Herald 250 with Carburettor Problems...
The first thing a good mechanic does when faced with a running problem with the machine, is diagnose the problem.
Anna had come to the conclusion that the carburettor needed cleaning due to the fact that the bike stuttered to a halt if the choke wasn't out for at least four miles of riding. Towards the end of the four miles, the engine was running fast and the choke could be pushed back in.
So the first thing that Graham did was look for holes or splits in the tubing surrounding the carburettor to see if air was entering the system.
Graham started to take the bike apart, looking carefully at the tubing that ran from the engine to the carburettor. This is part of the engine’s emission reduction system. Instead of venting crankcase pressure into the atmosphere, gases escaping from the crankcase are vented into the air filter box when high levels of vacuum open a diaphragm valve. The first thing to notice was the air intake in front of the engine which creates a vacuum when the throttle is closed. Any air in the system at the point of closing the throttle, is funnelled down the tubing from the air intake and directed into the air filter.
We examined the rubber hosing that directed the air into the air filter, and it was intact, with no corrosion.
A further consideration was that if the rubber diaphragm in the emission control valve was damaged or corroded, then air would be entering the system. So instead of air being directed through the air filter, it could be directed into the carburettor causing too much air to be going into the system at this point.
In order to test the theory as to whether air is entering the system, it is useful to blow into the pipe to see if the diaphragm is leaking. We did this and it was impossible to force air through it - so the diagnosis was that this system was functioning as it should.
At this stage, we took the whole carburettor off the bike in order to examine it further.
We looked at the air vent tube at the base of the carburettor which allows air to go into the float chamber which will let the fuel level go up and down without creating a vacuum above it. As well as the vent there is a further opening in the float bowl. It has a pipe attached to it to allow any excess fuel in the chamber to drain from it onto the road.
The 2016 Herald Rambler has a Constant Vacuum Carburettor. There is a slide (or vacuum piston). When the bike is idling (the throttle closed), the fuel is sucked out of the fuel bowel via the pilot jet and is mixed in with the air before going into the cylinder. The whole process is powered by the low pressure the piston creates inside the cylinder. When we use the throttle, the butterfly valve inside the carburettor opens up and the low pressure effects the top of the vacuum chamber. The diaphragm is suctioned up, and with it, the slide and the jet needle are raised. As the jet needle is raised, its tapered shape allows more and more fuel through the main jet, more air is allowed through the carburettor, proportionally to the quantity of fuel. Pulling the choke lever adds an extra quantity of fuel into the chamber, enriching the mix and allowing an easier start in a cold engine.
Old fashioned carburettors had the slide fixed to the throttle cable. The problem with that is that when you open the throttle wide, the vacuum drops and ceases to draw fuel out of the jet, and the engine stalls! With a constant vacuum carburettor, as the engine speeds up, the flow of air is constantly balanced with the flow of fuel.
We noted that there was quite a bit of muck around the pilot air jet. This is a small brass jet near the carburettor opening furthest from the engine. Its purpose is to control the flow of air mixing with the fuel as it passes through the idle (or pilot) jet.
We had a look inside the carburettor. We checked the rubber diaphragm to see if it was completely intact. It was
We took out the emulsifying or needle jet. The small holes in the jet are there to bleed air into the fuel. Quite often, these holes get blocked up, making the mixture richer.
We checked the pilot jet by squirting carb cleaner through it. This was fine.
We checked the other jets (Main jet, needle jet, choke jet), in this way too.
And then it was time to put the carburettor back together!
By using the new work bench space in the workshop, which has been moved to a mezzanine level with excellent natural light, we had put all screws and washers onto a white sheeted surface in an order, so that we knew exactly how many screws and washers needed to go back in and in what order.
This done, we moved back to the bike, which was still on the bike lift and began the process of reattaching the carburettor.
It is important to ensure that there is just enough 'play' between the throttle cable and the carburettor; too tight, and the throttle opens when you move the handle bars!
Off the ramp now and outside for the tuning!
Don's BMW K100RS gets a thorough going over.
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Don bought his BMWK100RS back into the workshop. He had noticed an oil leak. Don, Guy, Dave and I worked on the bike. Don had already removed the lower portion of the fairing but the left hand side of the fairing also had to come off to gain access to the cylinder head which is on that side of the bike. Once the left side of the fairing was off. Don and Co removed the rocker cover. This gave us the opportunity not only to renew the rubber rocker cover seal but also to do some service work. With the spark plugs removed it was possible to turn the engine over so the valve clearances could be checked. It is a 16 valve engine. The majority of the valves were within tolerance, with just three valves running slightly loose. As BMW have chosen to put the valve adjusting shims under the cam followers (AKA Cam buckets) it would be more trouble than it was worth to remove both camshafts to try swapping shims. If the valve clearances had been tight it would have been worth it. This is because lack of clearance can cause the valve to stop closing properly when it gets hot and for hot exhaust gases to burn away the valve seats as they rush through the gap.
New spark plugs were fitted and the tightness of the cylinder head bolts checked before Don and Co. put the bike back together. Don also drained the engine and would have changed the engine oil filter but unfortunately didn't have the correct filter with him. The oil filter itself is a canister filter with a convenient hexagon on the end for a socket. It fits into a circular opening in the sump which is protected from the elements by a round aluminium plate secured with three bolts. When we took the plate off, oil poured out from the gap around the canister, making me think the oil filter had not been secured tightly enough.
Piaggio Velofax - A little progress made...
After a great deal of online searching, Graham eventually found VE UK selling a throttle slide for the bike's Dellorto SHA 14 12 C Carburettor and was pleased to find that it fitted correctly. Jess assisted by fitting the top end of the throttle cable into the twist grip. The bike is fitted with a pump fed two lubrication system. The throttle cable divides into two near the engine. One cable goes to the carb to control the throttle. The other cable goes to the oil pump to control the amount of two stroke oil pumped into the engine, depending on the throttle opening. Unfortunately the oil pump cable on our bike appears to be seized and the screws that would give access to the pump are not only very hard to get at, but also likely to be heavily corroded. So a decision has been made to run the bike with a petroil mixture. In other words mixing a small but carefully calculated amount of two stroke oil with the petrol. There is a danger with this because the oil in the fuel decreases the amount of fuel in the fuel air mixture. On a high performance engine this weakening of the mixture could result in poor performance and even engine damage. On our humble Velofax I suspect it will not make a jot of difference.
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| Our humble Velofax before the engine was replaced in the frame. |
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