CB550 leaking float bowls solution and cheap O-rings
This is my solution to the problem of leaking float bowls I’ve had for years on and off. My bike is a 1976 CB550F1 with 069A Keihin carbs.
I’ve had varying temporary solutions with original Honda o-rings, pattern o-rings and no luck at all with gasket sealer (it dissolves despite claiming to be fuel ‘resistant’). I had already eliminated incorrect float height (22mm on these carbs), I tested the level with the clear tube technique and it was 4mm below the top of the bowls.
Solution 1Given the price of replacement rings at £15 for a full set per carb plus shipping when you only need the float bowl seals I decided to try sourcing from a local supplier which I did at 60p each. I couldn’t find a spec for the original Honda seals so I carefully measured the ones I had with calipers and so the ones I bought were 2mm round cross-section, 68mm internal diameter, and at the recommendation of the supplier, made from Viton.
Bearing Source
http://www.bearingsource.co.uk/ in Liverpool: 0151 374 2151 provided fantastic service in advising me and supplying these so I’m happy to recommend them. You may need to call or email first as these seals aren’t listed on the site (I’ve also had great service from them when I needed wheel bearings, top brand and cheaper than anywhere else I could find).
I had partial success with these and you may find that they do the trick for a few pounds instead of over £60.
Solution 2However due to a few stripped float bowl screw threads in the carb bodies I still had some seepage and wasn’t totally convinced that the seals were ‘meaty’ enough on the ones that seemed to seal ok so I decided on a more drastic solution: retapped carb bodies, larger screws and slightly oversized O-rings.
I selected stainless steel M5 socket screws, 10mm long. These are widely available on ebay for pennies.
The beefier rings I used are 2.5mm round cross-section, 68mm internal diameter, made from Viton. Again
http://www.bearingsource.co.uk/ can supply these.
My method.
Carefully drill out the original carb body threads with a 4mm drill. Tap the new threads in two stages, first with an M5 taper tap. The tap will not work on some of the holes due to the lack of clearance above the corner of the carb so grind off about 4mm from the end of the tap and ensure that it doesn’t foul on the carb body when through otherwise this will strip the new thread. See photo below before the end was ground off.
See photo 1 - Taper tap
Next chase the thread through with an M5 bottoming tap or convert a plug tap by grinding off the end. Again this is to ensure a fully threaded hole without the tap fouling on the carb body above the hole. See photo for home-made plug tap. In the UK Toolstation supply a 10 piece set which includes the small tap wrench and taper taps. You’ll need to find a plug or bottoming tap elsewhere (or buy two sets).
See photo 2 - Plug tap
Take great care when tapping the holes to use a light oil (e.g. paraffin) and to be very patient, use light force and nibble at the new thread about a quarter turn at at time, backing out each time to keep the swarf clear. Breaking off the corner of the carb would obviously be a disaster.
The new screws will now be a nice fit.
See photo 3 – New screw
Next, drill the clearance holes in the float bowls using a 5mm drill. Again, a spot of light oil and patience is required. Take great care otherwise there is a risk of breaking off the corner of the bowl. Not as big a disaster as wrecking a carb but it will still spoil your day and set you back a while. A drill stand is recommended. M5 socket screw heads will fit without the need to remove any of the float bowl body.
See photo 4 – Drilling bowl
The float bowls have four small retaining lugs in the O-ring slot, these are to hold the original 2mm rings in place during assembly.
See photo 5 – Seal retaining lug
These will need to be removed to enable the larger 2.5mm rings to seat correctly in the slot. I found a circular cutting disk in a Dremel does the job and finish off with a fine file. Take care not to cut into the bottom of the ring seat.
See photo 6 – Seal retaining lug removed
Clean and polish the ring seat and the mating face on the carb body to ensure a good seal. A wire brush in a Dremel will remove any debris and then follow this will some very fine wet & dry paper and then metal polish.
The new rings will sit nicely in the bowl with only a little persuasion without any need for glue or tape to hold them in place during reassembly.
See photo 7 – Oversize seal in bowl
The new beefy seals really look up to the job.
See photo 8 - New seal in place
When fastening the bowls into place don’t be tempted to bring the metal faces together at the first attempt as you may break off the corner of the carb or the float bowl. The screws can be tightened sufficiently with a small gap. Great care is needed with socket screws in alloy as it’s easy to over torque them, use fingertip pressure on the hex key only. Over the next few days check the tightness of the screws as the round seals bed into the square slots and you may get a slight turn on each one and bring the faces together. If you’re worried about the security of the screws use 12mm ones instead and add spring washers.
See photo 9 – Seal gap
See photo 10 – Finished assembly
I’ve had no more leaks with the new seals and the socket screws make it easy to work on the carbs while they’re still on the bike.
Bearing Source assure me the the Viton material is very resistant to fuel and ethanol but as the seals were so cheap I bought a few sets to have in stock for future use. Same goes for the screws.
This modification means you still have the option to use original seals in future, you’ll just have a bit more fun without the retaining lugs.
