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Issues running once warm


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Hello everyone, I need some advice on a running issue once the bike is warmed up.

 

The short version. My 1992 Honda VFR 750 feels like it has a fuel starvation issue once the bike is warm. Past 1/3 throttle it sounds like it is bogged down and wont accelerate under load. When the bike is stationary the issue only happens sometimes when warm but will usually rev through the full range. After a short ride when the problem happens I inspected my fuel filter and found bubbles in the filter. I primed the filter back to having no bubbles and after another ride they are back. I am stumped as to what the cause could be at this point. It will startup cold with 1/2 choke first try and will idle well.

 

The long version. I bought a 1992 Honda VFR 750 from a mechanic who was having the running issue but couldn't figure it out. The bike sat for two years before I got it. Before it sat he had a burnt valve that he repaired and rebuilt the head. He added a stage 2 jet kit and modified the exhaust. The R/R connectors melted and caught fire so he replaced it and ran higher gauge wire from the stator to the new R/R. The bike never ran right for him so he parked it then I picked it up. I did a good bit of work which includes the carbs cleaned and synced, new fuel pump and filter, plugs, another new R/R, vacuum piston diaphragms, and fresh oil and filter, chain, tires, brakes and fluid, fresh battery. The bike will start up with half choke and idle beautifully. I thought it was an electrical issue so i checked and cleaned all the ground and added dielectric grease to several connections.  I also swapped out the rectifier trying to fix the issue with no improvement. Finally settled it had to be somewhere in the fuel system but it has been cleaned several times. I changed the position of the needle on the jet kit to see if that would fix it but had no improvement. The fuel tank had some rust in it but it has been cleaned out and the holes have been sealed. Something that was strange to me is there is no filter or screen on the petcock and there is no reserve. I doubt the screen would cause my issues tho. I thought it might be a fuel line but I would think that would have a fuel smell while the bike sat. I also thought it might be a vacuum issue in the fuel tank but I have opened the tank while the issue was happening and it didn't change. The bike will run great if I start before its really warm for about 5-10 minutes then once warm the fuel issues happen every time. There are several threads with running issues that were similar but most either had issues at idle and startup also or were solved by replacing the R/R. I haven't been able to figure out my running issue and appreciate all help and advice.

 

 

 

 

 

 

 

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Might be worth bypassing the whole fuel system. Set up a temporary fuel supply that gravity feeds direct to the carbs. See if it solves the problem. Don't risk a petrol fire.

 

You could also check to see if it was any particular cylinder or carb that was playing up by pulling plug leads one by one and seing if the symptoms get worse.

 

Good luck

 

 

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I once had a similar problem, but it was more rpm related than throttle position related.  The problem was eventually determined to be clogged air filter due to over oiling the K&N air filter.

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On 8/11/2018 at 6:39 PM, Zarquon said:

Might be worth bypassing the whole fuel system. Set up a temporary fuel supply that gravity feeds direct to the carbs. See if it solves the problem. Don't risk a petrol fire.

 

You could also check to see if it was any particular cylinder or carb that was playing up by pulling plug leads one by one and seing if the symptoms get worse.

 

Good luck

 

 

I tried hooking up a bottle to see if air bubbles came up but since the issue is under load I haven’t been able to rig something to hold it while riding. 

Ive used a heat gun to check what cylinder is running cold and it ended up going back and forth between the two cylinders towards the front of the bike. 

 

On 8/11/2018 at 6:59 PM, COS_VFR said:

I once had a similar problem, but it was more rpm related than throttle position related.  The problem was eventually determined to be clogged air filter due to over oiling the K&N air filter.

Mine is a combination of throttle position and rpm. It will rev to 5500rpm and then it takes some effort to get it to 6k but won’t rev higher than that. I had a friend oil the filter so I will try cleaning it and see if that works. If it does I will be ecstatic. 

 

15 hours ago, squirrelman said:

check charging voltage b4 and after the problem starts.

Charging is good under no load. I may try to leave the leads hooked up on a ride to test under load when the issue happens 

 

 

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Maybe give it a brief run with no air cleaner at all? And make sure nothing can possibly block the snorkle.

 

Could be air starvation/over fueling causing the probs.

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My ST1100 was a bit like this, ran well at idle and below 4000rpm, but anything over that was accompanied by a lot of misfiring and surging. I traced the fault to a dirty electrical connector feeding power to the ECU, due to the corrosion it was only getting 10V and leading to weak sparks. 

 

That doesn't explain bubbles in your fuel filter, which presumably can only come from the fuel line to the tank. Is there a vacuum fuel tap in the circuit with a bad diaphragm?

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On 8/13/2018 at 10:34 PM, Terry said:

My ST1100 was a bit like this, ran well at idle and below 4000rpm, but anything over that was accompanied by a lot of misfiring and surging. I traced the fault to a dirty electrical connector feeding power to the ECU, due to the corrosion it was only getting 10V and leading to weak sparks. 

 

That doesn't explain bubbles in your fuel filter, which presumably can only come from the fuel line to the tank. Is there a vacuum fuel tap in the circuit with a bad diaphragm?

Where in the wiring harness did you find your issue and how did you find it? Also did you have the issue as soon as you tried to ride or once warm? 

 

I cleaned the air filter today and put minimal oil. Also used contact cleaner on the brain box and put dielectric grease on the connector. I’ve had to replace several connections on the bike due to melting while the previous owner had the bike. 

 

Bike ran great for first 5-10 minutes. I stabbed the throttle several times trying to get it to act up but it rode great. Then came to a stop sign and when I started off again it was bucking non stop under load. It would up as soon as I gave it throttle and wouldn’t rev past 5-6k. Under no load it will rev quickly but limit at 8k. 

 

I will note the diaphragms in the carb are aftermarket off eBay. They were exact replacements but spin on the plastic slide. I’m thinking these could be the issue but I can’t figure out why the bike being warm causes the issue. 

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On 8/13/2018 at 9:49 AM, Jajr said:

 

 

Charging is good under no load. I may try to leave the leads hooked up on a ride to test under load when the issue happens 

 

Quote

load on the engine doesn't affect charging.

i've seen many bikes that run great and have air bubbles at the top of the fuel filter.

 

 

 

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On 8/19/2018 at 11:28 AM, Jajr said:

Where in the wiring harness did you find your issue and how did you find it? Also did you have the issue as soon as you tried to ride or once warm? 

 

I cleaned the air filter today and put minimal oil. Also used contact cleaner on the brain box and put dielectric grease on the connector. I’ve had to replace several connections on the bike due to melting while the previous owner had the bike. 

 

Bike ran great for first 5-10 minutes. I stabbed the throttle several times trying to get it to act up but it rode great. Then came to a stop sign and when I started off again it was bucking non stop under load. It would up as soon as I gave it throttle and wouldn’t rev past 5-6k. Under no load it will rev quickly but limit at 8k. 

 

I will note the diaphragms in the carb are aftermarket off eBay. They were exact replacements but spin on the plastic slide. I’m thinking these could be the issue but I can’t figure out why the bike being warm causes the issue. 

On my harness I first checked the voltage at the ECU, there was a specific pin to check according to the manual. After that worked my way back through the connectors and specifically found corrosion at the red connector which is the one that feeds power to/from the right bar pod (starter, kills switch). With this cleaned I had full battery voltage at the ECU and the misfire was gone. This happened any time the revs were up over 4k whether hot or cold. 

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Those diaphragms should be fine, but you can check with the airbox off, engine running, sudden full throttle opening should see the slides lift most of the way. if they do, then its not the vacuum, unless sudden acceleration works, then bike slows down, as slides close !  Use your finger on the slide to ensure it has full movement before starting the engine.

 

There are three things that would generally stop an engine revving out. Lack of air, would be apparent when stationary. Lack of fuel, would be apparent when stationary & poor spark.

If the bike revs freely when cold, but not when hot, this is usually down to a failing electrical connector or devices. Coils suffer heat soak, connectors suffer from the same & the ignition advance is in the ECU which also can suffer heat soak, as it passes coil switching power through it.  The igniton advance would be rpm specific, with most bikes switching advance around 3500-5000rpm.

 

Just some items to think about.

 

 

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On 8/21/2018 at 7:22 AM, Mohawk said:

Those diaphragms should be fine, but you can check with the airbox off, engine running, sudden full throttle opening should see the slides lift most of the way. if they do, then its not the vacuum, unless sudden acceleration works, then bike slows down, as slides close !  Use your finger on the slide to ensure it has full movement before starting the engine.


There are three things that would ge

er from the same & the ignition advance is in the ECU which also can suffer heat soak, as it passes coil switching power through it.  The igniton advance would be rpm specific, with most bikes switching advance around 3500-5000rpm.

 

Just some items to think about.

 

On 8/21/2018 at 5:32 PM, squirrelman said:

Pickup coils are heat-prone too, so check resistances both cold and hot.

 

The diaphragms moved freely. When i stabbed the throttle and held it the bike had no issues while cold. But again once warm the slightest throttle was met with stuttering. 

 

The fuel and air are flowing freely. I've run a few tanks of good ethanol free gas through it so I don't think there is anything clogged in the system. I was told by the previous owner that the ignition coils were placed on a test bench and sparked over an 8mm gap for 30 min with no loss in continuity. I haven't checked the coils while hot since I've owned it but it's on my list. I've found a few corroded connections but have cleaned them. I need to check the connection in the fan assembly to see if the fan switch is bad.

 

I checked the pickup coils with an oscilloscope and ohm meter today to see if they were bad. I read online that the 87 vfr pickup resistance should be 450-550 but haven't found what it should be for a 92 vfr. The resistance was 307 and 314 when cold and 378 and 384 when warm. I rode the bike just long enough for the issue to act up so the resistance might change after a long ride.

 

I am running out of ideas so any and all suggestions are appreciated. 

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Some thoughts of what to test.

 

Does pulling the cold start out have any effect on the engine behaviour when its playing up?

 

Does disconnecting the plug leads one at a time affect the problem? Can you track the problem to a particular piston?

 

Can you use your osilloscope to check the low tension input signal to each of the ignition coils when its playing up.

 

Could also use the osilloscope to check the ignition timing output from the crank shaft sensors?

 

(If you trace both these signals together then you should be able to see if the ignition advance is working) It might be a funky ECU fault.

 

Are all the vacuum lines and carb insulators in good condition?

 

Does the bike always play up when it is worm?

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The 5th gen has grounding issues, I am not sure about the 4th. Nevertheless poor grounds do strange things and they can change when hot or cold. Clean and beef up all grounds. In the past I have soldered blocks, ran new grounds to frame including the ecu.

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