I recently installed a MSD offroad ignition as well as a holley truck avenger carb in my 67fj40 with a sbc 350 in it. Now my truck is backfiring when in neutral and in gear. It idles fine but over 2000 rpm I get backfires especially when coming off the throttle. I have my timing set at 10 degrees btdc, I have the carb set to how Holley has suggested (idle set at 1.5 spins out from full closure on the idle adjustmet screws), I have installed a sheilded magnetic pickup wire from the MSD box to the distributer. I am stumped. Can anyone give any insight as to why my engine would be backfiring now?
backfiring sbc.... please help (1 Viewer)
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I recently installed a MSD offroad ignition as well as a holley truck avenger carb - 670cfm- in my 67fj40 with a mild sbc 350 in it. Now my truck is backfiring when in neutral and in gear. It idles fine but over 2000 rpm I get backfires especially when coming off the throttle. I have my timing set at 10 degrees btdc, I have the carb set to how Holley has suggested (idle set at 1.5 spins out from full closure on the idle adjustmet screws), I have installed a sheilded magnetic pickup wire from the MSD box to the distributer. I am stumped.
1) would my truck idle correctly if I was one tooth off on the distributer?
2) I am in Tulsa OK, with an altitude of around 700 feet, would downsizing my jets help (Holley suggests not changing jets unless 2000 feet above sea level)?
3) Is there some adjustment to my idle mixture which would decrease the backfiring?
Can anyone give any insight as to why my engine would be backfiring now?
1) would my truck idle correctly if I was one tooth off on the distributer?
2) I am in Tulsa OK, with an altitude of around 700 feet, would downsizing my jets help (Holley suggests not changing jets unless 2000 feet above sea level)?
3) Is there some adjustment to my idle mixture which would decrease the backfiring?
Can anyone give any insight as to why my engine would be backfiring now?
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please see other post
It's probably something simple. Was the truck running correctly before the changes? Check to make sure the spark plug wires are routed correctly and in good shape. It only takes one to be bad or in the wrong place to cause a miss or backfire.
X2; check the wires. How is the power?
Yes, it can idle okay being one tooth off on the distributor. You shouldn't have been a tooth off if you didn't move the oil pump drive because the distributor would not have fallen into place. I have see people get 180 deg out when installing a distributor also. I suggest that you pull #1 plug. Remove your cap and coil wire and check your timing again. 8-10 deg btdc is fine for most motors, so you should note where tdc is when the tissue blows out of #1 cyl and where it falls for the rotor and your cap.
"Backfiring"
Do you mean backfiring out of the carb?
Or backfiring out of the exhaust?
The former indicates problems with timing and/or carb settings.
The latter, especially at idle, indicates cylinders not firing, thus dumping unburned fuel into the exhaust, where it is ignited.
In both cases, crossed plug wires could be the culprit. Or, a low quality plug wire could indiction fire another low quality plug wire. This may not be noticable with a lower energy ignition system, but may show up with a more powerful ignition system.
Do you mean backfiring out of the carb?
Or backfiring out of the exhaust?
The former indicates problems with timing and/or carb settings.
The latter, especially at idle, indicates cylinders not firing, thus dumping unburned fuel into the exhaust, where it is ignited.
In both cases, crossed plug wires could be the culprit. Or, a low quality plug wire could indiction fire another low quality plug wire. This may not be noticable with a lower energy ignition system, but may show up with a more powerful ignition system.
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- I was extremely merticulus about labeling both the distributer cap and the plug wires when I was installing. I guess I should pull each plug individually to make sure that each cylinder is getting spark
- I am certain that misarranging my plug wires is not my problem but that was a great suggestion. The plug wires I am using are Mallory 8.5mm brand new super zoot wires they should be good to go.
- If my distributer was 180 degrees off would my truck idle well? I don't think this is the case because my vacume port on the distributer and the orientation of my rotor is pointing in the same direction as it was when I started the project.
- The backfiring is coming out of the exhaust
- I actually got pulled over in Tulsa on the way to my first Green County Cruisers meeting today due to the backfiring. What a bummer, but no tickets which was nice of the officer. Just a suggestion that I get the problem fixed (which I was going to ask about at the meeting)
Thank you for the suggestions thus far. I am appreciative and really am looking foward to getting my cruiser running normally
Brian
- I am certain that misarranging my plug wires is not my problem but that was a great suggestion. The plug wires I am using are Mallory 8.5mm brand new super zoot wires they should be good to go.
- If my distributer was 180 degrees off would my truck idle well? I don't think this is the case because my vacume port on the distributer and the orientation of my rotor is pointing in the same direction as it was when I started the project.
- The backfiring is coming out of the exhaust
- I actually got pulled over in Tulsa on the way to my first Green County Cruisers meeting today due to the backfiring. What a bummer, but no tickets which was nice of the officer. Just a suggestion that I get the problem fixed (which I was going to ask about at the meeting)
Thank you for the suggestions thus far. I am appreciative and really am looking foward to getting my cruiser running normally
Brian
I have seen a ford run at 180 off.
I did a v8 conversion on another truck and had two plug wires crossed...it idled fine, but would backfire under load...
check them wires...
bk
check them wires...
bk
...plug wires out of order...
I had same problem, 350, 670 truck avenger, backfiring. I retarded my timing and it quit. I believe my timing degree indicator is either bent, mis-mounted, or otherwise discombobbled. I read that different years of the 350 had different marking flanges over the harmonic balancer. Maybe the motor and marking flange is mismatched.
1-8-4-3-6-5-7-2
Firing Order
Firing Order
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If your problem is not the wires crossed you could have an intake gasket leak. Did you just put a carb on or change the intake also?
The firing order listed above(1-8-4-3-6-5-7-2) is clockwise looking down on the distributer.
If it runs you don't have to worry about the distributer being in wrong. It does not matter how you turn it or which post you put #1 on(as long as they are in order). Just don't want to see you chasing after something that is not worth chasing.
Is your distributer cap and rotor button new?
Hope this helps. PM me if I can help further. I have over 20 years of race engine experience.
Have a good one; John
The firing order listed above(1-8-4-3-6-5-7-2) is clockwise looking down on the distributer.
If it runs you don't have to worry about the distributer being in wrong. It does not matter how you turn it or which post you put #1 on(as long as they are in order). Just don't want to see you chasing after something that is not worth chasing.
Is your distributer cap and rotor button new?
Hope this helps. PM me if I can help further. I have over 20 years of race engine experience.
Have a good one; John
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One other thing. MSD ignitions are not good about clearing up a fouled spark plug. They are hot, but actually do not fire well in that type of situation. (I have run one for years now) Put in some new spark plugs and make sure you have no fuel dripping off the carb boosters when you cut the engine off.
: John
: John
backfire on deaccel?
exhaust leak introducing oxygen into the exhaust.
Combine that with holly's propensity for over fueling and you get the occasional good backfire.
Probably running a bit rich. Check your plugs. If they're sooty, you're running rich. if you have one that's more sooty than the rest, you have a misfiring plug. bad plug or wire.
exhaust leak introducing oxygen into the exhaust.
Combine that with holly's propensity for over fueling and you get the occasional good backfire.
Probably running a bit rich. Check your plugs. If they're sooty, you're running rich. if you have one that's more sooty than the rest, you have a misfiring plug. bad plug or wire.
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WOW,
I can't thank you guys enough. Thankyou for all the replies Your suggestions give me a bunch of avenues to explore. I actually do have exhaust leaks at the coupling between the headers and the exhaust on both sides. That would be a simple fix.
I can't thank you guys enough. Thankyou for all the replies Your suggestions give me a bunch of avenues to explore. I actually do have exhaust leaks at the coupling between the headers and the exhaust on both sides. That would be a simple fix.
i think you might have a vacume leak on the intake manifold check your vacume port [the t fitting that a auto trans vacume line hooks to]on the manifold the engine will run at idle but when you run down the road unburned gas builds up in the exhaust then ignites
The Holley 670 Truck Avenger is indeed set up quite rich for a stock 350. It's really tuned for a 350 with a hot cam, big intake, big valves and a free flowing exhaust. The secondaries are especially set rich. This is because the power valve is so low (2.5) that with a stock 350 in an FJ40 you'll never open it. It can be considered the equivalent of a solid plug. And in that case, Holley says a solid plug has to be compensated for by around 10 jet sizes. I installed a 6.5 power valve, dropped the secondary jets by 10 steps from 89 to 79, and changed the primaries from 68 to 64. This caused an off-idle stutter, so I increased the accel shooter from .028 to .031. My plugs went from sooty black, and sometimes a slightly wet sooty black, to a clean very light beige. I also installed a heavier secondary spring. I don't recall the color, but it was one of the heavier ones. I may increase the primary jets up one size to fine tune it, as the power is a little flat at cruise. This carb came with my FJ40, but if I was to buy a new Truck Avenger, I'd rather get the 470 cfm for a stock 350.
