Don Cook Posted September 4, 2003 Share Posted September 4, 2003 Hello good people, I recently acquired a 1996 (5000 miles) 2L HPC with 48 DCOSP carbs. Most of the time there is a serious flat spot at about 3000 – 4000 but not always, occasionally it will run for hours without problems. I’ve checked the carbs, they are like new, the chokes and jets are as they left the factory. Generally it appears to get worse when the weather is warmer but not always. I’ve spoken to a couple of experts on the matter and they recommend changing the carbs to 45s and in principle I can do this, the point is though why does it sometimes run well and others not. Has anyone any advice. Cheers Don Cook. Link to comment Share on other sites More sharing options...
Roger King Posted September 4, 2003 Share Posted September 4, 2003 Don, The HPC road car came with 45DCOEs. The race cars had 48DCOSPs. The 48DCOSP is generally horrible to drive on the road because of very poor progression from slow run jets to main jets. In fact the 48s are really choked down 50DCOSPs. These carbs could indicate that your car was originally one of the race series cars, or maybe someone has been fiddling in the past. I'm not sure why it is sometimes OK, but 45s will normally be a huge improvement, due to fundamentally different design and internal dimensions. The 48s can be improved (maybe this has already been done), but will never be as good as 45s. You won't find any power loss with 45s either, assuming that the engine is standard. Just don't put the 45s on the official Caterham jetting - it's dreadful. Link to comment Share on other sites More sharing options...
k.russell Posted September 4, 2003 Share Posted September 4, 2003 So Roger, can you tell us what the best jetting set up is for 45's, as i may have the "dreadfull" set up,same as when i built the car 11year ago, and it does seem ok. Link to comment Share on other sites More sharing options...
Paul McKenzie Posted September 4, 2003 Share Posted September 4, 2003 You've got used to it, Ken ...all the other experts agree. By the way, love your films...Dr.Zhivago, Women in Love etc...shame about Glenda Slag though, although she talks a lot of sense nowadays. Paul Link to comment Share on other sites More sharing options...
Don Cook Posted September 5, 2003 Author Share Posted September 5, 2003 Roger, Thanks for the input. The car definately left the factory with 48s but was hardly a race spec car, full special leather trim and all the jing bang. You mentioned that it may be possible to improve on the 48s, could you tell me what that is or where I can get some info. I've prepared a pair of 45s to go on if all else fails but unfortunately I got the settings from the factory so I would probably end up with the horrible arrangement you mention. Again can you or any one advise. Thanks again Don. Link to comment Share on other sites More sharing options...
Roger King Posted September 5, 2003 Share Posted September 5, 2003 Don, With the 48s you'd have to fit smaller Aux Vents to improve progression. Ideally, smaller chokes too and then jet to suit. Unfortunately, I haven't actually done this for this spec of engine, so I can't give you any detailed info. For the 45s, try:- Choke 38mm Main 155 Air 170 Slow run 60F8 Pump 35 or 40 (NOT 45) The above is a starting point only and assumes the engine is standard. Jetting is affected by exhaust type too. You will probably need to drill extra progression holes too, but don't do this until you have tried the above because some carbs need it and some don't (no idea why - welcome to the wonderful black art of setting up carbs). Link to comment Share on other sites More sharing options...
Don Cook Posted September 5, 2003 Author Share Posted September 5, 2003 Roger, Again thanks for the follow up. The 45s I have are a couple of years old but un-used. They have 2 progression holes, if I have to drill more then do you know how many, what size and where. Thank, Don. Link to comment Share on other sites More sharing options...
Roger King Posted September 5, 2003 Share Posted September 5, 2003 I'm afraid that it's impossible to describe the extra holes in words. I have a jig, but have now stopped engine building. There are others who will do this job for you. Best to ask around. As stated before, try the car first. Because if it turns out that your engine doesn't need the extra progression holes, they will make it worse. What you are looking for is a flat spot somewhere between 1500rpm and 3000rpm on very light throttle if you need the holes. Edited by - Roger King on 5 Sep 2003 12:41:38 Link to comment Share on other sites More sharing options...
stevefoster Posted September 5, 2003 Share Posted September 5, 2003 VX's had more progression holes than 2. 3 or 4 and the Zetec like even more... Specialist job with a jig to get that drilling right I am told. You could always make the thing even better an fit Throttle bodies. I did this during last winter and the difference is amazing... Expensive unless like me you get all the gear cheap 2nd hand, youfit it yourself and you can map it yourself with a Wideband O2 kit from Mr Shurvington on the Se7en's mailing list.... Hants (North) and Berkshire Area club site here Light blocks here Link to comment Share on other sites More sharing options...
Don Cook Posted September 8, 2003 Author Share Posted September 8, 2003 Pardon my ignorance but what are "throttle bodies" Don Link to comment Share on other sites More sharing options...
Peter MarieEa Posted September 9, 2003 Share Posted September 9, 2003 Roger, what is the "dreadful official Caterham settIng" on 45 DCOE'S ? Have a '94 HPC and the low speed running is admittedly poor. Quick7 Link to comment Share on other sites More sharing options...
Don Cook Posted September 10, 2003 Author Share Posted September 10, 2003 I recently got this info from Simon Lambert at Caterhan. 45 DCOE settings Choke - 42mm Main Jet - 165 Air corrector - 175 Emulsion tube - F11 Idle Jet - 50F6 Pump Jet 45 Don Cook Link to comment Share on other sites More sharing options...
Don Cook Posted September 10, 2003 Author Share Posted September 10, 2003 Sorry Chaps the settings mentioned above are the Caterham ones for 48 DCOSPs. The settings for 45s are :- Venturi 40 Main 170 Emulsion F16 Air corrector 180 Idle 50F6 Pump jet 45 Don Cook Link to comment Share on other sites More sharing options...
rjc Posted September 10, 2003 Share Posted September 10, 2003 The webber manual gives the following for standard calibration on 45 DCOE's choke 36 emulsion f16 air corrector 155 main jet 145 idle 55f8 For the standard engine. My VX arrived from Caterham with the stated jetting as stated by Mr Cook above and I was never happy with it. I've tried various jetting changes and used f11 and f16 emulsion tubes to little effect. However I have found that the idle jets do make a noticeable difference and have run as large as 60f9 which I found decreased miles per gallon as noticably as they increased low throttle take up. I'm dying to know what other combinations are worth investigating ! Link to comment Share on other sites More sharing options...
StewartG Posted September 10, 2003 Share Posted September 10, 2003 It's all down to the progression holes. You can to some extent compensate with an over rich idle jet but this leads to poor petrol consumption and is not really the answer. My carbs were specially modified by webcon and have five progression holes any less and it runs too lean just above idle. There are all sorts of jetting combinations depending on choke size and engine spec but none of them can work optimally unless the progression holes are sorted. Link to comment Share on other sites More sharing options...
Peter MarieEa Posted September 10, 2003 Share Posted September 10, 2003 I'm tempted to try Roger King's jettings suggestions. But can't help wondering how much top end power will be lost by downsizing the Choke from the factory 40mm to 38mm? Does Roger recall dynoing a standard tune VX with side exhaust? To Stewart G. Are you running 40mm or 38mm Chokes in your reworked Webers? Quick7 Link to comment Share on other sites More sharing options...
Roger King Posted September 11, 2003 Share Posted September 11, 2003 You won't lose any power by downsizing the chokes. The standard 40mm ones are ridiculously OTT for an engine at this low level of tune. All they are doing is losing driveability. Believe it or not, but the Caterham upgrade package to (an alleged) 218bhp actually went down on choke size. The standard idle jet is ridiculous - the F6 air hole is so small that you don't get proper atomisation of fuel, even if the actual amount is approximately correct. Oversize pump jets are one of the commonest mistakes I have seen over the years on modified engines and coming down from 45 ones will improve throttle response. All references to standard jets are referring to Caterham's standard jetting for a Vauxhall, NOT the standard jetting that Weber supply in off the shelf carbs. The side exit race exhaust is a lot bigger than the otherwise similar road system and is surprisingly worth up to 10bhp, even on a standard tune engine. As I said earlier, it is worth trying the new settings without the progression holes first. If you have light throttle flat spots, have them put in. Link to comment Share on other sites More sharing options...
ian balson Posted September 18, 2003 Share Posted September 18, 2003 Roger, You didn't mention the your favoured emulsion tube value for your std HPC settings, would an F16 do the job? Thanks Ian Link to comment Share on other sites More sharing options...
Roger King Posted September 18, 2003 Share Posted September 18, 2003 Whoops, sorry. Yes, the F16 is a good starting point. You may want to try F11 or F7 too, but if you already have the F16s, don't buy any new ones until you've tried them. Link to comment Share on other sites More sharing options...
ian balson Posted September 18, 2003 Share Posted September 18, 2003 Great, thanks Roger, I've got all the other bits now. So, when my Chicken Pox ridden family allows me the time, I'll be off into the garage for an evenings spannering. Cheers Ian Link to comment Share on other sites More sharing options...
ian balson Posted October 12, 2003 Share Posted October 12, 2003 Took Roger's advice and changed the jetting on my standard VX to his recommended settings. I must say it's made a vast improvement. Progression is much smoother, driving around town is no longer a chore. The response to hard acceleration has also improved, no bogging down, I guess this is the smaller Pump jet. I went for the 35 pump jet. I haven't noticed any drop in top end performance from taking the chokes down from 40 to 38mm. I used my existing emulsion tubes (F16). For those interested, the whole lot cost £104 in parts. Dead easy to change, you need to take the carbs off to fit the 38mm chokes. You will also need some proper lock wire to secure the bolts that hold the Choke and Aux Ventrui in place. The one thing that did catch me out was that I needed a set of washers for the pump jets (old ones were unusable, and stuck to the old pump jets), the washers are not available individually from Webcon, they are only available as part of the service kit. Luckily I had serviced my carbs last winter and had not used the Pump washers so I was ok. Thanks for the info Roger, best £100 I've spent on the car. Rgds Ian Link to comment Share on other sites More sharing options...
Graham Perry Posted October 12, 2003 Share Posted October 12, 2003 We are all going to miss him now that he is having a break from engines. Link to comment Share on other sites More sharing options...
Peter MarieEa Posted November 3, 2003 Share Posted November 3, 2003 I installed Roger's Jetting in two stages. First just the Jets (the 38mmChokes I had to order separately).. While waiting for the Chokes to arrive I installed the Jets. I noticed an immediate improvement in the engines idle quality which I attribute the Idle Jet. Yes I know the Idle Jet controls low engine speed but it does have an affect on the idle as well. Before the car would stall an awful lot. But the car still bogged at anything more than light throttle under 3000 rpm. Finally got and installed the Chokes. Easy to do with out removing the Carbs. The Set Screws that locate the Aux Venturi and Chokes I found relatively easy to resafety wire in place with a Wire Twister. It was the 38mm Chokes that totally elliminated low rpm bog under moderate to hard acceleration. The increased torque under 3000 rpm is dramatic. I can now light up the rear tires at as little as 2000 rpm. Is there any loss of top end power, I'm not sure; maybe a little bit. I haven't had enough time see. Regardless, for road use the improvement is well worth it. When I made the Jet changes I went with the size 40 Pump Jet. I don't know for sure if it is related, but I do still have a light "miss" on moderate acceleration. So I might try the 35 Pump Jets. For those that are interested I will report back. Can't say when that will be as winter is almost upon us here in the 'Great White North'. Quick7 Link to comment Share on other sites More sharing options...
Glenn Wheeler Posted April 7, 2004 Share Posted April 7, 2004 can I relight this topic, I would like to change my jetting as per Rogers comments but where do I buy all the bits ? Link to comment Share on other sites More sharing options...
stevefoster Posted April 7, 2004 Share Posted April 7, 2004 You could get them from Southern Carbs in Wimbledon, From Weber or Webcon try a search on Google or try a post in the wanted section here. As new they will be expensive. You'd be amazed at the stuff folk have and never use... Hants (North) and Berkshire Area club site here My racing info site here Link to comment Share on other sites More sharing options...
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