10 replies to this topic

[Chris1275gt]

Hi
I have a 1975 1275gt. It has a bog standard Hs4 the 1275 engine has been honed to +40 thou the carb has been refurbished and set up by a mechanic, timing and fuel mix are spot on idles at 800 and all runs ok and smooth. Sometimes when stopping the revs drop to 400/500 and then slowly picks up to the set 800 revs and sometimes cuts out. Any one any ideas what needs tweaking?

[cal844]

Sounds like an air leak or a mixture issue

[luismx123]

I have the same issue. I vizard'd my carb and after install the idle drops down low after higher revs and it kinda wobbles around +-200 rpm every 2 seconds or so. I believe it has to do with some fuel pooling from the hs4 to the inlet mouth, but as cal said above it could also be an airleak - possibly somewhere in the butterfly spindle at the back of the carb as Ive seen one not sit properly in the predestined position. 

[andyapanel]

Check the carb piston can move freely.

[ACDodd]

Has it had the spindle bushes replaced in the carb?

Ac

[Chris1275gt]

Sounds like an air leak or a mixture issue

Not an air leak but adjusting the jet nut 1/2 a turn richer sorted it along with a sticky piston. Thanks

.

Check the carb piston can move freely.

Piston was not smooth but it is now. Thanks all sorted.

[cal844]

Glad its repaired

[luismx123]

what did you do to make the piston more smooth?
 

[Chris1275gt]

what did you do to make the piston more smooth?

I took the tar like oil that had been put in the damper by a mechanic friend of a friend, cleaned with petrol and put the correct su carburettor oil in.

[luismx123]

oh ok i thought you sanded/polished the piston itself... its something im thinking of as my "new" hs4 is really rough whilst my 30yr old hs4 is perfectly matched inside with polished smooth surface from constant use. I was wondering if it had something to do with this...

ALSO a point worth checking is the dashpot damper: there are multiple types and I found using the damper from my older carb works better than the newer. Its kinda round at the bottom vs the more square shape of the newer one... idk, just theorycrafting

[sonscar]

The piston should not touch the dashpot so should not polish it smooth.They should be matched and have a calibrated fall rate if you research it.There are lots of different dashpot lids with different characteristics,a minefield.Steve..