Oh wise ones, I am again working on a 1968 VW Beetle with a single port 1600. I have replaced the carb with a new Brosol and have tuned per specs. It idles and runs really well, except after it warms up after a bit of a drive, then the idle is very high. I have checked and set the timing, adjusted the fuel air mixture, etc., and it runs great until a 10 or 15 mile drive. I have verified that the choke shuts off, so I believe I have eliminated that issue, but is there anyting else I might be missing?
Those have a fast idle cam on the carb, could be sticking. It looks like a little eccentric with several steps and acts as the throttle stop. Just a guess but thats where I'd start.
Those steps that the idle screw rests on
To be clear, you are doing all the tune up settings to a nice warm engine, like after a 10-15 minute drive, and then after subsequent driving it's out of spec again? Or are you tuning up a cold engine and it becomes out of tune when warmed up?
I've only played with the dual port carb, 34PICT3, but I had a bunch of high idle issues. For me it would sometimes jump up on the high idle cam while driving and wouldn't go back to low idle without fiddling. Never could figure out what was going on.
To answer a few of the above questions, yes, only tuning it once warm. Auto choke and the stepped CAM are fine, adjusted fast idle screw per spec and used a feeler gauge. It opens and closes properly and is not sticking. I have adjusted the throttle cable also, and have slacked even more in case it was sticking, but it also seems to work ok. As long as I warm and it and tune in the garage, it works great. Even the first bit of driving it works great, no issues at all.
Now for the update. I went home last night and went through everything again, from setting the timing, to base lining and adjusting the carb and choke. I think I may have found something else however. It appears the Accelerator Pump linkage is tweaking where it connects to the carb "butterfly". It is twisting sideways under movement like it wants to pop off. I checked it for interference with the alternator, but can find none. My thought is that when during driving, when the carb gets floored or at some point, the linkage hangs up and dumps a bit of fuel down the carb. Does this seem reasonable. I am going to pull the carb off tonight and see what is causing the problem.
Ransom
UltimaDork
9/4/19 11:39 a.m.
I shouldn't think splashing extra fuel in would raise the idle.
If you've looked and verified that it's not sitting on a high-idle notch, I'd be pondering vacuum leaks. As in something that's developing a leak when everything warms up. Is the base of the carb nice and flat, as well as its mating surface on the manifold?
EDIT: kinds sounds like AC VWs have some specific misbehaviors, and I know nothing about that...
After working for years at an acVW dealer. Carbs and poor running were not a problem.
Now early FI .
