Guest E.Murray Posted May 18, 2004 Share Posted May 18, 2004 Hey all, My 260 has a relatively mild 350 that was running really rich and loosing power above 2500 RPM. This was a great excuse to spend some money (like I need an excuse...), so last weekend I rebuilt the carb (a 600 CFM Holley 4160), put on a new fuel pump and lines, added a Pertronix Ignitor II to replace the points and condenser, replaced the cap and rotor and added MSD-6AL and an MSD Blaster II coil. I never in my wildest dreams expected it to fire right up after all these changes, but last night I turned the key and it started almost immediately. Woo hoo! I backed out of the garage and started up the street for a test drive, but as soon as I give it any gas at all (anything above idle), it backfires through the carb like crazy. It seems to idle OK, but that's it. What do you think? I have the idle screws about 1 1/2 turns out (they were only 3/4 turns out when I pulled the carb, but I think the power valve was dumping in too much gas). Too lean? Timing off? It was late last night, so I didn't check the timing, but since I didn't pull the distributor, it should be OK, right? What is the usual cause for this? Any suggestions are welcome... Quote Link to comment Share on other sites More sharing options...
MrWOT Posted May 18, 2004 Share Posted May 18, 2004 Timing, hands down. Quote Link to comment Share on other sites More sharing options...
COZY Z COLE Posted May 18, 2004 Share Posted May 18, 2004 Couple things, did you use a HOLLEY rebuild kit? Any parts store kits are a POS. Did you replace the carb to manifold gasket? Double check plug firing order, vacum lines, and timing. Usual cause of carb backfiring is a too lean ldle mixture. LARRY Quote Link to comment Share on other sites More sharing options...
Guest E.Murray Posted May 18, 2004 Share Posted May 18, 2004 Yes, I did use a real Holley kit. I couldn't find one locally, so I ordered it from Summit. Funny you should ask about the carb to manifold gasket... When I pulled it off there was, of course, a big one with a big square hole in the middle that left the whole bottom of the carb open to the manifold. In the kit, though, there was one just like that, plus another one with just 4 round cut-outs for the throttles (covered the rest of the bottom of the throttle body). I couldn't remember if there was one of those on the original or not. Since there are some little passageways machined into the bottom of the throttle body between the throttle bores, I figured there should be a gasket covering them (wouldn't be much point in machining them if they were just open into the intake). I can't for the life of me remember if I pulled one off or not. I used both when I re-installed the carb. Should I not have? I will check the timing tonight, too. Quote Link to comment Share on other sites More sharing options...
COZY Z COLE Posted May 18, 2004 Share Posted May 18, 2004 I'd do this in the following order: 1.Check the timing and the plug firing order. 2.Reset the idle to original turn. 3.Remove one gasket ( either one) because two would be too thick and future tighting of bolts could cause body to warp and create vacum leaks/replacement of the body. 4.Check your power valve because of the backfiring. 5.Check for vacum leaks and idle adj. Good Luck!!! LARRY Quote Link to comment Share on other sites More sharing options...
Guest zfan Posted May 19, 2004 Share Posted May 19, 2004 Unless you have the anti blow out protection you probably blew your power valve. Timing could be an issue. What kind of vacuum are you making at idle? Your power valve should be around 2 inches of vacuum over the in gear idle vacuum. In other words if you make 6-7 inches of vacuum at idle in gear then run a 8-9 power valve. My money is on timing! Mike Quote Link to comment Share on other sites More sharing options...
jt1 Posted May 19, 2004 Share Posted May 19, 2004 Power valve should be LESS than the idle vacumn. John Quote Link to comment Share on other sites More sharing options...
Guest zfan Posted May 19, 2004 Share Posted May 19, 2004 My bad I meant less!! Mike Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted May 19, 2004 Share Posted May 19, 2004 Your timing is off. The other stuff may need to be done, but you can't expect to replace your points and not have it off. Quote Link to comment Share on other sites More sharing options...
Guest E.Murray Posted May 19, 2004 Share Posted May 19, 2004 OK, I checked the timing last night, and it was sitting on about 13 BTDC. I backed it off to about 8 just to be sure. No change. On a hunch, I just screwed the idle mixture screws all the way in. I have a standard idle circuit carb (clockwise lean) so this should have killed it. No change. Both the primary and secondary throttle plates seem to be closed far enough. What else could be going on? I can't find a vacuum leak. Is it possible that if I added an extra base gasket, that's affecting it? Then why doesn't the idle mixture do anything? I'm just spinning here... Also, I noticed something I hadn't before last night. It's probably normal, but I want to make sure. After I shut the car off, moving the throttle even a little bit causes gas to drip pretty quickly out of the primary boosters. Moving the throttle several times causes fuel to puddle inside the intake. Does it always do this and I just never noticed it? Up until a couple years ago, I always had FI cars. I really dig playing with carbs and tweaking them, but my lack of long-term experience really comes back to bite me every once in a while. Quote Link to comment Share on other sites More sharing options...
jt1 Posted May 19, 2004 Share Posted May 19, 2004 Double check the firing order, 1-8-4-3-6-5-7-2. Get a can of carb cleaner and spray it on all possible vacumn leaks, like carb base, vacumn lines, and around the intake. Don't let it get in the carb, use one of the tubes so you can get it right where you want it. If the carb cleaner causes the rpm to increase, you've found a vacumn leak. Fuel level in the bowls should be just at the bottom of the sight plugs. Slight movement of the throttle should cause fuel to come out the primary squirter, not the boosters. Was it backfiring before the carb rebuild? It sorta sounds like it may have jumped time, but hopefully it's just a vacumn leak. John Quote Link to comment Share on other sites More sharing options...
Guest E.Murray Posted May 19, 2004 Share Posted May 19, 2004 Would any of the ignition stuff explain why the idle mixture screws don't do anything? It wasn't backfiring before the rebuild. I think the power valve was just shot all to pieces (so maybe it backfired at some point in time...). Quote Link to comment Share on other sites More sharing options...
MrWOT Posted May 19, 2004 Share Posted May 19, 2004 Murray, the gas into the manifold when you move your foot has to do with your accelerator pump, depending on the carb you can adjust it with anything from a screw or nut, to changing a cam in the pump. It's a normal thing, and a good thing as well, otherwise when you romp the gas the car would lean out. Quote Link to comment Share on other sites More sharing options...
jt1 Posted May 19, 2004 Share Posted May 19, 2004 Would any of the ignition stuff explain why the idle mixture screws don't do anything? No, but a vacumn leak would. Are you sure all the vacumn ports you're not using are plugged? Especially the one on the back under the rear fuel bowl? John Quote Link to comment Share on other sites More sharing options...
jack46 Posted May 19, 2004 Share Posted May 19, 2004 Few times I have seen this problem it was firing order or timing and once was a bad valve. Hope it is simply the firing order easiest and cheapest to fix. What intake are you using? I was told NOT to use the gasket with just four holes unless it matches the intake. I had an RPM Performer at the time, and I was going to use that one, and was told don't. The gasket is not presently the problem, but could be in the future if motor sucks the gasket in. Quote Link to comment Share on other sites More sharing options...
Guest E.Murray Posted May 19, 2004 Share Posted May 19, 2004 I'm using an RPM Performer, too. Guess I should pull that gasket off there, huh? I will triple-check the firing order and vacuum ports tonight. If that doesn't work, I'll pull the whole thing apart again and blow it out to make sure I don't have a plugged passage or something. If THAT doesn't work, I'll look at getting a more aggressive pump cam or nozzle. What weirds me out is that this carb was running fair on the car with this accelerator pump, jets, etc. before I pulled it off and "fixed" it... That ought to teach me (won't). Out of curiosity, when I turn the idle mixture screws in all the way, aren't I essentially cutting off the fuel flow to the idle circuit? Why would it keep running with a vacuum leak if it wasn't getting any fuel? Thanks for all you guys' patience. Quote Link to comment Share on other sites More sharing options...
MrWOT Posted May 19, 2004 Share Posted May 19, 2004 Depending on how your particular carb is calibrated you might have high enough vacuum to pull off your main jets. This is not uncommon for some carbs, just depends on how your carb is slotted or drilled. Could also have the float level too high causing fuel to spill from main jets at idle. When you rebuilt it did you use a new needle valve? Quote Link to comment Share on other sites More sharing options...
Guest Tourniquet Posted May 20, 2004 Share Posted May 20, 2004 Putting some of the last few posts together... I dont think it is a vacume leak (if there is enough vacume to pull from main jets). Also you said that the idle screws did virtually nothing? You are noticing more leakage that before from the accelerator pump? I dont know much about carbs but I know that if I was use to FI cars and I tried to rebuild one, I would probably mess something up... Maybe the rebuild was not succesful? Quote Link to comment Share on other sites More sharing options...
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