Honda-SOHC

Quote from: Johnwebley on September 25, 2022, 02:34:35 PM

  Just to put another idea,

  if the oil was a modern synthetic,maybe the rings haven't bedded in?

  have you ridden it to gently?

  I believe you have to get pressure behind the rings ,to force them against the cylinder wall to bed them in,

 ideally firm acceleration to 6k revs,then shut the throttle,

 I think the idea is to pressure the rings on acceleration,then with a shut throttle it sucks oil mist up the bores to lubricate them,.

I have heard of modern engines using lots of oil because the new owner drives them to sedately to run them in.

you have well over 500 miles on the clock,with the mineral oil,start to work it?

 try running it up to 8k in 3rd then slamming the throttle shut,

  it can't hurt

 

 

Totally correct there John. At over 500 miles there should be no pussy footing around at that mileage. You stand more chance of polishing the bores, which will make it smoke, if the engine isn't put under load.

How I rode it was keeping between 5 and 6k preventing the engine labouring at all, up to 60mph in top and let off the throttle back on throttle etc so as not to have extended periods at constant rpm.

Then as I neared 500 miles I started going to 7 and 8k through the gears riding quickly and again not staying at a constant speed for too long which is pretty much where I'm at now except I'm changing up around 8 or 9k, haven't gone to the 10k red line yet.

Interestingly when I'm riding at 60mph or below 6000rpm the mirrors are clear. However get to 70 or 80mph 6500 to 8000rpm and I can see a fog lingering behind me on the road. 

So it's not burning any perceptible amount of oil below 6500rpm but above that it's really quite evident!

I'm going to check the level a day or two as I'm nearing 700 miles and see where it's at.



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"No idea what oil was in it but it came out very very black and thin so I suspect it was breaking down which wouldn't help."

Usually associated with running too rich, either from carbs being altered to excess or more normally far too much running with engine cold/choked to allow excess fuelling past the piston rings into the oil. The giveaway is oil smell (compare old to new) as it will generally stink of unburnt fuel.

The gearbox doesn't colour or dilute the oil in use but often it's pointed to as culprit in these scenario. Honda service interval is an acknowledgement of overall fuelling strategy which ultimately comes from the necessity to run slide carbs too rich ideally for a decent longer oil term, essentially it will dilute the oil and reduce viscosity.

Sensible to start with new oil and monitor condition, see what it looks like at 500 and 1000 miles etc.

With short runs and often on choke though you'd expect the oil to be knackered by about 500 and well short of Honda service specification.

Thanks for this really useful information K2-K6.

I couldn't believe how black the oil was considering its only done 500 miles. I'm used to only seeing that on diesel engines.

Have been starting the bike with a small amount of choke and running for 2 to 3 mins on the drive at a high idle of about 2-2.5k rpm.

After this I can knock the choke off and the bike will idle perfectly at 1200 rpm. Then I set off.

Is this excessive? I'm not one for constantly adjusting the idle speed screw, I like to set it when the bike is hot and leave it alone.

I'm pretty sure mixture when on throttle is spot on but I might whip the plugs out in a layby after a full throttle run to check.



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Surprised Julie didn't say yes or no for Graham or maybe that would be a bit cheeky, I hate it when my missus volunteers me for jobs.

Yep I know that feeling!




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So having just giving it a good thrashing home and got it nice and hot, unfortunately I can see now that it is smoking a bit looks like cylinders 3 and four.

I'm going to look at just the head first, assuming that the bottom end has actually been done properly as I'm told it has.

Hey ho the joys of old machines!



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..... especially when I can't see any smoke at all!

The old Mini A series 1275 S engine could burn a quart of oil in 200 miles without a trace of blue smoke!

I would definitely wait & see how much oil you use in the next 200 miles before doing antything hasty.
[/quote]Yep will put some more miles on it and then see where I'm at.

Thanks chaps



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Quote from: 350Simon on September 14, 2022, 11:10:37 AM

What would be considered normal for a 350/400 sohc?

I've done 500 miles of running in new pistons and rings, the bores were properly machined and honed and I know that my valve guides are on the upper end of the serviceable limit.

Oil level is just above the min mark on dipstick after 500 miles. Is this acceptable or should I really get the head reconditioned?

I'll be honest I can't detect any smoke from the exhausts in my mirrors while riding or at idle but my friend said the other day there's a faint haze when I'm giving it the beans.

Cheers
Simon

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Did you replace the 4 valve stem seals?

It has been my experience that when valve stem seals need replacing on a car engine then you tend to get visible smoke if the engine stands for a while from hot  and you then restart it 5 mins later - the smoke is often just for the first 5-10 seconds then all is good. Again on cars I have found giving it the beans also burns oil & produces smoke at high engine rpm.

I would not be too concerned until you carry out your first oil & filter change then see how it goes on the next 500 miles making sure you check the oil on level ground etc.
I would also check  if there a lot of excess pressure from the crankcase vent pipe system.

I didn't do any of the work I bought the bike as is. It's this one that was featured in this you tube video where the guy had real issues identifying the cause of smoke that turned out to be a loose guide in the head. He said the offending guide was replaced but the other 3 remain. All the stem seals were replaced. All 4 exhaust valves were replaced which brought the stem to guide clearance just within the serviceable limit.

https://youtu.be/YM15Gijr6Ag

It seems like a lot of oil to use in 500 miles, through the guides..... especially when I can't see any smoke at all! There's also no oil leaks anywhere.

Perhaps it isn't actually bedded in yet as someone said above.

Next question would be who could properly sort the head out for me?

Cheers

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As mentioned above, flexoid paper from ebay is the best, its oil and water resistant and comes in various thicknesses up to about 2mm. Years ago I bought some leather punches, 3mm up to 19mm diameter for all the holes, and to cut the gaskets out you can't beat swiss army knife scissors.

The floats and float valves will do their job irrespective of what position the fuel tap is in, the fuel can only pass down through the same delivery pipe.  Is there a chance that the tap is actually leaking in that position, and gravity is allowing the fuel to run down the outside of the pipe, running over the outside of the carbs, and following the easiest path downwards which would be via the overflow pipes.  Make sure everything is bone dry, kneel down next to the bike, then using a torch,(NOT A MATCH!!), turn on the fuel and see if the outside of the delivery pipe becomes wet and shiny. Have fun, we love this hobby don't we?  Late PS:-  I meant to say that it really does sound as though the tap has been wrongly assembled. There's an exploded parts diagram on the CMSNL website, might possibly help.

This seems like the most plausible answer to this conundrum. I shall try tonight when I get home and see what happens, a leak at the tap crossed my mind but I just imagined it would drip down onto the crank cases and so dismissed the idea but as you say the fuel may well be flowing down the outside of the feed pipe!

Ta