needle help

[pskipper]

Hi,

We're having troubles with excessive vibration from the HS4 carb we've fitted to the 1275cc midget engine in Lynda's police car. We have a HIF44 carb that we could fit instead but need the car tomorrow, can anyone suggest what sort of needle it should have for this set up so we can see what is in it and then decide whether to swap or vibrate!

Thanks

Philip

[bmcecosse]

Can't see 'vibration' coming from the carb ! Is this the engine you had out over the week-end ? HIF 44 is good carb - I would try it with the needle that's in it - it will likely be near enough to at least run the car. What needle is it ? BDL is suggested for slightly modified (ie MG Metro standard) 1275 engine.

[pskipper]

We tracked down the vibrations last night, there are loose componants on the plate which all the throttle cable attachments are on which are causing a vibration back down the throttle cable and to the accelerator pedal, makes driving around 50 like using a swarm of bees instead of a pedal.

[bmcecosse]

" loose componants on the plate " - Ehh ?

You still don't say what needle is in the carb at present ?

[pskipper]

The components of the bit marked plate vibrate into a blur between 3000 and 3500 rpm causing a vibration to travel down the throttle cable to the accelerator pedal.

The current needle is BDR

Hope this makes sense of the plate comment

[bmcecosse]

Aye - but the plate should not be 'vibrating into a blur' - this suggests some serious vibration in the engine - which is being transmitted into the cable (is that not the choke ?) - which is looking a bit frayed btw! I would replace the inner asap (if it is accelerator) - before it starts 'sticking' . Bike gear cables are v cheap (60p) - and ideal as new inners.
BDR 990 950 920 895 870 837 805 760 715 670 625 580 535 490 445 400
BDL 990 955 924 893 865 834 798 774 743 710 680 650 620 590 560 530

So - you're BDR slightly weaker mid range (which is where most driving takes place) and then goes quite a lot richer at the upper end! It may work very well - although could be v heavy on fuel if you use the 'top end' quite a lot. Run it and see what plug colours you get.

[MarkyB]

Is that plate flat?
Probably sympathetic vibration with something.
If you run a bead of weld down it or a 90 degree fold it should stiffen up and not vibrate.

[bmcecosse]

But they DON'T normally 'vibrate' !! Why would they!?!

[MarkyB]

But it IS vibrating.

Having looked at the picture again I think that a clutch return spring has been used.
This is putting the whole thing under tension and explains why you can feel vibration through the pedal.

Try a much lighter spring, it only has to close the butterfly valve after all.
Too stronger spring will put too much stress on the cable and your right leg.

[pskipper]

No, it's a standard return spring, I've replaced it with a brand new one since taking the photo and the new one is much tighter.

The plate isn't flat, there is a lot of play between the dowels which come out of the plate and the piece the throttle cable is connected to and the one next to it in all directions which means they move very freely.

We replaces the pancake filter with a standard mini airbox with a k&n filter in it yesterday and this has reduced the vibrations massively, only a little buzz on the pedal when decelerating, only audible if your foot is off the pedal.

If it is excessive engine vibration then what would be causing it?

Thanks for all your help so far

[bmcecosse]

Well - if the vibs are sorted, that's it solved. I have heard of the BDR being used on v highly modified big bore engines - but I think a BDL is likely to suit your (standard ?) engine rather better - the BDR may give heavy fuel consumption. But do please report back on plug colours aftyer a good hard run.

[MarkyB]

Darn, I really liked the strong spring theory.
It's unusual for cables to transmit much vibration.

[bmcecosse]

It's unusual for there to BE any vibration. I suspect there is more to come in this story!!

[Kevin]

As you have just swapped the engine I assume you have set the engine steady up correctly and also the gearbox steady cable because if they are adjusted up bow tight they can cause some interesting symptons.

[carlt]

bmcecosse - I'm interested where you got your su needle mixture no's from
and what do they represent [ mixture strength , but what exactly ? ]

[bmcecosse]

There are various sources on the internet - I down loaded some many years ago and have kept the print-outs. Just as well - cos I had to wipe my hard drive ! The numbers are the diameter of the needle (in thous) at equal steps down the shank - from the top down. Some say these are at 1/8" (125 thou) spacing - others say 1/10" (100 thou). I haven't got a definite answer - but as long as they are the same for all - it's ok for comparison. In any case - it's quite difficult to measure a tapering needle very accurately at a specific point!!
So - smaller numbers means more fuel (ie richer mix) can flow through the annular space between needle shank and jet, larger numbers means less fuel (ie weaker mix). If you want info on a specific needle - just ask!

[carlt]

Thanks for that ,
I would be very interested in a chart for the available SU needles ,
I do quite a lot of weird vehicles with even weirder engines ,
am just fitting up a 100E engine into a Buckler MK5 for trials -
+60 bore , worked ports and transfer chamber , twin su's on aquaplane manifolds .
Have been informed that a morris minor needle [do have the no somewhere] is the one to use
But rest assured it wont be correct for this engine ?!
Cheers
Carl

[alex_holden]

Google for "SU needle chart" and a number of sites come up. Alternatively you can use WinSU (you don't need to pay for it to see the needle profile graphs) or you can buy the official needle chart book from SU/Burlen.