kp910 user Posted April 5, 2016 Share Posted April 5, 2016 Hi, I need help to make my engine run properly for daily use and some track use. I´m serching for race applications of the SU flat tops, but can´t find anythng... help pls! My engine is a L26 Ported e83, balanced L28 crank, L24 flat top pistons, Isky stage 3 cam, 10:1 CR, MSD 6a, Quote Link to comment Share on other sites More sharing options...
Kurbycar32 Posted April 5, 2016 Share Posted April 5, 2016 I'm using a set of flat tops to hold down some paperwork in my garage right now. Seriously though my L26 came with flat tops and despite their bad reputation I tried to rebuild them anyway. After tearing down the carbs I found that many of the hard parts (bearings especially) had worn out and were no longer available. Since these terrible carbs werent used very long there is little information or parts available to us. your best bet is to change the carbs with something else. I used a set of webers DVG's with the isky cam for a while and they worked just fine, I have a set of modified 4 screw SU carbs on the new motor now. Quote Link to comment Share on other sites More sharing options...
Xnke Posted April 5, 2016 Share Posted April 5, 2016 (edited) The flat top carbs have a seperate choke circuit and power valve, plus the same style of needle-seat system the round top carbs have. IMO they're a superior carburator. The main jet is fixed location-you can't adjust it. You'll need to actually adjust the needle taper instead. Standard SU needles for 0.090" jets will work. With the cam and compression, you'll want to lean the main jet a little from idle to 3000RPM, and richen it up from 3000RPM to 6500RPM. Go slow. It's easier to go richer than it is to throw the needle out and start over because you need to go leaner. USE A WIDEBAND TO TUNE THE NEEDLE TAPER. It's really necessary if you want to really tune the carbs with any kind of speed and accuracy. The power valve/accelerator pump needs to be clean and the diaphragm and rubber sealing washer checked and replaced...they're normally junk. It's a vacuum-operated accelerator pump so there is no mechanical linkage to adjust. It works well when it works, and when the rubber parts are in good shape, it works. If they're in bad shape, then the carb runs badly, overly rich and way out of tune. The throttle shaft bushings need to be tight and smooth-no air leaks. If they leak air, the carburator will run lean and with poor vaccum signal the main jet and power valve will not operate properly-it will run badly. They need to be synchronized, both at idle and part-throttle. The damper oil *needs to be* SAE 20W, motorcycle shock oil is easily available in this weight and works well. Don't change this for whatever you have-it needs to be SAE 20W. The operation of the power valve and main jet depend on this damping rate. It's not like the round-top SU's where you can adjust for different viscosities easily. (nor is there a need to adjust the viscosity-the power valve makes that change useless and needless) For reference, this is the procedure I use for setting up Flat-Tops. It's basically straight from the manual, and it works IF your carbs are in good condition. 1. Make sure that the valve clearances are correct, the ignition timing is set properly and that the spark plugs are gapped correctly.2. Top-off the oil in the dampers to their correct levels using SAE 20 engine oil. 3. Run the the car until the engine is at normal temperature. 4. Unscrew the balance adjusting screw and the fast idle screw until they both are well clear of their stops.5. Turn the slow-running adjustment screw until the engine is running at 650 rev/min (700 of automatic trans) 6. Turn the mixture adjusting screw in both directions but with in the limitations of the limiter cap, until the engine speed reaches its highest point. If the adjustment procedure exceeds 1 to 2 minutes, the engine should be revved to clear the inlet manifold.7. Reduce the idling speed by unscrewing the slow-running adjustment screw.8. Screw in the fast idle adjustment screw until the engine is running at 1400 RPM. Use a flowmeter against the air intake of the front carb and turn the meter setting screw until the upper end of the float is in alignment with the meter index.9. Now place the flowmeter next to the intake of the rear carb and by turning the balance adjustment screw, set the rear carb air intake volume to match the front carb.10. Unscrew the fast idle screw until the engine resumes is normal idling speed and check is return to satisfactory idling by revving the engine 2 to 3 times. The fast idle screw should be set finally so the gap is 0.78 in between its tip and actuating lever. Should any difficulty while making adjustments, check the linkage. I slight amount of free movement in the auxiliary and main throttle shafts which corresponds with the clearance between the throttle shaft end stops.11. With engine at operating temperature, and the mixture and slow-running speed correctly set, check adjustment of the throttle closure dashpot (Automatic trans)12. Set engine to 2300 and 2500 rpm. The button on the dashpot should just be in contact with the lever attached to the throttle linkage. Adjust the position of the dashpot if necessary to provide this setting.13. Now set the engine to 3000 rpm and release the accelerator quickly. The time it takes for the engine speed to reduce to 1000 rpm should not be more that 5 seconds. Any wide variation form this indicates a worn out dashpot and must be replaced or repaired. Make sure your carbs are in good condition before you start. Make sure you don't have any vacuum leaks or cracked vacuum hoses, etc. Hard to make the car run right if half the stuff on it is no good-so make sure you go over it well! Edited April 5, 2016 by Xnke Quote Link to comment Share on other sites More sharing options...
RIP260Z Posted April 6, 2016 Share Posted April 6, 2016 Just to add.... The power valve comes apart, and the valves inside the aluminium housing are brass, and usually clock up with carbon deposits. There is an aftermarket refurb kit for the valves out there though I haven't seen one for a while. Here in the UK we had the flat top carbs for about 4 years during its production period, and there isn't any spares left, I grabbed what little was left. If I remember rightly, having good/correct fuel pressure is also key. Make sure the float is set correctly. These carbs have small fuel bowls and its good to have good fuel pressure to keep these full and refreshed with cooler fuel. Make sure your needle taper isn't worn (oval) as this means bad fuel metering. These carbs are good, just because the boat anchor crew say so doesn't mean it is. Its only adjusting fuel, air and vacuum...... 1 Quote Link to comment Share on other sites More sharing options...
Kurbycar32 Posted April 6, 2016 Share Posted April 6, 2016 The throttle shaft bushings need to be tight and smooth-no air leaks. If they leak air, the carburetor will run lean and with poor vacuum signal the main jet and power valve will not operate properly-it will run badly. That right there is what did me in. Many of the hard parts in my flat tops are worn out and dont seal correctly anymore. Quote Link to comment Share on other sites More sharing options...
Xnke Posted April 6, 2016 Share Posted April 6, 2016 So you are calling them a terrible carb, for being worn out? You realize that the round-top carbs that are so hightly vaunted suffer from exactly the same problems, and require the same work to get them to run right... Anyway, Hope you get your carbs tuned up, KP910! Quote Link to comment Share on other sites More sharing options...
Kurbycar32 Posted April 6, 2016 Share Posted April 6, 2016 Its a terrible carb to use here in America. It was only sold for here 1 year and parts availability is practically zero. Swapping them with anything supported by the industry makes life drastically easier. To be clear I'm not a round top activist like some of the people on here (hell I ran DVG's and loved them) but I do recommend using something that works and is serviceable. Nissan flat top carbs are basically the technological equivalent of the betamax; better than but the industry didn't adopt it so its orphaned. Quote Link to comment Share on other sites More sharing options...
johnc Posted April 6, 2016 Share Posted April 6, 2016 Properly tuned the flat tops can outperform tuned round tops. But the parts situation makes that a moot point. Quote Link to comment Share on other sites More sharing options...
Tony D Posted April 9, 2016 Share Posted April 9, 2016 The "Flat Tops" are the SU HIF-6 and are a superior carb and sought after in Europe. The SU models had adjustable jetsusing a screwdriver on an external adjustment. As noted above, they allow segregated idle and power mixes, something the old antiquated "Round Top" models do not. So many people listened to 'the experts' who confused EGR Overheating of the engine bay and intake manifold with the presence of the carbs (them new carbs the problem, sho'nuff, got to be!) that they were taken off immediately and service - garage replaced with Round Tops. Low Mileage, tight shafted Flat-Tops are out there if you look. Not so for the Round Tops. But both are rebushable. If you want flat tops, look into Europe for a set of HIF 6 Carbs and do a retrofit to the Japanese metal! Quote Link to comment Share on other sites More sharing options...
Tony D Posted April 9, 2016 Share Posted April 9, 2016 This being said.... Patton Machine makes drop-in Throttle Body Fuel Injection adapters that will support more HP than most will ever make on their N/A Z-Car and they're relatively inexpensive. On a MAP-Based system, a leaking throttle shaft can be your idle air bypass baseline... Quote Link to comment Share on other sites More sharing options...
RIP260Z Posted April 12, 2016 Share Posted April 12, 2016 The "Flat Tops" are the SU HIF-6 and are a superior carb and sought after in Europe. The SU models had adjustable jetsusing a screwdriver on an external adjustment. We have the "boat anchor brigade "( mainly through reading from other "boat anchors") over in Europe too, though thankfully things are a changin'. Though the damage has been done with Nissan selling little or no spares for these, most of what spares they held went to the dump. Also with set up, there are marks on some of the parts for correct line up positions. Those of you who want to persevere get hold of the Nissan workshop manual for proper setup, there is more in it than what Xnke put up. Quote Link to comment Share on other sites More sharing options...
Tony D Posted April 17, 2016 Share Posted April 17, 2016 Throttle Shaft Bushings are an easy fix. If that stops one from reclaiming a carb...well, lets just say our definition of 'mechanic' differs widely! That said, the ability to read the manual and the service brochures out there really help understand what was going on at the time. There are those who want to know, and those who don't want to be bothered and settle for what "works" and not what's optimised for the situation. What's 'easiest' always gets you in the running, you can always hope for someone to have mechanical troubles and grab a spot on the podium. But those who don't chase 'easy' usually are the ones people are hoping break down so they have a spot in the sun....some day! Quote Link to comment Share on other sites More sharing options...
RIP260Z Posted May 23, 2016 Share Posted May 23, 2016 And came across this, which may help someone... http://www.ebay.com.au/itm/Nissan-Datsun-240Z-260Z-Hitachi-SU-Carburettor-Kit/231473887086?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3De1859d6880134c7ebe32b6c25f73304a%26pid%3D100033%26rk%3D1%26rkt%3D4%26sd%3D231473887086 Quote Link to comment Share on other sites More sharing options...
Tony D Posted June 3, 2016 Share Posted June 3, 2016 Posts to Australia only? That Hoarder! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.