Oil pressure

[johnmc]

Hi everyone.
I'm trying to figure out why I am losing oil pressure when My 81 gsx 400 f heat's up
. I rebuilt the engine a couple of years ago and did everything as the manual stated . I even went up an oil grade from 10w-40 to 20-50 . It's better but the pressure is still low. Could the engine be over heating and causing an issue with the oil . Any help and advice would be greatly appreciated.
John
Sydney Australia

[Brendan W]

How are you measuring the oil pressure?

[johnmc]

At the front of the oil filter cover where the nut is .

[Big T]

GS motors are low pressure. What is the pressure you're seeing?

[johnmc]

12 psi / 82 kPa when it should be above 250 kPa .

[Grimly]

The four-cylinder GSX400F was notorious for wrecking crankshafts because of oil pump failure and bearing wear.
I swapped a GS550 lump into one because of that.

[johnmc]

I put a new oil pump in when I rebuilt the engine. I might drop the sump cover and have a look to see there is a bearing has let loose.

[bitzz]

GS motors are low pressure. What is the pressure you're seeing?

The GS motors with roller bearing bottom ends are low pressure. They don't NEED pressure, just flow.
The GS (x) motors with "shell" bottom end need the same oil pressure as any "shell" bottom end.
You WANT 30psi at idle, and it should increase 10psi per 1000rpm, to a max of around 80psi.

[Nessism]

The plain bearing 2nd gen 750 engine had oil pressure issues which wiped out heads and cranks. The fix was to shim the oil pressure relief spring. It wouldn't hurt to try on your bike to see what happens.

[Grimly]

The plain bearing 2nd gen 750 engine had oil pressure issues which wiped out heads and cranks. The fix was to shim the oil pressure relief spring. It wouldn't hurt to try on your bike to see what happens.

Good point, it might be possible to forestall any further damage, but if the running (hot) pressure has dropped off so much it's just buying time until rebuild.

[johnmc]

Thanks for the advice everyone. I have parked the bike and have decided to pull the engine apart to investigate what the issue is. Going from that I may do a bottom end rebuild or a complete rebuild. Thanks again,.
john
Sydney Australia

[johnmc]

Hi everyone
on Sunday I pulled out the engine and after removing the head and barrels I found the problem. Number 2 conrod has a lot of play in it . Something has happened to the bearing causing the low oil pressure. Probably me not checking everything twice or three times before installing. So now a full lower end rebuild.

[GregT]

If it's done no 2 rod bearing, check the shaft for round - must be less than .0005in out of round - and it's worth getting it crack tested too.

[johnmc]

Thanks you Greg t . Mate if the crank is out of roundness can it be machined or try to get another one to replace it.

[GregT]

Once upon a time Suzuki offered undersize shells for the first gen plain bearing 750. The only model they ever did this for. If it is out of round it might be worth scouring the internet for NOS undersize bearings. From memory rod shells came only in .010 and .015in undersizes. Mains went down to .020in undersize.
No idea of your chances of finding any now.
In Oz, an outfit in Queensland used to offer a shaft grinding service - and they'd sourced shells from somewhere too. Not just for Suzukis, they reckoned they could do anything.
Might have been Serco.