Fuel starvation at 3000 rpm?

I recently overhauled the 4.2 engine and carbs, including new needles. Compression is 150 in 5 cylinders, 110 in one, but it is improving as the new rings settle in. Timing advance reads as prescribed. Car runs very well at low rpm. No detonation, no pre-ignition noted in driving or shut down.

BUT - When I floor the gas pedal, with rpm’s going beyond 3k, I can feel a definite loss of power - or at least a failure of the car to accelerate beyond the 3k speed. It is as if it gives me all it has up to 3k, then fizzles out and additional throttle opening yields no additional power output.
Any suggestions would be appreciated

Highly unlikely it is fuel starvation, which implies a lack of sufficient fuel to the carbs. FAR more likely your mixture settings are off, or the dizzy advance mechanism is sticky or seized up, or the springs are failing.

You might need richer needles. That happened to mine when my state introduced ethanol. The standard ones didn’t cut it any longer at higher RPMs or hard acceleration. It would stumble.

Before you start replacing needles, try using a heavier oil in your dampers. That will give you a richer mixture when accelerating. Enrichment can be fudged by installing richer needles, but it’s a separate problem. This can arise because the stochiometric ratio of ethanol is slightly richer than straight gas. You can adjust to a richer steady state even with stock needles, but performance under acceleration is a function of the springs and dampers and can be dealt with directly.

Also, if you have a car with a tach generator (TC), verify your tach is accurate, just to eliminate that variable. The magnets in the TC can weaken over time, yielding an under reading tach. I.E. when chasing rpm related issues, it’s good to know the actual rpm.

I’d check that the throttles are fully open when your foot is on the floor!


and twenty characters

Vlad to see if it’s an enrichment problem turn the enrichment screws down 1 turn and see if that improves things, or alternatively pull the choke on as you start your run. That’s easy to do, but how many miles do you have on the engine - wonder if it just needs a little more to loosen it up.

Similar to Terry’s suggestion, try it with some choke, same as turning the the adjustment screws. I had the same problem, richer needles solved the problem.

Neither suggestion will help under acceleration, they would only change the steady state a/f ratio. To change enrichment, you have to do something with the dampers, springs, or pistons.

Though fuel starvation may be unlikely, it’s easy to eliminate by testing the fuel pump. Just disconnect the fuel line at the carbs or at the fuel filter and put the hose into a container. Turn on the ignition (but don’t crank the engine) and let the pump fill the container - I just use a 2L soda bottle. It should be able to pump at least 1/2 liter (1/2 Quart) per minute. Always have a fire extinguisher handy for such tests.

1 Like

The elementary check re the problem described Is the fuel filter in the glass bowl Vlad, if it’s the early type gauze, check for blockage, if the later “basket”or paper type remove and road test.
Is the factory air filter setup fitted?
Triple SU or other ?

I would think if it were fuel starvation, there would be a definite miss when the starvation occurs. It sounds to me like you are not having a miss, but you have a smooth engine that will not continue to pull strong. I also would advocate checking throttle opening, air intake restrictions, timing. etc.

Thank you everyone for the superb suggestions.
The engine had 0nly 20,000 mi on it when I overhauled it [as part of a ground-up resto]
I fitted a new fuel filter, so I will remove and road test.
The problem is not that it doesn’t respond to pushing pedal down rapidly - as in accelerating - which could implicate improper oil in the dampers.
The problem is that it just doesn’t seem to have the ‘‘energy’’ to go faster past 3k rpm.
I am going to try richer mix via choke during drive.

Remove the air filter and take it for a run. Could be air starvation rather than fuel starvation. Also check TDC is correct as the timing pointer is moveable and may be in the wrong position so your timing is out.

David , if the air filter is blocked black smoke will issue from the exhaust, if the air filter is removed the engine will run weak at precisely the revs mentioned, this with UM needles in the SUs, if fitted.

Vlad , is the factory air setup fitted ?

To correspond with what Les had suggested above about throttles being full open when the pedal is at the floor, make sure that there is little slop in the linkage. I just went taking the slop out with help from the forum. Before that, the pedal had to move too much before actuating the throttles and I wasn’t getting full power.

How have you tuned the fuel mixture at idle? Have you done it by ear for maximum idle speed and smoothness or by putting the jets a number of whole screw turns down from the bridge (eg 2 1/2)

The reason I ask this is that I have seen SU needles recently that have the shoulder machined in the wrong position such that you can end up with the maximum idle speed and smoothness found with the screws turned 5 or 6 complete turns down from the bridge. This means the jets can be 0.125 thou down from the bridge to get best idle mixture

Dennis 69 OTS with triple SUs

Check to see if all three carb spindles are rotating, and they are clamped tightly together, maybe you have only two carbs operating.

1 Like

Agree, try heavier damper oil…easy to do. I had a similar experience…now use 20/50 …made a huge difference