Series 1.5 carburettor setting

[Heronscourt]

I’m still struggling with achieving a smooth tickover. When the engine is up to temperature and the choke is off tickover bounces between 700 rpm and 1200 rpm and sounds lumpy.
Sparks are like lightening bolts on all cylinders (sparkright testers are brilliant). Timing is spot on so it’s obviously a mixture issue. The Jaguar manual states that tickover is controlled by the slow running jet. This is in the 3.8 section of the manual but the 4.2 and series 2 sections say that the same instruction applies to the 4.2 carburettors. The SU manual states that for HD type single carburettor installations the slow running jets are used however the SU workshop manual also says for HD type carburettors that if throttle adjusting screws are fitted the slow running jets must be closed and tickover is adjusted by using the throttle adjusting screws for multiple carburettor installations. My question is which method is correct. I’d like to understand this before I go through the complete carburettor set up again (for the umpteenth time).
Incidentally I’m using the Mangoletsi throttle linkage setup so there is no slop between carburettors.
Yours in hope,Alan

[mgcjag]

Hi Alan...are these rebuilt carbs....were they runni g ok befor you fitted the mangoletsi.......follow the detailed instructions in the Jag service manual...yes its in the 3.8 section...with no changes mentioned in the other sections....iv followed this many times and the carbs if rebuilt correctly will start up and run very well with just minor adjustments needed to make them perfect.....Steve

[Simonpfhc]

Hi Alan,

As Steve has said, the instructions in the workshop manual are very good. I’ve set up many sets of triple SU’s and when all the adjustments are set as per the manual, they are pretty close to spot on!

With the Mangoletsi linkage, make doubly sure that all the butterflies are fully closed and all open in unison by adjusting each drop link individually and then tighten the lock nuts.

After that, with factory settings, your engine should run pretty well.

Whereabouts are you?

[Heronscourt]

Thanks Steve and Simon. They are rebuilt carbs with new jets, needles, seals etc. The car has only done 3 miles around the block since a complete rebuild. I don't want to run it much more until I’ve got fuelling sorted. Mangoletsi set up was fitted during the carb installation, operation is very positive compared with the old linkage which was very sloppy and as I was converting to RHD purchase cost of a new linkage went well towards the cost of the Mangoletsi. All three butterflies are set fully closed and open simultaneously. This has been checked every time I’ve gone through the adjustment process.

I live on the Wirral

[cactusman]

I would agree with Steve and Simon. I rebuilt my carbs as part of the engine rebuilt. Set them exactly as per the workshop manual. Car started instantly.. Balanced them at idle with minor tweeks.....idles smoothly at around 700 rpm (presuming the tank is reasonably accurate)...

[abowie]

To answer your question, use the slow adjustment screws to set idle.

You should have the butterflies all completely closed at idle.

Get a syncrometerhttps://www.sumidel.com/shop/item/carbu ... nchrometer and set the flows so they are equal; I usually find a flow of 7 to 8 is where you end up.

[Heronscourt]

Thanks Julien and Andrew, it’s nice to have uniform confirmation of the way forward. I’ve ordered a synchronometer which is due tomorrow so hopefully I’ll make progress on Saturday. I’m also going to start set up from scratch including the choke linkages and Mangoletsi settings.
One final question, the SU manual shows a support cup washer and seal in each slow running valve but the exploded diagram in the jaguar manual does not show them. I’m sure I installed them when I rebuilt the carbs as I was following the SU manual but I don’t recall them being present when I stripped the carbs.
Should they be installed or not?

[mgcjag]

Hi Alan....when you say SU manual are you refering to the small SU pamphlet for HD carbs...if do its generic for HD carbs....the Jag service manual is spevific to the HD carbs fitted to the E type ....Steve

[Heronscourt]

Hi Steve, I have a quite thick booklet which covers all types of SU carbs but in accordance with everyone’s advice I’m going to follow the Jaguar service manual instructions for set up. Am I correct in assuming that as the exploded carb diagram in the manual doesn’t show the slow running valve support cup and seal I should omit them unless anyone can confirm that they are simply an omission in the manual?

Thanks everyone for help and advice, I’ll update progress after the weekend.

PS the synchronometer has arrived today

[abowie]

The o ring 45 and dished washer 46 make an air tight seal and must be fitted.

[Heronscourt]

Hi Steve, Julian, Simon and Andrew. Thanks for the advice to date. However having started from scratch several times to re set the SU carbs in accordance with the Jaguar workshop manual I am still unable to get below 2000 rpm no matter how I adjust the slow running valve ( even when fully closed) . Throttle plates are fully closed and choke control rods are set correctly. Jets are 60 thou below the bridge + or -2 thou.

With the engine up to temperature and the choke off revs are 2500 with slow running valve 2 turns open and the jets 2.5 turns below the bridge ( 60 thou down ) . The synchrometer isn’t much use at those revs but readings are 11, 15 and 20

I get a strong hiss from the centre carb but the other two carbs are almost inaudible. It must be pulling air from somewhere else but I’ve checked all the vacuum hose clips and they are tight. I did observe the hoses to be a very loose fit on assembly and wonder if the clips are actually capable of creating an adequate seal so intend to blank off the vacuum connection at the manifold to check if that has any effect. I’ve checked all carb and manifold nuts.

I really can’t think of anything else but any other ideas will be most welcome

[mgcjag]

Hi Alan...I your first post tick over was between 700 and 1200 and lumpy....now you cant get it below 2000... what's changed?....your still sucking in too much air and/or too much fuel...with the car running completely block the front intake of one carb at a time...see what happens....Steve

[Heronscourt]

Hi Steve, the only changes from that point have been new slow running valve seals and dished washers plus more accurate micrometer setting of the jet height instead of just relying on two and a half turns of the jet adjusting screw. I did clamp off the vacuum hose at the manifold today but that made no difference.

Tomorrows job will be to change the jet setting in small increments using the micrometer to ensure accuracy. Taking the dash pots off for each adjustment will be fiddly I know, but at least I’ll know they are all the same. If that shows any improvement I’ll then make individual adjustments based on the synchrometer.

Thanks once again for your patience.

Alan

[Allrand]

Hi Alan, did you remove or refit the trottle plates from the shafts? If those are slightly off center, rotated in the slot or reversed fitted, they won't close properly allowing a sliver of air to bypass the plate, significantly increasing idle speed. Hold the carb up to the light with the throttle fully closed, if you can see any light past the edges of the plate it'll show if you have a problem there.

[abowie]

I really can’t think of anything else but any other ideas will be most welcome

Are you sure you have your timing set correctly?