Hi all,

Carburetor backfiring...

In no small part, thanks to HDForums, I now have a running HD. I'm coming to you again with a need for advise. First I'll list the specs...

1. Frame off rebuild 1975 Shovel FLH
2. S&S Super B

 

separated into two posts... sorry...

 

3. New Autolite plugs, new wires, new advance, new points/points plate/condenser.
4. Points gap set, timing set.

As the bike warms up, the backfires slow down... This must be a carboratur issue... Its backfiring out of the carb, like a lean mix.

This is the question, should I run the idle mix rich to avoid the carb backfire, or do I have to live with rolling the throttle slowly sans accelerator pump???

Thanks so much...

If A/V helps... Heres a link to a youtube of the bike earlier tonight...

 

S&S B will not take the throttle blip like that with no accel pump it's a slow roll on throttle and likes a little idle higher RPM to keep the fuel moving. Have you done a leak check on the intakes and around the ends of the throttle shaft on the carb after she's warmed up ? Sounds like what I was hearing in the video, I'm an old S&S B man myself.

 

No, I haven't checked for air leaks... intake manifold, seals... all new. Makes sense though, I'll check them tomorrow.

And... To all, I apologize for the length of the video...

Table of contents

0:00 into idle
1:17 first backfire, then i let it warm up more.
2:00 really yanking it, no backfire.
2:12 starts to backfire again.
2:30 I start playing with mix
3:10 Mix increased the idle RPM's so I turned the idle down. Thought I should've been at the sweet spot at that point... still sounding junky... Turned it back the other way... got better results.
4:00 still backfiring some...
4:40 dropped the idle down...
5:27 more backfiring... maybe the idle is too low??? I can roll it with no problem...
6:55 The idle is low... I revved it, no backfire... turned it off until further consultation...

 

With the B the idles gonna need to set around 800 to 900 rpm to get the roll on right. The intermediate jet will be .031 the .295 is too small. Seat the mixture screw on top and open it 1-1/4 to 1-1/2 turns out . Fire the bike , warm it a bit and get the idle set. Now while running lean the mixture screw till it falls off a bit then fatten it till she starts lugging rich and back it off half way between the 2 with just hair on the rich side. If there's no air leaks you should be in range , that's where all my B's ran on 74 & 80 inchers. On the odd one occasionally I'd have to go to an .033 jet and 70 or 72 main.

The throttle shafts and the bores on the carb bodies on the B's did wear a lot so heads up it was always an air leak item to check and the old carb's all got some age & miles on them, I've got a box with 1/2 dozen B bodies the shaft bores are egg shaped.

One more thing on the side of the B's just above the float bowl line there's a small hole with a real fine port that's a bowl vent, I have seen crud and spider making a home block those and it can drive you nuts chasing a lean backfiring condition because of it.

Good luck.

 

Okay... I have a .031 intermediate and 72 main, the mixture screw is staying between 1 1/2 and 1 1/4... I may have the wrong main... I haven't looked at the throttle shaft, but I do hear air coming in to fast from somewhere... Thanks for giving me some things to look at. By the way, this wasn't and issue until I put the element and cover on... Makes no sense.

Thanks for the help... I'll keep looking

 

Maybe the the cover is causing a vacuum on a leak elsewhere...

 

I didn't watch the video...too slow of a connection.
Good advise so far from what I seen above.
Can add that a Bad condenser [even if New, and assuming you run points]
causes easy starting, good idle, No throttle response at all.......
Put an old, used, tested, condenser in it....

 

The little brace from the rocker cover to the backing plate can pull the carb off just enough to pull a leak at the intakes if it isn't shimmed or bent just right. Get a can of brake clean put the little hose on it and when warmed up at an idle give a couple fast small shots at the potential leak points I listed before, you'll know quick if somethings out and I've pinched my share of new o-rings fighting S&S manifolds.