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Exhaust reversion

charles d cannon jr

Active Forum Website User
VOC Member
I had recently placed a post asking about petrol dampened air filters and I appreciate the responses, but after some reading, I believe this is due to the phenomenon of exhaust reversion. If I remove my air filters and place my hand behind the carb throat while the motor is running there is a petrol mist that collects on my palm. I am running Mark I cams newly made by Gary Robinson, with exhaust and inlet valve overlap at 4 degrees BTDC, and an Overlander stainless muffler that appears to be spiral baffle pattern construction. My questions are is anyone else experiencing this, how restrictive is the Club's muffler, and could a less restrictive muffler cure this. I've heard of drilling holes in the spiral, using a long drill bit to change the character of the muffler. Seems like a Supertrapp muffler, with its "tunable" disc system might be a way to deal with this....Thanks.
 

passenger0_0

Well Known and Active Forum Website User
VOC Member
Hi Charles. With mk1 cams having little real overlap I'd be really surprised if the exhaust/muffler were playing any role in your problem. Many engines suffer minor blow-back at low speeds. In your case, if you think you have a problem, I suggest it is possible that your inlet valve closing timing is late and that the fresh charge is being partially blown back by the ascending piston. Slight variation in cam profiles can cause error in the method of cam timing you've mentioned although I have used a few of Garry's cams and they are very well made. I suggest you check your cam timing again for both inlet opening and closing as per the manual. If this is made right then the exhaust usually looks after itself. Cheers David
 

chankly bore

Well Known and Active Forum Website User
Non-VOC Member
If you are at all doubtful of your cam timing, try to get equal " lift" on your pushrods at 4 degrees B.T.D.C. exhaust stroke. If I remember rightly, only your front cylinder had the problem so it MIGHT be a relatively easy fix.Good Luck!
 

Howard

Well Known and Active Forum Website User
VOC Member
The last time exhaust reversion was mentioned here I spent some time reading up on it (because I hadn't a clue what it was (and I'm still not 100% sure - or even 50%), but as I understand it ....

Because of the valve overlap, at some revs there will always be some amount of blowback through the carb. The secret is to get the inlet tract of the correct length (or maybe volume) so that the fuel does not leave the inlet (Inlet length is tuned to suit the revs in the same way as exhaust). Apparently (and I have no proof) all air cleaners will have a mist inside, and this will reduce, as the revs increase. Putting the right bellmouth on in place of the filter creates a small "cloud" of fuel at the opening, which will expand and contract at various revs, but will stay within the influence of the engine.

If you take off the filter and don't replace with a bellmouth, you'll exagerate the problem.

If your problem is just one cylinder, it might need the valve timing tweaking, if it's both cylinders..... stop riding it so slowly!!! :)

If I've misunderstood this concept, will someone please explain - it's good to know how these old beasts work.

H
 

BigEd

Well Known and Active Forum Website User
VOC Member
VOC Forum Website Moderator
If you reread the original post Charles has used the equal lift method to set his timing. I found that the equal lift method was quickest, easiest and gave the best results. If he has only checked the timing on the rear cylinder it may be that the timing, as you suggest, could be out on the front cylinder. If it is out then he could use the alternative keyways on the timing side to split the difference and get a working compromise.

If you are at all doubtful of your cam timing, try to get equal " lift" on your pushrods at 4 degrees B.T.D.C. exhaust stroke. If I remember rightly, only your front cylinder had the problem so it MIGHT be a relatively easy fix.Good Luck!
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
If you reread the original post Charles has used the equal lift method to set his timing. I found that the equal lift method was quickest, easiest and gave the best results. If he has only checked the timing on the rear cylinder it may be that the timing, as you suggest, could be out on the front cylinder. If it is out then he could use the alternative keyways on the timing side to split the difference and get a working compromise.

Big Sid ask me on another forum, how to time a twin with out the cam timing marks. If you would go to Vincent.com , you will find my article. Might help.
 

davidd

Well Known and Active Forum Website User
VOC Member
Like David in the fist response, I would be surprised if the overlap of the MK1 would cause horrible reversion. I run 100 degrees of overlap on the Comet and while waiting to go out on the track I can look down and see the gas billowing out of the intake. On the track I run between 4200 and 7000 RPM's and there is so much momentum of the outgoing exhaust and the incoming intake that there is no reversion. I would guess that the MK1 has 50 plus degrees of overlap, about half of what I run, and you will get only wisps of gas coming out at idle, unless the cam has a lot more overlap than advertised.

You are also correct that a more restrictive exhaust will exacerbate the problem by making it more difficult for the exhaust charge to exit with a lot of momentum. I would think the original spiral baffle would be the least restrictive, short of a straight pipe. I know several owners who have liked the Supertrap very much, but I would not change unless your silencer is restrictive.

I am not sure that you would notice anything if you did not run an air filter. If everything is set up correctly, you will get a little mist escaping at idle, but not excessive. John Renwick dyno tested the Club silencer at the time it was first offered and I think I saw it recently in the PDF's on the Club site.

David
 

Kansas Bad Man

Well Known and Active Forum Website User
Non-VOC Member
I will try, when the fuel starts its burn, the fuel mass expands, pressure in the combustion increases , the exhaust valve starts its opening, the expanding fuel mass pressure follows the path of least resistance now with declining pressure but with a new characteristic which is velocity. so what you have is a accelerated mass forming a absent of pressure to its rear in the exhaust pipe . The absent of pressure is a pressure less then atmospheric pressure. The absence of pressure position in the exhaust pipe and combustion chamber is controlled by the shape of the exhaust pipe, ether increasing or slowing the speed of the mass.
 

clevtrev

Well Known and Active Forum Website User
VOC Member
I will try, when the fuel starts its burn, the fuel mass expands, pressure in the combustion increases , the exhaust valve starts its opening, the expanding fuel mass pressure follows the path of least resistance now with declining pressure but with a new characteristic which is velocity. so what you have is a accelerated mass forming a absent of pressure to its rear in the exhaust pipe . The absent of pressure is a pressure less then atmospheric pressure. The absence of pressure position in the exhaust pipe and combustion chamber is controlled by the shape of the exhaust pipe, ether increasing or slowing the speed of the mass.
And then you have the problem complicated with the two in to one exhaust, one gasp not being allowed to escape, before the adjacent one causes it to back paddle.
 

roy the mechanic

Well Known and Active Forum Website User
VOC Member
The problem you describe usually only happens with big hot-rod type cams, not with standard units. I would reccomend go back to equal lift at t. d. c. 4degrees may not sound much, but you will be surprised how much inlet valve lift you have created by the time the motor gets to t d c! Maybe you should measure the actual inlet lift at tdc and report back. Regards, roy.
 
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