Carburetor Problem?

[andyhodg]

After reurning my +2 to the road after being laid up for the winter I have a problem with the set up of the carbs (I think). Its a mildly tuned (ie hot cams and a big valve conversion carried out by Vegantune back in the 70's fitted with 40DCOE Webbers.

The problem is that when lifting of the throttle to change gear the revs stay high and take a few seconds to subside. It is not a stiff cable or linkage, all the jets are screwed in tight. I am now at a bit of a loss Any ideas?

Andy

[peterako]

Hi Andy,

How are you?

I know you've stated that the linkage and cable are free.....but
I sometimes get this problem (when returning to idle) due
to the return spring (the big one between the linkage and the
airbox) fouling slightly on the airbox.

I bent the spring a bit away from the airbox and lubbed well
and it almost cures it!

Just a tought,
Peter

[chrishewett]

Andy,
I agree with Peter. The standard setup is pretty poor and inclined to give problems. I am about to fit a weber throttle linkage which is much more up to the job. I use www.fastroadcars.co.uk
Chris

[andyhodg]

Peter / Chris

Thanks for the prompt response. I wish it was that simple. I have already checked the linkage and return spring. It happens at all thottle positions from changing gear at about 5500 rpm at full throttle as well as just tickling along changing gear at 2500 rpm.

Another related(?) symptom is that the idle speed increases substantially when the engine is hot. Checking the plugs I find that No 1 is black and the other 3 are quite a pale brown. Still at a loss though I think it is mixture related.

But I am still open to ideas.

Thanks again

Andy

[chrishewett]

Andy,
The guy at the link I mentioned in my last post is a mine of information and a weber expert. When I had carb problems I phoned him and he was very helpful and didn't seem to mind answering my stupid questions! It may be worth a phone call. (I am not suggesting your questions would be stupid).
Chris

[rgh0]

The "lazy return to idle" syndrome if not due to sticking throttle linkages is usually caused by incorrect location of the throttle butterfly versus the progression holes when on the idle stop. The first progression hole should be just upstream of the closed butterfly. You can check this via the plug over the progression holes. You need to check each butterfly as the shafts can be twisted and the carbs butterflies not properly aligned.

If the butterfly is to open and the first progression hole is seeing the idle manifold vacuum and flowing at idle throttle postion then the return to idle speed takes a little while. Fixing this problem on the older Webers without the throttle bypass needle valve that later Webers and Dellortos have is a problem and may involve drilling holes in the throttle plate or changing the throttle plate type to one with a different closed angle.

Why it should suddenly start happening on your engine is a bit of a mistery. Did you adjust the idle throttle postion for some other reason recently.

regards
Rohan

[andyhodg]

Rohan

Thanks for that. I think you may have found the problem. I have just balanced the carbs and in the process I found that the idle position had been achieved by adjusting the cable and not on the stop screw on the carbs. I corrected this and perhaps caused the problem as you suggested. I will have a look tonight.

Many thanks

Andy

[bill308]

Andy,

You may not have gone too wrong here. I balance my carbs just off the stops. This ensures that as you come off the stops, balance is retained.

But, to do this properly, you must use a proper gage. The best is a 4-bank manometer system as used by the motorcycle guys reading inlet manifold vaccuum. Later Webers and Dellortos have a vaccuum tap to enable this method. Earlier Webers and Dellortos do not. If your carbs do not have this capability, then it's best to tap the inlet manifold and install small hose barbs. The next most desireable method is to use an SK air flow gage. While not as good, it will give you a pretty good measurement of the flow through each carb. The down side is that you must move it from throat to throat rather seeing the intake manifold vaccuum for all throats simultaneously, as is the case with the multi bank manometer system.

Once balance is achieved just off idle, back off the cable adjustment and/or primary throttle stop screw to set idle speed. Done.

It's a good idea to check the relationship between the throttle plate and the progression holes with the engine stopped. This is easilly done by removing the brass cover plug over the Weber progression holes. All progression holes should be covered, but on the verge of uncovering the hole closest to the air box. Gently actuate the linkage by hand until you can just see the first hole being uncovered by the trailing edge of the throttle plate. The corresponding hole on each carb must be uncovered at the same time. One of my DCOE-18 carbs had a single hole mis drilled. I corrected for this by chamforing the trailing edge of the thrttle plate with a needle file so that placement of only this one hole was accounted for. The light filing extended about half way through the throttle plate edge. The engine was happy at idle and off idle.

Bill

[1964 S1]

Can
1. O rings on the carbs or
2. ignition timing

have something to do with the high idle?

[andyhodg]

Thank you all for your valued advice. I have finally got it sorted. I had tried to balance the carbs and the idle mixture and basically I got it badly wrong. I had opened the thottle stop to far and then held the speed down by going to lean on the mixture.

My own stupid fault but nothing serious to resolve. I can mow enjoy driving it in the current glorious weather.

Many Thanks again

Andy