Bangs from exhaust at 3500rpm

[nswizard]

Hi guys,

my old chap Vmax '97 has some issues
Not sure if problem is in carbs but it smells like it.
Few weeks ago it started to have low power on low revs. For example, if i am in first gear and moving up from complete stop, it takes quite a lot of throttle to make it go.
Last weekend I went on a ride and all was ok until i went 150km away from my garage It started to work like only 3cyl work and there were some bangs from one exhaust when i go 60kmh and release throttle.

Checked NGK spark plugs and they are all ok, gray color, dry.
To be sure i replaced them with new ones.

Today i tried again and removed spark plug cap from right rear cylinder and it kept working like it 3 cylinders work but bangs stopped.
Put spark plug cap back on and bangs returned.

Changed fuel completely.
Tried from fuel bottle directly connected to fuel pump.

Spark look the same on all cylinders. Think it can be bit stronger.
Also, i can see some small sparks even if it is not the time to fire that cylinder.

Checked for leaks in rubbers around carbs and all looks ok.

Audio is not the best one but maybe it will give you some clue:
https://www.dropbox.com/s/o4fuohll8sz3so4/Vmax.m4a?dl=0

When i rev vmax around 3500rpm those bangs show up and after like 4-5000rpm engine sounds ok.


Do you have some ideas what to check?


tnx
Vlada

 

[blaxmax]

If you think it needs more fire try these----
http://www.vmaxforum.net/showthread.php?t=40699
I would also try the shotgun---
http://vmax.lvlhead.com/tips/shotgun.htm

It could also be just needing a trim off the end of the spark plug wire.

 

[nswizard]

I've seen this COPS but to be honest all was ok since now and did not want to "repair while all works". Will try to find some Vmax around me to check the strength of spark.

Will try that idea with carbs.


tnx
Vlada

If you think it needs more fire try these----
http://www.vmaxforum.net/showthread.php?t=40699
I would also try the shotgun---
http://vmax.lvlhead.com/tips/shotgun.htm

It could also be just needing a trim off the end of the spark plug wire.

 

[wildweasel_pt]

I'm awed that with the plug cap in it pops and by removing the cap it stops. Should be the other way around. Popping is just unburnt fuel in exhaust usually. Noticed any leak in the exhaust lately? Your mix may also be out of hand on one cylinder causing incomplete burn.like mentioned I would start with a shotgun just clean everything up and then check also your manifold rubbers while at idle sparking it with brake cleaner and checking if idle is affected

 

[nswizard]

Yes i will have to clean carbs and entire fuel line, flushed floaters and bunch of dirt came out.

Noticed one more thing.
If you check this image:
https://www.dropbox.com/s/y4xfwrdvc6x3oxt/2016-09-01%2014.32.25.jpg?dl=0
you can see that there is some fuel around the diaphragm (moved it few times manually) while in other carbs it looks like this:
https://www.dropbox.com/s/dx0s1yht8sw1b2d/2016-09-01 14.32.20.jpg?dl=0

How it should be?


Vlada

 

[Dorney]

Yes i will have to clean carbs and entire fuel line, flushed floaters and bunch of dirt came out.

Noticed one more thing.
If you check this image:
https://www.dropbox.com/s/y4xfwrdvc6x3oxt/2016-09-01 14.32.25.jpg?dl=0
you can see that there is some fuel around the diaphragm (moved it few times manually) while in other carbs it looks like this:
https://www.dropbox.com/s/dx0s1yht8sw1b2d/2016-09-01 14.32.20.jpg?dl=0

How it should be?


Vlada

It's hard to tell by those pic. but the slide looks scored & the needle doesn't look straight. When you work the slide with your finger does it move smooth ? Also when you push the slide all the way in if you put your thumb over the large oval hole the slide when left go should move very slow if not check the diaphragm.

Dave

 

[nswizard]

Check this video:
https://www.dropbox.com/s/6oas6sdpt1h8eav/2016-09-01 17.44.45.mp4?dl=0

One carb sounds weird when i push slider.
I would say that carbs were never cleaned in this 20 years so i decided to open them completely and check how things inside look like.

You can never give too much love to your vmax


Vlada

 

[blaxmax]

Check the spring in the diaphragm caps to see if it is around the circle underneath the cap. It needs to be around the groove.

 

[nswizard]

Correct! That was the issue with one carb. Spring was not sitting well.

Was wondering if it is ok to put another smaller filter before main one? Will that damage fuel pump?
Also, do you know what fuel pressure should this pump produce? Want to take it to a service center for a quick check but could not find this data in manual or forum.


Vlada

Check the spring in the diaphragm caps to see if it is around the circle underneath the cap. It needs to be around the groove.

 

[Dorney]

Correct! That was the issue with one carb. Spring was not sitting well.

Was wondering if it is ok to put another smaller filter before main one? Will that damage fuel pump?
Also, do you know what fuel pressure should this pump produce? Want to take it to a service center for a quick check but could not find this data in manual or forum.


Vlada

On the video the top left one seemed to fast. I would take a look at all of them. Fuel pump psi I would think should be around 3 to 4 psi & not over 5 or 6 tops.

Dave

 

[blaxmax]

Correct! That was the issue with one carb. Spring was not sitting well.

Was wondering if it is ok to put another smaller filter before main one? Will that damage fuel pump?
Also, do you know what fuel pressure should this pump produce? Want to take it to a service center for a quick check but could not find this data in manual or forum.


Vlada

Putting another filter between the oem and the fuel pump is a good idea as I put a clear one to see how well the fuel pump was pulling. Also if all fuel bowls are filling up and the pump stops clicking after a few seconds it should be working fine. 2 psi is what the stock flows.

 

[nswizard]

Bad news
Finally got compression testing tool and checked.
On cylinder that is not working and produces those bangs i have 3 kg/cm2 (50psi).
Measured two other cylinders and they have around 8-9 kg/cm2 (110-120psi).

Seems i will have to take engine out and check if valves in engine head are burned.

Do you have some other ideas what to check?


Vlada

 

[blaxmax]

How many miles or km's on it?
Also check piston rings wear.

 

[nswizard]

30 000 miles, but i am not the first owner so who knows.

Will remove engine soon and hope it is just valve.


Vlada

How many miles or km's on it?
Also check piston rings wear.

 

[nswizard]

Seems that tool I used for checking is a bit weird. Checked with two more and in 3 cylinders i have 10 kg/cm2 and in that problematic i have 8 kg/cm2.
Definitely it is bit lower then other 3 but it should not be the reason for not working.

Can you tell me if idle screw in this carb is ok:
https://www.dropbox.com/s/qh0vbqhf614ck5s/2016-09-09 19.13.05.jpg?dl=0

O-ring seems to have some wear and needle part is completely black.
Should that tip be like this or it should look pointy like a real needle?
Spring has some oxidation.

Today I've changed the oil and filter because some fuel definitely got into oil and oil level raised a bit.




Vlada

 

[Dragmaxster]

Seems that tool I used for checking is a bit weird. Checked with two more and in 3 cylinders i have 10 kg/cm2 and in that problematic i have 8 kg/cm2.
Definitely it is bit lower then other 3 but it should not be the reason for not working.

Can you tell me if idle screw in this carb is ok:
https://www.dropbox.com/s/qh0vbqhf614ck5s/2016-09-09 19.13.05.jpg?dl=0

O-ring seems to have some wear and needle part is completely black.
Should that tip be like this or it should look pointy like a real needle?
Spring has some oxidation.

Today I've changed the oil and filter because some fuel definitely got into oil and oil level raised a bit.




Vlada

I'm sorry to say that a/f screw tip is broken off. By the looks of the o-ring it was tightened too tight. Yes it should look more like a needle. You can try compressed air to get it out. Good luck, That's a tough one.

 

[nswizard]

I was really gentle when i checked the number of turns before completely tightened.
What worries me now is if that needle tip got somehow into cylinder and caused that lower compression?


Vlada

 

[blaxmax]

I was really gentle when i checked the number of turns before completely tightened.
What worries me now is if that needle tip got somehow into cylinder and caused that lower compression?


Vlada

I doubt that with the taper it has it went through. I use a jewelers' headset with lights to see if it is still in there and like Drag said high compression to get it out.

 

[Fire-medic]

I bought a set of carbs from another rider, he'd taken them apart to clean, and in the process he found the tip of the air screw broken off. It was protruding into the slide bore. I used a long socket extension and a deep socket inserted into the bore to place pressure onto the exposed tip, and it popped right out.

Another time I had a broken off brass piece in a carb and that one my machinist removed it for me, good as new.

I suggest checking for valve clearances before removing the engine, maybe all you need is the valves adjusted. Don't forget the old trick of a bit of oil into the cyl to see if the compression improves, a sign of worn rings. A leak-down test should also help to determine the source of the leakage, air from the exhaust, exhaust valves, air from the carbs, the intake valves; besides informing you of the relative sealing capacity compared to a healthy engine, or your other cyl's.

 

[nswizard]

Hi guys,

I've got my replacement needle, did shotgun again.
Result is the same, left back cylinder does not fire up.
Tomorrow I will borrow pro compression test tool and try that idea with oil in plug hole.
Can valve clearance be checked on left back cylinder without taking engine out?


Vlada