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Old 04-20-2002, 03:43 AM
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Cool Back Fire Issues

I have a 406 CID SBC in a car I'm building. I wanted to be exotic so I got a Weber carb system from Inglese. I've been trying to get this engine tuned, but having some problem with backfire in the exhaust - I just can't seem to get rid of it.

Engine stats:
406 CID (punched 400 4-bolt)
Cam: Crane 244@.05, 109 lobe centers, .51 lift all around
10.3:1 compression

Exhaust stats:
home-made headers of parts made by Headers by Ed. 1 5/8 X 36 inch pri to 2.5 inch collector. 2.5 inch pipe to Model 80 Flowmaster (dual in/out crossflow). From header flange to muffler is about 20 inches with a balance pipe in between.

Induction stats:
Inglese 4 x 48IDF Webers on IR manifold. Supposedly tuned to the cam specs.
Idle mixture 3/4 turn as recommended. However, in a David Vizard book, he goes out to 2 turns? But I don't know what system he was adjusting. IDA? IDF? DCOE?

Ignition:
Mallory Unilite + HiFire 6
Recurved distributor for a fast advance (lightest springs avail), but kept distrib advance to 26 deg. The Webers need 36-38 Total advance @ 1800 RPM.

Engine response is good, the Webers are not sneezing, but it periodically backfires (and very loud too). What could this be?

Timing?
Do I need to change the distributor advance up to lower the initial timing at the crank?

Is it a carb issue? Could the jets be wrong?

Is it a cam issue? But nothing can be done here - I'm basically stuck with it.

I need help!

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Old 04-20-2002, 07:40 AM
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Andy, sounds like quite the setup I am envious. Weber carbs are notorious for loading up the engine at low idle speeds particularly with radical cams, you could try changing the intial timing and raising the idle speed a little and see if that helps. This is assuming it runs fine everywhere else.
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Old 04-21-2002, 04:37 AM
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Chuck,

I was thinking of doing just that. That means going inside the distributor and pushing the advance up to the limit. My only concern is if I lower the initial timing, the Webers will start to "sneeze". I haven't had the RPMs up too high, only to around 2000 - to check total timing. But it seemed to run fine and the throttle response is very, and I mean very, quick.

I was basically wondering what are the sources or causes of exhaust backfire - besides burnt valves.

Andy
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Old 04-21-2002, 07:27 AM
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An exhaust leak will cause exhaust backfiring as will burnt valves, I would try changing your timing and carb tuning first if your leak down tests show the engine to be in good health. Dropping a cylinder periodically can cause it also so check the entire ignition system, here's some links.

<a href="http://www.weber-carburetors.com/carbinf.htm" target="_blank">http://www.weber-carburetors.com/carbinf.htm</a>

<a href="http://www.teglerizer.com/dcoe/webertune.htm" target="_blank">http://www.teglerizer.com/dcoe/webertune.htm</a>

<a href="http://www.webercarburetors.com" target="_blank">http://www.webercarburetors.com</a>
Click on "technical info" for a detailed PDF file on tuning.
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Old 04-21-2002, 04:45 PM
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I agree with 4 Jaw Chuck, sounds like a valve or bad plug wires. Start it up at night and look at the motor,you will be suprised. I had some Accel plus 300 wires and didnt last a year. Every one had spark jumping and a backfire in the exhaust until I put on a new set. At least the wires are garenteed. I have a 406 with a cross ram and it is more responcive than a single quad. If you need any other info please send me a email.
LOL,
G.
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Old 04-23-2002, 04:06 AM
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Well I'm pretty sure it's not a bad valve since the engine is new. I only have about 3 running hours on it. But I will check the ignition system. It's all new: Mallory Unilite, HiFire 6, Mallory coil. My wires, however, are 10 mm Taylor 409s, and I'm using Splitfire plugs. On this note, I'm not sure of the plug gap. I initially set them at 0.040.

I checked the headers for leaks when I first started it up - couldn't find any unless there is a small leak where the 4 tubes (bundled) enter the collector - since I did make my own headers.
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Old 04-23-2002, 04:14 AM
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If you are interested in see my set up )and my project) here are where my pictures are located:

http://www.kitcentral.com/cgi-bin/sh...gi?builderid=2
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Old 04-23-2002, 04:25 PM
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Andy B, after seeing the crankcase evacuation system I would say you have a vacuum leak in the engine somewhere. This happens with these systems without a PCV in the system somewhere, the PCV will limit the volume through the evacuation system and prevent sucking oil seals and intake end seals into the engine. Also this system is not a good idea for a street car due to the exhaust back pressure keeping your anti backflow valve closed for most of the time until crankcase pressure exceeds the backpressure (this can be up to 5 psi depending on exhaust configuration). This might be what caused the leak to begin with.

It works real well on open header race cars because they operate at full throttle most of the time and there is no backpressure to speak of. Is this a race car? I can't tell from the pictures. Ideally your angled exhaust venturi tube would need to be at the exhaust exit to not be pressurized by the exhaust system, this would mean running a tube right to your exhaust tip.
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Old 04-24-2002, 09:18 AM
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Andy: I think you should buy a book called "Webber Tuning Guide" or something like that. It may be out of print because, as you know the primary application for 48IDF's was on Porsche 911's converted to racing. If you can't find it anywhere else, call Automotion and ask them about it. Also, a company called Red Line used to distribute webber parts.

You will have to spend quite a bit of time tuning those carbs and will probably end up changing your idle and main jets as well as the emulsion tubes. If you think Holley is bad, you haven't seen anything yet!

I would guess that your advance is too high at idle and fuel richness is causing unburned fuel at idle to slowly cook off when an accumulation of fuel builds up. I raced Porsche's, both 356's and 911's for a long time and this idle popping is very common. I don't understand the advice on so much advance so early.

One thing to keep in mind is that a big engine would run one venturi per cylinder directly into the intake port. Most Porsches or Ferrari's that used these carbs were 3 liters or less that flowed very high numbers. My Porsche operates best at 5000-7500 rpm when racing. So pay attention to flow velocity when tuning the carbs. But the carbs, if rich at idle, tend to pop off more.

I hate to say this, you don't see a lot of webber applications on rods because they are really tough to get running well. Good luck, I'm glad this is your problem and not mine, it can be aggrevating.
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Old 04-24-2002, 12:02 PM
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Do you have access to an air/fuel ratio meter... those will save you more tuning time than any other device i have ever seen... do they make adjustable main jets for webbers?
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Old 04-24-2002, 04:02 PM
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Andy. Just read your question,and all of the answers so far posted. The only thing that comes to mind,is it your cam or more pointedly your lifters ?. Could they be "pumping up" or adjusted just a little tight ?. When your motor backfires,is it left or right back ?,or do both banks alternate with backfiring ?.
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Old 04-24-2002, 05:19 PM
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There are at least 100 different size main, idle and emulsion tube options for the 48IDFs.
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Old 04-27-2002, 04:35 AM
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4Jaw,

I installed the crankcase evacuation system because I wasn't getting enough manifold vacuum. I originally had a PCV valve and a vacuum system to route to a vacuum reservour for my brakes. I now have a vacuum pump for the brakes and a crank evac for the engine. But I had this backfire problem even with the old system. I though that doing it this way would give a bit more vacuum and help with the backfire issue, but no luck. If necessary, I can always cap-up the evac system and return to PCV set-up I had earlier.

One thing I am a bit concerned about is the manifold vacuum system installed on the manifold by Inglese. It uses small tubes connected to each runner inside the pushrod well side of the manifold. It conects each cylinder to a common manifold for a common vaccum outlet. So there is interconnectivity (small as it seems) between the intake runners.
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Old 04-27-2002, 04:40 AM
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BarryM,

I know I get some "pump-up", but I do have plenty of slack on the rockers. I can "jiggle" them a bit, so they are not tight against the valves. My rockers are Comp Cams Pro roller rockers.
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Old 04-27-2002, 04:56 AM
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F1,

The high advance early was recommended by Inglese. It has to do with the fact that it set-up is for a small-block Chevy (SBC). They recommend a TOTAL advance of 36-38 degrees at around 1800-2000 RPM. The original Mallory distributor had an advance rate (springs) set top around 2800, so I got a recurve kit and reduced the spring rate. This helped the response quite a bit. But I still have an adjustment to the max distributor advance I can make by resetting the advance stops inside. This would allow a lower initial advance, but I run the risk of being low enough to have the IDFs "sneeze" on me. I'll be working on the advance end soon. I'll still have to re-synchronize the carbs and I have yet to do a barrel-to-barrel synchronization since I was concentrating on setting the timing correctly - per Inglese (and the HP Weber book) instructions.

As for jetting, I was hoping that Inglese set-up the carb correctly to begin with since I did send them all the information about my engine/trans setup. I would think that they would know what to do with the type of cam, compression, port size, intake port volume, gearing, header design, etc. I have and jet it accordingly.

As for exhaust popping, the instructions say to enrich the idle mixture because it is too lean. I tried it (at least about 1/8 turn richer) and the backfire (not simply popping) did not go away. Maybe I need some more? Or can they be wrong and I would need to lean it out? The current setting (what they say is the nominal setting) is 3/4 turn out.
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