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Old 28-11-2004   #1
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Question Carb flat spot Uno 60S (Solex carb)

Hi folk, I have a terrible flat spot in my carb, starting after the overun and then re-accelerating. There is quite a jolt. I have had the carb re-conditioned and the problem went away for a while. It is most notable when the engine is hot. I did find the crankcase to cam cover breather to the carb had become unattached and re-connecting this helped, but did not completely fix it. Out of interest I removed the hot air intake pipe to the air cleaner and so far so good..no flat spot..is this luck or am I dreaming. What happens if hot air is taken to the carb intake in hot weather, can this affect performance at all.
Your comments appreciated, cheers guys. (Whoops and Gals!!!)
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Old 28-11-2004   #2
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Re: Carb flat spot Uno 60S (Solex carb)

Im no engine guru but keep this in mind. When the engine breathes hot air you're gonna get less power mainly because you have a lesser volume of air goin into the engine.

One thing I would try is a carb cleaner which you just spray on the carb and apart from just cleaning it, it also helps lubricate it. Sorry if I wasnt of much help
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Old 28-11-2004   #3
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Lightbulb Re: Carb flat spot Uno 60S (Solex carb) <very long!>

Brymak, the thermostatic capsule in the air filter intake fails (or gets tired) and leaves the flap jammed in the 'hot' position. Try popping the casing apart and removing the flap (for this time of year...)

Also check that carb heat shield is intact (under carb base) and not broken off at the back, seems a common problem. You can improvise some sort of shield (I made one out of a sheet of brass, it was what I had to hand!) and rivet it in place to the remains of the original shield. This was to cure a hot-starting problem where, when the engine was shut off, excess heat in the carb caused it to 'burp' fuel into the engine (you could hear a gurgling at two-minute intervals) and the engine then took a lot of cranking before it would start.

This is all assuming that it is a heat-related problem which seems likely given your descriptions. However, you should also consider the possibility of blocked jets. You can clean the carb out yourself (and save having to have it reconditioned ) I can't remember which is which on the Solex, but if you take the air cleaner off you should have access to some brass 'screws' that are the jets. Take them out and check they're clear.

Incidentally, I personally prefer the Solex to the Weber that's sometimes used in this application - the Weber seems to have worse flat-spots!

To do a thorough job, take off the air cleaner housing. Take the carburettor top off, remove the jets, and blow everything out with compressed air (from your $120 air compressor :-)). Fit a new fuel filter to the hose where it leaves the fuel pump (your car may not have one of these, and they are a good idea.)

I've just realised something else (reading your flat spot circumstance again)... your car probably has fuel cutoff on the overrun (I'm 98% certain that having an econometer means that it does). This will possibly contribute to your flat spot (in theory, the fuel is supposed to come back 'on' once RPM drops below a preset limit; this might not happen during re-acceleration), but we'll hope that the above measures will make it bearable. Engine fuelling is often a sum of parts - an improvement in one item will mask the inherent problems in something else.

If not, you might like to try temporarily wiring the fuel cutoff solenoid to the ignition coil feed (coil terminal with red/orange wires) so that the cutoff is always 'on' with ignition. Unplug the cutoff solenoid (at base of carb) and use wire-tap connectors to join a length of wire to the wires on the coil. That way, you can always reverse the modification if it makes no difference.

It would also be interesting to know if you can detect any difference in fuel economy with the cut-off device disabled!

If the cut-off control unit fails it will usually prevent the engine from idling. There is a common trick where, when this happens, someone snips the end off the solenoid 'point' that's supposed to cut off the fuel. This might have been done to yours (unscrew it from carb to check) and, obviously, the cutoff solenoid will have no effect after such a molestation!

Yet another possibility for acceleration flat-spots is the accelerator pump diaphragm having failed. With the top off the air cleaner, you should see a strong squirt of fuel into the carburettor when you operate the accelerator linkage on the carb by hand. If not, the diaphragm needs replacing. However, I think this fault would be most noticeable with the engine cold (so probably not the problem here.)

You mentioned the crankcase breather - it's not really related to these issues. Even so, when you have time, take the 'pot' off the engine block (one 13mm bolt) and just check that the hole through it isn't completely blocked - also clean the pipe with the wire flame-trap near the top. If it's been disconnected, chances are it has become clogged through lack of airflow, which then leads to oil leaks and burning oil. Clean it out with engine degreaser... It can be a bit tricky to get the bolt back in, but you'll get it.

Anyway, that should give you a few things to try, actually it should keep you busy for hours!

-Alex
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Last edited by alexGS; 28-11-2004 at 15:55.
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Old 29-11-2004   #4
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Talking Re: Carb flat spot Uno 60S (Solex carb)

Thanks Alex, will try some of your tips especially around the fuel cut off solenoid. I have already had the carb completely rebuilt about 3 weeks ago and the prolem is not as bad but still that niggly flat spot.

Thanks also for your comments about my non functioinigs econo meter. I contacted Mal Simmonds and he sent me the vac sender after i emailed him a photo of my old one. he snt the wiring loom plug as well as it was slightly different to my old one. With a very slight mod to the wiring and hey its up and running again. Mal was very helpful and will use him again for other parts.

So hey thanks again and will keep you posted as to my progress. You certainly have a vast knowledge of things Fiat!!

Cheers for now, Bryan
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