1980 GN400 jet sizes

[Loops]

Hi All,

I'm new to this forum but I do float around on the Triumph Tiger forum every now and then.

I acquired a couple of Canadian import Suzi GN400, made a good one out of the parts (and need to sell it but that not the point of this message).

The second bike I customized to look like an Old Brit Trials bike (well to my eyes anyway).  Spoked wheels 21", 18", knobblies, solo seat with springs, aluminum guards, braced bars.  K&N and a BSA B33 'silencer' - trade descriptions might argue about that!

My question to the Thumper collective is anyone have an idea what jet size would be in the right ball-park ?

Other than pipe and air filter engine and carb are standard.  Original main jet was 132.5.  I've tried 140, 150 & 160 but the old girl won't pull to the red line in 3rd, 4th or 5th and the plug indicates very lean (as I'd expect).

How much bigger do I need to go?

Take care out there

Loops

99 Tiger
81 GN400
80 GN400
99 125
48 Douglas 350 (on it's way)

[squirrelciv]

Hi matey, I come as the bearer of bad news I'm afraid :-) If you haven't got the standard airbox you will never get a GN400 to run well. Trust me I know, I built a special (Beryl where are you) and had no end of problems. The thing is, the standard carb has a metal slide which is just too heavy for fine tuning once you cock about with the vacumn (spell?) Took mine to a Dyno man in the end and he couldn't get it to run well either. The answer is bin the carb or keep the airbox. I fitted the carb from a DR350 which we managed to jet OK and the bike ran well.

Best of luck with it though and hope I've been some help :-)

[Loops]

Thanks for the input - I think I won't waste time on the GN cv carb, i've got a DR400 slide carb somewhere in the garage which is probably an easier proposition.

I got that carb to put on my SP400 for more power but sold the bike before fitting it.  Kept the carb though, must be 20+ years since I got it.  Give it a good clean before I use it.

I'll let you know how it goes.........may be a few weeks beofre I get round to doing it though.

Cheers

Loops

[squirrelciv]

Best of luck, and keep us posted.

[cloggy]

Sorry To disagree but I ran a SP400 for several years with the vacuum carb and a no airbox.
I got a jeweler mate to drill out the main jet and by chance he got it perfect first time
I had the inlet weir ground off and a Phil Joy Protec copy road and track cam with Ford Cosworth Cortina springs and Protec  alloy valve keepers and a straight through exhaust to a supertrapp.
The carb is sitting in my garage.
I eventually put an 34mm amal on but that was just touch rich off idle, couldn't get a small enough cutaway
If you are using a standard motor with the DR carb[or SP370] you'll have to jet DOWN two sizes on the mainjet or it'll run rich. Add the above mods and I had to drop the needle as far as it would go too. And that's running  a DR400 with the same exhaust and keeping the airbox but ditching the snorkel.
Does anyone Know if Phil joy is still in business?
I want another cam

[squirrelciv]

Cloggy, is the SP card the same as the GN??? The problem with the GN carb was the wieght of the metal slide. Did the SP have a plastic one?

Either ways, sounds like a lot of work you did there to get it to run. When I popped the DR carb on (still a cv type but with plastic slide BTW) had it running very well inside 1/2 hour.

[guest7]

Can we have some piccies of this trials thumper please? :-)

Cheers
GC

[cloggy]

Originally I just threw the airbox  and exhaust away, well I say originally but it was a long time ago and it's more likely that I just replaced [or not] things as they fell off.
So for a long time I just ran a standard engine without the airbox [ but with a really big foam and mesh K&N] and with a straight through pipe and supertrapp.
When I came to lightly modding the motor I found I had to drill the main jet way bigger, ridiculously bigger, I think it may have been 1mm.
Anyway it all ran really well with no flatspots; but you don't get much instant power with a vacuum carb so I shoved on a 34mm Anal orifice which even with the smallest cutaway ran a tiny touch rich straight off idle, but fine for the road. And yes the SPand GN carbs are identical, possibly in all respects except the main jet.
The SP370 carb is pretty much identical to the DR400 carb apart from being 1mm smaller at 32mm[I've got one of each], not sure what this refers to as the venturi isn't round on these carbs

[guest27]

To concurr with GC - can we see some pikkies please - like a good picture me.

R

[cloggy]

No you can't have any pics and the GN and SP carbs are identical in all respects excepting the main jet and the starter jet

[guest27]

Awww go on - give us some pikkies to look at whilst doing nowt

R