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> backfire through the carbs ..., makes for nice fireworks!
SirAndy
post Jan 30 2005, 01:18 AM
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hey, it's me again, the "carb-hater" !!! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

i've been trying to get this engine to run right for what now, 10 months??? (IMG:http://www.914world.com/bbs2/html/emoticons/headbang.gif)

i still have a issue with backfires upwards through the carbs (looks cool at night with the GT-engine lid) !!!

now here's my question:
- i'm still running stock (well, SSI) heat-exchangers and a close to stock (Manta) exhaust.
- i have a cam with a lot of overlap.

could the stock(ish) exhaust setup cause too much backpressure and the engine doesn't flow and belches back through the carbs ????
(IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) Andy

PS: this happens all over the rpm range, but more often under lower rpm load and higher rpm "cruise" (steady gas) and only sporadically at WOT ...
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Aaron Cox
post Jan 30 2005, 01:21 AM
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what dizzy? what cam? where is your timing set?
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SirAndy
post Jan 30 2005, 01:30 AM
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QUOTE (Aaron Cox @ Jan 29 2005, 11:21 PM)
what dizzy? what cam? where is your timing set?

timing is fine, dizzy is a 050, all new parts. all that is checked about a 100 times.
carbs rebuild, cleaned more times than my underwear in the lascht year.
linkage is tight, jetting is gud.

everything says it should run just fine, but it doesn't ... (IMG:http://www.914world.com/bbs2/html/emoticons/headbang.gif)

so i'm looking for clues besides the usual.
so what about too much backpressure ????
it seems like with a lot of overlap, too much backpressure could spit hot exhaust up the intake.
right?
(IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif) Andy
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redshift
post Jan 30 2005, 01:35 AM
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But not in over-run, in decel with the clutch engaged?

DAMN YOU FAST BASTARDS!

Andy, I think you could see a small amount of that back pressure thing... maybe.. but that sounds real maybe..

How much overla... no man... it doesn't do it when you let off the gas in 4th from like 90 with the clutch out?

That's all I wanna know..


M
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SirAndy
post Jan 30 2005, 01:43 AM
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QUOTE (redshift @ Jan 29 2005, 11:35 PM)
it doesn't do it when you let off the gas in 4th from like 90 with the clutch out?

nope, never on decel .... clutch in/out doesn't matter ...

also, *lots* of power on WOT!!! no bucking, gulping, burping.

only on low rpm load and midrange cruise. fairly often ...
(IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif) Andy

oh, and have i mentioned that i also get backfires out the exhaust?
usually when coming off the gas.
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Jake Raby
post Jan 30 2005, 01:48 AM
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Toss the fuckin 050! The advance curve is probably causing much of the problem coupled with the 050s retarded running at low RPM.

Other than that a coughing carb is a lean carb... Or you have tight intake valves, or burned intake valves..
GET RID OF THE DAMN 050!
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redshift
post Jan 30 2005, 01:49 AM
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What's your fuel mixture doing, and what aboot the pressure?

I can't see that rig causing that much back pressure... maybe it's 2 or 3 unrelated things...

I say on the one hand, I am hearing about a wobbly dizzy, and on the other, sounds like a fuel delivery thingy too.

Spark, on time, all across the range.. OR LIKE JAKE SAID HELL I GUESS!

I hate carbs!



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SirAndy
post Jan 30 2005, 02:01 AM
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QUOTE (Jake Raby @ Jan 29 2005, 11:48 PM)
GET RID OF THE DAMN 050!

alright, that was clear ... (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)
brad just told me he has a mallory in his stash that i can try out. we'll see how that works out.

i'll keep you guys posted ...
(IMG:http://www.914world.com/bbs2/html/emoticons/aktion035.gif) Andy
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redshift
post Jan 30 2005, 02:03 AM
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We don't really give a shit.

(IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif)

(IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)


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SirAndy
post Jan 30 2005, 02:07 AM
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QUOTE (redshift @ Jan 30 2005, 12:03 AM)
We don't really give a shit.

i know!
it's called "torture".

but you wouldn't know anything about that, now would you ????
(IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) Andy
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Jake Raby
post Jan 30 2005, 02:22 AM
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Try the Mallory with Grey springs, 12 degrees initial advance and 28 full..

90% of carb problems are ignition related.

I Love carbs.
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sixnotfour
post Jan 30 2005, 02:25 AM
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A Shop next door had a speedster that did the same thing , They changed carbs , jetting, distributors, valve adjust. Turned out to be tight valve guides, on the rebuilt heads the previous owner had done.
It was hard to start sometimes and sometimes it fired right up , while idling it would sometimes pop out the intake and somtimes the exhaust was another symptom.

May not be your problem, but similar. Or maybe TMI as my wife would say. (IMG:http://www.914world.com/bbs2/html/emoticons/burnout.gif)
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Air_Cooled_Nut
post Jan 30 2005, 03:42 PM
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If it's backfiring thru the exhaust (post ignition) then try retarding the advance. Check the air:fuel ratio to make sure the mixture isn't too rich as well.

Here's a copy from a listee on the Type 3 list I'm on about carb backfire:
QUOTE
The basic problem is that the intake valve is open when the spark fires. The
fix is to redo your basic tuning procedures -- valves, dwell, spark
timing -- and check for simple mistakes like misplaced plug wires.

You'll also want to check for damage to the carbs as a result of the
backfire. Parts can be blown off and distorted by the blast.

Steven Ayres, Prescott AZ
'66 KG1600


A cold engine compression check could help spot a sticking intake valve (lower compression than the other cylinders).
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jwalters
post Jan 30 2005, 05:28 PM
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Yea, I had the same problem with my 1.7---stock except for dual carbs--engine was sooo worn out I had to run almost 50 degrees of advance to get power out of it--

What I discovered was my intake lash kept closing up on me--weirdest thing---I would set it to spec--.006 and it was good until the motor was hot--I found out by doing a speed check--I ran the piss out of it and pulled right onto a lift--up--covers off--and checked them--they were minus .002!!!!!!!---so I backed them off to .010, which made for a rhoads lifter kind of tick when cold--but when it got hot all was good---took me two months to figure this one out--and ten carb complete cleans in the process!!!

After this I could finally tune the carbs--I was so happy!!

Then I decided to tear the car down for a rebuild--- (IMG:http://www.914world.com/bbs2/html/emoticons/mad.gif) That is another story..............
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SirAndy
post Jan 30 2005, 06:15 PM
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QUOTE (jwalters @ Jan 30 2005, 03:28 PM)
Then I decided to tear the car down for a rebuild

except, my moter is brand new, i only have a few 100 miles on it.

this is going on since i got it 10 month ago ...
(IMG:http://www.914world.com/bbs2/html/emoticons/blink.gif) Andy
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jwalters
post Jan 30 2005, 06:20 PM
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I think my problem was the aluminum pushrods-the only thing I could theorize was they were expanding faster than the engine----but you are not running those are you??
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Joseph Mills
post Jan 30 2005, 06:32 PM
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QUOTE (SirAndy @ Jan 30 2005, 01:43 AM)
nope, never on decel .... clutch in/out doesn't matter ...

also, *lots* of power on WOT!!! no bucking, gulping, burping.

only on low rpm load and midrange cruise. fairly often ...
(IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)  Andy


Andy,

I recently had a somewhat similar experience - back firing thru the carbs under acceleration. At first I thought maybe clogged jets from sediment running too low on gas at an AX.

Then I noticed my tach acting a bit weird. I have a Unilite with no points, so I started checking the electrical in the engine bay. The second time around checking everything, I found a stressed connection that was just barely hanging on. I think the forward motion of the car would swing the wire back and occasionally disrupt the spark.

I repaired the connection and viola! Problem gone. Might be worth checking out. (IMG:http://www.914world.com/bbs2/html/emoticons/rolleyes.gif)
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SirAndy
post Jan 30 2005, 06:34 PM
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QUOTE (jwalters @ Jan 30 2005, 04:20 PM)
I think my problem was the aluminum pushrods-the only thing I could theorize was they were expanding faster than the engine----but you are not running those are you??

unfortunately, i have no idea what's inside the engine.
i bought it "ready to drop in and drive" ... (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif)

i have been thinking a lot lately about tearing it down and go through the internals, just to be sure my problem is not within the engine.

everything else has been checked and then double-checked about 20 times.
all external parts have been replaced, just to be sure.
tried different dizzys (haven't done the mallory yet), wires, plugs etc. replaced 5 times.
carbs rebuild, fixed, rejetted more times than i can think of.
replaced fuel-pump twice.

you know, all the usual stuff ...

no luck so far,
(IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif) Andy
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r_towle
post Jan 30 2005, 06:43 PM
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my gut says valves are either sticking, rockers might be the wrong setup...etc.
I would take off the valve covers and verify everything moves correctly, verify with a guage for each valve...

I might even go for a dial indicator to check the cam...

If you bought this from a reputable builder I would not suggest this...
Have you thought about the cam gear maybe not being bolted on correctly/tightly???

The other is wiring...I have had the same issue as above...a hidden bad/loose connection that made tha car run like crap only when hot and it skipped, sputtered and backfired out both ends...

It was of all things a short in the tailight wiring harness that was shorting out the relay board just enough to throw off the whole system.

Rich
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jwalters
post Jan 30 2005, 06:45 PM
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So it would be safe to assume you do not know the cam profile?? Maybe the lobes were ground with an unusually tight centerline---I was looking back thru your post--WOT the same thing?? What rpm are you maxing it to?

If these are not it--it might be a bad grinding on the valve seat or the face, know what I mean---not a burned valve, just not seating right due to the stone used being to worn when it was ground--I have come across this once....it was a 350 chev motor from chev!! in a truck---when it heated up it became very evident by blowing smoke out the tailpipe--different from what you are describing--but when this pup let go--lets just say I brought ALL traffic to a stop, it was like a mosquito fogger going by, times a hundred!!---that was the only vehicle I have ever been embarrased to be seen in! The cops tried to pull me over, I knew they were back there, but I was hell bent on making it back to my shop from the test drive--they found me by following the smoke trail--they could only go 2 mph behind me and could not catch me--but because there was no wrecks they laughed it up and let it go----whew!!!

I know this is making you tense--but from my experience, it is a two dollar part / process doing it----I hope you find it w/o pulling the motor apart--
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