Created by: | Orig. Posting Date | User Name | Edit Date |
| Feb 9, 2017 05:32PM | Dan Moffet | |
| Feb 9, 2017 04:23PM | TDirkdiggler007 |
Posted:
Feb 9, 2017
05:32PM
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Total posts: 9542
Last post: Apr 18, 2024 Member since:Aug 14, 2002
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It would depend on what type of carb you have. SU carbs are generally pretty stable once properly tuned.
A carburetted engine typically idles low until the engine (and some carbs) are fully warmed up.
Cars with SU carbs should be tuned warmed up - idle mixture and then idle speed. Most street engines in decent condition should idle in the 800 - 1000 rpm range... 1300 would be high.
Until it is warmed up, and idles smoothly, the engine will usually need some choke, to both richen the mixture and boost the idle speed - the choke linkage has a 'fast idle' cam for this.
If you have a SU HS type with the fuel bowl on the side, the 'choke' pulls the main jet down to richen the mixture. Some HS types have a Waxstat jet, which has a device on the bottom (with wax in it) that expands as the carb warms up, pushing the jet up slightly to lean out the mixture as an 'economy' feature. This means you have to take its action into account as you tune the carb.
If you have a SU HIF type, with the fuel bowl at the bottom of the carb body, the 'choke' activates a valve that controls flow in a secondary passage to let in additional fuel to richen the mixture. On these, the main jet is hidden inside, with its bottom end immersed inside the fuel bowl. The mixture screw (which you turn IN to richen, counter-intuitively) operates a lever to pull the jet down. The difficulty is in this lever, which is bi-metallic, and acts like the Waxstat to lean out the mixture as the carb warms up. The hard part is that as you try to adjust mixture, the heat from the exhaust manifold heat the carb up more than when the car is moving, counter-acting your adjustments.
You have to figure out what you've got and how to tune it.
My guess is your engine is tuned rich and its mixture improves as it warms up, resulting in the idle speed climbing.
A carburetted engine typically idles low until the engine (and some carbs) are fully warmed up.
Cars with SU carbs should be tuned warmed up - idle mixture and then idle speed. Most street engines in decent condition should idle in the 800 - 1000 rpm range... 1300 would be high.
Until it is warmed up, and idles smoothly, the engine will usually need some choke, to both richen the mixture and boost the idle speed - the choke linkage has a 'fast idle' cam for this.
If you have a SU HS type with the fuel bowl on the side, the 'choke' pulls the main jet down to richen the mixture. Some HS types have a Waxstat jet, which has a device on the bottom (with wax in it) that expands as the carb warms up, pushing the jet up slightly to lean out the mixture as an 'economy' feature. This means you have to take its action into account as you tune the carb.
If you have a SU HIF type, with the fuel bowl at the bottom of the carb body, the 'choke' activates a valve that controls flow in a secondary passage to let in additional fuel to richen the mixture. On these, the main jet is hidden inside, with its bottom end immersed inside the fuel bowl. The mixture screw (which you turn IN to richen, counter-intuitively) operates a lever to pull the jet down. The difficulty is in this lever, which is bi-metallic, and acts like the Waxstat to lean out the mixture as the carb warms up. The hard part is that as you try to adjust mixture, the heat from the exhaust manifold heat the carb up more than when the car is moving, counter-acting your adjustments.
You have to figure out what you've got and how to tune it.
My guess is your engine is tuned rich and its mixture improves as it warms up, resulting in the idle speed climbing.
.
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I have a 69 mini with a single carb. The idling is fine until I start hitting stop lights and the engine really starts to warm up. The temp gets to about the mid point mark on the gauge and the idling goes from 1300 to 2000 rpm. This seems like a carburetor issue but I just had the car serviced not too long ago and this has always been a problem with this car. Is this just a normal problem to have with a mini or can this problem possibly be alleviated?
