Hitchcocks Forum

I have a copper pre Monoblock carb that is fitted on a 1938 sidevalve motor that I have just got running again, got it started today but I'm having problems with the carb. History on this motor was that it got heavily burnt in a shed fire, and luckily the carby being copper didn't melt, tappet door was completely melted off, magneto melted and the timing cover had started to melt.For sentimental reasons the owner wanted it repaired, it isn't in a bike, it is just the motor, so anyway I now have it sort of running but the carby is giving me problems because I can only get it running with my thumb in the throat of the carby. Any thoughts as to where to go with this carb and no throwing it away is not an option

Norm would taking a look at this help:-

http://www.hitchcocksmotorcycles.com/hi ... remonobloc

As the history of this engine and carb combination is uncertain I think I'd start tuning from first principles. These bronze (not copper,) carbs are quite tough so internal wear shouldn't be too bad but pretty much everything else will be worn out. As usual more info would help, size of engine, carb bore, current settings of jets, slide etc.

REgards, Mark

Hi Norm, perhaps a small internal passageway in the carb has been distorted in the heat - I assume you have probably dismantled and checked/cleaned all the accessible components? A couple of weeks ago, I had a BSA Starfire here, which had not been run in nearly 10 years, different carb, I know, but that would only run with the tickler button pressed and overflowing or with the intake almost completely blocked off with my fingers. I had cleaned all the jets etc, but still no good. What fixed it was to run it up to about 4000 rpm with the intake partially blocked, as above, then whack the throttle wide open and put the palm of my hand right over the intake. Gradually things improved and after about a dozen repetitions, it was running fine. Get the engine revving, smother the intake and petrol gets sucked through all those little internal passageways and cleans them out. It might work for you. Cheers, Paul.

Hi Mark and Paul, the motor is a 350 sidevalve, I stripped it down, the crank pin had stain marks in between where the rollers were sitting so it had been very hot inside, oil pumps were just full of ash, no sign of any oil in there. I should have thought the carby was bronze, I was so surprised after it was hydroblasted that it just looked like copper with brass fitting and bits. The motor had been running with this carby setup before the fire. Paul we were starting it with a vee belt off a wood chipper so it was a bit difficult, I'm chasing a gearbox on ebay so that I can bolt it up on a frame so I can try kickstarting it. I'll give the carby another clean and see how I get on. Thanks Norm

hi norm,



seems i missed your welcome party, anyways welcome back;-)

Brass, and bronze, comes in different colours depending on the alloy. I would have thought that copper would be too soft for a carburettor body.

Norm, if the fire was so intense that it melted various engine parts then although it did not destroy the body of this ‘brass’ carb it might well have distorted the body and/or internal fittings. Experience shows that carbs of this age frequently give problems that can be very difficult to fix with new individual parts – but it’s always worth a try. ‘Thumb on the throat of the carb’ suggests weak fuel supply – so the focus perhaps should be on float, float needle, jets, internal fuel passages, and needle – any of those could well have been affected by the intense heat this carb has endured. Paul’s idea of letting petrol clean the carb could also be done more easily perhaps by using a solvent aerosol carb cleaner wherever in the carb you can spray it. Let us know how you get on.

I'm puzzled, what was a Pre monobloc called, before the Monobloc came along?

Well,275, 276 and 289, obviously!! [sorry couldn't resist] Cheers, Paul.