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Discussion Starter · #1 ·
I know its a chevy but I am need of help. Truck is a 78 k20 with a 350 and eddy 1405 carb. Just bought it and have a backfire through the carb when I give it some throttle. Cant even really drive it, no power. It idles pretty good and vacuum is about 17". The vac reading does flutter between 15-19 at idle. Start to give it throttle and it starts to backfire constantly. I goose the throttle and get a BIG backfire. I changed the plugs and set timing to 8*. Inspected the dizzy (HEI) and noticed some play moving the rotor back and forth, but the timing does not jump on the light. I also pulled the valve covers and noticed a lifter tick on the pass side #4, third valve from front. I looked for rocker movement with the valve covers off and noticed #1 first valve from front didn't have much movement at all. I can spin the pushrod and open the valve with a pry bar. I can jiggle the rocker arm around too. I pulled the carb off and in the process of rebuilding it. The carb spacer/ adapter is one of the 4 hole ones and the carb to adapter gasket was blown between the ports.

I got the truck cheap, the po said the noise/backfire just started, it was a daily driver. So, what I am trying to do is narrow it down to valvetrain or ignition. Because Im broke, throwing parts at it is not an option.

Although the cam might be going flat I dont think it would "just start" happening. Stuck lifter? So that brings me back to the dizzy, with HEI is there a way to test the dizzy? I plugged the vac advance, switch ports on carb, played with timing, check voltage at coil. The coil looks like one of those cheap chinese ones and the pickup looks like it was replaced. Cap seems good but I have some carbon tracking on the rotor. I have some play moving the rotor back and forth, more than I think there should be, but my timing holds.

Gas in the motor is about 3 months old and I dumped a bottle of iso heat in the tank. I also seafoamed the carb, absolutly no change. I have my plugs gapped to .044

Any ideas on the culprit here? Thanks.
 

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Before you get too excited, take the rotor off and look at the underside of it. Check out the inside of the cap for carbon tracking and cracks. Most often this kind of problem is ignition related.
 

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your timing is at 8 BTDC right? 8 degrees advanced?

becuase i once was having terribly backfires from my truck - which has a '71 360 motor with an edelbrock 750 carb, and it was literally as loud as a shotgun. pretty scary. the problem was that since i was a young 18 year old fool, i had retarded the timing quite far, which caused the truck to do these ungodly backfires.

i would worry about the cam and lifters after you straighten this problem out. sounds like your problem is definitely ignition/timing/fuel drlivery - carburetor problem.

could try increasing the timing a little bit...

tell us about your progress
 

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440 4spd Power Ram said:
Worn out cam and lifters, very common with small block chevys of the '70s.
I'm going with this diagnosis............ {yes}

I was gonna call it too but Leon beat me to it. ;D
 

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before you start tearing into it to much, get a propane torch, turn on the propane but dont light it. with truck running move the torch over and around the intake and the carburetor base and around any and all hoses.

if at any time while doing this the trucks idle speeds up, you just found your vacuum leak. fix that and see if your backfiring problem didnt go away.

after that, follow the proper procedure for adjusting valve lash on the engine.

both of those things would probably go a long way towards fixing the problem and wont cost you anymore than $15-20 for the propane torch if you dont already have one.

the fluctuating vacuum would be more of an indication of a leak than a flat cam lobe. especially since you say it wants to idle fairly smooth.

if neither of these fixes your problem then i would start looking more at a flat cam lobe.

eric
 

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Discussion Starter · #7 ·
timing is 8* btdc
I checked for vacuum leaks, I pulled the carb just to make sure, wont know about that till tomorrow. Intake looks dry, but...
From looking at the rocker movement I would defnately say at least one lobe is going, but this wouldn't happen overnight. The po drove the truck everyday, it sat for 3 months and was just used for plowing, then all of a sudden he supposedly started one day and it started backfiring. (at least thats the story).  
Im sure its ready for a new cam, but it feels like theres is something else here, at least Im hoping!   Thanks
 

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I wouldn't say it doesn't happen over night.

I had a company GMC van 1978 305 with 84K miles and it went in a week.  It started on a Thur and by the next Wed it was headed to the shop. It turned out 3 lobes were bad and 3 more were on the way out. I have heard that same story at least 5 times with the 77-84 chebby and GMCs 305's and 350's. :-\
 

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I've seen alot of 305's with worn lobes for some reason.
 

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Discussion Starter · #10 ·
Yeah my #1 was causing the backfire, on 7 cyl now. Still have absolutely no power, gutless as all hell. Replaced advance springs and that did not help, oh well, good enough for a plow truck! Thanks all!
 

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a2tripp said:
Yeah my #1 was causing the backfire, on 7 cyl now. Still have absolutely no power, gutless as all hell. Replaced advance springs and that did not help, oh well, good enough for a plow truck! Thanks all!
Oh heck Chebby cams are cheap......... ;D
 

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mopar65pa said:
Oh heck Chebby cams are cheap......... ;D
thats actually quite true. in my blazer i dropped a comp cams extreme 4x4 cam in it with all new lifters for about $150.

eric
 

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IntenseImages said:
thats actually quite true. in my blazer i dropped a comp cams extreme 4x4 cam in it with all new lifters for about $150.

eric
Chebby parts are son cheap you can build a motor with new parts for under $500 :p
 

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Discussion Starter · #15 ·
Cam cost is not the problem, I am by myself with limited expierence with putting in a cam. I helped a buddy out about 10 years ago on his 318 and it went flat in less than a week, we figured it wasnt broke in right.
 

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It's time consuming.


Pull the rad and grill, pull the front accessories off, timing cover, chain and gears, intake, valve covers, pull the rockers, pull the push rods, pull the lifters, and pull the cam. Done.

repeat in reverse for installation........  :p
 

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pay for the flight and i will bring my degree kit to alaska and help you out.  ;D ;D

it can be a time consuming process to do it right. the biggest thing especially on a chebby is to make sure the lifters are primed before install so you can adjust valves properly before start up, make sure you use plenty of cam lube, make sure it is ready to fire over the first time so you arent grinding on the cam trying to get it started and make sure after initial start up that it doesnt drop below 2000rpm for the first 20 minutes.

 

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You said you had some carbon tracking on the rotor, it's possible that the wires aren't too good either. Close the gap up on the plugs and see if things get better. If it runs better with .025" plug gap check the distributor & wires for evidence of cross-firing and leakage. Even with some wear on the lobes the engine should run without backfiring. Worn cam lobes tend to shorten up valve lift and duration numbers, up to a point that cylinder should actually idle smoother(and weaker).
 
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