MG Sports Cars

I have a 500 cfm Edlebrock installed on a 215 olds engine with 4k miles, when it is running it runs great. When it is cold it will start with the first few revolutions however when hot it takes 5 to 10 seconds to crank with the throttle held to the floor. The float levels are correctly adjusted the air fuel mixture are set lean and it still puffs black smoke when cranking hot. I do not have a heat shield between the carburetor and manifold just a thick gasket this may be the next thing I try. Looking into the throat of the carburetor when hot I do not see or smell raw fuel, so help what do I try next?



Edited 1 time(s). Last edit at 02/12/2009 03:39AM by coolmg.

My engine exhibits the same problem when shut off and left for a few minutes, but turning the ignition key with part throttle gets it started in about 2 or 3 seconds.
Sounds like a classic case of fuel perculation which most of us have experienced with a carburetted engine. I'd try adding a 1/4" (or more) phenolic spacer or nitrile rubber composite under the carb if you have clearance to the hood/bonnet, and see if that helps. Check Mr. Gasket carb spacers/gaskets at Summit Racing or Jegs for something suitable.
Example shown here.
[store.summitracing.com]

What Graham said. Ceramic coated headers & Y-pipe helps, as well.

Crank with the throttle floored? I restart the warm engine with my foot off the throttle... but to be honest I usually don't think about these things much. It's not easy for me to observe whether there's a puff of smoke on start-up. It starts easy though.

If you have a choice between straight gasoline or a 10 (or 15) percent ethanol blend, for a carbureted engine I think you'll find straight gasoline resists percolation and thus idles a little smoother. It also gets better mpg.

Mark,
They are all correct. Heat is the enemy. If the spacer does'nt work then wrap the headers and find a way to vent more heat out of the engine bay.
JB

Are you guys familiar with fuel pressure bypass valves? I think they're designed for cars with high pressure fuel pumps that are running a carb instead EFI set-up. Anyway, you run the high pressure line to the engine bay, connect the bypass valve to it. Off the bypass valve, a low pressure line goes to the carb & a return goes back to the tank... that way the fuel keeps circulating like in an EFI set up... I'm plumbing mine that way because I may switch to FI later so that way it'll be ready, but wondered if that sort of set-up would help this problem too since the fuel should stay cooler in general. Thoughts?

If interested, the two bypass valves I've found are from Holley (don't recall the PN) and QuickFuel Technologies. Quick-Fuel's PN's are 30-900 for the regular one & 30-1900 for a billet one.

That might help if there's a problem idling (like boiling the fuel out of the vent tubes) but it probably won't help much with heat soak. In fact, heat soak is one of the big problems with running a carb and the modern fuels just don't seem to help, as they seem to be formulated with injectors in mind. With a 40 lb rail pressure you can keep lighter fuels liquefied at much higher temperatures. Since carb'd cars are becoming more and more a niche market the time may come when they only run properly on their own specialized fuels or maybe avgas. A sad day but it wouldn't surprise me.

As for dealing with the heat soak, everything you can possibly do to keep under-hood temps lower will help some. Do enough of them and you may be OK. Plus, don't run winter fuel in the car.

Jim

What do you all think about the Holley 390 cmf carb as a fix to the hard starting problem?

There must be at least 100 Edelbrock 500 carbed cars on the BritishV8 site that don't have "hard starting" problems, so changing carbs doesn't seem to be necessary. Do you have a Holley 390 already?

I'm not so sure, Curtis. Even with ceramic headers & Y-pipe, I still have to start my hot MG with the pedal to the floor. I have a carb spacer, to boot.

Curtis I don't have a Holley 390 they run around $300.00 on line. On a second note just for grins I adjusted the floats up another .125 just to see if it made a difference and I adjusted the accelerator pump to it's lowest level. It might be well to note that after adjusting the floats again I ran the engine until it got to temperature, turned it off and let it sit for 15 minutes. During this time I shot my I-R temp probe at the base of the carburetor just to see the correct temp and it was at 100 degrees F. I don't think this low temp is and was enough to boil gas to the point where it runs over into the venturi. Again starting the engine required a full throttle and a lot of cranking. In conclusion the temp is not a problem I don't think neither are the other tuning aspects of the engine, timing, valve adjustment and all the other others. One question I have a six pound fuel pump do you all think this is too much pressure for the needle valve to handle? Yes I am still looking for the answer and no I don't want to change the carb.

Is this a new carb? Does it do this only when the hood is shut, or does it go away if you try it with the hood left open?

You may need to verify the fuel pressure. It could be borderline too much. 5.5 psi would be ideal.

The fuel supply could be an issue, particularly if it runs past the exhaust at some point, as heat there could force it into the float bowl during hot soak. Raising the float level isn't something I'd think would help here. Lowering it might, as might a fuel regulator with a pinhole sized bypass to bleed off line pressure during hot soak. You might try comparing your fuel level in the float chamber at shut down to the level after hot soak. If fuel has gone missing it's a fair bet it found it's way into the intake. If it hasn't the level may have been forced up by percolation in the supply line. Ideally there would be no change at all. But that black smoke at startup has to come from an over-rich mixture and for that to happen the fuel has to come from somewhere. If it isn't coming out of the float bowl then you have gremlins dipping it out of the tank and pouring it down the intake.

Jim

Where is your fuel filter mounted? In the photo gallery, I've noticed a lot of them mounted in relatively warm places and some of them in plainly hot places.

If your process is to hold the throttle wide open to start a warm engine... well, as a practical note, it seems unlikely that you can get the throttle wide open without the accelerator pump squirting. You might just once try starting the warm engine without touching the throttle pedal.