Reason for sudden drop in mpg?

[Bounder]

I have an 87 Venture. Prior to changing spark plugs and a new air cleaner I was getting 40+ mpg. Carb sync with Carbtune, and just added new caps and plug wires. Mpg in low 30's. I think it is running pretty good. I have been running with no side panels and no lowers. I am wondering if that can affect mpg. Asking for opinions.I started with an EMGO air filter. After reading some forums I thought maybe the EMGO filter changed conditions in the airbox so I installed an oem cleaner,mpg increased about 3 mpg.

[bongobobny]

Hmmmm, you may have lost a cylinder, these bikes run surprisingly well on 3 cylinders! OK after sync'ing the carbs, did all the vacuum lines go back where they are supposed to be?? Check for split vacuum line. I would invest the 10 bucks give or take in a spark gap tester to make sure you have a good strong spark going to all 4 plugs. Another possibility, and trust me many of us have done this on some vehicle, did you accidently bash the plug and close the gap on the plug?? A quick pulling of each plug and verify the proper gap may be in order.

 

Yup, the stock air filter works best, aftermarket ones can actually decrease MPG due to either increased or decreased air flow affecting the air/fuel characteristics. They run like crap with no airbox...

 

Sooo, buy a spark gap tester, pull the plugs and inspect, and check for a disconnected or misplaced vacuum line!

[Flyinfool]

It sounds like you are running on 3 cylinders.

From cold, start the engine and see if one exhaust pipe does not heat up as fast as the others. The cold one is the cylinder with the issue.

[Marcarl]

When you did the sync, did all the cylinders draw the same amount of vacuum?

[Bounder]

They were all the same when I did the sync. I will check the exhaust tomorrow. Thanks

[Bounder]

all 4 pipes get hot

[Bounder]

I did not check plugs with tester.I pulled the plugs before changing caps and wires.3 were slightly black and sooty, 1 was tan and white .I have not checked after R and R.

[bongobobny]

OK not sure you understand what I meant by checking the SPARK strength with a tester. A spark gap tester is a tool you can buy from any automotive parts store, etc and it is a box with an adjustable gap to see how far the spark can travel which indicates how STRONG the spark is. If memory serves me right the Yamaha TM calls for 0.109". You adjust the gap distance on the box, clip one side of the box to ground, and the other side goes to the plug wire cap. The spark should jump the gap when you turn the motor over. You can disconnect all 4 wires so the bike won't start on you while you are cranking it over. This is an excellent test to see if your wires are good, if your caps are good, etc. A weak spark will definitely affect performance!

 

Of course you know that you test the clearance on the spark plugs with a gauge, the proper clearance I believe is 0.032 - 0.036". Never trust the plugs to be properly gapped from the factory, they aren't necessarily gapped...

 

Hope you get to the bottom of this, you should be getting around 40 mpg which you were before you started working on it, so let us help you find out what went wrong!

[kevin-vic-b.c.]

OK not sure you understand what I meant by checking the SPARK strength with a tester. A spark gap tester is a tool you can buy from any automotive parts store, etc and it is a box with an adjustable gap to see how far the spark can travel which indicates how STRONG the spark is. If memory serves me right the Yamaha TM calls for 0.109". You adjust the gap distance on the box, clip one side of the box to ground, and the other side goes to the plug wire cap. The spark should jump the gap when you turn the motor over. You can disconnect all 4 wires so the bike won't start on you while you are cranking it over. This is an excellent test to see if your wires are good, if your caps are good, etc. A weak spark will definitely affect performance!

 

Of course you know that you test the clearance on the spark plugs with a gauge, the proper clearance I believe is 0.032 - 0.036". Never trust the plugs to be properly gapped from the factory, they aren't necessarily gapped...

 

Hope you get to the bottom of this, you should be getting around 40 mpg which you were before you started working on it, so let us help you find out what went wrong!

 

If I am correct you are looking to test the strength of the Coil output by doing this test more than testing the Spark Plug. On old oil burners we used a screwdriver across the terminals to draw the spark from the coil to see it was bright crisp and how far it would pull. I think same idea.

[bongobobny]

Yup!!

[Bounder]

you are right, I did not know about a spark tester ,I will look for one. I will try to verify .109" I checked plugs before installation. After syncing carbs and looking down through the airbox all seem to be uniform so I think vacuum is OK.

[Marcarl]

How many tanks of gas have you used for the test, sometimes 1 tank won't tell the right story.

[bongobobny]

I was looking for the actual spec for the 1st gen's but could not find it, I believe that spec may be for the 2nd gen. It does not matter, the point is you should get a spark at least twice the distance of the stock gap. Try at 0.100 and if it doesn't work try 0.080. The point being all the sparks should be around the same, if you have one wire that is much weaker than the others there is an issue with either the wire, the coil itself, or with the plug cap assembly.

 

Question on your new wires. Did you buy new assemblies, or did you buy separate wires and separate caps? Also, speaking of caps, on the coil side of the wire did you install the o rings on the plug wires under the screw on caps for the coil? These are important, they keep the plug wire from vibrating loose. Also, it is a good practice to snip 1/8" to 1/4" of the wire off each time you take the plug wire off. The reason for this is the metal prong spreads the center conductor apart, and when you reinstall without cutting the end off it may not make as good contact resulting in a weaker spark. Of course then the o ring has to be pushed further down the wire, the screw on cap will push the o ring back towards the end when you screw it on. Also, you should inspect the coil secondary connection as crud and corrosion can, and will get in there! There should be no green stuff in there!!

 

On the spark plug boot, there is an assembly in there that unscrews and has a spring, a disc, and a resistor element. They get dirty and cause problems as well. Somewhere on this site is a thread about how to service these...

[Bounder]

I have driven only about 100 miles since new caps and wires so not a very good test yet. Will be leaving Wisconsin for California on Monday so that will be a good test. I was getting about 30 + or - from California,have since installed oem air cleaner and new caps and wires. I did the sync before leaving CA, carbtune is in CA. At least gas is not too expensive. I bought new assemblies on EBAY. I did not inspect coil secondary connection. I am becoming familiar with the bike over time. I have new diaphrams but the existing look good. I considered servicing the caps. I will keep them and check them when I get back to CA out of curiosity. Thanks for all the advice.

[gggGary]

Check the slides all rise, especially the one with a white plug. the cylinder will idle and the pipe will get hot but it will never make much power. bike will seem to run fine.

 

 

On my bike I found this;

 

 

 

Very easy to do. A "pro" mechanic's oopsie. goes great now. Could also be dirt in a jet on that white plug cylinder.

[Bounder]

don't know white plug cylinder?

[gggGary]

don't know white plug cylinder?

 

 

I pulled the plugs before changing caps and wires.3 were slightly black and sooty, 1 was tan and white

 

I would suspect the diaphragm on that tan and white cylinder but it's easy enough to check them all.

Also float levels are hard to check, but can make a real difference in running and MPG

[bongobobny]

OK Guys we are starting to go a little astray! Carefully reread post #1 . He stated the bike WAS getting around 40mpg which is normal. All he did was a "tune up" consisting of new air filter, carb sync, and new plugs and wires and immediately after his mpg went south! Obviously something he did/changed went wrong!

 

Now gggGary does have a point about the one lean plug indicating perhaps something wrong with THAT carb before "fiddle futzing" BUT even so it was getting 40 mpg!

 

My point is to find out why all of a sudden the drastic change in MPG. Could be a weak spark, could be something not properly reassembled with the airbox (Keep in mind this is a 1st gen, not a 2nd gen), or a vacuum line that split when removed to sync the carbs or when reinstalled. Could be carbs were not sync'ed right, etc!

 

Remember these bikes run surprisingly well on just 3 cylinders...

[gggGary]

Quote; "I have new diaphrams but the existing look good." Can't know that unless the lids have been off.

 

that's why I was suggesting he check for the slides all rising.

[Bounder]

I ordered new diaphrams before new plugs and air cleaner but after checking 2 of them that looked like they were pretty recent I didn't change any.The tamper proof screws were gone so I think diaphrams were changed. Bike now has 63000 was in low 40's when I bought it.WhenI did the sync the "indicaters?" were straight across and in the upper half of the gauge.I don't have the carbsync with me so can not check that now. The only difference now is that I did not reinstall plastic side panels and lowers. I also had mounted new Elite 3 front and rear before leaving CA. I don't really know what happens with carb sync. Does it effect fuel air mix?or just sync slides.

[Flyinfool]

The carb sync just sets the throttle plates to be all open the same, it does not effect fuel air mix or the slides directly.

[Bounder]

Thanks Jeff

[Bounder]

checking spark, spark jumps 1/4" on all 4 cylinders

[snyper316]

Dont mean To Hijack your Forum here but do they make the spark gap tester for motorcycle specifically... all i have found is the nubbed ones... and if they do can someone point me in the right direction to get one then once i get this I believe my arenal of electrical testers will be complete....

[bongobobny]

Not that I know of. I just know about the generic universal variable spark gap box found at most Auto Parts stores...