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Big Dumb Viking
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Discussion Starter #1
Earlier today I was out riding, and I decided to see how my '89 VN750 ran at high (6000-ish) RPM's.

I noticed that as I approached this, the bike seemed to falter a bit and lose power. Problem went away as I switched to next gear.

Is this normal? If not, what would be the issue, most likely?
 

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Linkmeister Supreme
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Need bigger main jets??
 

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I know that my bike is EXTREMELY sluggish when I'm running on one cylinder. This normally happens because one of my jets is clogged up and after a few ounces of Seafoam and 30 or 40 miles in the 6-8RPM zone I'm able to blow the **** out.
 

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Heading to the DARKSIDE!
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177 Posts
Mine used to not be able to get to redline.......All I had to do was turn the air/fuel mix screws out 3.5 turns from seated......and that fixed the problem for my bike......it used to get starved for fuel at 7000 rpm.
 

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Big Dumb Viking
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597 Posts
Discussion Starter #6
Mine used to not be able to get to redline.......All I had to do was turn the air/fuel mix screws out 3.5 turns from seated......and that fixed the problem for my bike......it used to get starved for fuel at 7000 rpm.
That just might be it.

Because it runs perfectly the rest of the time.

So...where are those things? :) I'm a complete idjit when it comes to mechanic stuff.
 

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Premium Member
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120 Posts
Earlier today I was out riding, and I decided to see how my '89 VN750 ran at high (6000-ish) RPM's.

I noticed that as I approached this, the bike seemed to falter a bit and lose power. Problem went away as I switched to next gear.

Is this normal? If not, what would be the issue, most likely?
No, this is not normal, your bike should pull steady all the way to redline.

How did you approach 6k rpm - WFO or just sneaking up on it slowly?

Done any mods to your bike from stock?

As to the pilot screw...let me suggest that one turn of the pilot screw has about enough effect on your system at 6k rpm as removing 3 lbs of weight from your bike makes it faster. Technically, yes it's true, but it's so inconsequential that you'd never notice it - and neither will your engine at that speed.

The pilot screw is part of the pilot circuit - it has little to no effect on your engine after about 1/8th throttle.

If at a steady state cruise, your bike is hiccuping or missing a little, it could probably use the needles being raised by a washer or two.

If at WFO it misses noticably in the upper rpms, you likely need a bigger main jet.

If you haven't modified your bike at all and it's doing this, I'd check the condition of your air filters to make sure they aren't deteriorating and letting little bits of foam get sucked into your carbs and plugging up the air jet passages at the mouth of the carb inlet...and then I'd pull the ducts coming from your airbox and inspect those three little passages at the bottom of the carb mouth to visually verify that they're clean & clear, and maybe shoot some carb cleaner down each one for good measure.

Hope this gives you a little more direction as well as explanation.
 

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Big Dumb Viking
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597 Posts
Discussion Starter #8
How did you approach 6k rpm - WFO or just sneaking up on it slowly?

Done any mods to your bike from stock?
If I am understanding what "WFO" is, then indeed, that was what I was trying.

The only mods were done by Ron (Lance) where he coastered it, and relocated the R/R. A previous owner removed the goat's belly...dunno when though.

Ron put in new air filters a year and a half ago...so I doubt that's it.

Might just be the main jet you refer to.

Thanks!
 

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Heading to the DARKSIDE!
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Well.......all I can say is that after I rebuilt my carbs with 40 / 140 jets....I had the air/fuel mix screws turned out 2 turns from seated. I took it out for a test drive. I could not get it to rev in any gear past 6500 - 7000 rpms.......and the backfiring on decel was horrible. I went back to the garage and turned both air/fuel mix screws out 3.5 turns from seated......all backfiring completely stopped and I could then easily pull in any gear to 9500 rpm! That WAS the ONLY change I made.
 

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Old Truck Junkie
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4,133 Posts
I had those same jets, but I had 2 shims under each needle. As per Ron.
You can put the shims in just by removing the fuel tank. You are shaven right?
 

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Premium Member
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If I am understanding what "WFO" is, then indeed, that was what I was trying.

The only mods were done by Ron (Lance) where he coastered it, and relocated the R/R. A previous owner removed the goat's belly...dunno when though.

Ron put in new air filters a year and a half ago...so I doubt that's it.

Might just be the main jet you refer to.

Thanks!
Sorry, indeed WFO = wide f***ing open...

I noticed a little missing in mine around the 6k mark last week and found the old filters turning to dust. When I pulled the carbs for the shave last week I found about 1cc of foam particulates floating around in the carb in one place or another. Granted it still pulled to redline just fine once it was warmed up, but that doesn't sound like that's the issue for you. To be on the safer(er) side, I'd pull an air filter cover and check them out - 3 screws & takes less than a minute...

There is certainly and wide spread acceptance that exhaust mods require not jetting change (i.e. removing the goat.) I will not disagree with this accepted theory as I've not made any exhaust changes to the VN personally. I will say that this is the first carburetted bike for which I have ever heard such a theory so widely propagated.

However, the generally accepted practice amongst most bikes is two jet sizes for pipes, two jet sizes for intake (pods), & 4 jet sizes for jets and pipes. Point in case, I just did the ear shave to a single K&N and went from 132 mains to 137.5 mains for a 5.5 point bump or...2 jet sizes. Granted those going to straight pods are going up 3 or 4 jet sizes, so this practice may not be holistically true for these bikes, but it's a good rule of thumb.

I checked out your website - you may be an idjit when it comes to mechanical stuff, but you're no idiot, and the carburetor operation isn't that complicated. It may be a little hard to appreciate if you haven't ripped one apart personally, but it's actually pretty simple. That said, not complicated does not equate to not delicate - there is a difference...

While not a direct match for the Vulcan, and I'm sure there are other better explanations, this page does a pretty good job of going through the same basic functions of what our carbs also do. Again, note that the pilot screw setting has no substantial bearing on how the engine is tuned during full throttle operation...

Since this appears to be the first-ish time you've ventured into your upper rpm range and nothing else seems to be out of the ordinary, I'd order a couple pairs of jets 135s & 137.5 (they come in 2.5 increments) and try them out. It only takes about 15 minutes to swap out main jets. The rear carb can be a little difficult for the front screw because it's wedged against the throttle linkage, especially when trying to get that dang screw started again, but you don't have to remove it completely to get the needle jet out.

All of that said, I'd WHOLEHEARTEDLY recommend the ear shave so that yanking the carbs wasn't a suicide inducing proposition and you could regularly inspect things unimpeded. If you replace the top cover screws with allen head cap screws when you have them out the one time, you can actually access the top half of the carbs without removing them too!

In any case, that's my two cents... Let us know where you start, what you try, and what the results are and we'll gladly talk you through. :smiley_th

If you want to chase the pilot screw option, they (and likely the aluminum plugs blocking access to them...) are located here and here.
 

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Big Dumb Viking
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Discussion Starter #12
Haven't done the ear shave yet. Ron was going to do that, and even ordered the major parts for it, just before he passed. I have the parts...just need the time and the gumption to get to it. :)

I appreciate the info.
 

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Holy thread revival!!! But I didn't want to create a brand new thread for what seems to be the same issue that I am experiencing.

I have had my bike up to about 90 or so before I decided to slow down (read: chickened out). But I have always gotten up to that speed by slowly easing the throttle back. The other day I was feeling ballsy and at 60mph gave it a significant twist of the throttle..and the bike just bogged down. If I let off the throttle and then ease it back, I can get back up to 85-90 no problem. I tried this in other gears as well and when I get over 6000 rpms it does the same thing.

The first thing I checked was for POOGS. I was below half a tank when it happened at first. So I came home and cleaned out the vents and all on the gas cap. Went out again and the same thing was happening. Filled it up with gas and added some SeaFoam this last time. I've gone about 20 miles so far and haven't tried it again. So not sure if that will help anything.

Asbrand, how did you end up fixing this issue on your bike? Did you move up to bigger sized jets?
 

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I was feeling ballsy and at 60mph gave it a significant twist of the throttle..and the bike just bogged down. If I let off the throttle and then ease it back, I can get back up to 85-90 no problem. I tried this in other gears as well and when I get over 6000 rpms it does the same thing.
...truly does sound like fuel starvation for one reason or another...Im sure you checked the tank filter....if your air screws are open (not covered) try opening each up 1/4 turn at a time...this will richen your mix (keep track so you can return). Also, have you ever done a full carb cleaning ?....
 

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I rode it a ways last night after putting in the seafoam and then rode in to work today. It seems better, but I didn't get to really get on it. The carbs were rebuilt by the dealer (where I bought it) about 1 month ago (1100 miles ago). I haven't looked at the air screws yet. And when you say tank filter, are you talking about the screen on the fuel tap (petcock)? I haven't checked that either. But I certainly will be. Thanks for the tips Wolfie.
 

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And when you say tank filter, are you talking about the screen on the fuel tap (petcock)?
Exactly....an older bike, or even one thats been in the weather or was exposed to possible condensation can develop some rust in the tank....next time youre real low, pull the tank and petcock (easy) carefully removing petcock so as not to disturb the screen (so you can see any crud if there is)...throw in a half gallon of gas, and let it wash out...could be there might be a lil water in there as well....just a hint too...with gas in the tank, testing your higher RPM perf, switch the petcock to reserve early and see if that helps...if it does, its the petcock filter (worked on mine anyway)...few riders ever ride a tank down so low, that the reserve part is needed, so oft just the top part clogs and the bottom stays cleaner...doing that'll suck any water outta there too (will run like crap for a bit) and mebbe put in a half can of drygas as well...you can try all that before pulling the petcock....
 

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It is, but you got me thinking more about it. It seems that it was just hanging in the square opening and not pushed into one of the slotted holes. Based on another thread i was reading, it seems like it should be in one of those slotted holes right?
 

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romeobravo172
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842 Posts
The slotted hole close to your knee on top, also cut a 45 degree angle in the hose so as not to bottom it out in said hole, it has to compensate back and forth!
 
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