My 1995 VFR750F is surging & backfiring.

[4thGenVFR750F]

Hello All, 

   I had a problem with my VFR. I was riding home from work last and it started backfiring and feeling under powered. Also when I get on the throttle it surges and feels as if it loses torque. Especially in the higher RPM's it happens. Around 6000+ RPM it will surge more frequently and lose torque/acceleration. It seems to me that the RPM should also climb a lot quicker then it does. It will also backfire during compression braking, holding RPM and accelerating in the higher rpm. It usually runs aroun1000-1100 RPM at idel but now it idles anywhere from 1000-900 RPM I know it's not a big difference but it was enough for me to notice. 

Here are the things I have done to troubleshoot/Fix the problem. 

1. Checked the primary coils and secondary coils. (Within Service Manual's Ohms range)  

2. Checked the pulse generator (Within Service Manual's Ohms range)

3. Checked the Ignition Control Module and all test were negative.

4. Bought & replaced old spark plugs with NGK spark plugs. (I did not do a leak down test because there is no smoking or oil smell in the exhaust, Should've I done one anyway?)

 

I am starting to wonder if this is a carburetor issue or something else with fuel mixture or delivery. Come to think of it the fuel pump does project a loud repetitive clicking sound every once in a while. 

I really hope that it is something besides the carbs. It terrifies me just thinking about taking those off. 

I have the original service manual which shows excellent visual and verbal instructions. I have just never dug that far into the bike before and it makes me nervous!

 

If any of you have any suggestions on what to do please let me hear them. It will be greatly appreciated and Thank you in advance. 

 

[rangemaster]

It's the carb(s)-clogged idle jet, the one that doesn't come out. I use a piece of .012 guitar string to clean them out-works every time. 

 

Nice looking 4th gen.

[4thGenVFR750F]

1 hour ago, rangemaster said:

It's the carb(s)-clogged idle jet, the one that doesn't come out. I use a piece of .012 guitar string to clean them out-works every time. 

 

Nice looking 4th gen.

Thank you very much for the suggestions. I will definitely try that this week. I was just adjusting the Throttle stop knob and got the idle corrected.  I think I turned it by accident when taking the middle lower fairing off. 

 

Thanks I just got it about a month ago. Starting to find gremlins. It only has 20,000 miles on it. I think it was lack of maintenance that is causing these issues.  

[MadFrog]

I would start with the easy stuff first: replace fuel filter. Easy and cheap. Even if it's not the particular issue you are chasing, it may be time to replace at 20k, especially if the bike has sat with Ethanol fuel for any length of time.

Then check out the fuel pump - I would be suspicious of the noise you are describing.

Finally, check out the carbs, but it seems unlikely that such a sudden kind of failure would be carb related unless something got suddenly clogged.

 

Hope this helps

 

BTW taking off the carbs is not that bad really. A good length pry bar helps :) After that, it's just a matter of tediously keeping track of where shit goes, take your time and take pictures with your phone if you don't trust your memory.

[4thGenVFR750F]

3 hours ago, MadFrog said:

I would start with the easy stuff first: replace fuel filter. Easy and cheap. Even if it's not the particular issue you are chasing, it may be time to replace at 20k, especially if the bike has sat with Ethanol fuel for any length of time.

Then check out the fuel pump - I would be suspicious of the noise you are describing.

Finally, check out the carbs, but it seems unlikely that such a sudden kind of failure would be carb related unless something got suddenly clogged.

 

Hope this helps

 

BTW taking off the carbs is not that bad really. A good length pry bar helps :) After that, it's just a matter of tediously keeping track of where shit goes, take your time and take pictures with your phone if you don't trust your memory.

Thanks MadFrog, I have already changed the fuel filter. I think I'm gonna get a fuel pump when I get paid since you also think it my be suspect. Now that I think about it if the fuel pump is not able to supply the necessary fuel during open throttle and higher RPM that might explain the surging. However, Would that be a cause of the backfires that happen under compression braking? Hopefully you are correct about the carbs I would still hate to have to pulls those off. HAHAHAHA. 

I JUST WANT TO RIDE!!!! LMFAO!!  

[slovcan]

Just food for thought - sunk or stuck float. Is there any smell of gas or sign of a leak? Too much fuel can cause a power loss. It can also leave gas in the exhaust that can create a backfire. This could also account for the added running of the fuel pump.

 

Cheers,

Glenn

[Dutchy]

U can take the fuel pump out off the equation/bypass it. The 750 engine will happily run without it (unless the tank is running low)

[MadFrog]

As Dutchy said, you can try to bypass it first, or at least test it out before going out and buying a new one. Unless you just want to throw parts at it - that is certainly your prerogative! :)

 

Hopefully you'll be back on the road soon.

[Dutchy]

The "clicking"is absolutely as it should be.

I had a video here on VFRD untill Photofucket decided to change its terms and conditions

As the piston in the fuel pump moves up, it eventually creates a "short" through a set of points on top, causing the piston to drop back. and repeat.

These points do wear use/fuse eventually, but run the bike w/o the pump first and see if the issue persists

 

 

[squirrelman]

 

pull off the line from the tank to fuel pump and check for a good, full, free flow.  anything less and you might have some restriction in he tank.

 

pump will click alot if it's not getting enough fuel.

[Dutchy]

here ya go!

 

 

 

VIDEO0064_zpscda3d494.MP4

 

 

 

 

You did check the airfilter yes?  Over the years some members here have found entire mouse/rat nests.....

There is also a very small filiter that will desintegrate over time.

 

 

(#6,7,8)

 

 

 

 

Take the filter off and see if all 4 butterflies open/close ?

[4thGenVFR750F]

     So I just bypassed the fuel pump and voila it runs much better! Unfortunately, The first ride I took it on it died about a block and a half away from my house. I pushed it home and looked at it like WTF happened! Then it hit me I had turned off the fuel........So yeah at least I got some cardio in. 

    I turned the fuel on and had another go and it was absolutely better! The acceleration was much faster and I was able to bring up the rpm to 11000 with relative ease. There was a little hesitation at the higher rpm and with full throttle but I contribute that to it only being gravity fed and the length of the fuel line. Besides that it ran great, well better than it has since I got it a month ago. I am pretty confident it is the fuel pump. 

Dutchy, I watched the video and notice that yours was in somewhat equal intervals. Mine would click about 5-9 times then stop then 2-4 then stop intermittently. What are your thoughts on that?

Also thanks for the advice about the air filters I will check them. 

 

  

[Dutchy]

Which part of "The 750 engine will happily run without it (unless the tank is running low)" was unclear? That includes the fuel tap under the tank....  many many 

 

By the sound of it, the pump is not functionering right; as the engine idles, the "demand" for fuel should be pretty regular. 

the manual does descrive a test procedure you can go through. Did you also disconnect the pump electically from the loom (a red plug I recall)

 With the engine running ok'ish now and as long as you remember to not run near empty, you can continue to run gravity and look for a replacement pump w/o rush and pick one up cheap.

 

How do the contact points look? If they are burned through, a repair kit is available (as per my post).

 

 

 

[4thGenVFR750F]

       Alright, I got the new fuel pump today and installed it. Clearly the earlier pump was bad. There was a very clear difference in power and acceleration. However, with that said, In the higher RPM it seems to struggle. When I lightly and gradually apply the throttle it is not so prevalent. When I went full throttle it happened almost every time.  I know Dutchy mentioned checking the air box which I will be doing as soon as my new tank gets delivered. If not the air box what else could it be? I am thinking that maybe the carbs need to be tuned. Also, I talked to the original owner and he said he installed a Jet kit in it last year. He said he used the Number 4's jets?(Sorry for my ignorance, but I am not sure what this really means in terms of the "Number 4" jets.) Which he says were the ones with that had the most fuel delivery. Do the carbs need to be re-tuned once the larger jets were installed? 

 

And Dutchy the points on the pump looked ok. maybe the internal mechanism had a malfunction?

[JZH]

Speaking of points, there is an optical "points" kit available from a Dutch KTM guy that might fit (I hope so--I bought one!) these OEM fuel pumps.  I've fitted mine, but the bike it's fitted to hasn't yet been off the lift in my garage...

 

Ciao,

[Dutchy]

Numbah 4???

 

Spicey beef with fried rice???? 

 

 

 

I will dig out my dyno run from last year and get one from a 4th gen that was done this morning to see if there is a dip/flattening of the curve

 

[squirrelman]

On 8/7/2017 at 6:53 PM, rangemaster said:

It's the carb(s)-clogged idle jet, the one that doesn't come out. I use a piece of .012 guitar string to clean them out-works every time. 

 

Nice looking 4th gen.

That's not the idle jet !!  instead, it's the fuel pickup tube for the starting enrichment circuit.

[mk2davis]

I'd pursue the whole number 4 mystery. The link below is for reference only, but usually main Jets are in the 60-180 range, depending on the bike model. I don't know if I've ever seen single digits main Jets. And a bigger number usually equals more gas (except on Weber's I think).

What mods justify the jet change? Some people say the 4th Gen is lean from the factory. So you might go from a 110 (arbitrarily picked number) to 115? That would be big jump for a stock set up. Any adjustments in exhaust, intake, cam, or compression the would justify more?

http://jetsrus.com/a_jet_kit_street/honda_750_VF750F_INTERCEPTOR.htm

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[Forbes]

Don't discount electrical, my V8 kept picking up a slight miss/hesitation on medium to hard acceleration and wouldn't rev to limit easily about 20k after changing several sets...100k irridium plugs?? Eventually found some copper slip grease, put that on the tip where the lead connects, perfect! Worth a try..

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