Acceleration stops at 50mph - max speed=50mph

[Matrix]

While i am waiting for the parts - i got new needle valves, i am waiting for diaphragm, gaskets, floaters and valve springs -
i installed a fuel pressure gauge one hour ago and i got a reading of 5 psi from the fuel pump.

Does anybody know what should be the fuel pressure in our cars? (my version is a Stromberg 150CD, 1968 model, MK1)

I am suspecting that even if a fix the issue with the carbs, that i will have the same issue again in the future if the pressure is too high.
Do you believe that 5 psi will cause issues such as overflow of the carbs?

[Alec]

Hello Matrix,

If you measure the pressure with the engine off, I suspect you will have a higher reading than with it running. That said I dont beliecv that a functioning needle valve will not shut off the fuel flow at 5 psi?
You can do a practical test by disconnecting the coil and crank the engine for a while and see if the carburettor floods?

Alec

[Matrix]

Hi Alec, i just found from the operation manual that the pressure range of the fuel pump must be : [1,5 - 2,5] psi
So 5 psi is too much as it seems. Now i have two problems: carb leaking and too much fuel pressure.
I would not risk to fix only the pump issue. I think i will replace the valve nevertheless.
Perhaps a spacer between pump and block will mitigate the value of pressure from what they say - if not i will throw inside the pump a Depon!

[Mike Stevens]

Checking the Mk1 ring-binder manual agrees with the figure of 1.5 - 2.5PSI pressure but it also states a delivery of 1.5pints/minute (minimum). Although I'm not sure how you'd measure that without a proper flow meter. However, most analogue meters have an accuracy of 5-10% of full scale, so your 5PSI could really be anything from 1-10PSI! My vacuum meter has an offset zero so it can read above and below atmospheric pressure and has a full scale of much less than 140PSI so should give a more accurate reading.

On reading this topic, my immediate thought was "fuel starvation" such that the engine runs very weak at higher engine loads. That might explain the white plug colour. One thing that can happen with long periods of unuse is that dirt and/or rust in the tank can cause a partial blockage between the tank and the carbs thus reducing the fuel flow rate at higher engine loads. It might also explain a stuck needle jet. I test the needle jet with a carb removed from the engine and float chamber removed by blowing down the fuel inlet pipe holding the pin in the needle valve shut and also with it open. One can easily exceed a few PSI with this method!

Dashpot oil. The dashpot is used to slow down the rising of the piston (in both Strombergs and SU carbs). This has the effect of slightly enriching the mixture until the piston stops moving so that the engine can accelerate a bit easier. The amount of enrichening will depend on the viscosity of the oil in the dashpot: a heavier oil will slow the dashpot more than a thinner oil, thus producing a richer mixture, and vice-versa. The amount of enrichening required will depend on many things including engine and air temperature etc. I've always used ordinary engine oil (20-50) as it is in the oil can ready for use. For ordinary road use I can't really believe that what oil weight you use really matters. I had a friend who used to race the smaller triumphs and before each race he used to suck out the old dashpot oil and replace it with new. He also admitted to me that the real reason he did it was to put the opposition off!

Once you have the fuel leakage issues sorted it would be worth trying to measure the fuel pressure in a dynamic fashion so you can see what it is doing when you get stuck at 50MPH.

Good luck!

[Forkie]

I dont think any spacer is required between fuel pump and block - just a paper gasket. I believe a spacer will interfere big time with the timing of the pump, as it is offset to the cam rotation.
Re fuel pressure - ok, mine runs on S.U.s - the fuel pressure should be around 3 psi.
Re dash pot oils (Mike) - this has ALWAYS been a contentious issue. When i was running early BL stuff, it was always 'engine oil' ( 20/50). Nowadays, it would seem '3 in 1 ' oil is the preferred choice
I ask this, as even after a very expensive autobox rebuild,which has undoubtedly helped, the car still doesnt feel ' up there'.

[johnfrancis898]

I recently discussed what we put in our output dashpots at a car meet. For me it’s always been 3 in 1 oil. One guy preferred TQF (PAS/Auto fluid). The other guy used ordinary 20/50 mineral oil. I recently refurbed my twin SU HS6’s and talking to Burlens the guy recommended their stuff but agreed 3 in 1 was ok. I relented and bought some of their stuff but looking at it I’d say it looks the same as 3 in 1 or 20w/50 for that matter. He wasn’t keen on TQF. Apparently the SU recommendation was always 20w oil so I’m told. Having debated this we all agreed our cars ran ok when properly tuned!

[Matrix]

@Mike, Finally after some trials the spark plugs of the rear carb which leaks got a black color and the plug of the front looks healthier (see attached photos)
regarding the precision, i always thought that the 10% error is applied on the measured value always, which would translate to either 5.5 or 4.5 PSI.
If it is applied on the full scale, then i have bought the wrong instrument for my case.

@Forkie, i got 10 paper gaskets, and i am thinking via trial and error to start by applying 5 gaskets and measure the pressure until i get it right.
Question: do i need to apply gasket dressing on each of the 5 paper gaskets ? Or it is adequate to just install the gaskets without it?

@johnfrancis898, @Mike For the dash pot oil i am quite sure that i have read in two different sources (manuals) where one suggests engine oil, and the other light oil (Singer).
I am using the second.

Before the leak, a new fuel pump was installed and the mechanic used compressed air to clean the hoses. Maybe this is the point where dirt was inserted into the carb ?

I am waiting for the service kit of the carbs: i bought floater, gaskets, diaphragm, valve spring, and needle valve.
I have not removed carbs before and i am quite sceptical regarding the best approach.
I am thinking of just removing the fuel hose and remove the whole system as is with both carbs and linkages as they are in order to avoid tinkering with the linkages.
This or i could attempt to unscrew the linkage and remove only the rear carb. What do you think ?

As it is now i have no other option than to attempt to fix it myself. I cannot risk to travel to the mechanic with a fuel pump which *very possible* overflows carbs, and also rear carb
has these issues. I smell fuel when i start it, and this projects a dangerous situation to me.

[Forkie]

When you say you have '10 gaskets, but will start with 5' - only ONE gasket is required, at ANY joint.. looking at your plug pictures - they both look dry, which is a good thing, the rear plug does look a tad rich - have you tried weakening the rear carb down a tad?

[Forkie]

Just re reading from the start - you stated a few posts back 'the rear carb is leaking' . Leaking, i assume fuel, but leaking from where....

[Firewatcher]

Hi Matrix,

Have some confidence - we all have to start sometime, but yes take the two off together complete with linkage, just be careful when you are moving them about. Better to loosen the top diaphragm screws while still in the car to save struggling with them on the bench. Also have something to catch the petrol when you tip them over as you will have a cupful out of the bowls.
Nothing special about tuning them, for normal use:
Make sure the air valves drop freely all the way down when you have finished reassembling them (before putting the dashpot damper in).
With the Air filter off, on tickover, check the suck sounds even by holding a hose to your ear at each of the inlet ports.
With engine warm, (no choke) check the mixture by lifting each air valve about 3mm with a slender screwdriver through the inlet port - revs should stay the same or drop off slightly.
I have always been happy with Strombergs. In over 50 years, I have only replaced the balance tube once, and gone through quite a few top diaphragms. The main jets, needles and needle valves are all original. My motor is a workhorse and still used every day, so it has seen some miles. I just use whatever oil I happen to have in the can at the time for the dashpots and never had a problem.

As Mike said it does sound like starvation and these older motors do have some debris lying both in the tank and fuel lines - changing the fuel pump may have disturbed some that is now giving you the flooding. The rubber connectors also tend to perish, which again can get through and lodge in the needle valves to cause serious flooding. The filter will help in future, but it is definitely worth double checking the line from the tank through to the pump. While you are there just check also that your petrol tank vent is clear and you are not trying to suck on a closed tank.
From what I have read it began with a fuel starvation issue which the new pump didn't resolve so an unrestricted fuel supply is the first thing to establish.

Clearly you are concerned about the new pump pressure, but if you still have the old one it would be better get a repair kit and refit that. It is fairly straightforward, quite likely that the non return valves are still serviceable, so if you are worried about peening new ones in, just leave them and replace the diaphragm. I must say I agree with Alec that a clear needle valve should be fine with 5 or 6psi even though it is above that recommended for Strombergs, so I wouldn't do anything else with the pump until you have made certain that the needle valves and fuel lines are clear.

The colour of the plugs at this moment doesn't tell you much because you still have the fault.

Whereabouts are you, you might have one of our Elite on your doorstep.
Pete