Cota 247 Running Lean and Overheating

[David Lahey]

To hopefully give you a little confidence or patience or whatever, here are some things I did to a Cota 348 before I got it to run well. I bought it in almost unused condition in about 2001 so assumed that it would go well - ha!

The problem was that it ran lumpy just off idle most of the time and also went crazy lean when it got up to full riding temperature.
Here are the things I did that made no difference to the problem:
Replaced both crank seals
Replaced fuel tap
Fixed up the little hole at the rear of the gearbox where someone had blocked it off (this did fix the gearbox pressure buildup though)
Thermally insulated the carby from the exhaust pipe
Decarboned the head, piston and exhaust port
Fitted a new condenser, under the fuel tank

Here are the things that did make a difference:
Swapped the starting jet for the pilot jet (they were fitted in the wrong holes)
Replaced the (dell orto - what the...??) carby needle with a standard AMAL needle
Fitted a new standard AMAL needle jet
Fitted a new AMAL throttle slide
Replaced the B7ES plug with a BP7ES plug
Fitted a new carby-to-barrel tube
Despite all this it still didn't run as well as I wanted and still needed the air screw set just off the seat. I studied the arrangement of the pilot circuit and compared it (1976 Cota 348 with Mk2 AMAL concentric) with the arrangement in another MK2 AMAL concentric but from my (1978) Sherpa T M198 and found that the designs was different. The later pilot circuit had different looking holes where the mixture entered the main venturi. I took a hunch that the later pilot circuit design was a better design and tried the body of the carby off the Bultaco with the Cota 348 jetting and presto, the 348 now runs very nicely thankyou.

keychange, I hope this story gives you hope for getting your 247 to run right. It is just a machine after all. Whatever is wrong is just a "thing" and "things" can be fixed.

[keychange]

It would be a little less depressing if I was sure it was the carbie - I have cleaned and replaced and refitted and cleaned the bloody thing a hundred times - at this stage I still don't know if I am making it better or worse and I have spent probably 100+ hours ( no kidding ) and lots of dollars blundering from one possibility to another. I may have fixed the problem 3 times and created new ones on the way ... I have learned a lot and yet I know nothing. I don't think I am one step closer to a solution..... clean the pilot jet again, who knows.

I don't have a collection of parts or neighbours to swap parts with ( Ross is 90 minutes away) my nearest spark plug source is 3 hours return trip so I buy them by the half dozen but current rate that's about 3 weeks supply (doesn't that tell us something).

In 4 months not a single trouble free ride - not even a single hour's trouble free ride. I have worked in IT for 30 years as a developer and systems manager - logic, I like - patience I lack.

I'll be riding Brownie's TY250 monoshock this Sunday again and the allure of simple first kick Jap reliability is becoming hard to resist.

[keychange]

Guess what - I have mentioned before that the "Mk1 Concentric 600 Series 2 Stroke Major Repair Kit" from Amal (Burlen Fuel) had some wrong parts and Ross remember you said my idle adjust screw sat out too far - when I checked it was longer than the original - well I replaced that with the original and I should have have thought right then to check the idle air screw ..... I did today and it too was wrong so I replaced that with the old (put a new o-ring on it) and I ran for 10 minutes up and down the big hill at the back of my house and it ran fine and cool - the crankcases were only just warm rather than what they have been. I think we I have it ... I'll take it for a serious run tomorrow and let you know. Fingers crossed

Andrew

[keychange]

This morning I got all dressed up and headed into the hills where we played last week. It's about 2 kms along the gravel road and I took it easy riding up the banks etc and then I came to the first gate . I pulled over it was idling ok and then it died and refused to restart. I pulled the plug out and it was dry and lean and it had spark It was mainly downhill back so I rolled it back and eventually half way home on 4th or 5th attempt it jump started and I brought it home. I walked over to Brownie's TY - it started first kick and off I went for a very enjoyable, trouble free ride through the hills and forests.

When I came home I walked over to Monty - I wanted to use the fuel in carbie - it started first kick

[David Lahey]

You need to work out what is wrong at the time that it stops and won't start.
If it is making spark at the time, then the reason is probably a lack of fuel in the float bowl.

Did you test for the presence of fuel in the float bowl at the time it stopped by tickling it?

It is possible to be have fuel flowing into the float bowl and the rate of flow may be enough to keep up with usage from the bowl when riding around at slow speed, but not be great enough to be able to keep up with usage when riding along a road. It is very hard to tell from your descriptions of what is happening to the bike to know if this is what is happening to your bike.

[keychange]

David

It is running fine at medium to high speed - although obviously lean and probably still overheating (didn't run long enough to really tell). I mean it runs really well under power then as it heats up it become halting and then spurts forward.

When I stopped to open the gate today it idled for maybe 20 seconds then died. Would be nothing unusual with many bikes except this is then bloody near impossible to restart - I had a spare new plug and that made no difference. It's as if it overheats to the point that the fuel vapourises before it can ignite. If it was starving for fuel surely that would show up under power - and it doesn't.

[David Lahey]

David

It is running fine at medium to high speed - although obviously lean and probably still overheating (didn't run long enough to really tell). I mean it runs really well under power then as it heats up it become halting and then spurts forward.

When I stopped to open the gate today it idled for maybe 20 seconds then died. Would be nothing unusual with many bikes except this is then bloody near impossible to restart - I had a spare new plug and that made no difference. It's as if it overheats to the point that the fuel vapourises before it can ignite. If it was starving for fuel surely that would show up under power - and it doesn't.

No - sorry - not a true assumption. When the engine is under power, it will continue to operate with a much lower fuel level in the bowl than it can when idling. Brownie made a relevant comment about this earlier from when he rode your bike.

If you want to prove your overheating carby idea, why not just feel how hot the carby is next time the bike stops and won't run?

[keychange]

No I don't think it's the carbie getting hot - but surely if the engine and inlet manifold are hot enough, fuel mix will struggle

[David Lahey]

Yes, if a motor gets hot enough for the fuel to start to burn/explode before the spark, it will run strangely.

The point I am trying to get across is that if your motor is getting hot, it may be because the fuel level is too low, causing a lean mixture, creating more heat than it would with the correct fuel-air mixture.

If you have checked the ignition timing, and checked that the motor turns freely, and the fins are getting airflow over them as you ride, and the fins are not excessively insulated with paint, and you are using suitable petrol with at least 3% oil, general overheating while riding along dirt roads is probably due to the fuel-air mixture being lean.

[keychange]

I removed the head and barrel and took them with the rings down to Greg Harding's Sunday gig and met Steve who is a mechanic- He immediately noticed the excessive ring end gap - about 2+mm . Does anyone know what the manufacturers specs on this are? BTW - the piston is still Std.

So a new set of rings are the go - I don't suppose there are any after market options still available.

Cheers