It's been bothering me lately that my car seems to fall on its face just shy of 5k rpms. It pulls HARD up that point, then it just stops. I end up shifting it at 4600-4800rpms. In park, it'll easily rev over 5k rpms, but under load it just doesn't pull like I think it should. The motor is fairly stock, rebuilt about 12 or 13 years ago. Ported and "Doc Dual Planed" intake with an 850cfm Holley Double Pumper. Factory style mechanical fuel pump (gauge at rail reads 6-7psi at cold idle, but won't give me a good hot reading because it's a cheapo Jegs liquid filled unit). Ported manifolds with dual 2 1/4in Flowmasters. Pertronix Ignitor III ignition, coil, and wires. Fresh plugs gapped at 40 thousandths (according to the plugs, the carb seems setup well). Timing is set at 12* initial (haven't installed a re-curve kit yet), vacuum advance is hooked up to the "ported vacuum" source. Royal Purple 10W-40 full synthetic in the motor. Trans is a '66 ST400 with the stock converter. I installed a B&M 400-3 full manual shift kit, with a fresh filter and fluid last summer. Stock 3.23 open rear end. I get that it in its current configuration it won't be a high RPM screamer, but I think it should at least be capable enough to pull a little higher. Any suggestions? Or am I just asking too much? Thanks guys!
Andy, did it pull past 5000 before? What cam do you have in it? When I first got my 66 it was a stock cam and at about 4600 it would still be spinning but had nothing left. I just figured it was the old "nailhead out of air thing"
Back when it was stock (still had the Dynaflow, factory intake, factory 4GC, stock single exhaust, and a different tach) I seem to recall taking it to 5500rpms a few times. It wasn't pulling hard at that point, but it got there. Now it seems as if it just hits a brick wall. Should be a cam fairly similar to stock. My parents are the ones who had the motor rebuilt, and they can't find any of the old receipts, so I don't know 100% what's in the motor. Not sure on the fuel pressure, don't have a gauge in the car, only on the fuel rail itself. I don't totally trust that gauge any ways. Valve springs should have been replaced when the motor was rebuilt, along with the heads. Again, roughly 12 years and about 12,000 miles ago. Could it be something in the trans holding it back? It rev'd fairly easily to 5300rpms just sitting it the driveway. My other thought was that the factory fuel lines and fuel pump just aren't up to the task.
Not sure on the total timing, all I have is a cheapo timing light. According to the factory timing curve, should be around 30-34* by 3800rpms. I'm running this air cleaner... It's a low profile Specre unit. I have not installed the ram air ducts yet though.
if its 30-36 should be fine more than that will start hurting top end. nice air cleaner. whats the filter size ?
i had a stock 66 gs skylark it was all done at 4400 rpm, put a 750 edelbrock and open air cleaner it pulled to 4800 rpm, it could go higher but was not making any power after the 4800 rpm.
Basicly you are only trying for 500 more rpm.... probably is doable,,, but your cam is most likely limiting you.... On my original engine , I was using the 091 cam... and it would go to 6000 but after 5500 it was just windmilling and the car had 4.11 gears too... if you had real low gears in back it probably do it.... about all you can do is ''fine tune'' it.... recurve the dist,,, go to a direct vac source rather than the ported one , richen the secondaries up a couple of steps... hook up the cold air tubes... but I think you are dealing with a stock, bread and butter cam.... if you are using the old points system,,, that can limit you too, if they aint set up right....eliminate them if possible.....
I would check the valve springs for sure as this was the cause for the issues we had with the dragster. Weak springs led to 13 bent valves. Oh-well. Get your springs checked.......
First things first . . . pull your spark plugs. Check and see if they're wet or if the porcelain is cracked. I've seen a similar situation where everything was fine when not under load, but as soon as you put a heavy load on an engine, the thing would fall flat on it's face. Two cracked plugs on a 4 cylinder engine. The thing had me dumbfounded for about 4 hours until i figured out the problem . . . :rant:
My old race car 70 Post sedan 455 with trans brake, 4000 RPM converter, ported irons and TA SP1/Dominator combo did the same thing. Springs were weak and the car just nosed over at 5000. Also I had a bad cam and did not even know it with the bad cam it ran 12.25- 12.50. With the weak springs and a comp torque cam it ran 11.85 first time out but still nosed over at 5K
Shouldn't be. If anything, it should be rich. I've been told the jets in it (80/78) are way to big for such a stock motor, but the plugs have always looked good. I'll check out the valve springs. Thanks guys!
Not if it's running out of fuel at the top end. A real-time fuel pressure test would be a piece of cake compared to pulling valve springs if you ask me. Try the easy stuff first. A fuel issue would also explain why you can't replicate the failure in park, there's no load. Valve springs don't care if the engine's under a load or not; you'd see the problem either way at the same rpm. Devon
it dont much matter if a engine is stock or modified.... what matters is the fuel/air ratio... and a carb tends to lean out as the rpms go up.... this is why the factories jet the secondaries some richer than the primaries.... and to get better gas mileage while just driving.... Like Devon said do the easy stuff first.... If the fuel pump fades away at high rpm,,, the carb will lean out.... and if you have a fuel pump that is fading added to the fact that all carbs lean out at high rpms... see where this is going.....:Brow: :Brow:
Check the fuel hose that comes from the tank. If it's old & cracked it could be sucking air & NOT leak fuel. If the hose is old replace it anyway.
I had this problem years ago chased it for 3 years til one day someone suggested blocking the return line and problem solved!! put a pair of vice grips on the return line n go for a ride. Mine was bypassing too much fuel. had a few friends this helped also.
The engine is an air pump. Look for fuel supply or air restriction before you move on to finer points. Tom Mooney
Finally got a fuel pressure gauge installed that I could monitor while in the car. Under any condition, I have not had over 5psi. Idle's at about 4-4.25psi. Under load, and heavy acceleration it doesn't change much, but has dipped below 4psi. It stabalized at just under 5psi as I cruised home, and didn't change much after that. So in review, under ALL conditions I've had between just under 4psi up to just under 5psi of fuel pressure at the carb inlet. Between those pressures, the car ran the same, and still had the issue right before 5k rpms. It's been sitting in the garage for about 10min, and the pressure still sits at just over 4psi. And just to reiterate the problem; right before 5k rpms, the car just hits a brick wall. It pulls hard to that point, then just stops. I seem to recall taking the car over 5k rpms in it's previous configuration, which leaves me stumped as to why it has less top end now. Thoughts? Insights? Advice? Thanks guys.
