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Diagnosis?

Bob: IM S6Bob: IM S62016 Intermeccanica S6 Supporting Member
It's kind of quiet on the site these days, so here's a problem to solve.

My 2.2 litre roadster with 48 IDF Webers exhibits the following:

Slight backfire on cold start (no chokes), in one throat. Once warmed up, idles nicely around 1,000 rpm, and revs fine.

When cruising, the engine runs slightly rough while 'floating' and maintaining a steady speed, but accelerates and decelerates fine (sometimes a little rough when decelerating). The engine pulls fine all the way up to 5500 rpm, and I am getting good gas mileage, but it is smooth only when accelerating. There is no backfire when cruising, but it needs a slight gas pedal to keep it smooth. It seems the car is best accelerating, but not cruising.

I have checked the idle jets and cleaned them (didn't find any dirt), adjusted the mixture screws, checked the spark plugs. 6,000 miles on the engine/car. There is a full canister fuel filter up front under the tank.

Any other ideas? I can't find any exhaust leaks or other leaks.

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Bob: IM S6Bob: IM S62016 Intermeccanica S6 Supporting Member
It's kind of quiet on the site these days, so here's a problem to solve.

My 2.2 litre roadster with 48 IDF Webers exhibits the following:

Slight backfire on cold start (no chokes), in one throat. Once warmed up, idles nicely around 1,000 rpm, and revs fine.

When cruising, the engine runs slightly rough while 'floating' and maintaining a steady speed, but accelerates and decelerates fine (sometimes a little rough when decelerating). The engine pulls fine all the way up to 5500 rpm, and I am getting good gas mileage, but it is smooth only when accelerating. There is no backfire when cruising, but it needs a slight gas pedal to keep it smooth. It seems the car is best accelerating, but not cruising.

I have checked the idle jets and cleaned them (didn't find any dirt), adjusted the mixture screws, checked the spark plugs. 6,000 miles on the engine/car. There is a full canister fuel filter up front under the tank.

Any other ideas? I can't find any exhaust leaks or other leaks.

I Inter911
Funny, I've got the same problem in my 911. One cylinder coughs and sputters as it warms up and when at idle or cruising at sub-4,000 rpm wnen warm.

I had the filters off and you could see this one cylinder sputter up through the throat...a fine mist of gas each time it sputtered. Like its spitting up. Intermittent though...not regualar.

We ran out of premium gas here in town a couple of weeks ago so I had to use regular Shell and its way worse now. Even with Octane booster.

Brian
execk2execk2
Sounds like me at Carlisle...... BOOM and everyone hits the sidewalk. People are way too jumpy! I turned my left carb (leading carb) with the engine hot all the way out, then turned the right one down until it was almost out. Went back to the lead (left) and turned up the idle to about 1200 then went to the right and smoothed out the sound (sync) and it took the backfire out.

This after a bout of PA bad gas and you'd think at 2.19$ a gallon they would give you quality.
FM Former Member
when rich

it will have unstable idle
it will accelerate good
it will cruise backfiring or sputtering
it will run cold
it will backfire
it will smoke dark
it would use a lot of gas

when lean

it will have ok idle but a little low
it will accelerate crappy
it will cruise ok but lack power
it will run hot
it will destroy your engine soon if lean for long
it will lack power
FM Former Member
Make sure your not running rich and you may have to remove the carb in order to put a new gasket in. My right side carb was doing exactly what you describe and after insuring it was completely blown out and a new gasket it runs perfect. My carb manual says not to use the metal type gasket so we made them out of the proper gasket material and it now works perfectly.
FM Former Member
my guess---vacuum leak.
causing lean mixture on the one cyl. which causes backfire
through carb. also would cause floating rpm.
my disclaimer-don't do this at home--danger---(but works great) in the past i have taken a flammable liquid and sprayed
it around carb base, manifold base and throttle shaft areas
with engine running. once i heard the rpm change i found
my trouble.
good luck
FM Former Member
Robert, It sounds like one carb is running a little leaner than the others. That's the reason for the pop on idle. The others don't pop because the mixture is richer, like you would do if you were able to choke it(richening the mixture). This maybe caused by a vac. leak or just a slightly lean mixture. Try locating a vac. leak first, then try opeaning up the idle mixture screw a little , if this doesn't work you may need to try one step richer on the idle jets. Eddie
Bob: IM S6Bob: IM S62016 Intermeccanica S6 Supporting Member
Sorry to hear about all the bad gas some of you had. My problem has turned out to be loose nuts.

I finally had time to do some physical probing and sleuthing (after about three days of downloading articles on Weber carb jetting and tuning) After examining jets, adjusting mixtures, etc. I took Bruce's advice and tried spraying around the carb gaskets, etc. but no difference. I then noticed that when putting a little pressure on the carb with my hand, the idle speed changed.

With a 911 type shroud it's hard to even see the nuts, let alone get at them. Tightened up the nuts holding the carb to the manifold, took it for a run, and smooth running.

One problem solved. And I know a lot more about Weber carbs than before.

How come we usually overlook the simple solution?
Bob: IM S6Bob: IM S62016 Intermeccanica S6 Supporting Member
Steven: It was the gasket leak that was causing the problem, but it just needed tightening. Thanks for the suggestion originally.

Bryant: Took the car for a little run just before supper tonight, and much less hassle on cold start up. With no chokes, these carbs are rough until the engine warms up (a minute or two), but tonight was an improvement.

In a way, these little problems force you to learn about your car, and that's a good thing (as Martha S. would say).

Bob
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