Wednesday, February 20, 2019

Vintage 250-305 Honda Petcock repairs

Fuel Petcocks (sometimes referred to as Fuel Valves) generally have three positions to control the fuel flow into the carburetors. OFF, ON and RESERVE are the three options and all must be functioning properly for a safe, well-fed motorcycle. Most of Honda’s early petcocks had a fuel bowl attached to the bottom, which trapped water, dirt and other debris from heading down the fuel lines towards the carburetor float valve, the float bowl and jets. Simple maintenance is to turn the fuel valve OFF (check either a punch mark alignment with the lever retention plate or use the lever as an “arrow” to indicate the direction of the fuel flow in each position) and then unscrew the sediment bowl carefully (use a rag to catch the overflow).

Clean out the bowl, inspect the filter screen that is usually held in place with the fuel bowl O-ring and reinstall. Turn the fuel lever to ON and watch for any leaks at the bowl/body interface. If there are fuel leaks, either tighten down the bowl a bit more or turn valve OFF, remove the bowl, O-ring and screen and replace the necessary parts to ensure a good seal. DO NOT leave your motorcycle unattended with a leaking petcock!

The least that can happen is that the tank empties out all over your motorcycle and onto the ground, contaminating the area with raw gasoline, which is toxic and very flammable. The worst thing is that the bike catches fire from an external ignition source and you lose the whole thing.

When you have leaks around the fuel lever, drain the fuel tank, remove the tank from the bike, drain the fuel bowl, remove the O-ring, screen and then remove the two screws that hold the outer lever plate onto the petcock body. Again, use rags to control spills and be careful about the potential for fires and damage to your machine. Once the plate is loose, the lever, tension spring and plate will all come off as a unit. The 4-hole sealing gasket in the petcock body can be inspected for tears or flattening and replaced as necessary. In emergencies, you can rotate the gasket or turn it over and reinstall the lever parts to help control leaks. Use of a new gasket is preferred, along with inspection of the flat side of the lever surface, where it contacts the gasket.

Fuel can often etch the face of the lever surface and allow minute fuel leaks to pass by the edges of the gasket, causing continuing fuel seepage. If you see this condition, use a flat file or some mild emery cloth on a flat surface to carefully flatten the surface of the lever for a positive seal once it is assembled again.

It is wise to remove the entire petcock unit, periodically, to check for sediment contamination in the RESERVE passage at the bottom of the petcock body. The petcock, for most models, has a small brass rod that rises up inside the fuel tank, accessing fuel to a certain level. Once that tube end is exposed, it becomes necessary to switch the fuel lever to RESERVE, which will pull the remaining fuel from the bottom of the fuel tank, through a small passage in the petcock. Old fuel, condensation, rust and debris can easily clog the fuel passages in the petcock, causing fuel flow difficulties. Usually small drill bits, inserted by hand can clear most of the passageways, so the fuel can freely flow through the petcock body. 

Replace the petcock O-ring that seals the body to the bottom of the fuel tank. On the 160s, 175s, CB72-77s and many of the 350-450s, the nut that connects the petcock to the fuel tank nipple has a special flat sealing gasket inside that must be replaced and carefully threaded to both parts at the same time, ensuring that the ends of the two parts are sealed in the middle of the nut.

Later model machines, like 350-450s, use a special nylon screen that covers the petcock's inlet fuel passages and feed tubes. This screen must be checked for tears, damage and wear to ensure clean fuel feeding the carburetors.

                                              Honda Dream/Benly Petcock exploded view
                                                                 AHMC parts manual

The petcocks used on the 250-305 Dreams, CB92, CA95, CA160s are unique in design and require careful assembly with new parts to ensure that there are no fuel leaks around the lever. Many of the petcock sealing parts are interchangeable between the Benly and Dream models. The bottom of the fuel tanks must be clean and able to receive the petcock O-ring, plus the screw holes must all be sound, to insure that the three retaining screws will hold the petcock body tightly against the fuel tank. The 5mm screws are JIS thread, which are very difficult to find now. For best results use OEM Honda replacement parts for Benly-Dream petcocks. Most aftermarket kits do not fit well, especially where the three central holes fit into the base of the petcock body.

Thursday, February 7, 2019

The 250-305 Engine overview (Part 2)… the other stuff to know…

The engines have the same architecture among the three versions. All of the engine cases are basically the same, apart from the early ones not having a primary chain tensioner mounted. They are not “machine matched” at the factory. The 1960-61 cases and cylinders had a different oil supply feed for the top end, thus there are two different base gaskets. The 1960-61 engines had a “rear breather” system cast into the back of the top case, which allowed the crankcase to be relieved of pressure during operation. 1962 and later engines used a breather plate and drain tube coming from the top cylinder head cover.

1960-61 cylinder heads used a small 10mm spark plug, changed to 12mm in 1962. Pre-1965 heads had an inverted U-shape design in the forward fins, which was changed to a sharper V shape from 1965 onwards.

The die-cast cylinders, introduced in 1966, used a narrower camchain tensioner to allow better air flow between the cylinders. The cylinders will all interchange between the various engines, apart from the previously mentioned 1960-61 oiling passage differences.

Honda Dream intake valves are smaller in diameter than those used on the CB/CL models. They do share the same exhaust valves, after 1962.

The camshaft sprockets and locking nuts were changed in 1962 from right hand threads to left hand threads. There are about four different camshaft/camsprocket spline patterns which precludes any attempts to mix and match camshafts and camsprockets from other years.

Early transmissions had straight cut gear dogs, finally upgraded to back cut dogs which offer better dog engagement and fewer missed shifts.

The shift forks, shift drum, kickstarter shaft, kickstarter pawl/spring/plunger are universal parts fitting any and all 250-305 engines, except Dreams with rotary gearboxes. There are two different lengths of shift shafts, changed around 1962.

There was a change to “shallow spline” shafts in 1967 which included the crankshaft and primary drive sprocket as well as the transmission input and output shafts. You can use the later shallow spline sprockets on early deep spline shafts, but not the other way around.

Honda used both Nippon Denso and Kokusan ignition and charging system components. Using mismatched branded points is a no-go with these ignition systems. Use of aftermarket point sets from Daiichi on a Dream is also a no-go, as they do not allow proper timing adjustments. For best results, used ND points on ND point plates. ND parts have a -004 suffix, while Kokusan have -005 parts suffixes. Either brand might be found on a Dream engine, so pay attention before you order your parts.

The point cam/advancer shafts which run through the inside of the right camshafts come in 2 outside diameters which match the ID of the holes on matching camshafts. They don’t interchange! Check the return action of the camshaft sprocket return springs before assembly. The springs should return the weights back to resting position. Remove excess slop by pinching the ends of the springs very slightly. Check the camsprocket for any looseness in the rivets which hold it together.

The ends of the intake and exhaust rocker arm pins are 2 different diameters. They don’t interchange either! There are two types of rocker arms, but all will interchange.

I use Honda’s GN4 motorcycle oil in 10-30wt for most applications. Honda originally specified non-detergent oils, but that technology has been far advanced by current motorcycle-rated oil products on the market now. Full synthetic oils are really not necessary unless you are involved with racing activities and even those may not require synthetic oils. Normally, these engines are pretty oil tight, but synthetic oil molecules are so small that they can work their way past old seals and gaskets.

Speaking of gaskets, most of the original gasket materials were all or partially made from asbestos, so be very careful with scraping and grinding away on stuck-on gaskets from the past 60 years.

Last time I checked, Honda’s “suggested retail price” for ONE 250-305 piston was $186... for ONE piston. Tim McDowell has replacement WISCO forged pistons available with and without sleeves for less money.

You can interchange CA pistons with CB pistons and the other way around. Just find a matched set of whatever you can find and it will be fine. OEM Honda pistons have ART cast into the sides and CB/CL77 pistons have CB77 marked on the sides as well.  There are a fair number of aftermarket replacements, made back in the 1960s, which may not be quite right. I have found some that have too small piston pin holes. The piston crowns on early and late pistons are quite different. Whichever ones you use, get a matching set for installation. Aftermarket pistons might need a smidge more clearance than OEM pistons, which I usually have clearance at .0015” (one and a half thousandths).

MrHonda has the center camchain guide rollers in stock a which fit all 250-305s. Contact me through my site:

Wednesday, February 6, 2019

The 250-305 Engine overview – Part 1 of 2

The famous SOHC, 2-valve, 250-305cc engine has its design roots in the original 1957 C70- series 250cc Dreams. While the originals were dry-sump (separate oil tank) designs, many basic features were carried over into the 1960-later wet-sump engines, which power all the Dreams, Super Hawks and Scramblers, in various configurations. The engine’s rotating components spin on ball, roller and needle bearings in almost all cases. Low-friction element bearings only require a constant flow of lubrication, instead of high-pressure oil, which is necessary to maintain lubrication for plain-bearing engine designs.

Original Dream-series engines used 360-degree crankshafts, with a single carburetor. With the release of the high-performance CB72-77 Super Hawks in 1961, the crankshaft firing was changed to a 180-degree configuration and dual carburetors were adapted for high rpm work. The Scrambler engine is basically a modified Super Hawk design, lacking the electric starter and tachometer drive components.

The crankshaft is built up with a combination of ball bearings for the clutch end, and roller bearings for the middle two of the four main bearings. A heavy-duty roller bearing supports the crankshaft on the left side end for the alternator rotor. The connecting rods run on needle bearings for the big ends, but the piston pins run directly on machined small ends. The small ends of the rods tend to get oval-shaped, especially when they have a lot of miles on the engine or if the pistons have seized during operation. Oversized wrist pins of .004” are available to clean up minor wear issues.

Pistons are CB72 or CB77-based for Scramblers. Pre-65 engines have 9.5:1 compression, reduced to 8.5:1 after 1964. Cast pistons are used with chrome-plated cast iron rings. Early ring sets used a single chrome-plated compression ring, with the other two rings un-plated. Those ring sets were superseded to all-chrome plated rings for increased durability. Dream pistons originally had thick 2mm rings, but were superseded with a later design using 1.5mm CB ring sets. Some very early version CB72 pistons had 10:1 compression pistons.

The 305 and 250 crankshafts differ only in the size of the balance holes bored into the crankshaft counterweight assemblies. Honda used relatively long rod lengths to prevent piston rocking and encourage good rod leverage on the crankshafts. Heavy crankshaft weights helped to keep the momentum of the small displacement engines going between gear shifts, so that mid-range torque was favorable, despite the high horsepower peaking speeds. The use of a short 54mm stroke, allowed the engines to turn high rpms without creating excessive piston speeds (measured in feet per minute). Scrambler crankshafts lack an extra oiling hole on the rotor end, which is used to feed lubrication to the starter clutches on the Super Hawk applications.

Camshaft timing and valve sizes are the same for both 250cc and 305cc CB/CL engines. Cylinder head castings are machined differently between the two types, using a chamfered edge on the 305 cylinder head to add piston clearance for the 6mm larger bore size. The combustion chambers, spark plug threads and valve seats are a single cast-iron insert, surrounded by alloy castings. 1960-61 cylinder heads used 10mm spark plugs, which were replaced with 12mm plugs in 1962.

The camshaft assembly is composed of four major separate pieces. The camshaft sprocket contains the spark advancer weights and springs and is splined on both ends to accept the left and right side camshafts. Camshafts are retained by an expanding internal shaft on the left and a compressing nut on the right cam. The ignition point cam runs through the inside of the right side cam. An oil seal on the right side cylinder head cover contains engine oil that lubricates the camshaft and point cam combination. The camshaft is supported by four ball bearings.

The 94 link, 219 pitch camchain is guided by a lower roller, located between the cylinders mounted on the top of the upper crankcase, while a spring-loaded, mechanically-adjusted tensioner roller is mounted on the rear of the cylinder block assembly. There are “wide” and “narrow” camchain tensioners used on the cylinder blocks. The change to narrow versions came out in 1966. First version tensioners had the adjustment bolt on the left side.

The 26mm carburetors are almost identical between the Scrambler and Super Hawks, save for the calibration of the main jets and jet needles. On the 250 sizes, the carburetor types differ because of the use of the additional “power-jet” enrichening system on the CB72, versus a standard main jet design on the CL72s. On 250-305 Dream engines, they shared the same sized 22mm carburetor, but there were slight jetting adjustments made for each version

Transmission ratios are the same between CB and CL series engines, but they can be altered by “X-ing” the center gears on the main and countershafts. This interchanging of gears closes up the steps between 1st and 2nd gear, while increasing the jump to 4th from 3rd gear, which is beneficial for off-road riding. Dream transmission ratios are completely different than the CB/CL versions, so only a few parts interchange between the models.

The clutch assemblies are all interchangeable as a complete unit, between any of the engines, but there was a change to a “cush-drive” clutch outer for the 1965-on engines, which takes a bit of the torque loading off of the primary drive chain. There were 4, 5 and 6 plate clutch configurations, depending upon the year and application. Whatever you choose, the total stack height must be the same as what Honda installed originally. The inner clutch hubs have machined grooves for fine wire retainers to help trap a few of the plates in place when the clutch is disengaged. This minimizes clutch drag and the attendant problems of not being able to find neutral when at a stop light with the engine running. 

Each of the 4-5 clutch hubs are machined for the matching clutch pack setup. The friction and steel plates have different thicknesses, depending upon the total number of plates used in the assembly. The best solutions are to find a complete used 5 plate clutch assembly from a CB or CL77 and install the whole thing into your engine, no matter which model you have. Current clutch springs with a 323 code part number are equal to the original CB77 street bike springs. DO NOT use the 275-810 springs for street use.

The chain count in the Scrambler applications is four: Primary chain, oil filter drive chain and camchain and final drive chain. For a Super Hawk and Dream, the count is five, which includes the starter motor drive chain.  See notes below for replacements.

Ignition for the Type 1 (180 degree) engines is battery-powered, by use of dual points, condensers and coils. Ignition point sets are mounted on a single, adjustable plate, mounted on the right end of the camshaft. The dual-condenser set mounts on the ignition coil bracket, beneath the fuel tank. All Dream (360 degree) ignitions have a single twin lead outlet coil and a single condenser and set of ignition points.

Charging system components include a permanent-magnet rotor, air-cooled stator assembly, which feeds a Selenium or silicon-diode rectifier, to charge the 12v battery.


Common period modifications included 350cc big bore kits from WEBCO, Harmon-Collins racing camshaft profiles, including a roller rocker kit. Many other camshaft manufacturers offer various cam grinds. Cylinders can be bored to 3mm oversize (337cc), using stock cylinder liners. Due to the hemi-spherical combustion chambers, the valve angles prevent use of valves more than about 1 to 1.5mm oversize.

“Pops” Yoshimura created modified 250cc engines that were capable of beating CR72s at some racing events. His CB72s won an 18-hour endurance race, with sustained engine speeds over 10,000 rpms for the entire event. His CB72 engines were churning out 34 horsepower at 11,500 rpm, while the 305s were adding an additional 5 more horses, at the same rev limits. His bikes were built using modified factory production parts, which attests to Honda’s engineering prowess in creating durable and powerful engine components.

It is difficult now to grasp the full impact of these machines on the motorcycle market in the early 1960s. British, American and European-made machines dominated the marketplace, but their overall build quality and design expertise were sorely lacking, in comparison to the new wave of high-performance Hondas, which swept the world’s marketplaces. Advanced designs, coupled with superior engineering and materials enabled Honda to surpass the products of companies who had been in business for decades, in a matter of just a few years.  With the strength of the design of the Honda Scrambler engine, the task of riding from Tijuana to La Paz, Mexico in less than 40 hours, using production machines, was well within the design parameters of Honda’s CL72 dual-purpose machine, back in 1962.

Honda CL72 - CL77 engine specifications.
Total cylinder capacity:         247cc                                     305cc
Bore x Stroke:                         54mm x 54mm                       60mm x 54mm
Comp. ratio:                9.5:1 (’62-’64)                                     8.5:1 (’65-on)
Normal measured compression: approx. 150-175psi
Max. Horsepower:         24 hp                                28.5 hp @ 9,000 rpm
Max. Torque:              2.06 kg-m@7,500rpm /2.5 kg-m @ 7,500 rpm
Ignition: 12 volt battery and coil (dual points-Type 1, single points-Type 2)
Spark plug: NGK D8HA, except for 1960-61 models)
Carburetor:     CA/CB/CL72 PW22 (22mm) 26.5 float level
CB/CL77 PW26 (26mm) 22.5mm float level
Lubrication: Wet sump w/ gear oil pump
Clutch: Wet-type, Multi-plate
Transmission: 4-speed constant-mesh, foot shift
Ratios: CB/CL72-77: 2.788 (1), 1.661 (2), 1.171 (3), 1.00 (4)
Crankcase capacity: 1.6L (2.8 Imp pt/3.4 US pt) 30W, 10W-30/40