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Carburettor icing experience with XV535

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Carburettor icing experience with XV535

PostAuthor: Scorchio » Mon Apr 16, 2018 8:09 am


I’m just putting up a note to share my experience and solutions with carb icing which the Virago XV 535 carburettors are prone to in the UK climate and why Yamaha equipped them with automatically switched-on carb heaters. I have recently modified my heater system to make it manual and have explained why.

1. Symptoms of carb icing

• A cylinder stops firing when opening the throttle a little after a deceleration e.g. coming out of a slow corner, slowing into a 30mph zone etc.
• On an extended run the 2nd cylinder might stops firing too.
• You open the throttle a lot, say half, then the engine pulls strongly.
• You pull in to the road-side, the engine doesn’t stop both cylinders continue to happily idle. Within a minute the engine is happy to pull away and resume the journey as normal. However the problem may return sooner or later.

2. Why does this happen?

• When air conditions are high Absolute Humidity and low ambient air temperature.
• Absolute Humidity refers to how much water vapour the air contains and not Relative Humidity which is how close the air is to dumping either as mist, dew or rain. Relative Humidity (RH) is what you find on weather forecasts.
• The highest risk of icing is near the coast on a Summer evening when the day warming has evaporated water into the air which then cools in the evening leaving the moisture ready to condense (dew). In the UK no-one is very far from the coast.
• Carburettors can reduce the air temperature passing through the throttle slide / butterfly by 12°C to 15°C. This is worst when reducing the throttle during deceleration causes the strongest drop in air pressure in the carb.
• This leads to moisture instantly condensing and freezing inside the carb.
• UK being a small land mass surrounded by sea, increases the risk of carb icing more than most other countries. UK pilots of piston engine aircraft equipped with carburettors are acutely aware of this risk and they are equipped with carb or air intake heaters to deal with it.

3. Why is the Virago XV535 prone to carburettor icing which disrupts engine running?

• The XV535’s BDS34 carburettor has three small Transition jets in the inner wall which makes the carb so good at smooth pick-up when opening the throttle and it is these jets which supply the fuel during low speed cruise.
• The Transition jets are in the region of strongest pressure drop and hence icing.
• Further downstream is the Idle jet and upstream is the Needle jet and both of these are not affected.

4. About the XV535’s automatic carburettor heaters

• Yamaha recognised the icing risk and fitted UK models with heaters which warm the carburettor body near the transition jets.
• They fitted a controller which switched the heaters on if the ambient air dropped below 12°C and off again when rose to over 17°C.
• For UK bikes it meant the heaters were on nearly all the time the bike was used.
• Assuming the alternator was happy to supply this along with all the lights etc then all is OK.
• The heater units have a life limit and all units originally fitted to the bike will have burnt out by now and either have been replaced or left.
• You can check if they are healthy or dead. Disconnect the wire from the centre post and measure resistance from post to carb body. Haynes say it should read 5 or 6 ohms. 5 ohms is a 30w unit and by the way 7 ohms is a 20w unit.
• I have not been able to find a part number and wattage spec for the XV535 heater replacement. But I found a heater part number for every other Virago which is 5FU-83790-20-00 at £26 . This is for a 30w unit and has the same fitting.

5. Why have I fitted a manual switching system?

• With original automatic heating system the heaters are on virtually all the time in the UK and will likely fail sooner rather than later. So someday the engine will falter but as long as you recognise the symptoms, then once you nurse the bike home it’s simply a case of electrically testing the heater to confirm a fail then ordering and fitting fresh units.
• I do a lot of short journeys, slow running on single track roads and with lights on. I also do a lot of winter riding and although the temperature is colder, the absolute humidity is low too and hence much less moisture available for icing.
• The bike was without working heaters for years and only occasionally had the icing caused an issue, but when it did it was in the middle of no-where, cold and wet and recurring along the journey.
• I decided to change to manual control because I just wanted the heaters on when there is an icing event which is rare. As the heaters are rarely used I can expect they’ll last “forever” and therefore “always” be available for when I need them.
• I also have peace of mind that the load is significantly reduced on the charging system and battery particularly in poor charging low speed conditions when the full lights are on.

6. How did I change the Heater Control System

• I fitted a rocker switch with tell-tail red LED underneath the back of the right-hand pod. Like this but red LED - ... tSQyZch0b4 .
• I took out the original heater controller i.e Thermo-switch in the Haynes manual.
• I connected the thermos-switch’s +ve supply wire to the new switch.
• I disconnected the original wires from the heater centre-posts then taped and sealed them back along the wiring harness out of harm’s way.
• I ran two short wires from the switch to the new heater centre posts.
• The original heater earth wires I left connected as original.
• Currently I am running KTM heaters units which are just 20 watts to see if they are sufficient . They are a lot cheaper (£14) than the Yamaha 30w units (£26) and although icing is pretty much a non-event now, I have had the odd hiccup in particularly arduous conditions after a long deceleration coming into a 30mph zone but the motor cleared its throat and carried on happily.

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Re: Carburettor icing experience with XV535

PostAuthor: sbrbot » Fri Apr 20, 2018 12:26 am

Thanks for that post. It's good to know that original Yamaha's carb heater can be substituted with KTM's carb heater.

Yamaha's code for heater is 83790. You just have to run search with that code and word "Yamaha" on any part's site (like MegaZip, Boats, Partzilla) and all Yamaha's heater models will be shown:

3DM-83790-00-00 (20W)
4TR-83790-00-00 (15W)
5FU-83790-10-00 (20W)
5FU-83790-20-00 (30W)

The strange thing is that none of them is not dedicated for xv535 but probably most of them fit to xv535.
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