VFR400 lack of exhaust pressure

ace-card · Nova Slayer Joined: 22 May 2011 Karma :

Hi all,

Just got in from trying to help get my mates VFR400 on the road......its had a full strip down and rebuild. We came up against a couple of strange problems today:

1)there is much less pressure coming from the exhaust for the two rear cylinders than there is from the exhaust for the front two. The rear also seems to blow cooler. You can cover the end of the can up with very little consequence;

2)the engine is running Silkolene Super 4 10W40 oil , but upon taking the dipstick out, it seemed to smell slightly fuelly (if there is such a word!!)

With regards to point 1, we took off the exhaust headers for the rear and ensured they are properly sealing onto the manifold (uses tension springs to hold it on so we smeared some extra sealant on to be sure). We thought all the heat and pressure was escaping from there, but this has now been ruled out and the problem persists. As for point 2......weird! Any ideas??? I know there are some genius people out there with some answers! Wink

Cheers guys!
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2007 Skyjet SJ-125, 2001 Aprillia RS125, 1988 Honda CG125, 1997 Yamaha YZF600R
Mama said "life is like a box o' chocolates....have what's nice, then offload the crap to everyone else!" Wink

herulach · World Chat Champion Joined: 19 Apr 2010 Karma :

I've never had to diagnose it myself, but I'm gonna guess piston rings hence low compression and letting petrol into the oil past the piston.

Thats assuming you've checked daft stuff like the rear pistons getting a spark etc.
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YBR 125>FZS600

finpos · World Chat Champion Joined: 13 Sep 2005 Karma :

One of the back 2 cyls. not firing would kind of explain both - see both the exhaust outlets are getting hot.

f.

timsmith · Two Stroke Sniffer Joined: 09 Jun 2011 Karma :

An engine is simply a pump so if less is coming out less is going in (sort of!) do a compression check on all cylinders and also make sure that the rear cylinders are firing. It does sound as if the rear cylinders are letting fuel passed the rings rather than burning it if the oil is starting to smell of petrol. good luck.
Tim

timsmith · Two Stroke Sniffer Joined: 09 Jun 2011 Karma :

An engine is simply a pump so if less is coming out less is going in (sort of!) do a compression check on all cylinders and also make sure that the rear cylinders are firing. It does sound as if the rear cylinders are letting fuel passed the rings rather than burning it if the oil is starting to smell of petrol. good luck.
Tim

Frost · World Chat Champion Joined: 26 May 2004 Karma :

Excess fuel indicates it's not getting burnt. Cold pipes / gases mean nothings getting burnt. Start it up and run it for 20 seconds or so and spit on the rear down pipes, it should sizzle off pretty much instantly just like it does on the front.
If it doesn't then the rears aren't running, or are running badly. Plugs would be the first thing to check, along with seating of the carbs and airbox as these can be hard to get right. Checking around all the pipe joints to make sure nothing is blowing is useful too. Lick the back of your hand and feel about for moving air with that.

ace-card · Nova Slayer Joined: 22 May 2011 Karma :

Thanks for all the replies....we did suspect the piston rings due to the fuel smell in the oil....hope not though as that's engine out again and all he wants to do is ride the damn thing! Gonna compression test it tomorrow to rule it out.
____________________
2007 Skyjet SJ-125, 2001 Aprillia RS125, 1988 Honda CG125, 1997 Yamaha YZF600R
Mama said "life is like a box o' chocolates....have what's nice, then offload the crap to everyone else!" Wink

0l0dom0l0 · World Chat Champion Joined: 21 Oct 2009 Karma :

jimspeed · World Chat Champion Joined: 05 Mar 2010 Karma :

are the carbs all sync'd up? if maybe the rear carbs were closed more than the fronts it would do that too.
I often use an infrared thermometer gun to check header temps on rally cars which gives me an idea of how balacnced the throttles are (better with fuel inj as you know the same amount of fuel is getting injected to each cyl).
it would have to be really really poorly for rings to cause it to not run on those cylinders and you would have lots of blue smoke too
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Hyosung cruise 125(passed test on, sold) Kawasaki el 252 (better than expected but sold on) Kawasaki GPZ500S first "big"bike.(sold) ZZR600 E5..Z750 2007,ER5, currently on a 2008 Enfield bullet electra x and loving it..
,"Alpha-9: Is there any correlation between dyno rod and dyno kits?"