SF - Main Jet Size?

Haggis

Haggis

Hero member
Location
UK
Hi guys, I need to clear up upper end of carburation and welcome your thoughts.

Bike is late 71/early 72 SF, with GT (number eight) cams fitted. Otherwise standard engine. VHB30 carbs. Slash cut silencers with just a perf tube inside.

Let's start with the main jets. Main jets fitted are 128. I believe the factory spec jets are 122 for a GTL, 125 for an SF and 112 for a GT.

Symptoms... up 4,000 rprm mid throttle is OK, then wind open throttle and it bogs and will not recover. Only recovers when widing off throttle. Me thinks too rich up there.

Pointers?

Cheers, Paul H
 
Wouldn't you still be on needle at that stage...which together with atomiser jet is a wearing part and usefully replaced?
 
Hi Rob, needle and its jet are fresh. I want to work down from a sensible main jet, which excercises some control over the mainly needle controlled section. IMHO getting the main jet about right is the first thing to settle. Fine tuning starts after that.
 
Paul,

Have you got the correct V3 needle and AN265 atomiser?

The V3 is difficult to source, often a V6 is fitted instead.  Haven't had much luck with those.

Should be quite happy on a 125 main.

piet
 
Don't get me into main jets with an SF - You will melt pistons providing your main seals aren't burnt with free flowing exhaust. Keep a stash of Main Jets for the VHB 30's, 125 to 134's bet the the 132's will work just fine with Open Mouth Carbs - The 125's where STD -  BUT ;o)
 
Thanks Guys, here is what we have...

- Idle jet - 55
- Main Jet - 128 
- Needle Jet - 265N
- Starter Jet - 70
- Needle - can't find my note.

I'm going to do a few test runs, partially blocking the air intakes to confirm the rich/weak situation. Throttle is marked with pointers for confirming exact opening position. Just waiting for feckin rain to stop.  >:(
 
Went out on the test run today - just to make double sure my thoughts of going to rich at WOT were correct.

Closed off 1/3 and then 2/3 of the available air intake area with masking tape. 1/3 was a bit worse and 2/3 worse yet again. Excatly same characteristic, just worse. So now I can be sure I'm running rich at WOT. I'll get some smaller mains, check WOT and then start fiddling with the needle position if required.

Ignition timing is bang on, advancing nicely. Valve timing 'is what it is' for the rest of this season.
 
An interesting method of checking main jetting. Generally you get a good indication by pulling full throttle under load and quickly snap back about 1/8 throttle. In the time it takes to settle you will get an indication of mixture. If it is happy with less air for the main jet fuel flow the revs will pick up. If it is a bit rich and doesn't like less air it will stutter. If in the ballpark it will just slow a bit. If you want to be really specific fit an AF meter like in the recent post on the triple.
 
Hi All. I ended up at 124. Cleaned up lovely at the top end. In fact so much torque has been liberated, I have now exposed another problem -> clutch slip.  :)

Hi Tippie, completely agree with your method. Works just right. I like the blocking method as it is completely constant regardless of exact throttle opening, load or rpm and needs no inerpretation, other than... is it better or worse?
 
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