Airbox and Carb

P

pjcheeseman

New member
Can anybody tell me what size main jets I should be using for a UK 1978 180 Jota? When I got the bike there was no filter but the airbox etc was all in place, so I bought a Unicom foam filter. The carbs were fitted with 145 jets but it seemed to cause the motor to die at wider throttle openings. I changed these for 135 jets which seemed to improve the performance. However I have since read somewhere that the Unicom filter 'strangles' the engine. So, I'm slightly confused. Is it best to run with or without a filter? What filter should I use? What jets should I be using for these options? Any help much appreciated. 
 
I never used a filter back in the day.....the carbs did wear out though much to my ignorant surprise.
The bike had done a lot of miles though.
 
Is it oiled foam?
If so wring out as much oil as you can.
You can buy air filter material from air conditioning sales shops. A thick open cell foam, sort of like nylon pan scrubber material.
It's really there to stop rocks and small children being sucked into the combustion chamber.
Originally UK Jotas came with 145 mains but fuel was quite different back then. Today's fuel lets you get away with smaller mains according to a lot of folk on the forum.
Try 125s if the 135s gave an improvement, but I wouldn't go below 120s.
They work best with K1 needles too. Fit new ones as old needles wear out and change profile with time.
 
G'day Cheesy  ;)

My local dynamometer bloke and I did a bunch of dyno runs with my Jota using a range of main jets. My bike is a reasonably typical Jota - 4C cams, high comp pistons, 39/36 valves, original air box and low restriction filter, Jota exhaust.

We found that if you go above #135 main jets on otherwise standard carbs (32mm PHF Dellortos with AB265 needle jets), it won't flow any more fuel at wide open throttle. Just for interest, we did a run with no main jets at all and fuel mixture was similar to the 135 mains (around 11:1 I think). We deduced that the needle jet was limiting the fuel flow at WOT. 

If a previous owner of your bike fitted 145 jets, then he may have been using a methanol mixture and could have fitted a different needle jet too. Probably worth checking.

Standard settings are:
Main Jet 118
Needle Jet AB265
Jet Needle K1 middle notch
Slide 60/2
Pilot (idle) Jet 55
Starter Jet 70
Float Valve 250
Float Height 17.5 - 18.5
Idle mixture screw 1.5 turns
Accelerator Pump 4cc/20 strokes

Modern unleaded fuel is a bit different to the leaded stuff that was around in the 70's, so you may need to run bigger jets than 118.
I'd go with Grant's suggestion of 125. That's what my Jota ended up with after our dyno session. It actually ran marginally better with 120 jets (made about another ? HP) but was a little too close to running lean for my peace of mind.

Of course, there are all sorts of other variables such as cam timing, air filter resistance, general engine condition. Not to mention air pressure, temperature and humidity. So your optimal jetting may end up different to mine.

If in doubt, give it a run on a dynamometer with a sniffer up the exhaust.

Cam

 
Interesting info Grant and Cam.

Especially comments on modern fuel.

I rebuilt a 750SF3 that was purchased as a wreck some 6-7 years ago. Did the full rebuild, with everything pulled down, checked.
I did the usual clean and check on the carbies and it ran like rubbish.
When I costed putting all new jets, needles slide etc al, it was cheaper to buy new carbies from Bevelheaven setup as per stock.
The new carbs made a hell of a difference and allowed it to be 'tuned' properly.  Note that I am no Red or Batto.
 
DavidH said:
Interesting info Grant and Cam.

Especially comments on modern fuel.

I rebuilt a 750SF3 that was purchased as a wreck some 6-7 years ago. Did the full rebuild, with everything pulled down, checked.
I did the usual clean and check on the carbies and it ran like rubbish.
When I costed putting all new jets, needles slide etc al, it was cheaper to buy new carbies from Bevelheaven setup as per stock.
The new carbs made a hell of a difference and allowed it to be 'tuned' properly.  Note that I am no Red or Batto.
Click to expand...

Main, idle, choke or pump jets generally do not wear, they only corrode, and only if they're stored at the bottom of your garden pond...

New needles and needle jets are usually all that are required, sometimes a pair of slides and pump diaphragms for the twins.  Triples are a slightly different story.  Eventually, the carb bodies will also be scrap though.

I have not found modern fuels to make much of an impact, jets sizes today are much the same as they were 25 years ago... mainly dealer hype saying you needed 170 mains when a big-bore collector with jota pipes were fitted (that was what the gravevine here yielded in the 80's, was a load of bullshit, even back then!).  Don't recall how many bikes have since passed through the workshop requiring correction, for certain quite a lot...

I can supply a modern-type plate filter for the old triple air box, just slip it in and be happy.
piet
 
Just use a sheet of Isopon mesh gauze from Halfords cut to size instead of a filter,cheap and passed on to me by a big mileage Jota owner.
 
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