60 replies to this topic

[Marc13]

Went to check the oil in my dashpot and it was pretty much empty, so i put a glug in and filled it to just past the rim thing below the threads. I screwed the top back on and went round the block. Then took the top off to check the level and there was no oil to be seen. Pit another glug in to just below the threads again, drove round the block, checked the level and again there was none to be seen. Must have repeated this another 3 or 4 times....everytime after a quick drive or even after just letting it run on the drive, the oil level had dropped so none could be seen (you could see it on the plunger thing). Where is the oil going? And should I keep topping up, or is it never going to fill?

[sonscar]

You are overfilling it.Look closely down the hole and you will see the inner tube connected to the piston.It is this you are filling.Excess pushes out of the cap breather and is burned in the engine.Steve..

[Marc13]

Thanks Steve, just checking a few points:
1) so a coreectly filled dash pot, would not have the oil visible to the thread level, when you take the top off...ie when youbtake the top off, you cant really see the oil?

2) so the excess that i have been over filling with, gets mixed in with the fuel and burned off? It doesnt stay in the dashpot?

3)when its in the fuel mix would you expect the engine to run poorly for a while?

[Spider]

This sketch shows the oil level

 

 

[Marc13]

Thanks, so how does the excess oil that I put in, get out? And where does it go to?

[Icey]

It will just be drawn into the engine and burnt off. The amount you’ve put in is unlikely to cause any long term problems but might be worth checking and cleaning your spark plugs.

[Lplus]

Thanks, so how does the excess oil that I put in, get out? And where does it go to?

It runs down between the piston spindle and the dashpot inner tube surfaces and into the carb airstream through the two holes in the base of the piston.

[Marc13]

It has been running horribly since I overloaded the oil in the dash pot. I did also tweak the fuel/air mix with the screw on the side of the carb (that alters the position where the needle goes into the jet thing) but have tried to put that back, but still running awful. Is this likely to be down to the dashpot oil going into the carb? But eventually will solve itself??

[Lplus]

The oil will have disappeared into the engine and burned  up as soon as it got out of the centre of the piston and into the airflow.  There should not be any long term effects at all.  Not sure which screw you mean, The screw in the side of the carb body of an HIF series carb alters the height of the jet in the base of the carb.  That one?

[Marc13]

Yes, thats the one (that altera the height of the jet.

I did an oil and filter change.
Then oil in the dashpot.
Then tweaked the jet screw on the side of the carb.

Obviously I was expecting it to run like a dream! But its now running dreadfully. I was hoping it was the dashpot oil getting into the carb that was causing it to run so bad (as it would fix itself)....but it seams this isnt the cause?

[Ethel]

The oil just needs to cover the the plunger that you remove with the screw cap. If you overfill it the excess oil is displaced when the plunger goes back in. If it's the cause of bad running it's probably fouled the spark plugs.

 

It's unlikely, but not impossible, that the tube the plunger runs in has sprung a leak. Spider's HS cross section shows it as a drilled/bored rod, but I'm pretty sure at least some Hif's use a length of tube press fitted to the piston.

[Ethel]

Here's a bit more colourful diagran of a HIF SU. You can see the green plunger inside the damper tube with its open top about level with the shoulder of the dashpot. It, in turn, is inside another tube, represented by the dark, parallel, lines. It's not shown in the picture but, both those tubes are steel with a slider bearing between them. Excess oil would run down between both tubes, then follow the path of the light blue arrows to be sucked through the communication port (4).

[nicklouse]

The oil just needs to cover the the plunger that you remove with the screw cap. If you overfill it the excess oil is displaced when the plunger goes back in. If it's the cause of bad running it's probably fouled the spark plugs.

 

It's unlikely, but not impossible, that the tube the plunger runs in has sprung a leak. Spider's HS cross section shows it as a drilled/bored rod, but I'm pretty sure at least some Hif's use a length of tube press fitted to the piston.

Yep it has been known for the dash pot damper tubes to leak. 
 

not hard to check.

[Lplus]

Here's a bit more colourful diagran of a HIF SU

Some HIFs (6/44) use a small pair of ball bearings in a cage between the piston shaft and the dashpot tube.  Excess oil just flows straight past.  It seems unlikely that the quantity of oil lost would be enough to foul the plugs, but a quick clean should sort that.  The point of the oil is to slow the pistons rise, so if the plunger meets resistance when pushed down there is still enough oil in the piston to be effective.

[Ethel]

That's a better description of the bearing, thanks.

 

I read that the OP had tried to top it up several times. But they'll have an idea of how much oil they poured in - if any was needed, it would only be a thimble full.

 

I'd suggest removing the dashpot, it's just the 3 screws & it would be worthwhile if only to get a better grasp of how an SU works if you've never seen inside one before. Just be prepared, with somewhere clean to sit it down without resting it on the needle - a cardboard box with a pencil sized hole punched through the top will do nicely. Also watch out for the piston: lift the dashpot straight up and it should bring the piston & needle with it (there should be a retaining circlip on the bearing tube), but the piston could stay sat on the body or decide  to drop free at any point. Sticking your fingers underneath as soon as you can is all it takes to avoid that disaster.