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Rough running and backfiring

GaryBeu

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Still can't get trustworthy running in the '71 MGB. We've checked the points, new plug wires, dismantled and cleaned the carbs including needles and adjusted by the Bentley book as best we can. Idle is rough (engine sorta shakes) unless we increase idle to about 1500. On acceleration (at standstill or under load) it hesitates and then backfires at about 3k - 3.5k rpm. We don't know what adjustment to make at this point. Help? Suggestions?
 

davester

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I would check the following first:

1. General engine health: Have you done a compression check? Also, a vacuum gauge attached to the manifold might tell you something.

2. Vacuum leaks: When you rebuilt the carbs did you check the throttle shafts for leaks? While it's running you can spray carb cleaner where the throttle shafts exit the carb body. If the engine speed immediately changes (due to cleaner being sucked into the leak) you know that your carbs need rebushing. Also, do you still have the gulp valve attached...it might be leaking? Try blocking off the hose from the valve to the engine.

3. Bad spark plug? Try changing the spark plugs, or perhaps pulling one lead off at a time to see if it makes a difference. If you can pull a lead with no change to the idle then you've found your problem.
 
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GaryBeu

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Thanks guys...we'll try all of that. With it in the driveway, a quick acceleration of the throttle causes a backfire in the front carb.
 
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GaryBeu

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Uhhhh...we are total neophytes when it comes to mechanicing. Have no idea what an anti run-on valve or a gulp valve might be, what they look like, where they might be, or what they are supposed to do. Any assistance would be greatly appreciated :smile:.

We pulled the plugs and they were all dry but totally black. Cleaned them and went back to step one in Bentley manual and adjusted the carbs again. She seems to run really well right now. MAYBE we have her cured and hope to drive her to town tomorrow and get her registered...it's kinda scary even doing test drives with no license plate!
 

davester

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There's a section in the Bentley Manual on emission controls. I believe it has a diagram showing the gulp valve and anti-runon valve. I think there's also a section in the Bentley Manual that has the drivers handbook for the later cars which also shows these devices. In addition, you can go to the moss site and they have all sorts of parts diagrams.

Make sure you check those throttle shafts. If they've never been done I assure you that they need replacing and rebushing. You'll never get the carbs adjusted right with them leaking.
 

JPSmit

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black is rich which would do exactly what you describe. run it for a while and see what happens.
 

DrEntropy

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Taking it "outside the yard" is a first step to relying on your understanding of the beast. A bottle of water in a sprayer taken to the throttle shafts on the carb will tell if the shafts are worn. At idle it will change tune when squirted.
 
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GaryBeu

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OK here's where we are. Sprayed water on throttle shafts and had no change in anything. Plugs were black, sooty and dry. Cleaned them, leaned mixture and got black plugs again. We have set carbs by the Bentley manual (all of our emmission controls except the carbon canister are gone) again. Screwed mixture screws up until jet was level and brought down (lean) two full turns, etc. We are now THREE full turns plus two flats down to lean and drove 3 miles, had black sooty plugs and a "stutter" in the exhaust and rapid backfire in the carbs on acceleration to 2,800-3,000 rpm. I'm at my wit's end and HATE to give up but, I guess I need to take the car to the local "expert" who says he'll need to time it, adjust the valves and then tune the carbs for $200.00+ (which I don't really have but...don't know of a better solution right now. Grrrr!! :smile:
 

JPSmit

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are you turning the nut correctly? anti-clockwise for leaner clockwise for richer. you may be turning it out too much. I know you are frustrated but I think you are close.
 

DrEntropy

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Try setting the jets back to level with the "bridge" then use one and a half turns of the adjuster for a starting point. To lean it you turn the adjuster back the other way (raise the jet back toward the bridge).
 

JPSmit

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BTW Gary, if the car is running normally except this, I can almost guarantee it is neither timing nor valves. You may need to adjust them at some point, but, timing and valves don't really unadjust themselves. To get into that is to make it all too complicated. Deal with one variable at a time!!!! (don't ask me how I know)

Doc gives good advice but, (without insulting your intelligence) you might be asking yourself, what is the bridge? It is that part of the carb between the air intake and the manifold. So, there are two ways to measure the needle placement. 1. The typical way is the number of "flats" on the nut on the bottom - what you have already been adjusting. Each flat is one flat edge of that nut.

2. Distance from the bridge. To do this, take the bell of the top of the carb, remove the "sleeve and needle" so that you can look down into the carb itself. The brass ring in the middle is where the needle. By turning the nut below, you can bring the mechanism up flush with the bridge (it will make sense) by turning it anti clockwise. Then, as doc says, turn one and one half turns clockwise to get you in the ballpark. (this corresponds to 60-80 thou if you have a caliper). Reassemble the carb. (this is a good time to make sure there is oil in the bell - use engine oil for now) and fire it up. Assuming it runs, take it out, run it for a few miles, come home, when you pull into the driveway, shut off the car and coast to a stop. (don't let it idle) pull the plugs and see the colour. If black, turn the nut one flat to the right (anti clockwise) clean the plug and go drive again. Same drill till they look the right colour. Then go buy a new set of plugs, make sure they are gapped right, install them and drive the wheels off. You are close I promise you. keep calm and carry on.
 
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GaryBeu

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JP and Doc...THANK YOU SOOOO MUCH! I DID have a brain short and was turning the nut the wrong direction. My mind said that turning it down (clockwise) increased the air between the jet and the piston and vice versa. Once I drew myself a picture of the jet and needle, I actually understood! I'm going back to square one here in a few minutes and I'll report. We do have her registered now so we don't have to worry about driving on the roads. Thanks again for all of the support.

Gary and Carla
 

DrEntropy

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I wuz kinda thinkin' ya had it bas-ackwards from your prior post. Glad you didn't throw in th' towel!!

:thumbsup: :savewave:
 
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To plagarize Tom Hank's:

"Throw in the towel?? There's no throwin' in the towel in LBC/BCF land!!" :laugh:
 

davester

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DrEntropy beat me to it. Despite having done this countless times since the '70s, whenever I adjust the mixture I still have to do mental contortions to remember which way is lean and which way is rich.
 

Mickey Richaud

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"Righty-tighty; lefty-loosey"

Yer on yer own to proceed from there!

:jester:
 
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GaryBeu

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Guys, I really appreciate everyone on here being willing to admit their own mistakes from the past and help a guy who doesn't know much about this and is easily confused :smile:. Carla I are sure glad we found this forum and ~hope~someday to know enough to be able to assist someone else. We don't tend to throw in the towel...I can usually just get away from what I'm trying to do and maybe let my mind go to something else and, all of a sudden, something clicks on. Maybe just another something to try (that may or may not work). We are working on this between thunderstorms so it may be awhile before we have a report. Thanks again
 
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GaryBeu

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Well...we ~think~we have it adjusted! Seems to run really well at this point. We'll drive some more tomorrow and let you know. Thanks a million for helping us understand!! :smile:
 
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