TheSamba.com :: View topic - High fuel pressure fixed but new problems...too lean?

Sun Aug 16, 2020 1:31 pm

First time posting on here so please be gentle.

I've a fairly new engine running twin carbs and have had issues with some seriously high fuel pressure. The original fix was a fuel pressure regulator but a different mechanic suggesting shimming the fuel pushrod slightly.

This has solved the fuel pressure issue but now I have a new problem.

Firstly, the engine starts better when cold and runs great. After about 10 miles, however, feels lumpy and doesn't idle.

Are the two things linked? Does it sound like it could be running lean?

I've searched the site but can't find anything matching this scenario.

Any help or thoughts would be appreciated on this one...

ETA
One thing to mention is that I am not convinced by the oil thst was put into the engine (running in oil). Before I spend too much money, could it be as simple as changing the oil?

Sun Aug 16, 2020 4:27 pm

Welcome
You will receive lots of help here for every thing on your car but you will
Need to provide more informative info about this
Engine size
Carburetors
Ignition
Fuel pump
Keep a record of what the mechanic tells you. Fuel pressure for most carburetors on our cars is 3- 5 psi
If you are using an electric pump then it needs to also have regulator

Sun Aug 16, 2020 5:23 pm

I'm not a believer in the "shimming the fuel pump" thing to reduce pressure as this reduces the pump push rod stroke & reduces the VOLUME... Pressure is created by the spring in the pump acting on the diaphragm. If you reduced the pump volume enough to reduce the pressure, yes it will run lean because you do not have enough push rod (pump diaphragm) travel to supply fuel...

Sun Aug 16, 2020 5:48 pm

Keep in mind your dual carbs don't have chokes or "cold start enrichment" devices on them.

So, in theory if it starts and runs good cold but not good fully warmed up then your basic jetting and mixture adjustments are probably on the rich side.

Sun Aug 16, 2020 7:13 pm

klroger wrote: I'm not a believer in the "shimming the fuel pump" thing to reduce pressure as this reduces the pump push rod stroke & reduces the VOLUME... Pressure is created by the spring in the pump acting on the diaphragm. If you reduced the pump volume enough to reduce the pressure, yes it will run lean because you do not have enough push rod (pump diaphragm) travel to supply fuel...

Shimming the fuel pump will reduce maximum possible pump diaphram stroke a tiny bit, which is rarely of any importance. There is no direct mechanical connection between the push rod
and the diaphragm. The "stroke" of the diaphragm is determined by the fuel level in the carb and the pressure of the needle valve against its seat,
not by the push rod stroke unless the car is operating at the extreme level where it is using more fuel than the pump can normally provide.

Shimming the pump also results in a miniscule reduction in output pressure, because it will reduce the diaphragm spring compression by the thickness of the shim.
It is not a practical method of solving output pressure problems.
A single extra gasket under the pump can only reduce pressure by about 1 or 2 tenths of one psi.

Mon Aug 17, 2020 12:44 pm

Thanks for the replies everyone. Reading with interest.

I'm not mechanically minded in the slightest so bear with me :)

1915
weber 34 icts

The fuel pressure gauge my mechanic used showed a pressure of over 5 before he shortened the rods. The pressure is now 3.25.

After the problems, a week or so later, I did drive it for about 6 miles without incident in the hottest day of the year and nothing happened...I was a little nervous about going too far and the weather has been terrible ever since. I'm assuming there is something wrong though and I just didn't drive far enough.

Mon Aug 17, 2020 2:23 pm

Did you check to make sure your gas tank is vented properly and the vent hose is not plugged?

If it is there will be suction on the gas tank and the gas won't flow. If you remove the gas cap and hear a swoosh sound then there is suction on the tank and you need to fix the vent.

Tue Aug 18, 2020 4:07 am

74 Thing wrote: Did you check to make sure your gas tank is vented properly and the vent hose is not plugged?

If it is there will be suction on the gas tank and the gas won't flow. If you remove the gas cap and hear a swoosh sound then there is suction on the tank and you need to fix the vent.

Thank you for the suggestion.

I just popped into the garage and checked. There was no swoosh sound.

Wed Aug 19, 2020 11:03 am

I thought I would update this in case anyone has similar.

My mechanic has said the carbs were way off. One was running very lean with almost no fuel getting to it. The other was running more at 80%. He has fixed this and adjusted the idle which dropped when he fixed the carbs.

He isn't convinced this was the problem and wants me to take it for a proper drive on the weekend.

Will keep this updated :)

Wed Aug 19, 2020 11:44 am

Did you take apart the carbs and clean them initially and set the floats properly?

Fri Aug 21, 2020 12:28 pm

74 Thing wrote: Did you take apart the carbs and clean them initially and set the floats properly?

Yes.

I'm taking it for a proper drive tomorrow so fingers crossed...