Could this be causing my backfiring problem?

[CobraSix]

In case you don't know, I'm still only running the center carb of my Offy set up. I've been having problems with my carb backfiring through the carb about every 10-15 minutes or under heavy acceleration

So today, I finally got around to locating the problem.

I started with tuning.

8* BTDC at 700 RPM
17 inHG @ 700 RPM

Seems to be okay

I decided to check my underload timing. At 2500RPM, my total advance is about 10-12* BTDC. Should it be around 25 BTDC at that point? So I take the vacuum advance tube and hook it to the manifold vacuum source. No change. My vacuum gauge was reading 0 on the ported vacuum on the 1100 carb.

Then I put the vacuum gauge back on the manifold and the distributor on the carb. So I decide to remove the vacuum from the distributer and check the movement of the distributor as I sucked on the tube. The distributor seemed to move fine. So I start the car back up. Decide to leave the vacuum tube off of the carb for the distributor. Car ran noticably worse and when I hooked it back up the carb ran well. If I hook up the vacuum tube to the manifold and leave the carb blanked, the car runs the same, but still runs at 10-12*BTDC at 2500 RPM.

So...what is wrong?

Or should I look at getting this when I get my MSD/DS2 hooked up:

http://www.jegs.com/cgi-bin/ncommerce3/ ... rmenbr=361

It's a programmable timing computer...

Thanks.

Slade

 

[MustangSix]

If that's all the advance you get, I'm not surprised at the occasional backfire.

With the throttle closed at idle, there is no vacuum at the ported vacuum source. When you crack the throttle open, you should get something.

As I understand it, you believe that the diaphragm is good, yet you get no vacuum advance when hooked to the manifold? There has to be a leak.

 

[CobraSix]

When I suck on the same hose that is hooked up to the carb(or manifold), the car advances pretty good (even when running, yes I did it running too). As soon as I hook that hose up to the manifold, nothing happens again. Now, when I suck...I tested myself on a gauge and sucked around 15-20 inHG with no problems (what can I say, I'm a natural sucker ).

I guess it's time to change over to the DS2 and start from there. Maybe that distributor is just not working right anymore. My only other distributor (besides the DS2) is a dual vacuum advance. The only reason I haven't put the DS2 on yet is that I haven't gotten all the parts.

Slade

 

[Anonymous]

Slade,

You aren't getting enough advance. By 3000 RPM you should have 32-38 Deg of total advance.

Try this...
With car warmed up disconnect vac adv hose and plug.
connect vac gaude to manifold
loosen dizzy and adjust until you get the highest vac reading (It will also be the highest idle speed) Don't worry you cant give it too much without knowing it. If you go too far it will start to sputter, back off.
once you get highest vac reading...road test for performance... if it pings too much then back off just enough to stop the ping. Then you are set. I never use a timing light on my Mustang. I lot of guys will give Hg numbrs to set by, but these will vary based on engine age and condition. I know you have basicly a fresh rebuild so you should be somewhere in the range of 17-20hg (I know I said I wouldn't give numbers but this post is so long now anyway)

You might also look up one of Jack's post's outlining how to test your vac adv diaphram. If it is bad you can replace for about 12 bucks.

If you think you have a leak it shouldn't be too hard to find just spray some carb cleaner at the places it could possibly leak. If there is a leak the idle speed will increase.

I hope this helps

Bill

P.S.
By using this method my initial timing is at about 20 deg (this number will vary based on the centrifigal advance plates in the dizzy. I have 6 deg plates so I get 12 deg at the crank, then 20 deg advance at idle for a total of 32 deg total) BTDC at idle (Used light just to see)

 

[CobraSix]

Now I'm royally confused.

No, not from the advise, everything you guys have said is great info. I went back out to test and reset the distributor again. I tested the carb and the vacuum port is working fine. I then had my wife help me check my timing advance. This time it went to 35* BTDC at 3200 RPM. Before it would only go to 15*BTDC.

I did a 15 minute road test tonight and it ran great (even better then before). I'll do another road test tomorrow on the way to work (40 miles) and see what happens. Wish me luck.

Slade

 

[CobraSix]

All I have to say is wow...what an improvement. No backfiring on the way to work today. Also, it looks to only have taken 1.5 gallons of gas vise the 3 gallons it had been taking me (40 miles).

So, on my drive in to work, I started thinking about what I changed on the car to make it different. See if you can follow what i did and maybe explain why it worked.

After initial discovery that the dizzy was not advancing under load, I pull off the vacuum tube and check the dizzy using the good ole suck method. Dizzy seemed just fine. I tried hooking up the dizzy to the manifold vice the 1100 and still no change in the way the dizzy advanced. If I left the dizzy disconnect from any vacuum source, the car ran extremely poorly and almost wanted to die.

So, after these posts I decide to try one last shot at it. I figured I would take Bills advice and advance the car to the point where I would have 35* BTDC at 3200 RPM. Honestly, on a 40 mile highway drive to work, how often will I be at idle. So I advance the dizzy about 10 (about 20 total) degrees. idle RPM jumps from 750 to 1100 RPM. Vacuum goes from 17 to 21 inHG. So I decide to run the RPM down. The last time I set the dizzy, I used the throttle plate screw on the back of the carb to run the RPMs up to 700. So I remember that and ran the RPMs down using the same throttle plate adjustment screw. I checked the idle air mix screw and made sure it was still set.

I bring my wife out to help run the RPMs up while I check the advance. Then it advances to 35* BTDC @ 3000 RPM and my idle advance was at 10-12* BTDC.

So...riddle me this batman...what changed?

The only thing I could think of is that maybe I had it too retarded, causing low RPMs which I ran up using the throttle plate adjustment because the idle mix wasn't doing any good. This in turned cause my dizzy/carb/vac to be close to max total advance already meaning that under load it had no more room to move.

Am I making any since?

But the car ran so much better this morning. Started up easier. Felt like it had more power. By the gas gauge, used 1/2 as much gas as it had been. And best of all...no backfires through the carb no matter what I did.

Slade

 

[MustangSix]

The only thing I could think of is that maybe I had it too retarded, causing low RPMs which I ran up using the throttle plate adjustment because the idle mix wasn't doing any good. This in turned cause my dizzy/carb/vac to be close to max total advance already meaning that under load it had no more room to move.

Sounds logical to me. That would certainly explain the backfiring.

 

[CobraSix]

Maybe this weekend I'll put the outer carbs back on since it seems that the problem has been fixed.

Slade

 

[Anonymous]

Sounds like you got it....

There is a fine line in there with the air mixture, idle speed, and total advance... I had the same thing happen many times...

It is so easy to mask one that is not set right by over-compensating on the other... i.e... here you had retarded timing and covered that by added rpms.


I say go ahead and add the outer carbs... if you experience more trouble.. you can always pull them and isolate the problem again...

but you 1.5 gallons of fuel will no longer be accurate

 

[Anonymous]

Slade,

It really sucks when you know your close and everything you do just doesn't work. It took me months to thrack down the probs with mine. Combination of Autolite 1100 probs with pertronix troubles and not enough advance, almost the same thing you were dealing with. I got lots of gray hairs on that one.

Glad you got it working the way it shoud be!!