SOHC.co.uk Forums > CB350/400
Progress - She runs at last! (75 CB400F)
mike the bike:
If it's 2 and 3 not working, its4 more likely to be ignition related than carb related.because 2 and 3 carbs get their petrol before 1 and 4.
ManicGTI:
All 4 carbs are filling, so yeah I'm assuming I've got a dodgy connection somewhere on that coil
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Laverda Dave:
You appear to be getting somewhere with your carbs Dave. I thought I was getting somewhere with mine as well having made up a float height gauge similar to the one Chris described in an earlier post. I set the floats to 21mm, checked, checked again and once more for luck, all in order, bowls were re-fitted (a number of the screws had to be helicoiled as the threads felt 'weak'). I bench tested the carb bank with fuel and still two of them leaked!!!! I took them apart again and rechecked. I blew out all the airways with compressed air and checked each fuel valve with a magnifying glass. I must admit the quality of finish on the new valves looks a bit rough, not the smooth finish I would expect for something that is supposed to seal against fuel. I fitted the two 'best' of the old valves as these looked in better condition then the new ones! Carbs back together, bench tested and all was fine, happy days :).
I refitted the carbs back on the bike on Saturday having bench sync'd with a 1mm drill bit (fiddly!). Everything connected, all cables lengths correct, idle screws set at 2 turns, fast idle clearance set, all good. Morgan carbtune connected and I started her up. I was greeted with very rough running, carbtune only registered a reading on no.3 and no amount of adjustment brought the other three carbs to life although the engine note improved. By now it was getting dark so I gave it another go yesterday. I took the carbtune apart and re-polished the stainless rods and brass guides in case they were sticking (it made no difference anyway). Yesterday was going well, the engine was sounding better but I could only get a reading on the carbtune if I turned it upside down??? Reading was 20hg, and was in the range stated in the manual. I balanced all the carbs to the same setting +/- 2hg. Problem I have now is the engine races to 400rpm if the throttle is 'blipped' and it feels 'flat' when slightly revved. Just to add to the frustration no.2 carb became incontinent and gushed from its overflow. I also noticed exhaust no.1, 3 and 4 are yellowing at the head, is this a sign of running hot and a weak mixture, no.2 header is fine. Just a note, as the carb balancing was taking a while I stopped a few times to let the engine cool down. The bike was also a pig to start from cold on both days.
Any thoughts would be appreciated, I noticed I have a lump hammer in the toolbox, it might come in useful soon...... >:(
ManicGTI:
Sounds like progress anyway! Are the carbtunes worth getting? Do they come with the adaptors to screw into the balancing ports? I'd like to have a bash at balancing mine further now.
My new brother in law popped round a few days ago, turns out his first bike was a 400. Happily mine started on demand with no air box/filter on and he seemed to think it was running OK. I have to admit being used to more modern bikes sometimes I don't know if something sounds rough, or it's just the age of it.
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K2-K6:
Laverda120, I believe you've gone away from where you need to be.
I think you were right to bench sync them, are you making sure while you do that the slides are "hanging " on the throttle mech and not being supported by the drill? If so then at that point I would not adjust the slides again, at all.
The even slide height should make a healthy engine run correctly at all parts of throttle opening and it doesn't need these moving from each other.
I've always seen the carb "balancing " routine as a bit of theatre, which can cause more problems than it solves.
With the slides set accurately to the same height, then if it doesn't run properly at tickover these are not the tool to correct it. You just put an error on all other rpm operation that is not desirable.
The part you need to adjust is solely the idle screws. If you turn each screw while running at tickover, you should get a change in rpm, if not, then that one idle circuit is not clear and operating as it should. Again, the throttle slide adjustment just will not correct this. It needs the idle circuit to be cleaned properly.
There is a well documented routine in Honda manuals to carry out the idle circuit adjustments, and it works well. You may end up with slightly differing screw set points, this will either be because of the jets and passage being partly blocked or a real difference in the hardware. it's here that differences are adjusted out and not the slides.
Think of the idle circuit as a microbore carburettor in itself, and it should respond to opening and closing exactly the same as any whole carb, just at low speed. If they are not right, you'll chase your arse for ever on the main slides, and never get it ok. What's more, the wide open throttle is then incorrect which will give you another whole set of problems.
I'd say, step away from the carb balancers. Go back to basic checks.
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