Exhaust blow back

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Keith.owen
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Exhaust blow back

Postby Keith.owen » Thu Jun 13, 2019 9:27 am

Hi,
My VH is running pretty well since I did a rebuild.

The crankshaft has been balanced with an Omega 7.5:1 piston and HS cam fitted with normal followers. It seems to burst into life in top gear from about 45 mph.

I recently fitted a new exhaust and silencer from Feked. The silencer is a true copy of the burgess type and sounds nice. The pipe near the exhaust port has only mildly discoloured straw colour and this only for a couple of three inches. However, sometimes on the over run, slowing down from about 40 or above it will blow back out of the exhaust, normally just one 'pop' each time.

The plug looks o.k. if anything a little pale, but the exhaust colour doesn't suggest the mixture is particularly weak and the bike starts well.

Does anyone have any suggestions as to causes of the back fire? Is it over-fuelling or what?

Any thoughts appreciated.

Keith
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Re: Exhaust blow back

Postby david.anderson » Thu Jun 13, 2019 10:09 am

Keith
There are many possible causes.
1/ Are there any exhaust leaks. Not so likely with the clamp over flange iron head 1950 VH but a small leak will do it. Seal the pipe and flange with Maniseal or even silicone rubber will work
2/ is the timing out. If retarded it can backfire.
3/ is it a little too rich at idle as that can cause a small backfire but not really likely
4/ the most likely cause is too lean on the idle mixture screw. Screw it in a quarter turn at a time and see if that makes a difference. Too lean at idle will never effect the pipe colour.
David

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Re: Exhaust blow back

Postby Keith.owen » Thu Jun 13, 2019 10:21 am

Hi David,
Good to hear from you. I trust all is well.

I shall give these things a try - somehow I don't think this weekend though, we're having some very wet weather!

The bike seems to be running pretty well, but, not having ridden another it is hard to really know how well it compares. It does however seem to be 'willing' to go so I guess it must be pretty much alright - I've had it up to 70 ish on a couple of occasions, but just short bursts due to road conditions - the brakes are o.k. but not brilliant!

Thanks and BR,
Keith
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Re: Exhaust blow back

Postby pappleton » Thu Jun 13, 2019 11:11 am

Hi Keith,
I recently resleeved the monobloc on my VH. Same problem, bit of a backfire on over run. I can only put it down to being a bit richer on the idle. A bit of a twiddle with the idle mixture screw has sorted the issue. BTW who balanced your crank assembly? Did it make a difference? What factor did they , or you, use?
BR, Paul
Paul Appleton '53 VH plunger, '53 VHA rigid - in many boxes, '58 H.D. pan/shovel rigid

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Re: Exhaust blow back

Postby Keith.owen » Thu Jun 13, 2019 11:21 am

I had it done by Owlesbury in Winchester, 01962 855 969. I think it was 67% (or what ever Nev Hunter suggested) and yes, it has smoothed out a lot of the vibration.

Keith
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Re: Exhaust blow back

Postby Knud.Degnbol » Thu Jun 13, 2019 7:59 pm

I would first look at exhaust leaks.

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Re: Exhaust blow back

Postby nevhunter » Fri Jun 14, 2019 12:26 am

Yes exhaust leaks are the usual cause or a lean idle. The slots in the end of the pipe near the head may be too long or just a bit loose. Engines tend to do this "naturally" to a point especially if you have backed off at revs or saving the brakes down hill with a lower gear.. Richen the idle a bit. It will also make it a bit easier to start. Nev

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Re: Exhaust blow back

Postby Keith.owen » Wed Jun 26, 2019 10:54 am

Hi,
I have found a bit of a leak between the silencer and exhaust so tweaked that up. The clamp on the stub at the head is quite tight and I don't think anything is getting in/out there. I am a bit wary of over tightening it - I don't want to find out the hard way!

I have been riding her quite hard and when over running from 5-60 ish she does rather pop back; also the plug looks a bit light in colour. I have tried opening the throttle and closing it to see what effect that has and also trying the choke, but it's a bit difficult with so much traffic / general road conditions. If I raised the needle to make the mixture richer I guess this would have no long term effect (e.g better rich not weak)?

Bike goes well but seems to get a bit flat on larger throttle openings.

Keith
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Re: Exhaust blow back

Postby nevhunter » Wed Jun 26, 2019 12:11 pm

Your cut out on the slide is too large perhaps. Carbs go lean on big throttle openings on load, If you close it a bit and it goes better that proves it's a bit lean. Nev

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Re: Exhaust blow back

Postby brenton.roy » Sun Jul 07, 2019 1:36 am

Hi Keith, jetting will be a bit trail and error with your cam etc.
If it feels a bit revvy and isn't going as well as you would like, try a size bigger main.
As Nev says, cutaway is common for popping back, but you could also be running a bit lean at top end.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.


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