Carb backfiring under load.
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Mountain Pass
Joined: May 2012
Posts: 100
Likes: 0
From: South of Butler PA
Carb backfiring under load.
I just swapped a 1970 351w in place of a 1984 302. I completely rebuilt the 351w. Budget peformance build.
Engine overhauled.
Valves lapped in, new heavy springs.
Lunati cam + lifters Duration 269/296, Lift .517/.517
Weiland stealth intake.
Good dist, cap, rotor (duraspark II)
Holley 600 carb from 302, ran good.
Summit double roller timing gear.
Timing is set at 10btc
No vacuum leaks
When I assembled the engine, the timing chain was adjustable, but I left the dots pointing at each other, so 0* The cam has a 4* grind on it.
Engine idles AWESOME, 500 rpm all day long.
As soon as you blip the throttle it backfires through the carb, but revs up kinda okay. Took it out on the road and it fell flat on its face, backfiring through the carb and exhaust like crazy, no power at all.
If the engine is set to TDC, timing gear dots aligned, distributor pointing to #1, timing is 10btc, What else should I look at? Thanks
Engine overhauled.
Valves lapped in, new heavy springs.
Lunati cam + lifters Duration 269/296, Lift .517/.517
Weiland stealth intake.
Good dist, cap, rotor (duraspark II)
Holley 600 carb from 302, ran good.
Summit double roller timing gear.
Timing is set at 10btc
No vacuum leaks
When I assembled the engine, the timing chain was adjustable, but I left the dots pointing at each other, so 0* The cam has a 4* grind on it.
Engine idles AWESOME, 500 rpm all day long.
As soon as you blip the throttle it backfires through the carb, but revs up kinda okay. Took it out on the road and it fell flat on its face, backfiring through the carb and exhaust like crazy, no power at all.
If the engine is set to TDC, timing gear dots aligned, distributor pointing to #1, timing is 10btc, What else should I look at? Thanks
Postmaster
Joined: Nov 2015
Posts: 2,769
Likes: 7
From: O-O-O-Oreilly's
Sounds a lot like what I went through. I rewired my truck and after having touched nothing but wires, it did the same thing.
My solution was to verify timing (my balancer slipped and is inaccurate), and running very lean will cause backfiring through the carb. The combination of very retarded timing and lean mixture did it. It would start, idle, and free rev great, but just threw a fit under any load. Also verify the function of your vacuum advance.
My solution was to verify timing (my balancer slipped and is inaccurate), and running very lean will cause backfiring through the carb. The combination of very retarded timing and lean mixture did it. It would start, idle, and free rev great, but just threw a fit under any load. Also verify the function of your vacuum advance.
Postmaster
Joined: Nov 2015
Posts: 2,769
Likes: 7
From: O-O-O-Oreilly's
I have found that backfiring through the carb can also be an issue of retarded timing. It was in my case, at least. If retarding the timing doesn't do it, try advancing it.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
Likes: 83
From: NorCal
My FE didn't like 10°-12° BTDC... it likes stock at 6° BTDC.
It wasn't a backfire through the carb... it was just popping out the exhaust on downshifts.
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Postmaster
Joined: Mar 2011
Posts: 2,698
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From: Taxachusetts
Definitely too retarded. A bigger than stock bumpstick requires more initial - try setting the timing at 14* - 16* BTDC. Factory specs don't work any time you make a change like you did.
You'll have to check your total timing after you get the initial right. Might have to back off the vacuum advance, too. The DSII distributors have an allen wrench adjustment through the vac nipple.
You'll have to check your total timing after you get the initial right. Might have to back off the vacuum advance, too. The DSII distributors have an allen wrench adjustment through the vac nipple.
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