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Matt, Engineer
Category: Small Engine
Satisfied Customers: 20384
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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Replaced the carburator 56 hours ago on a onan marquis 5500

Customer Question

replaced the carburator 56 hours ago on a onan marquis 5500 and it has been running great but now the rpm's are flucuating and the carburator is cold and sweaty any ideas
JA: I'll do all I can to help. How long have you been dealing with this?
Customer: problem just started today ran great yesterday
JA: Do you have fuel in the tank? If so, is the shut-off valve open and the fuel clean?
Customer: its not old gas and there is fuel in tank
JA: Do you plan on doing the work yourself?
Customer: yes
JA: Anything else we should know to help you best?
Customer: no
Submitted: 1 month ago.
Category: Small Engine
Expert:  Matt replied 1 month ago.


This could be an airleak after the carb, any air dragged in here isn't 'seen' by the system and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

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