Miles Posted December 23, 2001 Share Posted December 23, 2001 Symptoms: engine (350 sbc) ran rough at all speeds with backfiring through exhaust when the throttle is released. Flat spot and rough running at 1500 RPM and up. Replaced wires, plugs, cap, points, rotor, condensor etc. Set timing 12 DBFTDC. Replaced old Carter 4 bbl with an Edlebrock 1406. Set timing at 12 DBTDC. Installed fuel pressure gage. Electric fuel pump is putting out 4-6 psi. Stays at 4 psi and jumps to 6 psi then back to 4 psi. Engine still backfires through open headers. Local speed shop guy told me that running without exhaust hooked up will cause back firing and can cause a flat RPM spot. Engine won,t start after it heats up. It will start if pedal is held to the floor and just keep cranking. Also, advancing the timing to 12 DBFTDC seems to make it hard to start (engine was hot when this happened). However, engine ran smooth through 4000 RPM with timing at 12 DBFTDC. Any help would be appreciated. Quote Link to comment Share on other sites More sharing options...
pparaska Posted December 23, 2001 Share Posted December 23, 2001 You sure the timing mark is correct? Vacuum leak maybe? Quote Link to comment Share on other sites More sharing options...
Guest Anonymous Posted December 23, 2001 Share Posted December 23, 2001 Like pete said, if you are sure that the timing is what it says it is, then I would check for a vacuum leak. In regards to the backfire when letting off the throttle, this is normal if you have no cat, and I'm going to assume your open headers don't have a cat built in... It is caused when gas puddles up in the hot exhaust pipes, if you have a cat this subsides most of the unburnt gas before it can ingnite, but my car does this with the L6, it has no cat. I've had someone ask if it did that because of my turbo, and if when I let off, the boost goes down, but it still dumps gas for a sec, which ignites in the exhaust. I told him why, and he couldn't beleive that it wasn't the turbo model, he thought for sure it was when I beat him Damn Hondas! Some more information would help with the rough running part. What is the setup? Carbed or FI? TB or Port injected? Just some thoughts, but I would check vacuum. Hope you get it figured out, Kris Quote Link to comment Share on other sites More sharing options...
Guest Drewz Posted December 23, 2001 Share Posted December 23, 2001 Intake gasket leak at manifold??? Aluminum manifolds can warp badly, also have you rechecked the rockers ??? Too tight?? The balancer could be from a diff. motor/year and may be giving you false readings. Alot of possibilities. Plug all vacum lines and pour some water on the sides of the intake when the engine is running and see if it gets sucked in to the head/intake seal?? You can also spray carb cleaner on the carb base and sides of intake to check for a leak. Vacum hose to the trans. come off down below?? Good luck man!!! Quote Link to comment Share on other sites More sharing options...
Miles Posted December 23, 2001 Author Share Posted December 23, 2001 More info: Set up is a 1974 350 out of a Monte Carlo with a performance cam according to the previous owner of the engine. Edlebrock 1406 4 bbl and point type distributor. Nothing fancy. Engine runs rough and lacks power when stepping on the gas. It tends to stumble as it reaches shift RPM in 1st, 2ed 3rd etc. For a brief period I was able to rev up to 4000 RPM (no load) and it sounded very tight and normal. Shut the engine off and it was hard to start. Held pedal to the floor while cranking for about 15/20 seconds and then it finally started in fits and spurts. Idles ok at 600 RPM. The electric fuel pump is putting out 4-6 psi. Mostly around 4 psi with spurts of 6 psi. Is that good enough for a 350? Quote Link to comment Share on other sites More sharing options...
Tim240z Posted December 23, 2001 Share Posted December 23, 2001 Sounds like the needle is sticking open letting fuel run unchecked into the inlet. With the engine running, or just the electric fuel pump running if hooked to a seperate switch, look down the carb and see if fuel is pouring in. Had that problem before with my Holley, with similar symptoms as you have. If your pump is run thru a switch, try switching it off while trying to start, or just disconnect it. If it fires nicely without fuel pressure, then you've got it licked. Tim Quote Link to comment Share on other sites More sharing options...
Guest Drewz Posted December 23, 2001 Share Posted December 23, 2001 If the fuel pump is hooked live with no oil press. cut off pleeeaaasseee get one since a guys van burnt here last year when the pump did not shut off and an engine fire ensued!!!! Quote Link to comment Share on other sites More sharing options...
Miles Posted December 23, 2001 Author Share Posted December 23, 2001 I was going to hook up the oil pressure cut out per JTR, but there is not much space where the senor is located near the fire wall. I noticed that there is a plug just above the oil filter boss and was wondering if a pressure sensor switch could be screwed into where the plug is. I have also located a supplyer for an inertial cut off switch that chops all of the power in an accident. It is the same inertial cut off switch used in Ford explorers. Quote Link to comment Share on other sites More sharing options...
Tim240z Posted December 26, 2001 Share Posted December 26, 2001 Miles, Have you sorted the problem? Tim Quote Link to comment Share on other sites More sharing options...
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