Rejet for exhaust? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 05-12-2012, 01:00 AM Thread Starter
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Rejet for exhaust?

So I got this 1995 vn750 about a month ago, and am going through some stuff to get it running right. The rear cylinder doesn't fire well at idle but runs like a banshee once its over 2500rpm.
I replaced the champion plugs with iridiums, new wires and did the coil relay mod, seafoamed and used gumout on the carbs to fix a sticky float. Changed the oil after.
I've put about 30 miles on the plugs and pulled them tonight. The front ones are "normal" with tan deposits on the tip. The back ones are black and smell of gas, not oil.
It has on some very short "racing" mufflers the PO put on it. I got the stock mufflers in the deal also though. I've heard some people warn about changing the exhaust and its effects. My question is could these little mufflers be causing my problem? The rear cylinder has the shorter exhaust so theoretically would be effected more by a muffler swap. Increased back pressure at higher RPM would allow the cylinder to function normally?
Do people have to rejet when changing the exhaust? If so I'm betting the PO didn't. I'm wondering if putting the stock mufflers back on might fix my problem, I would rather do that then pull the carbs.

4/12/12 1st bike ever! 95 VN 750 13k
New plug wires
Iridiums
Coil relay mod
MCCT mod
Marbled
EPA stuff removed
Rear signals shaved
Shinko 170-110 oversized tourmaster tires
Splines lubed
Front signals relocated.
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post #2 of 12 (permalink) Old 05-12-2012, 01:21 AM
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Changing mufflers will make no difference in your case. You need to clean your carbs! If you keep the goat/cat, any or no muffler after it will not change much if any carb sync wise. I hate carbs personaly, but are very easy to diagnose. Idle to 25%, 25% to 50%, 50-75% and 75-100% throttle problems are quickly pin pointed. I have yet to see any of those posted here but are common on every carb! Look it up. This could also be an electrical problem for you. Make sure you are getting spark, if you do then focus on the carbs.
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post #3 of 12 (permalink) Old 05-12-2012, 12:21 PM
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Sounds like your pilot jet is clogged in the rear carb. Worst case is opening the carbs up and having to use something to clean out the jet. Best case is some more seafoam.

1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
TOC MCCTs
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Tuxedo Mod
Rebuilt Forks w/ Progressive Springs
V&H Cruzers
VN750.com Grill Cover
Meanstreak Seat
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R


1990 Kawasaki Vulcan 750

1998 Honda VFR800 FI

2014 Honda VFR800F

1989 Pontiac Firebird Formula 350
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post #4 of 12 (permalink) Old 05-13-2012, 02:47 AM Thread Starter
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Well I decided to keep the short mufflers on for now. I drained the carb, took out the pilot screw and the diaphragm cover, removed the diaphragm. I shot gumout into everything I could. It seems to have done the trick, or at least helped for now. I'm trying to put off pulling the carbs until fall if I can.

4/12/12 1st bike ever! 95 VN 750 13k
New plug wires
Iridiums
Coil relay mod
MCCT mod
Marbled
EPA stuff removed
Rear signals shaved
Shinko 170-110 oversized tourmaster tires
Splines lubed
Front signals relocated.
skyboxer is offline  
post #5 of 12 (permalink) Old 05-13-2012, 02:54 AM
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Quote:
Originally Posted by skyboxer View Post
Well I decided to keep the short mufflers on for now. I drained the carb, took out the pilot screw and the diaphragm cover, removed the diaphragm. I shot gumout into everything I could. It seems to have done the trick, or at least helped for now. I'm trying to put off pulling the carbs until fall if I can.
Good news. My suggestion is to keep running some SeaFoam in your gas to prevent it gumming up in the future.

1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
TOC MCCTs
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Tuxedo Mod
Rebuilt Forks w/ Progressive Springs
V&H Cruzers
VN750.com Grill Cover
Meanstreak Seat
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R


1990 Kawasaki Vulcan 750

1998 Honda VFR800 FI

2014 Honda VFR800F

1989 Pontiac Firebird Formula 350
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post #6 of 12 (permalink) Old 05-13-2012, 07:08 PM Thread Starter
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Indeed. I've noticed that now both cylinders are firing, it runs a little hot like barely in the red. When I turn off the bike I hear it bubling into the reservoir. The fan switch doesn't work so I've been grounding it out myself for now, but running the fan doesn't seem to cool it down much. I have yet to drain and change the coolant, but I'll try some flush when I do. I'll likely just remove the thermostat too. It seems like one thing is leading to another, but I'm having a blast working on it so far. I've got to get it cool though. The entire rider's skills test is low speed maneuvering, and I need to practice to meet my goal of being licensed by the end of summer.

4/12/12 1st bike ever! 95 VN 750 13k
New plug wires
Iridiums
Coil relay mod
MCCT mod
Marbled
EPA stuff removed
Rear signals shaved
Shinko 170-110 oversized tourmaster tires
Splines lubed
Front signals relocated.
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post #7 of 12 (permalink) Old 05-13-2012, 07:23 PM
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Removing the thermostat can actually have the reverse effects on cooling, as coolant flows thru the radiator to quickly, fluid leaving rad to fast means hotter coolant going back in motor. Ck your thermo, you can do this in a pan of water on a stove, if it is working you can actually see it move or open. Removing a stuck closed thermo might get you home! Have actually done that on a few older cars.

04-3,200 miles bought July 2011 with 928 miles. Loaded with acessories to many to list here, see my albums for more details! I also make a custom tank bib specific for the 750 if anyone is interested, ck my albums for some pics.
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post #8 of 12 (permalink) Old 05-13-2012, 07:28 PM
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Quote:
it runs a little hot like barely in the red. When I turn off the bike I hear it bubling into the reservoir
Bleed the air from the system...I guarantee. if you get air again, then a coolant hose may have a slight leak somewhere.

As for pipes and rejetting, it all depends on "what pipes". If you decrease backpressure too much from stock, you will need to rejet (details on the forum here). If you keep close to stock backpressure, no rejetting needed. I removed the goats belly (had to, looked like swiss cheese) and added stock, baffled Sportster muffs. IMO quieter than stock, but no rejetting. To have both ends of the muffs match though, I had to drill 1 3/8 hole in the shorter (rear) run. This balanced the backpressure as the belly did before. Also, adding an "H" pipe before the muffs helps balance as well. I just dont like the new fangled crossover pipes, IMO thats just pisspoor exhaust engineering and an attempt to hide it. In effect, the total lengths of both pipes need to be equal. Even if drag pipes. If wanting to balance an unbaffled system and make the ends equal, then a homemade baffle needs to be installed in the one with the least backpressure.



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post #9 of 12 (permalink) Old 05-19-2012, 12:46 AM Thread Starter
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Well I drained the coolant out and ran some flush through it, then filled it with distilled water and dexcool. The temperature stays good and the fan switch works now! It starts to miss after riding for a bit. I thought it might have been a heat issue but now the heat is controlled. I'll check the vacuum lines first but it looks like I'll have to pull the carbs to get to the root of that problem.

4/12/12 1st bike ever! 95 VN 750 13k
New plug wires
Iridiums
Coil relay mod
MCCT mod
Marbled
EPA stuff removed
Rear signals shaved
Shinko 170-110 oversized tourmaster tires
Splines lubed
Front signals relocated.
skyboxer is offline  
post #10 of 12 (permalink) Old 05-21-2012, 05:15 PM
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Quote:
It starts to miss after riding for a bit. I thought it might have been a heat issue but now the heat is controlled.
"Miss" covers a wide range of probs. Generally a "miss" refers to electrical. If you REALLY mean popping and backfiring, then check for leaks in the entire exhaust system (closest to head most important), then check the air screw settings on the carb. Backing them off (counterclockwise) 1/2 turn will often solve some of the popping/missing. An earshave will work wonders on that also.
What can cause what you describe too, is loose camchains, bad plugs, defective pickup coil wires, et al. But try the easy fixes I suggested first.



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