Brainstorming things that affect fuel consumption

If you have seen my previous post (Damn Gas Hog) then you know that I am having problems with fuel consumption--specifically about 31 mpg. I've done all the normal stuff and now will start again--one at a time. Here are the things I think can have an effect on fuel consumption, most of which I have done. 31 mpg is ridiculous, so I'm determined to find a solution. Add to the list, think outside of the box, give me some ideas. Something is causing the carbs to suck gas--even at idle.

1. valves--especially intake
2. carbs cleaned--spotless
3. clunk test
4. pilot circuit
5. colortune
6 carb sync'ed
7. carb floats wet set
8. jet sizes and any modifications
9. needle jet
10. petcock vacuum system
11. air filter--currently Uni Foam
12. fuel enrichment system
13. compression test

What else? any tips?

 

Your foam filter could be too restrictive. Go back to a stock filter.
Post picture of your plugs so we can see.

 

Tabaka45,

IMHO all the things you've listed would affect fuel consumption. I recently cleaned and rebuilt my petcock and gas cap. I can already tell a difference in how the fuel is being delivered into my carbs.

Gary

 

Got a fuel leak anywhere? Does your bike smell like gas after it sits?

 

Here are some more factors: tire pressure, weight of the bike, condition of the wheel bearings, trying not to go everywhere up hill, etc.

 

Here they are--left to right--1 to 4. Pilot screws open between 1 1/4 and 1 3/4. these pictures are after idling to colortune # 1

 

No gas smell after sitting, and I can't find any leaks. No gas in the oil. I do smell gas just after starting for just a few moments.

 

The wheels turn freely, and I have the tire pressure set to the recommended pressure. I live in S. GA, on the coast, so the only hill (unfortunately) is a bridge.

 

Your plugs are showing a rich mixture, but not horribly so. #1 appears to be getting an extra shot of fuel.

Maybe you have an intermittent float problem. Could be that it when you wet set there was no vibration and no jostling and the needle valve closed tightly, but when you're on the road, parking etc it can move enough to not seal tightly. You shouldn't smell raw fuel at all anytime, unless your parking your nose by your gas cap. Are you running stock pipes? Is there ever any gas coming out the pipes? You could hold a piece of paper behind when you start it and see if it gets any wet fuel on it, there shouldn't be any.

 

I thought of the float issue you suggested. Any suggestion on how to check it out?

 

I would expect sooty black plugs. I see they are dark but not black. Try the air filter or no air filter at all and see what you get.

MN

 

I've taken the UNI foam filter that was in the bike when I got it and split it to about 2/3 of the original thickness and will try that. Haven't ridden it yet, but it didn't effect the color tune setting as far as I could tell.

 

As yet unmetioned.
Vacuum leaks from the intake boots/gaskets, airbox boots, throttle shaft seals, vacuum port caps.
Brake drag front, or rear
Fork, swingarm alignment
Poor fuel quality
Improper clutch adjustment
Worn clutch
Starter curcuit not fully disengaged. The levers should not be lifting/contacting the jets.

A unifoam filter should if anything cause the bike to run lean. Also the pilot mixture screw setting does not sound right to me. It should be more like 2 1/2 - 3 turns. Try a vacuum sync, and color tune again.

I know it is counter intuitive, but try running in fourth gear in this speed range. It could be a simple case of the rpms being to low, and therefore making the motor work harder in the lower torque band to maintain speed. These bikes really do like the higher revs. I seldom ever get into fifth until 60-65 mph. Mostly back country roads, or highway running.

Ghost

 

Lastly the Quote about running around and dogging the engine in high gear at lower speeds could be another problem. Try keeping the bike in a lower gear so your not bogging it down. Weather you realize it or not you are working the engine out of its power band thus introducing more fuel needed at lower rpms with a load on the engine. That alone could cause you to burn more fuel than needed.

Still the idle mix screws only out less than 2 turns is odd because if your air filter is not restricting and your pilot jets are correct ( for USA ) they should be somewhere in the 2.5 turns out area. But the fact is you can set them lower and not have a yellow color tune is still good.

MN

 

Yeah, you should never smell gas and 1 1/4 turns is insanely tight to still be getting a tan plug. Is there a chance your jets were ever drilled out to a larger bore by a PO, or have you already swapped them out yourself?

Try and trace that smell if you can, that might be the clue you need...

Also, high speed tire pressure for the rear wheel on a maxim 650 is 36. That's where I usually kept mine, when I had lots of weight and was going for hours straight at 90+ in Mexico... but I'd probably leave them at that if I kept the bike anyways.

How does your bike act when starting in cold weather?

No one mentioned exhaust back pressure. Are your header gaskets good, no air leaks there? Stock pipes? No drilled out baffles? That would contribute to increased airflow and thus worse mileage and the tight idle mixture screws...

Whatever the case, if the bike is running great otherwise, have you considered trying to compensate for the mystery factor by just getting smaller jets and seeing how the bike responds?

 

For finding an intermittent issue with your fuel level. You can do multiple plug chops under varying riding styles, but always under load. You can check for excess gas coming out the pipes at start up, and you can carefully examine your float/needle valve assembly. Inspecting the operation of the two in concert, examine the seat and valve to see if they match up well (magnifying glass needed here with a bright light). Finally you could try to simulate riding conditions with a hose attached and see if it begins to overfill. None of these methods are easy or fool proof but they might show it. Otherwise you could simply replace them to be sure.

I am suspecting it's a slow leak either in the carbs or somewhere from your tank to carbs because you really shouldn't be smelling gas. It takes very little gas exposed to be able to smell it, and over time even a little bit of gas leaking would destroy good fuel economy.

 

The exhaust is stock, and as far as I can tell there are no modifications or holes drilled. I measured the jets and my initial measurement indicated that they are the stock size, and they are marked as stock. I'm going to check them again if I end up removing the carbs again.

 

One thing that hasn't been mentioned is the ignition system. The plugs fire well and there is no skipping, so I have assumed that everything is fine in that area. I don't even know if a weak spark could even affect mpg that much. Any comments on that?

 

Rider weight, lol




Just sayin............

I don't know if it's been mentioned, but I'm just thinking out loud here. What about the tires? Proper pressure? Not a weird size for the rim?

What about the kind of fuel you're putting in there. Which grade?

And if you had spark issues, you'd probably be having other keeping-the-engine-running issues.

 

That's for the stock tire that came on the bike back in the 80's. If you have anything other than stock (I hope so), you want to go by the pressure listed on the tire. Every manufacturer is different.

 

Your mileage is off by more than 30% of what you should get, if you have combustion in all 4 the only difference that a plug, with a weak spark, can make is in the 5% range, so that's out. Compression would be a more likely culprit in that area, but you said it was good.

You can check primary and secondary resistance to eliminate the ignition as a factor. Some peoples definition of a fat spark is 'I can see it' but even a failing coil will still arc a plug-still I don't think it's your issue.

 

Actually, not really. The pressure listed on the tire is generally the MAXIMUM pressure.

What you DO want to go by is the new tire manufacturers' recommendations, not Yamaha's 30-year old recommendations. For instance, the Avons I'm running do need significantly more pressure than the stock recommendation (but still not the max pressure printed on the tire.) In the case of Avon, the recommendations are on their website.

 

Your weight.
Tire Pressure.
How you ride.

The prime factors for economy.

Engine "Condition"
>Compression vitally important.

Carbs:
>Diaphragm Piston "Stiction"
Search or Click-on CLUNK TEST

If the Diaphragm Pistons are not rising and falling without resistance;; the Bike performs poorly.
If the Pistons RISE under vacuum, ... but fail to fall when not under load ... gas gets wasted by being allowed to flow out of the Emulsion Tube at a greater rate than needed --> TOO RICH for low manifold vacuum cruising and coasting.

Look at your Spark Plugs.
They should be relatively NEW, ... Chocolate Brown, Dark Tan or Brown-paper Bag colored.
If they are "Sooty" or extremely dark in color, ... investigate why you have a RICH condition.

• Wrong Jets (Main and Pilot)
• Egg-shaped Emulsion Tube Orifices
• Worn or Altered Needle Valves.
• Tuning:
> Float Heights
>> Fuel and Air Jets
>>> Air Cleaner
>>>> Tire Pressures
>>>>> Leaks
>>>>>> Miscreants siphoning or stealing gas.

 

I did a good compression check and got 135, 142, 138.138. So unless someone sees a problem with that, I'm assuming they are ok and will eliminate that as a problem.

 

Very respectable.

You're in good good shape.

What do the Plugs look like?

 

Take all four and throw them in gasoline and see if one or more is out of sorts with the others. I did mine on my 454 and found out I had a bad float. A bad float or two would really kill mileage.

You don't even have to use gas. I used water instead.

 

These were taken after idling for a few minutes while colortuning. I plan to do a chop check today or tomorrow. The pilot screws are presently still set at 1 1/4 to 1 3/4.

In reading your carb instructions you seem to indicate that the enrichment plungers, or valves, were tapered at the end with a thin needle-like protrusion. The ones in my Hitacha 33's don't have any needle-like protrusion and don't appear to have ever had any.

 

Make sure your AIR PILOTS are in the correct place. If they're backwards they can cause this.

Air jets are BACKWARDS from fuel jets, in that you want a LARGE hole for pilot and SMALL hole for main.

Rick has a thread with pic of what goes where for this, in the top of your carb.

 

I'm pretty sure I checked this, but I'll look again. I'm getting desperate. Thanks

 

Did you go back to a standard paper air filter element yet?

 

Those Plugs are as close to PERFECT as you can get.
Nice coloration.
Color reached onto the Center Electrode Insulator.
If they were darker, ... I'd be concerned about wasting fuel.
But, since they are all Light Chocolate Brown, ... I'd say you were Dialed-in and can't improve on economy in a meaningful manor without DE-TUNING the situation to an "Undesirable" condition.

 

I am narrowing down the problem--I think. 1. Have not gone to the paper filter yet, using foam filter sliced in half. 2. The plugs pictures were taken after idling with the pilot screws at 1 1/4 to 1 3/4.

Rode this afternoon and did a chop test at about 60. #'s 3 & 4 were dark but not real bad--3 a little more than 4. #'s 1 & 2 were very black and sooty--like the inside of a stove pipe. I almost didn't get it started, but with the throttle wide open was able to get it going. Road it about another 10 miles and at times it ran a little rough even at speed. When I stopped to park at the ball park (grandson's football game) it would not idle and bogged down. After the game -- an hour--there was a puddle of gas under it and it wouldn't start even at full open throttle. ( I was running on primary since I had blocked off the petcock vacuum to see if that helped.) The plugs were soaked, but after drying them and spinning the engine to clear it, it cranked and got me home. So, at this point I definitely think that I have one or more--probably # 1 or 2--leaking float needles. The carbs are coming off and all will be replaced. When I get them off I will post pictures of the carbs just in case anyone sees anything else that looks wrong. Apparently the leak is not so bad as to show up on a static wet check, but leaks when the engine runs. That would seem to explain the very tight pilot screws I think.

The float seats look perfect, so do you think I need to replace them or just the float needles? The seats have the wire mesh top which I would like to keep if possible.

 

What kind of carbs on that bike? If they are The ones I think they are the o-rings around the seats for the floats are leaking by.

MN

 

You have to put-in the time and get everything copacetic before you can expect the best results.
I was thinking you had done the work and meet with disappointing results.

I guess you still got a ways to go.
Cleaning and rebuilding a rack of Carbs is a right of passage for all XJ Enthusiasts.

Order some Kits.
Go through 'em.
Get them all perfectly situated.
Polish the Bores.
Colortune, Sync and Ride like you stole it.

 

Right of passage yes, I would even call it the price of admission to being an XJ enthusiast. Seems like most who 'give up' or end up selling before succeeding just didn't want to dive in.

At least now you know why your mpg's have been suffering. You can keep the 'beanies' (screens) just need to clean them. If you're up for it go the 'full monty' on those carbs, new rubber (unless it already is), new needles & seats, and CLEAN! The decent kits come with the seats and needles, and some include new jets for about the same price. Whatever you need, XJ4Ever has it all. And Rick knows his stuff with the carbs on these bikes, his advice saved my inexperienced hide when I went in for surgery.

That way you know it won't be an issue again anytime soon. While you have them apart give your floats a good inspection and check that they all float well-I would also measure and inspect your float towers, make sure they aren't cracked.

 

I have done everything and I am sure the carbs are perfectly clean and the the slides pass the clunk test. The needle valves and seats appear to be good and I never had any problem with the static wet setting, which I did numerous times . Therefore, I didn't replace them. I didn't checked the float levels with the bike running which probably would have shown them to be leaking. And apparently the leak is not so large as to foul out the plugs under normal use, although it may have been getting gradually worse. In any case, I think I am getting to final solution--or at least I hope I am. Live and learn.

 

If you didn't move the petcock from PRI to RUN when you parked, the petcock was flowing full-strength to the carbs. Definitely replace the float valves/seats.

Another contributor that has not yet been mentioned: the battery/charging system. I know that my auto's fuel mileage is drastically affected when the battery is old/weak and continually needs charging. Easy enough to check after you fix your carbs.

 

I left it on primary on purpose since I was trouble shooting the situation. I had already checked the float valves several times, but always off the bike. From now on I'll try to simulate engine vibration some way, or even check the float level with the bike running to see if there is any change.

 

Right - and that makes sense while the engine is running, but when it is not running the fuel is still flowing through the petcock in the PRI position. So if you did not switch the petcock from PRI to ON when you parked at your Grandson's game and your float valves are iffy... You get a puddle (and maybe gas in the oil).

RE simulating engine vibration - maybe strapping a small-capacity oscillating sander, or muscle ache vibrator to the carb rack?

 

By the way, PRI stands for 'prime', not primary. (As in, to prime a pump'; it's not the primary petcock position.)

Just a small detail.

Sounds like you're finding your issue! New float needles is what my money is on. Dealing with that on my new-to-me FJ right now... sitting for a year dry dried them up, I imagine. Had a leaky petcock I just rebuilt, also--the two combined meant a puddle the morning after I brought it home...

..only problem was the puddle was in an enclosed parking garage. Apartment manager made a huge fuss and called the fire department, and then banned motorcycles from the parking garage... (Fire department said an hour later that there was never any risk, but the manager didn't care.)

Anyways, best of luck.

 

I have the float valve seat and needles out and will order new ones tomorrow. But these look perfectly good and seem to be soft and pliable, and they seem to seal well. I did notice the the valve stems had what looked like a dull finish which I assume might be varnish. I used 800 grit paper to polish the corners and also the inside of the valve. I wonder if that might have been causing one or more to stick? I never used carb cleaner on the needle because of the rubber tip, but I did soak the seats.

While I'm at it, what else could cause the float or valve to stick or get jammed? It all is very clean and everything moves freely.

 

Polish the Float Valve Seat Bores with 1000 or greater.
Roll-up a tight roll of Finishing Paper and cut the bottom off square.
Twist the roll into the Float Seat Bore and polish the Inside Diameter.

When done, ... Cut-off the inserted section colored by the tint of Brass before using the roll on the next Seat Bore.

 

I did that this morning even before getting your suggestion. Do you think it's worth trying them or just order new ones. The rubber tips look and feel good, even though they are a little shiny where they actually seat on the valve, which I would think is to be expected. It's a little hard to believe that the little bit of varnish or whatever was on the seats and pins would cause them to stick. They certainly seemed to move freely.

 

Replacing them is maintenance.
If they leak when setting Float Heights --> Replace.
If you can set accurate Float Heights; they're OK.

Some aftermarket Float Valves are sketchy.
Need polishing right out-of-the-bag.

They might be leaking because of GEOMETRY.
Eyeball the travel.
If the Wire Clip is causing the Pin to get c0ckeyed; Hang the Pin with the Wire on the other side.

 

Thanks. I'll give it a try tonight--after the Halloween ghosts and goblins clear the area.

 

I've polished the seats and the corners of the needles (mine are square). Is there any specific way the needles should be oriented?

I have them installed so that there is a corner exactly fore and aft. Tomorrow I will do a wet check with some source of vibration to check for leaks before trying them for the last time on the bike. Also, I have reversed the order of the float valve assemblies, i.e., put #1 in the #4, #2 in #3, etc. Since the leak seemed to be in # 1 and/or #2 I think this reverse order will help determine if the leak is a valve assembly or something else if it still leaks.

 

The original Float Valves are 30 years old.
If you go to Paris; you visit the Eiffel Tower.

If you Rebuild Carbs; you replace the Float Seats and Pins.

Begin by checking the Petcock for leaking of not.
New Kits.
Gaskets.

No need to experiment and wasting time figuring-out you have to replace a Petcock and put Kits in the Fuel Bowls.

 

I've already rebuilt the petcock wit all new parts and it doesn't leak. I already have the carbs back together, and can't get new valves by this weekend, so i will probably give the polished valve assemblies a try, and then order the replacements.

 

Got it back together and did the wet check, and all are about 2mm below lip (the specs say 1mm +/- 1mm). So I left it hooked up to the gas for a couple of hours to see if anything changed or if anything leaked. When I checked it there was some gas seeping out from under the lip on 3 and 4, but the wet checks still were good. I generally do not tighten the 4 float bowl screws, so I tightened them and the leaking stopped. The carbs were level, fore and aft, so I am assuming that capillary action was the cause of the seeping fuel--or something is wrong with my wet check method. Two mm seems to be a lot for capillary action. Any comments?

2 hours later. I left the gas hooked up and gas was coming out of the throat of # 1 and the wet check verified that. I guess I have a slow leaking float valve.

Should I get valve seats and needles, or just needles? Hard to see how a rubber tip could damage the brass seats.

 

Call me hard headed, curious, or just stupid, but having read some other posts on problems with valve seats and needles, I decided to give it one more try. Using 2000 grit wet/dry paper I have polished all needles, all valve seats, all float tangs, all float hinge pins and the inside of the float hinges, and even the tip of the spring extension on the needles. I also cleaned the rubber tips with a q-tip and carb cleaner and immediately washed with gas. I re-installed everything, hooked it up to a free flowing gas source and left it for 2 days, during which time I shook the carbs frequently. No gas leaks and the fuel levels stayed constant.

I re-installed the carbs last night and will sync them tonight. Wish me luck.