Getting a Old Honda CB400 Back on the Road

A friend of us runs a successful motorcycle repair shop and does general servicing. His bread and butter is servicing anything with two-wheels regardless the engine size. He loves it when non-abused Honda Wave or similar motorcycles come in for their annual services. What he does not live is elderly Japanese four-cylinders coming in with a string of problems, with multiple causes and a big bad dark cloud hanging over parts availability.

Personally we love nursing an ailing old Japanese gem back into good health and loathe working on bland modern big bike commuters, so when our friend had a problematic Honda CB400/4 he asked the customer if he would be happy with hiring some external experts, who were more geared up for errant classics. Of course we never will call ourselves experts regarding an old CB400/4s…

The elderly diminutive Honda four-cylinder had various medical complaints, but the most frustrating was its inability to start and run, making it paramount for us to collect it in one of our pickup trucks. Oddly, I had never ridden an Honda CB400/4 this old and that wasn’t about to change unless we could figure out what was wrong with it.

The plugs looked a bit ropey and it needed a good service, so we thought we may well bang in a new set. As with many old Hondas the spark-plugs are those tiny toy jobs that cost more than the big manly B8ES to be found on big Kawasakis. They did appear to have a bit of oil on them which didn’t bode well for the CB400 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 we turned our attention to the fuel system.

The engine gets its fuel charge from a set of four Keihin 20mm carburetors. 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 we 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. Some early Italian motorcycles suffer from the same illogical quirk, but then one expect that sort of nonsense from an Italian motorcycle…

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 we slackened off all the hose clips that secured the intake rubbers to the carburetors and the carburetors to the intake rubber mounts. We 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 carburetors 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 carburetors and pull it out sideways. We removed the throttle cables from the carburetor bank and then pulled the carburetors off the rubber intake stubs.

The intake stubs were very hard and looked to be in a poor condition as were the rubbers that connect the air-box manifold to the carburetors. We were able to replace these with new from that well know online e-commerce website, at a reasonable cost.

We carefully gripped the carburetor bank in the soft jaws of our bench vice to safely remove all the float bowls to give us an idea of what the carburetors were like inside. They were a long way from clean and it seemed pretty likely that there would be blockages aplenty. When we’re stripping carburetors we use numbered plastic storage bins to keep all the parts of each carburetor separate and safe. As this particular carburetor bank is so small we only split the carburetors down into pairs, but each carburetor had all its jets and components removed for cleaning.

When dismantling the carburetors it is of paramount importance that you use the correct size of screwdriver for the individual screws. The screw heads on old Japanese motorcycles like this one are JIS and the pedants would have you believe that the screws being destroyed on old motorcycles were as a result of people using Philips screwdrivers. We have been working on motorcycles for in excess of 35 years and we 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 Philip screw driver or using a posi-drive tool. The larger screws on Japanese tend to require Philips No. 3 drivers. Most screwdrivers and bit kits available in DIY stores and automotive 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 low quality. The same goes for bolt heads that are routinely rounded by cheap Chinese 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, we 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. It the screw s in an aluminum alloy, as they usually are on motorcycles, then you can try applying a bit of gentle heat which will expand the aluminum alloy more than the steel screw thereby loosening it.

We cleaned the carburetors, initially, with a jet of brake clean from our chemical spray bottle, agitated with paint and toothbrushes. Once they are as clean as possible and all the grease and oil is removed, we then put them, together with all the component parts into the basket an send them of for ultrasonic cleaning. After we received the parts back from the ultrasonic cleaning, we ensured that all the carburetor body holes and drilling got a serious blast through with compressed air to ensure nothing remains lurking.

The carburetor pairs were then returned to work bench and we installed all the jets to their correct places. We inspected the float valves and we were pleased to see that they were in good condition, so no need replacing them. Unlike the Mikuni carburetors found on some older Kawasaki’s, the main jet on these carburetors is held in place by a spring plate, held down by the float bowl. Having now ensured that the carburetors are as clean as they possibly can be, we reassembled them on the plate and then reconnected all the levers etc.

We removed an skipped the carburetor 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 behind. If you suspect a leak on the intakes, get a chemical spay bottle and set the nozzle to a fine spray pattern and spray it around the carburetor stubs while the motorcycle ticks over; If there’s a leak you will hear a slight change in the tick over, usually a slight increase in speed. We replaced all the rubbers in the air-box manifold with new. We gave the inside of the carburetor stubs and the air-box manifold rubbers a thin smear of red rubber grease to ease things along before mating then with the carburetor bank.

It’s the lubrication jelly of the motorcycle mechanics world and makes carburetors slide in a whole load easier! Being new and soft they are already a lot more compliant than the old hard items. We 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 carburetors, keeping all the bricks, dirt and dust out of the system.

Before balancing the carburetors, we 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. We first removed the tapped covers on the valve cover.

We then set the engine to TDC (Top Center) 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 degree (one complete revolution) and number 1 was on its compression stroke. In this position we were 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 tapped gap, the feeler gauge (0.05mm) blade is pushed in between the bottom of the adjuster screw and he 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.

In the Haynes manual for this motorcycle it shows someone mucking about with a magneto spanner and a ring spanner which is fine if you are a mutant with three arms, but no good for the rest of us. With the proper tool you can make the adjustment and lock the nut with one hand and use the feeler with the other. Once the first four had been set, we turned the crank shaft another 360 degree and checked the remaining clearances.

Next job was to synchronize the carburetors, a must if you’ve taken them apart. On the Honda CB400/4 the vacuum take offs are tapped holes on the inlet tract of the head, just in front of the carburetor stubs. Under usual are they are blanked off with screws sealed with fibre washers. We screwed the requisite adapters into the holes and connected the hoses of our trusty Vacuum Mate to them. We 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 carburetors so far out of balance it was running as rough as it possible could… Once the engine was sufficiently warm we adjusted the balance with the adjusters attached to each carburetor on the actuator arm. While we were about it we checked the ignition timing with our 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.

We removed all our test equipment and replaced the remote fuel tank with the original it came in with… When we returned the motorcycle to our friend shop and his customer came to check the motorcycle he was very impressed, and said he had never expected that the engine would run as smooth as it did now… The fact was that the engine was actual in a good condition despite being neglected for so long.