HS4 question.

[JOWETTJAVELIN]

I'm not an expert on carbs by any means, but have successfully rebuilt a few of them in the past. Rebuilding a HS4 the other day due to running too rich and idling roughly (mixture screw right up to no effect), I noticed the needle isn't fixed on the HS4, and removing the old jet tube the wear was plain to see.

Seeing as the needle isn't fixed and therefore presumably comes into contact with the jet tube, is the HS4 more prone to wear than other SU's with fixed jets which need centring?

[bmcecosse]

Yes - it does wear the jet over time........ New jet required - and safe to assume the needle has taken some wear too - especially at the 'thicker' end....... So ideally new jet and needle.......

[JOWETTJAVELIN]

Thanks. Yes I ordered a new needle aswell - they aren't included in the rebuild pack.

I am pleased to say it is all working superbly and now there is just the fun of fine tuning.

[bmcecosse]

If the throttle plate has the little valve (or a small hole) in it - remove and solder up the hole......

[JOWETTJAVELIN]

It did have a hole... and so did the one I removed...

What is the significance of it and why should it be blocked?

[bmcecosse]

Well - the little hole is not as bad as the valve ! It allows air to pass even when the throttle is closed - so can make it a bit tricky to set the idle speed. It was a 'frig' to try to help with emissions figures. The valve if present forms a significant obstruction to airflow.

[mike.perry]

Are you fitting it to the 803?

[JOWETTJAVELIN]

Are you fitting it to the 803?

Good heavens no!

It has gone back on an Allegro 1300.