40 idf idle adjustments, looking for advice

[david c.]

Hi all,

I just installed a set of twin Weber 40 IDF carbs on my 2.0. I've never played with carb installation before, let alone this model.

I've started the car, and it runs right up to about 3000RPM and stays there for a few seconds. Then it begins to climb very slowly upward.

The car hasn't been started in a few weeks, so I'm leery of running it at such high RPM's.

How do I turn the idle down to a reasonable RPM?

I found four screws on the carbs, one near each throat. I tried to adjust them to see what happened, and it didn't make any noticeable difference.

Thanks for all the consideration and help you may have!

Dave

[neilca]

The screws you were adjusting were probably the idle mixture screws. First thing is disconnect your linkage. Locate the idle adjustment screws on the side of the barrels were the linkage attaches. This is were to lower that idle. You will need to adjust both carbs and balance them (adjust such that both carbs pull the same amount of air) to get idle. Adjust and reattach the linkage.
Good luck,
neil

[Dave at Pelican Parts]

First thing is to set your timing. If the timing is too far advanced, you'll idle too high.

There is an article on setting the timing on a carb'ed 914 in the Tech Articles section of this site.

--DD

[JP Noonan]

If I had just put something as major as carbs on I would recheck the install. It is very easy to bolt down the manifolds in such a way that the tinware gets caught in-between causing a MAJOR vacuumm leak. Also see if any vac ports are open. Were the carbs set up for a similar engine at about the same altitude? I would think if they had been jetted for a much bigger engine, or for a place like Denver, it might cause problems. For it to idle that high it would have to be a combonation of these problems though.

BTW I also agree with the two previous posts.

[PeteD]

All good advice above!

When putting on new carbs, the very first step should be to BENCH TUNE the carbs. This is done by first setting the idle jet screws about 3 turns out from all the way in. (all set the same amount. Use the flats on the screws to count the turns exactly ie 24 flats = 3 turns)


Next, set the closed throttle butterfly so that all of them are just .003" between the butterfly and the carb throat. This is easily done with the carbs upside down on your work bench. This will make it obvious to you which screws set the idle. The idle is controlled by a screw that holds the throttle lever open a bit.

This setting will be *very* close to perfect for balancing the carbs at idle (assuming your cylinders are all pulling the same). The final setting should be done on a warmed-up engine with a *quality* carb sync tool (or better, a pair of them).

The linkage must be assembled very carefully such that the carb throttle levers are not pulled up at all at idle AND that both levers move in perfect synchrony when pulled. This can be done, again on a warmed-up engine, by holding the RPM at say 2500 (use a friend for this) and adjusting only one of the linkage rods to sync the carbs using two sync tools or one, moving back and forth across the engine quickly, over and over...

Be sure you don't change the linkage adjustment when you tighten the lock nuts down.

This all of course assumes you have the right idle, main, air, and pump jets in the carb and that the venturi is sized right for your engine... Too many people start with too large a carb and never get it right.

Good luck. Once you get it right, it is a sweet thing!

PS: Buy a book on the carbs and read it about 5 times over several weeks. This will help you really understand the carbs. Carbs are *NOT* a bolt-on accessory. They take lots of tuning to get right. To do this, you have to understand how they work, what changes what, and integrate that knowledge with your observations of how the car runs under various conditions.

PPS: Also, in tuning your carbs, you need to consider your distributor. The 205AA is perhaps the best overall road car distributor for carbs. It was (one of) the stock distributor on the 1.8L.


[This message has been edited by PeteD (edited 02-14-2000).]

[neilca]

Lets talk carb sizes. I have read a lot about not getting to big of a carb. The weber carbs have replacable chokes in them. If I were to buy a set of 48mm carbs I can choke them down to flow like a 40mm carb by putting in a 40mm choke. My point is don't throw away those big carbs choke them down to make smaller carbs.

One of our racing tricks is to buy 48mm carbs and choke them to 41mm. This eliminates the loss caused by the butterflies in the flow.
neil

Guys I was reading this post and trying to make sure I've got all my bases covered on the carbs. However, I'm confused by the first paragraph of "PeteD's" response. He said "Use the flats on the screws to count the turns exactly ie 24 flats = 3 turns"

I would think that 6 flats (or 180 degree turns of the screw) equals 3 turns. Am I right or am I missing something?

TIA
ShawnO