Compression tested my 91 vfr 750 is it okay ?

Hey guys, new to vfr's and this website, I have recently acquired a vfr 750 converted to a street fighter. It isn't running to great, hits about 5000 rpm and starts misfiring, kinda sounds like rev limiter. You can throttle just past the misfire but it just bogs down and refuses to accelerate any more after that. I have replaced all ht leads and plugs, an tested resistance through the coul and ht leads and they are fine. I currently have the carbs off and can't seem to find a fault with them, I will be ultrasonic cleaning them with new jets and so on. But just to be sure I cold compression tested it tonight without the carbs on and I got 165-175-170-162 psi is this okay for a vfr that's done 100,000 kms hahaha it seems sweet as to me! other then the difference in cylinders of course. Now the cylinder that read 162psi is the cylinder that's running slightly cold as I felt the exhaust when it was running.. I'm thinking a sticky valve or valve clearances ?? Also plugs foul up pretty sooty after a ride. Just a bit confused at the moment. Any help is much appreciated also it's running a small dump pipe- no muffler.

 

Hi and welcome to the forum. You should put your bike model and year in your profile for us to see. The guys here will also beg you to post in the introduction forum along with photos of the bike at hand.

YOU BETTER BE WEARING HEARING PROTECTION!

Do you have the service manual?

http://vfrworld.com/forums/showthread.php/3161-Downloadable-Parts-Catalogs-and-Service-Manuals

Are all four plugs sooty?

Have you tested the regulator/rectifier and stator output?

Airbox and filter are in place as you test?

So no muffler. Did the prior owner dyno the bike in this condition? What size jets are on it? Perhaps someone with this generation bike can verify a jet setup for no muffler. It will definitely run with less power and not approach the stock power and smoothness without a muffler. If you plan on installing a muffler than do so first thing. Otherwise your current jetting is invalid. I suppose if you are lucky the answer that the prior owner far over-jetted it when he removed the muffler. But alas the truth is usually closer to there are several things wrong and you will just attack them one by one.

I suppose the valve clearance is the final thing to check once air and spark are all verified. Sixty thousand miles? Were you provided any repair history of the bike to see if the valves were adjusted? On second thought, given the condition of the bike perhaps you want to dive right into a valve clearance check as a proactive measure.

Good luck. I look forward to following your bike refresh!

 

What Tinker said, these carbs are a touchy bunch and from what you wrote I concur, it's carb issues. Then on a second thought, I'm not sure my eyes could handle a another street fight VFR unless it's liike Sebs. JK But WHERE'S the PHOTOs or it doesn't exist.

 

Geeze, i thought NZ--even without a new flag--was a civilized country, but if they let you ride an obnoxiously loud bike, maybe not ?? So you don't give a damn about disturbing sleepers, scaring dogs and cats, small kids, and giving Honda a bad image ?? Ride on, hooligan. :blue:

In fact very few bikes without mufflers and some backpressure run properly throughout the rev range and usually bog down badly somewhere. That's why mufflers are a MUST !:eagerness:

 

Hey knight thanks for the reply I just put some info up and a profile picture, as you can see this bike is a project haha. All 4 plugs are sooty yes. I intend to keep the muffler off as it doesn't sound too bad I think. I was provided no prior information about the bike, il have to check the jets but I'm confident they are stock because the owner didn't mention changing them, it certainly hasn't been dynoed.. This bike was a daily work commuter for a 60 year ol man I do have a Haynes manual for this bike yes. Also I did a running charge test on the battery and the voltage was 18.40v ?? A bit high if you ask me.. Also I checked the pick up coils and they are fine. Also the airfilter is new and was fitted when test riding the bike

 

In reply to everyone else (because I don't really have time to reply atm) thank you very much for the replies and help, I also believe it is carb issues so I will rebuild and install them as stock. Squirrel an the bike is not straight piped but it does have a tip coming off the collector manifold thing just in front of the rear wheel, now the bike isn't obnoxiously loud at all, it's half as quiet as your average road going Harley ! .. Anyway the previous owner did the exhaust not me and that's not really the issue I'm here to talk about haha

 

Sure, it's likely a carb issue !! There's no easy way to tune a carb to run right at ALL rpms without a muffler !

 

Yes you need to run tests per the manual on the charging equipment, at a minimum it sounds like it will need a rectifier. Is that battery bulging? High voltage is going to boil it. Hopefully you did not add a brand new battery yet.

 

At that voltage you're gonna fry the CDI box soon.

 

Yeah that test was at idle too :/ dunno if the battery is bulging, il test the rectifier and related charging system components. Could the high voltage be affecting the CDI or spark somehow ?

 

You already tested the charging system, right ?? Now you need to replace the R/R b4 the CDI melts, OK ??

 

Yes I will replace the rectifier but I'm wondering if that's the cause for poor running

 

It is a system that is in need of lots of help. There are many different causes for it not running right. However to Squirrel's point, you should not run the bike any more without plugging in a new rectifier. The current voltage is going to do more damage.

Test the impedances of the stator before ordering the rectifier. This might detect if the stator needs replacement.

 

I'd guess probably NOT the cause, but you need to replace the r/r anyway.

 

I hope Knight doesn't smack you with his slide rule and 57 pages of quotes from a Chiltons for disagreeing.

 

Well I have found a fault in the carbs, seems as though someone before me has had a tinker :/ all carbs are missing the o-ring and washer for the idle mixture screw, so possible vacuum leak. It would also mess with any mixture adjustments. Also the slide piston that holds the needle has no sealing washer for the needle, so a vacuum leak from the throat through the piston via the needle to the vacuum seal housing is possible, would probably cause the piston to not operate correctly. Being a L model vfr, these are also supposed to have 130 jets all round.. But this has 128 in two of the carbs and 130 in the others ? Anyway certainly a possible cause for the crap top end.