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Weber Backfiring
- 1969 Boss 302
- 48 IDA Webers --main venturi 37 -- idle holder 100 -- idle jet 70 -- air corr 195 -- main 150 - Emulsion tube F5 - Runs fine up to 2400RPM - Belches and backfires on acceleration 2400-3200RPM - Runs fine above 3200RPM - Have leaned and enriched the fuel until the cows come how What is the jet/adjustment fix I need to implement for this problem that appears in the idle to main circuit changeover area? Thanx much |
Do you know the camshaft specs (?) is it Roller or Flat tappet and what compression ratio is being utilized? Altitude?
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What is your timing?
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- Timing is 36 total
- solid camshaft is: Lift Lash Dur .05Dur Lobe ctr I .528 .028 300° 250° 108° E .528 .028 300° 250° |
That is a cam that will not be friendly with Webers the exhaust duration is quite long and the lobe separation should be about 112 degrees.
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I think the original posters setup is all wrong. Wrong emulsion tube. And what size pump circuit parts does it have? |
I agree the cam has too much overlap for Webers
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Yes Gaz you are correct, LSA should be about 112 degrees to prevent valves from being off their seats at the same time too long. Webers don't like that, single plane 4 bbls love it, at the expense of a little torque.
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Thanx for the inputs...was able to mitigate most of the below 3000 rpm overlap issue (and bogging/backfiring) by putting in 160 mains and setting mixture at 1 1/8 turns. Motor runs pretty well now with only minor bog/backfire but clearly when time permits, a different cam will be needed.
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Have you experimented with different emulsion tubes ?
Z |
Mutt,
I had a similar problem in the rpm range of 2500 to 3000 which turned out to be a slightly overrich transionsition between the idle circuit and the main circuit. I discovered that it was overrich in this range after I installed an A/F sensor in my exhaust. To correct the problem, I changed from the original F7 emulsion tube that was provided with the setup to a F11 emulsion tube which has air holes high up and slightly leans out the transition. I would recommend installing an A/F sensor to help you with further tuning of your system. John |
How's the balance? If you haven't balanced the carbs with a synchrometer, you'll never get it to run right.
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bump that timing to 38deg.
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Emulsion Tube Selection
Mutt,
Have you solved your transition flat spot problem? If not, I do have a recommendation regarding the emulsion tubes. You stated that you have F5 emulsion tubes in your car. If so, the next emulsion tube to yours that has additional air holes above those in the F5 tubes would be the F3. The addition of 2 holes that are 1 mm in diameter is the only difference between these two emulsion tubes. If you do not have access to a set of F3 emulsion tubes available to you, you could drill two holes in your F5 tubes just above the existing holes to slightly lean out the transition to your main circuit. Figures showing these emulsion tubes can be found in the official Weber Tuning Manual. In any event, I would still highly recommend that you install a bung in your exhaust to install a wideband Oxygen sensor. I have found this tool to be of a great assistance in jetting Webers. Regards, John |
Since we are back at this. My experience was that one jet size too high or too low will cause this. Don't skip sizes when jetting. & yes, etubes can be of value once things get dialed in pretty close. Luckily, I was able to mine to transition smoothly with the stock emulsion tubes.
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Do you really need an idle circuit that rich for a 302? |
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Z. |
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Z. |
F11 emulsion tubes
I have a extra set of F11's you can try..
Send me a PM Jon |
Saltshaker,
I know that this is an older post but I have to ask if you still have those emulsion tubes. I have a 302 that I have just picked up a set of Webers for. I am having issues (mostly when the engine is warmed up). Then there is a lot of small backfiring through the side pipes while at lower RPM's. When I stomp on it the car clears up and runs fine. The issue is just in the transition. Any thoughts are welcome. Jim Stern |
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