G/T500 is stock air box best? Are K&N/Unifilter worse?

[Nicholas]

Hi all,
Just ordered a set of Jemco road pipes and was advised by the builder (John) that I should increase main and pilot jets. He also said to put some K&N pods on the bike so that it breathes better.

The jet increase I completely understand, however the K&N advice, is contra to what others who “know” G/T500 say.

My GT500 came with K&Ns, so I bought a used air box to bring it back to stock, but since ordering the Jemco pipes, perhaps stock isn’t always best?

I’d be keen hear from those with experience and knowledge in this area, is it the complications of tuning or the loss of performance?

I’ve got the stock VM32SC carb, standard GT500A.

My goal:Keep the power low, don’t turn it into a revving monster
I have spoken to a few racers and they think that the bike inherently maintains lower power, even with race pipes…

Many thanks for reading guys!

[Vintageman]

The Jemco’s will not make the later GT/T500 a rev monster. They do boost and you will feel a difference. They are kind of load pipes.

Stock air box, you should have to richen up jetting, pilot and mains to avoid pinging.

IMHO pods will make it harder to tune and too loud.

I can't say if they will add any performance versus stock air box. Theory says less resistance gives better intake charge delivery.

Pods themselves can cause weird resonances. Foam Uni may be better or longer K&Ns without metal back plates?.

Right before the Chambers kick in sometime the pipe boost pipe is off time and boost flows straight through to carbs causes a weird spot… a double dip of sorts or just kills intake charge.

The Air box dampens this undesirable affect for street. The Jemco s are long and have gentle cones so maybe no so bad

I would start stock air box and tune here. Then you can compare other ideas if you get bored.

I run full stock air box with the Jemco on a 75 T500, 35 pilots and 3 or 4 steps bigger on mains (forget now exactly).

Other Ideas
Remove the snorkel on back of stock for a little less resistance (gets loud).

Never seen anyone utilize Y boot on T 500 ( maybe early Y is rounder) and add the big single K&N/ UNI filer to it like RD fellows do (those guys go nuts with mains jet size after when compare to stock). They like the Y pipe versus individuals?

[Nicholas]

Hi thanks for the tips, CJS also mentioned the issue with the K&N caps and that the air box makes it easier to tune. I have a stock box, but it needs some work to clean and repair, so I would have preferred going with the pods as they would be easier to fit on the bike…but I’ll hold onto the air box for now. Someone mentioned water is an issue with pods…

I did think about using a single oval shaped filter too, but I am not experienced in this area…if you know of a good reading resource, that would be a good start for me.

When you say harder to tune, are the jets and needles all used in different phases? So I could build focus on the 1 to ¼ spot, then to ½ to full? Or does the pilot affect ½ to full? Not done this before, so I’ll be finding out soon!

WRT Carb
You mentioned increasing your main jets three to four…. I might update my order then as I went three sizes up on main and two on pilot…
Pilot (30 stock): 32.5 and 35
Main (97.5 stock): 100, 102.5 and 105.

I guess the pilot decision is good, but maybe I should do 102.5, 105 and 107.5 for the mains.

Thanks for sharing!

[titan performance]

Is your motor in stock trim on this bike Nick?
Do you intend to rag the motor mercilessly?
We all have different experiences, but if you are just adding the pipes and keeping the air box standard, I think jetting to 102.5 will be enough, 105 max....and it will run sweet on the stock 30 idle jet.
I ran a 500 on stock carbs with pipes for years, and it ran perfect, in fact it was probably the best 500 I have ever had. I melted pistons a couple of times when I got involved in long races with bigger bikes, but I could have ridden forever on the stock settings without prolonged thrashing. I never encountered too much pinging, and running on the very slightly side of lean, kept the motor very crisp and responsive.

[Nicholas]

Hey Paul, thanks for your thoughts.

Yes, the engine is stock and I don’t want to trash it. I want to be able to take it out on long tours with my mates who have BMW R80s….Need to show them that my bike is just as durable! :-/

I will take everyone’s advice and start with the air box and try the jetting with the following:
Main jets: 102.5, 105, 107.5
Pilot jets: 32.5 and 35

I don’t want to compromise reliability, so will possibly run it a touch rich to keep things nice and lubed. I really want to take care of it as when I am done I’ll have replaced every seal, ring and bearing in the engine. I am also rebuilding the carbs with all NOS parts, I was lucky to find a pair of 188 series needle jets!

So after all this, I am not intending to abuse it!

Actually, is there a good article out there on jetting a T500?
I am hoping to assemble the engine this weekend, just waiting on the barrels from my coater.

My final challenge is to fabricate oil lines, CJS recommended using PTFE, but I need to find an adhesive that can bond to PTFE or fabricate some fittings out of alloy…

[redline]

I'm still doing my first T500 build so don't have any real world experience, but just a couple of things I've read:

The bike Frank Whiteway won the IOM TT on was kept almost completely stock. They tried different filters but got the best performance with the stock airbox.

A. Graham Bell wrote a little about airboxes in the 2T tuning handbook. He said generally you'll get better overall performance with an airbox than with pods.

I imagine you'll have a slightly better top end with pods and if you ever take your bike to the salt flats, that's probably the way to go. However, if you want a more rideable street bike the airbox is the way to go.

[Nicholas]

I imagine you'll have a slightly better top end with pods and if you ever take your bike to the salt flats, that's probably the way to go. However, if you want a more rideable street bike the airbox is the way to go.

No, steet bike all the way, I'm no Bert Munroe!

But thanks for the vote on stock airbox. Seems like I made the right decision to get a used one, I'll bring it back to its glory this weekend. When you say the bike was stock, are you also referring to jets etc? Wondering if I have gone too large in my size selection. The stock is 97.5 and 30...and as I am replacing all the parts in my carb the smallest I have allowed is 102.5 and 32.5.... hope they are not too rich.

[Vintageman]

Nicholas

"harder to tune" What I mean is that VM Mikuni has 5 circuits pilot, slide cut, needle jet, jet needle and main (Have you seen Mikuni VM tuning guide?).

Lets say hypothetically with air box you have a 100 main and then you find you need a 200 main for WOT without air box, that is a significant change all due mainly to increased air draw through carb bell.
Reason this: The Jet needle circuit also pull air from bell mouth to mix with fuel it meters. It just makes since this is off too now (lower clip you say). Well you just richened the lower circuits too which doesn’t use bell mouth as much (air jet opening for needle jet, pilot air opening). Do you get a different needle jet (you find one let me know). Do you get a different slide cut (find one of these I am again interested).
So you have main jet, Needle Clip, Needle Jet and Pilot jet to cover all five areas. Not saying these can cover it, but you need to work at it.
I find it funny guys say the only upped mains and bike runs great? Sure WOT.

The next issue with the T500 is it has domed heads not squish band, so it susceptible to pinging. And those chambers you are waiting for when they boost raise compression and you will hear pinging not present with stock exhaust if it was on lean side to start (IMO it was shipped by Suz that way)

I tune for street ride (see other post below) and therefore all positions of throttle and full usable RPM band matter.

So you think you only halve to change jetting once. It may take a few and depends on how fussy/style or how sensitive you are. Your starting choice should be very close

Make sure float height is correct

Also make sure you Needle Jet is not worn or corroded inside due to decades of humidity cycling etc.
188 series needle jet is not available, 159 series works fine, but you should arguably use Hex jets instead of round main for they are designed for hex main threads which are different. There is a table to cross.

Way too much info and yup I write too long post sorry

[Nicholas]

Hey Vintageman, no not too much information, great stuff.

I haven’t read the VM tuning manual and will make sure I download a copy. I have ordered the tuning two stroke engines by Graham Bell, as suggested by redline.

I have been fortunate to find all NOS stock for my VM32 carbs. I’ll be replacing all parts, including a set of 188 Series Needle Jets.

You might want to contact Steve at MotoCarb, think he has a couple of needle jets and throttle slides for your carb. Interestingly I did not think to replace the slide…
http://www.motocarb.co.uk/" onclick="window.open(this.href);return false;

My needle jet uses the round 8M heads and that is what I will be using… I am just wondering if I should have ordered a step smaller on the main and pilot jets.

So WRT my chambers, what do you expect to be a reasonable start? 107.5? Do I need to start higher? I will put the needle clip on 3 as stock, interestingly when I got the bike it was on 4, prob due to the K&N pods and AllSpeeds

You mentioned a post? Did you forget to include it? Interested in hearing more about street ride tuning.

[ja-moo]

The thing with stock air boxes is usually they have too small of intake openings, this is from EPA noise level requirements, this a restriction in the system, reducing power. I don't know what the stock filter material is, but if it's paper, you need to find a suitable foam replacement. The necessity of bigger jets, just means there is more air going through your motor, and more power. Motors are just a pump, less restriction, more performance.

[titan performance]

Nick, 107.5 will be too big on the main. With the stock needles on the centre notch, use a 102.5 main and the stock 30 pilot with the standard air box.

[redline]

When you say the bike was stock, are you also referring to jets etc?

I don't know what jets they ran, just that they used the stock airbox and stock pipes.

Another great book is the two stroke tuner's manual by Gordon Jennings.
It's out of print unfortunately, but there are PDFs of it around if you care to search for it.

Ironically 2T tuning theory is a lot more complicated than 4T but once you start to get it the results are very addictive.
There's not so much you can do to 80s and newer bikes because they're so sorted to begin with but the older engines respond really well to a bit of fettling.

My T500 came with a brand new set of titan performance pipes and I've picked up a pair of heads modified to have squish with a central spark plug.
The plan is to keep the port timings fairly conservative and build a fairly high compression torque monster.

[Nicholas]

When you say the bike was stock, are you also referring to jets etc?

So, stock on a GT500A is on 188 series parts (non hex needle jet).
main jet 97.5, pilot 30, needle 5FP17-3, needle jet P-4, slide 30, VM32SC.
My allspeeds and K&N had moved my needle to 4.

I am going back to stock, bar the pipes and jets. I have a set of oversized jets, but not sure I need to go so high with the stock box…time and experience will tell…my new Jemco pipes should be with me in a couple of weeks.
Thanks for the advice on the book. I ordered two-stroke performance tuning by Graham Bell, it is a great book! I did not realise that blueprinting your engine can make such a difference, my crank as been rebuilt, but I am following some of that advice for my rebuild. Not playing with compression settings this time though….

I look forward to seeing you engine when you are done, did you get re-purposed heads or are they another make? Are you going to measure the clearances TDC then adjust with head gaskets? I was reading that the aluminium head gaskets don’t need to be used and the engine will seal better with some sealant…but that is one way…otherwise are you building a crank with fixed length con rods?