Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: sprinta on July 06, 2017, 03:29:41 PM
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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
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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.
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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.
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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.
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?
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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.
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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Here you go.
Hope it's all yours is .
Good luck
http://www.sohc.co.uk/index.php/topic,13165.0.html
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Try swapping 2 and 3 plug leads. As they fire the same time it won't make any difference to the running. If the problem migrates to no. 3, then it's a HT problem.
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Thanks Rich, I was in too much of a rush to look for it properly.
Something with these fours generally is that if they're built well then there's not much that really goes wrong with them, they are very reliable usually.
Sprinta, it may take a little sorting through to properly debug each example but each little bit of attention should gradually move it forward to running really well.
Also if it's been bored, honed, glaze busted it that rebuild with new rings, then you'd expect the compression readings to be improving still at this point and up until closer to a thousand miles. You may be getting a discrepancy with the unburnt petrol washing oil off the bores prior to testing at the moment so any correction of running and firing reliably should give you a better look at the compressions after a decent runout.
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Here you go.
Hope it's all yours is .
Good luck
http://www.sohc.co.uk/index.php/topic,13165.0.html
Thanks. Interesting read with several points for me to check.
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Thanks for all the replies and suggestions.
All new NGK plug caps which all give a reading of around 5K ohms
Secondary coil resistances both around 15K ohms. Have not tested primary resistances yet.
My initial worry was that it was a guide seal or piston/ring problem when I first looked at the No 2 plug, 2nd photo. But at this stage that plug had done around 300 miles and around 30/40 miles not firing. Following the plug change it did smoke but cleared up after 10/20 miles before fouling again at around 30 miles.
The photo of the No 2 plug this time shows a less oily deposits and more black carbon as it was firing intermittently so at this stage I am still at a bit of a loss as to the likely cause?
Will check the primary resistance of the coils and then swap the N0 2 and 3 ht plugs/leads over and see if the transfers the problem?
Also need to check the valve clearances as they may be a bit tight on No 2 and 3 resulting the slightly lower compression?
As previously advised all the main and pilot jets are clear. Yes I did have the throttles fully open and the tests were done with the engine up to full operating temp, still got the bl**dy burns trying to screw the hoses into the plug holes!
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Check your points as well as they are common on 2 and 3, 1 and 4.
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Check your points as well as they are common on 2 and 3, 1 and 4.
Same for condensers
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The bike has electronic ignition fitted, so points and condensers are out of picture.
I have receipts showing that gen head gasket was used.
I note that main jets are 110 and pilots 40. All air screws around 1/2 turn out? UK K2.
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Primary resistance of coil both 4.8 ohms.
Swapped the HT leads of No's 2 and 3 over and did around 50 miles. The black sooty plug has now moved No 3, see photos, No 4 on left. So is that indicating that there is something wrong with that particular ht lead?
With No 2 now showing a more normal colour can I now rule out seal or piston/ring as being the cause?
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Start it tonight in a real dark place and have a good look at that HT lead to see if it's tracking out to earth anywhere, it's one of the quickest ways to find a leak in HT systems. You may have to take the tank off to get a good view of the first parts of the leads.
Sounds like progress though as you've got it to move around and so giving you a direction.
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A further test is to swap the leads back and just swap the HT caps. This test will show whether it's the HT leads or the caps that's causing it. Any excuse for a rideout.
"I'm just going for another test ride dear, see you later"
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Thanks for the replies and suggestions.
Will follow them up and see what it brings.
Andy
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An update from my previous post.
As previously advise the bike is fitted with electronic ignition, so no points or condensers.
Fitted replacement pattern ignition coils.
The carbs have been ultrasonically cleaned and the needles dropped by going from fourth to third groove and the air screw initially approx 1 turn out.
All floats levels at 26mm. Pilots: #40 and mains: #110
Did around 50 mile run and removed plugs. No's 1 & 4 very lean and No's 2 & 3 OK?
No 1 air screw now around 1/2 turn out and still fairly light No 4 air screw around 1/4 turn out and still light?
No apparent leaks on any of the carb rubbers.
Confused!
So what to do?
1. They are OK leave alone?
2. Go back to the 4th grove for No's 1 & 4?
3 Go up a size on the pilot jets for No's 1 and 4?
4. Raise float levels on No's 1 & 4?
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Two and three don't look bad at all.
I'd lift the needles on one and four so that they are richer, then run it again for similar distance to see what you've got.
It may not be correct to run differing needle heights but in the interim it would be safer to make sure those two aren't going too lean while you consider what else to do.