Bad mpg

[stevel]

Hey guys, new to the forum.I have just bought a 97 RSTD. I am getting pretty poor gas mileage.Around 28mpg.Have changed plugs,synced carbs,checked air filter and changed fuel filter.Timing looks good.I think the bike vibrates more than it should at highway speeds,of course I don't know how much it should vibrate if any. It has after market exhaust but dont know if it has been re jetted. I get a lot of popping on decelleration. I am now looking at carb rebuild or a possible problem with the coils. The bike has 36000 mi on it. Any help would be great.

[ToRide1]

It sounds like you did every thing right and it should be doing better than 28 MPG. Can you ID the Pipes I'm sure someone here can tell you just what jets you need to go with them and you may want to get a cheap bottle of carb cleaner from wal-mart 4.Oz. will Of it is all thats needed.

[stevel]

Thanks .I have run seafoam through it twice. No change. Not sure about the jetting.The previous owner did not mess with jets however the first owner had the pipes installed and we don't know if he had it re jetted.

[pegscraper]

It sounds like jet sizes are the next thing to check. And aftermarket mufflers normally don't require a rejet. Have any modifications been made to the air box? Stock jets for a '97 Royal Star are #20 pilots and #95 mains.

 

The early Royal Stars do vibrate a little. They don't have a counterbalancer shaft in the engine, and their being way overgeared doesn't help the situation. They lug very easily, even at speed. Swapping for the VMX rear gear will up the rpms just a little and greatly help things out.

[hig4s]

Seems like a read somewhere (oh but don't quote me) the there is a throttle position sensor that if bad can cause bad gas mileage.

[pegscraper]

Yes, if that's bad, it will lose 3 - 4 mpg right there. That's a possibility, but that doesn't account for all of his poor mileage. It should be up right around 40.

[stevel]

Thanks again guys .I am going to remove carbs and check jetting and condition.Would this be the best time to check valves as well as I have heard a tight valve can cause bad mpg as well. I have 37000 mi on it.Also the covers are easier to remove when the carbs are off.Also heard about the TPS could be an issue. will check it as well. Since I will have it torn apart I though of checking the coils as well. Has anyone ever heard of problems with crossed primary wires on the coils.I know some later yamaha,s (not Ventures)had issues with crossed wires from the factory.

[pegscraper]

If you had a valve tight enough to be ruining gas mileage, the engine wouldn't have 37,000 miles on it. It would have died and needed a valve job a long time ago. If the valves have never been checked, it's not a bad idea to do it. It won't have anything to do with gas mileage though. The carbs are not in the way of the valve covers at all.

 

I've heard a bit about the crossed ignition wires. I don't know or understand the whole issue. I know some of the shop manuals have been wrong too, and have shown the wires for #2 and #4 switched. On some VMXs, the front left coil fires the front right cylinder and vice versa. I don't know why they did that. In the case of your Royal Star, the gray wire should run to the front left coil and fire that cylinder - #2. The white wire should run to the front right coil and fire #4.

 

I think you're digging a little too deep here. The crossed wire issue was straightened out by this time anyway. Check those jets.

[ToRide1]

I had to think about this one and I can be wrong but! The Throttle Valve Assembly (slider) could be hanging up and you may not catch it when doing the carb syncing. This will make it run ruff and kill gas milage. Just something to keep in mind when looking it the carbs.

[stevel]

Well I have pulled the carbs and the main jets are correct #95. Don,t think I will separate and tear into them as they seemed to operate fine.

A question on the pilot screw. The manuals all say to adjust it you need to remove the brass plug first to get to it. Mine have a rubber plug pressed into them. does this mean the pilots were probable adjusted at a shop and they replaced with the rubber plugs. These are on the out side of carbs to one side of the slider cap. I will check valves and throttle position sensor next.

[pegscraper]

Have any modifications been made to the air box?

 

What size are the pilot jets? Only above about half throttle do the main jets come into play. Anything below about half throttle and you're on the pilot jets.

 

The brass plugs over the mixture screws being replaced with a rubber plug means they have been adjusted. How many turns out from seated are they?

[stevel]

Thanks ,Will check the pilots.Their location is on top of throttle body right? Dont have a diagram with me.Get back to ya on the screw settings.

[pegscraper]

The pilot jets are in the bowls right next to the main jets. There will be a number stamped on the side of them. You may have to remove them to read the number.

 

The jets on the top in the air horn are air jets.

[stevel]

Well I just got around to checking my jets,The mains are #95 and the Pilots are #17.5. A little confused however. The replacement parts breakdown calls for a 17.5 pilot. Here on the forum someone said it takes a #20.and my manual has another number. Any one know what I should be looking for as far as a correct number? Also wondering if the needles on the vacuum diaphragms are adjustable on this model. Thanks Steve

[V7Goose]

I'd go with the parts breakdown for the right year, make and model. The carbs have changed on these bikes quite a bit (just based on the part number changes - I don't know the specifics of WHAT changed).

 

Make sure you set the float levels with the spring in the float needle NOT compressed. I cannot comment on the rubber plugs you mentioned - on the current models the mixture screws have cone-shaped brass caps on them, not plugs, and when the caps are removed the screws stick out, so there is no way to use a "plug" to cover them. I have never looked at the carbs on your model.

 

A tight valve can absolutely cause your problems - I have seen it on other engines. And we have no idea how long it might have been leaking, so it is quite possible that no damage has been done yet even if it is bad. They are due to be checked anyway, so best to do it.

 

These engines run VERY well on only three cylinders, so you want to make sure all are firing equally. Do a compression test (both dry and wet with leak-down); read your plugs and check how fast the header pipes warm up when you first start it. In addition, since you are messing with the carbs anyway, you should get a similar slow-down and speed-up from the mixture screws on each carb - that will be a big indicator of how uniformly each cylinder is working.

Goose

 

Oh, ANY significant vibration from these engines when they are not being lugged is a sign of a problem, usually just poor carb sync. And popping on decel is caused by lots of things that affect combustion (and poor gas mileage, of course). Make sure you check carefully for any vacuum leaks before you try to set the mixture and get it running again.

Edited March 20, 2009 by V7Goose
couple of other thoughts

[pegscraper]

G, these 28mm carbs never had cone shaped brass caps on them like the 32mm Venture carbs do. They came with brass plugs in them, and the fact that his now has rubber plugs in them means that someone has drilled out the brass ones to adjust the mixture screws.

 

How many turns out are the mixture screws?

 

My copies of both Y's factory shop manual and the Clymer's manual say these 28mm carbs have a #20 pilot jet. I don't remember what mine actually had in them anymore. My stock carbs are long gone. If yours have #17.5 pilots, I guess I'll say leave them there and work with that.

 

The needles on the throttle valves are adjustable with shims or small washers. There should only be one shim under the needles. There might be another one on top of the needle too as a spare or for other adjustment, but that one doesn't actually raise the needle.

 

How many shims are under the needles on your carbs?

[stevel]

I have checked compression on all cylinders. (dry test) All within specs. Do you think this answers my question about a tight valve or is it still necessary to pop the covers.

On checking the Throttle Body Sensor. I am within specs. on the test with idle closed. But upon testing the unit in the wide open mode I only read 3.5 ohms. It should read 4-5 ohms. Is this a sign of a bad sensor or can it be adjusted by loosening the mounting screws? These are fairly pricey so I hope I need not have to replace.

[pegscraper]

Your readings on the TPS sound good enough to me. It can be adjusted a little bit. You will need "security" torx screwdrivers for the screws though. But I doubt this is the issue.

 

V7G will have to answer on the tight valve possibility.

 

How about those questions above?

[V7Goose]

If your dry compression test is in specs, then all rings and valves have to be good (as in not too tight - they could still be too loose, but that won't cause bad fuel mileage). Now your main possibilities remain fuel mixture and ignition, along with carb sync.

Goose

 

One caveat on the valves - I hate to keep throwing doubts, but think it is important to cover all bases. There is a SLIGHT possibility that you have one or more valves too tight, but still read good when cold. If the adjustment is borderline, the valve will close fully when cold, but slightly leak when hot. I'd just store that info and not worry about it unless you can't fix the problem with anything else you try - then I'd probably want to check them just to be sure.

[stevel]

Thanks V7 ,I have removed pilot screws and cleaned them .Noticed they have been set to 1 to 2 turns out. This seems a little rich to me . I will re mount everything and re sync carbs. Cant recall the general settings on the pilot screws.Should they start at around 3 turns out?

Edited March 22, 2009 by stevel

[pegscraper]

So how many shims on the needles? Third time I've asked the question now.

 

It's difficult enough to diagnose problems like this in a format like this. But having to ask questions two or three times to get answers - I gotta say this is really frustrating. And I can think of one question back on page one that I asked at least twice which you still have never answered. I give up.

[V7Goose]

Thanks V7 ,I have removed pilot screws and cleaned them .Noticed they have been set to 1 to 2 turns out. This seems a little rich to me . I will re mount everything and re sync carbs. Cant recall the general settings on the pilot screws.Should they start at around 3 turns out?

Sorry, I do not know the answer to your question for your specific engine. I know that the settings on The RSV have all been a bit different on each carb, but that may be simply because three of the four carbs have different jet sizes (not so for your bike).

 

Tuning by ear (really by tach) is actually pretty simple and fairly accurate, in my experience. With everything else in good working order, turn out a mixture screw to find the highest RPM, then turn it slightly back in to just notice a drop (probably no more than 1/4 turn in). I used this method on an Venture recently, then when I checked it some months later with an exhaust gas analyzer, all carbs were VERY close - looked like I might have just slightly turned each screw in a tad bit too far, resulting in a final CO % that was excellent, but a tiny bit leaner than I selected with the analyzer.

 

If you do not have a shop tach, I know of a free one that you can put on a laptop and it uses the sound of the exhaust. It works really well, but ONLY if you know some details about it. It will only monitor a very narrow range of RPM for any specific setting, so you cannot just fire it up and rev the engine to see the RPM change. But that narrow band works great to watch for minor RPM changes from the mixture screws.

Goose

[wes0778]

My , How hard do you ride it? My mileage dropped from 42 to 33, on the first leg of my day trip, today. I did however make the 130 mile trip in an hour and 45 minutes:whistling:. could that be your culprit?

Walter

[ToRide1]

If this is a Mikuni; BDS28 [28mm] Pilot Screw Seting! 2 1/2 Turns out from lightly seated.

[ToRide1]

I wondered about these bikes being re-jetted like they do the 650 V-Star! Take the 650cc displacement 650cc X 2 = 1300cc displacement. Now both the 650 V-Star and 1997 RSTD use the same carbs 28mm Mikuni. What I'm talking about requires a less restrictive exhaust and using a 127.5 main jet, Pilot screw 3 1/2 turns out and two shims on the needles and Pod Filters. It should make get up and go and on the 650's they get better gas milage than stock about 3 miles more to the Gallon.