[Cosmo]

My 1968 Spider is original Euro spec with Webers and I have a problem with getting the engine to pull past 3000 rpm under load, but only when the car is on the first run of the day. Starting off from cold, I can drive for 10-15 miles and get the engine fully warmed up and still have the problem. But if I just drive a few miles, stop for a couple of minutes and then restart the engine the problem disappears and it pulls strongly all the way to the red line.

I did refurbish the carbs a while ago with a rebuild kit and checked all the jettings and float levels were standard. The rubber mounts were also replaced and I've tuned the carbs with a balancer and a Colortune - idle is very good. I'm thinking it is some kind of vapour lock or air bubble that is purged from the system when sat for a couple of minutes?

 

[Alleggerita]

The car may be running lean on the main circuit. Another thing to rule out is carb icing albeit that's rare on twin Webers.

 

[Cosmo]

Are either of those problems going to suddenly disappear just by switching the engine off for a couple of minutes?

 

[Alfacliff]

yes, thats the way carb icing is, I had a car with a missing deice hose and wondered why it died the same distance from home often, one day I finaly saw the ring of ice fall down the throat of the carb. icing can occur above freezing, up to 45 or so due to the cooling effect of the venturi.
cliff

 

[101 Alfa Mike]

Sounds like your car is a bit lean on the transition from the idle circuit to the main circuit. Try a slightly higher fuel level in the float bowls to make the main circuit come on a little sooner.

 

[Cosmo]

icing can occur above freezing, up to 45 or so due to the cooling effect of the venturi.

Well it is pretty miserable her in the UK but it isn't that cold outside here yet!

Would I be able to tell just by feeling how cold the outside of the carbs are? I also have a small temperature probe that I could use remotely.

 

[Cosmo]

Sounds like your car is a bit lean on the transition from the idle circuit to the main circuit.

Yes, the symptoms seem to fit a mixture problem at that rpm point but why would turning the engine off, and back on a couple of minutes later fix the problem?

 

[Alleggerita]

Carb icing is caused by air going through the venturi at high speed (more likely if car is undercarburated, eg venturis small for the engine). If you turn off the engine, no more air is going through and the heatsoak of the standing engine will clear the ice around the venturi. And yes, if it is bad, the carbs are icecold and in extreme cases can have ice build-up on the outside - though I have never seen that on dual Webers. Another tell tale is black smoke out of the exhaust. It is most likely to occur in temperatures slightly above freezing in air w/ high moisture content. Fuels containing alcohol don't help.

 

[Cosmo]

Well it is a pretty stock engine as far as I know with standard 40DCOE/32s so the venturis should be correct. It's getting colder here but still above 10C but I'll check the carb temperature when it next happens.

One problem I have is that the original engine had the crossover air filter arangement. The current engine has the air filter on top of the plenum chamber but none of the fuel feed plumbing was moved when the change was made. Consequentially the fuel filter is in the way of the hot air feed to the air intake from the heater plate on the side of the engine. So there is no hot air feed for winter running. Should I move the fuel filter (where is the correct location?) and reconnect a hot air feed for the coming cold weather?

 

[Cosmo]

More information.

The carbs certainly are getting cold. Starting out on a day with an ambient temperature of 15-17C the carbs were a little colder at 11C. After a few miles the engine was warm (not hot) but the carbs were still 11-12C and the spluttering was present. After letting the engine warm up fully and stopping for over 5 minutes the carbs got to 17-18C and the spluttering had mostly gone.

Then I let the car sit for about 30 minutes and the carbs had warmed up in the hot engine bay to over 35C. No problem at all now, with the engine pulling well. Towards the end of the test drive I thought I could detect the problem starting to reappear and when I checked the temperature the carbs were now down to 20C.

So it does appear to be related to the carb temperature, although it doesn't seem to go below 10C and there is no actual ice.

 

[Cosmo]

So I went through the process of relocating the fuel filter and fitting the cold weather trunking to the air intake but to very little effect. The rev "limit" now seemed to be a little higher at around 3,500 but the carbs still ran pretty cool.

So I started checking everything from scratch and found that with the engine hot and the carbs cold the carb to manifold nuts were not torqued up as tight as they should be. This gave me a only very slight vacuum leak but enough to mess up the mixture at high revs. I guess when the carbs sit in the hot engine bay with the motor off, they warm up enough to just about seal the leak. Weird huh?

 

[1.6 HF]

So I went through the process of relocating the fuel filter and fitting the cold weather trunking to the air intake but to very little effect. The rev "limit" now seemed to be a little higher at around 3,500 but the carbs still ran pretty cool.

So I started checking everything from scratch and found that with the engine hot and the carbs cold the carb to manifold nuts were not torqued up as tight as they should be. This gave me a only very slight vacuum leak but enough to mess up the mixture at high revs. I guess when the carbs sit in the hot engine bay with the motor off, they warm up enough to just about seal the leak. Weird huh?

Interesting, but not weird. Weber jets, air correctors, and emulsion tube drillings are calibrated to fractions of a mm, and are available in increments of fractions of mms for the simple reason that very small differences in either fuel or air can make dramatic differences in the mixture. What you had wasn't a vacuum leak--it was an air leak, which was adding way too much air to the mixture.

In other words, 101 Alfa Mike had it exactly right--you were running much too lean on the main circuit. On DCOEs, the so-called 'idle' jets control the low speed circuit that operates all the way up to 2800-3000 rpm, after which the main circuit takes over. You were dying exactly at the transition between circuits.

What you didn't originally tells us was how well it was idling; if the idle speed was 'hunting' (i.e., rising and falling), that would have been a red flag for an air leak.

 

[Cosmo]

What you didn't originally tells us was how well it was idling; if the idle speed was 'hunting' (i.e., rising and falling), that would have been a red flag for an air leak.

Actually I did say in my first post that the idle hasn't been a problem, it's been good since I rebuilt the carbs and set them up with a balancer and Colortune a couple of months ago. But now that I've found and fixed this problem I'll go back and check the idle tuning again to be sure.

 

[1.6 HF]

Actually I did say in my first post that the idle hasn't been a problem, it's been good since I rebuilt the carbs and set them up with a balancer and Colortune a couple of months ago. But now that I've found and fixed this problem I'll go back and check the idle tuning again to be sure.

Yes, you sure did--I missed that. You're right, though, you're better off rechecking the synch and the idle mixture.

 

[Cosmo]

Unfortunately I spoke too soon as the original problem has not gone away despite further checking for air leaks and redoing the tuning.

So I went back to check the float level as suggested by 101 Alfa Mike. It was a little low at about 9mm, so I made it 8mm. This has been a big improvement in the behaviour and there is only a slight stumble now before freely revving over 4000. I think I'll try 7.5mm and see if that fixes the problem completely.

The docs I have say that the 40DCOE/32 settings for a 1750 Alfa Romeo is 8.5/15 (min/max). Should I be worried that I have to go out of spec to 7.5? Need I also adjust the max value accordingly?

 

[101 Alfa Mike]

As you go to higher fuel levels, there is more risk of going too lean/rich when cornering. Not saying you can't go higher. You can. Confirm your fuel levels by looking into the emulsion tube wells. If you look carefully, you can actually see the difference between the static fuel level and the overflow passage to the venturi.

What idle jets are you running? My sources don't show 1968 Spider jetting as 1968 was a dry year here in the States. If its 50F11s, I would try some 50F8s which are slightly richer toward the upper end of the progression circuit.

Mike R

 

[VeloceOne]

Hi:
Jets as far as I know, should be the following for a 1750:
Main: 130 (Mine came with 125)
Air Correction: 200
Emulsion Tube: F-9
Idle Jet: 50F8
Starting Jet: 65F5
Needle Valve: 150
Accel. Pump Jet: 35
Accel. Pump Bypass: 60

Regards Ian.

 

[Cosmo]

Thanks for the figures Ian, they match what I have, including a 130 main and 50F8 idle. I'll set the level at 7.5/14 mm and see how that goes.

How does the needle valve affect the set up? A number of spec sheets list a 200 instead of 150 for the 40DCOE.

 

[101 Alfa Mike]

Generally, you want the smallest needle valve that is consistent with your needs as that gives the best control of the fuel level. Otoh, if you are making more hp than stock, then larger needle valves may be in order. Try to find the less common 175s. Even those have 36% more flow area than the 150s.

 

[VeloceOne]

Hi Cosmo:
If you do not have a modified engine, do NOT go to a larger needle valve. 150 works fine even on a stock 2L.
I know the stumble seems to be carb related, but are you sure that your ignition system is in good order? Do all the basics first, an not focus on just one area.
Regards Ian.

Note: My car did come from the factory with 125 main jets.