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My 77 midget, with original SU carbs; has some problems when accelerating over 3500 RPM.
It goes really sweet from idle, but when you keep on accelerating, it kind of "stalls" or "statters", around 3500 RPM.
Ideas?
How about fuel starvation? This could be caused by ...
(try operating it manually to find out what kind of flow it's giving; I once heard that a cam had worn so much that it was moving the leaver enough to pump fuel)
If I go up onto a dual carriageway or motorway within ten minutes of starting the car from cold (even in warm weather) in my 77 Midget, it dies over 3500rpm.
Trouser washing time when your speed drops suddenly to thirty or lower with lorries behind you. Sustained high revs without warming up the car on the choke seems to be the reason. I did read that waxstat jets cause this; are you having the same problem?
Had the same thing happen on my '72...one of the chokes was stuck. Hopefully its that easy for you.
I too have had problems with a stuck choke causing a problem similar to yours. I installed an extra spring to the choke on each carb to ensure that the jet came fully up when I released the choke. In addition, check out the ignition parts.
The stuttering is the sparkplugs misfiring. This can be caused by either a fuel or electrical problem. My Sprite used to eat spark plugs in about 3K miles (948 cc Mk II).
Bad spark plug wires will also cause this problem (look at them in the dark and see if they are giving off tiny blue sparks).
A bad distributor cap (carbon bushing or tracking), bad rotor or weak coil can also cause such problems. Also check the wires going to coil and from coil to dizzy. If they are getting old and cracked, they can deliver reduced power.
I had a very similar problem. In my case it was fuel starvation.
I had partial blockages in the jets; the hoses from the float to the jet had deteriorated inside. I had enough to idle and putter around town but couldn't get up a hood head of steam. Replacing the hoses and jets cured my problem.
Have you still got points in your distributor?
This could also be a condenser breaking down or if you are unlucky a coil breaking down. The problem with these things is it could be a multitude of things.
I had a similar problem and it turned out to be a burnt valve.
I went for a test run yesterday, and it still did the same. I noticed though, that when it "statters" around 3500 RPM, it lets go afterwards, and you can keep accelerating, but with a very strong smell of petrol.
I am suspicious of the fuel pump (does it have a filter inside?). I changed the fuel lines when I restored the car, I am thinking of 2 other possible causes:
- Replaced the long metallic pipe in front of the heater bod by normal fuel hose. Could this have any influence?
-Added an extra petrol filter in the hose.
Just to make it easier, I don't have any fuel leaks (at least that I see!)
Or ... how about:
When the engine gets to 3,500, the (mechanical) fuel pump delivers increased fuel pressure into the carbs, and one of the flo-jets doesn't fully shut off. The 'statter' is cos you have an over-rich mixture; the fuel smell is petrol leaking out of the top of one of the float chambers ...
Next time is 'statters', stop the car immediately (kill the engine, dip the clutch) and have a look under the bonnet.
Yes. There's a float inside those chambers which blocks the fuel inlet, so the fuel in there should be at a constant level. At least, that's the idea...
Take the top off the offender, and check that the valve shuts off when the float is raised (action should be obvious) - I always blow through them. Check also for cr*p around the valve (or flo-jet, to the real techies ...)
I had a similar problem at Snetterton on Sunday at the end of the Revett straight doing 6500rpm in top - about 100mph!
This was also accompanied by a strong smell of fuel and I'm fairly sure that the flo-jet in the rear carb is playing up.
In my installation the rear carb is the first to get fuel from the tank so any muck in the fuel causes a problem here first. I think I'll swap the fuel lines and float chamber lids around anyway as this might help with fuel surge/starvation during acceleration and braking.
Im looking into the same problem with my daughters 1500. It seems to me that with the spring loaded needles, that there is wear in the jet itself. If I adjust for a nice idle, I get 'stuttering'. If I adjust for smooth running at higher speeds, and 3500rpm is about where the 'stuttering' takes place, then I get a very rough rich idle.
My next step is to take out jets, check for wear. If worn oval I will replace. Next step is a trip to the local chassis dynomometer and get set up with new needles. ( I note from other postings that chassis dynomometers (rolling roads) seem to be rare in US, there are 5 in Auckland, including one shop that is able to set up SU's).
Twin carb setup is off a Tr**mph Toledo, replacing the original single carb setup (car is an import ex US.
While this is not an answer to the original query, it is worth while setting your car up for the correct mixture for a smooth idle, testing for 'stutters', and then winding down the jet adjusting nut 4 or 5 flats. I the 'stuttering' goes away, or reduces, then look for wear in the jet itself.
Don't forget that some of these symptoms (stattering, but not the petrol smell) can result from the dashpot sticking ... so I'd check that they both rise and fall smoothly, before going further ...
Could this be a air leak in the intake/carb(s) at high RPM? Ian says he adjusts for a smooth idle and gets "stattering", but adjusts to reduce the stattering and gets a rich idle. If it WAS a vacuum leak at high RPM you'd have to add fuel to get the mixture proper at high RPM, which would result in too much fuel at idle, and if you reduce to get the mixture right at idle you'd be starving for fuel at higher RPMs.
I haven't heard this possibility mentioned yet so I thought I'd throw it out there. Maybe it's worth checking your manifold gasket(s).
I fixed mine today. Identical symptoms. New points, plugs, condensor, and rotor. Don't know which one was causing the problem but it's gone. I have a new distributor cap on order but probably won't use it until the next tune up! Mines a '74. Safety fast!