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Discussion Starter #1
2003 VN750
In first or second gear at low rpm, or when backing off the throttle I experience what feels like and sounds like engine misfire. There is no loss of power or stalling. Can't tell if it happens at higher speeds/rpm, too noisy.
My mechanic replaced the spark plugs and caps and vacuum tested the carbs. tells me the carbs are balanced and voltages are good.
After I got it back, it still does it, but not as much.
What else could cause the misfire experience (if that's what it is)?
I run high test only and each refill gets Sentry gas treatment.
 

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Daily Rider
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Try running regular 87 octane, the hi test doesn't improve performance. Some one that knows more will tell you the same thing I'm sure. My 2000 always runs good on the 87.
 

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Super Moderator
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Part of your problem is likely due to running high test gas and sentry gas treatment.

These bikes need to run on Regular gas.

2nd...are you taking about a "popping" from the motor when you close the throttle??

If so.... This is normal. Do a search for "marbling"...

If the motor is in fact "missing" do you smell gas? Does it backfire loudly? If not, it's likely not missing...but the aforementioned de-accell popping due to the reed valve system.
 

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Captive New Yorker....
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Just a note here the popping on decel also occurs on bikes with out the emissions stuff on board, my 86 UK model does the same and has never had emissions stuff fitted at all.
 

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Sparky!!!
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Just a note here the popping on del also occurs on bikes with out the emissions stuff on board, my 86 UK model does the same and has never had emissions stuff fitted at all.
but not as bad.. i took my emission stuff off, coasted and still have pops, but not as loud.
 

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but not as bad.. i took my emission stuff off, coasted and still have pops, but not as loud.
Yeah I guess it does quite down some as a lot of you guys or most of you have coastered your bikes, but as you say it still pops and if you are knew to the bike you could think something is wrong when there isn't :)
 

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Kufbass
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Here is a thought,as it happen to me last night. I have alot of lights on my bike that I am going to replace with led lights. Check your battery and your charging of the battery. I know last night I ran out of charge on my battery. I run two fog lights that run at 55watts a piece. Luckily I was going down hill when I was giving it throttle,but was dogging. I was also coasting,so was not really charging battery. I turn off the fog lights and then took a little bike the bike started to run fine. The only thing I think is running these lights drain the battery. The spark plugs were getting no power to them. I could be wrong, but that is the way it seemed to me. I found 3 watt replacements for the 55 watts. I also ordered the 6 watt replacement for the 55 watt headlight that is on there now.
 

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Linkmeister Supreme
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Hey Sam, my `91 vn, made for sale in Canada, never had any of the emmission equiptment on it.
Canadian bikes came with the coaster plates installed instead of the fresh air reed valves, but mine still pops and backfires when declerating too.

Many members here have cut the popping way back by pulling the cover or seal off the top of the fuel/air mixture screws and turning them further out, to enrichen the mixture.
I believe the factory setting is about 1 5/8 turns out from closed.
The usual recommended setting to cut back on deceleration popping it between 2 1/2 to 3 1/2 turns out.
Put a little dab of silicone on the threads to keep the screw in place when done adjusting the screw.
 

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Here is a thought,as it happen to me last night. I have alot of lights on my bike that I am going to replace with led lights. Check your battery and your charging of the battery. I know last night I ran out of charge on my battery. I run two fog lights that run at 55watts a piece. Luckily I was going down hill when I was giving it throttle,but was dogging. I was also coasting,so was not really charging battery. I turn off the fog lights and then took a little bike the bike started to run fine. The only thing I think is running these lights drain the battery. The spark plugs were getting no power to them. I could be wrong, but that is the way it seemed to me. I found 3 watt replacements for the 55 watts. I also ordered the 6 watt replacement for the 55 watt headlight that is on there now.
110 watts is way too much draw for the bike. I don't think the posters problem is electrical ....although yours certainly is. Surprised the bike was rideable... The Vulcan has perhaps 70 watts of available power, and that's at 5000-6000 rpms. Two 55 watt lights will kill your battery. You say a 6 watt replacement? LEDs I assume. I just used 20w halogens.

Even without the "emissions" crap as you call it,the bike can still "pop" when you close the throttle while breaking. This is somewhat due to lean mixture, and response if the CV carbs.... And how "open" the throttle was before you closed it.

For the record, it's not really hurting anything and some folks like me....actually like it.
 

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It could but on these bikes they do it from new so must be part and parcel of owning and loving the Vulcan750 :smiley_th
Good article by the way.
 

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Discussion Starter #13
Part of your problem is likely due to running high test gas and sentry gas treatment.

These bikes need to run on Regular gas.

2nd...are you taking about a "popping" from the motor when you close the throttle??

If so.... This is normal. Do a search for "marbling"...

If the motor is in fact "missing" do you smell gas? Does it backfire loudly? If not, it's likely not missing...but the aforementioned de-accell popping due to the reed valve system.
good clarification. there is no backfire, popping is a more accurate description. it pops on closing the throttle as well as steady throttle at lower speeds. my mechanic swears it needs high test and sentry. When I run out on this tank, I will try regular.
 

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I wouldn't think that you would need to add Sentry in every tank full of fuel. It's just another gas additive/cleaner like Seafoam and others. Seafoam is good, but so are others, MMO, Lucus, and such. I use them just to absorb any condensation that might happen in the tank an do it every 6-7 fill ups. But then the gas doesn't stay in the tank long for me, I ride her everywhere. Had to fill up twice yesterday and the day before, car hunting for my baby girl and put 400+ miles on the bike the past two days. Normally I get gas twice a week, then fill up and ride on the weekend. I've put 20k on her in the past 19 months or so. If I'm not riding, it must be raining....
 

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good clarification. there is no backfire, popping is a more accurate description. it pops on closing the throttle as well as steady throttle at lower speeds. my mechanic swears it needs high test and sentry. When I run out on this tank, I will try regular.
You need to stop your mechanic swearing, these bikes run on low 87 octane fuel.
 

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Hi Sam47, I had a similar problem with my 95. I completely replaced my plug wires, not just the caps when I notice arching in the front left, and the right rear plug holes while the bike was running. It's a pain to do, you have to remove the tank, and drop the battery box to get at the coils, but mine purrs like a kitten now. I also went with ngk spark plug wires. I do have some concerns with them, they are not as well insulated as stock wires but I was told would perform better.
 

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Try the 87 fuel for a few tanks and see how she runs. Won't hurt it, it's designed for 87. Now if you have massive carbon build up, then that could increase the compression and require a higher octane fuel. But if the Sentry preforms as they state, carbon shouldn't be a problem. If you have no problems with the 87, and you shouldn't, just don't tell your mechanic so not to hurt his feelings.


Had an '86 Caravan that didn't like EXXON 87 gas in Texas, pinged like crazy, but all the other station's 87... Well it ran just fine. Go figure...
So give it a try and see how it goes.

Just saying.....
 

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With race cars, we always considered over 9.5 to 1 as high comp. The Vulcans are almost 10.5 to 1. We always ran a higher octane in the higher compression engines when racing (my Chevy Nova had 11.5 to 1). Sometimes even ran aviation fuel which was sometimes 110 octane.
I tried a cupla tanks of 93 in my '85 700. Ran better IMO (more power, better gas milage). But I have all of my finely engineered emissions crep removed and changed the finely tuned exhaust.
Willing to bet that a totally stock VN would in fact run better on Regular. But IMO and from personal tests, once modded ("de-engineered" ?) runs better on the 93 or 98 thats around. I mostly use Sunoco when possible.
Also, the lower the elevation your at, the less octane is required for proper operation. Thats why Rocky Mountain states usually have the 98 octane readily available ( I read that at the Petroleum Engineers Institute website I think).
 

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Actually youre correct CW...if you run say, a 98 octane gas crossing the Great divide. Your engine wont run well (less compression too) and will blow some black smoke. BUT...with a high compression engine like the VN's, once you get down to the flats between the Western slope and southern Cali (say, a trip from Boulder to LA), 85 octane can cause severe detonation in a high comp engine at low altitudes. Now...the time spent at the higher alts, is much less than the time spent across the flats. What one could do I suppose, is to fill with an 87 on the east side, then refil with higher (say 93 ?) for the flats. I'd much rather prefer to continue with the higher octane the whole trip to be safe. A lower octane is better if constantly running higher (say ya lived in Denver).
 
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