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Motorcycle Forum / Country Specific / Classic (UK Group) / August 2007More CB750 fun
So, gentlemen, the story so far is that my CB750 carbs were blocked. I took them off and cleaned out the jets - three jets per carb. Still doesn't work. It runs fine on easystart - if I keep spraying it runs continuously. Did I miss something in the carbs? What else could it be? I guess easystart is a lot easier to ignite and maybe a very weak spark will let it run on easystart but not petrol? Is this likely or should I be looking at the carbs again? Cheers, Nick  SignatureCCM R30, CB750 (broken) > I guess > easystart is a lot easier to ignite and maybe a very weak spark will let > it run on easystart but not petrol? Is this likely No. > or should I be > looking at the carbs again? Yes. Ultrasonic cleaning - the only way to be sure. SignatureBMW K1100LT 750SS CB400F CD250 CB125 SL125 GAGARPHOF#30 GHPOTHUF#1 BOTAFOT#60 ANORAK#06 YTC#3 BOF#30 WUSS#5 The bells, the bells..... > Yes. Ultrasonic cleaning - the only way to be sure. Ok but if the jets are clear, what is left to clean? Cheers, Nick SignatureCCM R30, CB750 (broken) >> Yes. Ultrasonic cleaning - the only way to be sure. >> > Ok but if the jets are clear, what is left to clean? First, you can't tell if the jets are really clean - they're open now but that doesn't mean that they haven't got some crud left in them that reduces the size of the jet. Also, I assume that you at least made an attempt at cleaning out the air passages in the carb body? Also, have you checked that the float needle valves are not sticking, that the float height is correct and all the other fun problems you can get with carbs? SignatureMorini Corsaro 125 | CB450 K4 | XL250 Motosport | 900SSD | K1100LT Laverda SF2 | Harley FXD BOTAFOF #33 The UKRMC FAQ: http://www.unixconsult.co.uk/bike/ukrmcfaq.html > First, you can't tell if the jets are really clean - they're open now > but that doesn't mean that they haven't got some crud left in them that > reduces the size of the jet. Hmm. I guess so. There was a little sediment in the float bwls but they are thoroughly clean now. > Also, I assume that you at least made an attempt at cleaning out the air > passages in the carb body? The only air passage I cleaned out is the idle air adjuster (I think). There are passages to make the piston rise but they are quite big and were clear. Are there other air passages? > Also, have you checked that the float needle valves are not sticking, > that the float height is correct and all the other fun problems you can > get with carbs? The float valves are OK. Oddly, I can see no means of adjusting the height. The floats are all plastic and pivot on a steel pin. The Haynes manual shows you bending a tang on the float but it differs from my carburators. Cheers, Nick SignatureCCM R30, CB750 (broken) >> First, you can't tell if the jets are really clean - they're open now >> but that doesn't mean that they haven't got some crud left in them that [quoted text clipped - 9 lines] > think). There are passages to make the piston rise but they are quite > big and were clear. Are there other air passages? Did you blow through all the passages that the jets screw into? >> Also, have you checked that the float needle valves are not sticking, >> that the float height is correct and all the other fun problems you can [quoted text clipped - 4 lines] > Haynes manual shows you bending a tang on the float but it differs > from my carburators. There should be a way of adjusting them - the float bodies themselves are always plastic in all Japanese bikes I've played with, but usually the bit the pivot goes through is metal. Of course the fact that Haynes shows something different isn't exactly news... SignatureMorini Corsaro 125 | CB450 K4 | XL250 Motosport | 900SSD | K1100LT Laverda SF2 | Harley FXD BOTAFOF #33 The UKRMC FAQ: http://www.unixconsult.co.uk/bike/ukrmcfaq.html > Did you blow through all the passages that the jets screw into? Yes, though they are quite big. I've got the carbs back off - I'll have another go at cleaning them and if that doesn't work I'll give up and book it in with my local man. > There should be a way of adjusting them - the float bodies themselves > are always plastic in all Japanese bikes I've played with, but usually > the bit the pivot goes through is metal. These are completely plastic. I'll take a photo. > Of course the fact that Haynes shows something different isn't exactly > news... To be fair, mine is a later model. There is a supplement for the later ones but no carb photos. Cheers, Nick SignatureCCM R30, CB750 (broken) > > Yes. Ultrasonic cleaning - the only way to be sure. > > > Ok but if the jets are clear, what is left to clean? All the little passages and galleries in the carbs. Ultrasonic cleaning is the next step. At least you're getting good at removing and refitting the bloody things. SignatureSimon > All the little passages and galleries in the carbs. Ultrasonic cleaning > is the next step. > > At least you're getting good at removing and refitting the bloody things. True. I find it quite easy now... Cheers, Nick > So, gentlemen, the story so far is that my CB750 carbs were blocked. > I took them off and cleaned out the jets - three jets per carb. [quoted text clipped - 5 lines] > easystart is a lot easier to ignite and maybe a very weak spark will > let it run on easystart but not petrol? Did you check the spark? Do you have ample compression? > Is this likely or should I be looking at the carbs again? If they were completely gunged up, yes. SignatureMorini Corsaro 125 | CB450 K4 | XL250 Motosport | 900SSD | K1100LT Laverda SF2|Harley FXD BOTAFOF #33 The UKRMC FAQ: http://www.unixconsult.co.uk/bike/ukrmcfaq.html > Did you check the spark? Do you have ample compression? I only checked the spark with one of those neon things. No I've not checked the compression but the bike ran well before it was garaged. As I said it runs OK on easystart so there can't be *that* much wrong with it. :-) >> Is this likely or should I be looking at the carbs again? > > If they were completely gunged up, yes. Hmm... I' don't know. They didn't look particularly dirty inside but all the jets were blocked. I removed and soaked the jets in carb cleaner, then blew them out with the carb cleaner aerosol. Cheers, Nick SignatureCCM R30, CB750 (broken) >> Did you check the spark? Do you have ample compression? >> > I only checked the spark with one of those neon things. No I've not > checked the compression but the bike ran well before it was > garaged. As I said it runs OK on easystart so there can't be *that* > much wrong with it. :-) That very much depends on how long you're running it on Easystart... >>> Is this likely or should I be looking at the carbs again? >> [quoted text clipped - 3 lines] > all the jets were blocked. I removed and soaked the jets in carb > cleaner, then blew them out with the carb cleaner aerosol. Yes, but did you only blow through the jets or also the various other orifices in the carb body itself? SignatureMorini Corsaro 125 | CB450 K4 | XL250 Motosport | 900SSD | K1100LT Laverda SF2 | Harley FXD BOTAFOF #33 The UKRMC FAQ: http://www.unixconsult.co.uk/bike/ukrmcfaq.html > > Did I miss something in the carbs? What else could it be? I guess > > easystart is a lot easier to ignite and maybe a very weak spark will > > let it run on easystart but not petrol? > > Did you check the spark? Do you have ample compression? If it runs on easystart it's a pretty safe bet it's fuel related. SignatureSimon >> Did you check the spark? Do you have ample compression? > > If it runs on easystart it's a pretty safe bet it's fuel related. That's what I'm thinking (and hoping). Cheers, Nick > >> Did you check the spark? Do you have ample compression? > > [quoted text clipped - 5 lines] > > Nick Does it run at all without the Easystart? You did remove the pipe(s) to make sure petrol is actually getting to the carbs? Oily > > So, gentlemen, the story so far is that my CB750 carbs were blocked. > > I took them off and cleaned out the jets - three jets per carb. [quoted text clipped - 11 lines] > > If they were completely gunged up, yes. Easy way to check the spark is to remove a plug cap from one plug and put a spare plug in it, rest this on the engine and try starting it, if there is a spark all OK, if not the ignition needs a look make sure the spark is a big fat blue one. SignatureXJ600S > So, gentlemen, the story so far is that my CB750 carbs were blocked. I > took them off and cleaned out the jets - three jets per carb. [quoted text clipped - 3 lines] > > Did I miss something in the carbs? What else could it be? Honestly, with modern carbs if all the petrol in the floatbowl has evaporated due to storage the only way to get the buggers working again is with an Ultrasonic cleaner. A few years ago I bought a GS500 that the owner had stuck in a shed with the petcock set on prime About 2 gall of petrol had evaporated through the carbs, that's when I bought my first Ultrasonic cleaner. - > So, gentlemen, the story so far is that my CB750 carbs were blocked. I > took them off and cleaned out the jets - three jets per carb. [quoted text clipped - 6 lines] > it run on easystart but not petrol? Is this likely or should I be > looking at the carbs again? Stupid question 1: are you sure the carbs are fully closed? The cables are not holding the carbs open. It won't start then. SQ2: Is the choke working? on etc? Signature-- ZX10R in the correct colour. Triumph Sprint ST for long two up touring. http://sportstourer.org |
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