Thread: New manifold,cam,and carb,engine stumbles badly at take off

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I have a Chevy 305 coupled to a Turbo350.I just put on a new Edelbrock Performer manifold,mild cam,and Demon 525cfm carb.It seems to idle okay,but sometimes when I punch it,the carb back fires.Everytime I punch it at a stop sign,the engine stumbles badly and almost dies.Once the Monte gets going it runs okay.However,at cruising speed sometimes I get a mystery back fire from the carb.What should the timing be,and how about the idle?Could I have bad plug wires?I've done everything the carb manual tells me to do.Any help would be greatly appreciated.

Which cam did you install? What's the lift? Did you degree it in? Are you a tooth off with the dist.? Sounds more like timing to me than your carb.

I don't have the specs on the cam right now,but the shop told me it was pretty mild.What do you mean by degreeing it in?If it was one tooth off,wouldn't it backfire everytime I stepped on the gas?It's a random backfire.The engine idles pretty good.One tooth off can also cause the engine to stumble badly?You're right though.I don't think it's the carb.What should the approximate timing be?Any more ideas?

I bought the parts from them.I did the installation myself.I've never run int this problem before.It has to be in the timing or plug wires.I've always had good luck with out of the box carbs.

What kind of distributor do you have, and where did you hook up the vacuum advance?)

The BG demon has two vacuum ports on the right front of the carb. Most distributors should be connected to the front port.

Yes,it's hooked up to the front port.The tranny shifts fine .I have a stock chevy HEI distributor.I'm going to check the plug wires,and make sure I dropped in the distributor right.

Disconnect the vacuum line to the distributor, plug the carb port, then check your timing at idle and at about 3500 RPM. You may have to add a timing tape. That should tell you a lot.

Re-check your valve lash. If they're hydraulics, set them at zero lash plus a quarter to a half turn.

Have you checked the float levels?

Demons are usually darn good right out of the box, and your problem sounds like something other than carb.

valve adjustment ???? sticking vavle?????

The engine idles good.Could it still be the valves?I'm thinking the distributor might be a tooth off.

sometimes you won't notice a valve problem untill you rev it up a little bit. As a fact thats how I got my engine. the guy was having problems with it backfiring through the TB, tried a lot of things and couldn't fix it, ended up pulling it for a crate motor, and selling the old one to me for $200. The problem? an Intake rocker arm pushed though it self to the valve stem. Moral of the story is I had got an engine with 60,000 miles, needing a $2.50 fix.

HOWEVER, you have a timing problem, I had a similar problem a while back, I changed the distrib. and reset timing, no problem sense then. timing is a 4 degrees advanced on the balancer. I would tell you total timing, but i never learned how you figure that out. sorry

Yeah,I'm pretty sure the problem is in the timing.I've also got the timing set at 4 degrees advanced,and the idle is good.Is this still possible if the distributor is one or two teeth off?I'm working on a 1970 Monte.What do you have in your car Monte Man? Anyway, I'm going to work on the car this Friday.I'll let you guys know what happens.Thanks

I finally had some time to work on my stumbling 305.I pulled the distributor and found that the weights were frozen.I dropped in a brand new one .The engine still stumbles the same.It just dawned on me.Could my mechanical fuel pump be bad?I'm thinking maybe it cuts out under load.Could my valves be too tight?What are the symptoms for both of these?Please help!

My son-in-law had the same problem with his T-bucket. It was the distributor. He had it recurved, and it worked fine. However, if your distributor is new, the curve is likely close.

Are you SURE you have the distributor installed correctly and all the spark plug wires in the correct order: 1-8-4-3-6-5-7-2, clockwise?

Bring the engine to #1 TDC. On the distributor base, mark the location of the #1 spark plug tower. Pull off the cap and make sure that the rotor is pointing at that tower. If it's not, the distributor is not installed in the correct position.

Also, 4 degrees BTDC sounds pretty low for initial timing. Have you tried kicking it up a notch or two? 12 - 18 degrees is not unusual.

By 12 to 18 degrees,you mean before or after TDC?

Okay,guys I got it!It was the darn fuel pump.I've never had trouble with fuel pumps,so I didn't even think about it at first.The Monte runs real good considering it's a 305.