QuestionQUESTION: I have a 67 Mg midget. It idles smoothly and revs smoothly but when I drive it around it runs fine until around 45oo rpms then it starts to bog down. When I give it gas at the pedal it feels like it is getting too much air but if I choke it it does not help.I have adjusted the jets on the bottom of the carbs both ways but it did not help. Any ideas?
Thanks
ANSWER: Hi Tommy,
First be sure the air filters are clean, then get a manifold pressure gauge. (they are not expensive, Wall Mart)
Put the gauge on a vacuum port on the intake manifold and run the gauge out from under the hood and place it under a wiper blade so you can read it while driving. Slowly approach the speed that it bogs and read the vacuum. At no time should it drop below 5" hg.
If the gauge goes to -0- first check the timing then if that is ok, suspect a partially stopped up exhaust.
You can either get a short piece of brake line (about 2" long) and drill a hole in the exhaust pipe close to the front of the car. The hole should be just a little smaller than the diameter of the brake line. Cut the brake line at an angle on one end and drive that end into the pipe with the open angled part of the brake line facing the back of the car. Attach the vacuum gauge to the brake line and road test the car again. Read the exhaust back pressure. I found that most British cars can not stand more than 1 1/2 PSI at high throttle settings. GM claims some of their cars can stand up to 3 PSI but I have found that all the cars I have tested could not stand more than 1.5 PSI. To close the hole get a large diameter short sheet metal screw for any hardware store and pull the brake line out and use the screw to seal the hole.
If that seems to be too much for you just remove the muffler and road test the car to see if it runs ok. I don't do that because most mufflers are rusted solid. Also other model cars have mufflers welded in and two mufflers and some have CATs.
Let me know,
Howard
---------- FOLLOW-UP ----------
QUESTION: Thanks Howard. I replaced the whole exhaust system from the manifold back in Spring of 2007. Think it could have clogged since then?
AnswerThe test is the only sure way to know. If you want to by pass that test you can test Ignition by connecting a timing light to the coil wire and put the timing light under a wiper arm and tape it in place and tape a piece of card board over the light end so you can see the flash in daylight. Then drive the car to the point of failure to see if you loose the flash. If that is ok, then do the same with a fuel pressure gauge put in a "T" just before the carbs so you can read fuel pressure at the time of failure (bog down) to see if fuel supply is a problem.
Let me know,
Howard
