daveh
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Posts: 346
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Post by daveh on Feb 2, 2022 11:17:54 GMT
Hello and welcome to the forum. Having needed to overhaul my CM250 carbs last year, I can thoroughly recommend Gee's sonic cleaning service, you'll get a first rate job done there.. From my own experience admittingly only having worked on two sets of these carbs, providing nobody's been in there before and lost parts, or worse butchered the internals you should be fine using the jets you already have, and can just get away with just some new float bowl seals and rubber bungs, my recommendations linked below. The bowl gaskets supplied in some of the replacement kits swell when they contact unleaded which is a major pain if you're removing the float bowls in situ, as you'll struggle to refit them. The ones in the link are made of vitreon which won't do this. Also I recommend you change the float bowl screws to an Allen Screw type, they make it easier to remove the bowls in situ if you want to alter the jets without removing the carbs from the bike. Good luck with your project. Disregard the parts description these fit your carbs, and you'll have a pair of spares.. www.ebay.co.uk/itm/224340069671?hash=item343bb5cd27:g:fEYAAOSwsNlgHLRz&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5337683463&toolid=10001&customid=kz4qlj0djq02ylrr00003www.ebay.co.uk/itm/222477904913?hash=item33ccb76c11:g:KMUAAOSwwMJfzvnSThanks Willy, Good to know about the seal material issue. On this point, what fuel are folk using on these older bikes, E5 premium or E10? Thanks for the links. Yes I had planned to replace screws with cap heads as some of the screw cross heads were already knackered. Dave.
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Post by steve mullineaux on Feb 2, 2022 11:23:34 GMT
I bought two sets for £15 total recently off ebay & had no problems fitting them . They are still available eBay item number:223689463153.
The only issue was they took 3 weeks to arrive but there was plenty of other jobs to get on with in the meantime.
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daveh
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Posts: 346
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Post by daveh on Feb 2, 2022 15:25:51 GMT
I've now realised that the jets in my carbs are possibly different to the standard, which makes sense as the exhaust had been changed to a single exhaust and need for larger jets. Also the carbs I have are VB30B and the standard for 1980 model says VB30A. Does anyone know the difference between 30A and 30B?
Thanks,
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Post by Graeme on Feb 2, 2022 17:48:22 GMT
I've now realised that the jets in my carbs are possibly different to the standard, which makes sense as the exhaust had been changed to a single exhaust and need for larger jets. Also the carbs I have are VB30B and the standard for 1980 model says VB30A. Does anyone know the difference between 30A and 30B? Thanks, Its just different frame numbers are specific to certain carbs (upto frame number etc) they all fit each other. VB30 denotes a 250 and VB31 denotes a 400. (250 and 400 not interchangeable) Both carbs you have should still be identical apart from what's stamped on them. Gee. www.cmsnl.com/honda-cb250n-1978-belgium_model15800/partslist/E19.html#.YfrBstXP2Ul
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daveh
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Posts: 346
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Post by daveh on Feb 2, 2022 19:37:59 GMT
Ok that's good info Graeme. I still suspect that the jets have been changed to suit modified single exhaust, so I'm going to clean out best I can to see if I can get the engine running so I know what I'm dealing with. Then later I can look at sorting our carbs properly after deciding if I will revert back to standard pipes. Thanks again.
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Post by steve mullineaux on Feb 6, 2022 1:11:08 GMT
I serviced my 250N carburettors last month using rebuild kits bought on ebay item number 223689463153 at a bargain price of £15 for 2 kits. They took 3 weeks to arrive but I had no problem fitting them.
in answer to your question about removing the jet this was confusing as my workshop manual says to unscrew it but having removed the bung there was no screw slot on it. Mine was pressed it and is not removable.
I think this contradiction is due to there being two different versions of the Keihin carbuerettor used on our bikes. VB30A & VB30B of which one has screwed in & the other pressed in jets.
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daveh
Full Member
Posts: 346
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Post by daveh on Feb 6, 2022 20:07:40 GMT
I serviced my 250N carburettors last month using rebuild kits bought on ebay item number 223689463153 at a bargain price of £15 for 2 kits. They took 3 weeks to arrive but I had no problem fitting them. in answer to your question about removing the jet this was confusing as my workshop manual says to unscrew it but having removed the bung there was no screw slot on it. Mine was pressed it and is not removable. I think this contradiction is due to there being two different versions of the Keihin carbuerettor used on our bikes. VB30A & VB30B of which one has screwed in & the other pressed in jets. Yes that's exactly what I found Steve. I've got the carbs all cleaned up with new gaskets and o rings and bench tested with fuel and no leaks, so shut of float valves are working OK. Not fitted yet as I have to replace one of the plug HT caps before I can check if it will run and if I have got all the jets sufficiently clean! Bike has not run for 18 years, so carbs were quite gunged up. Checked the engine top end and this is well coated with oil and turning over without fuel there seems good oil pressure and good compression in cylinders. Here's hoping......
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