I need some help diagnosing a high RPM stumble on my 71' 350 Buick. SYMPTOMS: 1. Strong acceleration under full throttle up until around 3500 RPM then the motor stumbles HARD and falls flat on its face. When I let off the gas it returns to driving normally. 2. Idles decent with no signs of a "miss" due to a bad spark plug wire, etc. 3. Idle seems low. 4. Car still has enough power below 3000 RPM to lay a good 20 foot patch. 5. Under light throttle the stumble is not present but the throttle response is a lazy compared to when it has a good tune up. WHAT I KNOW: 1. Fuel Pump is brand new. Old pump was leaking badly. New pump seems to have good flow at idle. I have a see through filter and fuel is flowing good at rest. 2. New soft fuel lines at tank, pump, and carb. 3. No fuel leaks. 4. Recently a pertronix ignition was removed and replaced with stock points. (NOT BY ME!) 5. Plugs, timing and carb were very recently tuned up. (NOT BY ME!) 6. Carb is relatively new and in a good state of repair (Edelbrock 4 barrel 600CFM) MY PLAN: 1. Replace points, condensor, check & set dwell and static timing. 2. Check function of vacuum advance canister. 3. Check & set carb idle speed, mixture. 4. Check for vacuum leaks at idle with vacuum gage. Any thoughts from the forum? Thanks!
"HEI swap and q jet for a start!" I do not believe this is a carb problem and I have no issues with the Edelbrock. The symptoms seem to be timing related. Any thoughts? Thanks!
When I had a problem with too much advance my car would fall flat on it's face at the top of first gear. And there is nothing wrong with points ignition. I always see people saying stuff in an HEI. If you take the time to keep things in good repair your points ignition is just fine. That's my .02
I kinda doubt it's timing this time, but thats easy enough to check. It sounds like lack of fuel even though you have a new fuel pump. It might be low float level too-- that would do the same thing.
I'm guessing fuel delivery, too. A gage is the only way to know what's up as the rpms climb. Even though the lines appear to be ok, another culprit can be the sock in the fuel tank, it's basically a screen that prevents coarse contamination from going downstream. May be partially blocked. Devon
Another shot from the hip... Check your advance wieghts and see if theyre extending into the lead that powers ya points.. Mine did same exact thing yours did and when the wieghts would fully open theyd ripped insulation from the wire and would make contact and arc out.. Car dropped in power significantly until lower revs and the wieghts came bak in. That wire starts to ground out it feels like its losing 100 horses. Time for HEI just a guess
Points are awesome... Especially with a windowed rotor to make running adjustments.. Talk about Fun.. You wanna see something really cool.. Adjust those points to about 20* and youll start shredding tires like steve mcqueen driving a trans am in a corn field backwards.( just don't forget to put them back to 30* or theyll burn up in about a month or two or youll need to get a new cap too) I love playing with my points.:laugh: *edit msg* Sometimes i like to mess around with things . I don't advise messing with your points unless you've got a dwell meter. Too easy to screw up point gaps with not having a dwell meter or feeler gauges. So if you're not familiar with playing with them don't bother. Youll be cursing me and my offspring for generations to come. Last thing i want is a newby to read this and go outside and screw up Dad's car tryin to make him go faster on his way to the newstand.
So what is proper timing for great throttle response, etc on a 350-4 with TA stage 1 intake, headers, holly 750cfm carb, pertronix ignition. Mine seems to run just fine, but it would be nice to know I'm gettng the most out of it. Can't recall what it's set at currently.
Most of us like to use 10-12 intial timing, 20-22 deg of mechanical (limited from 30 stock in my case using bushings), giving 30-34 total timing all in by 2200 rpm using the light springs in the crane vacuum advance kit, and the vac advance limited to 10 degrees on top of all that.
:grin: Anyone besides me think it may be wek valve springs, not closing properly and floating the valves at high RPM??:spank:
There is no doubt that any 70's Buick 350 has weak springs simply from age. I suggest cam/lifters/springs if you open up the motor. My vote is a problem with the points....
I think it's too few RPM's for that, and I inferred that it falls on it's face hard as opposed to sorta not pulling or a slight drop in power...
Its not valve float. Its way too low in the RPM range and I know what float feels like. This is a hard stumble almost like hitting a REV limiter. I have owned this car for 20 years and had it to 5500RPM with no float at all. My current GUESSES are: 1. Points-dwell-timing issue 2. Fuel flow from the tank to the pump (Possibly the new soft lines collapsing under suction pressure) 3. Coil problem Thanks for the feedback!
:spank: :spank: Sorry, I missed the 3500 RPM in your original post. I'll back up and regroup!!!:laugh:
I would advance the timing more and see how it reacts to that, if at all. If that makes it better then it could be an advance mechanism type of problem. I would also tend to think it could be a point bounce type of problem. Whens the last time you replaced them? Maybe you should just drop 7 bucks a replace them, its cheap and gets rid of one more possible problem, and check your dwell too. Maybe your distibutor cam is worn too, although im not so sure it was cause a problem like this. If none of that fixes it, I would move to valve springs. Does it do it if its in park or neutral and you rev it up, or only under a load?
