OK, got the new carb (company sent me a new one....same model) Holley 4160 600cfm elec choke vacum secondaries.put it on and ran great for a good 2 days. Now all of a sudden it is backfiring through carb. At idle runs great...smooth. 2000 rpm and up backfires big time. Its timed by ear but has to be very close....was running very smooth and starting very ez when hot. Doesnt have a timing mark or indicator since it was a stock CC and I changed out intake (cyclone), carb, and distributor (non cc hei). Any ideas would be appreciated. Sprayed starter fluiid all around intake and base of carb and no change in idle so no vacum leaks.

 

carb wont have anything to do with back firing. you really need to get your hands on a timing light. and go from there.

 

I had a worn cam in my car and it backfired through the carb as well. I doubt it's that big of a deal, but you never know.

 

carb can cause backfire through carb, as I've learned by numerous people today.....but I have taken carb off and tried on another vehicle and it ran great...so no problems w/carb. Had a crack in top of dist. cap....so put old cap from cc dist on and it helped some. Now it runs smooth up to 3000 rpm instead of 2000..but then starts backfiring through carb again, also when you punch it fast it backfires. This was a new dist that had a defective cap, it is an aftermarket 65,000 volt. Would it cause problems with the dist having less than the 65000 volt cap/coil if the dist was set up for 65,000? I wouldnt think so, but thats why I'm posting here lol.

 

just a thought but the timing sounds like its advanced too far. try turning back the dist. a hair and see if that helps. also make shure you dont have too much fuel pressure

 

Get that timing checked. It could also be a lean backfire, but I'd guess timing at this point, particularly with a new carb. Are you running your vacuum advance and is it connected to the correct port? you say at 2000 rpm it backfires.....at 2000rpm+ under load? accelerating only? off the throttle going down the road at 2000+ rpm? Part throttle? full throttle? Need more info to help more. But easy things to check would be your centrifugal advance weights being stuck, the vacuum advance working correctly, and your base timing.

The carb can cause backfiring under certain conditions. A common one is when coming off idle it backfires, this is almost always due to having the idle screw turned too high on the primaries, exposing the vacuum advance port and letting the vac advance come in before the fuel from the accelerator pump shot makes it to the cylinder (lean, and overadvanced backfire). But at 2000rpm, this would be strange unless you are driving down the road, at 2000rpm then try to tip in on the throttle.
If this is your case, then set the idle adjustment screw slightly more open on the secondaries, and turn back the idle screw on the primaries a little. If you can get away with just turning down the idle on the primaries a touch try that first and see what happens. I have seen this several times before, but is usually not that common unless you are running a large cam, and like I said, usually off idle. If its a lean backfire its a hole other ballgame.

My bets are on your timing, especially if you timed it by ear, and its way too advanced.

 

Ok, I dont know of any way to check my timing, since it was cc it does not have any marks or indicator point if it did have marks. I've timed alot by ear and I know this has to be really close if not dead on. As for when it backfires (pops) through carb...Not at all at idle. If sitting in neutral and accelerate slowly no backfire now till almost 3000 rpm. If driving down road very ez on throttle no backfire till almost 3000 rpm. Never coming off throttle. Always if punching the accelerator hard it immediately pops big time, under heavy load...ie if shift too soon to higher gear it starts popping. Any ideas???

 

Hmmm....
I only know of one way for it to do what you are saying. And that is the timing is too advanced. A car will idle well with a very large variation in base timing, its when you are under load, or the engine speed comes up that you get into trouble if its over advanced. Which is exactly what is happening to you. Try disconnecting you vac advance line at the distributor and plug the line. Take the car for a drive and see if the problem is much less severe. I bet it will take more throttle before it happens. If so, retard the timing some, leave the vac advance disconnected, and go for another short run. Keep repeating until you can floor it without it backfiring. Even then you may be a touch too advanced. But once you can do this, reconnect your vacuum advance. You will probably have to adjust your idle screw each time you retard the timing some if the vacuum advance was plugged into the wrong port on the carb too. The correct port should have no vacuum at idle.

 

The CC engines all had timing marks. The tab is probably behind the water pump, straight up from the crank. You're probably looking over at the 2 o'clock position where SBC's used to have the tab. There is a mark on the damper that will be at 12 o'clock when the #1 piston is at TDC if your timing tab is at 12 o'clock (which it probably is).

Take another look. Until you have the timing right, you can't begin to tune the carb.

 

ok, I found the timing indicator I will put a timing light on it tonite. As far as the vacum advance line going to carburetor...I have it plugged into the port on carb as per pic and instruction on carburetor manual, but it does change the idle when I unplug the line at carb and plug the port. I have double checked and firing order on cap are right as long as #1 is in right spot. Old cap or new cap I could not find a mark for #1 cylinder so I put it in same spot as old one had which is 2 spots clockwise of the tab sticking out of cap pointing to drivers side where wiring hooks up.

 

The vac advance CAN be plugged into the other port, but I don't recommend it....And that could be affecting how you set the timing. Anyway, when you check your advance, check it with the vac advance disconnected (I'm sure you know this), but it would also be a good idea to make sure that you are idling at 600 rpm or less too. I say this because, depending on the spring setup in the centrifugal advance, it may start very early as well. Just something to check.

 

That typically isn't marked (but some have been), and isn't that important. What is important is that the rotor points to the cap terminal that has the spark plug wire on it that goes to the #1 spark plug when the #1 cylinder is in the firing position, and so-on through the firing order.

 

Ok well I put the timing light on it and if I'm reading the pointer correctly...not sure because numbers are almost impossible to make out but I think it has 4 tabs on the pointer and I think the top tab reads 12*btdc and I am hitting right below the next tab...so I believe I'm right around 8* btdc. And still popping through carb bad, I went throughout the entire range of the distributors play and nowhere did it take away the popping. Is there any chance the the distributor itself has something wrong with it. I'm just about at my wits end with this.

 

Ok just an update...I put new plugs in and checked compression in all cylinders. 125-140 on all cylinders, so I assume no valve, spring, or cam problems does this sound correct? Checked vacum 18 lbs strong, so again I assume no vacum leaks. Timing set at 8*. Fireing order is CORRECT. Tried carb on another car..pontiac 350 ran great, so again I assume it cant be too lean on a 305 if not on 350. Have a new 65,000 volt HEI and new plug wires coming tomorrow, will try that. Any other ideas.....I am really stumped here.

 

For it to backfire through the carb an intake valve must be open when the mixture is ignighted.

I believe that # 7 is on the intake stroke when #5 is on the power stroke.

I would suspect that the problem is a voltage leak from the #5 plug wire to the # 7plug wire, since they are next to each other in the firing order. If the # 7 plug was lit when the valve was open this could cause it.

It does not happen at low rpm because there is not as much resistence. when the resistence builds due to the cylinder pressure the voltage may be hunting a better ground and cross firing.



Check your wires and make sure 5 and 7 are well seperated. Also check the cap for a crack or contamination acting as a conductor.

Look inside the cap as well for cracks and contamination.

 

OK well just an update and a thanks to everyone who tried to help. Looks like JDSimpson wins the prize. Finally broke down and took it to local shop for their opinon (I'm so embarrassed) lol. Shane knows his stuff pretty well and after checking through everything says he's 100% sure that the cam is bad. So now I guess I get to build 2 motors...One for my 85 Z28 I've already begun, and one for my sons 83 camaro w/bad cam.
Merry Xmas all
Bo