Saturday, May 26, 2001, 6:48:17 PM, you wrote:
BG> When I am trying to tune the carburetors I get a backfire out of both
BG> carburetor throats when I rise the RPMs. It happens almost
BG> immediately. With the backfire I get a white smoke/mist out of the
BG> carburetor throats which I think is a fine mist of gas.
I think I have found the problem. It seems to be that the timing is
advanced to far. Sound simple but here is how I got into this mess.
I was trying to read the timing marks on the engine and I found 0
correctly but I mis-read the rest of the numbers. I thought that each
point was 2 degrees but in reality it was 4 degrees. So I thought
that I had set my timing to 10 BTDC, but it was really set 20 BTDC at
an idle. Whoops. I should really put some paint on those marks so
that I can see them. I will know for sure when I get in the garage
tomorrow morning and retard the timing.
I would like to thank all the listers who replied with suggestions.
Below is a summary of the suggestions, I have included them since
they were very useful and I figure other people can use them.
Once again thanks.
--
Best regards,
Bill mailto:w.gilroy@verizon.net
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Be sure to check the timing before worrying too
much about the carbs.
Be sure that any vacuum system you have is
correct, and try 10 BTDC.
By "correct vacuum system", I mean that if the
distributor originally expected a vacuum retard
source from the ZS, don't connect it to the vacuum
advance source of the dual SUs, or your timing will
be bizarro.
If this is confusing or you aren't sure, just
block off the vacuum ports for now, set it
to 10 BTDC, and get it running.
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First - check the timing.... It sounds too advanced - remember it
advances even more when you try to "Goose" it. - (it's firing before the
intake valve is closed all the way) Just a guess.....
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usually a backfire from the carb when you blip the throttle is a sign that
either the mix is too lean, a vacuum leak exists, or the timing is too far
advanced. If you had an overly rich problem, the exhaust will be black, and
you will hear a rhythmic thumping from the tail pipe. excessive fuel
pressure will show up with fuel dumping into the carb throat and out of the
vent pipe.
before adjusting the carb (and making certain that the manifold and carbs
are tight) make sure:
1. points are set
2. static timing is set
3. valves are adjusted
4. engine is up to operating temp (if lean cond exists, watch for
overheating)
5. shut off and check oil in dashpots
then:
loosen both throttle linkage clamps
make sure the choke is all the way off and the jets are seated.
turn each mixture adj nut all the way up, then back off 12 flats (2 turns)
set each idle screw so that you have about 1200 rpm
back each off until you have equal airflow into each carb (hose or unisyn)
when you have the idle set at 750 and both carbs are equal:
-move front mixture nut either up or down a flat at a time until max revs
are reached then reset idle to 750.
- move rear mix nut either up or down a until max revs are reached and
reset idle to 750.
Next, raise the revs to 2000 for 30 seconds to clear the plugs.
when revs return to idle, lift front dashpot using the lifting pins
if idle goes way up and stays up, the mixture is too rich
if idle dies, or you get a backfire, the mix is too lean
if the idle rises then stumbles, the mix is right.
looking DOWN from the top of the carb, counter clockwise leans and cw
richens.
Once you have the mix right, recheck the idle, and recheck balance then lock
up the throttle shaft clamps. Lift the throttle cable and watch both
throttles at the same time to make sure they lift at the same time. A tiny
bit of lead or lag (.010") won't hurt, but more than that, and you can get
stumbling and a backfire during rapid accel.
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