I have a Tipsy Nipper with a VW1834 engine and Zenith Stromberg CD150 carburettor. I've been having some problems with the engine lately as follows:
1. Sudden application of throttle causes rough running and power loss (didn't used to).
2. Engine rpm (any setting) increases with application of carb heat.
3. Engine idles well but cuts out if the throttle is pulled back after a high speed taxi run.
4. Engine runs well but rpm increase causes intermittent rough running (better with carb heat on).
These faults are recent. I've replaced the inlet manifold gaskets (thought there might be an air leak) and the carburettor diaphragm but the problem remains.
I'd welcome any suggestions from those more knowledgeable than myself (that probably means pretty much everybody).
Thanks
VW 1834 RPM increase with Carb Heat on.
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John Austin
- Posts: 11
- Joined: Sat May 29, 2010 2:10 pm
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Noel Howard
- Posts: 34
- Joined: Tue Jan 08, 2008 11:06 pm
Hello John,
I suggest that the fuel/air mixture is a bit weak - ie too much air. When you apply carb heat, you reduce the air density and so reduce the amount (mass) of air entering the carburetter therefore increasing the petrol to air ratio. This gives you a richer mixture which is the reason for the increase in rpm. All the other symptoms you describe are conistent with lean mixture. Try applying a bit of choke when on the ground with the engine fully warmed up and if you get an increase in rpm, then that confirms that the mixture is too lean. It is also worth checking that there is sufficient oil in the dashpot on top of the carb.
Best of luck, Noel.
I suggest that the fuel/air mixture is a bit weak - ie too much air. When you apply carb heat, you reduce the air density and so reduce the amount (mass) of air entering the carburetter therefore increasing the petrol to air ratio. This gives you a richer mixture which is the reason for the increase in rpm. All the other symptoms you describe are conistent with lean mixture. Try applying a bit of choke when on the ground with the engine fully warmed up and if you get an increase in rpm, then that confirms that the mixture is too lean. It is also worth checking that there is sufficient oil in the dashpot on top of the carb.
Best of luck, Noel.
I think that you check the rubber diaphragm in the top of the carb body again, a split will give this as it allows the diaphragm to lift the slide when a restriction is put on the outside air inlet, heat valve shut. These carbs are very reliable and the number one fault was the rubber perishing, these are all that opens the slide as no mechanical link exists. vauxhall viva etc., cars did it regularly.
only takes four screws to replace but make sure the new one is fitted with the little locator in the correct place and that the slide screws are not overtightened or they cut through the new diaphragm. also check the damper pot for leaks and oil.
only takes four screws to replace but make sure the new one is fitted with the little locator in the correct place and that the slide screws are not overtightened or they cut through the new diaphragm. also check the damper pot for leaks and oil.
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Nigel Ramsay
- Posts: 298
- Joined: Mon Feb 25, 2008 5:13 pm
- Location: Middle Earth
Grabbed this from the Triumph site:
"To set the idle speed the air cleaner and damper must be removed and the air valve must be held down on the bridge in the throttle bore. Using a coin the jet adjuster should be screwed up until the jet is felt to contact the underside of the air valve, at which point it should then be turned down again three full turns. By doing this you can establish a working position for the jet."
"you only hold it down when screwing the jet up to meet the piston, this just get the jet level with the bridge,
you could take the air piston out and set the top of the jet level as an alternative, do by sight, feel, or a straight edge across the top of the jet .
that sets the mixture not the idle speed,
for initial idle speed unscrew the idle stop screw, close the throttle plate fully, wind in screw to just contact its stop and turn further 2 turns, once its has fired up you can alter this to give approx 650/700 rpm idle speed.
when she's up to temperature , screw the jet in or out 1/4 turn at a time to get the best running and slow the idle stop back down to keep the rpm sensible for idling. three turns down will undoubtedly be too rich but it will make it start ok, I tend to go for 2 turns"
A couple of other points: Re-checking for any damage to the diaphragm is essential, the tiniest pin hole will do it. I had a lot to do with these carbs in the 70's and it wasn't unknown for there to be a pinhole in a brand new diaphragm, often caused by a local extra thin area of rubber from the moulding process.
On mixture v revs, in-fact, a weak mixture will lead to a rise in revs, which will rapidly lead to a cough or dead cut when load is applied (more throttle). A rich mixture generally makes the engine sound fluffy and will equally cough under load!
Plenty to be getting on with
good luck!
"To set the idle speed the air cleaner and damper must be removed and the air valve must be held down on the bridge in the throttle bore. Using a coin the jet adjuster should be screwed up until the jet is felt to contact the underside of the air valve, at which point it should then be turned down again three full turns. By doing this you can establish a working position for the jet."
"you only hold it down when screwing the jet up to meet the piston, this just get the jet level with the bridge,
you could take the air piston out and set the top of the jet level as an alternative, do by sight, feel, or a straight edge across the top of the jet .
that sets the mixture not the idle speed,
for initial idle speed unscrew the idle stop screw, close the throttle plate fully, wind in screw to just contact its stop and turn further 2 turns, once its has fired up you can alter this to give approx 650/700 rpm idle speed.
when she's up to temperature , screw the jet in or out 1/4 turn at a time to get the best running and slow the idle stop back down to keep the rpm sensible for idling. three turns down will undoubtedly be too rich but it will make it start ok, I tend to go for 2 turns"
A couple of other points: Re-checking for any damage to the diaphragm is essential, the tiniest pin hole will do it. I had a lot to do with these carbs in the 70's and it wasn't unknown for there to be a pinhole in a brand new diaphragm, often caused by a local extra thin area of rubber from the moulding process.
On mixture v revs, in-fact, a weak mixture will lead to a rise in revs, which will rapidly lead to a cough or dead cut when load is applied (more throttle). A rich mixture generally makes the engine sound fluffy and will equally cough under load!
Plenty to be getting on with
good luck!I have a little special tool that goes into the socket in the bottom of the carb body to adjust the mixture, but only on the early models, the later one were cap sealed. there is sometimes a little slide lifter on the bottom side of the body that lifts the slide a tiny bit, to check the initial lift as mentioned before on the previous reply. It just makes it easier than fiddling around it should not stagger when lifted.