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Discussion Starter #1
'72 OIFrame 750 5 speed, amal concentric carbs

When I slow down, changing down gears and go to open the throttle, the bike has a slight pause before accelerating. Sometimes the bike stalls, or back fires.

I just cleaned out the carbies, by dismantling them and cleaning in petrol to see if that fixed it. I thought i had readjusted the idle and air screws properly, it seems ok but it still misbehaved badly when i went for a 5 minute drive.

Another clue is it sometimes doesnt need tickling when normally it always needed tickling. Does this mean running rich? or maybe a problem with the float?

Im going to see if i can tune it again today.

Any advice? Thanks!
 

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Check your float level and needle and seat. Make sure that they are functioning properly and not binding or sticking. Too much gas but it may not be a jetting issue. Just an educated guess.
 

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Discussion Starter #3
sorry, im a newbie...
how do I "check the float level and needle and seat" ?

any additional opinions advice would be great!

help!
 

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Discussion Starter #5
yes thanks for that. I have the manual. nothing mentioned about how to check the float level?

is it possible that it could just be idle and pilot screw adjustment that could be causing this problem.

i.e. a pause right when im accelerating. And upon decelerating, through the gears coming to a stop the engine stalls or if not stalls, it misbehaves backfiring and missing etc.

any help would be great!

thanks
 

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Discussion Starter #7
the stalling upon slowing down is completely new.

a slight shyness upon opening the throttle was probably always there, but now its really noticeable.

we just came out of a mild winter in to hot australian weather, then the problems started. I thought maybe the change in temp and humidity could be the cause?
 

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I'll stick my neck out and ask the silly question..........your chokes (air slides) are fully open, aren't they??

Just a thought from the "village idiot" (who has a tale to tell about that one!): Jim
 

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Discussion Starter #10
hey Jim,
Im not sure what you mean with the air slides. When i pulled the carbies to bits i was careful to reinstall the airslides in the correct orientation.

Its weird that this problem just started happening.

one of the floats had a thimble of fuel inside, which i drained and plugged the hole. i was thinking of replacing the floats.
 

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Discussion Starter #12
Does anyone think this could just be poor air/fuel mix due to incorrect pilot air screw / throttle screw positions?

My friend said the mixture might be too rich. Does this sound right?
Is this the same thing as too much fuel?

Call me the newbie, sorry gents
 

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No guaranteed fixes, but some suggestions on what to check. The hesitation makes me think you're too lean. Is the bike doing anything else, like struggling on acceleration or spitting back through the carbs?

This is what I would do -- other folks, please correct me if I've left something out:

- Check your valve adjustment (engine cold) to make sure that's in line. Valve adjustment and timing must be spot on before you start messing with carb adjustments. Since it makes it easier to rotate the engine with the spark plugs out, check the condition and gap on those while you're at it.

- Check your air filters are all stopped up with dirt.

- Check the inline filters inside the banjos on the bottom of the carbs.

- Check the float level.

- On my bikes, I've removed the choke system and plugged the hole in the carb top.

- Turn the throttle stop screws out till they're not touching the slides.

- Sync the slides in the carbs so that they rise and fall at the exact same moment (remove air cleaners so you can stick the fingers of one hand into both carb mouths and rest your fingertips on the slides while you start to open the throttle with the other hand).

- Screw the throttle stop screws in (one at a time, finger in the carb mouth touching the slide) until you just feel it start to move the slide. Once both are just touching, screw them in 2 turns each or so.

- Put the needle in the middle position/groove. While you have the carb tops off to do this, check that the slides are a nice sliding fit in the carb bodies. If they wiggle around, then the carb bodies are worn and you're going to need to resleeve the bodies or get new carbs.

- 1.5 turns out from full-in on the pilot screws.

- Fire it up and adjust throttle stop screws equally to get a good idle (best done when engine is warm, otherwise, engine will be racing when it does warm up).

- Check for air leaks around the manifolds.

This should give you a good baseline for fine-tuning the pilot screws and idle once it warms up to operating temperature.
 

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Hi, Oily,

The air slides are the choke mechanisms that drop down when you close the choke lever. They are in the rear of the carbs, thus you can see or feel them quite easily if one or both of them is down.

Hope this helps: Jim
 

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Discussion Starter #16
Hello McQueen, that is comprehensive! thank you very much. I will use this as my guide.

I have identified i need to replace the air filters and one float is taking on fuel.

Its a starting point anyway.
 

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Discussion Starter #17
Still having the problem

Hi guys, Im still having this problem with a new thing.

The bike has become a good backfirer both when starting and when riding. Also before the problem I always had to tickle the carbs, now I only have to sometimes. I think these are the best clues

Sometimes it wont start until it backfires then its ok.

I have replaced floats, all gaskets/ o-rings in the carbs, new air filters. The carbs slide up and down perfectly together and the needles settings are in the middle.

A few days ago after replacing the parts, I thought I had fixed it but today it wouldnt start for ages, backfired and then rode ok for a kilometre then strarted misbehaving, stalling, backfiring. i had to limp home.
 

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The tendancy for the engine to "sag" when you come back onto the gas is for me an indicator of lean mixture rather than rich. I would follow previous advice given and be particular about manifold seals as it might be sucking a bit of air with high manifold vacuum.

If all else fails try riching the mixture by adjusting the needle valve one position richer. I know this does not address the engine backfire. but it may be related?

Let's hope you find a solution to your problem soon.

Regards. RodG
 

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sometimes an extreme lean condition can cause a backfire. If the mixture is too lean to combust in the cylinder, it passses into and collects in the mufflers until there is enough fuel to ignite/backfire but it usually happens on deceleration. I suspect there is something else amiss here. If the tickling (enriching) is not necessary sometimes and it doesn't start until the backfire, it sounds like an intermittant electrical something not providing proper spark.
 

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'72 OIFrame 750 5 speed, amal concentric carbs

I just cleaned out the carbies, by dismantling them and cleaning in petrol to see if that fixed it. I
Any advice? Thanks!
Fuel is not going to remove any varnish that may be restricting the small passageways and jets.
It takes something more serious the fuel.
 
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