82 GS400 8 valve, poor running/excessive blow by

[Guest]

Hi all, I have an 82 GS400 twin with the 8 valve head, a Canadian model (I believe that makes it a GSX400)

This bike was given to me and I have no history on it. I've replaced the carb boots and went through the carbs twice. Its hard to start but once it gets going it idles on its own.

When it's idling, it has a lot of blow by, by that I mean white/blue smoke coming from the breather port on top of the engine, and the dipstick hole if I leave it off. Like, an excessive amount, enough to make you think your battery positive is shorting out and melting. Absolutely no smoke from either exhaust pipe, and no evidence of oil burning on the plugs.

I haven't done a compression test because I don't have an adapter to fit this head (in the process of making one) but by placing my finger over the hole while cranking they both feel even.

I'm just looking for advice on where to look next. Oil isn't overfull. Could it possibly be a bad head gasket or the seals around the head studs, or both?

Cheers, David

 

[GregT]

They wear the ring grooves. I pulled one down and found excessive side clearance on the rings - about .012in from memory.
This makes the rings - particularly the second ring - act as pumps pulling oil up into the combustion chamber.

A rebore is usually needed when it gets to this point. The one I did was a race engine so I went bigger with GSX750 pistons.
I believe normal oversize pistons are available.

Pulling it down and having a look at the bores and pistons is going to be your only way to tell what's happened.

 

[Guest]

Thank you for the quick reply.

I just finished making my M14-M12 adapter. Left side cylinder 65 PSI, right cylinder 145 PSI.

I'm just about to start pulling the top end apart

 

[Guest]

Top end is apart, the low cylinder has a broken piston, chunking from in between the rings


The bore looks ok save the average wear you get, no big scores or anything. I think I can get away with the 1st oversize. Are these bores Nika-sil?

You mentioned GSX 750 pistons, I admit I'm a little intrigued, but I think I'll stick with stock.

Cheers

 

[GregT]

That'd certainly do it. They appear to have a weakness in the pistons. The 450 with the same stroke even when revved to 12 grand has no problems with the ring lands. But the 8V 400's do.
Measure it up and see if you can find the next oversize pistons.

 

[Nessism]

There was some other guy here that had the same problem on his 400. Can't remember what he wound up doing though.

 

[Guest]

That'd certainly do it. They appear to have a weakness in the pistons. The 450 with the same stroke even when revved to 12 grand has no problems with the ring lands. But the 8V 400's do.
Measure it up and see if you can find the next oversize pistons.

I suspect that it is a problem with detonation, or rather, persistent pinging. These motors ping pretty badly on anything less than 91 octane, especially when the head gets hot - which is inevitable given the comparative lack of finning and 10:1 compression. Hence the ring land wear and fracture problems. I may be wrong, but the general piston construction doesn't seem different enough to be problematic. ??

The easiest cure is to go to a 450 topend. Or GS 500...

 

[Guest]

Can the 8 valve head be used with the 450 jug?

I measured mine and they are still standard size. If I could find a standard piston I think I could get away with a hone but no luck. I found 1st oversize pistons on Megazip, and the rings to match on eBay. Megazip also has all of the gaskets I need.

I see the 750 pistons are the same bore but different part number, perhaps different pin size?

 

[GregT]

Can the 8 valve head be used with the 450 jug?

I measured mine and they are still standard size. If I could find a standard piston I think I could get away with a hone but no luck. I found 1st oversize pistons on Megazip, and the rings to match on eBay. Megazip also has all of the gaskets I need.

I see the 750 pistons are the same bore but different part number, perhaps different pin size?

Not entirely sure about the 450 barrel swapping on. I suspect the camchain tensioner positioning may be different. Height may differ too.
The 750 pistons have the same pin size - but are approx 1.2mm higher to the crown edge. I used a hand made aluminium spacer under the barrel.

 

[Guest]

I see, well that's fine. I think I'll stick with stock and run the high octane fuel. I run 91 in my high compression DR350.

 

[Guest]

Looking closer at it, the bad cylinder (left) had white exhaust valves, and the mixture screw on the carb was 2 turns further out than the right carb. The left carb was also gummier than the right carb.

So I suspect the left cylinder was leaning out for some time, and coupled with possibly running low octane fuel, could have pinged that one to death

 

[Guest]

Looking closer at it, the bad cylinder (left) had white exhaust valves, and the mixture screw on the carb was 2 turns further out than the right carb. The left carb was also gummier than the right carb.

So I suspect the left cylinder was leaning out for some time, and coupled with possibly running low octane fuel, could have pinged that one to death

Those carbs were set up pretty lean because a 4 valve can usually tolerate leaner mixtures, but that just adds to the temperature and ping woes.

Before you get too far in, are your sparkplug holes cracked over to the exhaust valves? Drumroll....

 

[Guest]

I didn't see anything initially, but when I get back home I'll wire brush all the carbon off and have a closer look...

 

[GregT]

Before you get too far in, are your sparkplug holes cracked over to the exhaust valves? Drumroll....

Are we giving odds ? Drumroll continues.........

 

[Guest]

Are we giving odds ? Drumroll continues.........

I have a spare GSX 400 motor that was senior [85 year old] owned and seemed quite nice when I was running it. Someday, I will pull the topend off and put on a 450 one which I have in waiting, but given how well my current motor runs, my motivation to build it up has been low. I'd pull the topend off and sell it but it's doing such a great job of keeping the spiders out of the crankcase I'm in no rush. Besides, the 450 swap is a better long term bet. OTOH it would be lighter to lug down the stairs to my workshop if there was less there....

 

[Guest]

Good call guys, both sides cracked from spark plugs hole to exhaust valves.

So now what, I know of another 8 valve 400 parts bike, $150 but unknown condition. Anyone in BC have a 450 top end?

 

[GregT]

Good call guys, both sides cracked from spark plugs hole to exhaust valves.

So now what, I know of another 8 valve 400 parts bike, $150 but unknown condition. Anyone in BC have a 450 top end?

Before you go too deep into the top end swap, see what the condition of the lower end on yours is. Blowby can do nasty things to oil.

 

[Guest]

Well, when it was running it was quiet, I can't feel any play in the rods, and odometer says about 30,000 kms, so I'm willing to chance the bottom end.

Looking at part numbers and the base and head gaskets are the same between the 8 valve 400 and the 450. It's hard to say without having both of the heads and jugs beside me, but it might be possible to bore my jug for the 450 pistons and use the 450 head, for some money savings.

I know I sound cheap, but it's a free motorcycle and I'm not sure how much I wanna put into it...

 

[Gorminrider]

The GSX 400s are a lot more common than 450s in Canada. I think Canada only got 450s for 1 year, then Everywhere got the 500 twin for a long long run of years...(it's the successor to the 450)

I'd stick to the gsx400 myself- it's not a BAD engine. Mine hum along great.

 

[Guest]

Good call guys, both sides cracked from spark plugs hole to exhaust valves.

So now what, I know of another 8 valve 400 parts bike, $150 but unknown condition. Anyone in BC have a 450 top end?

Abbotsford is only a bicycle ride from the US. I got a 450 topend from Oregon a few years ago for cheap. Prices can be highly variable; I got jugs and pistons for $40 but paid $120 for a complete head, and with exchange it was still about $200 CDN. GSX head gaskets are scarce, as Suzuki Canada didn't seem to have any last I checked. 450 gaskets are still around.

I suspect that all the 8 valves will by now have at least the beginnings of plug hole cracks; the question is whether they will become serious enough to matter. 450 heads run a lot cooler and seem unfussy about fuel. More midrange torque and about the same power at 9,000. Because they are rugged, used parts in useable condition are still out there on Fleabay, last I looked.

What is your bike like? Condition, model, etc. Did you search it out or did it fall into your lap?