The VX800 FAQ: Engine
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5. Engine

5.1 Carburetion/Cold-bloodedness

The VX, like many motorcycles these days, comes from the factory with very lean mixture settings, to help reduce air pollution. While this is a great goal, the performance penalty is a little extreme in some cases, and leads to long warm-up times, bad off-idle performance, and poor cool-weather performance. Luckily it's usually pretty easy to fix the problem on the VX, either by adjusting the low-speed mixture screws, adjust the mixture needles, or replace the jets entirely.

By the way, most people seem to get 45-50 miles per (US) gallon.

Adjusting the mixture screws

Adjusting to richen the low-speed mixture, is a piece of cake -- neither tank nor seat needs to be removed. The only trouble is that the low-speed mixture screws are factory sealed (on US models) to prevent tampering, per EPA regulations. There is a little round brass-looking things on each of the two carbs which can be ("ULP!") drilled out very easily.

I used a 15/64 inch bit (which was the sharpest I had). The center of the small brass plugs has a little dimple just right for guiding the drill. Other folks have used a smaller bit: then you screw a sheet metal screw into the hole, and use it to pry the plug out.

What you will uncover the two itty-bitty pilot screws. I carefully counted turns as I screwed each one in: on my bike the rear was out 1-1/8th, and the front was out 1-1/4 turns. You will want to count your own so you can get back to where you started if things go awry.

The shop manual gives setups for every model of VX800 made -- about 10 of them. And it gives the factory backoffs for those screws for every model. Except the U.S. models. For those, it just says "PRESET". Because, after all, you're not supposed to drill the little brass plugs out. But I did note that on every model where they do give the settings, the rear carb is turned out more than the front, not less. Since it was the rear cylinder that seemed to be giving the most trouble (its pipe had yellowed, its exhaust pulse missed sometimes), I backed it out by 1/4 turn. Started the bike up, and voila!! It was warmed up and running off choke in about a minute, maybe two. This had never happened before, and it was quite a cool morning. I went for a nice ride, did some errands, and sure enough, I was able to leave the choke off for the entire rest of the day, even after letting it sit in a parking lot for a while.

Before making this change I often had to leave my choke on for my entire 7 mile commute. Afterward I could unchoke after a mile or two.

To really properly adjust the mixture screws one should use an exhaust gas analyzer (EGA). The traditional method of doing it without such a device, however is as follows:

You might want to go through it all again to be sure, but that's pretty much it.

Adjusting the carb needles

Possibly better results can be obtained for a little more work, by raising the needles that meter fuel into the carb, thereby allowing more in earlier: Andy writes:

"The needle is held in the slide by the small spring that fits around the needle pressing up on the needle circlip. The height of the needle is then limited by the nylon spacer (white plastic donut) above the circlip being pushed up against the hold down clamp (metal cover). Placing shims between the spring and the clip just pushes the needle into the clamp slightly harder. This does nothing to alter the carburation. No damage, and no improvement. Instead, I removed the nylon donut and replaced it with 2 #4 brass washers. This was about 1/2 the thickness of the nylon donut. Since the spring on the needle pushes it up into the clamp, this raised the needle about .050" which was my initial aim."

"There is a noticeable, not huge, difference in engine performance. Since there is very little taper in the needle, perhaps more can be gained by using only 1 #4 washer, but I've not tried this."

"When you do this, do one carb at a time. You don't have to remove the carbs from the bike. Be careful not to damage the rubber seal on the slide. Carefully observe which way all the parts fit together, to help assembly."


The experiences with rejetting I've seen are:

Harry "the Wrench" wrote:

"The DynoJet kit will improve your mid-range performance considerably without modifying the exhaust system. Adding a set of free-flowing pipes will enhance the mid-range even more. You can achieve improvement similar to that of the average DynoJet installation by simply readjusting your mixture screws and raising the stock needles 20-40/1000s with washers, thus saving enough to buy yourself a nice tankbag or something. A DynoJet kit is generally comprised of a set of adjustable needles, a drill bit (for modifying the slides for quicker response), and an assortment of jets. They are intended for professional mechanics or advanced amateurs who are intent on getting the absolute max performance from their bikes and who are willing to spend many (sometimes many, many) hours fiddling with different combinations of jets, exhaust systems, air box/filter mods etc. in order to get everything working optimally together. As a professional, my advice to you is to do the simple, inexpensive and very effective mods (mixture screw adjustment/washers to raise stock needles) and spend the $125 elsewhere."

Here's another Dyno-jet experience, this one from Ron Lotton. (The full article is at

"DynoJet did not have a kit specific to the VX800, but had one available for the VS800 (Intruder). Shawn provided me with a free sample if I agreed to test it on my VX800. As the motors are the same I saw no harm in trying the kit. It is a straight forward installation which took about 3 hours to complete. (a mechanic could probably do it faster). I don't know if it is any quicker, but the bike has crisp throttle response with no hint of a lean surge. It now warms up enough to ride without using the choke within a couple of blocks at the same temperatures."

And last, from Mike Heathman:

"Mine was horrible in cold weather when I first got it. I drilled out the brass covers over the idle mixture screws and fiddled with them a bit. I ended up running at 1-3/4 turns out. I think stock is 1-1/2. I also noted that the front cylinder pinged when the temperature was below about 50F and it was warming up. I ended up putting in a 42.5 pilot jet (stock is 40) in the front carb. The rear already runs a 45 and seems OK with that."
"My bike now runs alot better in cool weather. At temps down to about 55F I can turn the choke off immediately and ride away. Hope this helps."
Mike also added, later on, on the subject of cold-weather starting:
"I have to say that my VX was quite a bear to start in cold weather when I first got it. A couple things have helped. First of all I replaced the front pilot jet with the next size up, the front cylinder had a horrible cold ping when I first got the bike and this helped alot. Secondly, and probably most importantly, adjust your idle mixture. I run my idle mixture screws at around 1-3/4 - 1-7/8 turns out in the summer. In the winter I take them out to about 2-1/8. This helps cold weather starting and warm-up tremendously, but it does foul the plugs if you idle in traffic alot. So the last thing I do is run a hotter plug in the winter. Using a NGK DPR7EA9 in the winter solves the fouling problem and seems to help starting even more. My bike starts just about instantly in any weather and I can usually turn off the choke in about 15 seconds. I do let it idle for a few minutes while I get my helmet, gloves, etc on and it's ready to go."
"The last thing I've found with the VX that is different from some other bikes I've owned. Don't touch the throttle when you're starting it. Give it full choke and leave the throttle closed. With CV carbs opening the butterflies actually leans out the mixture. I never crack the throttle until the engine has idled for a few seconds."

Covering the radiator

If you do much riding in cold weather, you may want to try a trick suggested by Mike Heathman, which I have also used with success. To reduce airflow through the radiator and around the engine, block the radiator with (in Mike's case) an old license plate or (in my case) the cardboard flap from a case of beer. The radiator cover comes off easily, letting you slip whatever you're using in underneath. It's not clear whether this works because some coolant always flows through the radiator regardless of the state of the thermostat, or perhaps the engine just gets cooled sufficiently from too much air flowing over it, but in any case, cold weather (like down to 20F) warmups are much more reliable. Without doing this, it's possible to do a very long commute and not be able to take the bike off-choke even at the end of the ride.

Sticky slides

A couple of people (me included) have had an engine stumble caused by the carburetor slide (air piston) not moving freely enough. My problem was I think mainly on the front carburetor, which may be because the slide is horizontal and more prone to "stiction". The symptom was a slight hesitation at a particular engine speed, and a sort of "flat" spot in the power curve as I accelerated through there. The fix is to spray some carburetor cleaner on the piston and work it up and down to clear the stickiness -- I suppose in a really bad case you might need to disassemble the carbs to clean them fully. You can get at the front slide quite easily by simply removing the rubber intake manifold -- the rear is much harder -- I was able to spray some cleaner on it by putting my hand in through the air-cleaner opening, and using a flexible tube as an extension of the little plastic snorkel on the spray can.

At least one VX owner adds a little Marvel Mystery Oil to his gas every tankful or two to prevent this problem from recurring.

5.2 Valve adjustments

Not having done my own valve adjustment yet, I have to rely on others' comments for this section. Here, from various folks:

Poster 1:

"Screw type adjusters on the valves, 4 valves per cylinder. Most folks stretch the recommended adjustment period somewhat. Requires the standard Suzuki driver (square head). Not particularly easy to get at a couple of them, Using a motorcycle-specific feeler gauge made things much easier."

Poster 2:

"The job can be done with just feeler gauges and a simple wrench, but there is an inexpensive valve adjustment tool that makes it easier. This tool consists a handled wrench to loosen the locking nut and a concentrically located screwdriver. The idea is that you loosen the nut, adjust the screw and then TIGHTEN THE LOCKING NUT WHILE HOLDING THE SCREW'S POSITION. This tool can be found for about $10 (if memory serves) in places like the Chapparrel catalog. BTW, it is advisable to use TWO feeler gauges at the same time on four valve heads, otherwise the rockerarm swivels and throws off the measurement."

Poster 3:

"The information ... on valve clearance tools is on the mark. I bought one of the .004/.006" combo feeler gauges #315-0825, $3.99 and the "Tappet Adjustment Wrench Set" #315-0460, $28.99, from Chapparal last Oct. Work lots better than my hand-bent feeler gauge blade and the official Suzuki Valve Adjustment Tool #09917-10410, $13.26, from Ron Ayers. The latter "tool" is a round steel rod shaped like a large allen wrench with a square hole stamped into the end of the short leg. Said hole fits over the top of the valve screw, but has enough slop in the fit that its hard to tighten the lock nut without over tightening the valve screw. If anybody wants one, slightly used, I'll sacrifice and let it go for $10."

All valves are adjusted with their piston at TDC. There's an observation "port" at the front of the left sidecover -- remove it, and line up (by turning the crank through the plug in the center of that sidecover) the letters "RT" in the center of the port for the rear cylinder, and then turn the crank another 1-1/4 turns (1-1/3 for non-US models) to line up "FT" to do the front cylinder.

Don't use a screwdriver to get the big generator cover plug out, since the metal is soft and you'll probably chew it up. (that's the thing on the left side of the engine with the giant slot in it, which you need to remove to turn the engine over.) Use something that fits the slot pretty closely. I was able to use a piece of thick sheet aluminum filed to a curve to match the curved bottom of the slot. Rick Blunden says it can be replaced with the plug from a Suzuki DR650 that has a hex plug fitting.

Steve Ginthum writes:

A suitable tool is called a "drag link socket." It's an automotive front suspension/steering tool. I found one that had a blade width of 11/16" and thickness of 7/64" (2.78mm). The major tool suppliers make this, so I imagine that you can get one at a good auto parts store. I'm sure they can order it, also.

The advantage of a socket is that a really stubborn cap can be removed with an impact driver without damage and it can be torqued on without over-tightening.

Snap-On part #: F22A
MAC Tools # DLX1

The gap for all valves is .003 to .005 inches (.08 to .13 mm).

And of course, adjust when the engine is completely cold.

5.3 Air filters

The stock air filters from suzuki are pretty expensive, and unfortunately the big aftermarket places don't make direct replacements. Apparently there's a Japanese company called Meiwa that makes replacement air (and oil) filters that seem to be more available in Europe than in the U.S. One poster said to try the "Polo" catalog. Where those aren't available, here are a couple of different tricks:

The best solution seems to be to use a stock K&N part, which is apparently a cotton gauze filter that don't require the saturate-and-squeeze oiling method of foam filters, just a light film of oil. The idea is to dismantle a stock filter by removing the wire mesh (like window screening) that encloses the paper -- some people use cutters, others apply heat to soften the glue. You're left with a base to attach the new filter to. The K&N part number is RU-0160. It has a 1-1/2 inch flange, 3 inch diameter, 4 inches long). The price seems to be under $30.00 (US) for each filter. After the flange cools, the RU-0160 slides right on, and a hose-type clamp is provided to secure it. In some areas the K&N part comes pre-oiled (you can tell, because it has the red color of their filter oil, and says "pre-oiled" on the wrapper). Properly oiled, it has a 50-100,000 mile service interval for street-driven applications.

An even more promising piece of K&N news, from Raffaele Belardi:

"... I confirm that last friday I bought, in Italy, a K&N filter which is a direct replacement for the stock one on the VX (and the VZ800 Marauder). They came in the K&N box, with the K&N colors, the K&N stickers and the K&N instructions. And the K&N part number SU0009. The box also says 'Made in the United Kingdom'(!)."
Someone else reported that they couldn't locate that filter in the US, but this is still encouraging news. It implies that the air filter for the VZ800 Marauder is the same as that for the VX800, and being a current model worldwide, the Marauder may lead to more availability for this filter. [ Can anyone confirm this? - TRB ]

Rick Blunden reports good results with a set of UNI filters:

"The UNI UP4152 'Flex Core Pod' oiled foam air filter retails for $10-12 each and is a good replacement for the stock paper filter. Depending on what you read and what your superstitions are, the UNI traps smaller particles than the oiled gauze (K&N) or paper elements, but does not allow as much air to pass through as the K&N. Like the K&N, the UNIs should be cleaned in solvent and reoiled every 10,000 - 20,000 miles. They probably won't last more than 1 million miles :-). Rejetting is not necessary, and intake noise level seems the same as stock."

You can also use UNI filters designed for the GS500E and Madura 700. They replace the stock filters entirely (rather than reusing the metal base), but they don't really fit properly -- you need to shim them out from the air boxes with washers, since their flanges dont' fit over the airbox lip, and you need to glue or screw the rubber snorkels to the rubber tube on the filters, since the pipe clamp won't hold them anymore. All in all, having tried both, I'd use the K&N units. (The UNI part number, if you want it, is NU-2453, available from Dennis Kirk as 30-3026.)

5.4 Sparkplugs

The sparkplugs take an 18mm socket. Tim Morrow says "You can just barely get an 11/16 on it if you're careful."

Do-It-Yourself sparkplug socket: Take an 18mm deep socket, and pack some foam into the top inch (2.5 cm). Now take a piece of heavy wire and heat it up. Push the heated wire through the center of the foam. You now have a sparkplug socket that will securely hold the sparkplug, regardless of orientation. Works great to install plugs in your VX.

Those deep spark plug wells can collect road grit. It's really best if you blow out the holes with compressed air before removing the plugs.

The correct plugs for the VX are NGK DPR8EA-9. For a colder plug, use DPR9EA-9, for a hotter one, use DPR7EA-9. The correct gap is .031" to .035" (which is .8mm to .9mm).

5.5 Motor oil

Thanks to Raffaele Belardi for putting most of this oil section together.

Oil weight

No particular problems reported on motor oil types: people use with success oil ranging from 20W50 (apparently the most common weight) to 15W50, 10W30 and others. (Apparently 10W40 has fallen out of favor as an oil -- too many polymers to too little oil, or something like that.)

Most motorcycles, including the VX, recommend API "SF" standard oil. Apparently the American Petroleum Institute has created new standards, SH and SJ, which you probably shouldn't use in a motorcycle. To boost fuel economy, these oils are more slippery, and may not work with wet clutches, and may also not hold up as well in motorycle transmissions. To reduce effects on catalytic converters, most of the zinc and phosphorus has been eliminated -- these are apparently important as "extreme-pressure" additives in motorcycle oils.

Synthetic oil has much better breakdown and high temperature characteristics. it does, however, get dirty just as fast from combustion byproducts, so you can't just double your change interval, which is what one is tempted to do.

Some VXers' noted that synthetic oils cause 'grabby' clutch response:

Rob Opiela wrote:

"I switch off between Mobil 1 and Castrol GTX in my bikes. I put Mobil 1 in until I notice that the clutch is getting grabby, then I switch to Castrol. As a synthetic, Mobil 1 is VERY slippery. It has an energy saving rating by the API because of this. It also can cause glazing of clutches if you like to slip your clutch alot. If you notice that your clutch is starting to get a little grabby (glazing), switch to a dino oil an the next oil change."

Ravin31 adds:

"From my own experience I would say that synthetic makes the VX's shifting much smoother especially if you use sythetic hypoid gear oil. My mechanic says that switching between synthetic and regular oils can cause exsessive wear to the clutch. He recommends keeping to one or the other once you have made your decision. There is also some debate as to weather synthetic can hide or even cause clutch slippage."

Other owners report 65,000 miles on Mobil 1, without a problem.

Oil Filter

Rick Blunden writes:

"Fram makes an oil filter for the VX800 (#6018) that sells for about $6-10 depending on where you buy it. Suzuki's filters cost a little more. Fram had a problem with shallow threads on some filters about 3-4 years ago resulting in some filters blowing off the engine and dumping oil. Fram corrected the problem and issued a new part number for the new filters."

Changing oil

This is not really only a VX800 issue, but may be useful anyway... Oil should be changed with warm engine, as it flows more easily outside. Be careful it's not too hot -- it will burn you. Wear rubber gloves -- oil, particularly dirty oil, is a known carcinogen. I use disposable skin-tight gloves, to retain at least some "finger-feel". And, RECYCLE YOUR USED OIL. This is EASY. Do NOT pour it into the sewer. Most towns have recycling centers that will take it -- so does the Jiffy Lube place where I have the cars done. In some states oil retailers have to take it, but they may want receipts.

Remember that oil degrades with time as well as mileage. The combustion by-products will cause mild acids to form in the oil, which aren't good for your engine. That's the main reason you should put in fresh oil before storing your bike for an extended period.

5.6 Coolant

Only use coolant that doesn't contain silicates, which have been shown to damage some motorcycle water pumps. Only use distilled water when you're mixing your solution. For the quantities of coolant the VX takes, I find it more convenient just to by pre-mixed coolant at a motorcycle shop (Hondaline is what I got, I think.) The drain plug is on the lower left frame tube. There is a bleeder bolt which should be open while you're filling the system, located in the upper part of the left front frame down tube. Recheck the level (which should be to the top of the inlet hole) after running the engine, to be sure its full.

You should also inpect the rubber hoses regularly, and replace them before they cause you trouble. A couple of us have had a small leak develop at the little tube that runs between the cylinders. I was able to stop it by gently tightening the pipe clamps.

5.7 Swapping with Intruder

The VX uses the same engine that the VS800 Intruder does. Peter Harper had this to report from the mechanic who installed his Intruder engine:

"...he said that it was a simple bolt-in. Everything fit everything. With one exception. He said that on the VX800 the fuel pump is driven by a vacuum off the engine. The intruder is not. He manufactured a vacuum line, and all is well. But as far as the fit goes, he said it was clean."

5.8 Power/torque

The VX review in the July '90 issue of Cycle had this table:

      rpm    3500  4000  4500  5000  5500  6000  6500  7000  7500  8000  8500

      HP     27.9  33.0  37.4  40.7  43.2  43.8  46.5  48.7  52.8  47.5  44.6

      torque 41.9  43.3  43.7  42.7  41.3  38.4  37.6  36.5  37.0  31.2  27.6

This translates (roughly) to the following chart:

     60 +-----------+----------+-----------+-----------+----------+-----------+

        | H - HP                                                              |

        | T - Torque                                                          |

     55 +                                                                     +

        |                                                    H                |

        |                                                                     |

     50 +                                                                     +

        |                                              H                      |

        |                                        H                H           |

     45 +                                                               H     +

        |           T     T    T     H     H                                  |

        |     T                      T                                        |

     40 +                      H                                              +

        |                                  T     T                            |

        |                 H                            T     T                |

     35 +                                                                     +

        |           H                                                         |

        |                                                         T           |

     30 +                                                                     +

        |     H                                                         T     |

        |                                                                     |

     25 +-----------+----------+-----------+-----------+----------+-----------+

      3000        4000       5000        6000        7000       8000        9000


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