[Jack Ferris]

Im having issues with a Pro Street project in my shop. Its a BBC but I'm out of ideas as to what the problem is.

Heres what it is: 402 cid 8:1 BDS 8-71 Blower, 238 int./248 exh.@.050 .540 lift 114 LSA hyd. cam, dual Holley 80776 600 cfm 4150 carbs, MSD Dist, Blaster SS coil, no MSD box, 1 wire alt. The rest is stock stuff.

When it came to my shop it ran ok. We pulled the motor out to do the body and paint and re detail the motor. During that time I found 2 flat lobes on the cam. I then replaced it and put it all back together. I also took the carbs apart to clean them up and found all 4 floats were adjusted all the way down. I put them back to where they are supposed to be.

Heres the problem. It starts and runs ok. Not good but ok. When I turn the key off it runs for about a second, reves up about 500 rpm, shuts off and the it makes a huge bang out of both exhaust pipes. WTF? It did not do this before even with 2 flat lobes.


Any adeas would make my day.
Thanks

[Jack Ferris]

I did not mention this prior but the carbs have 73(p) and 80(s) jets. Out of the box it came with 66 and 73. Thats quite a step up.

[Tom Vaught]

Agree, The super low flow levels were adding a lot of emulsion air to the jetting
and helping with the excessive jetting.

Put the stock jetting into the carb and work only on the idle and transition side first.

By boost referencing the power valves to below the supercharger you can also get a clean idle AND the extra enrichment at the higher load conditions.

Tom Vaught



Tom Vaught

[TIN TIGR]

ima wit tom..your carbys are way off..is engine running hot?..it should turn off when spark pulled..

[Jack Ferris]

I have only been as far as cam break in. Headers were glowing red but temps were ok. I will see what happens after putting the jets back to stock. What is the difference between a blower carb and a non? These particular carbs are for a non blower appliction. Either way thanks for your guys help!

[Jack Ferris]

Ok I set the carbs back to the stock settings but didn't have a vacuum guage on hand to determine power valves. It calls for 65's but it they are 75 now. It starts and runs better than it did but it still has the run on issue. I tried dis connectiong the one wire alt and it did nothing. I did notice that when the motor shuts off once in a while it stops and will slightly rotate backwards. It seems this is when it bangs out the exhaust. Here is one more scenario. Can the electric fan back feed once power is cut to it and its still turning? On my bracket car I have the fan wired with a lamp that indicates whether its on or off. When I kill power the light slowly dims with the speed of the fans and does not fully go off until the fan stops. Will this keep the motor running for a second after shut off? None of this is is an issue on my car.

[BruceWilkie]

I've seen Accel and MSD ignition boxes keep running with as low as 3 volts backfeed from the altenator. A diode off the altenator or whatever other source supplying back voltage will prevent "feedback". MSD has the "keeps running fix"(add diode) instructions in there online troubleshooting. DC motors can turn into "generators" when coasting with power removed and opposite can happen where generators turn into motors if voltage gets applied.

[Jack Ferris]

Originally Posted by BruceWilkie

I've seen Accel and MSD ignition boxes keep running with as low as 3 volts backfeed from the altenator. A diode off the altenator or whatever other source supplying back voltage will prevent "feedback". MSD has the "keeps running fix"(add diode) instructions in there online troubleshooting. DC motors can turn into "generators" when coasting with power removed and opposite can happen where generators turn into motors if voltage gets applied.

I may try running it without the fan. It does not have a box just the MSD dist and the MSD SS coil. I did try it with the alternator wire un hooked and it still banged out the ex but like I said its when the engine rotated in the reverse direction about 30 degrees after it stopped turning the right direction. Whats weird is it it can be shut completely off for about 3-4 seconds before it backfires.

[62fatcat]

you did paint and body work,
recheck all of you grounds, make sure there is a solid ground from the battery to the engine, not a floating ground.

good luck,
Brad

[taff2]

If it's backfiring through the exhaust 3 or 4 seconds after it's completely shut off I'd say you have a backsurge through the ignition system,maybe a faulty module in the MSD distributor,and a spark at the wrong time could turn the engine backwards. Done any recent mig welding on the car? If you didn't completely disconnect the MSD dizzy it's possible you've damaged the module.

[taff2]

Just a thought, does it have a 4/7 cam swap? A local racer chased his tail for a few meetings after one of his crew refitted the distributor and put the plug leads on in the normal order when the engine had a 4/7 swap cam. It was in bits when I walked over and swapped the plug leads to where they should have been......

[marks73ta]

Are you running your power supply to the fans through a relay. If you do then on shut down the relay cuts any possible leakage back to the engine. Mark L

[Jack Ferris]

Originally Posted by taff2

If it's backfiring through the exhaust 3 or 4 seconds after it's completely shut off I'd say you have a backsurge through the ignition system,maybe a faulty module in the MSD distributor,and a spark at the wrong time could turn the engine backwards. Done any recent mig welding on the car? If you didn't completely disconnect the MSD dizzy it's possible you've damaged the module.

Indeed I did do some welding on the exhaust. I do not remember if I had the distributor all wired at the time or not. I have another dist but it only has 2 leads for use with the MSD box. The one in the car now has 3. Pos/neg/gd. The cam is just your bacic hyd blower grind and its not a 4/7 swap. Today I am running some grounds from the motor to the chassis and from chassis to the body. A damn good point that I over looked. When they come in running and then they dont its hard to chase down the most obvious problems. I will refire it tomorrow and if the problem still remains I will look into the distributor. Thanks to everyone for a lot of good information.

[flamedabone]

Can you point a timing light at it and record your initial and total numbers? The fact that it "runs OK, but not good" bothers me as much as the running on issue. Can you be a little more specific?

Good luck, -Abone.