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Username Post: dual carbs backfires        (Topic#374193)
hdbob5454 
Poster
Posts: 50

Reg: 11-29-23
12-19-23 05:32 PM - Post#2865063    

I just got a 40 with the 216 ,dual carbs and exhaust. starts up fine but even after its warmed up and you give it the gas it spits out the carbs ,checked accelerator pumps working on both , put in points and plugs manual says . 018 and . 040, turned in octane dail on distributor and timed with a light , old points were a little worn plugs were gapped a little smaller but still doing it with new stuff , any idea how to stop it?



 


hdbob5454 
Poster
Posts: 50

Reg: 11-29-23
12-19-23 05:36 PM - Post#2865064    
    In response to hdbob5454

I should have said that I took it for a short ride and once I got going down the road it drove ok but starting off is tough



 
1971BB427 
Contributor
Posts: 608

Loc: Portland,Or
Reg: 12-06-09
12-20-23 08:17 AM - Post#2865075    
    In response to hdbob5454

Have you checked dwell angle with the engine running to confirm the point gap is correct? Should be around 29 degrees dwell. I've never had much luck setting points with a feeler gauge and not having to adjust after I check dwell.
I set plug gaps at .035" on them. I'd have suspected accelerator pumps, but sounds like you've checked those, so try checking the timing, and see how it responds when revving the engine to ensure mechanical advance is working freely.


1969 Suburban 2wd 350
1939 Chevy pass. coupe 350/700R4
1937 Austin gasser 327/TH350


 
Skidplate 
Newbie
Posts: 1

Loc: Colorado
Reg: 06-26-24
06-26-24 06:34 PM - Post#2871274    
    In response to 1971BB427

If you bought it recently and you weren't the one who set it all up, I would recommend you just start from zero. Pick up one of those flow gauges. Not sure what they're called but they just set on top of the carbs and then you match the flow with the choke butterflies. There's a reason why four barrel carbs were so popular. You don't have to tweak the carbs to all match the same flow. Dual carbs are really cool but you need to accept the fact that you're going to spend some time getting them working properly. As in, there is no secret ingredient. Tweak. Drive. Tweak some more. Adjust everything then ... wait for it... and tweak some more. Then drive. They will work well eventually but not without some substantial blood, sweat and tears.



 
bobb 
Super Senior Member
Posts: 6683

Loc: paradise
Reg: 09-05-03
06-28-24 08:32 AM - Post#2871310    
    In response to hdbob5454

try advance your timing a bit.

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.


 
hdbob5454 
Poster
Posts: 50

Reg: 11-29-23
06-28-24 02:45 PM - Post#2871323    
    In response to bobb

turned out the accelerator pump arm had come off one of the carbs.



 
Keith_Knox 
Member #189 Moderator and "22nd Year" Platinum Supporting Member
Posts: 7210
Keith_Knox
Age: 82
Loc: Napa, Ca USA
Reg: 04-02-00
06-28-24 07:06 PM - Post#2871327    
    In response to hdbob5454

Glad you found it. Did it cure the backfire.

29-41, 42-48, 49-54 Moderator
1948 Chevy Fleetmaster Coupe Purchased 6/2010. Stock with rebuilt 52 216 installed May 1966.
1965 Chevy II 2 door post 350 3 speed manual
2019 Ford Ranger Lariat Super Crewcab


 
hdbob5454 
Poster
Posts: 50

Reg: 11-29-23
07-09-24 05:05 PM - Post#2871623    
    In response to Keith_Knox

yes it did, going to change carbs from W-1 to w-0 Carter's in the future.



 
hdbob5454 
Poster
Posts: 50

Reg: 11-29-23
07-09-24 05:06 PM - Post#2871624    
    In response to Keith_Knox

yes it did, going to change carbs from W-1 to w-0 Carter's in the future.



 


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