CB350 1971 Restoration Project

[BenPowell]

I’m fairly confident the diaphragms are fine, but they will be coming apart so won’t hurt to have another thorough check of them to be sure.

I do think it is a blockage somewhere, as I mentioned they were in a right state when I got them so in all honesty I’m surprised they work at all 

Just need to keep chipping away to find the root cause, like I mentioned I’ve checked and adjusted points and valves, going to run the bike tomorrow night to see if that’s improved anything, while it’s running I’ll check for vacuum leaks.

If the problem still persists, the carbs will be coming off for another thorough clean. Really don’t think it’s anything too major though, the bike will idle nicely at 1200rpm rev freely (no engine load) just the odd pop from one the right cylinder.

[AshimotoK0]

I was only thinking earlier this evening if anyone has ever fitted 450 style CV carbs to a 350K and lo and behold this pops up on eBay ... a real mish-mash of an example.

https://www.ebay.co.uk/itm/386144812818

Ben ... did you try checking for inlet leaks with easy start sprayed onto the inlet stubs ... should have let me soften them with my magic fluid 

[ Guests cannot view attachments ]

[philward]

Had a day on the 350. Rechecked all the settings - points bob on but timing (using the strobe) altered slightly, fiddled with carb setting and much improved but still intermittently hanging before settling to tickover. When I was building the engine on the bench and when fitting the carb manifolds, I thought the design of using JIS cross head screws and a gasket didn't seem a good design. The last thing I checked today was for air leaks on the carb manifold to head (using the carb cleaner method) and there was a clear movement in the revs when carb cleaner squirted at the joint.
Just spent the night stripping and re-assembled using a slight skim of silicon sealant and hexagon bolts.
I'll report back tomorrow
As an aside, its taken me back 50 odd years taking the tank off (when I had a 250k) and that bloody awful petrol balance pipe that goes from one side of the tank to the other under the frame - you have to juggle taking the tank off while trying to stop petrol coming out of both sides - its a bludy pain! I have put 2 tiny inline taps on the pipe (and disconnect the pipe in the middle between the taps) but there is so little space, its more trouble than the standard pipe.
Coincidence that Ash posted the same thing at the same time!

[BenPowell]

I was only thinking earlier this evening if anyone has ever fitted 450 style CV carbs to a 350K and lo and behold this pops up on eBay ... a real mish-mash of an example.

https://www.ebay.co.uk/itm/386144812818

Ben ... did you try checking for inlet leaks with easy start sprayed onto the inlet stubs ... should have let me soften them with my magic fluid 

(Attachment Link)

Will check for leaks tomorrow night, I hope it’s a leak, at least it’ll be fairly simple to do something with 

[BenPowell]

Had a day on the 350. Rechecked all the settings - points bob on but timing (using the strobe) altered slightly, fiddled with carb setting and much improved but still intermittently hanging before settling to tickover. When I was building the engine on the bench and when fitting the carb manifolds, I thought the design of using JIS cross head screws and a gasket didn't seem a good design. The last thing I checked today was for air leaks on the carb manifold to head (using the carb cleaner method) and there was a clear movement in the revs when carb cleaner squirted at the joint.
Just spent the night stripping and re-assembled using a slight skim of silicon sealant and hexagon bolts.
I'll report back tomorrow
As an aside, its taken me back 50 odd years taking the tank off (when I had a 250k) and that bloody awful petrol balance pipe that goes from one side of the tank to the other under the frame - you have to juggle taking the tank off while trying to stop petrol coming out of both sides - its a bludy pain! I have put 2 tiny inline taps on the pipe (and disconnect the pipe in the middle between the taps) but there is so little space, its more trouble than the standard pipe.
Coincidence that Ash posted the same thing at the same time!

Sounds like you’re getting to the bottom of the issue, just out of curiosity when trying to set up did you find the mixture screws made no change to the running of the engine? While mine was idling I was messing with the screws on both carbs, but didn’t seem to do much. So I just left them about 1 turn out.

To me that sounds like something in the idle circuit is blocked, additionally the idle speed screws are quite far in to maintain a steady 1200rpm idle.

The tank balance tube is a pain, I left mine a little longer just so i can prop up the bottom of the tank for access without removing it fully, ideally 2 clamps and a straight connector in the middle of the balance tube would work nicely.

[philward]

Did make a difference - more so on the left carb, but yes. Bit inconsistant and depends on the how I done the other settings. I first synch'ed the tickover screws using a feeler guage to balance the tickover. Then synch'ed the carb openings using the individual cable adjustments at the carb end. Then set the tickover - screwing the individual screws out equal amounts. The mixture screws responded better after the above setting sequence.

[philward]

Started up after actions taken in previous post - revs returning to idle better but still little response to slow running screw adjustment on R/H cylinder (and now no leak on intake manifolds) with misfire (pop) when slow speed screw backed out more than 1/2 turn. Should have checked that curcuit while I had the carbs off! I'll take the carbs off again and check the low speed curcuit for blockages - I'll feed any progress back Ben.
Also got a noisy tappit on R/H to sort when engine back to cold and strangely, the R/H silencer is far louder than the L/H (both new).

[florence]

Is it worth running without air filters attached for a test just so you can see if slides are popping back down in good time and together when throttle is released?

[philward]

Good shout, I'll try that - thanks

Sent from my moto g(50) using Tapatalk

[BenPowell]

Started up after actions taken in previous post - revs returning to idle better but still little response to slow running screw adjustment on R/H cylinder (and now no leak on intake manifolds) with misfire (pop) when slow speed screw backed out more than 1/2 turn. Should have checked that curcuit while I had the carbs off! I'll take the carbs off again and check the low speed curcuit for blockages - I'll feed any progress back Ben.
Also got a noisy tappit on R/H to sort when engine back to cold and strangely, the R/H silencer is far louder than the L/H (both new).

Thanks for the updates Phil, after adjusting the timing the other night and re-checking the valve clearances I can say it has made no difference to the popping except I now have a noisy valve which wasn’t before (shouldn’t have messed with them) pretty sure it’s just the one.

Checked for air leaks by spraying around the carb boots while running and that also made no difference to the RPM so it’s got to be something not right in the carbs, again mixture screws did nothing,

With regards to more noise from one silencer, to me that sounds like one cylinder is doing more work than the other, one of mine felt like it had a lower pressure from the exhaust so I increased the idle screw on that side and it brought it back inline.

I briefly unplugged the HT leads on each cylinder the one which was quieter cut the engine almost straight away, when i took the other off it continued  to run for a bit.

[philward]

Further update Ben. Today I stripped the carbs and found a few issues! Found both floats had small amount of petrol in and R/H slow running jet blocked (hence the lack of adjustment on that air screw) - soaked in some carb cleaner but can't shift what ever is blocking it. I should of spotted it first time. Sent for some new float and slow running jets.
So maybe your lack of adjustment on your air screw is a blocked slow running jet?

[philward]

Is it worth running without air filters attached for a test just so you can see if slides are popping back down in good time and together when throttle is released?

Checked prior to carb strip and all good - thanks

Sent from my moto g(50) using Tapatalk

[BenPowell]

Little update on the carbs tonight, I took them apart to inspect and found that the jet which is under the rubber bung (idle jet?) was blocked on both carbs. Must have missed it when I rebuilt them originally, because it took a fair bit of unblocking, gave everything else a thorough clean while they was apart.
Got them back on the bike with mixture screws 1 full turn out from loosely nipped up, and idle speed screws 3/4 turn from touch and the bike fired up straight away with no more popping from the right cylinder, in addition to this the idle mixture screws have a definite impact on the engine now, safe to say that was the issue there.

Before I removed the carbs I ran the bike with no air filters and could see the slides moving freely, and while off i checked the diaphragms and couldn’t see any holes, I also checked the slides and they are completely free, so I think the hanging rpm must have been caused by a lean running condition on one cylinder.

Tomorrow I’m going to spend some time getting the carbs set up, or as best I can without riding it.

Had the DVLA inspection this morning, guy was here 10 mins and took plenty of photos of the bike, hopefully I’m not waiting much longer for the V5, I just want to get out for a ride now 

[philward]

Sounds like we are having identical issues Ben! My slow running jet just wont clear. It's been soaking in carb cleaner for 2 days. I've ordered a .35mm drill. As a back up, I've ordered some pattern jets (as genuine not available) as a back up. Also ordered some pattern floats.
Hopefully I'll have your success!

[BenPowell]

Sounds like we are having identical issues Ben! My slow running jet just wont clear. It's been soaking in carb cleaner for 2 days. I've ordered a .35mm drill. As a back up, I've ordered some pattern jets (as genuine not available) as a back up. Also ordered some pattern floats.
Hopefully I'll have your success!

The radial holes on mine were all clear, but down the bore it actually goes to a smaller diameter and it was blocked there, I bought some 3D printer nozzle cleaners, turned out to be really useful for the carb, get a load in a pack for about 3 quid.

Link: https://www.amazon.co.uk/dp/B09L86LJ81?ref=ppx_pop_mob_ap_share

Was really quite tough to break through the blockage, and just for a final check I poured a small amount of petrol through to make sure the passages through the carb body were clear.