backfire through the carbs ..., makes for nice fireworks! Options

[r_towle]

sorry,
I wanted to ad one thing.
Carbs are stupid and simple devices.
They suck fuel in based upon the air volume..

If you where having consistant backfiring when you got off the idle jet and were going to the main jet, it would be obvious...
I think FI is harder to diagnose.

I would rule out the carbs in your mind and start looking elsewhere.

Rich

[jwalters]

QUOTE (r_towle @ Jan 30 2005, 07:45 PM)

sorry, I wanted to ad one thing. Carbs are stupid and simple devices. They suck fuel in based upon the air volume.. If you where having consistant backfiring when you got off the idle jet and were going to the main jet, it would be obvious... I think FI is harder to diagnose. I would rule out the carbs in your mind and start looking elsewhere. Rich

This is actually a good point--I failed to mention that during all my carb tuning I discovered one of the butterflies had moved on the rod a little and was actually holding the carb slightly in idle / main sequence----it DID partially cure the same problem with mine as yours, but not 100% the other was the lash--

Point--contributory factors = a big factor--

I was so pissed at myself for not noticing this until my 4th carb rebuild!!! Not saying you have the same problem tho--just a little more info for ya-------

[redshift]

It just sounds like late/early spark to me... and maybe weak in some ranges. (Also see above ref: Tailight grounds for the weakness... sounds like a winner..)

Seems like the curve on the distro would also cause something akin to that, so I defer to Jake E. Ngine.

But, I do think it'd be funny to try to help Andy get his engine back together, in threads full of UFO/Martian/Fiscally Responsible Polititian/Yeti pics.. and masked broken English profanity..

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


Skippy Skipperton

[tesserra]

Andy,
Did you try a manifold vacum gauge?
If you have vacum you should not be able to push exhaust gases back thru the carbs.
I too think it is a spark thing, maybe you have to retard the spark because of the overlap. It sounds like it is sparking with the intake partially open. Are your valves too tight for the overlap?

Just ideas, but I think the vacum gauge will put the backpressure issue to rest.
But if you want to use it as an reason to put on a new exhaust (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif) it sounds good to me.

George

[Carrera916]

Andy....

The key word you used awhile ago...overlap cam....that can really screw things up when you don't have a good ignition to begin with....

seems that jake hates 050, I haven't use it and I have no idea what it's sweeping curve are alike....when the advance or retard curves aren't smooth, you'll hit a flat spot that the engine speed and timing speed ain't timing right together, that can back fire a bit....

What I'm looking at is the camshaft itself being overlap, it leaves the intake and exhaust valves open at same time at a certain degree setting, now with the ignition spark fired, the flame will travel out of it's combustion chamber via intake sack or exhaust pipe, giving out 'Jumping Flash Flame'....

like brad & jake said, throw out the 050, try a different one...if problem persist, then look closely at the camshaft overlap & it's timing...that's something I would look at..

good luck....

[redshift]

CHANGE THE DAMN .50 ALREADY! AND REPORT!


M

[SGB]

Before ya tear it down, I would try opening up intake clearance a little at a time to see if the backfire goes away. That cam may be too agressive, but if it runs OK with a few hundredths more clearance at least it would break in right..... then maybe-

[machina]

maybe carbs are out of sync?

twisted throttle plate rod?

[redshift]

hehehe.. MAYBE IF YOU CHANGE YOUR ROTOR CAP....

ANDY!? ANDY!? WHERE ARE YOU!?

haha... I want you to change that cap RIGHT NOW!

I am serious, this is really bugging me... it's like hanging on at the end of a soap opera..

Will Bob have sex with Susan, the illiterate girl he doesn't know is his twin sister, that he met while running from the international drug lord Tybin Dally...


M

[SirAndy]

QUOTE (redshift @ Jan 31 2005, 02:46 PM)

ANDY!? ANDY!? WHERE ARE YOU!?

i'm here! no time to play with the car today ...

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

========================
will debbie do dallas?
will bob father a child with his long lost twin sister?
will tybin dally manage to evade police once again?
will brad figure out what is wrong with andy's motor?

tune in next week for the conclusion!
========================

[andys]

QUOTE (SGB @ Jan 31 2005, 02:34 PM)

Before ya tear it down, I would try opening up intake clearance a little at a time to see if the backfire goes away. That cam may be too agressive, but if it runs OK with a few hundredths more clearance at least it would break in right..... then maybe-

I'm no /4 expert, but this is some good old hot rodder advise. It effectively shortens the cam duration. Makes the valve train a little noisey, but will also help to eliminate one possibility (the cam). You can loosen up both intake's and exhaust's. BTDT on other motors to tame the cam just a bit.

Andy

[DJsRepS]

You should try another dizzy. Try to borrow a Mallory with pertronics eletronic points from someone on this Form. Im sure some one has one out on a bench. You could also try to tempory set your valves a little loose a little noisey just to see if the backfireing goes away. And do a compression check for good measure. A lean carb can cause a good motor to back fire. If it is only one side or one cylinder doing it try switching carbs just to see if the problem follows the carb.