Carburettor synchronising – 5 Jul 2016.

Having got the ignition timing right it was now time to balance The Racing Rhino’s carburettors.

I took the bike for a short run to get the engine properly warm so it would run properly without choke. On my return I parked the bike with the back end poking out of the garage door. It looked like it would rain but it never came.


Before getting started, I checked there was some free play in the throttle cables and slackened off the throttle damper so the twist grip would snap shut. I use the damper so I can make hand signals as the Rhino has no indicators fitted. I find it also eases the load on my damaged wrist.

The adapters for the gauges were fitted in place of the blanking plugs.


The gauges were placed on a stool beside the bike. The bike shakes too much to read them when they are balanced on the bike seat and they would probably fall off anyway.


My garage heater was set to blow cold air and aimed at the engine block.


I have a pair of Davida vacuum gauges given to me by a friend. I’ve been told many times that dial type gauges are impossible to use as the needles swing backward and forward. Not so with these. They give steady readings. I think they have some sort of damping built in.

Idle speed.

I started up the bike and let it idle. What I am aiming for is for the same reading on each gauge but I’m not interested in what the reading actually is. At the same time I’m looking for the idle speed to settle at something sensible. To begin with the left hand gauge was showing a higher vacuum which means it’s not working as hard as the right side. This is adjusted by turning the idle speed screw which is the big one with the spring behind it.


This screw must not be turned without opening the throttle a little to raise the slide off the screw. It is pointed and acts as the throttle stop. You risk damaging the slide, screw or both. Blip the throttle after each adjustment then let it settle back to idle. What you are doing with this adjustment is setting the two throttle slides to the same effective opening. Here my two gauges give the same reading.


To ensure my gauges are working properly I stopped the engine and swapped them over and checked to see if I still got the same readings. I did. Engine stopped again, I swapped the gauges back so I didn’t confuse my self with which was which.

Mixture setting.

The mixture for each carb is adjusted using the lower partly recessed screws. Working on one carburettor at a time, I screw in the mixture screw until the engine begins to falter then back out to where smooth running begins again. Then I back the screw out a further ¼ turn to make sure the mixture is slightly rich. If the mixture is too lean the engine will run hot and damage can occur.

Once both mixture screws have been set, again with much blipping of the throttle, The idle speed needs to be checked and adjusted again. In my case this was because the idle speed had increased.

I let the engine rest for a bit while I made a cuppa. It had been running for a while on the centre stand by now and I thought it should be allowed to cool a little.

Throttle balance.

Now both carburettors are supplying fuel/air to the engine in the same amounts. They will also allow each cylinder to idle at the same rate. The third stage is to ensure that when the twist grip is turned both throttle slides begin to lift at exactly the same time.

Move the rubber covers from the throttle cable adjustment screws on the tops of the carbs ready. Now with the engine idling open the throttle a little bit. Not a great handful! both gauges should show a decrease in vacuum at the exact same time. If one gauge drops first it means that throttle slide is lifting first. This is adjusted by either loosening that carbs’ cable or tightening the one on the other side. The adjusters have lock-nuts so, using a pair of 8mm spanners release the lock-nut and wind the adjuster in or out.

In my case the right hand carb started to work first but I couldn’t slacken off its cable as there was no more adjustment available. I had to tighten the left one. Remember that it’s important that there is some slack in the throttle cables.

I took my time to get the adjustment right and was rewarded with the needles on the gauges moving together and by the same amount.

Possible reasons for the blue chrome.

One reason for all this tuning was to look at why just the right hand exhaust header has turned blue. In the past both headers have discoloured on my Guzzis by a similar amount. Chrome discolours because of heat. In my case I think this is due to a combination of factors.

  • The ignition was a little advanced on the right. This can make an engine run hot. The left may have been advanced as well but I couldn’t tell once I had adjusted the right hand contact breaker points timing.
  • At idle the right hand cylinder was set to run faster than the left.
  • The right hand side was working harder than the left as its carburettor  throttle slide was lifting first.

Just standing in the garage the bike does seem to sound crisper and appears to be running even better than before.

Something I didn’t want to find.

Unfortunately I can’t take it out for a test run. While tuning the carbs I noticed oil dripping from the slot beneath the flywheel where the engine casing and gearbox join.


I put a pan underneath to catch it as it was dripping quite heavily. I sniffed the oil. It’s from the gearbox. It’s Tuesday and I wanted to take the bike to the Llandovery Motorcycle Weekend on Saturday. It looks like I have my work cut out to get it fixed in time. I might end up going on The Fire Bike instead.


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