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After replacing the intake manifold gaskets on my stock 67 327/350 I canít get the timing to settle down. I suspect the distributor is one tooth off --- but Iíve never had that problem before so Iím not sure Iím recognizing it. While the intake was off, the engine got rotated, so I used Larsí technique of ďwalkingĒ the distributor to get it correctly positioned in the engine. This technique seemed to work fine.
Symptoms: When I reset the timing to stock specs (10 degrees advanced, 700 rpm, vacuum advance plugged) the car idles very roughly. It will rev up but it wonít idle smoothly (as it did before I pulled the intake). It will idle much smoother if I turn the distributor to advance the timing, but by the time itís smooth the engine speed is 1000 rpm and the timing is 20+ degrees advanced (vacuum advance still plugged, or course). I drove the car around the block (vacuum advance hooked up) and it runs OK. Oddly, with the car idling in my garage, the rpm doesnít go down and the engine doesnít start to run ragged after Iíve unplugged the distributor vacuum line but before Iíve connected this line to the dist. advance can. This means something, just not to me!
Does this sound like a familiar problem to anyone? If I mistakenly put the distributor back one tooth off, what is the simple and easy way to correct the problem? :confused:
A lot of people "think" that putting in the dist. a tooth or 2 off, will cause a poorly running engine. This is simply not correct IF you can time the engine by rotating the dist. In other words, the dist. could be put in 55, or 165 degrees off, and as long as you can rotate the dist to that position (to time it), then the engine will run just fine. However, the dist. will be in the wrong position from a visual and connection (vac. line and ignition wire routing) standpoint (as well as the tach cable connection - if you have that type of dist.). In other words, you are simply lining up the rotor position (in 13 tooth increments), so that it then lines up with the correct dist. terminal/ignition wire when that cylinder needs to be fired.
That stated, now back to your initial question: Is the dist. (when timed properly and running correctly) in the same position as when you removed it? If it is, then all is OK. If the dist. is not in the same position, then you will need to determine which way to move it, and by how many teeth. Note that a Chevy dist. has 13 teeth, so each tooth movement represents about 27.5 degrees. In other words, if it is off 55 degrees, then that is 2 teeth off. Easiest method for readjustment is Lar's method.
As for the timing not settling down, this can be a function of several issues (something has changed with the cent. advance, vacuum leak causing varying engine RPM, etc.). We will explore that later.
As for the engine running roughly with the vacuum line to the can plugged, yes the engine will run rougher and need more RPM to smooth out. You are plugging the vacuum line coming from the carb/intake, and not plugging the vacuum can itself, arn't you? Once the vacuum can is reconnected, you will see quite a bit of advance if you check the timing (depending on vacuum advance unit, can have anywhere from 12 on up extra degrees! with most around 16-18 degrees).
Have you checked the vacuum amount? Is the engine drawing the same vacuum (at the same RPM) as before you changed the intake gaskets? Why were the intake gaskets changed?
As for why the RPM does not go down when you unplug the vacuum line, but have not reconnected it back to the vacuum can (when the engine is running), tells me something is sticking (like the cent. advance wgts.). I think you have cent. wgt. springs that are too weak or sticking due to lack of lube. At low RPM with the vacuum can disconnected, the wgts. should be retracted. However, with the engine running at a higher RPM, the wgts' rotate outward, causing additional timing increase. If these springs are too weak, then at idle (and slightly above, they are not strong enough to keep the wgts. retracted (zero advance). This would explain why your timing is bouncing all over the place at low RPM. Check your cent. advance springs with the cap and rotor removed. the springs should be strong enough to return the wgts. back to their fully retracted position. If not, then you have your answer and need to have someone check out your dist. (as a quick fix, try locating some "stock" springs, but you probably need to have the dist. checked and recurved correctly on a dist. machine, since springs come in all different strengths and will really screw up the cent. advance curve!). You could also have some bind to the cent. advance and a "little" lube on the wgt. pins and the pads under the wgts. can do wonders.
Being a tooth off wouldn't affect the timing reading. If it's actually set at 10 BTDC, then it is at 10 BTDC. Do you know that it was set at 10ļ before you removed the distributor? If not, it's possible that it may have been set higher than that before to compensate for another problem (slipped damper or some problem that required more timing in order to idle well). Increasing it's advance caused the idle to increase high enough to start centrifugal which further increased advance (kinda like the chicken/egg thing :)). It's important to lower the idle speed back down to spec in order to get accurate readings as you set timing.
If in fact the distributor is a tooth off, you need to pull it and reinstall it correctly. If you're not having a problem (vacuum advance canister hitting anything) being able to rotate the distributor while setting timing, it probably isn't off.
I redid the intake manifold gaskets because when I did this same job a few weeks ago I used Permatex spray-on sealer for the side gaskets and I ended up having a small vacuum leak at cylinder #5; so I used Permatex Ultra Black sealer from a tube this time, with a 1/8-inch bead around each port on both faces of each side gasket.
The timing was at 10BTDC for sure when I shut it off a few days ago to redo the intake gaskets. The distributor had just got dialed in on a distributor machine three weeks ago and has brand-new Chevy points, rotor and condenser and new vacuum can. The distributor is now in the same position as before I worked on the car, with the vacuum can in its familiar location between the intake manifold and the right side vertical spark plug wire loom. I can turn it thru its normal range of rotation while setting the timing. The timing does "settle down" at idle, in the sense that engine RPM does not vary, and the timing mark is steady when observed with the timing light.
I'm familiar with the engine idling rougher with the vacuum line to the carb plugged, and the situation now is different; yup, I'm plugging the vacuum line as it does into the carb, not into the vacuum can (not that I've ever goofed it up that way before!) I know the timing will advance quite a bit when I reconnect the vacuum line, but I didn't happen to check if that's happening this evening (no reason it shouldn't). Haven't checked the engine vacuum reading either today -- it was in spec in recent weeks.
MagicV8 -- no I didn't touch the dist. drive gear. The whole dist. was gone thru in recent weeks, then set up on a dist. machine by a pro (not me).
This has GOT to be one of those incredibly simple things that I'm overlooking. What the heck can it be? Is it still possible that I put back the distributor one tooth off. If so, then I should reinstall the dist. by lifting it up, rotating it CLOCKWISE a tooth and reinstalling it, right? Anyone got any experience with this fixing the problem I'm having?
Of course the engine runs rough when you disconnect the vacuum advance to set the initial timing! The speed drops and you loose manifold vacuum. If you can achieve the correct initial timing without the vacuum can hitting someting, the dist. drive gear is likely properly indexed and you probably have it on the correct tooth. With every thing properly set up the dist. window should be just about normal to the vehicles longitudinal axis.
Set the initial timing, connect the vacuum advance and go through the idle speed and mixture adjustment procedure.
One other item that you probably don't want to hear:
RTV (like Ultra Black) should never be used seal gasoline area gaskets! RTV is great for sealing the ends of the intake, and even around the water passages. But gas affects RTV by breaking it down slowly (will cause it to swell, soften, weaken, and acts as a great final cleaner for those areas where the RTV is difficult to remove), and should not be used around the intake ports. Most good intake manifold gaskets (like Fel-Pro) do not need any sealer around the intake ports.
I am new to the Forum and new to Corvettes but been around awhile. I agree with previous posts, the position of the distributor is irrelevant if you can set the base timing to spec. However I believe the vacuum advance functions like this. The vacumn advance should be ported above the throttle plate. By that I mean there will be little to no vacuum to the vacuum advance at idle. The vacu
\um advance comes in on light throttle, low load for fuel economy. Mechanical advance covers the performance side of the equation. If you had vacuum advance with the throttle closed when you stomped on the gas and the vacuum, dropped you would retard the timing by virtue of the loss of the vacuum advance. Therefore, I would not expect any timing change or idle speed change with the vacuum advance connected or disconnected and plugged. As far as the running problem, I would re-check for vacuum leaks, check carb adjustment and other basics. Advancing the timing will cover up other issues. All that said, I repeat. I am new to Corvettes. My background is on european stuff.
On most Corvette engines there is only ONE position that will allow the timing to be set to spec before the vacuum can interferes with some other components as long as the distributor drive gear is properly indexed.
The proper installation and wire indexing is documented in appropriate service manuals for each year.
"Ported" vacuum advance is an emission control stategy. Pre emission Corvette prior to '66 have full vacuum advance. Reducing total idle advance by using ported vacuum advance increases EGT, which is good for emission control, but increases fuel consumption and heat rejection to the cooling system.
Medium performance engines need about 20-25 degrees of TOTAL idle timing to minimize fuel idle fuel consumption, heat rejection, and yield a smooth low rev idle. SHP engines need in the range of 25-30 degrees. This total idle timing is obtained by the combination of initial timing and full vacuum advance, and since SHP engines produce less idle vacuum, they need specifically calibrated vacuum cans to achieve best idle performance.
There's lot's of discussion on this in the archives if you care to do a search. You have a lot of misimpressions.
Well I guess I do have allot of misimpressions. As I said I am new to Corvette. I will definitely check out the archives and my car, a 65. I am pretty sure the vacuum advance is hooked to a ported vacuum source on the carb. The car is however not correct including the wrong engine and it has obviously been screwed with. It runs ok but perhaps it will run better if I get the distributer deal sorted out.
Yes, it will - guaranteed! Once your distributor centrifugal advance is to spec and you have the correct advance can, connected to full manifold vacuum as it was originally, you'll feel the difference. These days, not many folks understand ignition timing, centrifugal and vacuum advance - you may find my articles on the subject in the October/November and December issues of "Corvette Enthusiast" magazine interesting in de-mystifying these issues and understanding how to optimize them at home with simple tools. :thumbs:
At this point I'm done fiddling with this problem. Mother nature let me know yesterday that the hernia operation I had 10 days ago is not yet up to being bent over the engine compartment quite so much. Tomorrow morning I'll drive to a Vette specialty mechanic and he'll figure out the problem. I'll ask if he does outpatient surgery on non-plastic bodies while I'm at it!
hmmm, Louie I feel your pain, here is a post [from Daren] and reply post [from me] from a recent thread on Distributors, you are living my fear:
535 posts [100%]
New York, NY 10154
Re: L79 Distributor Question (BigBlockThunder) 10:25 AM 11/17/2003
Put me in the afraid camp on pulling a distributer. When I change points, condensers, etc on my distributer I lay a magnet (on both sides) below where I am working atop manifold next to dist shaft. If I drop a screw or something the magnet gets.
165 posts [100%]
Re: L79 Distributor Question (Daren Schneider) 10:36 AM 11/17/2003
yeah, me too. Even though I would score myself as an accomplished home mechanic, something about pulling the distributor raises the gut fear of all diy-ers everywhere - having to make the dreaded "ok, come tow this car to your shop and make it run again cause I really f-ed it up" phone call.
65 327/365 4sp Milano Maroon vert
and then JohnZ or some other superman mechanic who can rattle off the firing order of a 327 in his sleep tells me to stop being such a baby and go ahead without fear . . . . :D
ps I now have a brandy-new / old stock still in box L76 distributor that I bought as a back-up part, who knows maybe if I become a pro at removing/installing distributors I'll drop it in some day and see if I notice a huge difference . . . I do have to send my intake off to get a slight crack magnafluxed (and ok reskinned while the boys are at it), meaning I will be playing "distributor roulette" :eek: soon . . .
Good luck on both the Vette and the hernia repair. When I had my first (of 2 - "had to have a matching set - one on each side") hernia operations, the Doc said I could go right back to doing what I normally did after a week. So the 2nd week I go out and start fertilizing the lawn with one of the push type Scotts spreaders. Well, the next morning it felt like I had torn the stitches back open and the hernia had ripped apart. After a day or so, it felt much better, but I learned that what the Doc thinks is "normal" is not necessarily what you consider "normal"!