Sorry, I forgot to mention again (I know it gets confusing in here) that my EGR currently has manifold vacuum. (Edit: But I see what you're saying; manifold vacuum EGR is recent, while the high idle has been with me the whole time.)
The bucking occurred when there was little/no vacuum for EGR (or anything else): heavy throttle low RPM. I didn't leave the hose plugged but it doesn't happen with the ignition advanced (advance hooked up to check valve constant vacuum). It does happen when the advance is hooked up anywhere else.
I'm confident that the bucking was a timing issue, and I'm confident that I'll find a happy medium by adjusting the base timing and hooking up the advance to manifold/ported.
It does look like I'll have to replace that EGR. Those are expensive. I'll see if the one from the S10 fits.
Disclaimer- I may have already asked these questions before and made these statements before- if so, please overlook my forgetfulness
So, just to clarify on the bucking: It bucks at low rpm/heavy throttle when the timing is more retarded (via a quick bleed off manifold or ported type vacuum source that releases vacuum and associated advance) and doesn't buck when the timing is more advanced (via a check valve advance source/type that maintains vacuum advance longer)- is that correct?
If so, I have never heard of retarded ignition timing causing bucking- usually it just causes low power and hotter exhaust.
Under low rpm high load conditions, it takes more voltage to fire the spark plugs (open throttle means there is more air "insulation" between the spark plug electrodes)- maybe there is some stray voltage jumping to another cylinder's spark plug through the wires or cap at just the right moment to cause preignition at a particular piston position when the intake valve is still open (that would explain the intake backfire).
I have had cars that would stutter (but not backfire) only only under those conditions- upon inspection I found a bad plug wire.
Please remind me- have you replaced those components?
LOL, don't worry about your forgetfulness, I'm way worse. I can't even always remember my own stuff let alone somebody else's. I appreciate your patience.
Yes, that assessment of the bucking context is correct.
I have not yet replaced the ignition parts. I recently acquired new wires and plugs but I'm waiting until my budget supports getting a distributor cap and rotor before I do the whole thing (also waiting for some accurate FE readings with my current configuration). It sure sounds like I should abort waiting and skip straight to replacing the components I already have, as a diagnostic measure, although the more advanced timing is probably better anyway.
FYI- You might be able to improve the performance of your current dist cap and rotor by taking some sandpaper (or a dremel wire brush) to the contacts, removing corrosion and deposits (just don't take too much off). Then spray the cap out with carb or brake cleaner to remove any conductive dust that might be coating the inside of it.
I haven't done anything with the stuff in question since the last post...but I have done some reading.
Originally Posted by theholycow
Distributor advance: Holds vacuum well; takes about 25 seconds to go from 20in/hg to 15in/hg EGR: Does not hold vacuum.
I tried disconnecting EGR and plugging the vacuum line, but it didn't affect my idle speed. I think my engine is not very sensitive to vacuum leaks.
Oops. I have a positive backpressure EGR, which means it won't hold vacuum unless it has enough exhaust backpressure. The vacuum test I tried was therefore invalid.
Well I've finally done some stuff. I didn't want to keep posting small worthless updates, so I'll post a few small worthless updates in one post. (Drat...the trouble with that is I've forgotten most of what I've done.)
A couple tanks ago I added a vacuum restrictor into the EGR vacuum line, which is connected to manifold vacuum. I previously observed better results with the EGR on manifold vacuum, but after learning that it's a positive backpressure EGR I figured a restrictor would reduce the EGR vacuum leak at idle. I think this configuration increased my FE.
I finally located my distributor clamp bolt, acquired a distributor wrench, and today I finally began adjusting my timing. I set initial timing to 15? (spec is 8?). Now with the vacuum advance connected to ported vacuum and the throttle partially open I get about 30?. Before today it idled at 22? with no additional vacuum advance for part throttle and no less vacuum advance for WOT; 22? was effectively the base timing. I'll keep adjusting it by ear until I'm happy, and I'll measure the result. Next time I fill I'll go back to 87 octane (and probably have to adjust timing more).
To compensate for my forgetfulness I decided to keep a log of thoughts and post when there's a significant chunk. I just wrote up my first entry and it was plenty for posting.
Driveability suffers at that timing. I got the old bucking thing while climbing a hill at heavy throttle low RPM so I advanced it some, roadside.
After that, I still wasn't quite happy (I don't remember the exact symptoms, it was yesterday morning) so I advanced it very slightly more.
Driveability still suffers a little, compared to what it was last week (before I advanced base timing and changed from constant check valve vacuum advance to ported). I had a stumble when I tried to launch hard into traffic; by "stumble" I mean it lost power once for a moment after I opened the throttle, almost like bucking but not repeated. Also, now when I go WOT at low-medium RPM I don't get the power I was getting last week, and I get a weak moment (not a stumble, but just flattened power).
When I was planning and executing my manual transmission conversion and asking about how it will combine with the carburetor to affect driveability and fuel economy, it was suggested that I should make my accelerator pump more aggressive or I would get symptoms like that. Perhaps with my timing in better condition I have uncovered the predicted symptoms.
Since I advanced the timing so much so soon I fueled up with 93 again. I'll switch to 87 when I think things are steady and I'm more confident in the timing, maybe after adding my CAI and modifying my EGR.