Misbehaving Weber?
Posted: Sun Jul 10, 2016 3:16 pm
Hello all, I'm new (again) to carburettors, after learning fuel injections systems for the last two decades and getting really into the test equipment that dealers use to diagnose them... Back to carburettors and it's all new again as the ones I owned 20 odd years ago never really gave any trouble.
The car is a 1968 +2 with Weber 40DCOE31s fitted. The engine was rebuilt in the early 80s by Dave Smith (a Lotus specialist from Ilford) and I have a rolling road result table from that visit, while that was 30 odd years ago, it was also approximately only 7k miles ago, showing a headline of 138bhp at 7k rpm. It's pretty certain that the car ran well at that point and in the rebuild, two CPL2 camshafts were fitted (can anyone explain the meaning?) and Dave recorded all the jet sizes fitted in the carbs, which I have compared to what is fitted now.
Maybe it needs a good set up, but the car has only recently returned from Barry Ely, who last year had it in to fix this issue, but was only able to improve it slightly, It idles pretty well, responds pretty well to a blip of the throttle but then is all lumpy on progression under load, resulting in sooty plugs and a fairly rythmic 'miss'. When I brought it home, I spent a little while at high speed on the M25 and after a couple of miles, the running cleaned up really well and it felt smooth, fast and responsive until I slowed down again, soon after which it was back to hesitation. So while I have no doubt Barry had a good go at sorting it, I'd like to get to the bottom of it and be sure it is right.
When Dave Smith built and tested the engine, the carb had the following:
Main Jets: 120
Air Correctors: 150
Choke: 32mm
Emulsion Tube: F11
Idle Jets: 45F8
Today it is fitted with:
Main Jets: 115
Air Correctors: 150
Choke: 32mm
Emulsion Tube: F11
Idle Jets: 45F8
So the only thing that looks different is the Main Jets (115 now, vs 120 before),
I know I need to measure float levels next (first?), so will read up on that and I've found a guide suggesting that the final position of the idle mixture screws should be in the range 1.5 to 2.5 turns out, so I checked mine:
1: 1 and 1/8 turns
2: 7/8 turn
3: 1 turn
4: 7/8 turn
Here it is: http://www.redlineweber.com/html/Tech/c ... _best_.htm Following that guide, the author suggests that the idle jets could be too large if the best idle settings were as above. I am starting to think I should approach this as though these were new carburettors rather than trying to assume that they are already correctly set up. I'll find the links and post them in this thread.
Somewhere I have a Colortune, so I will have a look at that and see what it suggests is happening.
I don't have a CarBalancer (yet) so I can't confirm the balance of the carbs and from what I have read so far, this is one of the more critical adjustments to get right before I can do much else.
Ignition control is Lumenition and the spark plugs are Bosch Platinum '8' range. I can order some NGK 6 but can't see them fixing this low speed condition.
Some things have changed over the intervening years - namely fuel quality. I know that Brian used to use Shell Nitro in it, to avoid the ethanol of other modern fuels and I haven't added any fuel yet since acquiring it.
Advice and pointers welcome. I've done some searches and this one gave me the most clues: lotus-carbs-f40/weber-jetting-t36196.html
Realising that carbs are specced depending on the driver's wishes and expectations, I'm looking for a smooth, pleasant drive more than a racing car at wide open throttle.
Thanks,
Ian.
The car is a 1968 +2 with Weber 40DCOE31s fitted. The engine was rebuilt in the early 80s by Dave Smith (a Lotus specialist from Ilford) and I have a rolling road result table from that visit, while that was 30 odd years ago, it was also approximately only 7k miles ago, showing a headline of 138bhp at 7k rpm. It's pretty certain that the car ran well at that point and in the rebuild, two CPL2 camshafts were fitted (can anyone explain the meaning?) and Dave recorded all the jet sizes fitted in the carbs, which I have compared to what is fitted now.
Maybe it needs a good set up, but the car has only recently returned from Barry Ely, who last year had it in to fix this issue, but was only able to improve it slightly, It idles pretty well, responds pretty well to a blip of the throttle but then is all lumpy on progression under load, resulting in sooty plugs and a fairly rythmic 'miss'. When I brought it home, I spent a little while at high speed on the M25 and after a couple of miles, the running cleaned up really well and it felt smooth, fast and responsive until I slowed down again, soon after which it was back to hesitation. So while I have no doubt Barry had a good go at sorting it, I'd like to get to the bottom of it and be sure it is right.
When Dave Smith built and tested the engine, the carb had the following:
Main Jets: 120
Air Correctors: 150
Choke: 32mm
Emulsion Tube: F11
Idle Jets: 45F8
Today it is fitted with:
Main Jets: 115
Air Correctors: 150
Choke: 32mm
Emulsion Tube: F11
Idle Jets: 45F8
So the only thing that looks different is the Main Jets (115 now, vs 120 before),
I know I need to measure float levels next (first?), so will read up on that and I've found a guide suggesting that the final position of the idle mixture screws should be in the range 1.5 to 2.5 turns out, so I checked mine:
1: 1 and 1/8 turns
2: 7/8 turn
3: 1 turn
4: 7/8 turn
Here it is: http://www.redlineweber.com/html/Tech/c ... _best_.htm Following that guide, the author suggests that the idle jets could be too large if the best idle settings were as above. I am starting to think I should approach this as though these were new carburettors rather than trying to assume that they are already correctly set up. I'll find the links and post them in this thread.
Somewhere I have a Colortune, so I will have a look at that and see what it suggests is happening.
I don't have a CarBalancer (yet) so I can't confirm the balance of the carbs and from what I have read so far, this is one of the more critical adjustments to get right before I can do much else.
Ignition control is Lumenition and the spark plugs are Bosch Platinum '8' range. I can order some NGK 6 but can't see them fixing this low speed condition.
Some things have changed over the intervening years - namely fuel quality. I know that Brian used to use Shell Nitro in it, to avoid the ethanol of other modern fuels and I haven't added any fuel yet since acquiring it.
Advice and pointers welcome. I've done some searches and this one gave me the most clues: lotus-carbs-f40/weber-jetting-t36196.html
Realising that carbs are specced depending on the driver's wishes and expectations, I'm looking for a smooth, pleasant drive more than a racing car at wide open throttle.
Thanks,
Ian.