Carb diaphragm or something else?

olivera · 24-09-2019, 08:53 AM

Can anyone share details of the symptoms of a damaged carb diaphragm please? I have an issue with midrange throttle where the bike (1995 M900) runs on one cylinder. The bike runs fine at low throttle position (I believe this is where the carb is using the pilot jets), it then runs on one cylinder during the midrange (usually where the carb runs on the needle jet) and runs fine when at full throttle (main jet). Before I start to strip the carbs, thought I see if anyone has had a similar problem or can advise me of the symptoms of a damaged diaphragm.

Any advice would be most welcome please?

slob · 24-09-2019, 09:07 AM

I'd guess something else, the carb in question would struggle to achieve full throttle with a split diaphragm, since lifting the slide relies on a pressure difference either side of the diaphragm.

I'd try swapping the igniter boxes round and see if the misfire changes cylinders as a first step

Darkness · 24-09-2019, 04:16 PM

Quote:

Originally Posted by slob

I'd guess something else, the carb in question would struggle to achieve full throttle with a split diaphragm, since lifting the slide relies on a pressure difference either side of the diaphragm.

I'd try swapping the igniter boxes round and see if the misfire changes cylinders as a first step

I’d try that first too as it’s probably more likely to be ignition unless we are seeing the start of ethanol related rubber issues?

Flip · 24-09-2019, 05:45 PM

I’ve seen those symptoms too on a mates BMW F650, so Rotax single motor fed by a pair of Mikuni CV carbs.

It turned out one of the diaphragms had split on the edge where it folded over.
On that bike it was relatively easy to get to them and pop them open to look inside so I would take Slobs’ advice and swap the ignitor boxes first as that doesn’t involve removing the tank and airbox to get to the carbs.

Quote:

Originally Posted by Darkness

I’d try that first too as it’s probably more likely to be ignition unless we are seeing the start of ethanol related rubber issues?

I’m a little surprised that we haven’t seen it on diaphragms already to be honest as I have to regularly replace the rubber inlet manifold from the Amal carb on my little race despite my best efforts to use Ethanol free fuel.

Darkness · 24-09-2019, 03:17 PM

Quote:

Originally Posted by olivera

Can anyone share details of the symptoms of a damaged carb diaphragm please? I have an issue with midrange throttle where the bike (1995 M900) runs on one cylinder. The bike runs fine at low throttle position (I believe this is where the carb is using the pilot jets), it then runs on one cylinder during the midrange (usually where the carb runs on the needle jet) and runs fine when at full throttle (main jet). Before I start to strip the carbs, thought I see if anyone has had a similar problem or can advise me of the symptoms of a damaged diaphragm.

Any advice would be most welcome please?

I’ve had those symptoms before, and it was a holed diaphragm, but that was on a Stromberg carb on a Vauxhall HC Viva.

Darkness · 24-09-2019, 04:15 PM

Quote:

Originally Posted by Darkness

I’ve had those symptoms before, and it was a holed diaphragm, but that was on a Stromberg carb on a Vauxhall HC Viva.

It was a fairly small hole.
With small throttle opening there was low vacuum, so the piston and needle didn’t lift, but wasn’t meant too anyway.

At full throttle there was enough vacuum to fully lift the piston and it felt fine.

The problem progressively became worse at anything between full throttle and idle as the air leak was sufficient to stop the piston and needle lifting as much as needed: the engine ran very lean with no usable power. I got home by red-lining it off the line in first, then changing up to top and trying not to press the accelerator as it would bog down and need another blast in first to keep it moving.

slob · 24-09-2019, 03:44 PM

I stand corrected

350TSS · 24-09-2019, 07:30 PM

The petrol should not be in the diaphragm chamber, just air below and air above so we should not get deterioration from ethanol. There may be a slight mist getting past the needle hole but you would have thought that the suction from the cylinder would draw it away?

Flip · 24-09-2019, 10:16 PM

Good point- I hadn't really thought that way and didn't really know if the vapour from Ethanol fuel would be enough over an extended period to deteriorate them.

Lets hope not

olivera · 25-09-2019, 08:56 AM

Thanks guys, igniter first and if that makes no difference i'll check the diaphragm.

24-09-2019, 08:53 AM	#1
olivera Registered User Join Date: Jul 2011 Location: Bedford Bike: M900 Posts: 55	Carb diaphragm or something else? Can anyone share details of the symptoms of a damaged carb diaphragm please? I have an issue with midrange throttle where the bike (1995 M900) runs on one cylinder. The bike runs fine at low throttle position (I believe this is where the carb is using the pilot jets), it then runs on one cylinder during the midrange (usually where the carb runs on the needle jet) and runs fine when at full throttle (main jet). Before I start to strip the carbs, thought I see if anyone has had a similar problem or can advise me of the symptoms of a damaged diaphragm. Any advice would be most welcome please?

24-09-2019, 10:16 PM	#9
Flip Registered User Join Date: Mar 2010 Location: Beachtown Bike: M900 Posts: 2,188	Good point- I hadn't really thought that way and didn't really know if the vapour from Ethanol fuel would be enough over an extended period to deteriorate them. Lets hope not __________________ You're perfect, yes, it's true- But without me you're only you!

24-09-2019, 09:07 AM	#2
slob . Join Date: Jul 2003 Location: East London Bike: Multiple Monsters Posts: 9,738	I'd guess something else, the carb in question would struggle to achieve full throttle with a split diaphragm, since lifting the slide relies on a pressure difference either side of the diaphragm. I'd try swapping the igniter boxes round and see if the misfire changes cylinders as a first step

24-09-2019, 03:44 PM	#7
slob . Join Date: Jul 2003 Location: East London Bike: Multiple Monsters Posts: 9,738	I stand corrected

24-09-2019, 07:30 PM	#8
350TSS Too much time on my hands member Join Date: Jan 2013 Location: Shipbourne Bike: M900 Posts: 1,422	The petrol should not be in the diaphragm chamber, just air below and air above so we should not get deterioration from ethanol. There may be a slight mist getting past the needle hole but you would have thought that the suction from the cylinder would draw it away?

25-09-2019, 08:56 AM	#10
olivera Registered User Join Date: Jul 2011 Location: Bedford Bike: M900 Posts: 55	Thanks guys, igniter first and if that makes no difference i'll check the diaphragm.