Ohhhh SU huh, going “Old Skool”, I like it. Plenty of tuning options and sizes around and then if you still have trouble there are side draft Strombergs you can play with. But I think @Murdo will be all over an SU setup like a fat kid in a Lolly shop Sent from my iPhone using Tapatalk Pro
I do have a bit of experience with SU's. Most people don't take the time to understand them and curse at them. How hard can it be for a carb with 3 moving parts?
Did you learn to synchronise them using a piece of garden hose held up to your ear... easiest way to go deaf... but once you knew what you were doing it was easy as pi$$
True, but I went up market and bought the proper manometer to synchronize them. Only need one 1 1/4" for this job.
Yes... should be ample considering twin 1.25" SU's managed to produce 75hp from the 1275cc Cooper S.. and while a sweet little engine the A series BMC was far from sophisticated or terribly efficient unlike the the ZXR250. There are also a big range of needles to ensure enough fuel can be dumped in.. well done @Murdo & @Frankster
That may be so Tigger, but the intake hole in the pump is only 1 1/4" and to fit a 2" carb would, I doubt, have enough airspeed to lift the slide fully and make needle selection and tuning nearly impossible. I have three 1 1/4" carbs and .... to tell any more would give away secrets. The worlds fastest Velocette uses a single 2" carb feeding a supercharger and nearly 600cc (although he has too small a fuel bowl and has gone about getting fuel to the carb a different way to what I would have) and it seems to go alright at 193+ mph.
Oohhh its getting more interesting.. hmm as it is being designed to run on Salt (we assume ) the power mostly needs to be top end only.. hence the basic log style ... so a reasonably large A/R will work well.. and I have no doubt @Murdo will have that all nicely sussed out..
Having the 1 1/4 SU's is a very good reason for using them. Talking to Justin Newell (He has just rebuilt Little David) the other day and he is using a larger SU than you. Little David has a diet of very high percentage of nitro, so needs less air. A small carb with give you better torque low down, but the salt runs would be full bore. Have you considered a dual choke progressive Weber or using two 1 1/4's with the second one cutting in at half throttle and both fully open at full throttle(like the Weber).
No room for downdraught DGV, DFV type (or even sidedraught DCOE), but I do have three SU's to play with. The use of a massive carb (compared to engine size) will not give the results we are looking for, so baby steps first. I've seen so many people shove bigger and bigger carbs onto engines trying to get more HP and only making their cars go slower. It's all about airspeed into and out of the pump. The SU (or any Constant Velocity carb) will keep the airspeed up by raising or lowering the slide unrelated to what the throttle plate is doing, but what the engine needs to make HP.
If you want spare engine/parts, I have 2½ zxr250c motors? The zxr engine has barrels separate to the block, so the barrels can be lifted to reduce compression, rather than a head spacer or changing con-rods. Starting it could be tricky with low compression and valves closing so late into the compression stroke. In standard tune, the torque drops from about 10,500rpm, dropping faster after 14-15k. I do believe the ignition advances when going faster, but will you be advancing it further? Maybe a tuneable ignitor from these guys: http://www.ignitech.cz/en/ (I was also considering their efi system)? Good luck with your drag bike (and whatever else) project!
A couple of bits arrived in the post. Oil cooler for the oil system. Alloy tube and plate for the plenum chamber. Trimmed the ends to fit, drilled the manifold holes and roughly cut out the port holes to be finished later when bolted to head. Made the mounts to hold the weight of the turbine and tacked up the pipes for trial fit. Am putting the pipe out the rear to use the hot exhaust gas to fill the vacuum created by the riders body, and reduce a bit of drag. Still lots of trimming and welding to do yet.
The heatwrap I ordered arrived yesterday so got in and did a bit before it got too hot. I bought 15me and still didn't have enough. Just to make sure it was fireproof I cut a bit off and hit it with oxy torch. Just went a bit brown. Made gaskets from some old manifold material I had on hand. That should help keep the rider cool.
Been getting out of bed in the 'Sparrow fart Dawn' and getting some work done before it gets too hot. Today I finished the welding of the fuel tank. I pressure tested it with 30psi of air. Had two pin holes to reweld. Cleaned up ready to go. After that I welded the ends to the plenum chamber, cut out with plasma cutter, sanded smooth and drilled for the top plate. I used a 3mm pilot drill, then one of these tricky drill and thread bits. Cut some gaskets and ready to fit.
We need a 'fuking brilliant' button on the posts Ill have to drop in and check out these creations ... great work I think u guys are worried the "Flyer" is going to flog u ... if it ever runs that is