Guest cplmike Posted April 15, 2013 Share Posted April 15, 2013 Hi. I own a 92 VFR 750. It's a jap import, rather unlocked.I changed the plugs, swapped to a k&n filter, oil and oil filter are brand new, the same as fuel filter. Carbs got dynojetted and cleaned. The problem is that the #3 cylinder, ...i'm almost sure, is not firing. spark #3 is the same as on the other plugs. So I assume the ht lead and ignition coil are working.However, downpipe of that cylinder gets a bit hot (but not as much as others). After unplugging coil or plug cap, there are no noticeable changes in idle speed or throttle response. It happend one time , on a totally cold engine that unplugging coil showed a difference. But it quickly stopped working again. The bike is likely to backfire or pop in exhaust. Color of the spark plug on that cyl is the same as other cylinders. Unplugging other cylinders makes a huge difference in idle speed,almost killing it.While the bike maintains a steady idle speed,and is pretty rideable , it seems very underpowered above 120 kph and tops out at 180, so it's likely that this cylinder causes all the problems. I'm totally disappointed and don't know what to do, i tried to solve this for ages, but I wanted to ask more experienced riders what could be the reason before I start buying and swapping things along with spending a fortune on the parts, probably ending in buying a new engine after all previous methods will fail Is it possible that the pulse generator is faulty ? I'm thinking of buying a new set of carbs, coil, cdi module ,wire harness and if these fail, i'll replace whole engine....as it will possibly be compression or valves. Link to comment Share on other sites More sharing options...
wera803 Posted April 15, 2013 Share Posted April 15, 2013 I'd do a compression test first and make sure that cylinder has a similar reading as the other 3. Link to comment Share on other sites More sharing options...
Graves Posted April 15, 2013 Share Posted April 15, 2013 Hello and welcome Before you jump on buying parts you'll want to do a few things. 1st - Do a compression test on all 4 cylinders - post the results 2nd - Make sure the spark plug actually firing. If compession is good and you are getting spark on the plug. It will likely be a little to no fuel issue. Link to comment Share on other sites More sharing options...
Guest Posted April 24, 2013 Share Posted April 24, 2013 Due to lack of time to remove the radiator, I only measured pressure on the rear cylinders. And what's weird , the results are : 150 on #1 an 175 on #3. Service manual claims that the pressure should be around 199. If the front ones have somewhere in between of 150-175 , do you think this could cause a 25% loss of power ? I discovered that #3 starts to react to unplugging it's coil while the choke is turned on. I suspect that slow jet is clogged. I took a look at OEM prices. ca. $20 each one.Ill try to clean them once again and post the results. thanks for your replies. Link to comment Share on other sites More sharing options...
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