Slow idle

[21window]

Hi

I have a 61 with zeniths in my vw bus. I have tried numerous times to get the engine to idle at around 900 rpm, but the issue I keep running into is that it will sit there and idle at around 900 for about 15-20 seconds then all of a sudden it drops down to about 250 rpm. If you leave the idle up higher, about 1300 rpm it idles perfect and never drops off.
I have rebuilt the carbs and tried the mixture screws all over the place. I have also set the vacuum on the carbs with a unisyn so everything seems fine. No intake leaks.

Is this a common issue with the zeniths? throttle shafts feel tight.

Thanks

[Christopher E]

I'll be interested in any suggestions as well - since my 912 does the exact same thing.

However Im on Webers40IDFs and my first suspicion was the distributor sticking/not settling back fully after advance? If I adjusted and setup my carbs at say, 10 degrees BTDC and then (after idling sometime) the distributor settles back to 5 BTDC - wouldn't I get these symptoms? When adjusting to a higher idle there is more centrifugal force in the weights and the drop would not occur? It's a theory anyway...?

How difficult is it to get to the weights on a Bosch .031 - Anyone have a link with pictures?

Thanks!

[21window]

Just to reply to your question about the distributor. I also suspected that as a potential problem so I completely rebuilt my distributor with new everything that was available for it so the advance plate returns perfectly. No improvement.

[C. Clark]

What distributor are you using ? The early aftermarket .009 and the .050 were ill fitted to these engines and mimic the issues you describe.
Chas.
356912911
Vieques PR

[21window]

i'm using the original iron bosch vjrb18 or something like that.

[Christopher E]

Ok - so... - encouraged that this was NOT a dizzy-problem I went over my carbs again. Did a tune-up from scratch - valves, timing - cleaned idle-jets and turned my Weber IDFs to standard startup positions (1/2 turn in on idle speed screw / 1 turn out on mixture) and started the procedure.

After fiddling a while and finally setting the CO to 4,5% I noticed the idle-dropping issue is gone. In retrospect the biggest change of all were the idle speed screws that were turned in quite a bit - and I had to unwind them 2-3 turns to back off clear from the throttle-shaft - and then start anew with a 1/2 turn in.

Seems like idle speed screws set to far in would open up other transition holes in the idle circuit - and maybe therefore causing a too rich or too lean mixture - and causing the drop.

Worth a look, anyway? Good Luck! // Chris

[21window]

Thanks I'll check that out. At least it's something.

[matt demaria]

If you look in the spec book or factory manual, the engine is supposed to be set at 750 RPM. It will actually cool down better at this RPM. The issue is that the dist advance is supposed to kick in about 900 to 950 RPM. When the idle is set to around here (900 area) the advance will kick in or out. If the idle is set after the advance is started the idle will drift lower and the advance will drop out---leading to a large drop in RPM. If the idle is set just below the kick in point, the idle will eventually drift up and advance will kick in driving idle way up.
You can either put stiffer springs in the advance system to increase kick in point. Or you can modify existing spring (take one coil out)
Or you can mistune the carb so that the response is so dead it will not drift around in RPM.
Or you can set the idle to 750 RPM where it shoild be and not have the problem in the first place.