GS425 Oil Consumption. Problem?

[brit7.11]

I'm hoping someone can offer some input on this issue. I recently rebuilt my 79 GS425, as it was using oil and leaking from the head gasket (courtesy of an iffy rebuild whilst with a previous owner), and was lucky enough to find NOS +0.5mm pistons and rings.

The cylinders were bored by my local engine place, and I gave them the Suzuki manual page with clearances on it to follow. When I got it back I checked the piston/bore clearance (roughly, wiht feelers) at between 0.0015 and 0.002 so all seems well. Ring gaps were checked and ok.
Besides the rebore, I fitted new valves with the seats recut, and a new cam chain. Everything else was in good condition. I had the cases vapour blasted, then the crankcases, head and barrels Cerakoted.

Nothing unusual was noted on the rebuild, and it started easily, runs perfectly, idles well and is quiet. It pulls really well, better than before, and I've had it WFO in top up to 95mph sitting up and there is no trace of any smoke, either to myself or a following rider, eith on start-up, on overrun or under hard acceleration. I tested the compressions at 140/150 psi cold, and there are zero leaks.

I ran the bike in sensibly (I think) taking it easy-ish for a couple of hundred miles, but not labouring the engine and allowing to rev, then gradually increasing the load. Since then I have covered 1600 miles, which has included some hard, fast runs.

Apart from this oil use issue, all seems well. The bike goes from max to min on the sight glass every 300-500 miles. I'm topping it up with about 250ml to bring it back to max, and have done this 4 times or so. It seems to vary, but over time doesn't seem to be improving much, as I'd hoped it would.

I checked inside the airbox, and there was some oil coating the inside of the front plenum chamber, but not what I would call excessive.
The plugs look clean and dry. The oil I'm using is Millers 20W50 mineral.

Anyone got any thoughts?

Apart from the oil use, the bike's running brilliantly, and if there's no smoke, where's the oil going?

Thanks!

[Cipher]

Is there any plumbing on the bike that drains from the airbox?
mine engine feeds frank gasses into the airbox but the airbox also has a drain with a tapered rubber end that appears to contain foam filter media.
any oil wetting the underside of engine.

is 20w50 suitable for the ambient Temps of your locale?

[brit7.11]

The airbox has an open drain pipe from the bottom. I have noticed a drip or two from this, but again, not a huge amount.

20W50 is fine for here. I have thought about trying another brand of oil to rule out evaporative losses.

If oil is going through the breather system, does it not cause smoke? And why isn't the oil separator catching most of it?

[Nessism]

How do the spark plugs look? Are both cylinders burning, or just one? If just one, maybe the oil control ring spreader spring is overlapping?

Screenshot 2025-08-07 044936.png

[brit7.11]

I did some further checking today, after a 250 mile run yesterdayafter which i topped up 500ml(!!).
The attached pic shows the plugs. The left hand plug has significantly more deposits than the right. I hadn't noticed this before.

Also, I held a clean white cloth over the end of each pipe while revving the engine hard. The right pipe produced virtually nothing, while the left side produced a good black circle of oily soot. There is a tiny amount of black smoke visible, when revving hard at a standstill, but I'd associate that more with richness.

The compression test also show slightly less compression on the left, whether done hot or cold.

I hope it is something simple like an incorrectly fitted spreader ring, which woukd be easily fixed however i was extremely careful fitting the rings.
Either way, it seems I need to strip the top end again.

[Grimly]

I was using 500ml for every 300 or so miles before I did the top end, and there was not a trace of smoke. New rings and valve seals certainly fixed it, though. The reason it came apart was the base gasket leak which had become chronic. If it weren't for that, I'd have carried on using it.

[brit7.11]

I was using 500ml for every 300 or so miles before I did the top end, and there was not a trace of smoke.

Interesting. Thanks.

[oldGSfan]

Yes I'd be wary of the top end, valve guides, seals (if they have them). I have a Norton with no rubber valve seals, just cast iron (NOS replacement) guides, and the thing doesn't use a drop. I think having that done at a good shop, plus providing the pistons and rings (also NOS .040 over) to the shop doing the cylinder bore work was the trick.

I've also done some Japanese bikes and had mixed results, some re-work. It's a learning experience for sure.

[Cipher]

The airbox has an open drain pipe from the bottom. I have noticed a drip or two from this, but again, not a huge amount.

20W50 is fine for here. I have thought about trying another brand of oil to rule out evaporative losses.

If oil is going through the breather system, does it not cause smoke? And why isn't the oil separator catching most of it?

The fsm states 10w40 as an alternate.
I tried Rotella t6 once and my engine seemed to drink it. No noticeable smoke.
went back to t4 and issue disappeared.

I've used a couple other synthetics no issues either.

how many drips is 250 ml of oil with engine at operating Temps and road speed rpm?

[Cipher]

It looks like it has a vacuum petcock.
does the vacuum line attach to the left carb??
a slightly leakin petcock diaphragm enriching the left cyl slowly diluting oil making it vaporize.?
mebbe?

[thiagosilva8]

Sounds like a clean rebuild—if there’s no smoke, the oil might be entering through the crankcase breather under high RPM. You could try installing an inline breather catch can or adjusting your breather routing to reduce oil mist blow-by. Here's a blog on oil leak causes and fixes that might help with deeper insight.

[Grimly]

https://www.vikingbags.com/blogs/new...gine-oil-leaks on oil leak causes and fixes that might help with deeper insight.

Sure thing, spammer.

[brit7.11]

Thanks for the inputs.

Given the carbon deposits on the plug, and the black muck coming out of the LH pipe (although no smoke), I've concluded that I need to strip the top end. I've got the head off so far......barrels are being reluctant!

There's loads of carbon build-up in the LH combustion chamber and on the valves, versus very little on the RH. At 1600 miles since clean rebuild, clearly excessive.

Bores seem fine, no obvious damage.

Next step is to get the barrels off without breaking any fins!

[Nessism]

Be sure to check the valve guide seals. There are some very good Viton seals these days. UK guys seem to love Viton, so hopefully, you can find some in your marketplace.

Oh, and if you can get your hands on one, a bore gauge is useful to check the cylinders. No more than .001" out of round or taper allowed. For a newly bored cylinder, it should be less than half that. Boring cylinders is not easy, so your shop may have messed up. Best to check them out carefully.