Sidedraft off-idle backfiring

[AustrianVespaGuy]

Any good dual sidedraft tuners out there? I'm getting backfiring through the carbs (DCOMs) at like ~5-10% throttle at cruising speeds. Most noticeable when I'm on the interstate going down a hill ~3500 RPM with just enough throttle to maintain speed. On the flat and uphill is enough throttle that it goes away, and also no problems when the throttle is fully closed. (Maybe a hair rich, as the exhaust thumps a little.) Also exists in lower RPM bands but less prominent. I've already played with the timing a lot which doesn't seem to help very much. I also tried richer idle jets 65 F8s which seemed to help some, but certainly didn't fix the problem and were way to rich everywhere else, so I put the 58s back in. Carbs are decently balanced so I don't think that's the problem either.

Didn't always have this problem and it seemed to start about the same time that I installed new plastic floats. I couldn't find any info on how high to set the plastic ones, only the brass ones, so I originally set them at the same level as the brass floats but then set them a little higher when I noticed this problem. Again didn't seem to change much of anything. Any other ideas? Cause I'm running out. Settings are:

30 chokes

58 F23 idle (~F7, weird DCOM number)

125 mains

160 air correctors

Can't remember what emulsion tubes but I've checked them before they are nothing way out of whack

Thanks for any help!

-Carl

[AustrianVespaGuy]

Oh yeah, just the stock camshaft distributor, hence the more mild jetting than many of you would maybe expect. Really idles and runs well everywhere else.

-Carl

[TobyB]

First- are you sure it's spitting back through the carb?

That's almost always timing.

But if it's truly backfiring,

A couple of things to look for-

air leaks, of course, both in the intake and exhaust.

Adjust your idle screws by feel. Don't keep them even.

I found that when I dialled them for 'best idle' they were

around a half- turn DIFFERENT, AND the car stopped backfiring.

Lean out your idle mixture. When the throttles close, the engine should

go way lean until it settles down to idle...

hth,

t

[AustrianVespaGuy]

Alright, thanks for the tips. I'll both check for air leaks again and lean out the idle mixture. I have been trying to keep all of the turns even, but I won't this time.

[TobyB]

yeah, like you, I was keeping them all the same,

but when I finally gave up and adjusted each one so it

'felt right' the idle settled down a lot AND it stopped backfiring.

Go figure, huh? You'd think it was float level or something,

but each throat had its own optimal setting.

t