Helping the boy get his "new" truck running better yesterday and came across an odd thing that I'm not quite sure how to progress on.

It's a '74 K10 Chev 350 - Rochester 4bbl - TH350 - all stock...

The truck runs rough and has a small exhaust leak among other minor issues, so step one is trying to overcome the idling problem. In Park/Neutral, it wants to idle up at what sounds like about 1000 RPM, and when dropped into gear it does not idle very well at all. He pretty much has to keep some throttle in it or keep it in Neutral or Park to prevent it from dying. (The throttle screw that sets the linkage is backed off all the way, so that's not the high idle speed issue)

Then, I took a look at all the vacuum lines and they appear fine. No real wear, they're quite soft and intact, no signs of chaffing, melting or wear. Finally, I put a timing light on it and the timing mark was so far up around the top that I couldn't even see it. We disconnected the vacuum line from the dist and turned the dist to bring the timing mark into view. It took a lot of adjustment to get it back down close to the timing marker and at that point it started backfiring through the carb and would not run at all. Had to put it back close to where it was initially just to get it to run and this of course brought the idle back up too.

Thoughts? I was sure it was just an over-advanced ignition, but it's not behaving the way I would have thought. I thought about moving the distributor a tooth, but it really doesn't look like this thing has ever been out, so I doubt it's a tooth off. Just that I've not done a lot of this and it's definitely not where I expected it to be. I have to go buy a tach/dwell meter to see where the hard numbers are for that, but for now this timing thing seems odd... :confused: :confused: :confused:

Help?

May have slipped the outer ring on the balancer.

Stick your finger inside the balancer and feel the keyway, if it's not inline with the timing mark on the OD then 1 of 2 things is going on. Either it slipped or is one of them that is 10° off from GM.

Make a paint mark on a radius with the keyway.

90° vee engine ok.... tdc is middle of driver side bank, and 45°btdc is vertical. Set it at about 15 to 20 with no vac hooked up to the vac adv.

22.5° is halfway between vertical and the center of the driver side cylinder bank deck.

Oh yeah, and get a new balancer if yours slipped.:)

Any chance the rough running is the result of a carb problem, or with the funny timing mark, is it more likely to be a timing issue?

Carb is a "new" Rochester 4bbl that the P/O (had?) put on. Not enough knowledge to say if it's rough or choked, or what. It's not smoking, but if one give it -any- gas in the process of starting after it's warm, it'll flood badly.

We'll check the HB this weekend, but hoping to have a list of "possibles" before getting dirty...

They say that most carb problems are in the distributor.

And re-reading your initial post sounds like yours is an old points unit in addition to maybe a slipped balancer. I thought that HEI came out in 73, but I also don't care when it came out, because it is available now.

But I didn't think about the part where you said the idle stop is all the way out and it idles 1000. That sounds like some fuel and air is getting in from somewhere. Like you say, choke needs checked first. And look at the high idle screw. I haven't fooled with a Roch in many many moons, so I'll leave that to others.

Flooding easily is a symptom of weak ignition, in addition to blubbering etcetera. A tune up starts with the ignition system.

And poke around that carb more

Also check that some on may have used a later 305 plug wire orientation on the cap.

Thanks Tim - this IS an HEI distributor btw... (and my son whom this truck is for is also Tim)

Main problem is that I just don't know how to adjust a carb other than the main idle screw that sets the linkage. I suppose there's a book or two out there, but for me, a picture book would be most helpful! :confused: Is it possible something in the choke is stuck? Would it behave this way? How to check/test/fix...?

Went out last night and fiddled with the timing and a light for awhile and found it runs best (albeit still like garbage) with the timing set where the mark is at 12:00. If the timing notches are in the ~2:00 position, that makes it approx 45* out of whack. Could not tell if it had slipped - the pulleys and other stuff are in the way and it was pretty dark. Stuck my finger all around in there (man, that sounds nasty!) and couldn't find the notch. I think there may be a large washer over everything and the pulley being there didn't help matters any. Here's a rough diagram of the proximity of everything:

http://i492.photobucket.com/albums/rr283/lerner5/Timingmark.jpg



We have a free weekend and I suspect it may require taking some things apart to see better.

Under gas, it's pretty smooth so I don't think there's a plug wire config issue like it's running on less than 8 cylinders or they're out of order. It's just that when it's idling in gear, it continually falls off to the point that it will eventually die. It also floods very VERY easily.

I'm trying to think of all the little things that I've noticed - figuring together, they may point to a root cause.

Thanks!

I concur that many carb problems live in the distributor :D

I would establish where TDC is on cylinder 1 to validate those timing marks.

Then I would check the vacuum levels against the vacuum advance. It sounds like in park you have enough vacuum to pull in the vacuum advance. Then you drop it into gear and lose some vacuum and thus advance. Without the advance the engine falls down. I would look for the vacuum advance to pull all in at 1-2" lower vacuum than you have in gear. A hand vacuum pump can be invaluable to check out the vacuum advance.

So then, what's the best way to test the vacuum level?

I'm guessing one can put a vacuum gauge right off the line that goes to the vacuum advance, but is that best? Should it be checked at various points along the way?

How much should I expect to see it at idle and how much (if any) should one expect when dropped into gear?

oops - double post~

I'd pull the cap, check #1 TDC is at the proper location both on the balancer and rotor. If that proves to be ok, I'd check the plug wire positions & routing next.

So then, what's the best way to test the vacuum level?

I'm guessing one can put a vacuum gauge right off the line that goes to the vacuum advance, but is that best? Should it be checked at various points along the way?

How much should I expect to see it at idle and how much (if any) should one expect when dropped into gear?
Yeah tapping/T-ing into the vacuum advance line would work fine. No need to check along the way unless you have cause to suspect something.

As to numbers you should see... that depends on much like cam timing and general engine tune. I am going to guess with a basically stock motor you should see close to 14" or so. I only see 11-12 in neutral on my engine... but than my cam dictates the numbers too :thumbsup:

I would expect to see at least a 1-2" drop in gear but that is a pretty wild guess as I do not normally deal with auto trannys.

Yeah Steve, there is a bolt and washer in the end of the crank. Should be a 5/8 head. You have to pull it to see the keyway. That keyway is indexed on the crank to #1 rod throw. When the key points at the center of the left bank, the #1 piston is at tdc or real stinkin close but definitely close enough by eyeball for your purposes. But a small screwdriver in the #1 hole would work as well if you don't want to pull the bolt.

A stocker should have 14" HG vac or more at idle.

Only reason it sounded like you had a points dist is you said you had to buy a dwell meter. I guess you just want it for the tach? I haven't used one in years so that didn't occur to me. I have a handheld phototach, and all my junk has tachs too. Sorry about the confusion.

Helping the boy get his "new" truck running better yesterday and came across an odd thing that I'm not quite sure how to progress on.

It's a '74 K10 Chev 350 - Rochester 4bbl - TH350 - all stock...

The truck runs rough and has a small exhaust leak among other minor issues, so step one is trying to overcome the idling problem. In Park/Neutral, it wants to idle up at what sounds like about 1000 RPM, and when dropped into gear it does not idle very well at all. He pretty much has to keep some throttle in it or keep it in Neutral or Park to prevent it from dying. (The throttle screw that sets the linkage is backed off all the way, so that's not the high idle speed issue)

Then, I took a look at all the vacuum lines and they appear fine. No real wear, they're quite soft and intact, no signs of chaffing, melting or wear. Finally, I put a timing light on it and the timing mark was so far up around the top that I couldn't even see it. We disconnected the vacuum line from the dist and turned the dist to bring the timing mark into view. It took a lot of adjustment to get it back down close to the timing marker and at that point it started backfiring through the carb and would not run at all. Had to put it back close to where it was initially just to get it to run and this of course brought the idle back up too.

Thoughts? I was sure it was just an over-advanced ignition, but it's not behaving the way I would have thought. I thought about moving the distributor a tooth, but it really doesn't look like this thing has ever been out, so I doubt it's a tooth off. Just that I've not done a lot of this and it's definitely not where I expected it to be. I have to go buy a tach/dwell meter to see where the hard numbers are for that, but for now this timing thing seems odd... :confused: :confused: :confused:

Help?

First thought is your setting the timing with an elevated engine RPM which will more than likely engage the centrifugal advance......need RPMs around 500 with the distributor vacuum disconnected. If you can't get the thing to idle down you could set total timing with a dial back light and let initial be what it may.

Does the throttle shaft in the carb have a lot of play. Sometimes the shaft leaks so much air you can't control the idle with the fast idle screw........Then you need to bush the shaft or different carb.

"Does the throttle shaft in the carb have a lot of play. Sometimes the shaft leaks so much air you can't control the idle with the fast idle screw........Then you need to bush the shaft or different carb."

I could take a look if I knew what/where this was. (sigh...) The point you bring up about the timing being advanced with the higher RPMs may be the case, but would it advance that far up the HB at only 1000 RPMs? I don't know the hard number yet, but that seems like an awful lot of advance for not all that much in engine speed. Still, we'll get out and look at it Sat morning. We'll also take a look at the vacuum and the various lines.

A question on vacuum though...
Wouldn't a vacuum leak cause the advance to NOT occur rather than occur prematurely?
Or is the vacuum a function that holds the advance in check and if the vacuum leaks, the advance occurs?

In any case, Saturday morning task will be:

1) Verify TDC against the timing mark
2) Verify the balancer -appears- to have not slipped
3) Check the vacuum level
4) Closer inspection of the cap, wires and plugs.
5) Likely change the plugs / rotor / cap (as needed)

6) Come back here and pose more questions :)

"Does the throttle shaft in the carb have a lot of play. Sometimes the shaft leaks so much air you can't control the idle with the fast idle screw........Then you need to bush the shaft or different carb."

I could take a look if I knew what/where this was. (sigh...) The point you bring up about the timing being advanced with the higher RPMs may be the case, but would it advance that far up the HB at only 1000 RPMs? I don't know the hard number yet, but that seems like an awful lot of advance for not all that much in engine speed. Still, we'll get out and look at it Sat morning. We'll also take a look at the vacuum and the various lines.

A question on vacuum though...
Wouldn't a vacuum leak cause the advance to NOT occur rather than occur prematurely?
Or is the vacuum a function that holds the advance in check and if the vacuum leaks, the advance occurs?

In any case, Saturday morning task will be:

1) Verify TDC against the timing mark
2) Verify the balancer -appears- to have not slipped
3) Check the vacuum level
4) Closer inspection of the cap, wires and plugs.
5) Likely change the plugs / rotor / cap (as needed)

6) Come back here and pose more questions :)

Centrifugal (mechanical) and vacuum advance are two separate things. When you disconnected the vacuum to the distributor you disabled the vacuum advance.

The centrifugal advance mechanism is the weights and springs the rotor is mounted on in the distributor and normally can't be disabled.

Vacuum advance mechanism moves the plate the points are mounted in relation to the point's cam. The centrifugal advance mechanism moves the rotor in relation to the plug terminals in the distributor cap.

When you set the initial timing at a lower rpm you have effectively disabled the centrifugal advance. The rpm at which the centrifugal starts to function is normally right off idle so at 1000 rpm it could/should be engaged. It's best to set initial at the lowest rpm it will idle to make sure centrifugal is not functioning. My best guess is <500 rpm.

I'd get a dial back timing light and set the initial + centrifugal at about 28-30 degrees or if it's a hot rod about 35-36 degrees. Then move on to the idle problem...........this will only work if the balancer hasn't moved, as mentioned above. To set total timing (vacuum disconnected) you probably need >3000 rpm. The rpm needs to be above the point where all the centrifugal advance is in.

One thing you could do, to get it to idle down, is have someone you trust in the driver's seat with a foot on the brake and in gear when you set initial timing. When in gear, the rpm should be lower but I don't know if it will be enough. Be sure not to rev the engine from under the hood while it's in gear and be sure to stand to the side of the fender while timing........

Dwell should be set or checked before timing the engine..........

You can tell if the choke is off by looking at the choke butterfly on the carb, if it's wide open it's off. There is a fast idle adjusting screw on the right side of the carb as part of the choke linkage. This is to set the engine idle at a higher speed only when the choke is on. Make sure this screw is not engaged when the choke is off.....This is different than the idle adjustment screw.

Throttle shaft - follow the throttle cable from the gas pedal to the side of the carb, it will be hooked to a lever that pivots at the base of the carb. The pivot point is where the shaft goes through the carb and could be leaking air. I think if you spray some WD or gas at this pivot point it may pick up rpm or check by wiggling it back/forth.

Did I forget anything? :confused:

Mechanical advance! DOH!!! I knew that...
:clonk: :clonk: :clonk:

Okay, I think I'm armed with enough to keep us busy for the morning.
Appreciate the knowledge and suggestions Steve and Pat!

Will report back with the findings - should be interesting... Other than a carb replaced at some point, it doesn't look like this thing has been taken apart since '74 :(

Here's a cool MSD timing class for us beginners: www.setyourtiming.com

Here's a cool MSD timing class for us beginners: www.setyourtiming.com (http://www.setyourtiming.com)

There are some real limitations or inaccuracies in that information. Especially with respect to vacuum advance. I would recommend a read through the "ignition 101" thread here on TC.

Well, only a little to report back on...

- Vacuum appears good - approx 15"
- Inspected cap/wires/plugs and found little to complain about. Did clean all of the contacts just because I was in there and then pulled each plug and cleaned/gapped them but nothing to write home about there...
- Balancer does not appear to have slipped. It's old, but it looks very intact.
- Went thru and carefully inspected all vacuum lines and could find nothing leaky, scuffed or cracked. I was actually surprised that given for the over-all dirty engine conditions, the rubber is actually in reasonably good shape.
- played with the dist a little more to adjust the timing and managed to get it to hold an idle somewhat better, but it still seems to want to eventually die. The dying still occurs as the RPMs fall off from a (relatively) normal range until it just dies. The timing mark still winds up just above the timing tab, but it's a lot closer than it was. I picked up the idle with the set screw on the linkage, but I forgot to pick up a tach, so I'm still just guessing as to where it's idling.

Not sure on the aspect of TDC - ran out of time to crawl under when the rains set in...


I keep coming back to a couple of things...

1) Is there something I can try to tweak on the carb itself? This is a fairly new carb so I'm not thinking anything is worn out, but I don't know the true history of it. Anyone in the past could have tried turning screws in hopes of making it run better. The plugs were not oily or wet, nor white - just a bit of a rusty shade of dull tan like I'd expect an older plug to look. Wires are in good shape too.

2) Is it likely that the symptoms are the result of the distributor needing rebuild? Engine is definitely tired and probably in the realm of 200K miles, but not sure how a balky dist would manifest itself.

I would track the vacuum and timing as it goes into it's death spiral and note changes.

From a diagnostic perspective you want to minimize variables. I am afraid that if you start tweaking the carb more that you may get things even more confused. Besides the carb tends to be either right or wrong and you have something that changes as the engine idles. My bets are still on ignition timing changing while it idles.