VFR 93 is hard starting.

Hi,
I have a 1993 Vfr 750, and since i bought it about 4 months ago the bike has been very difficult to start. It wont start or even chug when the choke is on although the choke operation seems smooth. I have twice changed the plugs and each time the bike benefitted and became much easier to start but deteriorated over the following days and weeks. It is not too difficult to start when being ridden everyday but when it is left for a day or two it become very difficult. I am a novice when it comes to working on the bike and would appreciate any ideas anyone has??? Cheers

 

you shouldnt have the change the plugs so often, but they may provide the clues as to what the problem could be. Have you checked for blockage/flow of the air filter? was the bike properly winterized (either stabil or drain carbs and tank)? would check them and run a tank of seafoam thru to see if that helps. If the plugs are fowling, sounds like air/fuel mix -> carbs to me

 

Black, sooty plugs= dirty air filter, choke stuck on, very low battery voltage, etc.

Hard starting could be broken choke cable. Do you feel a definite resistance to your pull ??

 

How about a little more detail. Mileage on the bike, maintenance history if you know of any, how does it run when you do get her going, etc. Does it hesitate or backfire when you accelerate or engine brake? Oh yes and pictures of the engine, carbs and the plugs as well as the air filter may provide a few clues for us to work with. Is there any indication that a backyard hack mechanic has worked on it?

Without knowing more than you provided, my first guess is that the carbs need a really good cleaning and perhaps even a few seals may need to be replaced. With the plugs out a compression test would give you a great indication of the general health of the engine. I have a box full of carb parts that may help but we need to identify what's up first.

Please provide some more details.

 

Yeah...more detail is needed. Do you have to hold the throttle open a bit to start it? Does it have a jet kit installed?

 

Had a similar problem...on one bike it was a broken choke cable on another it was filthy carbs...you could check the cable and it wouldnt hurt to clean out your carbs...for starters

 

Hi thanks for replys so far. Bit more info - Has been serviced at Honda dealership, millage is 18,000. Bike runs pretty good once it is going, there is a slight misfire in the low rev range. The air filter is clean and the choke cable is not snapped. Have spoke to the previous ownewr and it had the carbs ultra-sonically cleaned a while ago because the carbs got gummed up after being left stood for a long time. Have checked the battery which is fine. I do need to open the throttle very slightly to start it and i am unsure if it has had a jet kit installed, but the previous owner thought it may have been? hope this helps?

 

Ok...For starters...You will have to clean the carbs again...Fuel turns to a gummy varnish if left in the carbs too long... Certain jet kits are easy to spot...Dynojets have an adjustable length on the vacuum piston needle....There is also a number stamped on the main jet...Check the manual for the correct number...

I think you mentioned the plugs were fouled? Once you sort everything out with the carbs it should be much better.

be carefull of the screws that hold the carbs together as they are purposely soft and will damage easy...slow and steady with the carb dismantling...and cleaning.

I hope this helps

 

The fact that you need to open the throttle to get her to come to life sounds to me like it is too rich on the bottom end. Check your mixture screws are at 1 1/2 turns out as a starter and if the problem persists you need to open the carbs and see what jets are installed. Some jet kits have you open up the air bleed holes in the carb slides as well as replacing the pilot, main and jet needles. There are alot of variables when it comes to tuning the carbs to suit you engine. Do you have a pipe or aftermarket air filter installed? If not the bike should have stock jetting installed. I don't recall the actual jet sizes but I'm sure someone here knows and or we can look at the parts manual.

 

A jetkit should have no effect on starting if properly installed.

Likely the carbs sat and fuel has clogged slow jets.

Normally no throttle is used for starting a cold engine.

Problems to look for in your carbs would be dirty air passages, blocked jets, and improper setting of float levels or mixture screws.

 

It does when it causes excess gas to leak into the cylinder and foul plugs

 

sounds like the Idle jets are plugged.... does it run fully choked?

 

Plug fouling...needs air to start (throttle open)...yes it does seem rich.

'Properly installed' are the key words, Squirrelman.

Float valve(s) could be leaking or float height set wrong. Any multitude of things could be going on to make for a rich mixture. Sounds like a carb inspection/clean is in order. If a jet kit is installed, sometimes the best thing to do is put it back to stock settings if no obvious problems are found.

 

SAY WHAT ??

No jetkit mods cause " gas to leak into cylinders" but bad float needles DO.

 

I see your point Squirrelman. No mods are made to the enrichener circuit, and raw fuel in the CC is usually caused by float valve leaking. However, I've seen situations where jet kits were calibrated so rich as to leave spark plugs gassy and half carbon fouled when the engine is keyed off. NEXT time you go to start the thing is when the problem shows up...hard starting, fouled plugs.

Admittedly, this usually is the 'human' factor. Somebody clipped a slide needle to rich or installed it improperly. Installed a massively rich main jet, or went up a size or drilled slow jets unnecessarily. It's been my experience too, that jet kits normally come with cheap aluminum needles that work OK initially, but wear really fast. So over the course of several thousand miles the bike runs richer and richer.

 

That sounded like my situation.

 

Carbs are an amazingly intricate device. Very few people actually understand them. Small changes can make big differences. One main jet size can turn a 'lion' into a 'turd'. Jet kits are typically a pretty radical change in calibration...almost always the idea is to make plenty of fuel available and make the dyno graph look impressive compared to stock.

The thing is...there's more to jetting than dyno graphs for people that aren't at the 'track'. There's fuel milage, carboning issues, good idle and low RPM running to mention just a few. It's really hard to beat the factory at this game.

 

The thing is...there's more to jetting than dyno graphs for people that aren't at the 'track'. There's fuel milage, carboning issues, good idle and low RPM running to mention just a few. It's really hard to beat the factory at this game.

Thats what I have been preaching after the nightmare with my dynojetkit..

 

I like to call it driveability. It tends to suffer as a result of modifications and jetting work unless it is done properly. I too had trouble with a dynojet kit in my 91 VFR but I managed to tune it back where I wanted it after many hours of adjustments and changes including installing a new set of stock slides after I drilled the originals per the dynojet instructions. This was tuning validated by my ass-dyno which doesn't provide numbers to brag about but it does result in a driveable bike. I validated it later at the track and it performed admirably.

 

I really dislike when additional lift holes are required. I've had luck 'undoing' a couple of such modifications in the past by filling the holes with good 'ole' clear permatex silicone sealant and allow to dry. It seems to hold up fine. Those slides are quite expensive.

I guess dyno tuning can be useful...and lots of people swear by it. But I also know a couple occasions of reputably dyno tuned bikes that were said to be 'really' fast and somehow turned into 'slugs' on the track...horsepower track and good riders, too. I don't think you can always simulate 'real' conditions of temperature, air flow and several minutes of 100MPH plug riding on a stationary stand.