A strange one.

[kevin s]

A strange problem, 1098 on std carb, on several occasions it's lost all power but idles OK, pulled over, checked fuel supply to carb and there is plenty, big fat spark at plug too, tried to drive away no go then on each occasion topped the carb daspot up (still had some oil in) and it's fine again. I've never had an SU behave like this before, sure they stutter a bit etc but not undriveable. Piston does seem a bit loose in the carb body (piston and dashpot came from another carb) I wonder it it could be jamming somehow, any one ever had anything similar?

[mogbob]

Before removing the dashpot , when it " refuses to proceed " what happens when you lift the piston lift pin. Does it feel " jammed " ?
Possible causes damaged needle ? Is it perfectly straight ? Remove needle from piston , after slackening the grub screw.Place on a flat surface
eg sheet of glass , mirror or hard laminate surface at a push.Gently roll the needle and observe the very tip of needle.Is it rock solid or does it appear to wobble. Replace with a new needle ( "AN "standard specification ) if it wobbles.

Assumptions made that the dashpot and piston are perfectly clean and no visible physical damage.Correct red spring for the piston, although I wouldn't expect this to jam to piston action.
Other members may have other thoughts on the possible cause. Bob

[simmitc]

Debris in the float chamber? At load, sucked onto jet and cuts fuel, but at idle with less flow, falls back. Worth checking.

[James k]

I would guess that it's the action of removing and reseating the damper that's fixing it rather than adding oil. Definitely sounds like it's jamming. It might be worth removing the oil from the piston and fitting the damper to see if it slides up and down freely (without the resistance of the oil). I wonder if maybe the damper is slightly bent and causing the piston to jam against the side of the carb.

[NETTIE]

Debris in the float chamber? At load, sucked onto jet and cuts fuel, but at idle with less flow, falls back. Worth checking.

I agree float chamber related fuel starvation chamber not filling fully good luck

[kevin s]

I'm not sure it would have idled fine with the jet blocked? easy enough to check though so we we will have a look.

After only taking the cap and plunger out, putting some oil in (well actually unleaded fuel additive as that was all he had in the car) my son drove it 40 miles (half with a canoe on the roof) and it was perfect, last time he drove it with no problems for 3 months or around 600 miles after we just topped the oil up.

Perhaps over xmas we will just fit the twin carbs and LCB we have on the shelf waiting to go on.

[myoldjalopy]

Maybe the oil is getting out of the chamber (you mentioned the piston seems a bit loose - could the oil be getting past and out the hole in the piston cap?). If topping up with oil seems to cure the problem, check the oil more frequently and see if the level drops accordingly.

[svenedin]

The OP says it idles OK but the engine has "lost all power" when this problem arises. I take that to mean the throttle seems to do nothing? If the piston gets stuck down it could idle satisfactorily but when the throttle valve opens the piston can't rise and raise the needle with it to deliver more fuel. I would have thought the engine would run very rough or stall with an open throttle (more air) but no more fuel (mixture much too lean). A piston that fits well should go up and down smoothly in the dashpot like a plunger in a syringe.

https://sucarb.co.uk/technical-carburet ... troduction

[kevin s]

Yes it has no power and runs very rough when you try to accelerate.

Plan is short term check the oil every week, long term change the carb for the twin ones etc.

[svenedin]

Yes it has no power and runs very rough when you try to accelerate.

Plan is short term check the oil every week, long term change the carb for the twin ones etc.

Or you could get an exchange rebuilt carb as a direct replacement. Or find a used carb in reasonable order and rebuild it yourself with a rebuild kit.

[Mark Wilson]

I very much doubt that this is the answer here, but last year I experienced very similar symptoms towards the end of my rebuild. I could get a decent tickover, could back it out of the drive, but as soon as I turned it to go up the steep cul-de-sac it died, or with enough welly it moved very slowly as though the brakes were on. Jacked each wheel and all turned freely. I was convinced it was a fuel or timing problem, so changed the dizzy and carb, then pretty much gave up.

After the indignity of seeing my car leave on the back of a recovery truck my local garage (who are mini specialists but one of the guys knows minors) came up with the answer. A front wheel bearing had collapsed. It was ok unloaded, but under forward motion the shoes were forced into the drum. The hub was one of the very few parts I hadn't replaced or overhauled...

[oliver90owner]

….

After the indignity of seeing my car leave on the back of a recovery truck my local garage (who are mini specialists but one of the guys knows minors) came up with the answer. A front wheel bearing had collapsed. It was ok unloaded, but under forward motion the shoes were forced into the drum. The hub was one of the very few parts I hadn't replaced or overhauled...

But that should have been obvious, with a quick check while the wheels were jacked off the ground?

[Mark Wilson]

No, it wasn't obvious. As I said, I jacked each wheel and they all rotated freely. At that stage I still thought it was the engine losing power, but was checking for binding brakes as a possibility.

[jaekl]

Just to clarify, when opening the throttle the intent of the oil in the damper is to slow the rise of the dashpot and needle. The wider opening of the throttle puts more vacuum on the jet to pull more fuel in to initiate acceleration. With out the damper, the piston will rise rapidly and very high which will drastically lean the mixture and thus it stumbles. A stuck dashpot will also yield poor results and most likely will never be able to accelerate and with much less power. Whereas the first case, you may be able to slowly get up to speed. The OP mentions after adding oil, it behaved better. The question is where is the oil going? Check for holes in the center tube of the dashpot.

[myoldjalopy]

............which is similar to my line of thinking earlier. A damper with no/little oil will easily stutter/cut-out. In the OP's case, these symptoms are eliminated after topping up the dashpot with oil. So, if it needs topping up for the engine to run properly, it suggests the issue is a lack of oil, escaping the dashpot somehow.

[kevin s]

I think the carb is basicaly just worn out, the piston is way looser in the body than any I have had before, it may be better if I put the piston and dashpot back in their original body, the one that is in there is a bitsa of 2 or 3 carbs.

[islecastle]

Just to clarify, when opening the throttle the intent of the oil in the damper is to slow the rise of the dashpot and needle. The wider opening of the throttle puts more vacuum on the jet to pull more fuel in to initiate acceleration. With out the damper, the piston will rise rapidly and very high which will drastically lean the mixture and thus it stumbles. A stuck dashpot will also yield poor results and most likely will never be able to accelerate and with much less power. Whereas the first case, you may be able to slowly get up to speed. The OP mentions after adding oil, it behaved better. The question is where is the oil going? Check for holes in the center tube of the dashpot.

Would not an undamped piston cause a rich mixture?
Regarding the OP, I would suspect a stuck needle for whatever reasons would cause his symptoms.

[jaekl]

True, when the piston is up the opening in the gland is bigger to allow more fuel through. However, the SU principle is more involved. Due to the fuel level in the bowl is lower than the top of the jet/gland, the fuel needs to be pulled out of the gland. The steady state of the dashpot (jet - gland opening) provides an good fuel/air ratio with the vacuum at any particular throttle opening/rpm. However, when more power is needed, an enriched mixture is needed to get more BTU into the engine. This is done by increasing the vacuum at the jet. With the dashpot still low but with the open throttle, this increase local vacuum pulls air faster through the opening under the dashpot which also pulls the fuel through faster. Said another way if the dashpot would rapidly rise when opening the throttle, the pull on the fuel would provide basically the same air/fuel mixture has it would during steady state throttle. This would at best yield a slow increase in speed or power. Also consider the case when a hill is encountered while racing along at full throttle. As the rpm starts to fall, the engine pulls less air (less vacuum) and the dashpot starts to fall, but you want more fuel to crest the hill. This is accomplished because the air over the gland has increased, pulling more fuel.

[philthehill]

The venturi effect.

[islecastle]

True, when the piston is up the opening in the gland is bigger to allow more fuel through. However, the SU principle is more involved. Due to the fuel level in the bowl is lower than the top of the jet/gland, the fuel needs to be pulled out of the gland. The steady state of the dashpot (jet - gland opening) provides an good fuel/air ratio with the vacuum at any particular throttle opening/rpm. However, when more power is needed, an enriched mixture is needed to get more BTU into the engine. This is done by increasing the vacuum at the jet. With the dashpot still low but with the open throttle, this increase local vacuum pulls air faster through the opening under the dashpot which also pulls the fuel through faster. Said another way if the dashpot would rapidly rise when opening the throttle, the pull on the fuel would provide basically the same air/fuel mixture has it would during steady state throttle. This would at best yield a slow increase in speed or power. Also consider the case when a hill is encountered while racing along at full throttle. As the rpm starts to fall, the engine pulls less air (less vacuum) and the dashpot starts to fall, but you want more fuel to crest the hill. This is accomplished because the air over the gland has increased, pulling more fuel.

Thanls for that. I'm not really conviced though. Simple fact is that as the piston goes up, more fuel enters the engine.
I'll study your reply a bit closer and reply in due course. I don't really want to get into one of those "I'm right, no I'm right" pointless arguments on here.