Possible choke problem

[DWW]

Hi all, need some help or guidance on the choke operation. My understanding is that on applying the choke while engine is idling there should be a slight rise in revs, well mine has no effect. I am also experiencing cold start issues, it feels like the choke isn't doing anything. I had a look at the carbs (SU's) and while someone operated the choke I could see the mechanism working and the lever on the bottom of the carbs moving accordingly but having no effect what so ever. What could be the reason?

[mgcjag]

Hi Danny....your choke mechanism might be moveing but not correctly...,the service manual has drawings and a detailed explenation on how to set the choke mechanism......Steve

[Series1 Stu]

Hi Danny

Have you checked that you are getting full travel of the choke lever? I was fooled for a while with mine because the second half of travel lifts the jet and gives the enrichment you need for a cold start.

I only realised this when I was installing the choke cable between the dashboard and the centre carburetter.

Regards

[Simonpfhc]

Danny,

As Stuart has said, the choke has two stages; the initial part of the cable movement operates the butterfly and slightly opens it giving a slight rise in tick-over. Further movement of the cable then operates a lever and pulls down the jet assembly which enriches the mixture allowing easier cold starting. These movements should occur on all three carbs in unison.

You need to check that with the choke fully off, the three fast idle screws for the choke sit just above, and not touching, the operating levers. When there is then the slightest of cable movement, the choke levers are operated and the butterflies will open slightly.

There’s not really any adjustment for the second stage of opening the jets as it relies on the whole mechanism being correctly set up.

If you take off the dashpots and pistons, you should see all three jets lower with full choke.

Take a look at the service manual - there is a good setup guide.

[caveman]

Danny,

I had an issue whereby the second stage (cold start) wasn’t fully engaging/lifting. As the cable (akin to a bike cable) has an outer sleeve and thinner inner cable, I found when moving the choke lever from rest to full choke, the choke mechanism was stopping short.



The fix: between the two carbs (nearest bulkhead), you can see the cable at its final mounting point (circled & not my car). My outer cable was too far forward, like this picture and hence not getting full movement. I released the small nut/mount (pic below) at the end of the cable fixing (out of site in this picture & to the right) and withdrew the inner cable (accessed from under the drivers dash) enough to enable me to cut the outer cable shorter.



Once refitted, and when lifting the choke lever on the dash, the second stage kicks in at approx 3/4 movement and can easily be felt as the lever lifts the 3 carb levers. The dash lever, from rest to 3/4 (fast idle) moves freely in comparison. With cold start now, I only need a little over 3/4 choke to fire up. Fast idle picks up the revs nicely from 750 ERPM up, I don’t go over 1200 ERPM but it has the ability to go a lot further.

[DWW]

Thanks for your ample help, I will need to check all those points mentioned. The thing is when my car was at Guy Broads for speedo calibration (under warranty) they informed me they managed to break the choke cable and had to replace it, it is since then that the choke wasn't doing its job, I am thinking they may have not to set it up properly. and yes the choke lever does stop short of its maximum travel.

[abowie]

I am thinking they may have not to set it up properly

Sounds like that's the problem.

The section in the Service Manual describes the setup well.

[DWW]

Thank will investigate tomorrow

[abowie]

Thank will investigate tomorrow

Shouldn't be too hard to sort out.

Make sure that when off the choke levers are fully forward, and when choke is fully on they aren't getting caught on anything.

To set the fast idle screws, after making sure you have the 3 carbs properly synchronised, use a very thin feeler gauge to make sure the screw is just clear of the seat with the choke fully off.

[DWW]

Thanks everyone for your help, managed to fix the choke all working fine with the advice given. All carbs are balanced reading 10 on the vac gauge. The filter planum is still off as I need to check why when blipping the throttle the RPM's dip quite low almost stalling the engine before slowly climbing up to the set idle speed, any suggestions on why that is happening? again all help greatly appreciated.

[mgcjag]

Possible mixture too weak......how far down from the bridge have you set your jets....Steve.

[DWW]

Hi Steve, thanks for replying. I have not set any part of the carbs since they were restored and setup during restoration. the engine ran fine since, I think. The only thing that remains a mystery in my mind is setting the mixture on the SU, I managed in the past with Holley and Edelbrok corroborators to an extent so I may have to read up some.....if indeed the issue is with the mixtures. The last time I replaced the spark plugs, not long ago,there was no sign of lean running, if anything, I suspect she is slightly running on the rich side. One thing I did notice while the pistons in the carb drop ok as they should they are somewhat stiff to lift up, not by friction but by dampening, is that typical?
Any input will help.

[abowie]

Possible mixture too weak......how far down from the bridge have you set your jets....Steve.

Set for 5% CO in exhaust at idle.

[mgcjag]

Hi Danny....the pistons have oil filled dampers so there is resistance to them lifting.......as a minimum you should remove the tops and pistons andmeasure the depth of the jet on each carb..also check the needles are fitted in the correct position....you have had choke issues recently so your jets may not be correct.....PS what was the choke fix you did...Steve

[DWW]

Hi Steve, first and foremost the cable sleeve was too far forward restricting travel of the choke lever and cable, also found the idle adjuster screws were set leaving a gap to the idle operating lever. I adjusted the idles properly while balancing all three carbs. Next week I will be testing the mixture with the method lifting/releasing each pistons to determine rpm variation up/down or none to lean/rich per carb and adjusting where necessary. Hope it doesn't rain as the work needs to be carried out outside since it involves keeping the engine running to temp first....

[mgcjag]

Hi Danny.....the info i posted above will take 20mins to check....and you will then know how its set up.......in my opinion useing the lifting pins will get you no where.....its very hit and mis........Steve

[DWW]

I understand what you are saying but since the carbs were imposingly setup not long ago by the Guy Broad techs and I never had running issues before the new chock cable was installed I will first check by the method I mentioned and if not satisfied with the outcome I shall resort to what you suggested as I am sure sure that is the proper way to go about setting things up properly. Remembering I have to overcome my suffering from 'Carburetor Phobia' .

[bitsobrits]

also found the idle adjuster screws were set leaving a gap to the idle operating lever. I adjusted the idles properly while balancing all three carbs.

Danny,

There should be a small clearance between what you call the idle adjuster screws and the idle operating lever.

Those screws are used to set the 'fast' idle when the choke lever is in operation, and with the choke lever fully released they should be a few thou clear of the operating levers.

The idle speed is set by adjusting the large headed slow running screws (which manage an air bypass around the butterflies).

I would suggest you spend some time reviewing the adjustment procedure in the manual.

[DWW]

Thank will review the procedure. Slowly I will surely get there.

[caveman]

I never had running issues before the new chock cable was installed

Wise starting area and to avoid going down multitude of rabbit holes, which is easy done. I always ask myself, “what was altered/touched before this happened”, especially if it was running so well before.