Need Tune Up Help

[bluec10]

Need help tuning up my '69 GMC. Has a 250 w. Manual transmission. Truck came with AIR pollution control that I've removed. I can't get the truck to idle at the recommended 700 RPM. I've turned out the idle mix screw as much as I feel comfortable. Can't get the idle anywhere close.

The sticker says 700 RPM idle at 0 degrees. Do I need to advance the timing without the pollution control?

[geezer#99]

Yup, you need to advance it a bunch.
The last six banger I had liked 14 initial.

[Steeveedee]

What does the engine look like with the emission controls removed? It's possible that there is something amiss in the way that the equipment was removed and the vacuum lines or some such are messed up. Pics will help greatly!

The pollution control equipment doesn't affect the timing, btw. The timing affects the pollution...

[davepl]

Originally Posted by geezer#99

Yup, you need to advance it a bunch.
The last six banger I had liked 14 initial.

Hmmm... 14 initial plus 36 mechanical built into the distributor? Without recurving that'll knock so bad people will answer the door as you drive down the street!

They certainly do like that much initial, no doubt, but you really do need to back it out of the total mechanical if you do that. It's a really simple change to the distributor but I don't know how you'd lock it out at 22 or so without a distributor machine. I guess you could trial and error it!

I was only a baby in 1970, but I think that retarded 0 degrees initial was an attempt to keep the EGTs really hot (retarding timing increases temps, contrary to most guesses).

Hang onto the AIR pump system. They take about 1/3 of a horsepower to operate and folks with a stock restoration are probably looking for one. I'm trying to reassemble the complete smog system for a 427 I'm restoring now!

[My personal guess is that in removing the SMOG system you made an error somewhere and have introduced an air leak - when you find and fix that, it'll operate with less timing and the idle screws will work. Just a guess though.]

[geezer#99]

Originally Posted by davepl

Hmmm... 14 initial plus 36 mechanical built into the distributor? Without recurving that'll knock so bad people will answer the door as you drive down the street!

They certainly do like that much initial, no doubt, but you really do need to back it out of the total mechanical if you do that. It's a really simple change to the distributor but I don't know how you'd lock it out at 22 or so without a distributor machine. I guess you could trial and error it!

I was only a baby in 1970, but I think that retarded 0 degrees initial was an attempt to keep the EGTs really hot (retarding timing increases temps, contrary to most guesses).

Hang onto the AIR pump system. They take about 1/3 of a horsepower to operate and folks with a stock restoration are probably looking for one. I'm
trying to reassemble the complete smog system for a 427 I'm restoring now!

[My personal guess is that in removing the SMOG system you made an error somewhere and have introduced an air leak - when you find and fix that, it'll operate with less timing and the idle screws will work. Just a guess though.]

I’ve never seen any distributor have more than 24 degrees mechanical.
So that’s only 38 total.
Not near enough to detonate.
And likely all in at 3500 rpm.
With a lazy curve below 2000 rpm never a problem.
Easy to plot timing curves and adjust mechanical.
Several methods available to adjust mechanical depending on dizzy type.

[VWNate1]

These are dead simple to tune, I hope you have a dynamic timing light and a tach/dwell meter ? .

the carby has _two_ idle stop adjustments, one is electric and t'other is a stop screw .

First things first, open the spark plug gaps to .035" .

The initial timing is about 7 degrees with NO VACUUM SIGNAL, once it's running you can and should use the advance dial on the back of the timing light to set the full advance to 32 degrees, vacuum connected at 3,000 RPM .

Remember : it's critical to use the proper vacuum advance port on the carby so there's NO VACUUM SIGNAL at idle, and the vacuum comes in as soon as the throttle is opened .

If there are bolts/nuts at the very ends of the manifold, it's wrong and probably has an intake leak .

Easy enough to check, use starting fluid or other flammable aerosol when the engine is idling, don't forget to spray up from underneath as that's often where the minor leak is that causes poor idle .

If you're still running points, use the dwell meter to set the dwell to 33 degrees when cranking the engine with the plugs out and cap off, adjust the points as the engine is cranking, it'll be much more accurate .

Often a stock 1975 > HEI dist, from any junked i6 Chevy/GMC/Buick/etc. is under $50 and bolts right up ('75 ~ '77 uses a separate coil) and will *instantly* give you more power, better idle, faster starting cold or hot and increased fuel economy ~ no downside possible .

I used one on a worn out 250 that fouled the plugs every 250 miles, after adding the HEI it ran good as new again in spite of the oil use .

Tuning i6 engines isn't difficult but seems to be a lost art these days .

[davepl]

Delete!

[bluec10]

No leaks with the pollution control. I’ve had it running great. The issue is the carb. It’s a rebuilt unit and I’m not sure it’s setup properly. Any info about Rochester MV carbs?

[davepl]

This isn't a tuning issue. You can set dwell and mixture all day, not going to fix it.

If people would set their engines up mechanically as their were originally, they'd be a lot further ahead. But people figure they know better than GM and they start hooking vacuum advance up to manifold vacuum, stuff like that, without really knowing why.

If you fix the mechanical problems (like vacuum leaks) and set the timing to 12 degrees BTDC, idle to 750, then the mixtures two turns out, the thing should run pretty well. If it doesn't, then as I say, you've got a vacuum leak or something.

[VWNate1]

Failure to do things in the proper order is why everyone else has problems .

if you don't know, don't sling arrows .

I've been running and repairnig i6 engines for well over 50 years and doing things in the correct order usually reveals some basic mistake .

I use GM factory training and real world experience .

Agreed it's probably an intake vacuum leak, that doesn't negate the need to set it up right .

[davepl]

Originally Posted by VWNate1

Failure to do things in the proper order is why everyone else has problems .

if you don't know, don't sling arrows .

I've been running and repairnig i6 engines for well over 50 years and doing things in the correct order usually reveals some basic mistake .

I use GM factory training and real world experience .

Agreed it's probably an intake vacuum leak, that doesn't negate the need to set it up right .

I've been doing it 51 years, so I guess you should defer to me!

I don't know why you're agitating. He's got to fix the vacuum leak (or whatever) before he can tune it. You know it, I know it, and you agree with me. Why argue and be cantankerous when you agree?

As for "If you don't know", I do know.

[VWNate1]

Displaying ignorance / laziness isn't a good thing, clearly you want to short cut the job , I never do this .

[davepl]

Originally Posted by VWNate1

Displaying ignorance / laziness isn't a good thing, clearly you want to short cut the job , I never do this .

Yep, I'm just a crazy old fool who fixes vacuum leaks before setting mixtures.

You've called me stupid, lazy, and ignorant and it's not even 8AM yet. Also I "cut jobs short", sling arrows, do things in the wrong order, and don't have real world GM experience. Quite the day you're having.

The funny part is I'm right and haven't said a word about you. That makes me classy and smart. :-)

[bluec10]

Hey guys - no need to get personal.

I see myself as a "car guy" but it's a hobby, not a profession. Problem is that it could be so many different things.

1. I'm going to chase down vacuum leaks first off. Thought I did a thorough job when I replaced the manifolds but there may be a glitch. I'm a bit gun-shy about tightening these manifolds too much because they are cast iron.

2. VWNate - My manifold has a bold at the end, but the bold is in the factory location. I did have to make the manifold bolt hole a bit bigger so it would align.

3. Could it be the power brake booster?

4. I h haven't ever done timing at 3000 RPM. Is that common?

5. Timing first or idle-mix first?

[toolboxchev]

Old rule of thumb, Ignition first, then carb.

[HO455]

X2 80% of all carburetor problems are electrical.
But that being said...
If your throttle butterflies are open too far your mixture screws will not effect the idle quality. Post a picture of your throttle so we can see where the idle set screw is set at. If this is the case it is likely due to where the timing is currently set.
If you have a vacuum leak then we should see the idle screw allowing the butterflies to be closed too far and causing poor air signal to the venturies. This prevents fuel from being drawn into the engine and once again your idle mixture screws won't effect the idle quality. So let's see some photos of the throttle linkage and get you back on the road.