Mounting dual Webers in 1700 Supersprint

[AnkerB-S]

I was getting ready to balance the carburetors, removed the air filters and warmed the engine up to operating temperature. Barely touched the carbs and noticed that this immediately caused the engine revs to go up and down.

I think I made a mistake believing that this had to be caused by an air leak and took a look at the manifold nuts. As soon as I put a bit of pressure on the carbs I could see that they moved against the manifold and though they were too loose. Tightened the top four a little bit (the engine was hot so I didn't want to put my hands underneath the manifold) and thought I heard and saw some improvement. Ran the car up to a service station to have it inspected (the Massachusetts equivalent of the MOT) and thought it ran a bit better.

Only after this did I decide that I better check here and the various books and e-books I have and then realized that there is a thick, flexible gasket that provides vibration isolation. See the picture below.

I did a search on this site and found a thread from 2001 on the same topic and saw two different instruction posts. One specified, as far as I remember, 8 ft lbs, the other said something like 1 1/2 turn of the bolt after everything just touches. Of course I can't find this post any more.

I checked the assembly guide and the Weale book and didn't find anything there.

I should know better

Any help and/or pointers will be appreciated.

Thanks/Anker

 

[Roger King]

It's impossible to be specific on this because the flange of the washers varies so much in depth. There is certainly no torque figure for this because you should be using a nyloc that is not fully tightened. What is certain is that you need some movement of the carbs on the manifold in order to prevent fuel frothing. A rough rule of thumb is to set up with about 10mm of up and down movement at the outer end of the ram pipes without forcing it.

The best gasket to use between carbs and manifold is a Misab - you need one per barrel. This is different to what is seen in your illustration, being one large cross section 'O' ring bonded to an aluminium carrier that slots over the studs on the inlet manifold. This is more flexible, longer lasting and better sealing that what is in your photo (what you see there is a metal plate with a thin 'O' ring on each side).

If you initially start with things fairly loose you can easily tighten the nuts up a bit if there is an air leak around the gasket (spray WD40 or similar over the joint to see if it is sucked in), but if you overtighten, aside from poor running, you run the risk of permanently deforming the 'O' rings so that they will never seal.

Oh, and if that cold start enrichment cable is connected to anything, it's much better to remove it and give three pumps of the throttle when starting from cold.

[AnkerB-S]

Thanks Roger,

I think I'll go with the Misab, inexpensive and easy to procure.

The choke cable is necessary until I get the engine set up right. It takes full choke to get the engine to fire here in January.

I'll follow your suggestions tomorrow, get the carbs balanced and the idle mixsture set right according to the tuning instructions. I definitely tightened them too much earlier today.

Tomorrow we are having a record warm January day. I put a can of Sea Spray in the tank and will give the car a good run in the afternoon and see how that affects the running. When I get home from the run I'll do the carb tune.

Thanks for the post. I have been anxiously waiting for someone to chime in!

Anker

[SM25T]

Had to Google that. Like our Redex additive

[AnkerB-S]

I have a funny Redex story.

In the early 70's my brother in law owned a Wartburg, an east German DKW derivative, with a two stroke engine. They tend to get a lot of carbon build up, which Redex can take care of. We ran the car up to temperature, removed the spark plugs and poured some Redex into the cylinders to soak the carbon on top of the pistons. After it soaked for a short while i leaned forward to take a peek through the spark plug hole and see how it was doing. Just before I got my head over the engine the mixture in the cylinder I was going to check exploded and blasted the underside of the hood (bonnet) with a mixture of Redex and carbon. Taught me a valuable lesson!

[SM25T]

Interesting chemical reaction !! We used to squirt a bit into each spark plug hole in our Mini and let it soak. Then started it up and wiped out the whole street with white smoke. Also used up take the air filter off the big SU carburettor and pour it into the airflow while fast idling the engine. Sometimes it would almost stall, so a few more revs were needed. Again white smoke worthy of a Red Arrows air display. Maybe that's why we could never buy a house in Caterham on the Hill !!

[HendrixsWhiteStratReturns]

To the OP - good luck with it, I'm running twin webers on my 1990 1700 SS.

Just to echo what Roger said - remove the cold start gubbins (Weber do a kit) as soon as you can...those things are a damn nuisance and mine has run so much better since I had the carbs refurbed and blanking kits fitted.

ee

[AnkerB-S]

After cleaning the air filters, rebalancing the two carbs, adjusting the idle mixture and advancing the ignition timing to 12 degrees it starts much better and runs much smoother than before. I'll se how a start without choke works in the cold soon.

[Andrew Gilbert]

your best bet is to get it running best you can then have someone set it up on on a dyno, the difference can be fairly significant.

You don't want blow back through the carbs, although popping, banging and flames through the exhaust is more than acceptable...

[AnkerB-S]

The blow back through the carburetors is gone. That seems to have been caused by the ignition being too far retarded, and possibly crud in the carburetors. Since I adjusted the timing and did some spirited driving with Sea Spray in the fuel that's gone.

Good advice about the dyno. I know several independent Porsche shops with dynos. I am sure they will enjoy putting a Caterham on it.