5 replies to this topic

[nospamprl]

I felt the idle speed on my 998cc was a little to much at about 1200 RPM according to my tach. So I adjusted it to about 750rpm, but I noticed that when doing 2500-3000rpm on 2nd or 3rd gear there is a slight hesitation on the engine.

It doesn't makes sense to me that the idle screw adjustment has any effect when the throttle is pushed beyond that point, but Haynes says in the Fault Finding section that hesitation may be caused by idle speed adjustment.

Any thought s?

[CityEPete]

If you set the mixture with the idle incorrect it will mess it up across the range I suppose, I'd pull the spark plugs out and see what colour they are then adjust the mixture accordingly. Check that the choke is fully disengaging with the knob in and make sure the dashpot is topped up.

Edited by CityEPete, 28 December 2017 - 07:09 PM.

[hhhh]

Be aware that a plug chop will only tell you what the mixture was like in the few seconds before shut down. Also, with the new fuel formulations, the oxygenators make it harder to read plugs than it used to be. What appears lean now is quite possibly rich enough. The Haynes manual may be describing the lag that may be increased when the RPM falls lower during shifts. Alternately, if your needle is the wrong profile, CityEPete's answer regarding mixture setting would apply.

[CityEPete]

I still think a good run then leave to cool before checking the plugs is a good way to see if you're in the right area, if they are a nice biscuit colour and the car runs well I'm normally happy.

[Ethel]

The idle screw is just a travel stop for the throttle, it can't have any effect if the cable is already holding the throttle open instead.

 

As it's an imperfect world, you could get hesitation as the throttle lifts off the idle screw. Particularly if the carb is leaking in air that isn't going past the jet and the mixture has been adjusted to put more fuel in to air that is: to compensate. With the throttle more open there's less vacuum to suck air through leaks, like worn throttle spindle bushes. The mixture becomes richer as more of the air comes through the carb, as it should, - past the jet that was adjusted to fix the idling issue. 

 

Another possibility is a clogged air filter - its restriction to airflow will cause the mixture to get richer, like the choke on other types of carb. Carbs meter fuel by air speed, restrictions before them drop the air pressure meaning the air's less dense but still receiving about the same amount of fuel.  

 

You could also check the vac advance is working and plot the centrifugal advance through the rpm range.

[CityEPete]

Funnily enough I've got a brand new HS2 carb here to go on my morris minor as the tickover is too high even with the adjustment right down so it must be worn out and leaking past the throttle flap.