92 VFR750: Running Problems - engine not pulling

Hi guys,

I'm in need of a bit of assistance with diagnosing a running problem on my VFR, any help would be much appreciated!

For a bit of background on the problem; I recently picked up my 92 VFR 750 with 35,000 miles on the clocks, when I got the bike I fitted a new rectifier, plugs, filters, oil and checked the valve clearances. I've been running the bike for the last 6 weeks covering around 1200 miles in that time with no issues what so ever. However, last week half way into work I decided to open up the throttle, I got up to 3rd gear (70mph ish) and the power suddenly went.

With anything more than about 1/5th throttle there is not much response, the bike bogs slightly and feels like it's struggling to pull. It will run okay at low speed/low power and if you give it enough time to gradually get up to speed will cruise okay at 60/70mph, but anything involving a decent amount of throttle and it's not happy.

The bike will start, idle and rev in neutral no bother so i'm hoping it's nothing too nasty. The revs won't pick up under heavy throttle so It's not the clutch slipping. My initial thoughts are it's just not getting enough fuel, several days prior to the problem I did run the bike low into reserve so I'm wondering if it's picked up all the crap from the bottom of the tank and something's slightly chocked up? Going to investigate this tonight and have a look at the plugs also!

I'm just wondering if anyone has had a similar experience or is able to shed some light on the problem?


Many thanks,
Lewis

 

Fouled plug, check them. Your on your main jets the way you describe the bog, check those plugs and keep us posted

 

First thing i'd do on any newly-bought bike is to drain and FLUSH the fuel tank.

Check your fuel filter, and consider getting into the carbs for a cleaning.

 

he ran the bike with no issues, could be bad gas and fouled plugs, where as when I trailored my 93 home and adventually got it out on the road it would'nt rev out due to vacuum issues, fitted new carburetors and wah-lah - 150 mph motorcycle, Yes I am happy...

'oh, the previous owner/barbarian screwed up the carboretors by taking the top plate off, creating a vaccum issue, along with buggering up every brass part inside, it was more cost effective to buy replacement carburetors

 

Thanks for the help guys! So I managed to get a quick look at the bike last night.. but sadly not much improvement..

I took the tank off and flushed through the filter, took off and cleaned the fuel valve, put it all back and checked there was a good flow coming from the filter. Then with the plugs out hooked it up to the fuel pump and checked there was a good flow from there. So I'm confident there's good fuel getting to the carbs (filled it up with fresh petrol last night).

The plugs themselves looked okay as far as I could tell, maybe slightly blackened and rich, I took some bad quality photos maybe you can tell me otherwise? I did notice the number 1 cylinder looked leaner than the rest but I later found that the small vacuum nipple which is downstream of the carb on the cylinder head (presumably for syncing the carbs) was unplugged, I plugged that off so hopefully that'll richen it up.

As you've said i think the next best bet is pulling the carbs and giving them a good clean out, I did notice from the manual there's a small fuel filter on each carb before the float valve so hopefully that might limit most of the crap getting into them!

 

change your plugs, you can do the regular low cost replacement, yours are shot.

 

I recently had a similar issue on my 1987 VFR700F2. I CLEANED the carbs, but the diaphragm was not seated properly, so it would start and run fine but acceleration was poor. Had to go and reseat rubber and then it ran fine. It would go hwy speed but took a while to get there and faster! Check your vacuum related stuff in carbs. Do you have a manual? If not get one!

Sent from my LGL34C using Tapatalk

 

Unfortunately this will all have to wait until the weekend when I can get some garage time but I'll be sure to check all that you've mentioned and will keep you posted on the progress, thanks.

And yes I've got a manual, as far as the diaphragm and vacuum goes I did notice that the foam in my sub air cleaner has completely disintegrated.. from what I have read this is only there to filter the air being sucked into the carbs as the slider moves up and down, would this be correct? I also tried blowing on the sub-air cleaner hose which opened all four sliders, is this a valid test for the diaphragms operating as they should?

 

Bingo! Yeh that sub air filter is crap. Just take all that crap off and run it with the T exposed. When you blow air into that T all slides should open simultaneously. If not, you have a vacuum issue. You have a clogged main jet most likely. The deteriated air filter you referred to was sub assembly of the actual one in the box? I take off all the PAIR chit too and plug the air box/carb plenum/and cylindrrs. Keep us posted and dont get discouraged.

 

Yeah the deteriorated filter is the one from the sub-air cleaner assembly. And correct me if I'm wrong but the PAIR system is a USA only addition to the bike so I shouldn't have to bother with it?

 

Howdy Lovie, a couple of things you could try before pulling the carbs,

1 open the screw and drain the carb bowl and catch the contents to inspect for water or crap that may have got in when you went to reserve,

2 try running down the highway again and this time open the choke when the revs are up and throttle opened and see if the power returns. The choke circuit feeds extra fuel that bypasses the main jet circuit. Under 1/5 throttle is on the pilot circuit, which has tiny jets and usually clogs first.

i agree with the midnight rider that the main jets are somehow likely clogged and the choke test might prove that is the case.

 

Yeh, your bike is a euro one, so no pair stuff, i guess it has factory block offs and a different/modified air box. Your right hand switch turns your head lights on and off right? Does it turn your tail light off too? I have a granite blue 91 wearing 92 body work, i have been out on a few midnight rides, so i could be considered a midnight rider :wink:

 

Lovie, your plugs are showing a very lean condition, possibly lean enough to damage the engine, so get it fixed asap.