Dashpot Stuff

Filling the dashpot oil

Fill up the narrow tube that the dashpot damper slides up and down in; this is the piston rod that the piston slides in.

Fill it just below the top of the tube; if you overfill it, oil will get inside the dashpot and then into the carb body... lots of smoke!

If there is a metal bridge "retainer" across the narrow tube stopping the piston coming out, add SAE 20 oil down the tube, lower the piston then raise it, add more oil until with the plastic cap is just touching the threaded top. The oil is just visible at the bottom of the retainer recess.

Ball bearing chambers: withdraw the damper and fill with SAE 20 oil to within 6.5mm of the top of the hollow piston rod, NOT over the top!

Standard chambers: withdraw the piston damper, fill the hollow piston rod with SAE 20 oil until the oil is just visible below the top of the tube.

General theory of dashpot oil

"The main theory behind dashpot oil is that the viscosity should be thin enough to let the dashpot rise as quick as possible, but not too thin as to give a flatspot (temporary leanening). The engine needs a richer mixture when accelerating, and the delay whilst the dashpot rises gives this rich boost as the air flow over the jet gets faster.

Wear on the plunger is a large factor here. If the plunger is minutely smaller than another, then the oil will pass by it easier and a relatively thicker grade of oil will be required.

Every carb/engine is different, so you can't really say "I use xyz oil, it WILL be great for you too!" I use 3in1, but saying this, it is the thinnest oil I have tried. I'll try out the auto/folk/olive oils and report back."

Other dashpot oils

"I bought some Castrol Fork Oil (LIGHT) the other day. 3 quid in Halfords. I'm using it in the dashpot. It's very thin, so the dashpot rises very quick.

Here are my findings:

• It's thinner than 3in1 or other stuff
• It gives you lightening quick throttle response and better acceleration
• It hasn't given me flatspots on flooring it, but may do if you run lean
• It eliminates gear-change flat spots caused by the dashpot not falling quick enough

The only bad thing is I now find myself straining to keep a steady foot. The slightest bumb in the road makes the throttle go "rrRRRrrrRrRRrRr". A bit of a pain for road use, but great on the race track."

Dashpot oil viscosity

Thicker oil gives a richer boost when flooring it as the dashpot takes longer to rise and the air speed over the jet increases higher and for a longer time. The engine will be less sensitive to the throttle. The down side is acceleration response is slower. By a thick oil, I mean eg: 80w Gear oil

Medium dashpot oil eg: 20w50 engine oil is a good all rounder and is recommended by haynes etc, but....

"For better performance, use a thin oil. I use Duckhams motorcycle folk oil which is ~5w. This gives lightening acceleration response, but you don't get so much of a rich boost. If you don't run rich normally you'll get a flatspot when depressing the throttle sharply. Another downside of this is the throttle becomes very sensitive and you need a steady right foot."

Dashpot springs (I)

"One time I measured a couple of springs and I also had a black one, which was the stiffest. I have to look if I can find this list again."

This list was found in a tuning book:

So you can see, for smaller carbs, where the airflow is greater and also more suck, you need a harder spring as for bigger carbs.

Dashpot springs (II)

(BB) indicates ball-bearing suction chamber, ie HIF44 type

Dashpot Springs (III), for HS6/HS8

For HS6 and HS8 carbs there are four small diameter springs - blue, red, yellow and green. The HIF44 and HIF6 have larger diameter springs, in red, yellow and green.

A stronger springs compress the range of the used needle profile to the lower part and the air flow under pressure is higher, so it gives a richer air fuel mixture on the upper end of the needle profile used.

"My 1100 runs with a green spring (small) and a ABB needle, the 1293 runs with a BDL and yellow spring."