HONDA CB400 FOUR
Honda CB400 Four part 1 Kwak expert Ralph has got a middleweight Honda to put right!
Ralph Ferrand on part one of sorting a super Four.
Afriend of mine runs a successful small bike shop and does general servicing and MOTS. His bread and butter is run of the mill modern bikes wanting standard servicing and MOTS. He is geared up for this with shelves laden with filters, plugs, brake seals kits etc. for all the common bikes. He loves it when non-abused Blandits come in for their annual services and MOT from his menu pricing system. What he does not love is elderly Jap fours coming in with a string of problems, with multiple causes and a big cloud hanging over parts availability. Personally I love nursing an ailing 1970s gem back into good health and loathe working on bland modern commuters, so when my friend had a stroppy 400 Four rock up he asked the customer if he would be happy to deal with the wild man of Pensford with all his character peculiarities, who is more geared up for errant classics. I don’t know where he sends grumpy old British nails because I won’t touch them either. The elderly diminutive Honda multi had various medical complaints, but the most frustrating was its inability to start and run, making it paramount for me to collect it in the van. Oddly, I had never ridden a 400 Four and that wasn’t about to change unless I could figure out what was wrong with it. The plugs looked a tad ropey and it needed a good service, so I thought I may as well bang in a new set. As with many old Hondas the plugs are those tiny toy jobs that cost more than the big manly B8ESS to be found on big Kwakas! They did appear to have a bit of oil on them which didn’t bode well for the little four. As it had a decent aftermarket electronic ignition system, it didn’t seem likely that the ignition was the cause of the reluctance to run, so I turned my attention to the fuel system. The engine gets its fuel charge from a set of four Keihin 20mm carburettors. They are pretty basic with a butterfly choke, but they looked as though they were in the market for a good clean out. To remove them I had to remove the tool tray and the air-filter box top under the seat, which for some inexplicable reason was hinged on the left-hand side.
The Laverda Jota suffers from this illogical quirk, but then one expects that sort of nonsense from the Italian bike industry! The air-filter itself was rattling around in the air-box, missing the all-important spring clip securing it to the intake making it completely ineffectual. Next I slackened off all the hose clips that secured the intake rubbers to the carbs and the carbs to the intake rubber mounts. I also loosened the large oval clip that holds the rubber connector from the actual air-box to the air-box manifold that connects to the carbs and removed the large oval metal guide from the middle of this large oval tube through the air-box. At this point it is easy enough to pull the air-box manifold from the rear of the carbs and pull it out sideways. I removed the throttle cables from the carb bank and then pulled the carbs off the rubber intake stubs. The intake stubs were very hard and looked to be in a ropey condition as were the rubbers that connect the air-box manifold to the carbs. I was able to replace these with new from that well known purveyor of all things Honda, David Silvers, at a reasonable cost. I carefully gripped the carb bank in the soft jaws of my bench vice to safely remove all the float bowls to give me an idea of what the carbs were like inside. They were a long way from clean and it seemed pretty likely that there would be blockages aplenty. When I’m stripping carbs I use numbered plastic storage bins to keep all the parts for each carb separate and safe. As this particular carb bank is so small I only split the carbs down into pairs, but each carb had all its jets and components removed for cleaning. When dismantling the carbs it is of paramount importance that you use the correct size of screwdriver for the individual screws. The screw heads on Japanese motorcycles are JIS and the pedants would have you believe that the screws being destroyed on old bikes were as a result of people using Phillips screwdrivers. I have been working on bikes for in excess of 35 years and I have yet to own a JIS screwdriver and don’t destroy screw heads. What will wreck the screw heads is using the wrong size of Phillip screw driver or using a posi-drive tool. The larger screws on Japanese (M6 +) tend to require Phillips No.3 drivers. Most screwdrivers and bit kits available in DIY stores and auto shops only have No.1 and No.2 so folk use the No.2 as it’s the biggest they own and then allied with a lack of technique, the screw head gets buggered. The screw is then blamed for being made of cheese. The same goes for bolt heads that are routinely rounded by cheap far eastern spanners that don’t fit properly. I mostly use removable bits in a top quality ratcheting screwdriver, so if they get worn, they get binned.
If a screw is threatening to be over tight, I remove the bit from the tool, sit it in the screw head and give it a light, but sharp tap with a hammer and then return the bit to the screwdriver. You need to press down hard on the driver and simultaneously turn the handle. If you feel the blade start to ‘cam-out’, stop immediately before you cause damage. Resettle the bit and try pushing down harder as you turn, but If that doesn’t work, give the head another tap with the bit. If the screw is in an aluminium alloy, as they usually are on bikes, then you can try applying a bit of gentle heat which will expand the aluminium alloy more than the steel screw thereby loosening it. I cleaned the carbs, initially, with a jet of brake clean from my chemical spray bottle, agitated with paint and toothbrushes. Once they are as clean as possible and all the grease and oil is removed, I then put them, together with all the component parts into the basket of my ultrasonic bath. The bath itself was a big bag of money, but it is a top quality industrial tool and has been worth the investment over the years. I have tried various fluids, but the best I have found is a Mykal product that comes in five litre bottles that can be diluted from 5% to 20% with water. I use 10% and have found that it works very well. Being an industrial product, it’s not very easy to find, so we have now started stocking it at Biker’s Toolbox. After the half hour ultrasonic treatment, the parts were thoroughly rinsed in warm water – cold’s nasty on my tender pinkies! I then blew all the parts dry with the air blower on the airline, ensuring all the carb body holes and drillings got a serious blast through to ensure nothing remains lurking. The carb pairs were then returned to the vice and I reinstalled all the jets to their correct places. I inspected the float valves and was pleased to see that they were in good condition, so replaced them. Unlike the Mikuni carburettors found on my beloved Zeds, the main jet on these carbs is held in place by a sprung plate, held down by the float bowl. Having now ensured that the carbs are as clean as they possibly can be, I reassembled them on the plate and then reconnected all the levers etc. I removed and skipped the carb to head stub mounts and fitted the new ones. Although not strictly necessary, I always fit them will a thin smear of RTV silicone sealant, just to make sure that they are all fully sealed. Air leaks on these can be a right royal pain in the grinner. If you suspect a leak on the intakes, get a chemical spray bottle and set the nozzle to a fine spray pattern and spray it around the carb stubs while the bike ticks over; If there’s a leak you will hear a slight change in the tick over, usually a slight increase in speed. I replaced all the rubbers in the air-box manifold with new. I gave the inside of the carb stubs and the air-box manifold rubbers a thin smear of red rubber grease to ease things along before mating them with the carburettor bank. It’s the KY jelly of the motorcycle mechanics world and makes carbs slide in a whole load easier! Being new and soft they are already a lot more compliant than the old hard items. I reassembled the air-box and this time the new element was held in place with the spring clip, so will actually filter the air feeding the carbs, keeping all the bricks out of the system. Before balancing the carbs, I decided to check that the valve clearances were as they should be: which of course they weren’t. Compared with checking bucket and shim clearances, the tappets on the Honda are a piece of cake. I first removed the tappet covers on the valve cover.
I then set the engine to TDC (Top Centre) ensuring that number 1 was on the compression stroke i.e. both valves are closed. Naturally, obeying Sod’s Law, first time round it wasn’t so I turned the crankshaft round a further 360° (one complete revolution) and number 1 was on its compression stroke. In this position I was able to check the inlet and exhaust valve clearance on 1, the exhaust on 2 and the inlet on 3. To measure the valve clearance or tappet gap, the feeler gauge (0.05mm / 0.002in) blade is pushed in between the bottom of the adjuster screw and the top of the valve stem. If it won’t go in it’s too tight and if it has no resistance at all then it’s too loose. I have seen people bugger about with ring spanners etc. when adjusting tappets, but for the cost of a proper tool it’s really not worth the hassle. The correct tool has a socket with an arm on it the size of the lock nut and the inner part has a socket that fits the top of the adjuster screw as shown in the photo. The Haynes manual shows someone mucking about with a magneto spanner and a ring spanner which is fine if you are a mutant with the three arms, but no good for the rest of us. With the proper tool you can make the adjustments and lock the nut with one hand and use the feeler with the other. Once the first four had been set, I turned the crank shaft another 360° and checked the remaining clearances. Next job was to synchronise the carburettors, a must if you’ve taken them apart. On the baby Honda the vacuum take offs are tapped holes on the inlet tract of the head, just in front of the carb stubs. Under usual use they are blanked off with screws sealed with fibre washers. I screwed the requisite adapters into the holes and connected the hoses of my trusty Vacuum Mate to them. I set up an auxiliary petrol tank to feed the fuel system and started the engine. It fired into life at the first press of the button, but with the carbs so far out of balance it was running as rough as a badger’s rear end. Once the engine was sufficiently warm I adjusted the balance with the adjusters attached to each carb on the actuator arm. While I was about it I checked the ignition timing with my trusty Snap-on timing light and made a small adjustment to get it perfect. The engine now sounded sweet and was running like a well-oiled sewing machine. I removed all my test equipment and replaced the remote fuel tank with the bright yellow item it came in with. Next month I will be addressing the fact that the chrome on the fork stanchions had more holes in it than a politician’s integrity.