The MGA With An Attitude
VAPOR LOCK and Hot Air Flow -- CB-104

At 07:52 PM 8/28/04 -0400, rsperbeck wrote:
>"I have a MKII .... I have put a sidedraft Mikuni carb on it and a newer radiator from Moss. If I drive it on hot days 85 and get stuck in traffic it will vapor lock .... I cannot remember having vapor lock this bad with the old SU carbs. Any suggestions?"

When the MGA is standing still or moving slow and idling, the engine bay acts like a Dutch oven to bake the carburetor(s) over the exhaust manifold (any pushrod engine MGA). It will need a heat shield between the carburetor and the exhaust manifold, and should have phenolic isolators between the carbs and the manifold. If that doesn't fix it, set up a blower in the 4" air hose in front to blow cool air on the carburetor (similar to the MGC setup).

There is a magic transition speed around 25 MPH where the air flow through the cowl vent reverses. At lower speed the air comes out of the cowl vent, and air is supplied to the vicinity of the carburetor(s) from the 4" hose in front. At higher speed dynamic air pressure ahead of the windscreen forces air to flow into the cowl vent. This apparently does a good job of cooling the rear carburetor on a dual carb car. I notice that my rear carb is considerably cooler than the front one after a hard run at speed, and the front carb is first to suffer from vapor lock (when it happens). Many times the front carb is too hot to hold a hand on while the rear carb is only warm to the touch.

That doesn't mean the air stops flowing in through the front hose at speed, but it may be inhibited some. Chances are that air flows in the front and also in at the cowl vent and flows out the bottom at speed. But there may be a condition near that magic transition speed when air flow stops completely at the cowl vent, and air flow slows considerable through the front hose, and the carburetor(s) simple get steeped in the remaining near stagnant hot environment.

A fan shroud helps a lot to keep coolant temperature within reason at slow speed, but it doesn't do much for the vapor lock problem. I don't have any personal experience with the Mikuni side draft carb setup, but if you don't have a heat shield in there, I suspect you're cooked (pun intended).

Addendum, May 2007:

On 18 May 2007 at 17:01:50 UK time, Marc Vernackt in Belgium wrote:
>>"I have problems in starting my MGA from 1962 when the engine is hot. My car runs well for the first time. If I shut down the motor, let's say after a half hour driven, it will not start. If I open the bonnet and wait for 10 minutes, the engine starts again. What can be wrong?"

Hot carburetors cause fuel to bubble in the base of the carb around the main jet. This is aggravated by alcohol content in the fuel. The bubbles make the fuel mixture go very lean so it may sputter and run bad or may not start. Solution is to pull the choke full out, give it partial throttle, and crank it up. Treat it similar to a lean running cold start, giving it just enough choke to keep it running well until fresh fuel flow and air flow can cool the carbs down a bit.

At 09:49 AM 5/19/2007 +0200, Marc Vernackt wrote:
>>"Does it mean that something is wrong with the cooling of the carb? Or is it just a common problem for the MGA? Any modifications you know to prevent this failure?"

It is the nature of the beast, generally aggravated by modern fuel formulas containing highly volatile solutions with high vapor pressure (evaporate easily with a little heat). When I was driving MGA regularly in the late 60's they had no overheating problems and no fuel related problems. So you tell me what has changed in the last 40 years.

I have learned to tolerate it. Up until about 10 years ago I could often find alcohol free fuel in another county farther away from the big city, and I would fill it up there either going or coming from farther places. I cannot remember finding any alcohol free fuel anywhere in the past several years, and it is not likely to get any better in the near future.

Most of my driving is on open roads, not so much in town or on stuffy expressways during rush hour, so I do not often encounter this problem in any serious way. It can happen if I have a hot run on the expressway, then stop for 5 minutes to fill up with fuel. It can also happen if I am autocrossing, take a hot lap around the track in warm weather, then shut it off while waiting for the next lap, and have to fire it up again 5 minutes later. Pulling the choke out for a minute or so is not such an evil thing when you get used to it. Although it can be a little embarrassing when other people notice your nice vintage car running like crap in tight traffic.

There are some things you can do if you want to spend time with some non-standard modifications. See here:
Intake-Exhaust Heat Shield - Carburetor Heat Shield - Cold Air Intake Box

You could otherwise install a computer controlled fuel injection system in place of the vintage carburetors. Aside from being a major modification, it would sort of take the edge off of your concours points at a car show. And it's not the sporting thing to do. That is one of the best charms of an MGA, shear simplicity leads to reliability and easy maintenance. But if this bothers you enough, you might try a better heat shield.

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