'79 300 backfiring thru carb. - Ford Truck Enthusiasts Forums

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'79 300 backfiring thru carb.

 
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Old 01-07-2009, 07:36 PM
79 Stepside
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'79 300 backfiring thru carb.

Hi First time posting on site. I have a '79 300 with comp cam 260 , dual port offy & 390 holley. About three weeks ago went to get a load of fire wood. And on the way home lost power.Valves were chattering pulled valve cover noticed #5 intake rocker loose went to toruqe and stud turns. Going to have machine shop pull all studs and install 3/8 screw in studs. Do i need to recurve distributor and what should i set the time at on sticker on valve cover. Also can not seem to get the carb adjusted right . Intake and carb only three months old. Vacuum reading is around 10.
 
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Old 01-08-2009, 03:09 PM
Harte3
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Check for vacuum leaks and do a compression check. The timing is going to be whatever the engine likes. Since it is modified the factory specs are out the window. A recurved dizzy is probably in order but set the timing first with a vacuum gauge with the dizzy vac source plugged. Turn the dizzy until you get the highest steady reading and back off 1-2 hg as a starting point. Also try switching the dizzy between a timed vac port and a full vac port...should be both on that carb. Then go to the carb, turn the idle mix screws in until gently seated and back out 1 1/2 turns as a starting point (See Holley Tech FAQ"s on carb adjustment). And again with the vac gauge adjust the idle mix screws until you get the highest steady reading making sure you move both screws the same. Then go to the idle speed screw and set to the desired speed. If you have a tach that will be probably 600-700 rpm or wherever you get a smooth idle. Then repeat the whole process starting with the dizzy again. Then drive it. If you get ping under load then retard the dizzy in small increments and test drive again.

If you haven't upgraded the ignition a cheap mod is to use a 4-pin HEI module mounted on a heat sink. Info here: Dirt cheap Ignition and the next time you change plugs use the stock EFI's for a 300 and open the gap to .050. Idle speed may change and require an adjustment and it probably wouldn't hurt to go back through the dizzy/carb adjustment procedure again.

My engine pulls right at 19 hg at 2400 feet elevation when fully warmed up. Vac gauges are theoretically calibrated at sea level atmosphere and engine vac drops about 1 hg per 1000 feet of elevation. If your engine is in otherwise good shape you probably shouldn't see less than 17 hg at idle. Mine will run with the dizzy hooked to either a timed port or full vac source. On the timed port I have to turn up the idle speed screw to get a decent idle so I run on the full vac source...no problem.
 
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Old 01-08-2009, 07:02 PM
79 Stepside
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Compression good motor is rebuilt w/6000 miles.Im also running efi manifolds with two 2 1/2 pipes in to dual inlet magnaflow single 2 1/2 outlet. Sounds like small block at idle.I have seen the site about recurving the dizzy seems like something that could be screwed up easy. Do you know what is the most common curve marking to use . Example 12L & 18L. The site i seen went pretty indepth about recurving ,you need to find full advancement and mechanical degrees. Test drive a tweek.
 
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Old 01-08-2009, 07:18 PM
Harte3
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Re-In-"Car"-nation High Performance and Automotive Repair / contact Give that outfit a call. He furnishes detailed instructions for a recurve and after talking to him he can probably point you in the right direction as far as which slot numbers to use. Or check around the shops in your area that do distributors and you might get it done fairly reasonable.
 
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Old 01-08-2009, 08:54 PM
79 Stepside
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Thank you Harte3 . Just one more. I found your post #758 you talked about duraspark II w/ smog control. Replacing one spring ,same curve as police interceptor. WAs that replacing one spring from Mr. Gasket kit and recurving or just replace light spring w/ one from kit, Thank You.
 
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Old 01-08-2009, 09:45 PM
Harte3
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I'm not sure since it has been some time ago but I think just the light spring from the Mr. G kit is used and I don't recall which one it replaces.

Hold on, just found the article...Classic Inlines Performance Parts - Duraspark Swap. Scroll down and you will find the info. That is for a 200 or 250 I6 but should be the same principle for a 300. On heavier vehicles plus using them for work I think the usual recommendation is for full advance to come in between 3000 and 3500 rpm and anywhere between 36* and 42* full advance. Mine comes in right at 3000/41.

BTW I found that mine liked 8* initial timing but your results may differ.
 
 
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