Big bore GS(X) 400

[MuzzaB]

Caught up with Greg on Friday, it didn't take too much to get it running pretty sweet. We enlarged the pilot and increased the main to 135. Greg took it for a spin and did a plug chop. He said it had good acceleration and the plug looked good as well.

On Saturday I took it to Ruapuna for a CAMS club day. I took it out in the practice session and was really happy with how it went, it has far more grunt than it ever has.
Which is going to take a bit of getting used to, Ruapuna B-Track is pretty short, but because the bike gets off its arse a lot quicker I'm approaching corners faster than I'm used to. Which (as Greg suggested) is showing up the limitations of the brakes. The only place the bike is not happy is when you hold the throttle half open, it feels like it is running like crap, but as soon sa you open the throttle it takes off like a rocket. I tried raising the needle but it didn't make any difference. Unfortunately I crashed my bucket racer in the next race and that knocked the stuffing out of me for a bit. I took the GSX out again later in the day and did two races back to back, the bike has had a slightly leaky tappet cover for a bit (the gasket is not the right one for the model and has had a repair before), when I came back in there was oil over my boot and that was it for the day as I wasn't wanting to go down again. I spoke to Chris who has the GS450(500) with VM32's, he said his bike does the same thing at half throttle, so it may be that that's the way it is. Once I get used to it it will be fine.

Cheers
Muzza

[GregT]

I note both yours and chris's have Sagar pipes. My 450 did too but i altered it. Ted Sagar was pretty good at header length and diameter but past that they're whatever fitted in. I've already suggested to chris he makes up a "proper" tailpipe and muffler. The extra length is well known to smooth out exhaust pulse activity which can be a problem with big, short tailpipes.
For the benefit of those interested, Sagar pipes were made in ChCh NZ in the late 1980's. No longer available. Characterised by a huge tailpipe with a baffle included.

[Chuck78]

Do some plug chops at around half throttle (whatever position makes it stumble the most) from 3800-8500rpm, or if it is really sluggish and that takes a while, maybe 4500-8500. I'd suspect you need a larger diameter needle jet. Might be able to get by with a skinnier more rapid tapering needle if you can track down the right size. Look for that German jet needle chart I may have posted a link to. Or just find what series needle jets those vm's take, and go up or down in size (the o-2, n-1, o-6, etc size notations) from there. I'd suspect you're lean, but an AFR gauge or plug chops will tell you.

Good luck fellas. Sorry to hear about the crash, glad you're not too banged up.

[MuzzaB]

Cheers Chuck,

Not too banged up, just a few bruises here and there, need to repair my leathers though, took one knee out down to the last layer.

I am interested to do a half throttle plug chop to have a look. Rolling the throttle open from closed to full throttle it just goes and feels great. I tried going round the corners in different gears, I wasn't really looking at the rev counter, whether it was revving more or less didn't matter. It was where you were holding the throttle steady before powering on out of the corner that it was running badly/oddly. As soon as you opened the throttle more it willingly takes off. It is not actually much of a problem, it's just different, once it becomes "normal" I won't even think about it.

[pete]

What carbs do you have on it Muzza? Are they the stock ones? I know the stock ones on the 450's are renowned for a very on/off throttle response down low and I can attest to that.

If it has a needle like my stock ones (4C3), it resembles a piece of wire rather than a truly tapered needle and alternatives pretty much don't exist I'm told...

[Chuck78]

He has new aftermarket VM34's that were set up for XS650 Yamaha's

[pete]

Perhaps I should open my eyes and read a bit more in the thread before posting in future... I recall that from earlier in the thread now...

[MuzzaB]

A bit of an update, got a new valve cover gasket from the UK, it was advertised as for a 1981 GSX400ex, but once again it had holes where it didn't need them and was missing some it needed. But I fitted it with liberal lashings of silicone and it appears to be oil tight.
I raced in a street race on the weekend and the bike went really well. I'm getting to grips with what is required when twisting the throttle.
Twist it too quick and it goes OK but fells like you are waiting for the rush, twist it on progressively and it's great, the pull out of tight corners is awesome. You need to ride it like there is a string attached to the rev counter pulling the needle around if that makes any sense. I'd be suspicious that I didn't use full throttle, but the bike revs out through the gears using up to half maybe 3//4 throttle.
The track is short and I'd only get to flat out in fourth, or just into fifth gear on the main straight.
I got third in my class, Pre82 Junior (up to 600cc), so was really pleased with that. The bike didn't miss a beat all day.
GSX Action Methven.jpg

[MuzzaB]

Trophy.jpg

Silverwear

Here's a link to some footage. http://www.ashburtontv.co.nz/cms/hom...nder-2015.html
I make a few appearances.