Carb Timing Question, Backfiring... Options

[yarin]

I'm trying to work on the timing on my car. I bought a Sears Induction Advance timing light and i'm going based on Pelican's tech articles. Basically the car backfires like crazy over 3k rpm at any decent load. I hooked up the light, i think i see the major 34deg mark on the fan.. is this the wide fan blade? Do I set the timing light to 27 degrees at 3k rpm and line up the large fan blade with the mark on the housing?

Anyhow, i changed the timing around a bit and it still backfires like crazy out the exhaust. It also huffs on the intake side here and there.

I don't have any electronics to set the dwell.

This problem came up after two autocrosses. Any ideas? Fresh plugs, cap, rotor and wires already done.

This is my first time really getting into carbs so pardon my ignorance but I can't seem to find the "vacuum advance line"? Where would it come from and where should it connect to? Thanks!

[nebreitling]

first off: if you have points, you need to set dwell before you time it. period. get a dwell meter, or better off -- install a pertonix/crane unit.

secondly: 34 degrees is a lot of advance. try 28 for now.

thirdly: *most* carb setups have no vaccuum advance setup. some do, though. shoot a pic of your engine bay, carbs, and dizzy and we'll let you know.


good luck!

[tommy914]

QUOTE (yarin @ May 18 2005, 06:09 PM)

...Basically the car backfires like crazy over 3k rpm at any decent load.......This problem came up after two autocrosses. Any ideas? ....

If there was no backfiring before the autox's, then I would suspect that during the autox, some dirt that was in the carb ended up clogging a jet, probably a main jet.

I would pull the jets to make sure they are clean.

[lapuwali]

The timing mark is a notch on the edge of the fan, not the wide "blade" (which is actually a support). The notch is on the aft edge of the fan, so it's very hard to see through the "porthole". You almost need to lie across the rear trunklid to see it.

It sounds to me like your timing is retarded too much.

[jwalters]

Or possibly you have one or more valves which have recessed that few thousads of an inch and are getting too tight when hot---Discovered this on a friends motor--reset the clearances and away she went--

[scotty914]

and with an advance timing light you can use it 2 ways, one set to zero and time it to the 27 degree mark at 3 k

or set it to 27 and time it to the tdc mark at 3 k

but how i would do it ( i think the easy way ) would be to set the light to 8 degrees and set the timing at idle, then set the light to 27 degrees and check the tdc mark at 3 k, if it is off figure which way and reset the idle by that amount... if it is too adavnced set the idle at 6 and retest at 3 k or if too retarded set the idle at 10 and retest at 3 k

but make sure you dwell is right even if you have replaced the point with some thing electronic

as for carbs (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

[ematulac]

Check for exhaust leaks, too.

Mine backfired for months and I couldn't figure it out. I checked and rechecked my valves, timing, and carb settings. Turned out I had a hole in my exhaust. I got a cheap patch kit and sealed it up and the backfiring stopped.

[yarin]

Here are a few pics:

[yarin]

And another...

[yarin]

and another of the rotor

[Aaron Cox]

went through this about a month ago when i did the mallory set up.

with a regular timing light- you would see your timing mark at 3k assumed it was timed dead on.

with an advance timing light, i set mine for 28 degrees, and when it is spot on, you will see the TDC mark.

[yarin]

It almost feels like the advance is messed up. Isn't this setup based on centrifugal advance? It only backfires above 3k rpm at a decent load. Wouldn't that be where the distributor advancing timing?

[nebreitling]

okay, you've got points/condenser. get a dwell meter and check that. replace them while you're at it.

or better yet, yank them out and see if they float. replace w/ pertonix or crane xr700.

points can wear out and cause problems. you need a dwell meter to set them. without fresh points which are set to spec all other tuning is wortless.


once you've done this and timed the car, it might behoove you to adjust the valve clearances, and to take a look at your carb linkage, make sure things are synced, etc.

[rhodyguy]

is the distributor an 009? is that a rev limiting rotor?

k

[Jake Raby]

The big issue with carbs is that you need more initial (idle and low speed) advance than with EFI. This means that at idle you are probably at 6-7 degrees to attin 27 degrees at full advance with the stock dizzy.

Ultimately with carbs I choose a setting of 12-13 degrees at idle to better burn the less atomized fuel from the carbs and then retain the 27 degrees at full advance. This can be done with a stock distributor, but not easily. This is where the Mallory Unilite shines and I sell them already set up and optimized for your Type IV engine application.

The Mallory uses an optical trigger as a pick up instead of magnets, or points to do the job.

[rhodyguy]

jake, any chance you could write up a little tech article on the correct way to achieve those timing specs for carbs? my mallory came to me via dave hunt set for m.s. i'm pretty sure it's close, but a little squeeze couldn't hurt down low.

k

[yarin]

Thanks for the replies guys. Yes that is a rev limiting rotor. Not sure which dizzy i've got. I'm going to pickup a dwell meter today.

When i first brought the car home i took it up the road for a drive. The gas pedal bent and got stuck on under the sheetmetal, went to shift into 3rd and the tach shot to the right. After that the car wouldn't restart. Eventually it started again and ran for a while, but not without random ignition failures.

2 weeks later i found the old rev limiting tach mechanism in a few pieces sitting on the distributor. Ooops. Problem solved.

When i put the good motor in i plan on going with an optical or magnetic points replacement. (IMG:http://www.914world.com/bbs2/html/emoticons/beer.gif)

[nebreitling]

QUOTE (yarin @ May 19 2005, 08:48 AM)

Not sure which dizzy i've got.

you've got a stock Djet dizzy. you can look up the part number to see what year, etc.

[Jake Raby]

I am working on a series of "mini- books" and ignition is one of the topics in the tuning book..

Its impossible to tune any stock dizzy like a Mallory.

[kenpolives]

Yarin, you should have bought that orange car up north.....still runs great. Just kidding, good luck. Isn't a 914 a fun car to have.