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H2 Problem...

Posted by Andy C <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Andy+C' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 9:39 am

Was out riding it yesterday evening, and there was a marked reluctance for it to run properly.

Lots of bucking / surging (they all do it to a degree but this was really bad) also a lack of power just lots of vibration after about 5000 RPM just as it should be coming alive.

Ah well I thought I have bought a "pup" - knackered engine.

Anway, pulled the plugs this morning, they were all white, where they had been a nice brown / tanned colour the last time I looked.

I have done nothing to cause this.

I decided to take it out for a quick run this morning to see if the cooler air would fix it - 2 strokes run better in cooler air.

Anyway I popped in to pick up some fuel and switched the tap to the "prime" position accidentally.

Normal service was resumed.

The tap works off a vacuum, in the prime position it bypasses the vacuum system but means you have no reserve.

I suspect it it the 40+ years diaphragm in the tap that was causing the problem, so will be ordering a replacement.

In the prime position it has run better than ever so the tap must be the issue.

You live and learn.

Who needs a signature anyway......

Re: H2 Problem...

Posted by DBDBrian <iframe src='https://members5.boardhost.com/classicbikemart/info/user=DBDBrian' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 9:56 am, in reply to "H2 Problem..."

Simple and inexpensive fix then Andy.

Brian
Made In England

Re: H2 Problem...

Posted by Andy C <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Andy+C' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 11:04 am, in reply to "Re: H2 Problem..."

Dont know about inexpensive 60 odd quid for the diaphragm kit

Think I will pull it apart and check for any obstruction in the airways before splashing out.

When I checked it during recomissioning the diaphragm looked fine, but perhaps if not used for years it could have cracked.

I also sucked on the vacuum pipe and it seemed to work OK.

In the menatime I'll be riding it in the prime position and keeping an eye on fuel level !!

Who needs a signature anyway......

Re: H2 Problem...

Posted by calidavid <iframe src='https://members5.boardhost.com/classicbikemart/info/user=calidavid' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 11:59 am, in reply to "Re: H2 Problem..."

I keep a note of the mileage on my Ducati as the delusional gauge reads full, half full and warning light often in quick succession. They really do “all do that sir”.

Re: H2 Problem...

Posted by JERRY <iframe src='https://members5.boardhost.com/classicbikemart/info/user=JERRY' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 10:00 am, in reply to "H2 Problem..."

Glad it's an easy fix Andy. Seeing the age of the bike and lack of use I'm not surprised.

The oldest rocker in town!

Re: H2 Problem...

Brian
Made In England

Re: H2 Problem...

Posted by 6TDen <iframe src='https://members5.boardhost.com/classicbikemart/info/user=6TDen' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 10:07 am, in reply to "Re: H2 Problem..."

I agree Bryan, I think.

Den

Re: H2 Problem...

As they all say when cautioned Den, No Comment

Brian
Made In England

Re: H2 Problem...

Posted by Triton Thrasher <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Triton+Thrasher' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 11:42 am, in reply to "H2 Problem..."

I think vacuum taps fell out of favour eventually, for reasons like that.

Re: H2 Problem...

Quite possibly. It's hard finding a good quality non vacuum tap with a decent flow rate to replace it.

There are plenty of run of the mill ones, but their quality is dubious, the most widely used alternative tap is one made by Pingel, but they have no reserve and they cost around £240 quid.

Who needs a signature anyway......

Re: H2 Problem...

Posted by geoff the bonnie <iframe src='https://members5.boardhost.com/classicbikemart/info/user=geoff+the+bonnie' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 12:02 pm, in reply to "Re: H2 Problem..."

To keep an eye on my fuel level, i have to look in the big hole in the top of the tank. Lucky i have a 3.5 gallon tank so good for an easy 150 mile ride without worries. Dartmoor run in July over 100 miles so i know if i fill up, i wont run out. I have old lever taps and they just keep working year after year

Re: H2 Problem...

Posted by Greeves Rider <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Greeves+Rider' width='330' height='300' border=0 style='border:none;'> on May 26, 2026, 3:18 pm, in reply to "Re: H2 Problem..."

Just re set the trip on speedo every time you fill up and top up again after 150 miles

Re: H2 Problem...

And cheaper that £250 for a tap !!!

Good tip.

Since I usually put in 2 gallons a time 75 miles is probably my top up point.

Who needs a signature anyway......

Re: H2 Problem...

Back in the 70s, a Yamaha 350 YR5 my mate rode was using a litre of 2T oil every 300 miles. Smokey old thing but a lot of fast fun

Re: H2 Problem...

Does the tap actually have a reserve? On my Yamaha T80 the vacuum operated part of tap just opens up the main supply, the prime position just manually opens up the same supply level as the vacuum. There is no reserve.

Re: H2 Problem...

Posted by Leon Novello <iframe src='https://members5.boardhost.com/classicbikemart/info/user=Leon+Novello' width='330' height='300' border=0 style='border:none;'> on May 28, 2026, 5:12 am, in reply to "H2 Problem..."

I have fixed lots of things over my lifetime, but I don`t know what I did to do most of it. I fear that I would be a failure as a teacher.

Leon Novello

Re: H2 Problem...

Posted by 6TDen <iframe src='https://members5.boardhost.com/classicbikemart/info/user=6TDen' width='330' height='300' border=0 style='border:none;'> on May 28, 2026, 8:06 am, in reply to "Re: H2 Problem..."

It’s a fact Leon the older I get, the thicker I get,
Den

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