GSX400 to GS450 conversion

[John Park]

After another few tankfuls I decided to raise the needles one more notch. It was still running somewhat lean and surging in the midrange until really warm. I know that if it runs lean on ethanol free 94 it will be worse on 'swill' , and one of the reasons for doing the swap was to make a more fuel tolerant engine, so I raised it one more. This involved moving the big spacer to the bottom, with the clip now in the second lowest groove and the small washer on top. The needle went from 37mm projection to 36mm.

As T Kent pointed out, running too lean can result, counterintuitively, in worse gas mileage. Lower emissions, possibly, but there is an efficiency of combustion speed and completeness to consider. Regardless, it runs even better and smoother now.

I think the swap costs about 10% in fuel economy, but I can live with that.

Just bought another 450 head, and plan a GSX400 to GS500 conversion for next winter's amusement. Always wanted a 500….

[John Park]

Another thing to consider is that the GSX400 runs 20 degrees advance at idle and the 450 is 10 degrees. Just changed mine over to a 450 advancer, and it's definitely worth doing. The idle speed went UP, which shows that the advance was too much. Its happier at idle now and starts more gracefully. Quite why the GSX is 20 degrees at idle is beyond me, as it always felt too advanced was well, but the combustion chambers and cam profiles are so different that who knows.

[Raffspeed]

Hiya,
I realise it's been a while since you put anything in here but I'm in need of some advice. I recently picked up a 450 engine and frame for not very much. The seller actually thought it was a gsx400 but going by the numbers it's definitely a 450. I've been doing some research on swapping the head for a 4 valve version but then I came across your thread where it seems you've gone the other way. Could I possibly bother you for some advice on the pros and cons of going either way??

Cheers mate

[John Park]

Hiya,
I realise it's been a while since you put anything in here but I'm in need of some advice. I recently picked up a 450 engine and frame for not very much. The seller actually thought it was a gsx400 but going by the numbers it's definitely a 450. I've been doing some research on swapping the head for a 4 valve version but then I came across your thread where it seems you've gone the other way. Could I possibly bother you for some advice on the pros and cons of going either way??

Cheers mate

The 4 valve heads are on the edge thermally at 400cc. A stock 450 has more torque and the same top end hp. Just use 500cc pistons and cylinders and up your main jets a bit. You should get about 10% more midrange torque and close to 50 hp. That said, if you aren't good at building up engines you may not get the full potential.

Greg T [here on the forum] did a bored out 400X motor for a race bike, but it wasn't straightforward and the pistons were almost impossible to get. So just punch it out to a stock GS500 and then work from that; easy parts and good cooling and tough and reliable to start with.

[John Park]

One more entry to this thread for anyone who wants to do it. I took the front chain guide out to see how it was doing and also adjust the valves. Because the 450 head is taller, the guide doesn't sit as far into the block and it looks like the chain has been biting it a bit at the bottom.

I didn't have a 450 one but did have an old 400 roller bearing motor one which is the right length. I had to sand a bit of a ridge off the very top where it hit the valve cover gasket, but otherwise it dropped right in. It's now missing a funny little whir noise that's been annoying me for the last year.

So, the 400X front guide will work, sort of, but given that 450 guides are cheap and available you would be better off to use the right part.