. I hope it works out for you on the salt & you don't burn it down. Golden rule, start out fat.
Sid.
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You can get a constant fuel pressure with a gravity fed system with an intermediate tank between the fuel tank and the carb. My recollection is this system was used on the AJS porcupine. Maybe someone over there knows the details. My recollection is about the concept.
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I don't have a problem with fuel pressure just need to know a % of who lean to go. The reason we don't use float bowl on the SU is because you can't get Methanol needle valves for it.
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Growing up in a place where SU's were pretty much everywhere we used to hang them on everything
HS4s make pretty Christmas tree decorations . . .
But seriously, I'm confused as to how you're going to maintain any tune consistency without floats. I realize that methanol will require more fuel than a stock float set-up might provide, but even with an intermediate tank, if the tanks gets warm and pressure builds, you could be rich in the heat of the afternoon, and then suddenly lean a mile down the course.
There are enough variables to compensate for with varying adjusted altitude - and the carb will compensate somewhat for that - but for what will likely be inconsistent fuel pressure for 3 or more miles with no effective measure to moderate it sounds . . . well, kinda scary to me.
It may work on drag bikes in Great Britain, but the distance is shorter, and the temps are cooler.
For Bonneville, I think you'd want to be looking at what road racing bikes are using, not drag bikes.
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It seems to me that without a float bowl to keep the fuel at a constant level relative to the jets, the air velocity won't be creating a low pressure venturi to suck the proper amount of fuel into the air stream. Using a pressurized fuel supply without the float bowl assembly seems to be converting it to a low-pressure fuel injection system. Does this sound plausible?
Regards, Neil Tucson, AZ