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#1 |
Registered User
Join Date: Apr 2002
Location: Manitoba, Canada
Posts: 663
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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?
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Present: 2015 Tacoma. Yeah, not a GM, but I love it. 1969 GMC 32,000 - fix, drive, relax, fix... 2019 BMW R1250 GSA - Yahoooooooo 1979 Honda GL 1000 - retro touring at its best. Past: '05 Sierra 4x4 - Had 270,000 KM and running well when it was written-off by a stop sign runner. '94 F-150 from the "F word" company. I'll admit it...good truck. Sold what was left of it for $800 to a guy who came to pick it up at 11:00 PM with cash in hand. Hmm. '79 Sierra Grande (Black) organ donor - perfect rebuildable 4-bolt 350 and a good TH350. '76 Sierra Grande (Orange) - hate isn't too strong a word. Kid who bought it turned it into a hot rod. '68 C-10 R.I.P. - Dad's old truck...too far gone to resurrect. '59 C-50 - with hoist. Truck is gone, wife isn't. Nuff said. |
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#2 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,615
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Re: Need Tune Up Help
Yup, you need to advance it a bunch.
The last six banger I had liked 14 initial. |
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#3 |
Who Changed This?
![]() Join Date: Jan 2013
Location: Simi Valley, CA
Posts: 10,397
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Re: Need Tune Up Help
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...
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~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA |
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#4 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Need Tune Up Help
Quote:
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.]
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#5 | |
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Join Date: Aug 2009
Location: Bowser
Posts: 13,615
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Re: Need Tune Up Help
Quote:
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. |
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#6 |
Registered User
Join Date: Feb 2012
Location: So. Cali.
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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 .
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-Nate Geezer '49 3100 235 W/ Muncie SM420 SOLD '69 C/10 shortbed sidemount survivor 250 L6 W/ 350TH ![]() |
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#7 |
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Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Need Tune Up Help
Delete!
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible Last edited by davepl; 06-04-2018 at 10:45 AM. |
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#8 |
Registered User
Join Date: Apr 2002
Location: Manitoba, Canada
Posts: 663
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Re: Need Tune Up Help
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?
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Present: 2015 Tacoma. Yeah, not a GM, but I love it. 1969 GMC 32,000 - fix, drive, relax, fix... 2019 BMW R1250 GSA - Yahoooooooo 1979 Honda GL 1000 - retro touring at its best. Past: '05 Sierra 4x4 - Had 270,000 KM and running well when it was written-off by a stop sign runner. '94 F-150 from the "F word" company. I'll admit it...good truck. Sold what was left of it for $800 to a guy who came to pick it up at 11:00 PM with cash in hand. Hmm. '79 Sierra Grande (Black) organ donor - perfect rebuildable 4-bolt 350 and a good TH350. '76 Sierra Grande (Orange) - hate isn't too strong a word. Kid who bought it turned it into a hot rod. '68 C-10 R.I.P. - Dad's old truck...too far gone to resurrect. '59 C-50 - with hoist. Truck is gone, wife isn't. Nuff said. |
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#9 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Need Tune Up Help
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.
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#10 |
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Join Date: Feb 2012
Location: So. Cali.
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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 .
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-Nate Geezer '49 3100 235 W/ Muncie SM420 SOLD '69 C/10 shortbed sidemount survivor 250 L6 W/ 350TH ![]() |
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#11 | |
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Join Date: Aug 2006
Location: Redmond, WA
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Re: Need Tune Up Help
Quote:
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.
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#12 |
Registered User
Join Date: Feb 2012
Location: So. Cali.
Posts: 2,644
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Re: Need Tune Up Help
Displaying ignorance / laziness isn't a good thing, clearly you want to short cut the job , I never do this .
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-Nate Geezer '49 3100 235 W/ Muncie SM420 SOLD '69 C/10 shortbed sidemount survivor 250 L6 W/ 350TH ![]() |
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#13 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Need Tune Up Help
Quote:
![]() 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. :-)
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1970 GMC Sierra Grande Custom Camper - Built, not Bought 1969 Pontiac 2+2 427/390 4-speed Coupe 1969 Pontiac 2+2 427/390 4-speed Convertible |
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#14 |
Registered User
Join Date: Apr 2002
Location: Manitoba, Canada
Posts: 663
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Re: Need Tune Up Help
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?
__________________
Present: 2015 Tacoma. Yeah, not a GM, but I love it. 1969 GMC 32,000 - fix, drive, relax, fix... 2019 BMW R1250 GSA - Yahoooooooo 1979 Honda GL 1000 - retro touring at its best. Past: '05 Sierra 4x4 - Had 270,000 KM and running well when it was written-off by a stop sign runner. '94 F-150 from the "F word" company. I'll admit it...good truck. Sold what was left of it for $800 to a guy who came to pick it up at 11:00 PM with cash in hand. Hmm. '79 Sierra Grande (Black) organ donor - perfect rebuildable 4-bolt 350 and a good TH350. '76 Sierra Grande (Orange) - hate isn't too strong a word. Kid who bought it turned it into a hot rod. '68 C-10 R.I.P. - Dad's old truck...too far gone to resurrect. '59 C-50 - with hoist. Truck is gone, wife isn't. Nuff said. |
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#15 |
Registered User
![]() Join Date: Jul 2015
Location: 2nd left past the stump on a dirt road.
Posts: 2,629
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Re: Need Tune Up Help
Old rule of thumb, Ignition first, then carb.
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#16 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,120
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Re: Need Tune Up Help
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.
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Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. ![]() RIP Bob Parks. 1967 Burban (the WMB),1988 S10 Blazer (the Stink10 II),1969 GTO (the Goat), 1970 Javelin, 1952 F2 Ford OHC six 4X4, 29 Model A, 72 Firebird (the DBP Bird). 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377 |
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