Flat spot?

[William]

So, we've noticed a slight flat spot in the rev range, right around 3K rpms. Very strange, as it feels like the engine's not getting enough gas at that point-cruising along at 3K, the car feels like it stumbles just a bit, but put your boot in it and it pulls away. We think it may be some sort of air filter strangulation-our mechanic mentioned that the swanky chromed pancake filters the car came with are pretty restrictive. Does any of this make sense?
-William

 

[aerog]

What carbs are on the car? You said "filters" so I guess HS4s or HIFs?

I've got pancake filters on mine and I don't have any 'flat spot', nor is it noticeably lacking any power at all.

 

[Guest]

I noticed that on my '72 B which has HIF's. My problem was they were running lean. I used to take great pains to get them adjusted just so which involved enough time for them to get very hot.
I thought this was the way they should be done until I saw in the manual to adjust them just after it comes up to the thermostat opening temp then do it quickly before they get too hot. They turn out too lean if adjusted too long.
These carbs have a bimetal on the mixture adjustment. I'm not sure if ones other than the HIFs are effected in the same way.
It runs great now.

 

[William]

Mmmm, forget which kind of SUs are on this particular car. It's a '71,and has some sort of performance cam in it, installed by the PO. We've no idea what kind of cam, so carb adjustment may very well be off a bit. It's weird, but you'll be cruising along, on part throttle, and the car feels like it's stumbling a bit, rather like when it's cold and you give it too much gas and it chokes a bit. But like I said, hit the gas and it just goes. I was out in the car today (quite a bit, actually), and it was very hot outside, and this little hesitation/flat spot/stumble didn't seem to occur, at least not much, and certainly not when accelerating from a stop, going through 3000rpm up to 3500 or 4K (I very rarely go much higher, and don't need to in traffic). Just a little weird. Frnakly, it would not surprise me if it was the air filters (they're the chrome pancake style ones with embossed MG crests on them, very dress up). Our mechanic suggested getting a K&N or similar, as he said that the engine "just screams" without them.
-William

 

[aerog]

A side-effect of going with the K&Ns is you might need to reneedle the carbs, but I think that depends on the type of carbs, the HIF's aren't supposed to have as much adjustment as the others.

Regardless, you shouldn't have the symptoms you describe because of the filters. They're not the best, but they're not causing the engine to suffer either.

You can tell if you have HS4s by looking for a separate float bowl off to the side of the carb and the base of the carb jet will be visible on the bottom of the carb (the jet has a large jam-nut on the bottom).

The HIFs have an integral float bowl located in the bottom of the carb and no exposed jet. The mixture adjustment is done via a small adjustment screw on the side of the carb.

I have HIFs and haven't had much trouble with them, except that bi-metal temperature-sensitive mixture control can throw you off as was mentioned. Rather than try and find the "just right" temperature I let the engine warm up, set the mixture, then gave them about 1/8-turn adjustment on the rich side.

The bi-metal plate is an integral part of the HIF's mixture adjustment mechanism and is present only in the HIFs as far as I know. When you turn the mixture adjustment screw you're actually pushing or pulling on the "bi-metal" adjuster. The adjuster is "L"-shaped and the base of it connects to the base of the jet. When you move the adjustment in and out the "L" pivots, pushing the jet up and down. When the temp changes the "L" is forced to pivot, moving the jet up and down. When everything is adjusted properly it seems to work pretty well.

 

[aeronca65t]

Try running it without the filters for an hour or so (it won't hurt anything)....see if it's improved.......if it runs better, it's "new filter" time.
You *do* have oil in the carbs, right?
...and a clean fuel filter?

 

[Guest]

Another thing you can do is look at the plugs after a run at highway speed. If they are black and sooty, you are running too rich. Mine were very light gray when I was too lean. Also, when the filters are a problem, the car usually runs rich as the clogging of a filter is like closing off the intake to the carb slightly which will make it pull more gas hence darker plugs.
I played around with the jetting on a previous hot rod Dodge in the sixties that I put a big holley on. When it was too lean it did the exact same thing. Thats why I knew when my MG was doing it.
However, having said that, points can cause similar problems and can really fool you.
Both of my catalogs say the HIF was from 72 to 74 so I doubt the problem is like mine was. I think if it was me, I would try a tune up first before I do anything to carburation.

 

[Duane]

If it's not as noticeable when the ambient temp is hot, then it would seem it's running too lean. Cooler weather means denser air, and the need for more fuel in the mix. Try giving it a little choke; if it helps, there's your answer.

Duane

 

[William]

HS4's-they have the separate bowl on the side. We're thinking of going for a spin sans air filters. The car was running a bit rich before, so it may have gone over center to too lean. Dunno, really, and don't have the tools to adjust them anyway. It can't be too rich, as there's no real visible exhaust fumes. We have the old plugs somewhere (recently replaced), which I seem to recall were a bit blackened, but have not bothered to pull the new ones yet. I'll keep futzing.
-William