BritBike Forum logo
BritBike SponsorBritBike SponsorBritBike SponsorBritBike SponsorBritBike Sponsor SteadfastCyclesBritBike SponsorBritBike SponsorBritBike SponsorBritBike SponsorHepolite PistonsBritBike Sponsor
Upgrade to: Gold Membership | Premium Membership | Vendor Membership | Site Sponsor Membership
Member Spotlight
68triton
68triton
London Uk
Posts: 51
Joined: September 2012
ShoutChat Box
Comment Guidelines: Do post respectful and insightful comments. Don't flame, hate, spam.
Search eBay for motorcycle parts in following countries
Australia, Canada, France, Holland, Italy, United Kingdom, USA
Random Gallery photo
a word from..
Manuals on DVD (Shipping included)
Previous Thread
Next Thread
Print Thread
Rate Thread
Exhaust manifold sealant
#812307 06/12/20 7:53 pm
Joined: Apr 2019
Posts: 4
Novice poster
OP Offline
Novice poster
Joined: Apr 2019
Posts: 4
I have rebuilt a 1960 DBD34GS. When the bike is slowing down, the engine backfires two or three times when shutting the throttle. I have been careful with the carburettor mixture and settings. The bike starts easily, runs well enough (although I am still running it in) and examination of the spark plug indicates that all should be well on the intake. I have set the ignition timing at 39 degrees BTDC fully advanced.

Consequently I suspect that there is an air leak at the exhaust manifold. The pipe has a simple push fit into the exhaust port (easily to do) and is held in place by the bracket to the engine plate. The exhaust pipe and silencer appear to be standard and the pipe is rather a loose fit in the cylinder head.

I will ask a colleague to spray soapy water on the exhaust manifold and see if there is a leak. If so, will a non stick, heat resistant gasket sealant do the trick?

Regards
Rod

Support Your #1 BritBike Forum!

Check out British motorcycles for sale: British Motorcycles on e-Bay UK, British motorcycles on e-Bay North America
Re: Exhaust manifold sealant
Rod Leeming #812347 06/13/20 7:21 am
Joined: Feb 2012
Posts: 1,365
Likes: 4
Britbike forum member
Offline
Britbike forum member
Joined: Feb 2012
Posts: 1,365
Likes: 4
Yes, I'd suggest that the backfiring is due to a small exhaust leak.

Others here will have more elegant and probably effective solutions, though a narrow strip of shim steel around the end of the pipe has made a difference for me in the past. I've found that most of the usual heat-activated exhaust sealers 'set' and break-up with the engine running, do the the small amount of relative movement between the largely engine-mounted head and frame-mounted pipe. Do you run a head steady on the back of the head?

Nothing to be lost in trying one of the common sealers out there -am sure halyards would have something in a tube.

Incidentally, I assume you are running unleaded fuel? 39* advance would be good on leaded, though I personally prefer (and others have allegedly proven) that a degree or two less advance is better on the unleaded, as it burns faster.

KW


No generalisation is wholly true, not even this one.
Oliver Wendell Holmes
Re: Exhaust manifold sealant
Kerry W #812357 06/13/20 11:13 am
Joined: Apr 2019
Posts: 4
Novice poster
OP Offline
Novice poster
Joined: Apr 2019
Posts: 4
Many thanks for the useful reply Kerry. I will try a metal shim as you suggest - then some sealant if the backfiring persists.

I had not thought to retard the ignition timing. I do use the highest quality (most expensive) of unleaded petrol here in the UK. Also the bike has the standard support to the frame at the rear of the cylinder head.

I am interested when you say that unleaded burns faster. I wonder if it evaporates faster. The bike usually starts at first kick from cold - with two short dabs of the Tickler; choke half closed and the ignition slightly retarded. It starts easily if shut down for up to a few minutes or so.

It does not like to start if being left standing for about 30 minutes in warm weather when the engine is cooling down. I am reluctant to tickle the carb again and do not want to flood it - but perhaps the fuel has partially evaporated and it does need to have that bit extra in the float chamber.

Thanks again

Rod

Re: Exhaust manifold sealant
Rod Leeming #812407 06/13/20 9:56 pm
Joined: Feb 2014
Posts: 683
Likes: 24
Britbike forum member
Offline
Britbike forum member
Joined: Feb 2014
Posts: 683
Likes: 24
Hi,
A pipe expander used in the engine end of the pipe can be used to expand the end to the pipe to tighten the fit
Silicone sealer also provides a cheap and effective cure and lasts better than hard setting goo's
Do not allow the sealer of any kind to be pushed ahead of the pipe into the head
A blob of sealer in the port can cause the valve to lose its head !!!!!!!!!!!!!!!!!

John

Re: Exhaust manifold sealant
Rod Leeming #812459 06/14/20 10:17 am
Joined: May 2004
Posts: 4,457
Likes: 17
Britbike forum member
Offline
Britbike forum member
Joined: May 2004
Posts: 4,457
Likes: 17
Double up on Chaterlea25's answer.
Pull the head , or tak e the complete bike to a muffler fitting shop with a pipe expander.
Ask them really nice to expand your exhaust pipe to be a tight fit in the head
When you are ready to fit it, run the bike without a pipe for a couple of minutes to expand the exhaust port then slip the pipe in Quickly


Bike Beesa
Trevor
Re: Exhaust manifold sealant
Rod Leeming #812500 06/14/20 2:56 pm
Joined: Nov 2011
Posts: 6,152
Likes: 71
Britbike forum member
Offline
Britbike forum member
Joined: Nov 2011
Posts: 6,152
Likes: 71
Originally Posted by Rod Leeming
I have rebuilt a 1960 DBD34GS.
When you insert the pipe it should hit a hard stop against the internal step in the head. If you feel the hard stop, is the mounting bracket directly in line with the engine mount stud, or do you have to slightly pull the pipe back out of the head in order to insert the stud? If the latter, or if you don't feel a hard stop (because it binds before reaching that point), that may identify the source of the air leak, i.e. the mounting bracket is either bent or, in the case of an aftermarket pipe, incorrectly located.

Any air that makes it as far as the internal step has to then make two 90° turns as well as squeeze through the remaining gap between the pipe and step, which is pretty hard to do if the pipe is pressed against the step. Note, if clearance stops the pipe before hitting the step it doesn't mean the fit is tight around the entire circumference, only that a minimum of two points are tight, with the rest of the circumference free to flow air.

Originally Posted by BSA_WM20
When you are ready to fit it, run the bike without a pipe for a couple of minutes to expand the exhaust port then slip the pipe in Quickly
Having cold air sucked back over a hot valve stem isn't a good idea, and this would be especially the case with the large valve overlap of a Gold Star. Also, if the extra ~0.005" of a hot head is needed to install the pipe, the fit is too tight anyway and only would cause future headaches whenever the pipe needs to be removed. It doesn't have to be a hermetic seal to eliminate the backfiring problem.

Re: Exhaust manifold sealant
Rod Leeming #812525 06/14/20 7:25 pm
Joined: Aug 2015
Posts: 1,604
Likes: 13
Britbike forum member
Offline
Britbike forum member
Joined: Aug 2015
Posts: 1,604
Likes: 13
The current practice on modern sports bikes is virtually identical apart from using springs to hold the pipe in place


BSA B31 500 "Stargazer"
Greeves 200 "Blue Meanie"
Greeves 350
Greeves 360
Suzuki GSX1100 EFE "Sorcerers Apprentice"
GM500 sprint/LSR bike "Deofol"
Jawa 500 "Llareggub"
Aprilia RSV Mille "Lo Stregone"
'35 & '36 OK Supreme
Kawasaki ZZR1400 "Kuro no senshi"

Moderated by  Rich B 

Link Copied to Clipboard
Home | Sponsors | Newsletter | Regalia | Calendar | Bike Project | BritBike Museum | Spiders Cartoons | DVD- Manuals & Parts books
Upgrade to: Gold Membership | Premium Membership | Vendor Membership | Site Sponsor Membership
Powered by UBB.threads™ PHP Forum Software 7.7.4