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Author Topic: '73 CB250 - Doing it right this time round.  (Read 85691 times)

Offline ponieswhee

  • Posts: 28
    • CB175 build
Re: '73 CB250
« Reply #30 on: Jul 01, 2012, 15:37:51 »
I'd check the condition of the plugs to see whether you're rich or lean on that side then adjust the carbs accordingly. And verify the float levels are ok as well.

Offline Green199

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Re: '73 CB250
« Reply #31 on: Jul 06, 2012, 14:10:56 »
I'd check the condition of the plugs to see whether you're rich or lean on that side then adjust the carbs accordingly. And verify the float levels are ok as well.

Put new plugs in a few days ago and the left side is much darker and sootier than the right. So I adjusted the mixture and it now runs a little better, but it still seems pretty temperamental. Once its revved and the throttle closed off, it still revs for a second after and doesn't fully shut off....Its difficult to explain so Ill try and get a video up as soon as.

So while I waited to do that I decided to start on the seat today. Managed to strip the old paint and put the first few coats of primer on.
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Green199

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Re: '73 CB250
« Reply #32 on: Jul 06, 2012, 14:11:48 »
More....don't know how to put more than 4 pics in a post.
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Green199

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Re: '73 CB250
« Reply #33 on: Jul 08, 2012, 18:25:13 »
Still having a few problems with the bike running. Left side header is much hotter after a 5 minute run than the right.

Took apart the carbs to change float level. Whilst they were off the bike I gave a few parts a lick of paint. The throttle arm is Hammerite smooth black with new a stainless bolt and the tops are Plastikote gold. (Even though its plastic paint, it seems to be holding up well)...sorry for the poor quality, they were taken on my laptop.
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Green199

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Re: '73 CB250
« Reply #34 on: Jul 10, 2012, 17:25:29 »
Did a bit more work on the seat. It only had a few scratches and knocks in it so luckily I didn't need to do a lot of filling. I used Loctite body filler as it was the cheapest on the shelf (£6 for 280ml of filler paste + 15ml of hardener) and it seemed to hold up well and was easy to use (I suppose filling was never going to be too hard in the first place anyway! haha)
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Green199

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Re: '73 CB250
« Reply #35 on: Jul 10, 2012, 17:29:12 »
Started sanding it back with 180 grit wet and dry. Then moved on to 240 then to 320. Seemed to leave a nice enough finish to give it another few layers of primer, but  I decided to leave it til tomorrow as it was getting a tad dark. Need to be setting up a paint booth with some decent lighting next I think haha!
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline ponieswhee

  • Posts: 28
    • CB175 build
Re: '73 CB250
« Reply #36 on: Jul 10, 2012, 22:10:51 »
When I first got my 175 I had to play with just about everything to get it running right. Firstly the floats were off so bad the gas would just pour out of the overflows, fixed that then dropped the needles a notch which helped lean it out a bit. Also the throttle cables had been routed badly so the revs would rise when the bars were turned. The little CBs are very sensitive to throttle cable routing, might want to make sure the cables are moving freely. What helped the most was nailing down the ignition timing, which was way off...

Offline Green199

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Re: '73 CB250
« Reply #37 on: Jul 11, 2012, 08:52:37 »
When I first got my 175 I had to play with just about everything to get it running right. Firstly the floats were off so bad the gas would just pour out of the overflows, fixed that then dropped the needles a notch which helped lean it out a bit. Also the throttle cables had been routed badly so the revs would rise when the bars were turned. The little CBs are very sensitive to throttle cable routing, might want to make sure the cables are moving freely. What helped the most was nailing down the ignition timing, which was way off...

I don't think the timing is too bad at the moment (famous last words) but I'll have a lil tweak later and see if it helps. The biggest problem I'm having is definitely with the carbs. The fuel literally just pours out the over flow no matter what height I set the float to so I'm beginning to think the float needles aren't the correct ones (the rest of the bike was a bodge job so it wouldn't suprise me). I can't seem to find anywhere on line where I can buy them either. The carbs are the Keihin 725b model.

The other issue I'm having is occasionally when I take the carbs off to clean or reset float height, once assembled again, the bike starts and just revs itself almost full throttle (Cable routing is ok I think). And then one of the headers runs MUCH hotter than the other. Could this be down to the mixture or any other carb issue?

Bloody carburetor's are beginning to get on my nerves to be honest! haha
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline ponieswhee

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Re: '73 CB250
« Reply #38 on: Jul 11, 2012, 10:31:49 »
Setting the float level is a pretty important part of the carb tuning. If you're still getting overflow that means your gas level in the carbs is too high so you'll be running rich, which is consistent with the black, sooty plug. It could be that the floats are getting hung up on something or that they have holes in them and don't float any more. Generally the leaner the mixture the hotter it burns, so if one header is way hotter than the other it's probably leaned out or might be leaking air into the mixture from somewhere.

Offline Green199

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Re: '73 CB250
« Reply #39 on: Jul 11, 2012, 15:20:37 »
Ok, was going to check the timing but I couldn't find my multimeter or a timing light. But the left point seems to just open on LF (on the stator) and has a pretty consistent 0.3-0.4mm gap throughout the process, the same with the right. So until I can set it properly and double check its right I just cleaned the contacts up with a bit of emery cloth and re-installed them.

I was wondering why there was a constant flow of fuel from the overflow pipe no matter what height the floats were set to. So I took the carbs apart again to give them a clean and check and found a small hair-size crack running down the brass fuel overflow pipe. So I'm guessing that was the problem. Anybody know of a way to fix it? I was just going to solder it up but I'm not too sure if that would work.

I also found a small crack on the rubber intake boots that connects the carburetor to the head. So as a temporary fix until I can get another one delivered I covered the crack with silicon bathroom sealant. (It's white in the pictures but when fully dry it's transparent).
« Last Edit: Jul 11, 2012, 18:02:11 by Green199 »
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Green199

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Re: '73 CB250
« Reply #40 on: Jul 11, 2012, 15:25:30 »
While I had the stator cover off for the timing, I gave the side covers a bit of a clean and polish. Just gave it a once over with 320 grit wet and dry and then used some wire wool to bring out the shine. I didn't want it too polished because it would be a nightmare to keep clean and because I want this bike to look like it gets ridden and isn't just a show piece.

I also replaced most of the bolts with stainless allen heads, but there are a few I still can't get out and a few which I didn't have the correct size replacement for so they will have to stay in for the time being.
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Green199

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Re: '73 CB250
« Reply #41 on: Jul 11, 2012, 15:30:19 »
And finally...(I really need to find out how to put more pictures in a post haha)...

...put another few coats of primer on the seat and let it dry overnight. Just need to give it a wet-sand and should be able to spray some colour tomorrow as long as the weather holds up.

I moved the mounts for the seat about an inch forward. I should of really put the tank on the bike as well when I took the picture but I wasn't really thinking. Oh well.

I'm not too bothered about the quality of the base (the bit which gets covered by the seat) as I'm going to give it a thick white undercoat and leave it as it is.
« Last Edit: Jul 11, 2012, 15:32:59 by Green199 »
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline ponieswhee

  • Posts: 28
    • CB175 build
Re: '73 CB250
« Reply #42 on: Jul 11, 2012, 18:29:30 »
Seat's looking good. That crack in the intake manifold was probably leaning out the mixture pretty badly and would partially explain why the revs don't drop when you let off the throttle. I had the same issue with my GPz and ended up caulking it too until I could get a replacement :P

Offline Green199

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Re: '73 CB250
« Reply #43 on: Jul 13, 2012, 17:20:13 »
I had the same issue with my GPz and ended up caulking it too until I could get a replacement :P

So I'm guessing caulking it works for the time being?

Soldered up the crack in the overflow pipe. I sanded it down since I took the picture so the solder isn't as clumped up and bumpy as is shown...

Hopefully this should stop the fuel overflow leak...only one way to find out...
1973 CB250 'Doing it right this time round' - http://www.dotheton.com/forum/index.php?topic=36795.0

Offline Bert Jan

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Re: '73 CB250
« Reply #44 on: Jul 14, 2012, 05:11:36 »
please let us know if the leaking has stopped. I have a similar problem and going crazy over it.
Oh one more thing:

KEEP THE FAIRING!!!
You can't spell fairing without fair. So give the bike what it deserves!
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Netherlands represent!

Luctor et Emgo - I struggle and give up

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