I wouldn't discount an exhaust blockage yet.

As a quick test, when the car is idling, use your hand to feel the exhaust coming out the pipe. When the Cat collapsed in the SPi (with very similar symptoms) my best description would be that instead of it feeling normal like pulses firing out against your hand, you couldn't really feel anything hitting your hand - more like just some hot air drifting out the pipe.

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I would suspect fuel / carb too, is dashpot free and can you lift it right up and it falls smoothly back?

What type of fuel pump? split or pinhole in mechanical pump diaphragm?

Is oil level high, does oil smell of fuel?

I like Davids suggestions, some good ideas there!

If an electric pump, maybe you could cut engine at the speed it's happening, coast to stop and look in float bowl..... check as per Davids recommendation.

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Sir Alec! we are not worthy!

As another route to check, re fuel starvation. My Commer van had a similar problem and it was rubbish in the fuel tank, which couldn't pass into the fuel line. It became obvious only on highway running, which meant the first time I tested it on the motorway, I ended up with a police escort.

Coupled with the issue about the capacity of your fuel tank, I'd remove the tank completely and check it out. Even fill it and see how fast it drains. Someone may offer another idea for testing the tank.

Thanks for the replies everyone!



David!! Glad to see you're still hereabouts I'll definitely have a look at the fuel level, you've got some good suggestions there

Yes, when idling the mix definitely changes from bunsen-blue to bright yellow with about 2 full turns on the mixture nut.



The exhaust seems free enough at low revs, enough to flutter the leg of my jeans when I walk past

The leads I have on at the moment are known to work perfectly on my 1152.. and the leads which were on the van are working perfectly on the 1152 as well.

It's a mechanical fuel pump, and this is the only component of the system I've not had a look at.

With the idle mix set correctly (I tend to run it a touch on the rich side because it goes better) yes, using the choke when the stutter sets in definitely helps. With the mix richer to begin with, it doesn't help quite so much.



This would be the only component of the ignition system I've not changed, but like others suggest this "feels" like a fuel problem to me, not electrical.



I think this will be my next line of investigation after looking at the things David suggested.



The carb piston falls freely, albeit slowly due to the heavyweight dashpot oil. It's a floating needle, so there's no jet-centring to be done.

It's a mechanical fuel pump, and as mentioned above this is the only part of the fuel system (tank excepted) that hasn't been replaced. I'll check the flow rate over the weekend...

Oil level is normal, doesn't appear to be contaminated with fuel.



Yep, that was going to happen soon anyway, but I think I'll drop it out over the weekend and see what's what.

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Thanks for the suggestions everyone, I feel like there's a couple of possibilities I've not yet investigated which makes me feel better about it! Nothing like beating your head against a brick wall (no pun intended) to build the exasperation level

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Simon

The adventures of an owner builder in the Tallarook Ranges

Have you tried DextronIII auto trans fluid as the 80 sounds a bit heavy to me and may be stopping the piston from rising fast enough or even stopping piston travel (eg. like compression lock).



Just give me a call so we can sort out a time next week to get this sorted
I'll check to see if i have enough or any Penrite damper fluid left.

What spring is in the carb? Blue or Red

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http://www.youtube.com/watch?v=nDMkVq1jRGU

A little trick I do with the floating needles is to remove the bias on the needle. The little steel insert that holds the needle has a very small raised bit on the end that goes into the piston. Very carefully file this off. The needle will"self center" and will not wear a groove in the jet.
I have found with some carbys that the "bias" tends to stick the needle in the jet and does not allow it to drop back to the same postion.
This means that the needle is not returning to the correct position after you accelerate. You find that the mixture is all over the place when returning to idle. This is more prevalent if you have fitted a new jet or as you have done use a thicker oil or a weaker spring. The piston will not return to the bridge properly.
IMG]http://i339.photobucket.com/albums/n476/davidnrosenthal08/0411091958-00.jpg[/IMG]

These are a ram tube and a set-up that I use on SU's. The air holes on the atmospheric side of the piston are now connected to the outside of the air filter. The piston lift rate is controlled by the differental air pressure on either side, but if these holes are connected to the inside of the air filter ,then you have a fluctuating air pressure on both sides of the piston.
These ram tubes keep constant pressure[14.7 psi] on the atmospheric side. The piston rises more uniformally and does not flutter as much.

Smooth uniform piston opperation is the most critical thing on a SU along with the correct fuel level
It was done on cars 70 years ago when they ran the ram tubes instead of air filters

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com

I've tried everything in the dashpot from nothing, all the way up to the gear oil and in its present state the heavy oil produces the least symptoms. It's obviously a band-aid fix, since it's definitely too heavy than it should be indicating something else is awry. I like the sound of David's theory about the fuel level in the float bowl, I'm definitely going to check that out first.



I don't know what the spring is as there's no evidence of any paint on it, but I suspect it's a red one as everything else on this thing is bog-stock, so I see no logical reason why the spring alone would have been changed.

I'll have a look at the float level, fuel pump and tank in that order over the weekend and if I get no joy, I'll give you a call and it's all yours...

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Simon

The adventures of an owner builder in the Tallarook Ranges

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Your silliness has been noted.

Someone hasn't filled your tank with sand or gravel (got any stray kids running around) that could slow your fuel flow down and your capacity of the tank. Stick something in there to see if you can feel anything (or drop a match in there so you can see, that would fix everything,Just joking)
Phillip

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https://t.me/pump_upp

Yeah, I reckon there's something oddball going on in there, I just need to drop the tank to find out. This weekend, hopefully.

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Simon

The adventures of an owner builder in the Tallarook Ranges

Another thing to check is for any kinks in the fuel line from the tank to the pump.
My first Mk 1 S had this problem. Turned out that the fuel line had been flattened by a rock near the rear sub frame. Enough fuel was getting thru until certain speed was reached.

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1967 Cooper S - new front bearings to do.
1965 Cooper S shell - Slow progress. No time or money!
1966 Deluxe- next rustoration!
Mk 2 & XJ6 Jags. Less said the better.

Yep, will do this when I crawl under to yank the tank.

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Simon

The adventures of an owner builder in the Tallarook Ranges

Did this, the depth gauge on the caliper just touched the fuel maniscus at 25.1mm, close as damn it to a full inch.. so this might be a little low.



The jet is 150 thou / 3.8mm from the bridge, set just on the rich side of stoich at 1200rpm. I managed to put 16cc of fuel in the float bowl before it came up to the top of the jet, or nearly a full 10mm higher than the level was when I removed the float. My metric eyeball says the shoulder of the needle is about 10 thou proud of the base of the piston.



3 & 4 - both clear.



I didn't fit a new float, and although I swear I replaced it, the float needle is showing its age but is sealing up ok. The float is a fixed plastic type, but appears to be positioned in the range you specify -- although difficult to measure, I reckon I get about 150 thou of feeler gauge between the centre of the float and the locating shoulder of the lid. Was your measurement to this point, or to the recessed face of the inner part of the lid?

If the float level is wrong, can this be adjusted with the plastic float?

Now, here's another question: What is the expected clearance between the tip of the rotor electrode and the dizzy cap electrodes? I've got what looks like 5mm.. which intuitively seems a little wide to me... ?

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Simon

The adventures of an owner builder in the Tallarook Ranges

The only way to adjust float level with the 1 piece plastic float is add or remove washers under the valve seat.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Doc's spot on there. The float bowl tang that pushes against the needle spring is supposed to be pre-set, but I have found that some kits had a thicker sealing washer that goes under the needle assembly. It's not the correct thing to do ,but the metal bit that pushes on the needle spring, just bend it a LITTLE to raise the fuel level.

I only use the "fuel miser" kits that are made in england[ they are about $30 from auto-pro] and the washers are the correct thickness. If the washer is too thick it will lower the fuel level. You have to consider the "leverage" between the pivot, needle and center of bouyancy of the float.

Another trap is that some after market needles are a little longer than the original ones. Try pushing the needle and steel holder right up into the piston. The needles that were a little longer actually had a parrallel section at the top, so the first two 1/8" segments were the same size. You had to screw the jet further down to compensate for this.

It definately sounds like a fuel starvation problem to me ,but that's all I can think of

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Research is the difference between speculation and investment. Anyone who copys some one else will always be second
www.minisprintgt.com