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#1 |
Registered User
![]() Join Date: Jan 2005
Location: Eastern Nebraska
Posts: 611
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New crate 350, backfires...stumbles...
The crate 350 is in and running...but not very well. Here's link to complete details:
http://67-72chevytrucks.com/vboard/s...d.php?t=198802 We set timing to 4 BTDC and she runs, but idles rough. When you drive her she stumbles low in the RPM range, and even backfires at times. But once you get her reved up a little it screams! It's got Holley 1850-2 600 CFM carb that came off old engine. Didn't do much to carb besides de-grease. HEI from old truck. New cap / rotor / plugs and wires. Suggestions? Could the harmonic balancer have slipped and not be giving me true timing mark? Should I be timing differently with the HEI dist.?
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'72 Chevy K-20 Crate 350, SM465, 4.10's |
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#2 |
Senior Member
Join Date: May 2002
Location: NORTHEAST OHIO
Posts: 493
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Re: New crate 350, backfires...stumbles...
If you put the key in the balancer it probably is ok
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#3 |
Senior Member
Join Date: Mar 2003
Location: St robert Mo
Posts: 2,001
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Re: New crate 350, backfires...stumbles...
I would advance it more untill the idle smoothes and see how it runs. 4 seems really low to me but not sure. I think mine is around 12 but mine is not stock.
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1968 c10 lowered 3" 4" 355/Th400 built by Hatfield racing in joplin MO Last edited by 76bonanza; 06-11-2006 at 09:03 PM. |
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#4 |
*************
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Location: Pensacola, FL
Posts: 17,846
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Re: New crate 350, backfires...stumbles...
Sounds like timing to me. Did you replace the distributor? Some time the weights and springs get worn. Maybe check all of your vacuum lines.
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Rob - https://www.instagram.com/hart_rod_c10 As Iron Sharpens Iron, So One Man Sharpens Another. Proverbs 27:17 FOR SALE: DBW pedal bracket - http://67-72chevytrucks.com/vboard/s...d.php?t=651123 FOR SALE: Hood Brackets http://67-72chevytrucks.com/vboard/s...d.php?t=679945 1987 Silverado SWB - 34.5K original miles http://67-72chevytrucks.com/vboard/s...d.php?t=801834 1969 SuperBurb - http://67-72chevytrucks.com/vboard/s...d.php?t=200387 1968 Farm truck - http://67-72chevytrucks.com/vboard/s...d.php?t=358692 1968 SWB - http://67-72chevytrucks.com/vboard/s...d.php?t=551258 1948 Chevy - http://67-72chevytrucks.com/vboard/s...=122164&page=3 |
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#5 |
Registered User
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Location: Eastern Nebraska
Posts: 611
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Re: New crate 350, backfires...stumbles...
HEI dist is off the old engine. I'll check vac lines. Other thoughts?
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'72 Chevy K-20 Crate 350, SM465, 4.10's |
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#6 |
Registered User
Join Date: Sep 2003
Location: Rayne, LA
Posts: 2,782
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Re: New crate 350, backfires...stumbles...
Did you run 4 degrees of advance on your old engine with the HEI? I have the same engine in my truck with an HEI in it and the timing is set at 14 IIRC.
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Steven "If dogs don't go to heaven, when I die I want to go wherever they went." -- Will Rogers R.I.P. Oscar R.I.P. Ratchet 1967 Pontiac GTO - Dad was the Original Owner 1970 Chevy 2wd SWB "Oscar's Truck" 1970 Chevy 2wd Blazer "Ratchet's Blazer" 2013 Chevy 2500HD Crew Cab Duramax/Allison Z71 LTZ "Brown Sugar" 2017 Chevy Suburban "BDB" 2020 Chevy Blazer Premier "Foxy" Member of Louisiana Classic Truck Club (LCTC) Shop Build |
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#7 | |
Registered User
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Location: Eastern Nebraska
Posts: 611
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Re: New crate 350, backfires...stumbles...
Quote:
14 degrees behind top dead center? (IIRC = if i recall correctly?)
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'72 Chevy K-20 Crate 350, SM465, 4.10's |
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#8 |
Registered User
Join Date: Sep 2003
Location: Rayne, LA
Posts: 2,782
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Re: New crate 350, backfires...stumbles...
14 BEFORE top dead center and yes you got IIRC correct. When I put my engine in is when I switched to HEI, I did alot of internet research and found that HEI's like more advance than the old points type distributors, most guys were running betwwen 12 and 16. I finally found my sweet spot at 14 (after 2 weeks of messing with it.)
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Steven "If dogs don't go to heaven, when I die I want to go wherever they went." -- Will Rogers R.I.P. Oscar R.I.P. Ratchet 1967 Pontiac GTO - Dad was the Original Owner 1970 Chevy 2wd SWB "Oscar's Truck" 1970 Chevy 2wd Blazer "Ratchet's Blazer" 2013 Chevy 2500HD Crew Cab Duramax/Allison Z71 LTZ "Brown Sugar" 2017 Chevy Suburban "BDB" 2020 Chevy Blazer Premier "Foxy" Member of Louisiana Classic Truck Club (LCTC) Shop Build |
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#9 | |||
Union Electrician
Join Date: Feb 2005
Location: Louisville, Ky.
Posts: 3,356
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Re: New crate 350, backfires...stumbles...
I agree with smbrouss7, the PO of my truck had already installed a 290 hp crate 350 with HEI and it was set around 12-14 btdc, well after replacenig the harmonic balancer I found that out. I have been running mine around 12-14 depending on the weather for 2 yrs and it runs great, has plenty of power and with an edlebrock 1406 it's pretty good on fuel 14mpg for my 69 and has 3:73 non posi rear.
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#10 |
Registered User
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Location: Eastern Nebraska
Posts: 611
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Re: New crate 350, backfires...stumbles...
12-14 degrees before top dead center sounds like a lot. Wouldn't retarding the timing hurt low end, but help high end RPM?
This is manual tranny truck (4spd). It backfires under load at low RPM's, just off idle. Seems to love the higher end of the RPM scale at this point in time. What plugs you running? I think the autozone guy sold me R44TX plugs. (I'm setting the timing with the vac advance hose disconnected from dist)
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'72 Chevy K-20 Crate 350, SM465, 4.10's Last edited by Daubs; 06-11-2006 at 10:33 PM. |
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#11 |
Senior Member
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Location: Centennial, CO
Posts: 2,946
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Re: New crate 350, backfires...stumbles...
I've always started with 10 deg and adjusted from there, 14 isn't too much unless you're pinging. Do you have vacuum advance? If so, did you pull and block the line to the dist when you timed it? I usually set advance w/o vacuum. Then I connect the vac line to the ported vac fitting (no vac at idle, vac when the throttle opens up).
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ClusterTruck: 68 C10 Bought in 89, wrecked in 03. Slow low $ rebuild started '17. 6.0 & TKO http://67-72chevytrucks.com/vboard/s...d.php?t=788602 93 K1500 Short Step, 350, NV4500, EBL flash ECU. Vortec heads & roller cam someday... 05 Jeep Wrangler Unlimited. 3” lift & 6-speed 02 E320 the insurance total “free car” 13 Tahoe LTZ Last edited by clay68c10; 06-11-2006 at 10:46 PM. |
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#12 |
Registered User
Join Date: Sep 2003
Location: Rayne, LA
Posts: 2,782
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Re: New crate 350, backfires...stumbles...
Going from 4 to 14 degrees before top dead center is advancing the timing. I don't remember exactly which plugs I'm running, I know they are splitfires. Don't be afraid to play with your timing. Just make small adjustments of about 2 degrees each time, then go for a drive and see how it responds. Your engine will let you know when you are getting too far out of whack (backfiring, pinging, etc.) Also just because we both have the same engine part number doesn't mean EVERYTHING is exactly the same in our engines, not to mention we live in different climates, so our timing settings could be different. But, I do think that you're going to end up with at least 12 degrees of advance.
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Steven "If dogs don't go to heaven, when I die I want to go wherever they went." -- Will Rogers R.I.P. Oscar R.I.P. Ratchet 1967 Pontiac GTO - Dad was the Original Owner 1970 Chevy 2wd SWB "Oscar's Truck" 1970 Chevy 2wd Blazer "Ratchet's Blazer" 2013 Chevy 2500HD Crew Cab Duramax/Allison Z71 LTZ "Brown Sugar" 2017 Chevy Suburban "BDB" 2020 Chevy Blazer Premier "Foxy" Member of Louisiana Classic Truck Club (LCTC) Shop Build Last edited by smbrouss70; 06-11-2006 at 10:55 PM. |
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#13 |
Registered User
Join Date: Sep 2003
Location: Rayne, LA
Posts: 2,782
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Re: New crate 350, backfires...stumbles...
I just thought of something... did you re-use your harmonic balancer by any chance? If so, the outer ring could have slipped causing your readings to be off. Based on the time and effort you have put in to your engine I'm doubting that that's the case but I just have to check.
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Steven "If dogs don't go to heaven, when I die I want to go wherever they went." -- Will Rogers R.I.P. Oscar R.I.P. Ratchet 1967 Pontiac GTO - Dad was the Original Owner 1970 Chevy 2wd SWB "Oscar's Truck" 1970 Chevy 2wd Blazer "Ratchet's Blazer" 2013 Chevy 2500HD Crew Cab Duramax/Allison Z71 LTZ "Brown Sugar" 2017 Chevy Suburban "BDB" 2020 Chevy Blazer Premier "Foxy" Member of Louisiana Classic Truck Club (LCTC) Shop Build |
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#14 |
Registered User
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Location: Eastern Nebraska
Posts: 611
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Re: New crate 350, backfires...stumbles...
Yep, using the old harmonic balancer.
Okay, I'm confused...Time for "stupid engine question #1": Timing set at "0" top dead center mean's it's getting spark as the piston is at highest in the cylinder. "4 degrees Before top dead center" means sparking before the piston is at top dead center, correct? The spark is actually firing before the piston has reached the top-dead-center, or still moving up in cylinder(?).
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'72 Chevy K-20 Crate 350, SM465, 4.10's Last edited by Daubs; 06-12-2006 at 12:06 AM. |
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#15 | |
Junior Member
Join Date: Apr 2006
Location: Eufaula, OK
Posts: 103
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Re: New crate 350, backfires...stumbles...
Quote:
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Clif '68 C/10 LWB, 327, TH400, 12 bolt Last edited by clif1968; 06-12-2006 at 08:19 AM. |
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#16 | |
Registered User
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Location: Eastern Nebraska
Posts: 611
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Re: New crate 350, backfires...stumbles...
Quote:
And thanks 68Stepbed and meathead95 for plug recommendataion.
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'72 Chevy K-20 Crate 350, SM465, 4.10's |
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#17 |
Registered User
Join Date: Sep 2003
Location: Rayne, LA
Posts: 2,782
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Re: New crate 350, backfires...stumbles...
correct, it's only a stupid question if you don't ask
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Steven "If dogs don't go to heaven, when I die I want to go wherever they went." -- Will Rogers R.I.P. Oscar R.I.P. Ratchet 1967 Pontiac GTO - Dad was the Original Owner 1970 Chevy 2wd SWB "Oscar's Truck" 1970 Chevy 2wd Blazer "Ratchet's Blazer" 2013 Chevy 2500HD Crew Cab Duramax/Allison Z71 LTZ "Brown Sugar" 2017 Chevy Suburban "BDB" 2020 Chevy Blazer Premier "Foxy" Member of Louisiana Classic Truck Club (LCTC) Shop Build |
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#18 |
Registered User
Join Date: Mar 2005
Location: 10-Uh-See
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Re: New crate 350, backfires...stumbles...
My engine runs best with 10 deg. BTDC, ACDelco R45TS plugs with .045 gap, with HEI.
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#19 | |
computer illiterate
Join Date: Jan 2001
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Re: New crate 350, backfires...stumbles...
Quote:
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72 2wd blazer 72 swb 4x4 |
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#20 |
Hittin E-Z Street on Mud Tires
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Re: New crate 350, backfires...stumbles...
A question that I have that may help Daubs as well is this: When you set the timing, you should unhook the vacuum advance to the dizzy, right? Once it is unhooked, does the line back to the carb need to be plugged off or left open? So for those who set their timing at 10 or 12 or whatever, that's with the vac. advance unplugged?
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Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride ![]() American Born, Country by the Grace of God ![]() 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications! |
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#21 |
Junior Member
Join Date: Apr 2006
Location: Eufaula, OK
Posts: 103
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Re: New crate 350, backfires...stumbles...
Alot of people unhook it and plug it off to set the timing, but if it is connected to the correct vaccum port there should be no vaccum at idle anyway and you could leave it hooked up. If it is hooked straight to manifold vaccum you would have full vaccum advance at idle and it would be useless.
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Clif '68 C/10 LWB, 327, TH400, 12 bolt Last edited by clif1968; 06-12-2006 at 10:37 AM. |
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#22 | |
Hittin E-Z Street on Mud Tires
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Location: Greenville, SC
Posts: 23,090
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Re: New crate 350, backfires...stumbles...
Quote:
Thanks! I put a new edelbrock on a crate motor two years ago and made sure I hooked the vac adv. to the port that has no vac at idle, and left it hooked when I timed it to 8 degrees BTDC. I thought 8 was the max your supposed to go on a stock motor, but I might bump it up to 10 to see what it does! It runs great now, maybe I'll be able to free up some horsies!
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Jesse James 1967 C10 SWB Stepside: 350/700R4/3.73 1965 Ford Mustang: 289/T5-5spd/3.25 Trac-Loc 1968 Pontiac Firebird: Project Fire Chicken! 2015 Silverado Double Cab 5.3L Z71 2001 Jeep Wrangler Sport 4.0L 5spd 2020 Chevrolet Equinox Premium 2.0L Turbo 2011 Mustang V6 ~ Wife's ride ![]() American Born, Country by the Grace of God ![]() 1967 CST Shop Truck Rebuild! My 1967 C-10 Build Thread My Vintage Air A/C Install Project "On a Dime" Trying my hand at Home Renovation! 1965 Mustang Modifications! |
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#23 | |
Junior Member
Join Date: Apr 2006
Location: Eufaula, OK
Posts: 103
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Re: New crate 350, backfires...stumbles...
Quote:
Just make sure that it don't ping under load. If you have to much advance it will ping usually in third gear at slower speeds with light acceleration. That's a sure way to blow a hole in a piston, or blow out a head gasket. I normally time engines by ear, advance a little at a time until I hear it ping then back it down a little. This is usually were I find the "sweet" spot in any motor.
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Clif '68 C/10 LWB, 327, TH400, 12 bolt |
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#24 | |
Registered User
Join Date: Jan 2005
Location: Colorado Springs
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Re: New crate 350, backfires...stumbles...
Quote:
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1971 GMC K1500 SWB Fleetside, 4" skyjacker softride and, 14 Bolt FF, HD Dana 44 4.10's 1970 K5 TBI conversion lifted etc SOLD 1968 C20 camper truck SOLD “To ban guns because criminals use them is to tell the innocent and law-abiding that their rights and liberties depend not on their own conduct, but on the conduct of the guilty and the lawless.” Jeff Snyder |
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#25 |
Senior Member
Join Date: Apr 2005
Location: metro Phoenix, AZ
Posts: 241
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Re: New crate 350, backfires...stumbles...
Howdy;
when I first got to working on my truck after limping it home after buying it, I got one of those Mr Gasket advance curve kits for around $9, with the new weights and three sets of springs, for my HEI. I ended up using the lightest set of springs, initial advance of 12 degrees, and a plug gap of .060" For whatever reason, my engine liked .060 better, and it was inexpensive to try. Went from stumbling hesitation to ,well, really decent! Just my 0.02 cowboy33713 |
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