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By way of introducing myself to Sideways I proffer this description of SU operation. 'Mixture spread' is a little known feature of SU operation. It means that a given position towards the top of the needle will give different mixtures according to the butterfly opening, at the same air flow. https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/ (pdf to download) Happy to discuss on here as I dont use the wordpress facility Peter
Hello, I'm pretty close tuning my dual HS4 carbs on my '78 1500 Spitfire. I've purchased and read several books, scoured the web, but just can't hit the sweet spot on my carbs. I'm setting them up for aggressive driving fpr solo Autocross events. I can't seem to solve for a stumble under full throttle at the upper RPM range 4000 RPM and above. Can anyone suggest a course of action to assist me? I know the tune of the car matters to setting, so here is what I have. Newly rebuilt engine, 270 cam, flat top pistons, valves adjusted and readjusted to .012 thousands, NGK sparkplugs with .030 gap. Header is 4-2-1 with standard diameter tailpipe into a standard muffler As I said, I purchased dual HS4 carbs and manifold, rebuilt the carbs and had Joe Curto put new bushings in, all should be well. no air leaks that I can determine. Butterfly is not the pop off type but solid. running 92 octane premium gas Timing has been played with, from 10 BTDC to 14 BTDC... would like suggestions here. Distributor is a "Flame Thrower" electronic ignition with matching Flame Thrower Coil. Standard Green colored wires. I have several HS4 Needles to use: ABF - AAR - AAF - AAM - AAB I am using the AAM at this point. (this selection of needles is an indication of how long I've been putzing with these carbs trying to get the car to run. ATF in dashpots, is that ok? So with that, assume valves are set, timing is at 14BTDC at this point, I warm up the car. Disconnect the Choke. Pull the carb tops and set the jets. I've started with the '12 flats down' setting which if I remember comes out to about .08 on the caliper? (not sure if I have the decimal or the proper unit of measure, just that I use it to match both sides, analog dial) I've started at .05 as well. I balance the carbs using my Uni-Syn.. tweaking the idle to about 1000 to keep it running without stutter and air flow is balanced. As far as the needles go, I've learned they all are about the same at idle, 1st and 2nd stage on the needle. I've used the Minty-Lamb comparo-rama charts to compare needles. From this point, my next steps to attempt a tune vary depending on the source book, website, manuals I have. this is where I need some help. When I go full throttle in 4th gear, as the car is pulling or gaining speed, as the RPM's rise, I start to get a stutter and power falloff at about 4000 RPM. (this is the same if I'm at an autocross and at 4000 in 2nd gear, full throttle) I'm no genius, but I'm assuming Rich as I do smell gas, or exhaust slightly at this point.) and of note: I've never driven the car on the road, cut the engine at 4000, coasted to a stop and pulled the plugs. Once I"m at this point, How do you move forward and what do you do? I know that's a lot to take in, but I'm pretty close.. 'cept for the stutter at the higher rpm which I'd like to solve for. Thanks in advance and hope to get a reply with some ideas.... Mike Omaha, NE USA