Anyone any ideas on this one...

Just finished fabbing up an inlet manifold for my tr1 based chopper this morning and chucked my 1.5" SU carb off a mini on to see how it would run.

After some tweaking and dashpot refilling I can get it to run and rev freely but I cannot get it to idle or to settle on steady revs at all.

Ive been experimenting and it seems that if you open the throttle, the revs rise as you would expect then, as the carb piston rises it stalls (or at least runs very badly and hunts).

I reckon weak mixture needing a richer needle.

What do you think?

Any ideas appreciated since Ive got another day to tinker tomorrow

Cheers

Geoff

Try adjusting the mixture a little more rich,
But you may just have a mismatch.
Steve.

[Edited on 3/2/07 by stevec]

Long time since i did one of these but carnt you move the circlip on needle to richen or lean of.

No , but you should be able to lower the jet at the base of carb to richen it up but you may struggle to get the correct needle. Get an SU manual , should have all the needle profiles in it. HTH

Change the damper spring?

Fozzie

Lots of information on setting them up here:
http://www.manchester-minis.cwc.net/technical/sucarbs.htm

Which SU is it? The earlier ones have an adjuster on the bottom of the jet housing which can be turned to later the overall mixture and the later HIF's have a x headed screw on/in a recess the left hand side (looking from the front/air filter side) which does the same thing. Also, as fozzie said, changing/doubling up the dash pot spring will richen thing up a bit.
PS, the later HIF version are the better ones to have as they have a bi-metallic strip which alters the mixture slightly according to the carb/engine temp. I've also found they are a bit easier to set up.

[Edited on 3/2/07 by DIY Si]

Its the 1.5" HS4 carb.

Im basically trying to do what this guy has done:

http://cgi.ebay.co.uk/YAMAHA-TR1-STAINLESS-2into1-INLET-MANIFOLD-SU-CARB_W0QQitemZ190075718623QQihZ009QQcategoryZ10490QQssPageNameZWDVWQQrdZ1QQcmdZVie wItem

My carb does have a hex nut under it which I think regualtes the needle up and down.

[Edited on 3/2/07 by jambojeef]

That hex nut is the mixture adjuster you want. Not too sure about the flow you'd get through that manifold though. You want a much smoother bend into the two side bits if possible. I appreciate this may not be possible due to space though. More of a Y shape thingy, rather than what on mini's would be called a log manifold.

Exactly my thoughts so I made it like this.

Ive emailed the seller asking him if he can give away his secrets but Im guessing he ownt. You never know though I guess...
Rescued attachment DSCN0122 copy.JPG

The nut on the bottom of the carb raises and lowers the jet, but it's only really useful for setting the baseline mixture at idle, the profile of the needle and the strength of the dashpot spring controls mixture when the throttle is open.

to be honest the chances of the needle from a mini suiting the TR1 are slim to none. To get the best out of this conversion you need to take the bike to an SU specialist who should be able to either select a suitable needle or modify one to suit.

FWIW virtually any exhaust or cylinder head mods on a 998 mini cause the mixture to run very weak, and the solution is invariably to fit either an AAA or an AAU needle. If you can find one of these it may fix the weak running, but it's no substitute to getting it set up professionally.

Funnily enough I have reached much the same conclusion after todays experiments.

I ordered an AA needle earlier and we'll see what differences that makes but according to everything Ive read - all the needles for these carbs have much the same profile at the top of the needle meaning idle wont be much different from needle to needle.

So really I should be able to get it to idle even if my needle is messing up the fuelling everywhere else in the range shouldnt I?!

quote:

---

Originally posted by jambojeef
So really I should be able to get it to idle even if my needle is messing up the fuelling everywhere else in the range shouldnt I?!

---

Can you tell by feeling for play in the spindle or are the tolerances closer than that?

IMHO the dashpot spring doesn't affect the mixture, its function being to ensure the piston returns to the idle position when the throttle is closed particularly if the SUs are mounted at an angle

Vacuum on the engine side of the piston is transmitted to the top side of the piston through a drilling and the piston then rises until there is enough air admitted to reduce the difference in pressure to near enough zero. Open the throttle and the engine sucks more, the piston rises more, more air is admitted and the system finds a balance point.

The amount of air admitted to the engine is determined by the position of the piston and, since the tapered needle is fixed to the piston, this then admits the right amount of fuel to match the amount of air. The dashpot spring doesn't come into it.

Beautifully simple

chances that a mini needle will work on a motorclce are probably quite small.

the needle profile is more of an issue are higher revs/power demands.

almost all, if not all, the needles at the base (non pointy bit) are the same diameter - the bit that we are interested at idling.

check of air leaks.
is the jet istelf worn? they do wear into an oval over time so a new needle isn;t going to help - you need to replace jet.

also you check your float is set up correclty?

the HS type - separate float chamber - need to have the foat horizontal - you can buy the gommets that allow the carb to sit at different angles but keep the float chamber horizontal.

also the way the need is held in the piston can be adjusted and it needs to be done correctly - the needle holder hs flsuh with the base of the piston (that is true for HIF type, canlt rememebr if it is the same for hS).

Dis you put 20/50 engine oil in the dashpot? Mine on the Mini didn't like anything else, it let the damper rise too quick and you didn't get the richening you needed. We used to file a flat on the needle if we couldn't get the profile we needed, a bit rough and ready but it worked.