Cannot rev past 6500 RPM under load

[n00bzi11a]

So. I've received a nice tip from the Carb Man itself via DM, asking about the aluminum carburetor slides that were installed on the bike.

It turns out that according to him, these slides were originally developed to fit the Venture (and I'm going to guess the Virago 535 that also uses this carbs) and the hole on the slide is too small for the Vmax and usually he drills them out to 5/32" or at least 9/64" if the exhaust is stock.

So. I've took the slides out. And measured the holes: they were 2.15mm wide (around 0.086").

I've then checked the old slides that were on the bike when I've bought it and guess what… they were small as well (exactly the same size as the new ones).

If this is the culprit, it would make sense since the replacement parts have the same issue as the old ones

I've then drilled them out to 3.5mm (around 9/64") since my exhaust is stock. Picture bellow with the before and after size.


I'm going to test ride the bike tomorrow. I'll also try to record the ride.

Sorry for not taking any more pictures of the remaining questions but it's already 1 AM and tomorrow is going to hurt in the morning.

 

[dannymax]

So. I've received a nice tip from the Carb Man itself via DM, asking about the aluminum carburetor slides that were installed on the bike.

It turns out that according to him, these slides were originally developed to fit the Venture (and I'm going to guess the Virago 535 that also uses this carbs) and the hole on the slide is too small for the Vmax and usually he drills them out to 5/32" or at least 9/64" if the exhaust is stock.

So. I've took the slides out. And measured the holes: they were 2.15mm wide (around 0.086").

I've then checked the old slides that were on the bike when I've bought it and guess what… they were small as well (exactly the same size as the new ones).

If this is the culprit, it would make sense since the replacement parts have the same issue as the old ones

I've then drilled them out to 3.5mm (around 9/64") since my exhaust is stock. Picture bellow with the before and after size.


I'm going to test ride the bike tomorrow. I'll also try to record the ride.

Sorry for not taking any more pictures of the remaining questions but it's already 1 AM and tomorrow is going to hurt in the morning.

Just a slight revision...I don't know that the aluminum slides were developed for any specific bike but guessing they drilled them small to fit a range of bikes. Easier to drill out than add on. The small hole appears to be similar to the Venture 1200 slides.

 

[n00bzi11a]

Good Morning everyone,

Unfortunatly, the issue still remains. No changes at all. (but seem to be more responsive in the midrage were I still have power).

Would having the rear ignition coils switched cause a similar issue to mine? Because the it really seems like a redline limiter and the noise it makes is quite unpleasent. I'm not sure if it misfiring or pinging (due to wind noise and everything i't har do hear) but a can feel the bike jittering, and it's independent from the throtle input (WOT, half throtle, doesn't matter)

However, it revs up to 9k and beyound in neutral whitout any weird noises.
I've brought the bike to work today and I got some cameras here, I'll try to post a video with the issue later.

 

[Mr. Max]

I want to hear how this bike idles and revs. I would also pull the spark plugs to see if they are all the same colour and then measure temperatures of each cylinder exhausts at idle or steady 2 k rpm maybe. Just to get the basics checked.

 

[n00bzi11a]

Here is a quick clip from today's lunch break, going 2nd and 3rd gear WOT:

 

[n00bzi11a]

And Here the idle and revs in neutral (second start of the day, bike is cold but not ice cold).

The bike could rev higher and quicker when I'm just holding it around 8 or 9k to prove that it can reach it.

 

[n00bzi11a]

This is a recording of the bike idling. A bit further back from the back tire, then closer to the bike, and then going away from it.

After a couple minutes all the headers were pretty much the same at around 90 C°

 

[Fire-medic]

when I'm just holding it around 8 or 9k

Your bike will live longer if you don't run it above 9,000 rpm. Actually, it's probably better to shift before 9,000 rpm. The OEM engine management does not have a rev-limiter! That's why many people use either a DYNA ignition box, or an Ignitech one. The con rod big ends don't like over-revving, so let your conscience (and your wallet) be your guide.

 

[MaxMidnight]

If it is a carburation issue I don't understand why it would be able to rev 9K without load but fail at 6 1/2K under load.
As it only seems to have an issue under load I'm still wondering about ignition advance.

 

[n00bzi11a]

If it is a carburation issue I don't understand why it would be able to rev 9K without load but fail at 6 1/2K under load.
As it only seems to have an issue under load I'm still wondering about ignition advance.

It could be. The timing advance box that's operated by vaccum changes it's resistance by applying vaccum. However I cannot tell if the vaccum to resistance ratio is correct. All I've tested if it does react to the vaccum by increasing or decreasing the resistance (I've followed the Haynes manual and everything seems to match)
However I don't know the timing of the engine

 

[Jack Hammer]

I cannot remember, did you check compression on all cylinders?

Your idle sounds decent but acceleration is non-existent.

Cam timing needs to be checked.

Obviously your carb barrels open all the way when under load, do they?

Did you check the exhaust for blockages?

 

[3-Max]

Just a comment based on what I've been doing to mine. You mentioned you sync'd the carbs via a caliper. I did something similar on mine & thought that OK but last wk I sync'd with vacuum gauges & they were way off. I was having to reset the idle half a dozen times while doing it as it going to 3-4K+. Also set the air/fuel mix pilot jets to standard at the same time. Massive difference in performance.

 

[n00bzi11a]

I cannot remember, did you check compression on all cylinders?

Your idle sounds decent but acceleration is non-existent.

Cam timing needs to be checked.

Obviously your carb barrels open all the way when under load, do they?

Did you check the exhaust for blockages?

I didn't had the chance or equipment to check the compression. I need to find someone that has the gauge for it. Same thing for the cam timing.

I think the carb barrels open. But I've never looked at them under load. They slide very easly, the diaphragms are in exelent shape, the little oring on the diaphragm cover is brand new and when I blow into this holes (image bellow) they do open. So I think there is no reason for them not to open all the way. But I'm going to shove a mini camera over the carb and go for a quick run (I think it would be an interesting video.)

I'll try to remove the exhaust this weekend and test it to see if the issue presists.

 

[n00bzi11a]

Just a comment based on what I've been doing to mine. You mentioned you sync'd the carbs via a caliper. I did something similar on mine & thought that OK but last wk I sync'd with vacuum gauges & they were way off. I was having to reset the idle half a dozen times while doing it as it going to 3-4K+. Also set the air/fuel mix pilot jets to standard at the same time. Massive difference in performance.

You are right.

There was a significant change when "dry Syncing" with the calipers and doing it for real with the gauges.

Just to be clear, the carbs are now synced and i was done properly with gauges.

 

[Mr. Max]

What about final drive, is it original? Or venture, which I believe has longer gear ratio? I think it should still rev past 6,5k though. Mysterious case.

 

[n00bzi11a]

What about final drive, is it original? Or venture, which I believe has longer gear ratio? I think it should still rev past 6,5k though. Mysterious case.

It is an original Final Drive. But has nothing to do with it. It's "only" a severe lack of power on high rpm.

 

[n00bzi11a]

I'm suspecting it could be the CDI since there were some significant changes made in the carbs and none of them affected the behaviour.

Changing the cables also did nothing and running with or without the air filter also made no difference.

So, if it has spark, fuel and air, and none of them seem to solve the issue, it must be something electrical, and maybe related to my lack of oil level light and fuel light.

Speaking of that, here is a video of the behaviour reported before:
Note that if I start the engine, neither of the oil or fuel light come on.

 

[Jack Hammer]

That is a short somewhere. Could be a self weld bulb or something really weird.

One time, roughly 5 years ago, a local Chevrolet dealer contacted me about a brand new Silverado they had in stock that nobody was able to get started. It took me 2 weeks and the reason was a perfect wire in a harness all covered up OEM and a spot you'd never expect in a million years. They had a huge service department.

Touch that part of the wiring while someone cranking and the glorious thing storms to life. I called the store general manager (who assigned me) and asked him "Can you quess which car is running in the background?"

 

[one2dmax]

Without re-reading the entire thread. Did you rebuild the carbs and if so did you replace the emulsion tubes (brass sleeve part that the main connects to). There are some aftermarket stuff made wrong with the hole in the side misplaced which blocks proper fuel flow. Especially the Keyster parts.

 

[n00bzi11a]

Without re-reading the entire thread. Did you rebuild the carbs and if so did you replace the emulsion tubes (brass sleeve part that the main connects to). There are some aftermarket stuff made wrong with the hole in the side misplaced which blocks proper fuel flow. Especially the Keyster parts.

Hello.

I've just had it cleaned and replaced all the soft parts. Every jet hat the correct size and the main jet I recall having the original mikuni markings in them.
Because of it, I'm assuming that the emusion tubes are stock, but i could be wrong and if they're the cuplprit, they're still there!