1988 Moto Guzzi Le Mans Mk V
Q:
I have owned this bike for three years and renewed or overhauled: bearings, oil seals, timing chain, tensioner, clutch, forks, carbs, brakes, swingarm, final-drive, battery etc. The bores, valves, pistons, rings and compression where good so the heads were given a good clean, tested and left at that. She’s not cosmetically perfect but a joy to ride. I have recently changed to a Mistral Race exhaust and here is the nub of the problem; she starts on the button with a dab of choke all weathers, performance is good, acceleration and response excellent, she blows no smoke (oil of fuel) and burns no appreciable amount of oil. The plugs are a bit sooty but I would rather run a bit on the rich side. Recently a crusty deposit has started to build up on the plugs between the electrode and insulator which does not look like any of the chart photos. The nearest looks like ash build-up from additives. The plugs are not carbon fouled or oiled up and the deposit is a crusty brown to black not ashy or white. (I’ve just cleaned them but forgot to photo before) I’m wondering if it is burning a bit of oil but before I strip the top-ends and check rings I wondered if you can shed any light on additives and modern fuels possibly being the culprit or wisdom on jetting. Some say you can run most old Guzzis on unleaded but I have been using super unleaded with a lead replacement or combined lead replacement and octane booster. The current set-up is: Mistral 40mm straight through reverse cone megas, 150 Main Jet: up from 145 standard, 65 Idle Jet: up from 57 standard, Needle: K19 on third notch down, plugs: BP7ES, K&N Filters. Charles Cooper
A:
Your plugs are those recommended by NGK but you could try using the iridium tipped version which is the BPR7EIX. However, I think the key here is that all was okay until you changed the exhaust so the thought must be that there is something wrong with your current set-up. I am assuming you are running the new Mistral exhaust without the removable baffles fitted, in which case the recommended carburation changes are merely to increase the main jet size by about 10%. So, in other words, following this you should put back the 57 pilot jets, though there is also the change of air-filter to contend with. The other difficulty with the main jets is that what is not specified is whether that is a 10% increase in diameter or area over the standard 145. If it is diameter: that would equate to a 160, but if it is area that would make it a 152 (if there were one: your 150 would be about right). I think there are a couple of things you could try: fit the baffles as then less change is needed to the jets and see whether you still have the problem. Alternatively, as you could try to put the jets up to 160. You can get hold of all sizes of jets (among lots of other Moto Guzzi related parts) from Gutsibits on 01484 841395.