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Old 09-10-2003, 11:15 AM
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350 Backfire

Hello, I just installed a new 350 into my 67 c-10 that the 307 died in last week. It started right up, idles beautifully, but it backfires through the carb if you open up the throtle at all. If you very slowly increase the rpm, it wont do it, if you give it 1/4 throttle or more it backfires. This means it is getting to much advance right? If it backfires through the carb that means the plug is firing when the intake valve is still open correct? Well the distributor is a new points distributor, with a pertronix conversion in it. I put the stiffest advance springs on it that you can get and it still does it. This backfire happens at about 2000 rpm. Any suggestions would be greatly appreciated.

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Old 09-10-2003, 01:47 PM
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Is it vacuum advance? if it is, check the vacuum source you have it hooked to. Might not hurt to use a vacuum guage to find out what the engine is pulling on the timed ports and open ports.
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Old 09-10-2003, 04:40 PM
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What size carb is on there? I think that back fire through the carb could be a lean effect also...
What's your timing set at also..
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Old 09-10-2003, 05:32 PM
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I had a backfire problem similar to yours. I found out it was the rear bowl of the Holley dumping fuel. The needle wasn't closing right.
Just a thought.
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Old 09-10-2003, 06:01 PM
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A back fire through the carb is usually a lean condition, this occurs usually when the throttle is bliped quickly esp when the engine is cold and sounds like a poping noise and feels like a hesitation the smooths out. If this is the case check for vacuum leaks and check your timing. Also make sure you have a decent shoot of fuel from the accelerator pump. The idle circut maybe a little lean but at 2000 rpm it should be on the main circut. If it is occuring when the engine is reved and held at a steady rpm and sounds like exhaust noise then the exhaust valve is not opening and when the cylinder is on the exhaust stroke the only place out is through the intake valve and out the carb. Pull the valve covers and make sure all the valves are moving. Adjust the valves and check to see if the exhaust rockers line up in hight. If you find one that is a little lower than the other, that one is your problem and unfortunely this usually means--no easy way to put it-- a bad cam lobe.

Steve
hope I'm wrong
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Old 09-10-2003, 06:32 PM
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If you have messed with your wires you might check 5 and 7, I believe. The engnie will still run but when you open it up it will back fire or pop through the carb. I've had it happen to my engine.
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Old 09-10-2003, 10:59 PM
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Well, I will check the lean issue, The carb is a holley 750 (4160 i believe) and was set up for the 307 and is now on the 350 so I am going to up the accelerator pump a lot and see if that helps. I do also know that the distributor needs to be recurved ( I found I shop in town that still has a distributor machine) I have the timing at idle set at 8 BTDC, and with all the mechanical advance coming in it only ends up at 6 ATDC so I am not getting enough advance, and this is with the lightest springs available. This explains why when I open the throttle gradually it has plenty of power and no misses, but when I get on it hard, it backfires because of the lean problem, and then after has no power because I am loosing the vacuum advance. When it does this if I slowly let off the throttle, it will hit a point where the vacuum advance comes back in, and it gets power again. Thanks!! Time to go to some tweaking!
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Old 09-10-2003, 11:08 PM
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6 ATDC!?!?!?! Umm.. anyone but me think that this is bad?
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Old 09-11-2003, 12:11 AM
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I say check all the plug wires again, I`ve ran into this problem many times as it`s too easy to overlook, 5 and 7 are the most common to get backwards, but others are easy to cross as well.as said in before posts, lean conditions cause backfires, but too early spark advance, a valve not opening enough due to a loose rocker, or over tightened rocker can cause this, a broken rocker arm, or a broken valve spring, or in a worse cause, a cam lobe is gone. being your running a holley 750, I highly doubt the pump shot is too small. it is possible the pump isn`t giving enough fuel due to a ruptured pump or faulty seal above the pump. if the pump shot appears okay, I would look elsewhere, as in the places I mentioned before. check the cap wire routing and check the plug wire routing, take your time and check them 1 at a time. as for spark advance, shoot for 12 degree`s BTDC and 38 BTDC all in by 3000 RPM, if detonation is detected, retard the timing 2 degree`s at a time until detonation is no longer heard and it`s set. last year I had just did a fresh rebuild for a customer, I did the drivers side plug wires, a friend did the passengers side, he left for work and I finished, I started the car and it ran fine, until I gave it fuel, and it would backfire unless I did so real slow, not knowing what was going on, i decided to check all the plug wires, 4 and 6 were crossed, problem solved.
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Old 09-11-2003, 12:38 AM
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As alot of you may know, my tunnel ram does the same, and I have yet too find the problem. I've tried adjusting the secondaries (they're mechanical, couldn't fit vacuum w/ linkage) and adjusted the accelerator pump shot and nozzles. I currently have it @ 10BTDC and w/ the pro-billet the total timing is still at the factory 32 total. I also have the heaviest springs on it still. It's a stock 350 w/ a 270H magnum comp cam, edelbrock pro ram II, and 2 holley 450's. I was wanting too play w/ the distributor but am afraid that I won't know if I'm overdoing it. It has vacuum advance hooked up to a manifold vacuum port ( I don't know where a timed port would be on these holley's) Should I have the vacuum somewhere else? Any ideas on a good total timing? Should I start advancing the mechanical advance with lighter springs? If I do, what kind of differences should I be looking for? BTW, I also have a 6AL box and use the magnetic pickup wires on my particular pro-billet. I currently have 63 jets, and 30 metering plates, w/ 6.5 power valves. Also, how do I check the manifold vacuum? My fuel pressure is currently at 6psi and all of the needle seats are adjusted per Holley specs. The only other piece of info I can think too tell you is that there is no black smoke, and the plugs have tan tips and the porcelain is black.
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Old 09-11-2003, 12:50 AM
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Your timing specs should come in the same as above, 12 degree`s BTDC and 38 total. the vacuum advance should be connected to a ported source, and on every holley I seen the ported source is on the passenger side of the carb on the upper part of the primary metering block. you check manifold vacuum by connecting a vacuum gauge to a constant source or one that pulls vacuum at idle.
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Old 09-11-2003, 06:10 AM
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Adrenaline, take the vacuum advance off of manifold vacuum, it has to be on ported/timed vacuum slots to operate properly.
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-Holley 600 cfm 4-barrel
-Flowmaster Afterburner headers
-Turbo 350 Transmission
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Old 09-11-2003, 01:54 PM
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Well, I double checked the plug wires and they are right, and I re adjusted the timing to 12 btdc and it still does it under throttle. It backfires and tries to stall untill you let off the gas. The distributor is only getting 14 degrees of advance which is way to little so I am going to a shop and getting the distributor re curved and hopefully this will help. We shall see.
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Old 09-11-2003, 03:16 PM
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All fixed The weights in the distributor were incorrect and were only allowing 2 degrees of advance. Runs awesome now, its night and day compared to the 307.
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Old 09-12-2003, 12:29 AM
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That's where I initially had the distributor hooked up (on ported) but moved it because that vacuum port wasn't pulling anything at idle, stupid me, I forgot that it won't pull until the throttle is opened. I'll put it back and see how it does. Another reason I thought it wasn't a good spot was because I had the transmission hooked up to the same port on the front carb and it would go into nuetral when driving it, it wasn't getting enough vacuum signal. i placed both on the manifold vacuum and the transmission works great. My final question regarding the timing is, should I put lighter springs in? I can't remember where total timing comes in with the MSD's heaviest springs, but I'm sure someone here does. Well thanks for the response. Sorry for jumping in here subman, I've just always hated seeing multiple posts for the same subject, especially when everybody following this thread is allready discussing it.
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