Carb set up

[Peetee]

I have a 1275 A+ engine and 1.5" carb and I'm trying to set the mixture as per the Haynes Marina manual. The car is still running a fast idle (about 1000rpm by my ear) but the throttle adjustment is backed off so far the cam is resting on the casting boss of the carb body.
Any ideas how I can get the idle down further?

[paulk]

Could be the mixture mate

Joes exhaust has always smelt a little rich. might be worth just backing the mixture down and seeing if he's still fast.

[Peetee]

Rich? Hmmm, funny thing is he always struggles to run when cold - any amount of choke (or none at all) and you have to keep prodding the throttle to stop him from stalling.

[Packedup]

Could be an air leak.

Can't see it being mixture, to lower the tickover through fuel control would surely result in far too rich or far too lean?

If the carb has the breather hose go into it, then check that for splits, perishing, good seal etc. Check the manifold is nice and tight to the head, and that there's no leaks (spray a bit of carb cleaner or something around it), make sure the carb to manifold join is leak free.

[rayofleamington]

It certainly does sound like it could be an air leak, however it could also be symptoms of a very worn carb.

[Willie]

PEETEE... you certainly have the symptoms of a weak mixture and,if it is
possible to enrich it this will automatically lower the tickover revs. Presumably
you find the car tends to stall when you ease off the throttle and also when
you blip the throttle at tickover? Do you have access to a Colourtune kit?

[Peetee]

Presumably
you find the car tends to stall when you ease off the throttle and also when
you blip the throttle at tickover

Once the car is up to temperature it does not try to stall. The revs are a little high (as mentioned). During the first couple of minutes of warm-up it is hard to get the right mixtue using the choke and it will eventually stall regardless of how much choke you use unless you get the revs up either by resting your foot on the pedal or driving it. Once warm, it does seem reluctant to go up through the upper revs (I have no rev counter but would judge it to be from 3500 onwards) and running at speed it feels and sounds a bit rough although there is still plenty of power in comparison to a 1098.

[aupickup]

i think i would go along the lines of weak mixture.
seems famialar to mine a while back.
certainly see if the breather has any splits. i fitted a new breather pipe.

good luck, the 1275 is a nice engine, sometimes , when it is running ok.

[Cam]

Is the breather on the carb open to atmosphere or blanked off? If it's open to atmosphere then that could cause the 'choke' problems as well as the high tickover. Stick your thumb over it and let us know what happens.

[bmcecosse]

Some of these carbs have little valves/bleed holes in the butterfly disc - these are better removed/soldered over. there should be a seperate idle screw for use when the choke is pulled - so run this up to give a bit more idle speed when on choke. Otherwise - look for possible air ingress after the butterfly - ignition advance hole/servo hole. Maybe a leak at the carb flange although don't see how.

[Peetee]

breather on the carb open to atmosphere or blanked off

Blanked

[bmcecosse]

Is the piston free to rise and fall - and does it fall with a nice 'clunk'?

[Peetee]

To summerise:

The piston falls nicely.
The carb vents are all blanked off nicely
There is no obvious leak areas (all bolts are tight home).
I have adjusted the jet as recommended in the manual with no reduction in revs. Likewise with the throttle cable.

Thinking sideways on this one, Could this be a timing problem? High tickover/stalling when cold(not enough choke available)/runs a little roughly at high revs.

[MikeNash]

Regarding timing (and seeing you're getting desparate), check the timing with a strobe. If you've got it too far advanced you get a faster tickover. I suspect its meant to be only 3-5 degrees at idle cos otherwise its liable to be dodgey to hand crank. And too much advance (despite wot that bmceccosse advises!) would tie in with it being rough at high revs too. Best of luck, MikeN.

[Willie]

To me the fact that you cannot get enough choke when cold indicates that
you have a mixture or induction leak problem. I don't see how too much
advance would affect the choke? Have you actually removed the carb and
observed where the butterfly is in the 'closed' position?

[bmcecosse]

If you close the idel screw right down - the engine should not run at all! So - check the throttle really is closing down properly. Ignition advance setting cannot give a high idle speed if the throttle is closed! Somehow air is getting into the engine - you need to find where. If you put your hand over the carb intake at idle - the engine must stop - if it doesn't, then enough air to let it idle is getting in somewhere else. Don't follow your comment Mike - ignition should be set to 'not quite pinking' for best performance.

[MikeNash]

bmcecosse,
Go along with your carb comments but your ignition advance proposals bother me as universal advice. Eg my standard 1098 with too much advance gets rough running and pre-ignition long before pinking sets in (even on cheapest fuel). I think that your engines (tuned and all with raised CR?) are probably more clear cut, and being modded need non-standard set ups anyway. In your skilled hands (and no sarcasm meant) you can judge this point well.
But if you've low compression eg the sidevalvers you might only get proper pinking under extreme conditions long after you're already doing mischief. (I remember a paper long ago by Shell entitled something like Silent Ping, Rumble and Knock.) Extreme examples are my Ariel 600 sidevalver at 5:1CR and 1935 Oxford (at 6CR?) neither who've pinked in their lives. But they let you know it with too much advance - but only if you're observant and sensitive to these things. Sorry for of topic, MikeN.

[bmcecosse]

Ah - well you could be right with very low CR Mike. At 5:1 i'm surprised it runs at all!!! It is how 'modern' engines get their better efficiency - constantly advancing the timing till the sensor detects the pinking then back again - many times a second of course. On my Vauxhall engines with the window open I can just hear two or three 'pings' when I put my foot down.

[Matt]

At 5:1 i'm surprised it runs at all!!!

Hmm well the veteran car and the colleges 1916 fire engine (9.12L 4 cyl ) both run at 4:1. Obviously these never pink either ut you really can tell if you advance the ignition too far!!!

[bmcecosse]

I bet they don't have any automatic ignition advance either - probably a hand control ?