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MG MGB GT V8 Factory Originals Technical - 2 Qs: idle speed and air cleaner sufficiency


What is the lowest I should set the idle speed? My engine has a bit of a cam in it; it's my understanding that you don't want to idle any slower than the amount it takes the lifters to "pump up" with oil, or the lifters can slam into the cam lobes with resulting premature wear. True? ... and if so how fast is this?


Next, how do I tell if my air cleaner's too low and thus choking the motor? To get the bonnet to close, I am forced to use a very, very, very low air cleaner base and a 2" filter. With the choke tower, etc., it looks like there's almost no room left for air to get in. Is there any test to determine whether or not it's enough?

Could you provide a few more details on the engine, carb, how much cam is "a bit" etc?
Not much to go on by what you have posted so far.
Cheers, Pete.
Peter Thomas

As a response to your q2, can I suggest you take a look at the Summit catalogue (USA) look up the K&N page, they do a 14" dia top called an Xtreame, kind of very gentle cone shape, this replaces the metal chrome top you find on many US engines. It is made of the K&N filter material. When I did the sums the sq in surface area is pretty much the same as a 2 1/4" high "wall" type filter with really no restrictions and breathing directly through the top. I have a mate in Boston who sent me one and appears to work just fine. I don't know what carb you are using. I have a 4 barrel and the actual carb inlet areas are pretty small, all you are looking for is filtered free air flow. I should add, the car has C bonnet, louves and 2 1/2" cold air pipe up from below front valance. Might help Regards Richard


Thanks for the info. I'm kind of surprised at your conclusions. As to the X-Stream, it won't fit because it's quite a bit thicker than a regular steel air cleaner lid. The choke tower gets in the way. If I use a perimeter filter high enough to make the X-Stream clear, the bonnet won't shut. But even if it would fit, I'd still be dubious because it's right up there against the bonnet so not a whole lot of air is going to be sucked in from up there.

I have to say I'm just amazed that a 2-1/2" pipe is enough to get air in! I mean cold air is no doubt just super but that's like breathing through a straw, no?

The best answer, methinks, may be to get an RV8 bonnet -- sure to be a real PITA stateside. But I was just wondering if there's some kind of test I could run to determine whether I'm choking off the air supply at higher rpms.

If it revs freely to redline under load, especially in higher gears, you don't have much of a problem.
George B.

I had the exact same problem and found out that indeed air supply for my engine was chocked off at high rpm using a 2 inch filter and "low rider" air assembly. I think that there was only about 1/4 - 1/2 inch from top of carb to bottom of air cleaner lid. Lacking an test equipement, try a few passes without an air filter. A little risky, but hopefully you'll see a difference in performance.

If you want to e-mail me, I can send a photo of one person's solution - a remote air filter that works quite well.

So what did you do to solve the problem seeing as you had the same one as Bill?

Joaquin, good call. I did take it out on the highway for 10-15 miles -- was a little damp last night so hopefully there was not any material amount of dust. Without question it revs better without the air cleaner and keeps pulling all the way up to -- and through - redline. Have to figure out a better setup.

I'm not the most knowledgeable carb guy, but I've heard of people milling off the choke assembly to lower the carb, would that give you more room for an air cleaner? Will a lack of a choke hurt you in North Carolina or do you not use the car in the winter?

Just a suggestion,

No question you can do that, but two things: (i) in the winter it gets plenty cold and I think a choke is seriously helpful, and (ii) my carb is a new old stock Carter 400 that I found in the back of an auto parts place and is probably worth a fair amount so I'd really hate to go cutting it up!

I cut a long rectangular strip of sheet metal, about 2 1/4 inches tall, to place between the top and the air cleaner base to make an enclosed housing. Brazed the strip to the base and cut out an oval opening on the side to allow a large plastic elbow (skinny oval on one end and round on the other) then ran a 3" flexible tube from elbow to enlarged hole where the heater fan should be, finished off with a cone filter to allow positive airflow from cowl. Easier drawn than described, but its basically an empty housing with a remote filter. You'll get cooler outside air plus eliminate the restrictive 2 inch tall filter (if you look at the filter you'll probably see the it's not even 2 inches as the rubber top and bottom probabbly take up 1/2 inch from that 2 inches).

sorry, that was a 4" flexible tube

In regards to the 2.5" inlet pipe. I do not have exact sizes, but look at any late model mustang. they are 5.0L/302CID and they use inlet equipment in the 3" range. In fact I have a mass air sensor here and it is 3" While I would aggree a larger tube is nicer for max airflow, you don't need something that huge if you don't have room.

I am using a 2" K&N with the Extreme filter all on a 1" drop base unit. My carb is a Carter AFB (625cfm on my litly warmed 302) I have not had too much trouble with WOT, though the mechanical secondaries Holley was a bit beter top end. It could come down to that milled chock tube on the holley though....
Larry Embrey

I have a 350 Holley with a 2.5 inch filter which has 1.75 inch drop base on my 3.5 Rover. Initially I had the same problems at idle and under hard excelleration. I found that the inside top of the air cleaner was actually placing enough pressure on the choke butterfly, that the choke was approx half to three quarters closed. I removed the choke assembly and had the housing milled down enough to clear the inside of the filter, then so as not to interfere with the breather hole or the retainer thread for the air cleaner top, machined two cut outs to the front and two to rear down to the base of the choke housing. Doing this fixed the problem and it has run perfect since. It obviously does take a bit longer to warm up on cold mornings (it gets down to around 0 - 4 degrees celsius in winter here in Adelaide) but I haven't found that a problem.

Kim Porter

This thread was discussed between 23/09/2002 and 27/09/2002

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