The good news: It starts easily hot or cold, idles relatively smoothly at 750 rpm (and below actually), and is scarey strong and smooth above 1700 rpm. Operating temp is around 180 (IR). But between idle and 1700 its a little uneven but only in gear. In neutral, it seems to run up fine, and at constant throttle the rpm is steady at all engine rpm . But in any gear (4 spd) there is a little jerking thing going on at constant speed or when decelerating. (The jerking is non existent while warming up, but is noticable at irregular intervals of 2 to 5 seconds. Acceleration is strong and steady from 1200 up. What Ive done (after the hair pulling): Ignition- verify the timing map (idle, cruise, wot )set properly- per Duke, change plugs, rotor, cap, splug wires new. Carb: disassemble more times than I can count, and along the way verify that venturis, jets, rods, and rod springs, acclerator pump are all correct part #s and floats, choke, idle adjustment are set properly. Vacuum @ 750 is 12-13 inches and steady. One other thing, I have experimented with leaning out the idle, and advancing the initial timing without much sucess(initial advance greater than 10 seems to worsen it), but one thing of note- it does not detonate or ping on pump gas. I "assume" this indicates the mixture is too rich, hence Im focusing on the carb. Im just about out of hair, what am I missing? Thanks Les
-
-
Re: 327/340 Off Idle Problem - I think Addendum
Forgot to mention it is a 63, original engine with 3461S AFB . -
Re: 327/340 Off Idle Problem - I think
In case you missed it, refer to this thread: Recommendations for distributor vendor, 3 Mar, 10:07 AM , a little down the page for my discussion of some "issues" with the '63 L-76 OE timing map, particularly the vacuum advance.
In my experience Duntov-cam engines should idle at 800-900. I don't see how GM could have claimed that they could idle at 700 with any degree of quality/stability, especially with the "incorrect" vacuum can they used in '63; 700-750 was just not feasible with my engine even after I "fixed" the timing map. Before I installed the 8" vacuum can, the idle would often destabilize and drop, and the loss of manifold vacuum would cause the metering rod pistons to "dither", creating a rattle that I could hear as they banged into the covers. It took me awhile to figure out what this "idle rattle" was.
My experience is that once you get the ignition map set up to '64 specs, the '63 L-76 runs very well from off idle to peak revs with decent idle quality and stability and excellent throttle response everywhere - never ran across a Fuelie that could beat me, but a few could keep up.
The AFB seems to work best at idle and off-idle with the idle screws near the nominal 1.5 turns out from the seat, but this could vary with altitude and individual carb idiosyncacises, wear, or dirt.
As far as timing is concerned, set it to the detonation limit. Back in the good 'ol days of Chevron Custom Supreme I ran 16 degrees and with the '64-'65 centrifugal springs it was 40 degrees total WOT timing at 2350-up and screamed.
Notwithstanding the fact that these engines prefer to run at higher revs, mine would pull the 3.08 axle smoothly from 1000 revs (about 26 MPH) in top gear. Low speed driving was somewhat strange as even dragging it down to about 700 in first gear with the brake you are still doing about 8 MPH. It's not a great car for heavy stop and go traffic!
The only serious problem I ever had was when I tried blocking the heat riser - not good! After the choke had backed off in the first couple of minutes after a cold start it would take a good twenty minutes of driving in mild weather to get enough heat in the manifold to allow a start from a dead stop without stumbling. I pulled the shims and it was back to normal.
I can't finger a specific cause of your reported anamolies - just trying to give some insight and benefit of my experience.
DukeComment
-
Re: 327/340 Off Idle Problem - I think
Les,
I had the same problem with my first 63 with an AFB. Went to a fellow in Memphis that did all the distributors. Arnold Bonner was the master of distributors and carbs. He fixed the distributor and drilled out the accelerator pump nozzles to a larger size. Don't know what size. This kept the old buggy from stumbling as it gave it a little extra shot of java off idle and solved my problem. Doing the distributor alone did not solve the stumbling problem but helped considerably. Adding the extra shot of fuel did the trick.
Regards,
JRComment
-
Re: 327/340 Off Idle Problem - I think
Duke Thanks. I saw the thread, and I do have the 8" can (1810). I did a little work on the distributor (added shims for the end play, and changed centrigal springs to bring the advance in by 2400) and it helped the responsiveness, but did nothing for the uneven problem I was talking about.Comment
-
Re: 327/340 Off Idle Problem - I think
I had a similar problem with a 66 L79. The engine was recently rebuilt but the distributor was not. What was happening in this one was the end play on the distributor shaft was huge and the shaft was moving up and down and changing timing with gear poaition. When the end play was removed the problem ceased.Comment
-
Re: 327/340 Off Idle Problem - I think
Could be a little lean off idle. It always amazed me that '63 L-76 used the exact same carb as L-75 (with manual trans) even though they have very different manifold vacuum characteristics. Off-idle and transition to the main system is a black art.
One test is to use the throttle to raise and lower the revs in 100 RPM increments from idle to about 2000 If you can do this without the revs hesitating or wavering, the off-idle mixture is probably okay.
Try setting the idle adjustment screws at 1.75 and then 2.0 turns out from the seats and see if this helps. Also check that the accelerator pump linkage is set to spec in the '63 Corvette Shop Manual.
DukeComment
-
Re: 327/340 Off Idle Problem - I think
Les,
I think this problem of jerking or surging that you descride can be caused by to lean a mixture or two much advance at low rpm. I would start by checking the TDC mark on the balancer to make sure this is really TDC. This has to be done with a piston stop in #1 cylinder and you have to turn the engine by hand at the bolt that holds the balancer with the spark plugs removed. I would also verify that you have a 24 degree distributor by checking the weight base/cam as it should have 724 stamped in the underside but keep in mind that someone may have filed the slot at one time but you may be able to tell by looking close. Next, I would check to be sure the rubber bushing is still attached to the main shaft pole piece that rides in the slot of the weight base/cam. After you put the distributor back together make a accurate piece of timing tape showing TDC-12-24-36 degrees and tape it to the balancer corresponding with your new TDC mark. Plug vacuum advance and remove springs on weights inside distributor, start the car rev just slightly as this will advance centrifugal to the limit and time for total advance (36 degrees). After that, install the springs and re-check timing at the lowest possible idle as to insure the centrifugal is not starting to come in (vacuum can unhooked and plugged). If your initial is 12 degrees then you know your centrifugal is correct at 24 degrees because you timmed for 36 degrees total. Now you know for sure what centrifugal and total advance you have in your car. The only other way is a distributor machine but this is a easy process.
Hook the 1810 vacuum advance up and go for a ride, if the car still jerks and bucks at low speed (off idle) I would then look at the carburetor primary venturies as they may be dirty or the idle jets have been closed up leaning the mixture. The idle jets should be set at .035. If I got any of this wrong Duke will correct me but I did this on both of my cars and they run great and I know exactly what my total advance is.
Sorry I am so long winded and I hope I explained everything OK.Comment
-
Re: 327/340 Off Idle Problem - I think
Duke Ive previously raised, lowered the speed while in neutral, and it was quite stable from 1000 to 2000. The tach needle would jitter about +/- 10%, but it did that above 2000 too. I have tried a leaner idle, and a richer idle. The leaner seems to buck a little more which surprised me. I thought one of the tell tale signs for an excessively rich mixture was the absence of preignition, and it doesn't ping-even when I put my foot down.Comment
-
Re: 327/340 Off Idle Problem - I think
Tim Not long winded at all. In fact I thought it was well done. Thanks for taking the time. I was afraid I was looking down the wrong rabbit hole, so I'll give the distributor another shot. LesComment
-
Re: 327/340 Off Idle Problem - I think
John My end play was huge too, but shimming it to spec wasn't the silver bullet. Good suggestion though. LesComment
-
Re: 327/340 Off Idle Problem - I think
A lean off idle mixture can cause hesitation at low revs and light load, but has no meaningful effect on detonation, which is usually not an issue at light load. Preignition is premature ignition due to a hot spot lighting the fire before the spark plug. It's the same as advancing the timing. Preignition can lead to detonation, which is the pinging/knocking that can be heard. Preignition and detonation are two different phenomena, but they are related. Preignition can lead to detonation, and detonation can lead to preignition by overheating the combustion chamber surfaces making the detonation ever worse.
As I said earlier, when I blocked my heat risers the engine would stumble when starting from a dead stop until it had been running for about 20 minutes. The reason is that the lack of the hot spot under the carb reduced fuel vaporization so much that it would go dead lean - beyond the ignitibility limit, which is why it stumbled. Idle and off idle fuel metering is based on manifold, not venturi vacuum, so when you let out the clutch and manifold vacuum drops, fuel flow is reduced and can cause a stumble unless the hot spot is there to quickly vaporize the available fuel.
As Tim stated the idle jets in the 3461S are .035". and these are pressed into the primary venturi clusters. (BTW I think the follow-on 3721S has .037" idle jets - a slight richening.) The venturi clusters also have idle air bleeds and the main body has passages for the idle/off idle fuel air emulsion along with the idle openings and transfer slots in the throttle bore to admit the fuel air emulsion for idle and off idle operation. Any deposit build up in these passages or any form of damage will upset calibration and will usually cause a lean condition.
Any kind of vacuum leak will also lean out the idle/off idle mixture, so checking vacuum cans, power brake boosters, etc. for vacuum leaks is a good idea as is shooting propane around the manifold runners and carb/manifold gasket. If engine revs pick up, there is probably a leak. Another thing to check is the carb gasket. L-76 uses a different gasket than L-75. If you use a L-75 gasket with the heat riser slot, you could have a leak at carb/manifold interface.
Tach flutter is independent of engine flutter. From idle if you can used the throttle to raise or lower the engine revs in 100 RPM increments, the off idle mixture is probably okay. You can hear/feel any flutter. Don't just go by the tach.
Years ago I developed this test for my Honda CB1100F. It was very jerky in on-off throttle transitions, which made it difficult to ride in low speed traffic. No amount of idle mixture screw adjustment would cure the problem, so I finally drilled the .38mm pilot jets to .40mm. Problem solved. I can throttle up and down between 1000 and 2000 in 100 rev increments with no hesitation or wavering of revs, and the on-off throttle transitions while riding are butter smooth. The four 33mm CV carbs now act like a good fuel injection system.
So what have you found to be the best idle speed mixture screw setting and is the heat riser passage open?
DukeComment
-
Re: 327/340 Off Idle Problem - I think
Duke Thanks for the info.. I can see more carb work on the way. Your Honda's drivability problems sound a lot like mine. My exhaust crossover is blocked, but the stumbling existed before blocking it. (Blocking the crossover eliminated a percolation problem). The stumbling (and jerky low speed), is actually worse after the car has been driven at 180 degrees for about 20 minutes than it is after it initially comes up to temp (180). My "best" idle screw position is about 1 3/4 out. If I back it out much further it doesn't seem to help the stumble much and there is a tendency to run on after shutdown . If I go leaner, it seems to worsen the stumble. One more thing.. the ignition has a Mallor Unilite system (point replacement only). Since the car starts, runs fine otherwise I didn't consider it to be a likely candidate. Would you?Comment
-
Re: 327/340 Off Idle Problem - I think
I never had any significant percolation problems with the heat riser passages open, but I wired the valve open in 1968 after the OE RH muffler rusted out. The first thing to check if percolation is a problem is whether the heat riser valve is functioning, and most of the time, I think it's best to wire it open, especially if most of your driving is in warm weather. I don't think the heat riser passage need to be blocked to solve a percolation problem.
If your problem is more prevalent after the car is fully warmed up, then you are experiencing something different than I did when I blocked my heat riser passage since my starting stumble would occur until BEFORE the engine had been running for about 20 minutes.
I'm beginning to lean more in the direction of a vacuum leak - one that may be temperature sensitive. Get a Mighty Vac and test the vacuum can, power brake booster if you have one. Also check for leaks around the inlet manifold runner/head interface with propane or starting fluid, and are you sure you have a correct L-76 carb gasket?
I don't think the electonic ignition is the source or your problem, but I don't have a great deal of faith in aftermarket conversions, so if you can easily swap in a set of points, that's another test worth running.
DukeComment
-
Re: 327/340 Off Idle Problem - I think
My carb was still percolating with the heat riser wired open. In the process installed new intake manifold gaskets, but I'll double check with a propane torch. Another data point: after its hot, when Im starting in low, I have to slip the clutch around 1200 to get up to speed otherwise the rpm (and vacuum) drops suddenly and threatens to stall. I think I have a 4:11 rear (60mph, 3000rpm), so the bog on start up puzzles me. The base gasket has 8 holes (4 mounting studs, 4 barrel), and no "smile" . Is this correct?Comment
Comment