trapezeartist

It's a little tinkling noise from the engine under load, "pink, pink, pink,.......". It's the result of the fuel mixture igniting due to excessive heat in the cylinder before the spark plug sparks. As it happens while the piston is still going up it causes very high loads on the piston, small end, etc and a loss of power. The root causes are excessively high compression ratio, poor quality (low octane) fuel or a build-up of carbon in the combustion chamber.

With my riding skills, top end power is of negligible interest. My main concern is "driveability". I would like to think that my riding is getting better, so it can be easy to confuse improvements to the bike and improvements to the rider. Nevertheless a slow throttle, adjusting the slow-running mixture, adjusting the idle speed and (possibly) sorting out the breather pipes have been the things that have brought the improvements.

Well done! It was too tricky for me. But one bolt for shock, one for the CDI and two for the regulator just seemed easier than even more knuckle-skinning wiggling. And an opportunity to clean behind/under/around the said parts too.

Some people have suggested to me that I should use 127.5 main jet but 120 seems to be working fine for me. There's nothing tricky about the spring. The difficult bit is removing and refitting the carb on the engine. I wouldn't suggest this is definitive, but it goes something like this: Remove rear mudguard. Remove airbox (The jubilee clip may be difficult to access and there are two hidden rubber pipes under the main part.) Disconnect the top of the rear shock and lean it back. Remove CDI unit on right hand side. Remove the finned aluminium ignition box that lies on top of the gearbox. Remove the rubber connector to the carb. (Another tricky jubilee clip). Disconnect the fuel pipe and throttle cable. Undo the jubilee clip fixing the carb to the engine stub (this one may be easier). Twist, fiddle, wiggle and turn the carb until it comes out, probably on the left side. When you come to refit the carb, take care to make sure none of the breather pipes are going to touch the exhaust and try to point the jubilee clips in the direction that is easiest for access. I found it was easiest to leave all the jubilee clips loose until everything was together, then tighten them all. It helps with the alignment.

This is the table I found somewhere with the recommended settings: I followed the 300 competition settings and got a lot of popping in the exhaust. I've now wound the idle mixture out from 2 to 2.5 turns (it's actually a "fuel screw", not an "air screw") and that stopped it. The engine is obviously a lot happier as the idle speed went way up, and had to be adjusted down with the normal idle adjustment screw. As you can see there is no allowance for altitude. I would guess that these would work up to 1000m. Above that, you're on your own. Taking the carb off is a pig of a job, but once it's out the rest is easy. Remove the plastic top cover (2 screws) to get to the spring and diaphragm. Remove four screws on the bottom to drop off the float chamber and get access to the jets.

I've done that and it worked for me too.

I stayed with 120 main jet but took the air screw (which isn't an air screw, it's a fuel screw) out to 2.5 turns. That stopped the popping and pushed the idle speed up quite a lot, so I adjusted the idle back down on the normal idle adjuster. It ran well today.

I copied thall1's pipe layout and it worked well. Absolute swine to find a place to route everything though, without cooking something on the exhaust.

Having returned to trials in the last year, and starting off by observing to get my feet back under the table, I have been somewhat bewildered by the lax interpretation of stopping. Most clubs have suggested to me that up to 3 seconds stop is OK, but then told me it's up to me. So I could apply a strict interpretation if I want to. Some say that's OK as it's equal for everyone. But it's a bit harsh on the rider who hesitates in my section (5) while someone else who hesitates in another section get's away with it. I would prefer a strict interpretation as that is easier to observe fairly.

Gaerne claim to be waterproof. In my limited experience so far, that appears to be true. Last time out they were a little damp but falling off in a stream may have been the cause of that.

That's very helpful. Probably explains why mine dribbles petrol, and leaves a persistent smell in the garage and car . I'll try to get those changed before putting the carb back on.

The carb on the Beta 4T seems to sprout tubes in all directions. Does anyone know what they do, what they should be connected to, and how long they should be? The one marked in yellow is fuel-in. That's the easy bit. The one marked blue is a large diameter tube that communicates with the underside of the diaphragm. In my case there is a plastic cap on it, but is that right? The green one has a short 2-3 inch tube that points downward. Possibly the float chamber breather? The red one on the left has a very short tube. The red one on the right has a long tube which is blocked off, so seems pretty pointless. They look as if they are possibly intended to connect together in a loop from left to right, but if so, why?

That's interesting, 2T4T. I was similarly thinking of making an infill for the footwell. Now you've confirmed it works, I'll definitely do that in the next week or two. I hadn't thought of compressing the forks to aid loading and unloading. I'll try it next time. Thanks.

I'm guessing you won't have too big a problem with height. I carry my bike in a Citroen Berlingo car and it just squeezes in on height. If I lean the bike or turn the steering the handlebars did into the roof lining. Like-for-like a van with no headlining is going to give you a couple of inches extra headroom. In the Berlingo, the front passenger seat has to be pushed right forward do the front wheel can go down into the rear seat footwell. In a van with a fixed bulkhead you wouldn't be able to do that. Someone told me you can get two bikes into a Berlingo but I can't see how. Of course, all my thoughts are specific to the Berlingo, but you could probably get comparative dimensions for the two vehicles in order to see how relevant my comments are to your situation.

It won't stay white for long.

OK, you guys are convincing me. I'll get the carb out again (PITA) and wind it out another half-turn. See if that does it. If it does richen it, it might run a bit cooler as well. PS I run the fuel tap on the solenoid setting and only open it manually when I need to top up the carb before starting. I am slightly suspicious that it leaks a bit as the garage continually smells of petrol, but I need to do some more investigation there.

I'm afraid I do have to make a judgement. Riding illegal bikes on the road without tax, MOT or insurance is, well, illegal. It won't just bite the riders on the backside: it could knock on to the whole sport. It's "bringing the sport into disrepute". A note of common sense. Thank you. Nevertheless I still think it would help if there was some "marketing" of the sport. Kick Start and the televising of World Championship and Indoor trials did (does) that to some extent but it perhaps puts off as many people as it encourages. Some people will look at the sections and think that it is far too dangerous and way beyond their potential capabilities (probably rightly). Then they won't look any further and see what fun can be had wobbling up and down banks and around trees.

Without the AWD requirement, you have a huge choice. Insisting on AWD impacts in two ways: 1. Far fewer models to choose from. 2. The rear platform is likely to be higher in comparison to FWD, so it's a greater height to lift the bike and less headroom inside. If you're thinking of carrying a trials bike in a "good" car, you should also consider the dirt that gets carried into it and possibly the damage to the interior trim. (I carry my bike in a Citroen Berlingo and it is marginal on headroom. Hence I have put quite a lot of marks in the headlining with the handlebars. It's also got very muddy inside, but I'm not too concerned as it is only a knock-about car.)

According to this it's an air screw. And it was my understanding that this little table originates from Beta, but I may be wrong.

Unfortunately they didn't. I just googled it (because I'd never heard of it) and it is pig ugly.

Out! More air? Leaner?

Best to tell us where "local" is then.....

What did you do to get rid of the "dreaded backfire"?

Do you mean you couldn't shift the pipe, or couldn't shift the springs? To help remove the springs you can by a T-handle with a hook on the end. It's much better (and ultimately less painful) than struggling with a screwdriver or other bodge-tool. If the problem is removing the pipe, I'd suggest remove the springs and remove the silencer. Then the only thing holding the pipe to the cylinder head is the stiction, baked mud, exhaust sealant, or whatever. A good wiggling and thumping should get it clear.