Engine Misfire/cutout

[Joseph K.]

I have been working on a engine misfire/cut out. On my last leg of the 200 mile ride back from Wildwood my engine began to misfire at 2,500 RPM under a load. Hear is the data.

1. 69 - 350/350 Manual transmission
2. rebuilt motor with only 1,200 on the motor
3. all the wires, cap, rotor, points replaced at rebuild (service replacements)
4. Fuel pump replace during rebuild(service replacement)
5. Rochester carb miles ago
6. Just replaced the coil
7. points and timing spot on
8. I can see the plate on the points plate rotate when the engine is reved up
9. the vacuum 20 at an idle with a little wiggle in the gage
10. vacuum increases when the engine is reved.

Now with that said, the engine begins to misfire at 2,500 under load and if I push the pedal to the floor the engine will die if I do not let up on the pedal. Other wise it runs great. I think is gas related. any ideas?

Joe

[Timothy B.]

Re: Engine Misfire/cutout

Joseph,

I think you should swap out that coil and try the original or a known good coil if you have it.

[Tom H.]

Re: Engine Misfire/cutout

Possibly a rocker arm has come loose ????

[Clem Z.]

Re: Engine Misfire/cutout

check the spark plugs,if white and clean you have fuel related problem and if the plugs are dark you have ignition related problem

[Paul O.]

Re: Engine Misfire/cutout

Joseph

Had a similar problem happen about 9-10 years ago with my LT-1. Driving back from Bowling Green when accelerating heavier then normal loading the engine would misfire and almost die. Found 5 spark plugs fouled with a tan or beige heavy crusting. It could have been incorrect heat range plugs or some bad fuel replaced the plugs it then ran all normal. The plugs that were in the car had only 1800 miles on them. Hope this helps.

Paul 18056

[Kevin G.]

Re: Engine Misfire/cutout

Possibly the fuel sender has an crusty old clogged sock on the pick up tube?

[Paul J.]

Re: Engine Misfire/cutout

Joe,

You can try the stuff previously mentioned, I like Clem's answer. But I'd start with the simple things, replace the fuel filter first.

Paul

[Joseph K.]

Re: Engine Misfire/cutout

Thanks gents,

Tim, The coil was 2 years old and I replaced it last week with another new one. I had the same thought but there was no change after I changed it.

Tom, I have not adjusted the rocker arms since I rebuilt the motor. I originally adjusted them 3/4 of a turn after the clatter stopped. The engine is quiet no clatter and no missing until loaded.

Clem/Paul O, I like your spark plug idea. I will check them after work. You and Paul seam to be on the same wave length.

Kevin, Good thought on the fuel sock but I replaced all of that on the rebuild.

Paul J, I will check the plugs first then replace the fuel filter. I replaced them (carb filter and canister) on the rebuild but I might have gotten a bad batch of gas in Wildwood. The canister filter is very expensive.


Any body had a fuel pump go bad in 1,500 miles?

I will get to work and let you know how I make out.

Thanks Joe

[Clem Z.]

Re: Engine Misfire/cutout

Thanks gents,

Tim, The coil was 2 years old and I replaced it last week with another new one. I had the same thought but there was no change after I changed it.

Tom, I have not adjusted the rocker arms since I rebuilt the motor. I originally adjusted them 3/4 of a turn after the clatter stopped. The engine is quiet no clatter and no missing until loaded.

Clem/Paul O, I like your spark plug idea. I will check them after work. You and Paul seam to be on the same wave length.

Kevin, Good thought on the fuel sock but I replaced all of that on the rebuild.

Paul J, I will check the plugs first then replace the fuel filter. I replaced them (carb filter and canister) on the rebuild but I might have gotten a bad batch of gas in Wildwood. The canister filter is very expensive.


Any body had a fuel pump go bad in 1,500 miles?

I will get to work and let you know how I make out.

Thanks Joe

make sure the gas cap vent is working. try driving the car with the gas cap loose. i had trouble with my riding mower shutting off and i found the gas cap vent plugged

[Clem Z.]

Re: Engine Misfire/cutout

another thing to check as i have seen this happen when some one installed the heat riser valve upside down and it could not open

[Domenic T.]

Re: Engine Misfire/cutout

Joseph,
Can you get high RPM's if you slowly give it the gas?

If not then I can paint the worst.

I worked on 2 cars with new engines that never got over 2500 RPM's.

I checked all, fuel filter, fuel preasure, timing, valve adjust, distributor, exhaust restriction, carb, and had good spark.

Found it in the cam timing.

Believe it or not the pin on both cams that index the big gear were in the wrong place on both cars.

These happened about 2 years apart (the cam problems).

The cars ran great and idled just fine, they just wouldn't go past 2500 RPM.

Both cams were new and I spent a few days finding the first bad cam pin.

I think the error was equal to about 3 teeth on the gear.

If you can rev over 2500 slowley then that will eliminate the worst case.

DOM

[Clem Z.]

Re: Engine Misfire/cutout

if it is a stock GM cam i would check for split overlap. both tappets should be equal lift at TDC on that cylinder. not a fool proof way but should tell you if the cam is way off. you would have to remove the intake to do this

[Stuart F.]

Re: Engine Misfire/cutout

Under WOT, one tooth off will get you up to about 5600 RPM like a bear, then cutoff. Don't ask me how I know, but a shop did that to me once and I threw everything I could think of at that engine working the cutoff up from 5300 to 5600, then I found bent push rods. That was a 335hp 348 engine.

Another friend for whom I built a set of wicked tri power carbs for his Pontiac called at 2 am one morning to claim my carbs were crap as he couldn't get his new engine to go over 5600. I yawned and told him he put the cam in a tooth off. The next day at the strip he sheepishly asked me how I knew that. I told him just intuition.

Stu Fox

[Jack P.]

Re: Engine Misfire/cutout

check the spark plugs,if white and clean you have fuel related problem and if the plugs are dark you have ignition related problem

I am experiencing the same problem. L-79 , My plugs look great in that they are white and clean,

So Is it the Holley, fuel pump, or fuel filter

Jack

[Jack P.]

Re: Engine Misfire/cutout

Joe,

You can try the stuff previously mentioned, I like Clem's answer. But I'd start with the simple things, replace the fuel filter first.

Paul

Who sells the filter that goes in my Holley, attached to Carb on inlet fitting

Car started missing and backfiring after I ran low on gas. Will accelerate steady but will miss if I put foot down fast, almost sounds starved for gas between 1800 and 2800, but will cruise at 3200 and above no problem

Jack