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Top Speed?

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Top Speed?

PostAuthor: Flaekingr » Fri Aug 09, 2019 9:25 am

So I bought a 95 xv750 last week and I finally had a chance to give it some proper welly tonight on a long, straight, traffic-free road. And, to be honest, was a little disappointed when it topped out at 5k and about 130kph. I had it faster than that on the freeway going to work just cruising along. What do you rev out to before changing up? What revs does your top gear rev out at? How bloody fast can this thing go in a straight line on flat bitumen?
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Re: Top Speed?

PostAuthor: Flyingdog » Fri Aug 09, 2019 2:56 pm

Flaekingr wrote: I had it faster than that on the freeway going to work just cruising along.


Not quite sure what you mean going going faster cruise'n than what the bike tops out at, but at any rate... yep..80mph (top speed) is pretty slow for 750. Should do "at very least" 100mph, give or take. Air/Fuel/Spark is what makes it go. Listing out "all" the possibilities why you can't go beyond 80 would take some checking things out on your part. Specially since you suggested all you did was try to wring out some speed. Good, unrestricted air intake, good spark (coils, plugs, connections, wires), good fuel flow.. both from the tank (how's the filter) "and" inside the carbs. How did the bike react at this top speed? Cough'n, sneeze'n, chugg'n, variations in rpms at any point before WOT and when ya got the throttle pinned? Does the throttle cable "fully" open the carbs? Choke "not" sticking? No clutch slipp'n? Can't give ya any particular rpm number for the 750, cuz I never look or pay any attention too rpm numbers. Unless I'm sync'n the carbs. I just shift when I "need" to. Just as a comparison, my 750 will top out round 110 - 115mph. But been looong time up there. But the ambient temp, wind, "mechanical tune", tires, altitude, etc etc etc probly lot diff than you. Guess, at this point, just gotta start checking things out and report any odd-ball findings. Hopefully you'll get the problem pinned down with the help around here. Got a manual? They'll have the numbers for various checks.
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Re: Top Speed?

PostAuthor: Flaekingr » Sat Aug 10, 2019 12:00 am

What I mean is exactly that, cruising the freeway, going up and down hills and whatnot, the bike went beyond 5.5k revs and didn't feel like it was working hard at all really. I've since done a bit of investigation and the rear cylinder isn't firing. I've swapped the plugs to see if that makes the difference, it doesn't. The bike is new to me, I picked it up last week. Riding it home, the front cylinder stopped firing and the power bogged right down, I limped the rest of the way home at about 60kph. Last weekend I had the carbs off and cleaned, they were actually in pretty good shape, the jets certainly needed clearing but it wasn't filthy. I got them back on and put some fresh fuel in the tank and it fired right back up and I've been doing my 60km commute on it this week. It sputtered a bit on the first ride but hasn't missed a beat since. My thinking is that that sputtering was the rear cylinder ceasing to fire. I just got back from a ride, it sputtered again and bogged down when it went to reserve and ran crappy on reserve until I got it fueled up whereupon it ran beautifully again. I had the petcock out and blasted it with air last weekend. I haven't yet changed the fuel filter, but I'm thinking if that was the issue then both cylinders should be running sh!t? The inside of the tank isn't great, I'm sure it could use a clean out. To answer other questions, there was no adverse reaction at "top" speed last night, no chugging or variations in rpm or anything like that, it just felt like it had reached its max speed. So yeah, any points in the right direction would be very much appreciated. I've got a virtual manual and will be going through it and I'll be replacing the spark plugs as well.
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Re: Top Speed?

PostAuthor: Flyingdog » Sat Aug 10, 2019 4:12 am

You might wanna remove the plug caps and snip off bout 1/4" of wire, so the caps have a fresh bite to the wire's core. Don't know if your caps are still the rebuild-able type..meaning take a resistance reading of "just the caps". And, of coarse take readings of the whole coil-plug set up. Primary, secondary wiring and the connections to coils. You could possibly have some corrosion going on "inside" the caps. Or the little pill/type resistor (inside cap) is breaking down. Replace with like-sized 16# nail or something conductible.
One other thing you mentioned bout how crappy it ran going to reserve. That sounds like the petcock filter is messed up. There are two filters in there. A tall and short one. Both have to be clean and free flow'n. Specially the shorter filter..that's what feeds your reserve.
Last edited by Flyingdog on Sat Aug 10, 2019 4:19 am, edited 1 time in total.
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Re: Top Speed?

PostAuthor: Flaekingr » Sat Aug 10, 2019 4:18 am

That's interesting. I'll add a multimetre to my shopping list and get on it... and also check out the cost of replacement coils and leads. So it sounds like you don't reckon it's a carb issue? I forgot to mention that I checked the spark plug after my ride and it was dry.

And to respond about the petcock, I gave the main and reserve tubes a good blast of air. I'll have to have it off again and check it out.
Last edited by Flaekingr on Sat Aug 10, 2019 4:26 am, edited 1 time in total.
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Re: Top Speed?

PostAuthor: Flyingdog » Sat Aug 10, 2019 4:25 am

Flaekingr wrote:So it sounds like you don't reckon it's a carb issue?


Whelp..that's a toughy..can't rule it out. But ya gots to pick one way or another to eliminated all possibilities. Even then, you usually have to go back and re-check things. Not always a joy, but that's the price of owning a bike and DIY. :bg: As I'm sure you know..many a times it can be the simplest of things that can throw ya off..like petcock diaphragms. Why shops get the big bucks.
An engine that's firing correctly..the plugs "will not" be wet. Just a nice (even), light/tan color and "dry".
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Re: Top Speed?

PostAuthor: Jake » Sat Aug 10, 2019 8:11 am

Flaekingr wrote:I had the petcock out and blasted it with air last weekend. I haven't yet changed the fuel filter.


This has me a little puzzled.

You had the petcock out ? The fuel filter is on the petcock and is not replaceable, or at least there isn’t and never was a replacement available.

If there isn’t a filter on the petcock, then your reserve will not function, not if the filter was removed. If it was removed and an inline filter was used in its stead, then fuel line routing may be an issue.
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Re: Top Speed?

PostAuthor: Flaekingr » Sat Aug 10, 2019 9:00 am

I meant the fuel filter between the petcock and the carb, not the filter on the actual petcock.
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Re: Top Speed?

PostAuthor: Jake » Sat Aug 10, 2019 9:20 am

It wasn’t designed for an external filter. Depending upon how the fuel line is routed, it can restrict your flow, especially when the fuel is low. Good gravity flow needs to be as direct as possible.

As you can see from the diagram, there isn’t an inline filter, there wasn’t any room allowed for it, otherwise Yamaha wouldn’t have placed the filter on the petcock.

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_______________________________________________________________________
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Re: Top Speed?

PostAuthor: Flyingdog » Sat Aug 10, 2019 9:23 am

Nother thing you might wanna check valve clearance. It would be to a lesser degree, as I've never heard of these bikes getting any type of valve float action..but anything is possible, once the obvious is eliminated.
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Re: Top Speed?

PostAuthor: Flaekingr » Sat Aug 10, 2019 9:44 am

Now that is interesting. It never occurred to me that an inline filter would be superfluous. I'll post a photo of the petcock.
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Re: Top Speed?

PostAuthor: Flaekingr » Sat Aug 10, 2019 10:03 am

As you can see, I've got the outlet for a vacuum hose but no hose on it.
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Re: Top Speed?

PostAuthor: Flyingdog » Sat Aug 10, 2019 11:08 am

Ish..that petcock doesn't look in the best shape. Even leak'n? With that much rust on the mounting screws, hate to see what the inside of the tank looks like. Might just have to bite the bullet (if you don't rebuild) and just get a new one and lose the extra filters. Those type of elements, while ok, are not as porous as the petcocks screens. When they gunk up, everything stops (or slows to a trickle)..and that first pic sure looks like the fuel is struggl'n to get thru.

https://www.mrcycles.com/oemparts/a/yam ... /fuel-tank

Edit: Ah..the ole PO switch-a-roo..so much fun. :;:
Last edited by Flyingdog on Sat Aug 10, 2019 11:17 am, edited 2 times in total.
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Re: Top Speed?

PostAuthor: Jake » Sat Aug 10, 2019 11:13 am

Flaekingr wrote:As you can see, I've got the outlet for a vacuum hose but no hose on it.


That is a manual petcock, someone switched it for the vacuum petcock, no vacuum line needed. I did my switch to manual back in 2008, before anyone knew what could be used.

The vacuum petcock has ON, RES and PRI. No "off"

EDIT: I noticed you have the petcock in the "ON" position. That's a bad idea if the bike isn't running, you are dependent on the carb floats to stop the flow of fuel. You could potentially flood the crankcase with fuel causing hydro lock.

Check your oil level now ! An overfilled crank case could cause running issues, speaking strictly from experience.
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Re: Top Speed?

PostAuthor: Jake » Sat Aug 10, 2019 11:22 am

Flyingdog wrote:Ish..that petcock doesn't look in the best shape. Even leak'n? Might just have to bite the bullet (if you don't rebuild) and just get a new one and lose the extra filters.


A new manual petcock can be bought for about the same price as a rebuild kit for a vacuum petcock, in which case the rebuild kit would be pointless, since it is a manual petcock.

You could replace the o-ring behind the face plate, that is the reason it's leaking or just buy a new good quality one from Yamaha. Manual Petcocks for the Virago

My first one was a good quality pitbike petcock, and they can be bought for as little as $8 USD. A few people here bought the really cheap ones and they had trouble with the face plate leaking. Mine did too eventually and I replaced it about a year ago, with a new one for an XVS650.
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Re: Top Speed?

PostAuthor: Flyingdog » Sat Aug 10, 2019 11:50 am

Thanks Jake..forgot to add "or an equivalent", got distracted by a shiny object.
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Re: Top Speed?

PostAuthor: Flaekingr » Mon Aug 12, 2019 4:02 am

Aw man, I wrote a reply to this last night but it's been lost somehow. Ah well.

To paraphrase, cheers everyone for the replies, I really appreciate them. I've changed the oil and filter, bought a new o-ring for the petcock and am thinking seriously about replacing the tank. I've also put in new plugs, the PO had the wrong bloody spark plugs in, BPR6ES instead of BPR7ES, the spark is stronger but still no joy for the rear cylinder. I picked up a multimeter today and will report back on my findings.
I'm debating whether to pull the carbs out again and go over them, even though they were done last sunday and were actually pretty clean apart from the pilot jet for the front carb. So yeah, things to check. I was wondering, is it hurting the engine to only run on a single cylinder?
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Re: Top Speed?

PostAuthor: Flyingdog » Mon Aug 12, 2019 4:22 am

Flaekingr wrote: wrong bloody spark plugs in, BPR6ES instead of BPR7ES, the spark is stronger but still no joy for the rear cylinder.


In My Opinion..I wouldn't consider the 6es as a wrong plug. It's just a step hotter. Fine for bee-bopp'n around. In fact, throughout this sight, you see many many reference to Autolight 63 or AP63 (platinum). Well that's the same as the 6es. JMO-again..going a step hotter or platinum plug than OEM specs is really no biggie, it's just compensating for something else atad outta wack. One of those 6 of one/1/2 dozen other things. :bg:
Flaekingr wrote: is it hurting the engine to only run on a single cylinder?

Not really, bout the only thing going on is putting a strain one the non-firing cylinder's coil. But not as much if ya just left the plug cap and/or plug dangl'n without being ground as the bike is firing. (if that made sense?) But they're pretty robust things.

If the reason for non firing cylinder "is" electrical and the carb "is" working..could be loading up the head and washing the cylinder walls with unburnt fuel.
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Re: Top Speed?

PostAuthor: Jake » Mon Aug 12, 2019 9:00 am

The stock spark plug is a BP7ES ( non-resistor ).

BP6ES ( non-resistor ) is fine I would refrain from using the BPR6ES ( resistor ) or as you currently have the BPR7ES ( resistor ). You are effectively using two resistors, the plug cap has a resistor and the plug designation “BPR” is a resistor spark plug.

I use the Autolite AP63 ( also a resistor plug ) which Flyingdog mentioned, the equivalent of the BP6ES, but I removed the resistor in the spark plug cap. Much easier starting in my case.
_______________________________________________________________________
“It ain’t what you don’t know that gets you in trouble.
It’s what you know for sure that just ain’t so.”

~Mark Twain


There is an old Chinese proverb - "Man with one watch knows the time. Man with two, not sure."
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Re: Top Speed?

PostAuthor: Flaekingr » Tue Aug 13, 2019 3:26 am

That's interesting. I'll have to pick up the non-resistor type tomorrow and see how it goes. Cheers.
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