CARB BACKFIRE

[d1970]

I had a friend come by the house today with his 340 Demon. From a dead stop or rolling takeoff , when he floors it, the carb (Eddy 625 Thunder Series) backfires, then it catches and takes off like a bat out of hell. This is the only time it does this, it work great when he is cruising and floors it, car goes like hell (no backfire). Any ideas guys?

 

[krazykuda]

Timing is off...

 

[d1970]

KrazyKuda. He said the inital was 14 degrees, 20 in the dist and 16 in the can, all in at 2500. He is coming over tomorrow and I can check it with the timing light

 

[kiss]

try plugging the vacuum advance and see what happens

 

[70aarcuda]

check the accelerator pump ...lean misfire due to misadjusted pump...or to small of squirter

 

[d1970]

check the accelerator pump ...lean misfire due to misadjusted pump...or to small of squirter

I tried putting the rod in the bottom hole to give it more fuel from the acc.pump and it still did it

 

[70aarcuda]

does it squirt gas as soon as you move the throttle linkage?

 

[krazykuda]

KrazyKuda. He said the inital was 14 degrees, 20 in the dist and 16 in the can, all in at 2500. He is coming over tomorrow and I can check it with the timing light

Those numbers don't sound right... If he's got 20 + 16 = 36 in the distributor, he has 36° total and should have it at TDC to get the total timing at 36°...

Disconnect the vacuum advance and set it around 17° - 18° initial and 34° - 36° total by 2500....

 

[d1970]

Those numbers don't sound right... If he's got 20 + 16 = 36 in the distributor, he has 36° total and should have it at TDC to get the total timing at 36°...
Disconnect the vacuum advance and set it around 17° - 18° initial and 34° - 36° total by 2500....

he has 14 inital + 20 in the dis for a total 34 + 16 in the can for a total of 50

 

[d1970]

does it squirt gas as soon as you move the throttle linkage?

it squirts as soon as you move the throttle.

 

[krazykuda]

he has 14 inital + 20 in the dis for a total 34 + 16 in the can for a total of 50

That sounds like too much total advance... See my first post on where to try...

Those numbers don't sound right... If he's got 20 + 16 = 36 in the distributor, he has 36° total and should have it at TDC to get the total timing at 36°...

Disconnect the vacuum advance and set it around 17° - 18° initial and 34° - 36° total by 2500....

 

[AJ/FormS]

What carb?
You didn't say it backfires several times or just one time.
Several backfires points to a lean condition. Usually the secondaries coming in without a corresponding amount of fuel, or just flat coming in too soon..
Of course everyone wants to play with the pumpshot, but don't forget the powervalve.
However, If it backfires just once, and then goes, that is a different problem.And I'm searching my memory banks for a solution. Hang on it'll come to me........................................................................................
Oh yeah that's a bad intake valve or a valve not fully closed, sometimes accompanied with too much advance.

 

[rumblefish360]

AJ,he has a Carter/Edelbrock carb, nonpower valve.
He starting with the EZ things first before opening it up.

Timing and pumpshot.
Heck, gear ratio could be numerically to low. Cam to big.

 

[AJ/FormS]

The timing was detailed in post #3, and it is close enough to not be the problem.
The pumpshot he detailed in post #6, and reiterated in #10
All carbs have power valves. Or at least all common automotive carbs do.Sometimes they just have different names.
But yeah, I missed the Thunder series which is AFB based, In which case it would be difficult to imagine the secondaries coming in too soon..difficult but not impossible.
And to add to the list are;a faulty T-port sync, a weak coil,plug-gaps too wide,too-tight a valve-lash,a too-large intake plenum,a restricted exhaust, and a too tight TC.
But the underlying root is still the same;an instantaneous lean hesitation; you just need to figure out which is causing it.
Since it only happens at tip-in from a dead stop/slow-roll, IMO we can narrow it down to the transfers being too far closed, the power valves being tardy, or the pump shot too small, or some combination of those; unless the secondary air valve is tipping open too soon.

 

[rumblefish360]

Please point out the "Power Valve" on any Carter/Carter style carb.

 

[Abodybomber]

check the accelerator pump ...lean misfire due to misadjusted pump...or to small of squirter

This.... It's "tuning time"...Fighting this myself, on a new combination... You build a new combination,... You need to retune it...It's doing the research, in learning....

 

[Abodybomber]

The timing was detailed in post #3, and it is close enough to not be the problem.
The pumpshot he detailed in post #6, and reiterated in #10
All carbs have power valves. Or at least all common automotive carbs do.Sometimes they just have different names.
But yeah, I missed the Thunder series which is AFB based, In which case it would be difficult to imagine the secondaries coming in too soon..difficult but not impossible.
And to add to the list are;a faulty T-port sync, a weak coil,plug-gaps too wide,too-tight a valve-lash,a too-large intake plenum,a restricted exhaust, and a too tight TC.
But the underlying root is still the same;an instantaneous lean hesitation; you just need to figure out which is causing it.
Since it only happens at tip-in from a dead stop/slow-roll, IMO we can narrow it down to the transfers being too far closed, the power valves being tardy, or the pump shot too small, or some combination of those; unless the secondary air valve is tipping open too soon.

Please point out the "Power Valve" on any Carter/Carter style carb.

Agreed.....

 

[d1970]

the secondary air valve was sticking abit for some reason, played around with it and it works great now. Thanks for all the help guys.

 

[kiss]

the secondary air valve was sticking abit for some reason, played around with it and it works great now. Thanks for all the help guys.

Can you explain more? How could you tell it was sticking?