Acceleration stops at 50mph - max speed=50mph

[Matrix]

Hello all,

My issue is that when accelerating, the car reaches max speed ~50mph and no matter how hard i push the pedal the speed stays there.

Carbs have been rebuild 4 years ago.
4/10/2024 : I have replaced the original AC Delco glass fuel pump with a Powerline MK2 style pump : i had fuel starvation issue, severe deceleration even at 35mph, losing power when pressing the gas pedal.
Also a humming noise i heard when accelerating, was eliminated after the fuel pump replacement.
I have changed the diaphragm and hoses of the EGR system - one hose was ripped and the existing diaphragm was too thick seemed like two diaphragms connected. Diaphragm change was
a suggestion by my mechanic.
I have checked all spark plugs and they have a white color. Spark plugs are replaced with new ones 2 weeks ago.
I had replaced coil a month ago to defeat the issue of difficulty with engine start after a trip of 20 miles. (from what i understand, this had to do more with the overheating of fuel pump,
but i left the new coil in place)

Any hints what else to check please?

[johnnydog]

Is your car is a late MK1 or early Mk2 2000 with Strombergs?
Plugs that are white would suggest an extremely weak mixture which certainly would be one cause of the symptoms you describe regarding the lethargic performance.
Although the carbs have been rebuilt, it does not mean they have been set up correctly afterwards. Following a carb rebuild, they do need properly setting up - throttle linkage, mixture, air flow, needle valve (float) height, balance etc etc. A sticking piston in one carb (not rising / falling freely with a nice clunk as it contacts the bridge) can also show similar symptoms.

[Matrix]

It is a late mk1 (1968) which has Strombergs. The strange thing is that after the rebuild and setup, the plugs always had a dark brown color indicating a setting, slightly richer than the optimal setting.
But nevertheless i completed regularity rallys, run hundreds of kms without issues.
After the issue with the pump* i also replaced plugs and added thin oil in carbs (singer oil). With the new plugs, the color remains white, as if the settings were changed.
But i have not touched any carb settings and what is weird is that all plugs are white which means that both carbs were "detuned" somehow.


*pump issue: at first max speed was restricted to 75 mph. Then one day it decided to max at 50 mph and decelerate as i stepped more on the gas pedal. Then the same happened at 35 mph.

[johnnydog]

Are the new plugs the correct spec and the correct heat range?
What plugs were in originally? What did you replace them with?
As this appears to have happened since the plug change that's why I'm asking about the type of plugs you've fitted compared to what was in previously.
There are different schools of thought about the correct oil for the dash pots - some state engine oil 20w/50, some say a thinner oil. I personally go in the middle with a 10w /40 or a 10w/30 (depending what is on the shelf!) and they seems to perform ok.

[2500-Dreamer]

Could it be that the acceleration pedal cant travell all the way? Something shifted in the throttle linkage? Carpet?
Or does it realy cut out.

[Matrix]

I always use Champion N9YC as the manual dictates.
The accelerator pedal is ok, no obstructions. The needle stays at 50mph (the speed is not reduced, the acceleration is being cut out)
and after stepping down the gas pedal, it managed to do 55mph at some point but the engine worked overtime for this

[Alec]

Hello Johnnydog\Matrix.

The S.U. manual states 20W straight oil. In practice I used 20W50 engine oil when I had carburettors.

What is the acceleration like in the lower gears or doesn't it matter? A thing to check is that the distributor mechanical advance is not seized.
If it is still lack of fuel supply, a simple check is to find a quiet road with a layby, and run the car until it stops accelerating, cut the ignition and declutch simultaneously and coast to the lay by and check the float chamber fuel level. Also, the colour of the spark plugs may give a clue.

Alec

[Thorsten]

Hi all,
Regarding the carburettor oil, I'm quite sure the recommended oil for the Strombergs was ATF. This certainly won't cure the acceleration issue I suppose.
Best regards
Thorsten

[johnnydog]

Hi Thorston, yes I have also seen recommendations about using ATF in the carburettor dash pots as opposed to oil. Not tried it myself, and not sure how it would be affected by temperature changes? Probably stick with oil personally!

[Matrix]

Hi all,

My car (late 1968) has Stromberg carbs, and my mechanic suggested using thin oil such as
Singer All purpose oil, which i did. Maybe i overdid it? Is it possible to remove it ?
(but i did not fill it all the way up - only put at the base of the carb)

Updates that were done to the car:
1. The new Spark plugs i installed remain white : before and after new fuel pump.
(new fuel pump cured other issues of severe deceleration even at 35mph, and a humming noise when stepping on gas)
2. Coil in replaced with a new one i had (Lucas from Witor)
3. Diaphragm of EGR system is also replaced.

The car accelerates normally up until 50mph. When it reaches this speed it does not increase. Its max speed is 50 mph now.

My mechanic says that when the weather changes from winter to summer and vice versa the carb needs a tune. But i do not believe this! My father never had the carbs tuned, they had a standard setting and the car travelled easily at high speeds whether it was summer or winter!

I would assume that something happened to the carb. But all spark plugs are white so i find it strange to have both carbs malfunctioning.

I will check the distributor as you suggested.
I am very puzzled with what happened and my mechanic told me to visit him and have the carbs tuned to use a richer mixture. But i am not sure that my issue has to do with carb tune up...