Oil for the carbs

(peder) #1

Am I right in claiming that too thick oil, such as the 20-50 engine oil, will hamper acceleration, as will an insufficient amount?
Can´t find the SAE 20 oil, but understand that ATF will work just as well!?!
One of my engines, in a 420G, hesitates when I press the pedal at low speed and revs. Will check on the level and viscosity tomorrow.

(Christopher Potempa) #2

If topping it up with the correct oil doesn’t fix it then consider the vacuum and centrifugal advance on the distributor. It was frozen on mine and as a result, acceleration was very ragged. Even worse if I really gunned the motor. After freeing up the distributor centrifugal advance weights (much dirt and congealed/ glue-like oil) and replacing the vacuum advance unit it was back to smooth running and acceleration. I once read that nine times out of ten we automatically go to the carbs when the majority of the time it’s an electrical issue. Not saying this is where your fault is for your motor, but it’s something to consider. Good luck to you.

(Roger McWilliams) #3

S.U. Workshop Manual on page A.3 for the oil reservoir in the hollow piston rod “… with thin engine oil S.A.E. 20 preferably (but no thicker than S.A.E. 30), and this topping-up level is not critical; …”

Straight 30 engine oil can be found. Also, 10W-30 is readily available, the W weight not being likely important in most S.U. uses.

Ignition timing, distributor advance (mechanical and vacuum) curves often are checked before focus on carbs. Gummed up action, incorrect springs, vacuum not connected or operating as assumed are common in old engines.

(Old Ed) #4

Have used 3 in 1 oil for years, it’s 20 weight, available in just about any store.

(69 FHC ) #5

I’ve always used Marvel Mystery Oil, but from what I’ve been reading today it’s viscosity is too low… I think I’ll pick up something labeled as 20 wt. oil, 3 in 1 perhaps.

(Rob Reilly) #6

Google turned up a number of sources for this stuff, but Moss was the lowest price.


(Christopher Potempa) #7

Motorcycle front fork oil in 20 wt is another source as well.

1 Like
(Lee140FHC) #8

And since fork oil comes in various weights, you can make your own custom blend. BTW, ATF is generally considered to be about 10 wt. Also, see the thread about slightly modifying the damper rod piston assembly.

(69 FHC ) #9

I bought a 1/2L bottle of 20W fork oil yesterday at a local motorcycle shop. Ten bucks for a lifetime supply of dashpot oil. To the eye it appears to be significantly thicker than the Marvel Mystery Oil I was using in the dashpots. I’ve no idea if my butt-dyne will notice any difference while driving.

(Paul Breen pay palled it) #10

Have just been through a similar issue with my Mk2. I fixed several things together: front carb was lean; vacuum advance elbow was loose/leaking; vacuum advance was not centred in its travel - too many vernier “clicks” in one direction over time. Not having the unit centred hampers its movement. Not sure which of these assisted the fix - mostly lean carb, I expect. As for oil Briggs and Stratton oil is SAE 30 and 3 in 1 (sewing machine oil) is SAE 20. Mk2 FSM calls for SAE 30. Paul

(David Bruce) #11

ATF is the way to go.

(- 1950 MkV, 1959 XK150,) #12

Fyi here’s a useful chart comparing the different ways of measuring viscosity… I currently using hydraulic jack oil in my dashpots, with a viscosity of ISO 46.