simon metcalfe Posted June 11, 2008 Share Posted June 11, 2008 1700 X/Flow running on 40 Webers with Luminition ignition. A strange fuel mixture "leak" or spitting has started at 4000 rpm or above esp when cruising on motorways etc. It's not the usual "occasional" audible spitback but a film of fuel mixture (not just petrol) which I think is coming from the front carb but it's very difficult to tell - curiously the engine runs fine - no misfire or problems starting etc - but rather annoying and the whole right side of the engine bay is now covered having driven some 500 miles back fm Germany..Any ideas please? I have changed the coil and fitted new sparking plugs. Link to comment Share on other sites More sharing options...
alextangent Posted June 11, 2008 Share Posted June 11, 2008 Could be a leak around the venturi. Mine did this; Hylomar carefully applied to the venturi fixed it. They're held in with a grub screw. You'll need to take the trumpet out first. Don't leave it much longer; you don't want a carb fire 🙆🏻 Alex McDonald Loud, louder, loudest... Link to comment Share on other sites More sharing options...
millsn Posted June 11, 2008 Share Posted June 11, 2008 TADTS Run at a diffeerent rpm. that's why the engine sounds like thwacker thwacker. iT's the inductiobn charge bouncing in and out of the carbs Link to comment Share on other sites More sharing options...
Pezky Posted June 11, 2008 Share Posted June 11, 2008 check the carb balance, mine does this when it needs adjusting. Link to comment Share on other sites More sharing options...
alextangent Posted June 11, 2008 Share Posted June 11, 2008 One other thing to look for is a brownish film of liquid forming on the filter backplates at the bottom. It's the blowback of fuel that gets sent between the venturi and the carb body, rather than down the venturi tube and into the manifold. The lighter hydrocarbons vapourise off (if you're lucky), leaving a sticky oily brown fluid. Lots of it indicates a problem that needs fixed; 99.9% plus of the fuel should end up in the cylinder Takes 10 minutes to check Alex McDonald Loud, louder, loudest... Link to comment Share on other sites More sharing options...
simon metcalfe Posted June 17, 2008 Author Share Posted June 17, 2008 hello Alex and many thanks for this suggestion - you're quite right re the build up of unburnt deposits on the backplate - quite thick & oily if this is what you mean? Please could you explain exactly what I need to do re the hylomar - is it to go between the venturi and the trumpet - is this where the leak is happening? (I've noticed some of the trumpets are looser than others) Also do I have to remove the venturis do apply the hylomar - is this easy? And finally, I'm a bit concerned that the hylomar may somehow get into the inner workings of the carb and block it - is there any danger of this? Many thanks Simon. Link to comment Share on other sites More sharing options...
alextangent Posted June 18, 2008 Share Posted June 18, 2008 See the diagram here Remove the trumpets by undoing the little nuts and tabs. Watch out you don't lose the nut, washer or tab that holds them in. Fiddly little b#ggers. The venturi (17) is made of gunmetal and it's very soft indeed. On 40s there's a grubscrew and nut that holds them in; situated at the lower side of the carb body (it's part number 24 and 28). Once removed you can take out the venturi. The venturi has a slotted hole/flattened pipe that matches up with a hole in the carb body; it's that bit that leaks. It's opposite the grub screw side. Use as small amount of hylomar as you can around the venturi where it seats against the carb body (the exterior shiny bits and the stepped ridges), and slot it back in. The idea is to not block the hole, but to stop fuel passing the venturi where it gets sucked out the front. They're a pretty lose fit, so you should be able to get it in without smearing hylomar everywhere. Fiddly again, and use as little hylomar as you can. Less is more. When tightening the grub screw, don't over do it. You can drive it through the venturi. I cleaned up the indent on mine with a needle file to get a better seat. To stop the trumpets rattling, use a little hylomar around the step of the trumpet, Again, less is better. Try turning the tabs over as they're not flat and you may get a tighter fit. Then add a drop of locktite to the screw body before putting the tabs and nuts back on; they have a nasty habit of unwinding and getting sucked into the cylinder. Properly done, and with the right (minimal) amount of hylomar, you shouldn't have any problems. It resists petrol, and as long as gobbets of it don't end up in the carb, it will do a good job without dissolving or ending up down the cylinder. Took me about 2 hours with the carbs in situ (and clams -- jeez, do they get in the way or what ). Alex McDonald Loud, louder, loudest... Link to comment Share on other sites More sharing options...
simon metcalfe Posted June 18, 2008 Author Share Posted June 18, 2008 Alex, this is a big help - thanks very much - i'll have a go and let you know how I get on..Simon. Link to comment Share on other sites More sharing options...
simon metcalfe Posted June 21, 2008 Author Share Posted June 21, 2008 Hi Alex, just a note of thanks - I did as you suggested and it's worked - carbs are fine now with not a drop of unburnt fuel! I'm still not really sure where the air leak was from but your instructions were clearly spot on - thanks again - was beginning to think I'd have to take the carbs off and give them to a Weber agent to look at...£££ etc! 😳 Link to comment Share on other sites More sharing options...
alextangent Posted June 23, 2008 Share Posted June 23, 2008 You're welcome. Carbs are miracles of engineering; every time I take mine apart (twice so far) I marvel at the fact that they work at all. Then I swear at them anyway. 😬 Link to comment Share on other sites More sharing options...
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