Jetting for Jemco 3 into 3 expansions?

[apoole6]

Hey guys so im getting ready to order some Jemco 3 into 3 pipes for my gt380 and I was wondering if anyone else on here has them, what is the jetting used with them? Jon from Jemco told me 2-3 sizes up.

[Craig380]

Will you still be running the stock factory air filter and filter box? And is the motor stock?

If so, I would start with the standard jetting and see how it goes. I fitted a set of period J&Rs to my 380 a few years back (everything else stock) and it actually runs a shade on the rich side, so I left the jetting stock and just played with the pilot air screws to get the off-idle running right - a quarter turn got them dialled in. Of course, different pipes have different effects, so your mileage may vary, etc.

I do tend to ride the bike quite hard, so I'm comfortable with it being a little rich. I just wanted to pep the bike up a bit and have the wonderful sound of a triple running chambers but without affecting reliability. I'm sure I could get more power by changing things on the intake side but much as I like tinkering, I like riding better.

If you retain the stock air filter & box, the air/fuel ratios can't get too lean as the intake restrictions are as per the factory, so just take it easy to start with, and keep checking the plugs until you're confident the carb settings are right.

Of course, I'm assuming that you'll only be using the bike on the road and not racing. If you're looking for ultimate power, then it's a different ballgame

[Vintageman]

I run those pipes on a 74 gt380. Airbox all stock and new oiled filters. I did not change my jetting for a long time. I must have a few thousand miles like that. Have driven for hours at a time. I never stay WOT for too long, but have WOT for sure and highway driving. Last year i did up the pilot jet one size. My gt380 was on the lean side running small throttle cruising via plug color. It may not hurt to go up on mains. 3 sizes? going by 2.5 each per step may be a lot ... maybe not. It is a decison you need to make... every bike a little different and how you drive. Also temp and altitude always a factor. I use this bike mostly on warmer days and under 2500 ft.

My bike may be lean or just right, It is one of the best tuned ones I have in my opinion. You are going to love those pipes!

Another tip that may work for you: Run fine tip spark plugs like Iridiums . Another 1000 RPM pull. Not always do Iridiums do that for me but, did on that bike

[tz375]

The 380 and 750 pipe designs may be different, but on a 750, J&R have a large diameter stinger and then a very restrictive muffler design. JEMCo have a slightly smaller stinger and appropriately sized muffler section. JEMCO on a 750 with pods run very lean and need to go up on the mains and needle.

I'd speculate that if the 380 design is similar, you will probably want to start off with main jets 2-4 sizes larger and adjust the rest when teh top end is right.

[Craig380]

The stinger assembly on the set of J&Rs I've got is a straight-through perforated tube (approx 25mm diameter and maybe 200mm long) which can be wrapped with wadding. This engages with an internal, removable collar inside the pipe. The collar has a spring behind it, which sits on the end of the convergent cone. It's all held together by the end cap and a big circlip.

[tz375]

That's interesting. On J&R 750 pipes, there's an internal stinger 28mm ID which ends just after the last cone. Then there's basically a 25mm gap followed by a spring loaded perforated tube which is only 22mm bore. It sounds as if they used the same perf core in small and big pipes. On a 750 that section is restrictive but on a 380 it's probably not too bad.

There's a 750 J&R cut up on Ian's Field Guide.

[apoole6]

yep filters are stock, porting, standard bore, stock jetting. I'm also getting a non nickel plated set of pipes cause its cheaper so idk if powdercoating them is good or something else?


But yes settings are stock.

[Craig380]

That's interesting. On J&R 750 pipes, there's an internal stinger 28mm ID which ends just after the last cone. Then there's basically a 25mm gap followed by a spring loaded perforated tube which is only 22mm bore. It sounds as if they used the same perf core in small and big pipes. On a 750 that section is restrictive but on a 380 it's probably not too bad.

There's a 750 J&R cut up on Ian's Field Guide.

I couldn't find the the J&R pic, but from my own explorations, there is an internal welded-in ring of approx 25mm ID just after the cone.

It doesn't look very thick, however, it seems to be more of a bracing point at the end of the last cone, to support the external chromed end-section that the baffle tube sits in, and to be the base on which the internal spring collar sits. The baffle tube does not go right into the welded-in ring, there is a small gap there.

I'll have to get my measuring stick out

[tz375]

Craig, the pictures are on this page http://www.oldjapanesebikes.com/GT750_f ... -A55#pipes" onclick="window.open(this.href);return false;

Scroll down to J&R and click on the picture. "Behind" that one are some others including a 750 pipe I cut open. It is as you deccribe it. There's a flat plate to act as a spring seat to tension the muffler section. The spring itself looks like it was cut from a car coil over - heavy and thick.

[Craig380]

Aha, now I see, thanks for that

Mine's a little different to the pictures in that there's no solid stinger tube. The convergent cone simply meets that internal ring. The only 'stinger' is the removable, perforated baffle tube, which is under spring tension from the removable collar. The tube has no flutes or anything, it's 100% straight through.

Of course, I've no idea if the internals were like that when they left the factory, but all the parts look mass-produced

I've never checked the actual pipe dimensions and cone angles against any software. I'm sure they are far from ideal, but they sure do work very well on the road

[tz375]

Their design is interesting on the 750 in that the stinger is basically all inside the chamber. When you look in from the rear end of teh pipe with the baffle tube removed, can you tell if there's a stinger inside that last cone? It was hard to see on that 750 pipe which is one reason why I cut it open to take a peak inside.

At first it just looked like the end of the cone, and when I put a borescope inside, I realized there was a tube (stinger) hiding in there. None of this helps the OP to get his jetting right but it does add to the pool of knowledge.

Jemco also uses an internal stinger IIRC and the muffler/baffle tube slips over that.

[Craig380]

I have shone a light down the pipe before, with all of the removable stuff pulled out, and I couldn't see a stinger inside the chamber itself at that time ... but maybe I missed it.

I remember an article about chamber design which mentioned placing the stinger partially inside the final cone. If I recall it right, providing the inner end of the stinger is at least halfway along the length of the final cone, it doesn't affect performance or temperatures, but does significantly reduce the typical expansion chamber 'crackle' ...

I know Allspeeds use this stinger-in-the-chamber design, which is why they're so stubby relative to, say, Bassanis ...