C90 cleaning the carburettor: Chinese substitute Kaiwan PZ19 setting the float height.

Carb fitted: Kaiwan PZ19

When I got this bike it would start fairly easily although the electric start was a bit hit and miss. However, although it would fire up easily enough, it wouldn't run without virtually full choke. The plug was very sooty indicating rich running. The PO had said that it took a long time to warm up and he didn't know why... he didn't add that in fact it never warmed up and simply wouldn't run without choke. This obviously pointed to a carburation problem but to my surprise the carb  had actually been replaced already, it was clearly newer than the rest of the manifold and inspection showed it was a Chinese Keihin copy by Kaiwan, a PZ19.
In the past I've found this sort of problem to be due to a blockage somewhere in the carb so it seems like a strip, clean and a check are in order.

Remove the legshields, 4 bolts and a lower catch clip on this bike to reveal the carb. Honda fitted Keihin carbs, this bike seems to have been fitted with a cheaper Chinese substitute. In this case a Kaiwan PZ19 but I think this is a direct copy of the Keihin item.
First clamp the fuel hose then disconnect the choke cable.

 Loosen the screw

 Free the cable from the clip at the top of the bracket

 Wind it round to release the nipple

 Unscrew the carburettor throttle cap.

... and pull out the throttle slide, spring and needle.

I left this hanging on the bike whilst I removed the carb

 Pull off the two overflow hoses

Noting their routing down through the frame. Both pass through retaining clips, one drains to the left whilst the other drains through the frame.

Right hand drain tube passes through clip and enters frame

Left hand drain tube passes through guide below the block

Detach the air hose by unscrewing the jubilee clip, then unscrew the 2 x8mm bolts holding the carb to the manifold. Pull it away and out of the air hose as it comes. When refitting note to position the clip before re-attaching the carb as the air inlet spigot fouls the screw of the clip!

withdraw the carb and pull it off the air hose

Invert the carb and unscrew the 2 diagonally opposed screws that hold the float bowl onto the base. This pops off easily- excess fuel will drain out.

Float chamber detached

Check inside the bowl and clean out any sediment.

 Undo the large 17mm nut cap under the tap chamber with a spanner

 Lift off the cover, remove the filter cage and clean out any dirt, make sure the lower O ring on the filter cage is present and in good condition. Replace the filter- making sure it pops fully home and refit the cover plug-checking the O ring beneath.

Looking at the top of the carb, the float and needle valve are held by a free-sliding pin

 The pin just pushes out to release the float

 So that the float and needle valve can be lifted out. Note that the needle just hangs from a metal bracket on the float by a wire clip. In other carbs the needle has a lug on the top which slips into a groove in the float, these are non-adjustable but the type fitted to my bike type can be altered.

This exposes the two jets which on this carb are removable. The larger central is the main jet which screws into its holder and the smaller is the slow running jet.

Central jets in the carb. The slow running jet  has been unscrewed and is being removed.

In this carb the main jet is in two pieces which unscrew, in others they are made as one. In any event if the jet is clear its not necessary to separate the two parts. Both jets unscrew and can be blown through with carb cleaner. I found that the  slow jet was blocked which would certainly account for the problems in running. I realise that these are of critical size but when they are blocked like this no amount of blowing will clear them so either replace or poke them through. I used a high "E" electric guitar string which crunched through removing the crystallised white blockage.  I then blasted carb cleaner through all the passages moulded into the body and verified all were open.

Reassembly is the reverse of dismantling and all went well. However when reassembled the motor would now run off the choke, but throttle response at low openings was poor. Opening the throttle just a little led to a misfire which picked up again as the throttle was opened further. There are probably a number of explanations for this but in the past I've found this can be caused by too low a fuel level in the float chamber and this can be addressed by adjusting float 

I understand that the original Keihin carb may not have been adjustable, but the PZ19 clearly is. Checking Haynes manual float height is specified as 10.7 mm for this bike. This seems a little optimistic as I doubt anyone can set it within 0.1mm anyway. This value is for a genuine Keihin, but think it should be the same in this copy-cat carb. 

I stripped the carb again to check float height. Carb height is defined as the distance between the carb body flange base and the base of the float when the float is in contact with but not pressing hard on the needle. You can arrange this by holding the carb just off vertical so the float hangs against the needle. 

However in this carb the float is pivotted at one side and clearly doesn't hang straight so it makes a difference where you measure its height. By convention you check across the main jet. I have a float height gauge which you set to bridge the float making contact with the mating flange on each side and then depress the slide until it just contacts the float. However, it seemed to me that it was set much higher at 15mm. I readjusted the float height by bending the metal needle tab- in this case upwards,away from the float, so that the float would sit lower when the needle was seated.

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Resetting the float height to 10mm

I adjusted the float height to 10.1mm. The motor starts in choke but warms up and runs off choke quite quickly.

Many people recommend setting fuel level rather than float height. This is easily done using a clear tube on the drain spigot that can act as a U tube and indicate fuel level in the float chamber. This sort of adjustment can be awkward on the bike but the level should be checked at least in position because the carb isn't horizontal in use. Trouble is I can find no info on what level to aim at. There is a moulded mark on the lower section of the float bowl to the right of the fuel tap. Experimenting I found that setting the float height to 10.1mm gives a level above this mark when horizontal and higher again when in position on the bike. I'm not able to find the "real" height expected so I'll just show the height my fuel rises to in position with the float height set to 10.1mm.

Moulded mark on the carb bowl rim

Level indication mark highlighted in blue

The level observed in situ with float height 10.1mm is shown below. 

One remaining problem was that the choke cable outer is about 1cm too long for the cable clamp position on the copy-cat carb. This meant that the clamp couldn't grip the ferrule and the cable body was too thin for a tight fit. Consequently the cable kept working loose and jamming the choke which may well have been contributing to the running problems. I slipped a short length of split fuel pipe over the cable behind the ferrule and tightened the cable clamp onto this and problem solved. Cable now has a natural run and operates without moving the outer.

Choke cable clamped onto packer behind the ferrule

Still not sorted

Well  I took the thing out for a run and it runs like a dog. It starts and moves off easily but there is a decided lack of power, a real hesitation and even pushing the motor doesn't give me much past half throttle. 

Reading around I discovered that many of these cheap carbs come jetted for something other than the C90. Maybe a chainsaw or something!! 

The jets in a Keihin are simply specified by the orifice diameter and for the C90 these ought to be pilot-40 and main 80-85. Using a set of jewellers hand drill bits (and checking them on a micrometer) I found my carb is jetted at pilot 30 and main 70. You can use these drills to carefully enlarge the main jet at least to 80 although this is a rough and ready approach that could destroy any venturi effect.  The pilot was too small for this approach to work without breaking the drill bit! Having enlarged the main there was some improvement but still no real response to full throttle and the overlap between pilot and main as the throttle is opened seems rough. 

I ordered a Keyster carb set 0323 which comes with both correctly sized jets and a new needle. Sadly, and to my mind oddly, it doesn't come with the needle jet female counterpart so I'm not sure whether the Keyster needle will work in the Chinese carb hole but I'll try fitting this lot and report back. 

I fitted the pilot and main jets plus the needle from the Keyster kit, setting the needle clip to the middle position which should be right now. I readjusted the carb. Surprisingly the bunsen blue is visible over a wide range of air screw adjustments so I had to set it for best running at idle (eliminate misfires) and good response to throttle blip. Road test shows the bike is now running the best I've ever had it. The new pilot jet gives much smoother response at low to mid throttle but really full throttle is still unresponsive so the main hasn't improved matters. 

I'm going to explore non carburation faults now I am beginning to wonder if this is an ignition fault where the coil breaks down at higher revs or the cdi can't respond quickly enough. More serious issues might be a burnt or leaky valve and badly adjusted tappets. I did check the compression on this motor which was 160-165 psi, so pretty much perfect and this eliminates any leaky or burned out valves. Checking the tappets did throw up one problem in that the inlet tappet should be 0.08mm and the exhaust 0.1mm. The inlet was too big and accepted a 0.13mm feeler so I reduced that to 0.08. Exhaust was fine.  Sadly none of this helped. 

I am now reduced to electrical and ignition faults so I will swap out the coil and cdi unit. Whilst I'm in there I will swap the flasher because this one's a bit fast. I'll start a new post for this electrical stuff. 

The acquisition!

I found this C90MT on Ebay. I was after a cheap bike to run into town, not a project, so running with an MOT was important. This one seemed to fit the bill. I bid and won it for £847 travelling to Kent to collect .The pics and text of the ad are reproduced below.

For Sale I have my Honda C90M Cub. Now 17 years old it has only covered just under 16000 miles. There is no rot on the frame although it shows a few discoloured areas mostly due to age. Runs very well and has been used by me on a regular basis going backwards and forwards to the shops etc. Some of the paintwork shows signs of wear but this can easily be solved either with a can of paint (Supplied) or a replacement item (Readily available) Gears move smoothly and the clutch shows no sign of wear. It has just had an MOT with no problems which expires in September 2019.

Viewing is welcome, please contact me to arrange. These little bikes are very popular at the moment with the price rising daily.

Inspecting the bike showed it was grubbier than expected with surface corrosion on the frame around the fuel tank and a bit heavier on the rhs swing arm The plastic of the handlebar nacelle was chipped but a new 2 part nacelle and a new full spraycan of matched blue paint were included. The blanking plug was missing from the sw arm and a screw missing from the front indicator which rattled badly when the engine ran. The two blanking plugs  from the rhs of the legshield were missing. All the lights worked.

The electric start did work but the bike would only run on the choke, attributed by the PO to a slow warm up (?). The battery was new. Toolkit missing. These things however were I thought relatively minor and outweighed by the running condition and MOT. Anyway, this was an auction sale, hammer prices are final so faults and all- it was already mine. I loaded it up and carted it home.

My plan was to halt the rust using Jenolite replace the missing plug and waxoyl through the sw arm. Sort out the carb and use the bike, reserving next year for a full respray. Red on to find out how I got on.