Since putting The Racing Rhino back on the road it has performed beautifully. It’s strong, pulling right through the rev range, ticking over properly and is just a joy to ride. However, almost immediately the right hand exhaust pipe turned yellow then blue. I’m not sure, but I think it’s continuing to slowly get worse.
Chrome turning blue is caused by it getting hot and there are various reasons why this might be. A weak fuel mixture is a common cause but, in this case, if anything the bike might be running slightly rich as it now has air filters where originally there were none. Carburettor jetting is still standard. Both spark plugs are the in same condition – a sort of brick red with the odd sooty spec on them. Ignition timing has been checked twice and the valves readjusted after re-torquing the cylinder head nuts. I have also successfully removed all the leaks from the exhaust system using silicone sealant.
Interestingly, my previous experience has been that both exhaust headers will go blue pretty quickly on these bikes. So the problem side could be the left, possibly colder one. Another suggestion has been that the quality of the chrome is different on each pipe and that is why they behave differently. The cylinders of the Guzzi engine are offset with the right hand one further forward. This means there is a shorter distance for the very hot exhaust gasses to travel to the bend in the exhaust header on that side, and so more blueness.
Just before writing this post I saw that Hans, another Guzzi WordPress blogger, is having the same issue with his newly restored 850GT here – I paste the link into Google translate to see what’s going on.
Anyway I decided that one last thing I could investigate further was the carburettor balance. Perhaps there is more load on one side than the other. I wanted to use my vacuum gauges but there were no vacuum take off points. I’ve done this on The Fire Bike which has had them added at some point by a previous owner. Later Guzzis have a drilled “boss” on the intakes for the purpose.
Here is the drilling added to the left hand intake manifold of The Fire Bike
and this is the same point sans drilling on The Racing Rhino.
I had considered making this modification when the bike was being rebuilt but “chickened out”. This time I plucked up the courage and reasoned that, if I made a total mess of it, I would buy a set of later inlets with the fittings already in place.
I disconnected the battery and removed the fuel tank so that I couldn’t damage it with my ham-fistedness. I took off the air filters and then set about removing the left-hand carburettor.
I undid the clamp that holds it on the manifold and managed to wiggle it free without removing the throttle cable or fuel line. The three allen bolts holding the inlet manifold to the cylinder head were undone and the manifold taken to the bench.
These are my vacuum take off adapters which have an M6x1.0 thread
I spent a lot of time measuring where the hole was drilled in the V7 loopframe manifold and copied that carefully although I’m not sure how critical this actually is. I was finally happy(ish) and marked the spot with a marker pen. You might be able to see in the photos that I had made several attempts at this!
I don’t have any expensive tooling but managed to get the manifold clamped in a machine vice, supported at what looked like the correct angle and positioned in my hand-cranked pillar drill. It’s old (90 years old?) but can be relied upon to drill a hole accurately. I drilled a small pilot hole first.
Cutting the M6 thread requires a 5mm hole so, after checking that the pilot hole was at the correct angle, the larger diameter was drilled.
The thread was then cut.
I carefully removed any swarf from the manifold as well as any sharp edges. I measured the thickness of the manifold casting through the hole as 5.7mm. The blanking screws fitted to the V7 loopframe manifolds are 10mm long. Fitted to my newly tapped hole it’s too long but, again, I’m not sure how much this matters.
I made a shorter blanking plug (about 7mm), added a fibre sealing washer then refitted the manifold and carburetor.
I repeated the job on the other manifold. Here the carb is off and hanging by the throttle cable.
Once this manifold had also been drilled, tapped and plugged everything was reassembled – air filters, fuel tank and battery ground cable.
I checked that the bike started and ran properly. I don’t think the plugs in the take off points leak at all. With the engine running, I sprayed them with “easy start” to see if the engine “picked up” and it didn’t.
I think the modification has been a success and hope to soon have a go at balancing the carburettors using my vacuum gauges. I did this on The Fire Bike a while ago with good results.