1993 VFR misfire only @ idle

Hi,
My 1993 VFR w about 15k miles on it backfires at idle, or possibly it is better described as an intermittent mis-fire. (makes a POP sound instead of normal)
I put in new plugs and it did not change.
Otherwise it runs great.
Should I sync the carbs? Can carbs cause this symptom?
Or possibly it is some ignition system issue??

Thanks
fish

 

Hi,
Thanks for the reply!
My bike is completely stock, OEM everything as delivered when I bought it new in 1993.
(OK not totally OEM, I have changed tires/chain-sprockets, put tapered rollers in the steering head)

The plugs are tan on the ceramic part.

I suspect something in the carbs, only because the last thing that went wrong w/ it was the choke linkage on the carbs was sticking "on".

Possibly the linkage on the front carbs is still "on' a little, I can't see the choke linkage for the front carbs, was only able to verify the choke linkage for the rear carbs is moving freely now.

I fixed that sticking problem with lubrication and larger springs to push it all to choke "off" position.
I made no adjustments to the carbs, just freed up the choke linkage w/ lubricant.

fish

 

Its gonna be fun to find that one. Lets see---
1. Just started recently --- suspect bad tank of gas (drain and fill)
2. One idle screw may have backed out causing a rich cylinder at idle (check them if not blocked off)
3. Blocked air pre filter (in small box next to air filter)
4. Cracked or loose intake boot
5. Misfire more at idle more when cold or warm (choke circuit)

Another trick is to slow the idle way down and using heat insulated gloves check each exhaust pipe near the exhaust port being careful not to burn your hand checking for the resonance through the pipe to track down the offending cylinder. Note, if you find all four are misfiring and fuel is ok that should get you away from the carbs and into electrics. Hope this helps.
-Lee
BTW 2 h2's, man am I jealous, those bikes were a riot to ride albeit in a straight line (grin)

 

Hi,
It has fresh gas, I'll check to see if one or more cylinders are running at significantly different temps w/ an infrared thermometer.

I don't think there is a pre-filter on a 93, just a snorkel into the top of the air box.

It only misfires when hot.

The H2's are really primitive and simple compared to the VFR, but that makes them easy to troubleshoot & repair. I wish the VFR was less complex, but it sure is worth it on the road!

 

Tight or sticky valve? I'd check the compression when cold, then again while hot, looking for any changes.

 

Hi,
That is an interesting idea, the valves have never been adjusted as it only has 15k miles on it from new.
One or more could be leaking when hot.
It looks easy to check the compression & get to the cam cover on the rear, but the front looks like a big job.
Might be time to sell it and move on to something newer that I don't have to work on. (Local dealer is not an option, already been used & abused by them, not going back)

 

Adjust the valves, and sync the carbs yourself before you decide to sell it. I'm sure a carb-sync will help - especially if it hasn't been done in 15K miles.

For the choke, if you can push any of the choke valves in at all, they are not shut...just another thing to check.

 

OK, Thursday is going to be "VFR troubleshooting day".

I'm starting w/ the carb sync.

Last weekend I dug out the 1970's Mercury filled glass tubes (might have been called "Carb Sticks"???) that I last used to sync the carbs on my 1973 Z1.

(It had a Yoshimura "Hand Bent" pipe, done by Pops himself, sounded like Heavens Angels chorus....)

After only 35 years, the surgical rubber hoses that connect the glass tubes to the manifolds had turned into a brittle wad'o latex
(Hey, that might be a good name for a band...)

Today McMasters-Carr delivered 25' of new hose, and the Mercury seems to have survived the decades OK.

Now I just have to rig up a remote fuel supply and find some way to get the vacuum plugs out of the manifolds.

This could get ugly.

 

If you go to a lawn mower repair shop, they usually have a couple of used plastic gas tanks and on/off fuel switches off of an old engine laying around, and sell them cheap (like five dollars). Then you just need to get clever with some temporary tie-wraps.

And syncing the carbs before checking the valve clearances can be counter-productive. If you do need to adjust a few, it will change the way the cylinders breath. But syncing the carbs isn't too tough.

 

You don't need to remove the tank to balance the carbs on a 93 VFR750?

 

It might be possible for an experienced person to connect the vac gages w/ the tank on, but I took it off because I have never done this before, could not see how to get to the manifold caps any other way.

What I have learned so far:

- The manifold plugs on the rear cylinders can be removed by using a Philips bit and some extensions to reach in from the side.

- There are no manifold plugs on the front cylinders, there is an existing vacuum line already connected, to each front manifold. A hose from each runs to what might be some type of EGR valve.
So I just removed these existing vac. hoses from the EGR and connected the test vac gage hose to the existing vac hose, easy!

Here is the reading is has w/o any adjustment



- There is no way to get to three of the carb adjustment screws w/o a special tool (I ordered one)

- One of the carb adjustments can never be synchronized, as it connects directly to the manual idle speed knob.
(Whenever I adjust the idle speed it effects that one single carb only)

So now I'm waiting for the tool to adjust the other three carbs.

 

Found a valuable clue: while running the motor the backfire started when it came up to operating temp.
One of the surgical rubber vac tubes was turning black inside from backfire soot & pressure going into the intake manifold for that carb!!!!

So I'm guessing that intake valve(s) are too tight on that cylinder????

found a shim kit on epay

 

The steps in tuning a bike (in order) are:

1. Replace the spark plugs
2. Adjust the valves
3. Carb sync
4. Pilot Screw Adjustment

 

Hi,
Shim kit arrived Friday, so I took the rear cam cover off and checked the clearances.
Both of the #1 intakes were too tight, so thinner shims were installed
to provide proper clearance.
All the rest on the rear were within specs after 15,535 miles from factory new.

Ran out of daylight to do the front, looks harder w/ the radiator in the way.





Idles better already, much less backfiring, but there is still a little when it warms up, so I expect to find some intake valve clearances on the front cylinders have tightened up.