Tuning SU Carbs

[ALAN COCHRANE]

Hi Everyone

Well following a carb rebuild and a lot of adjusting I've finally managed to get the E-Type purring again. It's been a real struggle with everything seeming to go wrong at various points. Anyway here's my question and please remember it's been a long day:-
Why do you need to disconnect the throttle linkages before balancing the carbs? The HS8 carb uses a secondary circuit for the idle speed which operates when the throttle disc is closed and the carbs are linked by a balance pipe between the three inlet manifolds so are always interdependent anyway.
Sorry if the answer is blindingly obvious but hopefully I'm not the only one who's wondered about this.

Cheers

Alan

[abowie]

You do this to ensure that you have all 3 butterflies completely closed before you start.

[malcolm]

To be fair, I looked at what was involved in disconnecting and did't bother. Just left it all connected. Runs and ticks over very nicely, so maybe I was lucky?

[christopher storey]

+1 for Andrew Bowie : his reply has it in one .

[ALAN COCHRANE]

Thanks Gents that seems logical although I ended up doing what Malcolm did and gave up balancing the carbs individually. I couldn't get rid of an intermittent misfire no matter how much adjustment I did.
Tuning the carbs when linked together made it disappear. The car now idles very well. Although I was surprised that I only had to adjust the slow running screws and not the jet screws. They were left at at the base 2&1/2 turns down from the bridge.
I did have one false start though, when I followed the advice in an article written by one of the CMC guys. It stated that you should start with the jets 0.125" down from the bridge. I was surprised how easily the car started with no choke and the slow running screws had no effect whatsoever. I then realised that 0.125" equates to about 6 turns down on the jet screw. I'm assuming this was a typo and not what they actually do when tuning the carbs at CMC! 2&1/2 turns equates to 0.055" on my carbs- quite a difference.
It just goes to show that you shouldn't believe all you read especially on the interweb.

Cheers

Alan

[mgcjag]

Hi Alan...0.125 is the measurement given to me by David Lonsdale....very well known to the forum as one of the best carb rebuilders........The 2.5 turns as specified in the service manual is only to be done once the jet has been adjusted to flush with the bridge......I set mine useing the 0.125 and they work fine......Steve

[ALAN COCHRANE]

Hi Steve

Sorry I don't understand how setting the jets down 0.125" can make the carbs run normally. When I did it the engine started instantaneously from cold with no choke-not that there would be any even if I'd used the dash lever. The car was revving at about 1600rpm and obviously the slow running screws had no effect. Where do you go from there? I'm assuming you have to raise the jets until the idle speed drops?
I levelled the jet with the bridge on the rear carb and then wound it down 2&1/2 turns and measured the depth with a micrometer-0.055". I then set the other two down the exact same amount. I've left them at that and after balancing the carbs with the slow running screws the car idles very well.
I'd be very interested to know how you ended up with a normal idle speed.

Cheers

Alan

[mgcjag]

Hi Alan...what needles are u useing and are they set flush in the piston... Steve

[MarkRado]

The same is true for me: 0,042 for the UM (standard filter) and 0,052 for the UE (mangoletsi filter). Having read similar posts about carb adjusting, I tried going that much richer- impossible, I immediately get heavy rhythmical misfire and black smoke. Following the spark plug colour with my setup, I am nowhere near the lean side. It is a 3.8 engine with AUD112 carbs.

[christopher storey]

I think that somewhere, the jet diameter ( 0.125") has got confused with approximately how far below the bridge the jet is . My own experience has been with UM needles that I start from about 0.065" below the bridge and when I have reached the point where both pipes produce about 5.5% CO, take the dashpots off again , measure the depths, and then set all 3 to the average reading that the 3 collectively give me.This is usually in the 50 to 70 thou down bracket. I should say that this is with 4.2s, which have much bigger balance pipes than the 3.8 manifolds

[abowie]

I set them at 60 thou as a starting point.

[mgcjag]

Hi All...i wasnt refering to the 0.125 as the final position of the jet from the bridge but as a starting point.....when i rebuilt my carbs and float chambers i was recommender to start with the jets 0.125 down......the car started immediatly...they could then be tuned up.......i never remeasured to find the final resting point as that would have involved removing the pistons........ so my comments were made to back up the CMC artical that you should start with the jets 0.125 down rather than dismiss it as wrong..in reality the start point is not really that critical..... Steve

[ALAN COCHRANE]

Steve

Thanks for the clarification, it's certainly an alternative method to consider the next time around. I have rebuilt the carbs with the Burlen kit so the needles are UM and I'm using the standard air filter arrangement.
When I started the rebuild I found that the old needle in the middle carb was sitting low in the piston. The top lip of the needle groove was level with the piston base. This would result in the carb running lean compared to the other two.
I'm not sure if I was the culprit or if it had been like that since before my ownership. All my previous tuning efforts would have been hampered by this.
Anyway today is the big day-I finally get to take the car out for it's first test drive of the year. It's been a long journey since last November. What started off as a clutch change morphed into a virtual front end rebuild. On more than one occasion I've felt like walking away and closing the garage door. But on the positive side I now know the car so much better.
One final thing-where can I get a set of the metal tags that fit under the float chamber lid securing bolts?

Cheers

Alan

[mgcjag]

Hi Alan..Burlan sell them.. Steve