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weber 45 Dcoe side draft set up back firing


Tv808

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i have a fully built engine with a web 292 cam. intake opens 49 degrees before tdc exhaust 53 degrees before bdc. since there is no adjustable cam in guessing i need to change the degree on the sprocket to compensate. also i’m using the factory distributor with vacuum advance which also may be causing a issue since i have to compensate for that. i’m stumped.

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i have a fully built engine with a web 292 cam. intake opens 49 degrees before tdc exhaust 53 degrees before bdc. since there is no adjustable cam in guessing i need to change the degree on the sprocket to compensate. also i’m using the factory distributor with vacuum advance which also may be causing a issue since i have to compensate for that. i’m stumped.

Try removing the vacume advance to start... side drafts don't have provisions to hook up the vacume advance.. my guess is you are hooked up to manifold vacume which is different than what the distributor wants to see...

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Right, good question. One or two carbs?

 

Another couple of questions: did this setup work well before?

 

If dual carbs, carb synch is crucial and if out too far, can cause what seems like a backfire, but is really more of a lean spit out of the carbs. If you tried, but can't get the carbs to synch, I have seen many many times a twisted throttle shaft cause the same issue. A throttle shaft can get twisted if your throttle stops are not in place, if they are out of adjustment, if your linkage is not setup properly. They can also get twisted when working on the carbs. If removing one side of the linkage, you grab the shaft on the other side to hold it, you can very easily twist a shaft.

 

Others here mentioned the vacuum advance. Disconnect it and reset your timing. Ideally, you should re-curve your distributor to make sure you get the right amount of advance at the right time, but without the possibility of injecting too much timing at the top end of the curve.

 

If this is a setup that has worked well for you in the past, what happened to cause the issue? Recent repairs? Sitting un-driven for a long period? Something had to happen.

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it’s a fresh build with ka24e forged pistons 40 over and rods, lightened and balance crank, scneider valve spring and retainers, web 292 cam, weber 45 dcoe dual side drafts, fuel pressure regulator 3.5psi doug theory headers.

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Guest Mojo510

Can you tell us where the back fire is coming from, intake or exhaust? Out intake lean, out exhaust rich. When does the back fire happen, hard accel, during cruise, at idle, on decel or just random, etc

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i have a fully built engine with a web 292 cam. intake opens 49 degrees before tdc exhaust 53 degrees before bdc. since there is no adjustable cam in guessing i need to change the degree on the sprocket to compensate. also i’m using the factory distributor with vacuum advance which also may be causing a issue since i have to compensate for that. i’m stumped.

Unless you have the cam timing messed up bad i doubt that's the problem.I have a Webcam and ran it at all three marks on the cam sprocket,no problems like you describe.Ended up running it at #2 mark.

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the vacuum advance is not hooked up on my distributor so i’m going to try an electronic destrbitor without vacuum advance. thank you

 

Vacuum advance only affects part throttle driving. If you connect to intake vacuum it may advance too much. If advance is disconnected it's not that.

 

Probably a lean backfire. One of the jets is blocked.

 

Pull the plugs... a single lighter one is the carb barrel that's lean.

 

Check the plugs first, it's cheap and easy.

 

Next check the valve lash specially the intakes. A tight valve may not be closing tight. 

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A typical advance curve for DCOEs will put timing at 20 degrees BTDC at idle.  Try that to see if it eases the transition off-idle.  You may need a custom distributor set up rather than buying another junk Chinese copy off Amazon/Ebay.  

 

If you really want to tune your setup, look at the LM-2 air fuel meter from Innovate Motorsports, along with the tailpipe clamp for the O2 sensor.  Its the only way you'll know if you'r running rich or lean in each circuit of the DCOE, without doing all your tuning on the dyno.  $$$$  You need to degree in that cam with an adjustable timing chain set or it'll end up totally screwed up.  In order to grind a Datsun blank to a 292, you have to shift it off centerline or you run out of material on the blank.  That means when you reinstall it in a "stock" position, it'll be set up all wrong.  You need to go through the process of degreeing it or you won't appreciate the cam for what it is.  

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This is already headed where so many threads end up.

 

OP needs to do the homework and follow the steps to knock down the possibilities.At least give us some solid info to go on.

 

If we knew exactly what it was, when it was built, has it ever run in this configuration before, did something happen to cause it, then we can help. Without this knowledge, we're all pissing in the wind.

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This is already headed where so many threads end up.

 

OP needs to do the homework and follow the steps to knock down the possibilities.At least give us some solid info to go on.

 

If we knew exactly what it was, when it was built, has it ever run in this configuration before, did something happen to cause it, then we can help. Without this knowledge, we're all pissing in the wind.

 

C'mon! It's built! That is all you need to know!! And obviously not built by him  :geek:

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