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Author Topic: The AVL does it again - carb woes. Help!  (Read 512 times)

Machismo

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The AVL does it again - carb woes. Help!
« on: September 25, 2013, 05:33:44 PM »
Gentlemen, I think am one of those who post more to ask questions than answer.
After ages of experiments with the BS 29 carb and jets, I had finally settled on a BS 32 carb, 17.5 pilot and 120 main. Goldstar mimic exhaust. No PAV and D-0100 K&N in the stock side box.
Well this set up ran great until about a month back when i did some changes to the breather. Erratic running, loss of acceleration, in short - PITA to ride.
Since I reverted some of the breather changes, thanks to the guys here, the bike runs much much better.
However, there is this strange expulsion of air(?) underneath the carb flange that attaches the carb onto the intake manifold.
While in idle, there is this sudden,intermittent noise(just like those buses do, on releasing air) and the bike tends to stall. Same issue when riding as well.
If i place my fingers on the underside of the manifold rubber, i can feel some agitation and the set up really vibrates(although its firmly connected).
No crack in the flange rubber or the rubber connecting the airfilter and the air box. Fuel supply is good.
I am baffled. Please suggest..

Sudhir


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Re: The AVL does it again - carb woes. Help!
« Reply #1 on: September 25, 2013, 06:36:58 PM »
You clearly have an air leak somewhere on the intake manifold.  My guess is that if there is no cracking, splitting or tearing on the rubber then the throat on the carb is slightly smaller than the opening on the intake manifold.  If this is the case what's happening is the clamp is holding the carb tight, but somewhere the rubber is pinched and bunched up causing your leak.  I would pull the carb and intake manifold off and give everything a thorough inspection.  Look particularly close to the areas where the rubber mounts to the carb to see if the rubber looks distorted.

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barenekd

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Re: The AVL does it again - carb woes. Help!
« Reply #2 on: September 25, 2013, 06:38:01 PM »
A little hard to tell by the description, but I'd try sealing the the carb/manifold joint with some gasket sealer first. Then, if that didn't work, I'd look for more esoteric things like timing and checking the intake valve for leaks.
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DanB

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Re: The AVL does it again - carb woes. Help!
« Reply #3 on: September 25, 2013, 07:27:25 PM »
I had a leak from the Oring gasket on the rubber flange. Replaced it and it was good. But mine issue was not intermittent. Does it happen all the time?  Notice any difference with compression?  Your inlet pushrod is not too tight, right?
Suppose I were an idiot, and suppose I were a member of Congress; but I repeat myself. ... Mark Twain
2006 AVL Electra

Machismo

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Re: The AVL does it again - carb woes. Help!
« Reply #4 on: September 25, 2013, 08:08:08 PM »
I had a leak from the Oring gasket on the rubber flange. Replaced it and it was good. But mine issue was not intermittent. Does it happen all the time?  Notice any difference with compression?  Your inlet pushrod is not too tight, right?
Dan, I always notice this issue while doing a cold start in the morning. Its my habit to idle the bike for a couple of mins. That's when the sudden puff kinda sound(akin to when one loosens the valve pin of a tire) is heard and the idle immediately falters, sometimes dies.
No real difference in compression. Recently loosened the inlet pushrods.

You clearly have an air leak somewhere on the intake manifold.  My guess is that if there is no cracking, splitting or tearing on the rubber then the throat on the carb is slightly smaller than the opening on the intake manifold.  If this is the case what's happening is the clamp is holding the carb tight, but somewhere the rubber is pinched and bunched up causing your leak.  I would pull the carb and intake manifold off and give everything a thorough inspection.  Look particularly close to the areas where the rubber mounts to the carb to see if the rubber looks distorted.

Scottie
The rubber flange is new - in fact, was installed as part of this new BS 32 carb installation.
Had to smear some oil on the carb's mouth and use brute force to get teh rubber on.
So I am clueless how its leaking the air. Might as well remove the parts and have a look.
Does your flange give violent vibrations while the engine is on?

A little hard to tell by the description, but I'd try sealing the the carb/manifold joint with some gasket sealer first. Then, if that didn't work, I'd look for more esoteric things like timing and checking the intake valve for leaks.
Bare   
I understand Bare, too difficult to pin point without actually hearing the noise.
Could you tell me how timing can have an effect similar to this?
Is it possible to check intake valve leak without removing the head?

Thanks a lot guys!

DanB

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Re: The AVL does it again - carb woes. Help!
« Reply #5 on: September 25, 2013, 10:44:48 PM »
What's interesting is that sound you describe as 'air being released from a tire'. That represents a pressure build up and release. I can't believe the intake and the draw from the piston would create that kind of pressure. Could it? 
I'm wondering if there is a back pressure being created back into the carb as the piston rises and the inlet valve is not closed all the way. That could be timing or a valve leak.

Since timing is electric, then I can only think of the timing gears being off. But I'd think youd notice the prob all the time and not just intermittently. Therefore, the inlet valve could be leaking for some reason. When you pull the rubber flange to seal it, look inside the carb mouth. Any deposits or signs of blow back into the carb?

Just sharing my line of thinking... For what it's worth
Suppose I were an idiot, and suppose I were a member of Congress; but I repeat myself. ... Mark Twain
2006 AVL Electra

Machismo

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Re: The AVL does it again - carb woes. Help!
« Reply #6 on: September 26, 2013, 05:00:44 PM »
What's interesting is that sound you describe as 'air being released from a tire'. That represents a pressure build up and release. I can't believe the intake and the draw from the piston would create that kind of pressure. Could it? 
I'm wondering if there is a back pressure being created back into the carb as the piston rises and the inlet valve is not closed all the way. That could be timing or a valve leak.

Since timing is electric, then I can only think of the timing gears being off. But I'd think youd notice the prob all the time and not just intermittently. Therefore, the inlet valve could be leaking for some reason. When you pull the rubber flange to seal it, look inside the carb mouth. Any deposits or signs of blow back into the carb?

Just sharing my line of thinking... For what it's worth
Well Dan, your line of thinking is very much sensible to me and coming to think of it, I do remember seeing a bit of pale orange'ish deposits on the cylinder head inlet. I never imagined this could be the result of blow back but the other way round - carb towards the cylinder head. So could it be that the inlet valve is leaking?
What could cause a blow back from the inlet valve?
 Today morning, just as I was ready to ride to work(about 6miles), plonked in a NGK BPR7ES spark plug. The bike's response seems better. Perhaps the old 9ES plug was too cold for teh short rides in the city.

TejK

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Re: The AVL does it again - carb woes. Help!
« Reply #7 on: October 01, 2013, 07:04:45 AM »
Try going up to a 25 pilots - that 'air'  pucking noise used to come on my BS32 carb on cold starts while warming up as it was running lean. If its a 350 then try out a 125 main and 25 pilot. that works really well. Adjust the fuel screw to 1 3/4 -2 turns out.

If its a 500 then a 25 pilot and a 130-140 main works quite well without any cold starting issues. Fuel screw with 25 and 140 main open about 3/4-1 turn.

Hope this helps - in some way!