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Advice on engine problem
sprinta:
Have just bought a 73 CB750K2. It is one that has had a complete engine rebuild, re-bore, chains, guides etc and had completed around 350 miles before purchase.
Over the weekend did around 200 miles, nothing exceptional to note.
Took it out on Tues and did notice a bit of smoke and a slight engine oily smell on startup but did not identify from which pot(s) it was from?
After around 30/40 miles it started sputtering and thought it need to go onto reserve. However, it did not improve so stopped to have an inspection. No 2 downpipe was only lukewarm so clearly a problem with No 2 pot, bugger.
Limmped home on three and removed the plugs, see photos, No 1 on left. No 2 totally black and carbonated. No's 1 and 4 OK but No 3 slightly black.
Decided to check the compression. No's 1 and 4 around 150 psi but No's 2 and 3 around 135 psi. All just within 10% of each other but with my understanding that they should all be within 150 to 170 psi No's 2 and 3 are low? See photos.
No spark at all from the No 2 plug so got a replacement which I fitted today.
On startup a lot of smoke from No 2 but all 4 pots now firing. Went for a ride and stopped after 10 miles or so. Now only slight smoke from No 2 so hoping that it was just a build up that was now burning off following the 30/40 ride home with No 2 not firing?
Anyway, carried on as the bike seemed OK so was hoping that it may have just been a duff plug? Stopped again after 20 miles and No 2 now practically free from smoke.
However, after around 30 miles it started to splutter again and was now firing intermittently on No 2.
Got home and removed the plugs again. No's 1 and 4 still OK but No 2 black again and No 3 also blacker than one would expect, see photo.
Is it a piston/ring/seal/valve/guide problem and excessive oil or is it carburetion or possibly a coil problem as both 2 and 3 are on the same coil?
Any advice would be appreciated.
Andy
adriangsmith:
Looks like a carb setup issue to me.
Running way too rich.
Are you running the standard airbox or pods?
Had it on mine a couple of times until run in.
Once was a stuck float valve,another a blocked idle jet and the other time was because the plugs soot up and won't spark through the carbon buildup.
Could also be plug caps or coils.
Also at only 350 miles it's not yet run in.
Needs 500 - 750 miles for everything to be settled in properly.
Once at 750 miles make sure you do an oil change.
K2-K6:
As you've got commonality with ignition on 2 and 3, as you point out, I'd look at that to eliminate it first of all and before you chase carburation to any degree.
This is partly mirrored in the recent post by RGP on his restored 750 with a spark drop out causing very similar effects, worth a read through to see discussion.
Also, a quick check up the carb throats to make sure the choke flaps are being fully removed from airflow when retracted.
sprinta:
--- Quote from: adriangsmith on July 06, 2017, 03:55:34 PM ---Looks like a carb setup issue to me.
Running way too rich.
Are you running the standard airbox or pods?
Had it on mine a couple of times until run in.
Once was a stuck float valve,another a blocked idle jet and the other time was because the plugs soot up and won't spark through the carbon buildup.
Could also be plug caps or coils.
Also at only 350 miles it's not yet run in.
Needs 500 - 750 miles for everything to be settled in properly.
Once at 750 miles make sure you do an oil change.
--- End quote ---
Thanks for the reply.
Using std air box with new filter, so that should not be causing any restrictions? Have managed to removed the main and pilot jets without removing the carbs and the jets, at least, are all clear.
Will check the plug caps for 5K resistance?
sprinta:
--- Quote from: K2-K6 on July 06, 2017, 04:09:55 PM ---As you've got commonality with ignition on 2 and 3, as you point out, I'd look at that to eliminate it first of all and before you chase carburation to any degree.
This is partly mirrored in the recent post by RGP on his restored 750 with a spark drop out causing very similar effects, worth a read through to see discussion.
Also, a quick check up the carb throats to make sure the choke flaps are being fully removed from airflow when retracted.
--- End quote ---
Thanks for the reply.
Certainly hoping that it a carb/coil problem and not engine issue?
Don't have another coil to try but will check the various resistances including the plug caps to see if that pick anything up?
Has electronic ignition fitted but don't see that as an issue as No 1 and 4 seem OK?
Good point on the carb choke flaps.
Do you have a link to the post you were referring to?
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