'97 VFR 750 F - No 'powerband' - Please help!

Hi All,

I'm new to the forum, only acquired my Viffer a week ago.
I've done a near 200 mile run today and the 'powerband' seemed to kick in on only 5 or 6 occasions - like hitting a Nitro button when it did but it didn't do it most of the time and I was really giving it the beans!
I'll give as much background as may be useful:
First ride out, I had problems when the revs got up to about 8000, seemed to be 'stuttering' with power.
It had a full tank of fuel and I had no idea how long it had been standing.
Advice from the pub was empty the tank, put in fresh fuel, try some redex and change the plugs.
I stripped the fairing and tank off over the weekend to change the plugs and check them and found number 4 would not spark.
A bit of cross testing with leads and caps got me to the point of finding a loose terminal on the coil for that cylinder and after cleaning it and putting it back on firmly, the spark was there.
I wondered if I might have knocked it loose while stripping down but the original plug was wet and a bit black so it does look like it wasn't firing but now all four plugs are replaced with NGK CR9EH9s (the ones I replaced were CR8EH9s) and were all firing before I put it all back together.
The air filter looked quite clean and I brushed it off before putting it back.
I put in some new fuel with a bit of redex, used that, then I admit I did put the fuel I took out back in with Redex again and have used that up and since refilled again so the petrol is pretty much fresh.
I also sprayed some carb cleaner into the carbs before I put it all back together.
All this seemed to resolve the stuttering but now having had literally just a few tastes of the power boost around 6000 to 7000 RPM, I know what I'm missing.
The rectifier is a SH691-12 with the cooling fins so I believe it's an after market replacement but wondered if this could be on the way out and whether that would have any relevance?
Does anybody have any experience of this problem and can anybody please offer me some guidance as to how I can fix it and make my grin permanent?
Thanks.

 

Hi RowdyPete

Firstly a warm welcome to the MadHouse:welcome:

So in short, the bike is 16 years old and intermittently down on power.

I take it you have no idea whether the bike was being used regularly and running fine with the previous owner, or is this a barn find you are struggling with? Certainly I would suspect if its seen little use (how many miles does it show on the odometer?) over those 17 years then the fuel system could need a through clean through/overhaul.

Hopefully some of the folks more familiar with the 750 carbs will be better able to advise on what is worth doing, and in what order.

Certainly if you have emptied a tank full of ancient fuel through the carbs, then you may be pulling heaps of rust and other gunk from inside the tank. Sadly Redex and the like can only do so much - so if the jets get bunged up then they tend to get progressively worse and a full strip down and rebuild of the whole fuel system may be needed.

If the bike is running on three cylinders then it is hardly surprising its down on power. So hopefully after your recent investigations, you had another look at the plugs after the bike has been running for a while - as the state of the plugs should give a hint as to whether one or more cylinder is still not firing or is running lean/rich.

I doubt many 17 year old VFR's still have OEM RRs - they fail with alarming frequency, but if the battery is not loosing its charge then its not likely to be pertinent to the bike lacking power/acceleration.

Take care



SkiMad

 

Hi Skimad, thanks for the warm welcome and the quick reply!
The bike has a full history of services and extras and has done 76000 miles (new second hand clock with 40000 on it last year but documented) so it's been used regularly.
The guy I bought it from had it for a few months but decided he didn't want to take up road biking again after all and put in new front pads and replaced the front caliper pistons and seals AFTER I had bought the bike (pistons in wrong way round and had to rectify last week but that's another story!).
The bike is spotless and the inside of the tank is shiny and clean as new as far as I can see.
I think it was only running on three when I first took it out before doing all the steps I mentioned in my first post but that initial problem is now gone and I was able to lead the way today without holding my two mates up so it does seem to be running smoother but it's just this power boost at 6 to 7000 revs that only happened a few times that bothers me.
I haven't had the plugs out since replacing yet as it's such a long winded job as you know, but I may try that again tomorrow so I have all the facts.
My brother has suggested that it may be an issue with one or more of the coils but I don't know if that's right or even how to test them.
My gut feeling was that if the power boost does kick in on occasions, then I would have expected the carbs were pretty clear, once it's cleaned out the crud has gone kind of thing but again, I bow to greater knowledge if this is wrong.
I have to say that I wanted one of these bikes after reading reviews that said they were the best all-round sports tourer and now I've got one, I'm delighted with it.
It handles really nicely, corners well and you only have to think of where you want it to go and it does it.
It's a beautiful ride.
I can see what all the fuss is about.

Thanks,

RowdyPete

 

Run a few tanks of fuel thru that bad boy and see how it does, you'll know it better after a few thousand miles.


motorsickles are not like fruit--fruit don't talk, fruit just sits and listens. Motorsickles need exercise to keep in shape and they will let you know when something is wrong.

 

You check the fuel filter on the left side to make sure its not pluging up? Also there is a strainer in thank too. if its idles good then probably not a coil but fuel supply issue with filter or carbs. There is a secondary foam breather filter mounted on the left of the air box back corner. I checked mine this weekend and it was gone and deteriorated. I havent had any running issues but others have from this filter getting sucked into the carb system.

 

Hi,
check the coils/HT leads/caps with a multimeter. I has a dodgy cap on mine when I got it so it was only running on about 3 1/2 cylinders.

Maybe balance the carbs and check the sliders are moving freely.

Not sure how much of a powerband you are expecting tho...V4s are renowned for a very linear power curve.
cheers
Rich

 

Thanks for the replies.
kennybobby - I like the idea of just getting loads of miles in, hope to do this anyway.

SLOVFR - It ticks over ok with no spluttering but I'm not 100% convinced it's completely right. I checked the in-line filter and cleaned it out while the tank was off. Blasted out both ways with carb cleaner and the canned air but it did look clean and the carb cleaner blew out the other end quite powerfully. I'm not aware of this other foam filter, I didn't notice it when I had the air filter out. Is it part of the air box or to the left of the box?

richbike - This sounds very much like my problem. I tried a multimeter on no. 4 lead at the weekend but just continuity and it didn't register anything but I think that's resistance in the cap. TBH I'm not really sure how to easily test the coils/leads/caps with my multitester. I haven't checked the sliders but will hopefully do that tonight.
Regarding the power surge, I felt it a few times as I mentioned and have seen on forums and reviews that it kicks in about 6,500 and goes up to the top.
I will try and do some more checking tonight and report back.
Thanks again all.

 

Hi,

I'll dig out the numbers from the Haynes manual later. If you are planning to do much yourself I recommend getting one.

Its worth giving the suspension and brakes some TLC
cheers
RIch

 

Only the VTECS have the extra kick at 6300 RPM. If you were feeling it, it is probably because it was running right, before it seemed to do it.

 

Why did you switch heat ranges on the plugs ?? Open bowl drains on 2 sample carbs, whilest holding a paper towel under looking for any rust or dirt particles that might emerge.

My best guess is that a really proper carb clean would fix the problem.

 

Hi,
so the primary (LT) resistance should be 2-4 ohms (across the 2 spade terminals)

with the HT leas and cap the secondary should be 17k - 24K ohms (across either spade and the cap)
without the lead and cap 13k - 17K ohms
Bare in mind the coils are not grounded to the mounting so measuring to that is meaningless

OR... pull off enough plastic to get to the coil connectors, get the motor running up to temp on idle then disconnect each coil in turn and see how much it affects engine

cheers
Rich

 

squirrelman - I bought the plugs that were stated in the service manual I downloaded and then took the bike apart and got the old ones out. The CR8's it says are for colder climates, below 5 degrees but I didn't know they were in when I bought the plugs. Bear in mind I would very very rarely be using the bike in those temperatures. Would I be better putting CR8s in than CR9s?
Rich - Thanks again, that's a stroke of genius, pulling one coil at a time to narrow down the likely candidate.

I checked the sliders and they appear to be working fine. I gave another few blasts of carb cleaner too (tip of the day, don't let it spit in your eye!) and checked the sub air cleaner filter which was, as SLOVFR mentioned, dirty and deteriorated. It fell apart in my hands when I tried to clean it so I sorted a replacement, sprayed it with filter oil and put it back.
I have drained the carbs and there was minimal crud in there, just a few tiny black specks.
I tried firing up with each of the plug caps removed in turn but the only one that seemed to make a difference was number 1.
I checked the spark again on each and it seemed strong but I haven't checked it under load.
Got two used but supposedly good front coils/HTs/Plug caps coming tomorrow so I will have those to try.
It has been suggested that the DataTools gear indicator on the bike may possibly have an influence but will have a closer look at this later.
Can anybody please tell me what revs should I be seeing when idling?
*Edit* Sorry, scrub that, just found in the manual that it's 1000RPM +/- 100 and it's on 1000 at the moment.

 

Thanks to all that took an interest and helped.
I think it's running sweet now.
Tickover was sounding a bit irregular so I did some bits and bobs without taking it out but I just went for a spin and the power seems to be All there.
Smooth, consistent power curve, and plenty of guts. Hit 110 in 5th without realising.
I reckon draining the carbs, carb cleaner and redex through it did the trick.
Still baffled why when disconnecting the plug caps one by one, only no. 1 seemed to make a difference in engine sound but hey, it works and its awesome!
Big grin factor!
Thanks again all.

 

I`ve got a 97 model and mine takes off like a rocket when you get to about 6,500-7,000 revs