Hesitation on Half Throttle
Discussion
Hi Folks,
Long term PH'er, but a new member to this area of the forum having recently purchased my first Caterham.
It's a Beaulieu with the 1.6 Vaux SOHC on Webbers, and is so far proving to be good fun in the decent weather we've been having.
I do, however, have an issue that I'm struggling to get tot the bottom of - on Half throttle, it has a huge flat spot and miss- fires quite badly. At idle, or full throttle all seems well.
Experience with older cars would have suggested maybe timing advance, but this has electronic ignition so I'm not sure whether the same would apply ?
I'd be grateful for any advice from the forums, and any recommendations for someone with good experience of setting up twin carbs, preferably with a rolling road, in North Berks.
Thanks in Advance
Long term PH'er, but a new member to this area of the forum having recently purchased my first Caterham.
It's a Beaulieu with the 1.6 Vaux SOHC on Webbers, and is so far proving to be good fun in the decent weather we've been having.
I do, however, have an issue that I'm struggling to get tot the bottom of - on Half throttle, it has a huge flat spot and miss- fires quite badly. At idle, or full throttle all seems well.
Experience with older cars would have suggested maybe timing advance, but this has electronic ignition so I'm not sure whether the same would apply ?
I'd be grateful for any advice from the forums, and any recommendations for someone with good experience of setting up twin carbs, preferably with a rolling road, in North Berks.
Thanks in Advance
I had the same with mine which I sold about a year ago. I put it down to poor carb set up, and we never really got it right to be honest. It was running far to rich originally, but we never got rid of the missing and flat spots. It had had more aggressive cams installed, which might have had made things worse. I sold it to Sevens and Classics and they must have sorted out it, as they commented on how well it drove. I guess any of the proper Caterham specialists would be worth a call to get some help
"Half-throttle" may not be specific enough to diagnose from, but a couple things can affect weber smoothness in the beyond off-idle and not close to wide-open-throttle areas of driving.
First possible candidate; especially if it is closer to 30% throttle than half, is the progression circuit. Webers can have a bit of a flat spot between the idle circuits and main circuits coming on. Especially with a bit of a hot cam. One tends to find that running a slightly richer idle jet than optimum for idle, or setting the idle mixture screws a 1/4 turn richer, may cover that flat spot. It can occur at slight throttle openings from around anywhere about 1200 to 2800 rpm, depending on engine and idle speed and camshaft. So if the car seem happiest idling with a 45F8, you may have to go to a 45F9 to cover the dead spot, and have a bit of a throaty idle. (newest 45 DCOE carbs can be had with up to 5 progression circuit holes drilled in them to match modern engines better, the stock style is 3 holes, so that is a much greater continuation of the progression progression.)
Secondly, if it is definitely well past the progression circuits, then it may take a slightly richer or leaner main jet. Trying a shift in size is all you can do to test that, or run an AFR gauge to check.
Another solid contributor to air fuel ratio curves over an engine's RPM is the emulsion tube, and you may find one with a better fit. If you are lean near the middle throttle, then you need an emulsion tube that has less holes close to, say, 30% down the tube. As carb venturi air speed increases, the fuel drops in relation to the emulsion tube, and where the holes are will lean those areas out more, as the fuel level drops past them. So holes up top lean out the initial main circuit action, and may also retard it coming on a bit, holes down low will lean out higher RPM areas. Classic e-tubes start at F11, then F9, then F15, then F16, and F2.. running form individual cyclinder capacities of, say (not too accurate here from memory), 250 ccs, then 300, then about 400, and F16 is for about 450 and F2 is 500cc (and richer acceleration). So a 2 litre engine would usually run an F16 or F2. (yet and F9 or F15 may work for it). F11 have middle air holes, and an F9 is for similar engines, but has no middle holes so runs richer at around 3500-4000 rpm.
Thirdly; carbs are a bit of a compromise, for getting most power you do tend to not be optimal for idle and off-idle performance. Usually due to a larger carb and choke picked for WOT power versus driveability. This does tend to leave a flat spot and also a gap in the progression, due to a weaker vacuum signal. You could fit smaller chokes and re-jet the whole carb, if it works out as being a bit over-sized for the application. There are tables and equations for working that out online.
Fourth, carb balance can put things a bit off, so check air flow from the carbs and balance them out.
Fifth, you might have an air leak past manifold mounting gaskets, or loose bolts on the carbs, or even loose jets in the carb. Perhaps even a blockage in the carb.
Other problems may be cam timing slipping, or your crankshaft pickup timing changing somehow (depends on pickup and ECU adjustment). You might even have a cranky coil that doesn't like a certain RPM, though bad coils I've had have tended to be weak at the higher RPMS, not in the middle.
If you have a throttle position sensor, it may be off and feeding the wrong timing in. *shrug*.
I have a cosworth engine with huge weber 48 DCOP carbs that ran 42 mm chokes. I've played for ages with them, and changed everything from the all-out-racing setup, and the car is so much more driveable, and with no real-world noticeable loss in power. In fact, having it more tractable and responsive makes the car quicker and safer.
Hope this helps, or at least gives you places to research.
First possible candidate; especially if it is closer to 30% throttle than half, is the progression circuit. Webers can have a bit of a flat spot between the idle circuits and main circuits coming on. Especially with a bit of a hot cam. One tends to find that running a slightly richer idle jet than optimum for idle, or setting the idle mixture screws a 1/4 turn richer, may cover that flat spot. It can occur at slight throttle openings from around anywhere about 1200 to 2800 rpm, depending on engine and idle speed and camshaft. So if the car seem happiest idling with a 45F8, you may have to go to a 45F9 to cover the dead spot, and have a bit of a throaty idle. (newest 45 DCOE carbs can be had with up to 5 progression circuit holes drilled in them to match modern engines better, the stock style is 3 holes, so that is a much greater continuation of the progression progression.)
Secondly, if it is definitely well past the progression circuits, then it may take a slightly richer or leaner main jet. Trying a shift in size is all you can do to test that, or run an AFR gauge to check.
Another solid contributor to air fuel ratio curves over an engine's RPM is the emulsion tube, and you may find one with a better fit. If you are lean near the middle throttle, then you need an emulsion tube that has less holes close to, say, 30% down the tube. As carb venturi air speed increases, the fuel drops in relation to the emulsion tube, and where the holes are will lean those areas out more, as the fuel level drops past them. So holes up top lean out the initial main circuit action, and may also retard it coming on a bit, holes down low will lean out higher RPM areas. Classic e-tubes start at F11, then F9, then F15, then F16, and F2.. running form individual cyclinder capacities of, say (not too accurate here from memory), 250 ccs, then 300, then about 400, and F16 is for about 450 and F2 is 500cc (and richer acceleration). So a 2 litre engine would usually run an F16 or F2. (yet and F9 or F15 may work for it). F11 have middle air holes, and an F9 is for similar engines, but has no middle holes so runs richer at around 3500-4000 rpm.
Thirdly; carbs are a bit of a compromise, for getting most power you do tend to not be optimal for idle and off-idle performance. Usually due to a larger carb and choke picked for WOT power versus driveability. This does tend to leave a flat spot and also a gap in the progression, due to a weaker vacuum signal. You could fit smaller chokes and re-jet the whole carb, if it works out as being a bit over-sized for the application. There are tables and equations for working that out online.
Fourth, carb balance can put things a bit off, so check air flow from the carbs and balance them out.
Fifth, you might have an air leak past manifold mounting gaskets, or loose bolts on the carbs, or even loose jets in the carb. Perhaps even a blockage in the carb.
Other problems may be cam timing slipping, or your crankshaft pickup timing changing somehow (depends on pickup and ECU adjustment). You might even have a cranky coil that doesn't like a certain RPM, though bad coils I've had have tended to be weak at the higher RPMS, not in the middle.
If you have a throttle position sensor, it may be off and feeding the wrong timing in. *shrug*.
I have a cosworth engine with huge weber 48 DCOP carbs that ran 42 mm chokes. I've played for ages with them, and changed everything from the all-out-racing setup, and the car is so much more driveable, and with no real-world noticeable loss in power. In fact, having it more tractable and responsive makes the car quicker and safer.
Hope this helps, or at least gives you places to research.
Thanks!
Um, I should probably address the electronic ignition issues. :-)
Early electronic ignitions just use electronics to replace the points, traditional points wore very quickly. The electronic ones used a hal effect magnetic trigger, or an optical trigger. Yet all the traditional problems of timing still exist.
So if you have a distributor, despite it being electronic, you can still have it come loose and change static timing, plus it still uses springs that stretch at higher rpm's to advance the timing, and maybe also a vacuum tube to advance timing more under partial throttle.
If you broke a spring in the distributor, timing won't advance the proper way - it may surge to max quite early - and if you have a vacuum leak, you will run less timing than optimal for economy (and proper lean fuel burning) at partial throttles and overrun.
If you don't have a distributor, but just a coil pack - then crank trigger problems and a slipped position, and wiring and ECU programming are all that effect timing. Throttle position sensors are the other thing, if you have 3D timing maps that adjust to load.
If you have a distributor it is easy to use a timing gun to check if timing advances with RPM in a reasonable manner. You can also suck on the vacuum timing hose and see if it shifts.
You can use a timing gun for distributorless ignition too, but what you see as you speed up the engine may not be a nice roughly linear curve at all. It's smarter than that. :-) You can check static timing, though, quite well.
You may even have a loose coil wire that only skips firing at a certain vibration harmonics, and is ok at low engine speeds and fast vibrations. Weirder things have happened, lol.
Usually odd engine probs are electrical, more than petrol based, but webers do have that mid-area weakness at part throttle if not tuned perfectly, or are set up for total max power. Mine still are a bit lean around 4000 rpm at smaller throttle openings; when driving it like a race car, it never shows up. Holding it at that RPM with throttle maybe at 20%, and it weakens and pops a bit. *shrug* Still trying to make it perfect, but that's how it seems to go when you are using max sized carbs/chokes on a souped up engine. It hates you when you drive it like a civilian. lol.
I'm not saying it is something you have to live with, but that at the extreme racy edge of weber application, it does tend to show up more readily. Your system should come quite close to perfect, with systems checks for faults, and then a lot of time and some money for experimenting.
Um, I should probably address the electronic ignition issues. :-)
Early electronic ignitions just use electronics to replace the points, traditional points wore very quickly. The electronic ones used a hal effect magnetic trigger, or an optical trigger. Yet all the traditional problems of timing still exist.
So if you have a distributor, despite it being electronic, you can still have it come loose and change static timing, plus it still uses springs that stretch at higher rpm's to advance the timing, and maybe also a vacuum tube to advance timing more under partial throttle.
If you broke a spring in the distributor, timing won't advance the proper way - it may surge to max quite early - and if you have a vacuum leak, you will run less timing than optimal for economy (and proper lean fuel burning) at partial throttles and overrun.
If you don't have a distributor, but just a coil pack - then crank trigger problems and a slipped position, and wiring and ECU programming are all that effect timing. Throttle position sensors are the other thing, if you have 3D timing maps that adjust to load.
If you have a distributor it is easy to use a timing gun to check if timing advances with RPM in a reasonable manner. You can also suck on the vacuum timing hose and see if it shifts.
You can use a timing gun for distributorless ignition too, but what you see as you speed up the engine may not be a nice roughly linear curve at all. It's smarter than that. :-) You can check static timing, though, quite well.
You may even have a loose coil wire that only skips firing at a certain vibration harmonics, and is ok at low engine speeds and fast vibrations. Weirder things have happened, lol.
Usually odd engine probs are electrical, more than petrol based, but webers do have that mid-area weakness at part throttle if not tuned perfectly, or are set up for total max power. Mine still are a bit lean around 4000 rpm at smaller throttle openings; when driving it like a race car, it never shows up. Holding it at that RPM with throttle maybe at 20%, and it weakens and pops a bit. *shrug* Still trying to make it perfect, but that's how it seems to go when you are using max sized carbs/chokes on a souped up engine. It hates you when you drive it like a civilian. lol.
I'm not saying it is something you have to live with, but that at the extreme racy edge of weber application, it does tend to show up more readily. Your system should come quite close to perfect, with systems checks for faults, and then a lot of time and some money for experimenting.
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