Hey guys I reinstalled the Pertronix MR-183 (both modules in one dizzy) and coils in my 78GTS (carb) and got it working great. Turns out all the problems I had before were self induced. Anyway it starts better, runs great with no misfire but (there's always a but) it idles a little fast, about 1150 rpm with AC off and 1050 rpm with AC on with the idle speed screws backed all the way out. With the r1 and r2 points I had plenty of adjustment. The problem is, I think, that the the initial timing gets bumped from 3degrees atdc as required with the r2 points to 7 degrees btdc for which is the setting for R1 only and is necessary to get 34 degrees total at 5000 rpm. The net affect is advancing the initial timing 10 degrees which, of course, speeds up the idle a bunch. I synced the carbs (again) which are now flowing about 3.5 to 4 kg/h at 1000 rpm (I sync it when its cold and move the fast idle cam out of the way so it will run slow enough). All the air bleed screws are closed except one which is only 1/4 turn out. There is slack in the throttle cable so no problem there. If I put an extra spring on the rear bank it will idle down to 1020 rpm. I attach the spring to the carb balancing screw so it puts extra closing pressure on both rear carbs. Probably most of the extra closing pressure is on the right carb due to the left carb being closed against the idle stop. I don't like the extra spring as it puts side pressure on the throttle shafts and it only works on the rear carbs. The spring drops the hg/hg to about 3.25 to 3.5. Thinking it is a vacuum leak I put a restrictor in the vacuum line going to the charcoal canister. The rest of the vacuum system is closed, I think, except for the power brakes, air cleaner flapper valve and exhaust diverter valve. The brakes maintain pedal pressure and position after shutting the engine off so I think they are OK. The flapper valve is working as expected with no vacuum leaks. I'm not sure how to check the exhaust diverter valve and I think its working OK as it passed smog with no problems but I guess I'll just try blocking it off and see if it makes a difference. My question after all this is is this typical? If I retard the timing a couple of degrees it idles down OK. Just wondering if there is any other source of air flow through the carbs or vacuum system which would cause the fast idle or is it just that the throttle blades are not seating? I've cleaned the carbs several times and there is no build up which would prevent the blades from closing. I've read about "resealing" the carb shaft bearings but don't really know what that means as the bearings are original and there is very little slop. I've never had the blades loose or adjusted from new. Thanks Barry
Mine has same ignition and does the exact same as described, no amount of adjustment will bring it below 1100
The stock bearings are an open bearing, but there are two other pieces (items 38 and 39 on page 178 of the Weber Carb Book) that are added on each end of the throttle shaft to form a close-fitting (but non-contact) rotary seal that should only leak a little. These pieces can wear and let in unwanted air (and you wouldn't want item 39 to be put in backward). "Resealing" the bearings I think refers to going to sealed bearings, but this adds drag to the throttle shaft motion (but refreshing items 38 and 39 would also technically be "resealing"). Also, it's super easy to warp the base of the carb when over-torquing the nuts -- are you using fresh gaskets/spacers? Maybe a smoke inhalation test there if they've ever been off (if blocking everything else has no effect)? You can certainly just block the vacuum ports supplying the airflap, the diverter valve, and the brake booster to take them out of the idle picture at idle with the airbox removed. Didn't have any problem getting 1000 RPM warm idle at 7 deg BTDC on my ex-308 (at ~3.5 Kg/hr/barrel), and a lot of carb 308 are now at 7 deg BTDC without widespread reports of trouble (except Vonbarron's ) so you must have something else going on. Good Hunting!
Image Unavailable, Please Login Image Unavailable, Please Login Couldn't find anything wrong so I made a smoker (you gotta say the internet is a great thing) to test for vacuum leaks and found none. I ended up putting an extra spring on the rear bank and of course it idles fine. So probably have some misalignment of the throttle blades, even though they have never been loosened. Anyway on our way out for a drive up the coast. Barry
Forgot to say I checked the vacuum at idle and it's pulling between 11 and 12 inches of vacuum which is about right based on a couple of threads I read.
