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lwrdr69
Novice
Dec 1, 2009, 4:29 AM
Post #1 of 6
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1969
Chevy
Truck
Stock 307
122,000
Carberator backfiring
Replaced manifold and carberator-engine set to top dead center on #1 cylinder, rotor pointing to #1 on cap, timing mark on 0 degree's, engine starts and runs fast but backfires occasionally, Am I still off a tooth on distributor?
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Hammer Time
Ultimate Carjunky
/ Moderator
Dec 1, 2009, 4:32 AM
Post #2 of 6
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Re: Ignition Timing
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If you have the ability to set the time where you want it, then you are not out a tooth. I would bet that calls for more than a "0" setting though. If "0" is the maximum adjustment your able to reach, then yes, you may still be out a tooth.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
(This post was edited by Hammer Time on Dec 1, 2009, 4:32 AM)
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lwrdr69
Novice
Dec 1, 2009, 4:56 AM
Post #3 of 6
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Re: Ignition Timing
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Thank you, I also noticed my distributor cap is hitting my stock coil mounted in the stock mounting bracket which I'm assuming the mount holes are the same location as my stock manifold? This nay indicate the distibutor is actually off by a tooth or so?
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Hammer Time
Ultimate Carjunky
/ Moderator
Dec 1, 2009, 5:08 AM
Post #4 of 6
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Re: Ignition Timing
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Yes, I believe it does indicate that
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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Loren Champlain Sr
Veteran
/ Moderator
Dec 1, 2009, 5:58 PM
Post #5 of 6
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Re: Ignition Timing
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lwrdr; Since I don't know how much you know, I'm going to start with the basics, so please bear with me.
Pull number one spark plug after disconnecting coil. Turn engine over until you feel the push of compression from the number one cylinder. That will be the compression stroke. Line the timing mark up on TDC. The rotor should, then, be pointing to number one on the distributor cap. If off, raise the distributor out of the manifold, turn the rotor so that it points to 1 and, if you are real lucky, it will drop into place. As the dist. drops into place, you'll notice that the rotor will want to 'turn' clockwise as the distributor drops down. Many times, you'll have to 'drop' the dist. with the rotor a tooth off, so when it bottoms out, it will end up in the proper position. I've even had to 'bump' the engine over for it to drop all of the way, which will, of course, mess up your timing marks and have to start over if it didn't 'land' right. The vacuum advance unit will be apprx. 45 degrees to the passenger side of the engine. If it is pointing straight forward, it won't give you ample adjustment as you seem to be experiencing, now. Make sure that the points are adjusted to 30 degrees dwell before you attempt timing. For every two degrees of dwell, timing will change one degree. So, the point adjustment is crucial. Once the points and timing (4-6 degrees BTDC) have been set, then you can adjust the air/fuel mixture. (timing is set with vacuum advance disconnected and hose plugged, at or below 700 rpm). To adjust air/fuel, at idle: Turn both mix. screws out about three turns. Then, turn one screw clockwise until the rpm just begins to drop, then back off just enough for the rpm to recover. Do the same with the other screw. Redo the first one, then redo the second one. This will give you your best lean idle mixture.
Will all of that said.....small block GMs are known for the cam lobes (usually 7 or 8) going flat. When this happens, the valves can't open all of the way and will cause a backfire through the carb, especially on acceleration. Hopefully, this is not the case, here.
Loren
SW Washington
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lwrdr69
Novice
Dec 2, 2009, 12:17 AM
Post #6 of 6
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Re: Ignition Timing
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Thanks for all the information. All this was done prior to pulling distributor to change the manifold. The distributor was off a tooth and may be one more as well to get the full rotation for setting the timing. The distributor cap hold down screw lug is resting against the coil at the recommended 2 degrees timing, so I will try one more tooth for more clearance between the cap and coil. The backfire is gone now that I have moved it one tooth but distributor cap is still preventing the full rotation for more degree variety. Thanks again
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