Monday, January 21, 2019

Honda 250-305s… The Rules

Honda churned out somewhere around 250,000 of the 1960-67 250-305cc twins. 60+ years later, they still turn up from the back of someone’s garage or at a swap meets, etc.; and people have questions! Because of the depth of changes within each of the three models: Dream, Scrambler and Super Hawk, I can only skim the top of the subject,  but hopefully this story will help to defuse some of the confusion about these classic machines.

Rule #1
They are NOT all the same. There are “early” and “late” Dreams, Scramblers and Super Hawks with changes that are external and internal. Always use the serial numbers for reference when seeking parts or just asking for help. For the most part, the overall engine and tuning specifications are quite similar within each model, but there are deviations that can be important if you are going in deep on a repair or restoration.

Rule #2
Like cars of the era, these are battery-powered motorcycles. While the permanent magnet charging systems can generate sufficient spark to get an engine started, they really need a fully-charged battery and functioning charging system to operate normally. If you jump start any of these bikes and then let the charging system wrestle with a dead/dying battery the charging system will pump out as much as 40volts AC into the 12v electrical system blowing up all the light bulbs and often damaging the rectifiers. 

Because of changes in the 12N9-3A batteries used in CB72-77 and later model CA77 Dreams, you MUST look for the ones with the extended posts, not the ones with small vertical posts. There are TWO different batteries used on the Dreams. They DO NOT interchange and the correct one must be used to match up with the different battery box, tool tray, battery ground and side cover options for each type.

Rule #3
Because of poor storage or neglect, the engines are often found to be seized solidly. Moisture build up inside the cylinders will cause water to corrode the cylinder walls and piston rings into the ring lands. Depending upon just how badly seized it is, using a penetrating oil to help unstick the pistons can be successful in allowing rotation of the crankshaft again. Some people have been successful in getting these engines running again, but usually the cylinder bores are damaged, causing further damage to the pistons/rings.

Invariably the piston rings are stuck in the piston ring lands and are unable to do their normal function of sealing up against the cylinder walls. Because the piston clearances are so small  (often just one or two thousandths of an inch), the engines appear to have compression and will run. The corrosion is generally so severe that even running the engines for hours will not release the rings from the pistons for normal operation. Eventually, the lack of piston ring control and damage to the cylinder walls causes blowby and exhaust smoke. 

Excess oil in the cylinders will foul the spark plugs and eventually use up the 1.5liters of oil that the engines depend upon for normal operation. Plan on re-boring the cylinders and replacing the pistons and rings, at the very least. OEM pistons are increasingly hard to find and when you do the prices are beyond silly ($200 each). CB and CA pistons can be swapped into other engines, with only a slight change in compression readings.  Early Dream pistons had thick 2mm rings, but eventually were superseded by a version using thinner 1.5mm CB rings. has forged piston kits available as replacements for the cast OEM pistons.

Rule #4
Clean the centrifugal oil filter! Depending on the year of production, cleaning the filter may or may not require removal of the whole clutch cover. Later “big hole” clutch covers facilitate filter removal and replacement without cover removal, but it is a bit tricky to accomplish. The spinning oil filter is driven by a small crankshaft sprocket-driven chain. There is a thrust washer on the shaft that must be placed on the outside of the filter to prevent the locating pin from gouging the outer cover.  There is a small o-ring that seals the cover to the filter body. Replace the o-ring if damaged. They can often be reused if installed correctly. The filter body must be thoroughly cleaned, along with the inside of the cover. Any leftover scraps will go directly into the crankshaft bearings, so be diligent in your cleaning efforts.

In proper working order, these engines use very little oil, but some will be burned in normal operation. Make sure that the crankcases are filled to the top mark on the dipstick before riding out for any distance. These engines do not have valve stem seals; however a clever air bleed system on the intake valve guides helps to lessen the oil consumption past the valve stems.  Honda’s GN4 10-30 oil is more than sufficient for normal operating conditions. 

When these engines were originally built, oil technology was in its infancy in many respects. Honda specified non-detergent oils originally, counting on regular oil changes to keep the engine clean inside. The problem with non-detergent oils is that they don’t help suspend particles in the oil like today’s detergent oils do successfully. 

It isn’t a bad idea to pull down the oil pump to check the screen for damage and for the presence of thick, mucky oil deposits and contaminants. Virtually every engine I have torn down has had a thick layer of deposits lying in the bottom of the engine cases. Even with detergent oils, when these engines sit for years unused, the suspended contaminants eventually fall out of suspension and wind up in the bottom of the engine.

Rule #5
ALWAYS ensure that the engine spark timing/advance is set properly. Check the timing with the engine running, using a dynamic timing light. Static spark timing is fine to get the engine running, but you must check it with the timing light when the engine is running to prevent piston seizures and help smooth out idling speed problems and carburetion setting issues. The spark advancer system is built into the camsprocket inside the engine.

Many things can and do go wrong with this design. Wear can develop between the spark advancer shaft and the camshaft that it rides in, causing sideplay in the points cam. The return springs can weaken or break, causing lazy spark advance return. The camsprocket is riveted together and eventually the rivets loosen up, allowing the camchain sprocket to loosen and walk back and forth within the mechanism. There is also some slop between the end of the points cam shaft and the plate that it engages in, within the camsprocket assembly causing some initial spark advance on start up. Honda Dream camsprockets have heavier weights and lighter return springs to speed up the spark timing on those models.  Camsprockets used on CB/CL models have lighter, smaller weights and stiffer return springs compared to those of the Dream models.

Rule #6
There are some “universal” engine parts that are shared within all three engine types. The kickstarter shafts, shift drums (except rotary gearbox types), primary chain tensioners, shift forks and shift selector shafts are usually the same in all models. 1960-62 shift shafts were shorter than later versions, however. Dreams have a completely different set of transmission ratios than the shared CB/CL gearboxes. There are 4 different clutch assembly setups, depending upon the application. The camchains, camchain guide rollers and camchain tensioners are all interchangeable, however the camchain tensioners have two different bolt patterns, changing in 1966.

Rule #7
There are “round bowl” and “square bowl” carburetor types, but they will interchange on various models. The round bowl versions came first, replaced by square bowl designs around 1964 for most models.  You can interchange the CB and CL77 carburetors if you swap out the jetting components.
Carbs come in 22mm and 26mm sizes for 250 and 305cc models, respectively. The 22mm carburetors for the 250s (and 305 Dreams) are all different in function and mounting points. Only the CB72 Super Hawks have the “power jet” carburetor functions. The CL72 Scrambler carburetors look identical, externally, however the power jet functions were deleted.  

Dream carburetors for 250-305s are interchangeable with minor jetting adjustments. Good carb parts source, as well as other 250-305 repair parts is  or The most common problem with carburetors is that the mounting flanges become warped, causing air leaks. Second most common problem is that the slide bores become distorted, causing the slides to stick when the engines heat up. With careful work, both conditions can be overcome.

Rule #8
Crankshafts all differ between the three models, as well as having a different balance factor between the 250 and 305s. The biggest challenge in rebuilding these engines is that the wrist pin holes in the un-bushed small rod ends get out of round, causing a part-throttle “chatter” sound. Honda made .004” oversized pins, however machining the pin bores to match is quite difficult with the crankshaft in its full assembly mode.  Competent machine shops can press the crankshafts apart, bore the rods to the oversize and/or re-bush the rods back to stock size again. Obviously, this will become an expensive repair step, if needed. The majority of the engines I have taken apart have had loose pin fits after 10k miles. If the engines have ever had a running piston seizure, the chances of rod damage are quite high.

Rule #9
The electric starter versions of these engines (CA/CB72-77) have a total of 5 chains in place to operate the motorcycle. The original primary chains, which connect the crankshaft to the clutch outer, stretch easily and begin to strike the inside surfaces of the clutch cover when they are out of spec. OEM primary chains are pretty much extinct, however a chain/sprocket company in the UK has come up with an endless 3/8x3/8 chain which works perfectly in these engines. They can also supply the tiny oil filter drive chain, camchains and starter chains. In a pinch, camchains can be cut up and shortened for use in the electric starter models.

Rule #10
Rebuilding the petcocks on the CB and CL models is fairly straight forward and plenty of repair kits are available.  Reserve tubes can crack or become broken off after 50 years of use. Overflow tubes can be replaced with 5mm brass tubing from a hobby shop. Dream petcocks are a whole different animal. They share a lot of internal parts with the 125-150 Benly models, however the outlet fittings are on opposite sized from each other, so the bodies are not interchangeable. Most aftermarket repair kits are not properly made, so cause installation and sealing problems.  Most of the internal sealing parts are still available from Honda warehouses and are highly recommended. A few parts are NLA in the US, but can be sourced from eBay sellers in Thailand and other Asian countries. Look out for warped petcock bodies that no longer seal properly against the bottom of the fuel tanks.

Obviously, there are many areas of these bikes that require careful study so that any parts purchases are not wasted due to a lack of knowledge about the kinds of changes that have occurred over the years. I offer comprehensive digital download packages of restoration reference files that are targeted for each of the 3 models.  See my site: for details and to order. I always welcome questions and can often guide people in the right direction for parts purchases and other restoration resources around the world.
                                                                       Bill Silver aka MrHonda


  1. Rule #1 applies to other 60's Honda models as well. I rebuilt a '65 S90 and thought I was losing my mind when I found that there were four different styles of of right case covers for that engine, each just unique enough to keep them from being interchangeable. Actual model years for the S90 are typically best guess and I finally based my particular example on a list of various serial numbers provided by many other owners who had varying degrees of documentation on them. But that alone is of little use as Honda made numerous 'running changes' within model years.

    1. Absolutely! I have TSB bulletins going back to 1959 for Dreams and Benly models, where they made dozens of tiny changes to the size/shape of the crankshaft rollers and cages, among many other details in the engines and chassis.

  2. I bought a '65 S90 basket case in '68 or '69. I learned a lot about how engines work by building it back up and converting it to a TT racer. Found out quick to keep the engine number with me whenever I ordered parts. I think there were three or four different crankshafts. I was still a teenager and made the mistake of keeping all my receipts, which when I added them up, I could have purchased a brand new S90 for about the same amount. But it was a great learning tool that was cheap in the end.

  3. Great article Bill. Can you describe the "air bleed system" on the intake valve guides? I had never noticed that feature.

    1. Except for the 1967 models, the two small fittings that are located beneath the intake ports are connected to long drilled passages to go all the way to the base of the intake valve guides. The guides are cross-drilled to allow atmospheric air to help break the oil film on the valve stems reducing the tendency to suck oil down the stem and into the intake port. In 1967 they discontinued the process, so the late heads are not drilled for the fittings. see for a image of the intake guide.

    2. Thank you. I recognize it now, 1966 was a long time ago.

    3. Are these air fittings supposed to be hooked up to anything? Most of my Honda twins just had them open with no hose. One had hoses, but they just terminated over by the air cleaners but were not attached to it. Service book is not much help.

    4. They are basically drain tubes, but do allow the equalization of atmospheric pressures within the channels all the way to the valve guides. Early on, Honda extended the tubes out to the air filters, but decided that it wasn't necessary. There is no vacuum source to attempt to keep clean.