Brand new Webers . . . 1700 Super Sprint

[werthie]

greetings!

 

I've tried to search previous posts, with sketchy results (different engines, different chokes, etc), so here goes:

 

after an engine fire, I bought and installed a set of brand new Weber carbs, with decent but not great results . . . without putting the car on a dyno and measuring the A/F ratio, I know for sure that I have a major stumble coming off idle, including after every gear change.

 

for those of you with a 1700 Super Sprint running 30mm chokes, what are you running for mains, airs, idle and emulsion tubes?

are there any other variables?

 

thank you for your help!

 

alan in West LA

[xflow7]

Any particular reason you are using 30mm chokes? I think normally a 1700 SuperSprint-ish spec would use a 32mm-34mm choke. Here is a link to a reference for sizing chokes:

 

http://7faq.com/owbase/ow.asp?HowDoI%2FTuneWeberCarburettors

 

I think I have my jetting (for a 1700 with, I think, 32mm chokes) posted somewhere on the interweb, but I haven't managed to find it just now. If I come across it I'll post it here.

 

In the meantime, can I assume you've already gone through the basic setup of syncing carburetors, lean idle mixture, etc? Either of these being out by much can cause all sorts of off-idle misbehavior because of the idle position of the butterflies not being correct with respect to the progression holes.

 

Dave

[bigdog]

Any particular reason you are using 30mm chokes? I think normally a 1700 SuperSprint-ish spec would use a 32mm-34mm choke. Here is a link to a reference for sizing chokes:

 

[/url]http://7faq.com/owbase/ow.asp?HowDoI%2FTuneWeberCarburettors

 

I think I have my jetting (for a 1700 with, I think, 32mm chokes) posted somewhere on the interweb, but I haven't managed to find it just now. If I come across it I'll post it here.

 

In the meantime, can I assume you've already gone through the basic setup of syncing carburetors, lean idle mixture, etc? Either of these being out by much can cause all sorts of off-idle misbehavior because of the idle position of the butterflies not being correct with respect to the progression holes.

 

Dave

 

My 1600 sprint that's rated at only 100HP or so came from Caterham with 30MM chokes.

[TheDingo8MyBaby]

I guess the first question is: How do you know that the carbs are causing the stumble?

 

Have you checked your ignition curve to see where the advance comes in at? If so, where?

 

The link xflow7 provided is a good starting point for our cars, but doesn't tell the whole story. A common cause for the stumble is transition between the idle circuit and mains. This transition can be cured by going to hypojets from keith franck. I don't think the 30mm chokes are your issue, but may not be the right size for power in high RPM.

 

If you really would like to spend the time understanding your carbs, I suggest doing some research here: http://autos.groups.yahoo.com/group/sidedraft_central/

[xflow7]

I guess the first question is: How do you know that the carbs are causing the stumble?

 

Have you checked your ignition curve to see where the advance comes in at? If so, where?

 

The link xflow7 provided is a good starting point for our cars, but doesn't tell the whole story. A common cause for the stumble is transition between the idle circuit and mains. This transition can be cured by going to hypojets from keith franck. I don't think the 30mm chokes are your issue, but may not be the right size for power in high RPM.

 

If you really would like to spend the time understanding your carbs, I suggest doing some research here: http://autos.groups.yahoo.com/group/sidedraft_central/

 

Thanks for pointing that out. I did not intend to imply that the information there was the be-all end-all. I also don't think the 30mm chokes, in and of themselves would be a problem. Rather, just pointing out that most 1700 engines of that spec run 32's and so the OP may have an easier time finding jetting combinations that work well.

 

Checking the ignition is a good idea, too.

 

I have heard good things about the hypojets, but I will say for the OP's benefit that my 1700SS runs very well with stock Weber parts, and doesn't stumble at provided I keep the carbs balanced and idle mixture set properly.

[bsimon]

Take a close look at your old Webers if you still have them. There will probably be a series of custom drilled progression holes in the idle/mid curcuit access port above the throttle plates. I'm not sure what Kent block drillings are speced by Caterham, but there are between 4 and 6 holes in each port of the 48DCOE and 45DCOE carbs found on the VX engined cars. Since the 1700SS is a rather specific engine to Caterham, I would suspect they had established some custom tuning requirements for the webers as well.

 

Off-the-shelf Webers usually only have 1 hole.

 

Google "Weber progression holes"