![]() |
Register or Log In To remove these advertisements. |
|
|
![]() |
|
Thread Tools | Display Modes |
|
![]() |
#1 |
Registered User
Join Date: Nov 2016
Location: Greeley, CO
Posts: 17
|
402 backfiring hard start
So, I've been browsing the forum and haven't seen anything about the issue I'm having. I have a 72 c20 with the 402. All stock minus an HEI. Truck was running and driving just fine. Drove it a good 30 miles for thanksgiving, left to head back home and it idled normal, but when I put a heavier load on it the carb began to backfire. I took it slow the rest of the way home. Went to start it the next day and nothing. Absolutely no fuel was pumping. Replaced fuel pump. Got it started and idles good, but as soon as I put the gas pedal down it backfires again and dies. Then it's hard to start. I've checked for vacuum leaks and can't find any. Pulled the cap off of the dizzy to see if there was moisture and it's dry. Starting to pull my hair out with this one. Any help is appreciated. Thanks.
|
![]() |
![]() |
![]() |
#2 |
Registered User
Join Date: Sep 2015
Location: portland, OR
Posts: 48
|
Re: 402 backfiring hard start
I'd try a new HEI module because they are cheap. Good to have on hand anyway.
Maybe clogged fuel filter or needle / seat float bowl issue leaning it out real bad. That might explain why fresh fuel pump helped but didn't solve. Check timing, distributor could have slipped although unlikely. Check plug wires for cracks and aging. Unlikely they went bad abruptly but good to check anyway Good luck. |
![]() |
![]() |
![]() |
#3 |
Registered User
Join Date: Nov 2016
Location: Greeley, CO
Posts: 17
|
Re: 402 backfiring hard start
I'll check the filter inside the carb tomorrow. And if that doesnt work I'll look into the control module. Thank you for the input. I'll keep you posted
|
![]() |
![]() |
![]() |
#4 |
Registered User
Join Date: Apr 2015
Location: East Tn (In the heart of the Smoky Mtns)
Posts: 1,868
|
Re: 402 backfiring hard start
x2 check to make sure your distributor is tight. Your symptoms sound like the hold down came loose and it advanced too far
|
![]() |
![]() |
![]() |
#5 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
|
Re: 402 backfiring hard start
|
![]() |
![]() |
![]() |
#6 |
Registered User
Join Date: Nov 2010
Location: vista ca
Posts: 116
|
Re: 402 backfiring hard start
|
![]() |
![]() |
![]() |
#7 |
Registered User
![]() Join Date: Oct 2011
Location: Victoria, B.C
Posts: 3,794
|
Re: 402 backfiring hard start
I've also seen this with engines that have just scrubbed some of the teeth of the timing chain pulleys...thus resulting in timing chain slip. Not a good thing but fixable if figured out early before full part failure occurs.
As mentioned....heavily and thoroughly check your timing....TDC, #1 cylinder the whole deal...top to bottom because it does sound like a timing issue to me as well. All good Coley
__________________
....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500 |
![]() |
![]() |
![]() |
#8 |
Registered User
![]() Join Date: Oct 2011
Location: Victoria, B.C
Posts: 3,794
|
Re: 402 backfiring hard start
Geezer....you are the king and grand prince of timing issues!....lol
__________________
....for some men, there is experience, skill and effort....for the others...there is visa and UPS LOL 1966 Chevy 1/2 ton (Florida- Red/white) 1972 Chevy 1/2 ton (California- Blue/white) 2005 Chevy Silverado HD2500/Duramax 2000 Dodge Ram 1500 |
![]() |
![]() |
![]() |
#9 |
Senior Member
![]() Join Date: Jun 2005
Location: Winona Lake, IN
Posts: 6,277
|
Re: 402 backfiring hard start
My vote is for the fuel filter.
__________________
70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Power windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear |
![]() |
![]() |
![]() |
#10 |
Registered User
Join Date: Nov 2016
Location: Greeley, CO
Posts: 17
|
Re: 402 backfiring hard start
I'm going to look into it today. I'll check timing first. My tools are at the shop unfortunately, so i can only do so much. If timing is off it will most likely be advanced, so I'll try retarding the distributer a little and see if it makes it driveable. I'll keep everyone posted.
|
![]() |
![]() |
![]() |
#11 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
Re: 402 backfiring hard start
I thought of that, but on a Q-jet the filter is somewhat ingeniously spring loaded so if you get lazy and never change it, it eventually gets pushed out of the way and still flows, even when completely plugged.
He could have an inline filter somewhere that's clogged, but from the description as soon as he blips the throttle he has an issue, and the fuel supply for that should be covered by the fuel bowls, not the supply line. That was my reasoning, anyway!
__________________
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 |
![]() |
![]() |
![]() |
#12 | |
Senior Member
![]() Join Date: Jun 2005
Location: Winona Lake, IN
Posts: 6,277
|
Re: 402 backfiring hard start
Quote:
__________________
70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Power windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear |
|
![]() |
![]() |
![]() |
#13 |
Registered User
Join Date: Feb 2003
Location: Santa Ana, CA
Posts: 2,191
|
Re: 402 backfiring hard start
Don't forget to check you accelerator pump in the carb. If you are not getting an extra shot of fuel when you try to accelerate or pull a load, the engine will bog down from a lean condition. This could lead to a backfire. A faulty accelerator pump can also make for hard starting as well. Just a thought.
|
![]() |
![]() |
![]() |
#14 |
Registered User
Join Date: Oct 2007
Location: Sammamish, WA
Posts: 2,115
|
Re: 402 backfiring hard start
My guess is a worn cam lobe. If motor is tired, the cam and timing chain could be the issue. especially if it is backfiring through the carb. They run ok and then under a load will backfire.
|
![]() |
![]() |
![]() |
#15 | |
Registered User
Join Date: Dec 2009
Location: El Dorado Ca
Posts: 3,374
|
Re: 402 backfiring hard start
Quote:
|
|
![]() |
![]() |
![]() |
#16 | |
All about them K’s
![]() Join Date: Jun 2014
Location: Land of Entrapment
Posts: 6,374
|
Re: 402 backfiring hard start
Quote:
__________________
Tyler '57 3100 http://67-72chevytrucks.com/vboard/s...d.php?t=813888 '72 K20 Cheyenne http://67-72chevytrucks.com/vboard/s...d.php?t=662879 ‘69 K10 SWB http://67-72chevytrucks.com/vboard/s...d.php?t=805206 '98 Silverado LT K2500HD ECLB Vortec 454/4l80E: 6" lift 315/75/16's ‘87 IROC-Z all original 50K mile survivor TPI 305 IROC Blue ‘10 Camaro 2SS/RS Aqua Blue Metallic #93 -version 2.0 ![]() |
|
![]() |
![]() |
![]() |
#17 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
|
Re: 402 backfiring hard start
Carb's don't backfire from advanced timing.
Retarded timing plus lean fuel vapor will make it backfire. Being lean can be a result of a cold intake too. What do you run for initial timing? |
![]() |
![]() |
![]() |
#18 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
Re: 402 backfiring hard start
If it's advanced enough that it's firing when the valve is still open it sure can. How you'd keep it running I have no idea, but on principal I guess if you were -right- on the verge of advancing it too far maybe one cylinder or another would pop back through the intake valve.
Not saying that's the problem here, doubt it is, just saying it's not impossible. Extra retarded timing would be out the exhaust valve, extra advanced should be out the intake valve... no?
__________________
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 |
![]() |
![]() |
![]() |
#19 |
Post Whore
![]() Join Date: Feb 2016
Location: Portland Oregon
Posts: 11,111
|
Re: 402 backfiring hard start
Check for carbon tracks or oil inside the distributor cap. Could be causing a cross fire problem. If your unable to see any signs of tracking, swap caps and see if the problem goes away, or clean the inside with electronic cleaner and let dry COMPLETELY before starting. It's not a good idea to fill your distributor cap with an explosive vapor and then try to start it. Trust me on this one.
__________________
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 |
![]() |
![]() |
![]() |
#20 |
Senior Member
![]() Join Date: Dec 2013
Location: Gods country East,Tn
Posts: 8,545
|
Re: 402 backfiring hard start
The first question is how many miles are on the engine ? If it's new or a recent rebuild then you would consider ignition or fuel . If it's been some time and miles start at the timing chain . Easy check , pull off the distributor cap grab hold of the crank pulley and (looking at the engine from the front ) turn the crank pulley to the left BY HAND while watching the rotor , once the rotor moves turn the crank to the right and watch the rotor to see if it moves in concert with the crank or is slightly delayed . Any delay in movement of the rotor would suggest the timing chain has stretched or the old gears are worn . I'll bet a donut that's the problem and it takes 5 minutes to find out .
![]() Many times a worn timing chain will slip with the added load of just pulling away from a traffic light . Last edited by Grumpy old man; 11-28-2016 at 06:35 AM. |
![]() |
![]() |
![]() |
#21 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
|
Re: 402 backfiring hard start
At your elevation I'd start at 14 initial.
But with that said, a half dozen other posts will come up saying you don't need that much. You live at high elevation. You need to start the burn way earlier than those at sea level. |
![]() |
![]() |
![]() |
#22 | |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
|
Re: 402 backfiring hard start
Quote:
So the amount of advance you need is really tied to the vaporization rate of the gasoline, and the elevation may already be accounted for in the fuel you've purchased. But if not, if I were racing in Denver I'd try 4-6 degrees extra. Since I'm in Seattle and my 402 likes 12, I'd pick 16 if you had to pick something.
__________________
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 |
|
![]() |
![]() |
![]() |
#23 |
Registered User
Join Date: Nov 2016
Location: Greeley, CO
Posts: 17
|
Re: 402 backfiring hard start
Ok, got my truck to my shop. I took Grumpy old man's advice and turned the crank while looking at the rotor. There was a delay in movement. The truck has about 110,000 miles on it. So looks like I need to get it apart and see what the problem is. Thanks for all of the help guys. I appreciate it.
|
![]() |
![]() |
![]() |
#24 |
Senior Member
![]() Join Date: Dec 2013
Location: Gods country East,Tn
Posts: 8,545
|
Re: 402 backfiring hard start
Glad you have a direction on it , that's a old diagnostic trick I learned in the mid 1970's after my 68 Camaro had the same symptoms , I'd retime it and it run for a while then I'd retime it again ..... it's the only way to gauge timing chain slop without pulling it apart . If you would post some pictures of what you find once you open it up .I'm not sure what gears are in your engine but many had the nylon timing gears to reduce engine noise and after many miles (back then 100,000 was many miles ) or a few over heating episodes the nylon gears would wear allowing the stretched chain to slip and present the symptoms you had .had it been a fresh rebuild ignition or fuel would have been first on the checklist .
![]() |
![]() |
![]() |
![]() |
#25 |
Registered User
Join Date: Nov 2016
Location: Greeley, CO
Posts: 17
|
Re: 402 backfiring hard start
I will post pictures of it apart. I should get to it this weekend if all goes well.
|
![]() |
![]() |
![]() |
Bookmarks |
|
|