Vintage Dirt Bike Q & A

Hi Guys

Firstly, as a newbie to the site, allow me a quick intro. I'm from South Africa and have been riding/racing offroad for 40 years. I had a MT back in the day and recently bought a semi restored bike with the intention of completing the restoration.

All has gone well but there have been the inevitable carb issues. The carb is old and worn and the previous owner had fitted a carb kit. Does not look like OEM Honda when I compare the old parts with new. I managed to get her running quite nicely on the weekend and took her for her maiden drive. (Btw - she started first kick every time). Parked her and removed carb to change mj to 122 (I had fitted a 115 which seemed a touch lean)

Now she will not start.

There is spark, plenty of compression (140psi).

Jetting: 115 mj, 60 pj, needle is an 003 (leaner than std which I fitted) dropped all the way (top clip position), air screw was 2 turns out. Float level is 20mm. new needle and seat and o ring (does not leak - have tested this). Floats are ok.

The carb was dribbling btw - cracked brass overflow tube which I replaced today - dribble gone.

what now?

I noticed that the little brass tube of the choke circuit has a tiny hole in the side. I presume this is a siphon breaker? Maybe the float level is just below this and the choke circuit is not working as a result?

I am at a loss.

Ray? Clever guys - help a newb

Thanks
Neil (aka Fario)

Neil,

Since the bike was running fine beforehand, I take it something must have happened during the jet swap? Is the bike flooding during starting now, or the opposite (no choke action, or no fuel)? Was there any rust inside the gas tank, that might have loosened up during the ride? I've seen rust plug up the petcock so tight not a drop got through, and also stick the float valve open leaving a gallon of gas on the floor.

I mentioned in another post about a clogged gas cap vent, but I don't think that's what's going one here - maybe a good idea to check it anyway, though.

I've got two of these 28mm carbs open on the bench, both have four holes in each choke tube (maybe for an emulsion effect on the choke mixture?).

BTW, Keyster makes a very nice rebuild kit for these carbs (# 58-0009) with the large-knob FL250-type idle speed screw, 32mm-long #58 pilot jet (yours may use the 28mm short-style jet), etc.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray

Thanks for the reply.

Firstly, tank has been derusted and epoxy coated. I had also disassembled and inspected the fuel cock. All clear. Also fuel tank filler cap vent is clear. I have also fitted a small, clear inline filter and I can visually see the fuel flow.

I suspect that the bike is really flooded. I have tried the old method of removing plug and kicking away.

Tonight I checked spark, timing etc. all ok. The plug gets wet when screwed in and kicked over. I also help the plug over the plug hole and kicked her over. The external spark was enough to ignite the mixture escaping from the plug hole. Little burst of flame.

I am going to leave the plug out overnight and remove carb as well. Maybe enough fuel mixture wil evaporate from the crankcase.

I suspect that once started everything will be ok.

I have ordered another carb from e bay today as this carb is tired. Even with new brass.

I had similar issues with my WR250f until I discovered that carb cleaner had perished the viton coating on the float needle. Replacement sorted that out.

By the way, the cracked overflow pipe issue was resolved by cutting off the brass pipe. A small plastic tube from an ear bud is a press fit into the remainder of the brass tube. Works perfectly.

Neil

Just removed the carb and opened it up. Very wet everywhere. Lot of fuel in the throat. Float level appears right judging by the fuel I drained out of the bowl.

You are correct Ray. 4 little holes in the choke tube.

Now why is my bike so rich on startup and how do I dry out the crankcase?

Btw. Tried kicking her with choke off, the with throttle held wide open.

Nothing

So many of these bikes have excessive-fuel carb issues! The 004A MT250 carb I used on my MR175 for a while never leaked at all under normal conditions, though.

If you have the intake manifold off, you may be able to slide a clean rag or super-thick cotton rope down into the crankcase through one of the intake ports and wick the fuel out, or at least get an idea how much is in there. The piston should be at TDC first, obviously. A crankcase explosion could be followed by a flame-thrower situation, so be careful.

Does your MT lean way over on the kickstand? This was another of my theories on the carb leakage, as my MR would start leaking too if I leaned it over as far as the MT.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray

Thanks for all the replies.

I have been considering putting a modern 28mm carb on the bike.

Searching e bay I have found a new OKO PWK Flat side carb (28mm)

Dia 50 airbox
Dia 35 Spigot (inlet I presume)
8.5cm length
14.5 cm height
7cm width

PJ N424-21
MJ 99101-357
Needle 4J13

What are your thoughts on this? I do'nt have major issues in terms of authenticity but really want a new(ish) carb.
Playing around with old, worn carbs is an exercise in frustration

To continue my questions:

I see that there are a number of 28mm keihins from YZ/RM/CR 85's on e bay at good prices.

Will these fit? Has anyone tried this?

The attraction to me is that I will be able to get jets locally.

Those OKO carbs look pretty decent, and are also available in 30mm which is unusual. The OEM 28mm has a 35mm spigot, 44mm mouth, and 98mm overall length, so the airbox end might take a bit of work. I don't know about the throttle cable, you might get lucky and it will go right on, or you might have to remove the adjuster nut, etc.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric