I have rebuilt a V50 Monza. It is very reluctant to start. Compressions are good, carbs are rebuilt, there is fuel in the float chambers and the float heights are correct. It has electronic ignition (Piranha) which is giving a good fat spark and the static timing is correct. Plugs are new, valve clearances correct. I have removed the airbox and fitted K&N type cone air filters. All I am getting is an occasional backfire in the exhaust. If I take off the air filters and give a squirt of carb cleaner into the carbs, it will fire up immediately and run for a few seconds.
All suggestions welcome
Is the fuel getting through, check the pipes from tank to carbs and the breather hole in the fuel cap?
As above. It sounds very much like the fuel is not getting through. Pull the fuel pipes of the carbs and run some off into a jam jar, this will check it is flowing freely, then check the fuel in case it has water in the bottom. That is never good for running an engine on. Look for bits in the fuel to see if you have rust in the fuel tank.
If all the rest look good, then it will be a strip and clean of the carbs, particularly the idle circuit as that is what you will be using on start up.
I assume you are using the chokes. This is an enrichening device that opens a fuel way direct from the float chamber.
The air screws should be about 2 turns out as starting point. These are the smaller of the 2 adjustment screws on the side of the carb. The other (larger) one is the tick over adjustment.
I’m not an expert but I’ve heard that if you replace the standard air filter box with K&Ns/bellmouths, then the carb needle setting position should be different? I’ll defer to Don-Spada and Chris 950s on that one though!
It will probably need slight upgrade of the main jet, but should still start up fine with K&N’s fitted. 10% increase is the general recommendation.
It might just run a bit weak at full throttle openings.
Just another angle - the electronic ignition. You say that the static timing is OK and you have a good spark. You may want to take a look at my "Targa starting problems and burnt coil’ thread further down on the small block page on the forum, which describes my electronic ignition woes with my Targa. I sometimes had a good spark with it but it was the pesky electronic ignition that was the problem, in that it degraded over time but there was no way to test it and I went through all the likely fuel and other ignition issues before replacing the electronic ignition. My Monza has points - at least you can physically see what’s going on with them even if they are somewhat fiddly! I presume that the Piranha e.i. is an after market item, as I think that the Monza was based on the V50 Mk III which had moved over to double contact breaker ignition. Not sure whether any Monzas were based on the V50 Mk II which had the electronic ignition - does anyone else know?
No there were no Monza Mk 1 with electronic igntion. I have replaced the points on all the small blocks I have owned, as I found I have ended up having to slightly misadjust the gap to get the timing OK. I have fitted, Dyna, Lucas Rita and Newtronic/Piranha on various bikes and once setup they have been faultless.
Yesterday I stripped both carbs again and cleaned through everything minutely. Gave it a fresh tank of fuel and she fired up. I can only assume there must have been a tiny amount of residue from the soda blasting left somewhere in the carbs.
Glad you managed to resolve the problem, it did sound like a fuel block somewhere
Good to hear you have it solved and it didn’t live up to the rule that 90% of carb problems are actually ignition faults
Good news - I hope you get some decent weather to enjoy it. I absolutely love my Monza - the best bike I’ve ever had. Light, responsive, fast enough for most mortals and looks and sounds fab! A seriously underestimated motorcycle.