Nemett Jaguar--Canadian style

[Jasonb]

Might be worth a try on my twin I don't see anything like the amount of blowback that you had on the videos and I've only got short exhaust pipes but being an opposed twin the intake pipes between central card and heads is a lot longer so may "soak up" any blowback

 

[Brian Rupnow]

I replaced the exhaust pipe with the nifty 180 degree bend this morning, with one which is basically a straight pipe. This was supposed to cure the problem with vapour coming out of the carburetor intake. It didn't!!!
[ame]https://www.youtube.com/watch?v=8Rb9rNZgkrc&feature=youtu.be[/ame]

 

[canadianhorsepower]

I replaced the exhaust pipe with the nifty 180 degree bend this morning, with one which is basically a straight pipe. This was supposed to cure the problem with vapour coming out of the carburetor intake. It didn't!!!
https://www.youtube.com/watch?v=8Rb9rNZgkrc&feature=youtu.be

Hi Brian
I would put the exaust 180 up like zommies on a dragster.
the fact the the more RPM the more is does it makes me think
that your valve are floating (need a stronger spring):hDe:
or your points are floating, then again a spring bending should cure this. :hDe:

cheers
Luc

 

[swnf]

Long time lurker but it may be reversion

 

[Brian Rupnow]

And what, pray tell, is reversion?

 

[swnf]

Unwanted reversal in the normal direction of the gas flow.
"Intake reversion because of late closing of intake valve"
This is from the book Reher-Morrison
Championship engine assembly

I think this forum is the best
Just trying to learn all I can from you guys. Not trying to offend just a guess

 

[Brian Rupnow]

Swnf--No offence taken. I suppose that could be the problem. Very hard to analyze that one.---Brian

 

[Jasonb]

Brian, do you have your points on the crankshaft or cam shaft? If on the crank and you are using lost spark then maybe on the odd occasion the lost one is igniting the mixture?

Does it do it if you adjust advance & retard the spark?

I've not seen it on mine and that has the same cam profile and timing give or take a bit for how we have set the timing gear.

http://youtu.be/9L64ItB3xgw

 

[Brian Rupnow]

Jason--EDIT--I had to take a closer look. The points are operating off the cam shaft, so I am not getting a "wasted spark".---Brian

 

[Brian Rupnow]

swnf-The exhaust cam and the intake cam are fixed in position relative to each other by being Loctited to the camshaft. I can not change much in relationship to the intake cam because anything affecting the intake cam also affects the exhaust cam. The cam timing on this engine is given by the original designer, and there is a definite overlap of valves both being open at the same time as the piston approaches top dead center on the exhaust stroke. When an intake valve first begins to open, there is very little flow through it because inertia tends to keep the column of air in the carburetor intake "at rest" until the piston begins it's descent on the intake stroke. That is why the intake valve actually begins to open before the piston reaches top dead center on the exhaust stroke. As well, in order to fully remove the exhaust from the cylinder, the exhaust valve stays open for a few degrees after top dead center. The theory is that the column of exhaust in the exhaust pipe is already in motion, and inertia will keep it flowing until the valve closes, and this flow will actually help to start pull a fresh charge of unburned fuel through the intake valve which is beginning to open at the same time.. The same effect takes place when the piston approaches bottom dead center on the exhaust stroke. Most of the power has been extracted from the burning charge of fuel by the time the piston has traveled roughly 2/3 of the way down the bore, so the exhaust valve actually begins to open before bottom dead center on the power stroke so that any expansion left in the still burning charge of fuel will start the gasses flowing out the exhaust system. The intake valve, which began to open before top dead center on the intake stroke of the piston stays open all the way through the intake stroke, and stays open after the piston passes bottom dead center on the intake stroke and begins to ascend on the compression stroke.

 

[Cogsy]

Based on what you explain Brian, and on some small commercial stationary engines I have experience with that actually hold the intake valve slightly open at very low rpm (supposedly makes it easier to pull-start as it reduces compression), it seems to me that your valve timing is slightly out.

If this was the case then your exhaust valve would be closing slightly before TDC, meanwhile your intake valve would be open for slightly longer before TDC than designed (this probably wouldn't matter if the exhaust valve was still open). Then the residue exhaust gases would have nowhere to go for the last bit of upward piston travel except out the intake.

I don't know if it's possible with this design to minutely adjust the valve timing (I know it must be close now as it runs so well) but if it is that's what I'd try. I imagine there is a little more hp in the engine once you fix this issue as an added bonus.

 

[Brian Rupnow]

Cogsy--Nice to hear from you. I guess that will probably be the next thing I do, is try to adjust the valve timing. Since the largest gear is held to the camshaft with dual set screws, it is possible to make very fine adjustments. It's just that the way this engine is built, it requires a major disassembly of the engine to get at the adjustment. I'm just killing time these days, farting around with small engines and hoping for the weather to warm up so I can get outside a bit and do other things. We have just had the coldest February ever recorded in this part of Canada, and so far March hasn't been any better. I don't feel like building anything, and I'm starting to get "cabin fever" from being in the house so much all winter.---brian

 

[Charles Lamont]

Any chance the inlet valve is occasionally failing to close properly?

 

[Brian Rupnow]

Any chance the inlet valve is occasionally failing to close properly?

No Charles, I have quite beefy little springs on both valves, and the valves slide very smoothly in their guides. I run a small percentage of 2 cycle oil with my fuel to keep the Viton o-ring on the piston lubricated, and consequently all of the top end is very well lubricated inside, and I apply lube oil manually to the exposed rocker arms and any external pivot points.

 

[Jasonb]

One thing that would be easy to try without a strip down is to slacken off the adjusting screw on the inlet rocker a fraction, this would mean the cam would have to take up the gap before it starts to open the valve so it would open a bit later. Hopefully the fact it closes a bit earlier won't affect things too much.

 

[petertha]

Brian, is there a chance either the locktited cams or set-screwed gear train gave shifted slightly? Is there a way to validate valve Int/Exh open/close timing relative to TDC with a protractor wheel or whatever like you had it originally set up?

 

[swnf]

Could it be the intake valve bouncing or "float"

 

[Brian Rupnow]

Float would be caused by excessively high rpm or too weak of valve springs. It's not that. The gears have double set screws with "back-up" set screws behind them to prevent loosening off. I still have the degree wheel I used for my original valve setting, and I will see if anything has moved, although I doubt it has, because the engine has had this issue with vapour coning out of the carb inlet right from day one.

 

[swnf]

Will follow to see the outcome

 

[Charles Lamont]

Sounds like time for some wilder ideas. Is there a hot spot in the head causing the charge to ignite during admission? Is there a bad earth causing as spark inside the carb?