Advice jetting a 1976 GT500 with original all speeds pipes

[Nicholas]

Hi, I’ve just bought a GT500 that has been customised with some really nice vintage 70s parts.

The last guy I bought the bike from didn’t really do a good job restoring it, or the guy he bought it from didn’t… but it wasn’t running well, due a few things and mainly insufficiently fastened bolts not compressing seals resulting in numerous oil leaks.

I am just going through a complete rebuild on the engine and will patch my pipes etc... I don’t really know about the right carb and jet ratios for this bike, especially when fitted with All Speeds from the 70s and dual K&N air filters on the carb.

Does anyone have experience with this (simple?) setup or can point me to some info on the optimal jet sizes?

Thanks!
Nicholas

[titan performance]

Have you got the stock 32mm Mikunis?

[Nicholas]

Hi, yes sorry, yes it's the stock carbs for the one and only 1976 model. I'll confirm the exact config this weekend as its in a garage. I'd really like to know some of the technical details if there is such a thing, but I do understand that tuning expansion pipes is more of a dark art and down to experience/trial and error.

[Pete O'Dell]

where in the UK are you Nick? have a look at http://www.tr500.co.uk" onclick="window.open(this.href);return false; for contact details

[Nicholas]

I'm in London, but if he is a good guy then it's not a problem. I had my crank sent off to someone in London by my mechanic to have the seals replaced and they cameback with a massive bill....

Apparently the con rods had "hardened" and so they need to be replaced, I had no idea such a thing was possible? So so I'm having to replace the con rods, big end bearings and obviously the seals, over double what I was expecting to pay....

So, is this kind of thing common?

[Pete O'Dell]

Nick, you have been told complete and utter bullshit, where for instance are they getting the rods???? unless they have by some miracle found good new old stock then they are liars simple as that.

As for the hardening bullshit get your crank back as they are ripping you off as is your "mechanic" who if he knew anything would know this is crap, your being taken for a ride mate.

New T500 con rods are only available from one source on this planet........ ME, and I know where all of them have gone and in who's engines.

[tz375]

It is possible that they fitted used rods and that the explanation was lost in translation. Rods can wear or could have suffered from a few problems but work hardening or heat hardening are not on any list I have seen. In order to determine if hardness has changed they would have to have a hardness tester and that leaves a small indentation in the surface. You can pretty much guarantee that they don't have one because that is not something that engine builders need.

If they have built your crank, demand to see and get back the old rods and other parts that they say they replaced.

Do they claim to have fitted new rods? Do you know who your mechanic sent it to?

[Pete O'Dell]

I have since spoken to Nick and his mechanic is not at fault and he has been told by the crank builder he has fitted new rods, we shall see when crank comes back, I shan't hold my breath for new rods.

know who its gone too and his work is usually good, but still dont believe new rods

[Nicholas]

I thought I would consolidate discussions.
My setup 1976 GT500A:
VM32SC
Main Jet: 97.5
Needle Jet: 188-P4
Jet Needle: 5FP17-4
Pilot:30
Slide:2.5

My carb setup is slightly tweaked for the K&Ns and AllSpeeds. The jet needle is positioned at 4 as opposed to 3, so more fuel coming in...

I wanted to replace the jets etc.. on my carb and as the gringo that I am...I am discovering that its a bit of a mission owning a classic bike! oh well makes it that more personal, bike and his owner!

My current problem is that I cannot find any 188 series P-4 needle jets for my 5FP17 jet needles. I am no expert on carbs (yet), but are there other setups. I think mine is more on the leaner side...so need to be careful that I don’t go too lean...

What is everyone using out there? Can I get by with a different config on the same carb?

Or should I just replace the needles and take it to a tuner who can probably sort me out.... just afraid of the costs...yeah we can put a flat side carb on that etc...

[tz375]

Nicholas, those carbs appear to be completely stock jets and needles but raised one clip position.

A rule of thumb starting point would be to go up one size on the pilot jets and two sizes on the mains and work down from there. You mentioned a bent needle. That should be replaced if you can find one. If you have to change needle jets at some time to say P-8, you will need to change to 159 series and Large hex jets and the flow rates are different, so you will need to check the chart if you have to cross that bridge.

[Nicholas]

Nicholas, those carbs appear to be completely stock jets and needles but raised one clip position.

A rule of thumb starting point would be to go up one size on the pilot jets and two sizes on the mains and work down from there. You mentioned a bent needle. That should be replaced if you can find one. If you have to change needle jets at some time to say P-8, you will need to change to 159 series and Large hex jets and the flow rates are different, so you will need to check the chart if you have to cross that bridge.

Yes, I intend to replace all bits...hence trying to find the right needle jet. Thankfully the needle jets are still around so I will order a coupe. I can also get my hands on 188 P-8 jet needles....can I just get a thicker jet needle? How is that addressed?

So I guess if I can get new parts gong down the 159 series path then perhaps I should do this, and again what kind of jet needle would I need to use here? I'll rtfm any pointers to info will be appreciated! Thanks again for your input!

[tz375]

A larger needle jet is proportionately richer at low throttle opening than high. To get the same increase in annular area through the range would require a thicker needle at all points along the shaft and a lot of calculations or just good luck.

I don't have dimensions for the stock needle but I seem to recall a posting here that someone thought they were the same as another needle but one clip position different. If that is the case, I'd rather use 159 series in stock size and get an equivalent main jet size and an equivalent needle as a starting point.

[Nicholas]

Hi, just thought I would share that for those who are looking at replacing their needle jets and need to go down the 159 Series route for P-4, then the hex main jet should be 110, if your main jet was 97.5 on a 188 Series P-4 needle jet.

Dont hold me responsible for this info, it was recommended by someone at http://www.sudco.com" onclick="window.open(this.href);return false;. i.e. I havent done the maths.

[Vintageman]

I run Jemco chambers on a 75 with 159 Needle Jet and stock needle jet and jet needle

After many tries my jetting conclusion is:-> the low opening was too lean and that was fixed with larger pilot. 1/4 to 1/2 OK. 3/4 to full was a challenge If you fixed one the other was a bit off.

For example if full perfect ala main jet, 3/4 or so was a bit too rich. If 3/4 just right (move clip or change main jet) full a bit too lean.

I have stock intake. Your pod may exaggerate this more or not.

Lot’s of discussion hear on T500. The only other needle I know of worth considering is from the early T500. Its tip is not as thin as the later and one reason is why a large main jet is called out on the early T500. That my help the ¾ to full issue tuning. I have not tried it.

I will say that the 3/4 - Full tuning issue is not bad at all though with what you have for parts. I just like everywhere to be perfect and every day as I define it.

I choose to fix full and put up with a little rich 3/4. Once the bike warms up and depending on the day it perfect there too. Good luck and come back a share what you do please

[Nicholas]

I just like everywhere to be perfect and every day as I define it.

I agree with this philosophy, unfortunately it is also the reason I am in the mess! I managed to track down a set of 188 series NOS needle jets so I ordered 2 sets! I have also found a GT500A air box so the bike will be back to stock!

My expansions are a nightmare!
I have wasted a bit of time and money on my old AllSpeeds. Had the holes repaired by a magician of a welder then sent them off for a ceramic coating and the guys put holes through the old mild steel when they were blasting my pipes, maybe too much pressure… I am where I am and cannot waste any more money on these terminal things! I must get a new set of pipes.

You mentioned you had Jemco, have you tried other pipes? I am reluctant to get another set of AllSpeeds as I have read they are not good for the G/T500.

I have found SAMRACING, Cor seems quite knowledgeable in the art of creating pipes for T/GT500s. Makes racing and road versions and said that with a 120 main jet on stock air and carb his road pipes will increase power by 20%… they are just so long and horizontal, I hope he can make an upswept version.
http://www.samracing.net/" onclick="window.open(this.href);return false;

Anyone here used his pipes? I have attached a photo of his road versions...

Are is there a list out there of good road pipes? I have been told that some pipes suffer clearance issues with rear sets, but it was a competitor talking of another’s pipes…