Re: [AMC-List] backfire through carb

[Tom Jennings <tomj@xxxxxxx>]

On Wed, 2006-06-21 at 08:28 -0700, Jim Blair wrote:
> Too retarded is more likely to cause backfire under those conditions. Also 
> vacuum leak, weak spark, too lean, cam walk are also in the running. Then 
> there is sticking valves ( a bit of Marvel Mystery oil in the tank helps 
> that!) and vacuum advance with a hole in it. If you're running points, the 
> dist internal ground could be seperating, points getting too much 
> resistance, and a whole lot of other things. (I could tell what it was if I 
> could hear/see it in person)

Thanks for the tips. I'll start diagnosing this weekend (sigh, I have
other things to do).

I've got a Duraspark II, I'll check distro and timing and eliminate all
that. Engine should be like-new (20K mi), so sticky valves seems
unlikely, it's lived on synth oil changed religiously.

I have it pretty lean, so that's a candidate; I'll re-check the squirter
for stroke and leaks (I had a bad squirt pump diaphragm last year). It
does seem to happen only when the secondary is open, so I can rich that
up a jet size or two or three (it never cruises with secondary open;
it's really easy to feel in the pedal).

I hope it's ignition or sec main jet cuz it's easy to fix and I'm lazy.

Thanks again!

tomj


> 
> Jim Blair, Seattle, WA '84 J10 Black Jack (getting lifted and stroked!), '73 
> J4000 304/TH400/QT tow truck http://www.virtualjeep.com
> 
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> From: Tom Jennings <tomj@xxxxxxx>
> Subject: Re: [AMC-List] backfire through carb
> To: amc-list@xxxxxxxxxxxx
> Message-ID: <1150867004.5747.57.camel@xxxxxxxxxxxxx>
> Content-Type: text/plain
> 
> No suggestions on the backfire? I think I'm wrong on it being
> through
> the carb, it just "sounds like it" while driving. I'll pull the
> air
> filter (annoyin Weber clips) this weekend to make sure it's not
> in the
> carb. Could be I suppose.
> 
> If it were through-carb backfire, on WOT/2000rpm, that would
> only be too
> much advance, right? So it would be firing before the intake
> fully
> closes?
> 
> Through more test driving I realized it backfires a lot of the
> time,
> always wide open throttle under relatively low-speed, high-load
> conditions.
> 
> I'll re-check timing this weekend, but I know the total was 40
> or so,
> and that mech and vacc advances were at least working.
> 
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