First thanks to all contributors and moderators for the help and encouragement that you've wittingly or unwittingly given to me. A great forum in a world of (mostly) dross, well done.
I had one of these back in the day and the Force was strong. So, a 1977 400/4 was purchased a few years ago in substantially complete and original condition. After a while it became clear that not all was well with the engine, it turned out that the detonation problem had done for a couple of piston rings and pistons, and also damaged inside the cylinder head. Rebuilt it was including things like laser welding of the heads to take out the pits, dynamic balance and I couldn't resist a yoshimura 455cc kit either, plus full cosmetics on the rest of the bike. Your various contributions were most useful to me for example at the time Nurse Julie was posting on her rebuild. I only carefully cleaned the carbs as a set, and did go through three iterations of main jets for the Yoshi kit.
Fast forward and after a thousand miles or so I'd had a couple (or 3 or 4) of instances of leaks from the carbs which were down to various things including swelling or shrinking of the O rings- I drain the fuel in winter when I park the bike up. Because I needed to do something else to the bike (don't ask) I thought I'd strip the carbs right down and check and rebuild. I won't tire you all with the four or was it five times that I've fitted and removed the carb bank in the past couple of days or so but I thought I'd give you the condensed tips below. Some of these are borrowed from other contributors. The carb to head rubbers on my bike are still reasonably flexible, the airbox is standard as are the airbox to carb rubbers. The latter have gone a bit floppy over time.
Removal. Put masking tape over frame tubes near airbox on RHS. Remove airbox drain tube from the airbox, the elbow holds it up. Slacken the carb rubber clips. Remove airbox top, air fliter clip
air filter. Slacken the hose clip on the filter box to airbox adaptor. From inside the filter box take out the metal tube then the tapered (conical) rubber, the hose clip and finally the airbox intake rubber adaptor. Remove the airbox out of the RHS of the bike. Remove the throttle cables, pull the drain tubes up and pull the carbs back. Remover the carbs from the RHS of the bike.
Carb rebuild. I wasn't convinced by the DS carb rebuild kits (Keyster?)partly because the float chamber o rings had swelled or shrunk, or both. So I purchased a nice set of Viton rings from Nurse Julie. Quite a few of the o rings on the carbs were in very poor condition. I refitted all new brass ware from the DS kits and used the stainless float pivots, resetting the float heights as required and bench synchronising. I checked that the float valves were working properly by extending the feed pipe and blowing down it operating each float in turn, checking it again by turning the carbs over when the float chambers were on. On the first refit two carbs leaked from the drains. One had a brush hair in the float valve only found after using a magnifying glass, the other no fault found. Next refit all four leak. So I had a good look through the forum. I recovered the OE float valve assemblies from their bags, polished the seats, cleaned the needles carefully (small wear ridges on tips), took out the DS valves and replaced them, resetting the float heights. Interestingly there are some dimensional differences- on the DS kits the needles are longer. Checked valves worked as above. Refit- leaks from 1 and 2! Quick tap with hammer handle on float bowls stopped that. Warmed the bike up and synchronised the carbs with vacuum gauges. A ride of 10 miles or so this morning showed no problems, phew.
Carb refit. Fit the airbox to the carbs on the bench as another contributor said. This showed me that it was impossible to get the intake rubbers to fit if I tried to do it in the bike. Using a sturdy screwdriver and pressing from inside the airbox it is just possible to get a snug fit of all the rubbers. On my bike they are decidedly uncooperative, a smear of engine oil inside was helpful and working along the bank and back again eventually worked with various adjustments to the clips.
Pop a couple of~450mm lenghts of wire between the engine and the swing arm where you want the drain pipes to go. Rub a bit of engine oil on the inside of the head rubbers. Fit the carb assembly from the RHS, keep the intake stubs level with the rubbers. With a bit of fore and aft twisting to start with you need to compress the airbox to get the intake stubs past the rubbers without the stubs popping into the wrong rubber. The stout screwdriver is handy between the frame tube and the airbox for this. Once you're a couple of carbs in from the RHS the process gets a lot easier. Once in the right place push the carbs into the cylinder head rubbers, making sure you don't push the bike off the mainstand. On my bike carb 1 (LHS) goes in first, followed by the rest in a sort of fore and aft rocking motion. Tape two of the drain pipes to one on the wires and carfeully work them through, then repeat with the other pair. Refit the rest of the gubbins, tighten up clips etc.
Phew, hope that's helpful.
Arfa.
