Tick over too fast

Colthebrummie · Apr 24, 2017

Hello fellow series lovers/slaves
Ever since I've owned my series 3 lwb station wagon (petrol 2.25) I have never managed to get it to keep a nice low tick over. I've adjusted the mixture screw and the throttle stop screw and I can get it ticking over nicely but after a few miles it speeds up again. I estimate it to be about 1000 rpm. The throttle returns to its stop and the mixture screw isn't loose so why does it keep speeding up? I suspect wear in the carb but the landy has only done 24,000 miles and I have most of the mot certificates to prove it, also there is no sign of fuel leaking from any part of the carb. I have a spare carb (Zenith) but it is set up for a cable throttle and it would need servicing. I'm reluctant to swap it over I case I introduce even more problems. I should also mention that the plugs are always a bit sooty no matter what I do with the mixture screw. In every other respect the engine runs well. Any ideas?

Col

John S · Apr 24, 2017

I think your right. Not familiar with Zeniths but a carb overhaul might do the trick.

Blackburn · Apr 25, 2017

Air getting into the join between manifold and carb when heated up.

wammers · Apr 25, 2017

Yep you have an air leak somewhere.

discool · Apr 25, 2017

Maybe a sticky throttle, cable/linkage, anything that moves, pedal pivot, as got the plugs are they the correct grade for the engine? If so u could try a hotter grade.

tottot · Apr 25, 2017

The mixture screw should be set for the general running of the motor,that is not rich or lean.A petrol motors spark plug colour is a good guide,black too rich,grey/white too lean,light brown just right.
The mixture screw has nothing to do with the slow running and should be left alone once set right for general driving.
Some carbs have a separate slow running jet screw while others rely on a mechanical link on the choke system to open the throttle a little to keep motor running while it is cold.This type needs the slow running screw on the linkage set while motor is hot and then the choke is kept on some to keep the revs up when cold.As others have said any air leak will upset it
2.25's can be set to a low almost silent tick over,just the hiss of air going in the carb.

Colthebrummie · Apr 26, 2017

Thanks for all your replies, they give me plenty to think about. I had a look yesterday to see if there was any sign of a leak at the joint between the car and the manifold. Couldn't see or feel anything g but I did discover that part of the choke linkage has come adrift. The choke opens or closes the butterfly but doesn't act on the throttle. It's in a tricky to reach place so it looks like I'll have to get the carb off. I might as well service it at the same time unless all the spindle bushes are worn in which case I will service the spare instead.
I'm not sure about the plugs, can't remember the exact grade but I'll whip one out and check. Generally, the engine seems to run a bit cool if the temp guage is anything g to go by.

I'm not a fan of tinkering with carbs, I used to do it a lot before I knew better and I never once managed to improve one.

Blackburn · Apr 26, 2017

When you take the carb off check the flange to manifold for warping.

Colthebrummie · Apr 26, 2017

Blackburn said:
When you take the carb off check the flange to manifold for warping.

Will do.

wammers · Apr 26, 2017

tottot said:
The mixture screw should be set for the general running of the motor,that is not rich or lean.A petrol motors spark plug colour is a good guide,black too rich,grey/white too lean,light brown just right.
The mixture screw has nothing to do with the slow running and should be left alone once set right for general driving.
Some carbs have a separate slow running jet screw while others rely on a mechanical link on the choke system to open the throttle a little to keep motor running while it is cold.This type needs the slow running screw on the linkage set while motor is hot and then the choke is kept on some to keep the revs up when cold.As others have said any air leak will upset it
2.25's can be set to a low almost silent tick over,just the hiss of air going in the carb.

Only adjustments on fixed jet carbs are idle air volume screw and idle speed screw. Volume should be set in conjunction with idle speed screw to produce a steady idle. It has nothing to do with fuel mixture at higher RPM or through the rev range.

Tick over too fast

Colthebrummie

Well-Known Member

John S

Well-Known Member

Blackburn

Well-Known Member

wammers

No longer with us, RIP

discool

Well-Known Member

tottot

Well-Known Member

Colthebrummie

Well-Known Member

Blackburn

Well-Known Member

Colthebrummie

Well-Known Member

wammers

No longer with us, RIP

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