24 replies to this topic

[OllyD]

Hi guys.My other topic was getting no love, and I'm still without a solution, so I figured I'd rephrase my question more simply

So, what things would cause touching the accelerator to kill the engine?

Obviously being too lean is one, but what else?
Cheers TMF!

[dklawson]

If you can start the car and get it to idle but it won't rev, it does sound fuel related.
Have you confirmed that the piston inside the vacuum chamber of the carb is free to move?
Have you looked at the level of fuel in the float bowl(s)?

On the other hand... how does it die when you rev it? Does the engine actually seem to stop or does it refuse to go above a certain RPM? If it completely seems to die... do you have a tachometer on the dash?

If you have a tach, what is its needle doing when the engine seems to die? Does it drop immediately to zero or does it drift down? If the needle drops immediately to zero, this would indicate an ignition problem that would justify looking at the ground lead and points lead inside the distributor (assuming you have points). If the tach drifts to zero, that sill points to a fuel problem.

[Jammy]

Do you know which carb you currently have? Make sure the bit underneath, the jet I think, freely moves up and down. Its common that these get bent, meaning you can run rich at idle, but once you hit the accelerator, you open up the butterfly, allowing more air in, but the jet can't move so you don't get any more fuel, causing a lean mixture and the engine dying.

Next thing to do is to set up the carb properly, rather than just screwing the mixture from lean to rich and hoping that somewhere it will run correctly. The next bit is taken from another topic, and will work regardless of which SU carb you have on there:

The initial position in an SU carb is where it is set to tune it from, and will fire the engine PROVIDING the needle in use is good enough. It's the same for every SU carb.

First, wind both the speed adjustment screws back right off. If you have twin carbs make sure the throttles and chokes are synchronised as far as you can. Turn in the idle adjust screw until it is just about touching the throttle cam, and then turn it in 1.5 more turns (1 turn on each for twin carb installations). Take off the dashpot and look inside the choke. Wind the just adjusting nut/mixture screw until the top of the jet tube is flush with its bearing/the top of the bridge in the carb. If you can't get the jet flush get it as high as it will go but it must never be higher than the bridge. On twin carbs they must both be the same height exactly so flush is the best option. Now turn the nut/screw two full turns richer, that's moving the jet down. Must be exactly two turns as close as possible and exaclty the same on both in twin carbs. Leave the fast idle screw well clear of the choke cam until setting the fast idle speed later. Once it's running the most important and first priority is to balance the cabs if you have twins.

Also make sure the fuel height is right in the float chamber.

You should also check that your float chamber isn't flooding or stuck.

After you've done all this then maybe look for an air leak somewhere?

[Jimmyarm]

the piston being stuck in the dashpot, opening the throttle lets lots of air in but the piston doesnt move up allowing more fuel in therefore leaning the mixture out excessively and completely ruining the gas flow into the cyclinders.

[OllyD]

dk, to answer your questions. The float bowl is filling up and sealing fine, i've stripped and checked that twice.

The piston is free to move as far as I can tell. I stripped the piston, cleaned all the muck out, then put it back together clean. I filled the dashpot and it dribbled down into the main chamber, but I think this was overfill.

The engine dies when i touch the accelorator as if it's being strangled, or restrained. It drops in accordance with how hard or softly i push. It's like acceleration in reverse. If i rev it properly, it loses revs all down to 0, or i can tap it and have it dip then resettle.

Jammy,

The carb is a HIF38
The jet, i struggled to tell if i'd put it back on correctly when i stripped the carb. It slides in, then you place the bimetalic stip over the top of it, which fits around it and holds it in place, correct? It's got a bit of wiggle in it within the slot on the strip it fits in, but it's locked in place from going up or down by the strip fitting over it. Is this right?

i'm going to head out there now armed with your tips and see if i can make any progress.

Edited by OllyD, 15 June 2007 - 01:35 PM.

[Jammy]

To be honest, I've never stripped or rebuilt a HIF carb, so I leave someone more knowledgable to answer that.

If you haven't fixed it by Sunday, I *may* be able to pop over quickly. I've got a colourtune which will at least let us know whether it goes lean or rich when you hit the accelerator!

[dklawson]

Do I understand that you removed the piston to clean it and remove "muck"? What part did you find you needed to clean? Was it the OD of the piston, the ID of the hole for the dashpot, the OD of the part of the piston that rides up and down in the domed cover, or on the needle itself?

If the car was sitting for a long time and you had to clean the needle, you also need to flush out the jet tube itself. Any heavy deposits you found on the needle will be duplicated on the inside of the jet tube and would only be allowing a marginal amount of fuel to pass through.

[OllyD]

I actually found the carb didn't need cleaning- a problem with cutting out that I've now blamed the coil, led me to check the carb- the only real muck i found was around the piston area,where there was oil leaking down the sides of the piston, but nothing solid or blockable. The car has been in use regularly.

Essentially I think i've caused this problem myself in the process of cleaning the carb. Either that, or could it be related to my coil issues? (Coil arrives tomorrow hopefully)

PS. Thank you very much indeed for the offer Jammy, but I'm smack bang in the middle of revision at the moment. Fixing my car is happening in the spare five minutes i get here and there, so my weekends pretty much already fully booked. I appreciate the offer none the less.

Edited by OllyD, 15 June 2007 - 10:25 PM.

[dklawson]

It's extremely unlikely to be a coil problem. It starts and idles? Are we talking about a points ignition system or electronic? Do you have a tach or not?

What did you do when cleaning the carb parts? What did you take off, what screws and nuts did you turn?

[OllyD]

The car only starts and idles with the choke almost fully out, even then the revs are lumpy as hell, and sometimes give out all together. Adjusting the mixture makes this no better or worse, not even in terms of revs or speed.

i have got a tach, and the needle drifts down following the engine dying, though obviously being linked to the revs, as the revs dip so to does the needle. Thus, hitting the accellerstor pulls the needle down hard. It's an electronic ignition.

When I cleaned the carb, i first opened the floatbowl underneath, removing the float, jet, mixture screw, bi-metalic strip, needle vaalve and needle valve guide with the little filter on. Gave all this a quick wipe (though it was all pretty clean), and installed back as it was. Jet went back in place, bi-metal strip was fitted over the top of it, engaging it.


Then i unscrewed the dashpot, removed the little piston clip and took the piston and the spring apart. I left the needle in place on the piston. I gave the internals a wipe clean and put the spring, and piston and dashpot back together. I then refilled the dashpot (though this seems to be continuosly dissappearing).

[dklawson]

Well, it sounds like you took the carb completely apart for cleaning. Following that you need to go through a complete carb tuning, I see no other choice at this point. I assume you have a manual such as Haynes. You really need to read through it and/or an SU tuning manual and go through a complete adjustment procedure at this point.

Since it will run on the choke, but only when nearly full out, it does indeed imply that the mixture is way lean. If you hadn't taken the carb apart I would be suggesting you look for a major vacuum leak such as a displaced brake servo hose or similar. Again, this has none of the symptoms of an electric failure.

[choodalls]

One thing else - need to check that the piston inside the dashpot cover actually match as a pair now. What I mean is they might move too freely if they are not now a close fit (having removed the muck!). I can't remember how many seconds it should take, but the test (if I remember rightly) is to remove the dashpot/piston assembly from the top of the carb so it is separate, then push the piston up into the cover and see how long it takes for it to "fall-out" of the cover under gravity. If it does it too quick, then the piston won't work properly with that cover....
I noticed someone else on another forum had similar problems to your, so you could read through their thread here and see if anything there helps. He was running twin carb.s, but the checks and issues with this are still pretty much the same....
Good luck.

Edited by choodalls, 16 June 2007 - 08:45 AM.

[samwell]

have you put u throttle cable on backwards... i thats possible. Sounds like ur closing the throttle when u put ur foot down causing it to die?

[Bass Man]

have you put u throttle cable on backwards... i thats possible. Sounds like ur closing the throttle when u put ur foot down causing it to die?

Won't that make you go backwards?

[Ethel]

Won't that make you go backwards?
[/quote]

Only if you put the battery terminals on the wrong way round too