Topic: cb350F rebuilt but wont run more than 2 mins!

[draven3020]

Hi, I am a newbie but thought someone out there may have come across this before.

I have just finished rebuilding a '72 CB350F imported from the states so the history of the bike is minimal. The engine kicked over when I got it with some compression but on investigation I found most of the rings were stuck in the pistons so I stripped down the top end and overhauled it. The valves were re-seated, new rings fitted and everything was cleaned and re-assembled. Finally got it all back together a week or so ago, put in some petrol and hit the starter. She fired within a second and seemed to run smooth and quietly. But then as it warmed up it started to slow and eventually stopped. I thought it was just usual adjustments, fuel etc so I re-built the carbs with new jets etc and tried again. Same thing. The bike fires fine when cold but after a minute or so it starts to struggle then stops. It wont re-start straight away but does within 10 mins. The choke setting makes little difference other than upping the idle speed when closed but the bike runs fairly well with it fully open virtually straight away.
I have noticed that once warm it feels harder to kick over and this makes me think the engine is being starved of oil. However the oil light works fine and goes out as soon as the engine fires. There is plenty of the right grade oil in her and a visual inspection through the tapped holes show oil is making it to the head which I believe is at the end of the oil pathway. I have checked and reset the tappets twice with the engine cold and all is ok. There is plenty of fuel in the carbs.

Anyone got any ideas?

Cheers

Derek

[draven3020]

Thought I would post a pic of her...looks good but un reliable.....reminds me of an old girlfriend....

[draven3020]

Got the bike in March this year as project and began by stripping it completely. The good news was that the corrosion was extremely light and the only major holes were in the seat pan and battery box. The motor was fairly clean and kicked over. The mudguards were good but there were a few things missing such as throttle cables, wiring etc.
Repainted the frame in black enamel, re-wired it (I am a sparks/controls engineer luckily) stripped the engine and fitted new rings to all four pistons, the bores were ok and the odo showed only 9K so I was hopeful the engine was still fairly young. Sourced most parts from around the world via ebay. Refaced the clocks with the help of some vinyl reproductions. Local paint shop owner is a Honda fan and new the correct way to paint the shiny bits which was good as my painting skills are limited. Worst part of the whole project was getting the old gasket material off the bottom of the barrels! Best bit, if not short lived was the moment it fired first time. Pics of some of the work stages below.

[Bowswell]

Open the gas cap...and start again?
And check petcok screen.

[draven3020]

hi Bowswell,

No gas cap fitted so tank is open, new petcock screen and remains clean.

Cheers
D

[strynboen]

try to vire the coils direkt from battery..to thek for bad konnektions in safty kontakt and main svitch(key)..
and thek voltage before doing it..its the 2 Black/hvite vires on the + on the coils..and put a vire direkt from battery plus ...

[draven3020]

Hi Strynboen, your English is ok, spelling needs work but it makes sense!

Thanks for the comment, I will give this a try tomorrow but the bike has had the ignition switch replaced and the fault was the same after as it was before.

I have also swapped the coils from my Yamaha and the fault remains the same.

Will update again once I have tried it.

Cheers

D

[calj737]

From your description, I would almost suspect ring end gap is too small. Once the motor warms up a bit, the rings close tighter due to expansion. IF they're too tight, then they are causing a seizure in the cylinder. Engine cools, rings shrink, and the cycle repeats.

Can you confirm you properly measured the gap prior to installation?

If its not this, then I would replace the condensers and verify fuel level (bike starves for fuel due to improper float height or function). I'm leaning towards my first diagnosis though, as the problem pre-existed the carb rebuild, and afterwards.

[Flyin900]

Derek,

If it turns out to be the end gap on the rings then hopefully you kept the old Honda rings. With only 9K on the clock those rings are like new and should only need a cleaning and reinstall. Most Honda rings are good for well over 50K + with no issues.

I just restored a CB 350F with 13K on the clock and it had mostly sat for 20 years yet it was run occasionally. The compression was on the low side, yet uniform. The valves were sealing well yet the rings were gummed up from sitting, so I soaked the combustion chambers over the winter in Deep Creep continually introduced through the spark plug holes. I didn't disassemble the engine, as I felt it ran well and just needed some TLC on the rings to clean up the carbon and gummy old oil.

I fired her up this spring and ran some Seafoam in the oil after draining the Deep Creep and all is golden. Compression is back up in the 150 range across the board and the motor runs beautifully.

Sounds like you are a Canadian too. I am located in the GTA in Ontario.

[draven3020]

Hi Flyin900, I think you may be on to something, that fits all the symptoms and I didn't check the end gaps very carefully, just checked there was a small gap and assembled them (yep idiot I know)

I will strip the barrels off again once I get a replacement gasket set and take a look.

Cheers for the idea.

No not in Canada i am afraid, am in the UK so a bit further away! Have relatives in Edmonton though!

will update again in a week once I get it to bits.

Regards

D

[calj737]

Derek - if you did have too small a ring gap, it might be a sound idea to re-hone the cylinders while you're in there. If the motor has been "seizing up" due to heat expansion, you should see some scoring on the walls. Best to de-glaze that with a light hone so upon reassembly you get a proper seal.

[draven3020]

Yep, thats what I thought so I have gone the whole way and ordered a replacement set of oversized (0.25) pistons and rings and will have the barrels bored to match.

Will let you know how I get on....

Cheers

[draven3020]

Well Flyin900 wins the day, thanks for the advice, forums do work! Stripped the top end to find three cylinders fine, but no2 lower compression ring has been seizing in the bore. You cant see too well in the pictures but the ring is blackened and the bore is badly scored.

All because I didn't check the ring end gaps during the rebuild.

Barrells now going off for a re-bore they didnt need and I am waiting for the new pistons/rings.

A sad state of affairs considering how good the engine was before I put it back together!!

Another lesson learned at great expense.

Will post a pic once I get it all back together.

Thanks for all the help.

D

[draven3020]

Thought I ought to get some good from stripping the engine again so today i repainted the top end. I wasnt going to make the bike all new and shiny when I started the restoration but since it was in bits again anyway I added a little paint to spruce the engine up to match the rest of the bike.

[Don R]

The machine shop might want to measure the pistons and check the clearance before finishing the boring and honing. Do you know the proper clearance for your bore? Some shops don't know and guess. I'd hate to see you back in the same boat.

[draven3020]

hi Don,

Yes I will wait until the pistons arrive and give the barrels and pistons to the shop for a correct fit. Worth the wait to get it right (second time around!)

Cheers

D