1979 MPG

[ajs350]

My recently acquired 79 is only doing 21mpg compared to my 80 which does 33mpg.

I do not see any leaks and have had the carbs off for a clean which gave me a chance to confirm the jets were all standard when referencing the manual. All other maintenance items have been attended to, timing set, valves adjusted, carbs synced, new plugs, oil & filter.

Other than just a little hesitation off idle the bike runs like a champ so I'm not quite sure why the mpg is so poor.
Any clues?
Ross

[Don]

I'm sure Dave will chime in with some things for you to check once he gets back into town . . . . I can't think of anything myself - You should get about 30 or maybe a bit better when everything is sorted out

Don

[EMS]

Maybe the choke butterflies are partially closed?

[Rick Pope]

Mike could be onto something. It's possible to get the chokes out of time, right v. left. Take your tank off and look down where the right engages the left, in between the 3rd and 4th carbs. There's a little pin that engages a hole in the joint between the carbs. If the pin is against the outside edge of the flex joint, half your carbs have the choke partially activated.

[ajs350]

I have the engine tilted and the choke butterfly's are operating correctly. I'll be disassembling the carbs again later so maybe I'll find something else.

[Larry Zimmer]

Ross, float levels? I'd think it has to be floats, jet sizes or chokes (as Rick mentioned).

One other thing, are all cylinders firing? Do you have anything to measure header pipe temps? (Some sort of infrared sensor.) Run down the highway 15 minutes, pull of at a ramp and measure temp immediately. Measure as close to the head as you can; same place on each pipe. The 6 runs rather smoothly on 5; even fairly smooth on 4 at highway speed. Or, it could be a partial mis-fire on two cylinders if a bad coil. (Two bum cylinders is, obviously, about 30% loss. That sounds about like what you are experiencing.) If temps are all close, back to the carbs. Good luck.

Help, Dave! Mike N!

[daves79x]

Ross:
When I get home I'll get back to you - this weekend hopefully. Still working flood restoration.

Dave

[steve murdoch icoa #5322]

Ross, what air filter are you using?
Maybe the slight hesitation off idle and mpg have been affected by your filter. I have always achieved best performance with the stock Honda filter.
Just another item to consider.

[ajs350]

Thanks for all the suggestions. I'm going through them one by one.
Carbs are off and the jets are std: #35 idle, #65 and #98 mains. Choke is working properly.
I did do a temp test before I took it all apart and header temps at idle were 135, 120, 125, 125, 120, 125 for #1 thru #6 respectively.
Air filter is K&N. I do have the orginal filter but no foam so I'll have to get that. I have a paper filter also which I can try first.
Float levels are next although I did these 2 weeks ago I'm going to check again.
Air tee's are missing but I do not believe that is material?
Ross

[ajs350]

Mike could be onto something. It's possible to get the chokes out of time, right v. left. Take your tank off and look down where the right engages the left, in between the 3rd and 4th carbs. There's a little pin that engages a hole in the joint between the carbs. If the pin is against the outside edge of the flex joint, half your carbs have the choke partially activated.

The float levels checked out fine at 15.5mm so I went through everything again. While the choke operated normally going from open to closed I noticed that when open butterflies 4,5 & 6 wobbled a bit. I disconnected the actuating pin that operates the 2 banks and the right butterflies tightened up. That's great I thought except the pin that controls 1,2&3 is now out of alignment with the hole in the arm that controls 4,5&6.

Rick, was this what you were refering to?

How do I get the pin and the hole to line up to sync the 2 banks?

First picture shows the missalignment of the Pin and the hole on the 2 choke arms.
The second shows the rotation of the choke shaft on the right for 4,5&6 if I reconnect the pin which then slightly closes the butterflies.

[daves79x]

Ross:
Your choke misalignment (stop making me spell these big words correctly!) could be part of the problem. I think you may have inadvertantly taken a couple of the correct 'kinks' out of the right-hand choke arm. It is supposed to have a couple of bends in it that allow correct alignment. If it is too 'straight' it may not align and result in what you have there. I know a very slightly tight choke cable will cause the chokes to be very slightly closed and reduce mileage, so you may be on to something. You can see upon close examination where the bends were. Just re=bend the arm so it aligns.

However, my initial thought is that someone drilled the pilot (#35) jets out a bit somewhere along the line. Did you actually gauge them to make sure? They may say #35 on them (I assume you extracted them), but you need to carefully gauge them to be sure. K&L makes a gauge set with the smallest wire being a #35. That's where I'd look further after you see about the chokes. And actually gauge the other jets also - after 30 years, no telling what someone did to them.

All that said, I have a set of VB62 ('80) carbs here that I completely cleaned and checked 2 or 3 times. They run great but only return a little over 20mpg. I replaced the #35s with new on es just to make sure and still no change. I plugged the aircut valve to eliminate that, still nothing. I gave up on them and they still sit that way. A mystery I may look back into sometime. I'm missing something, just don't know what.

I'm sure something you can find will fix yours. Did you shim the needles? I usually don't on the '79 carbs. Even so, you should get over 30mpg. Call me if you want.

Dave

[daves79x]

Ross:
Also, your K&N will just further lean out the entire mixture, as we've spoken about before. You are obviously looking for something causing richness. Just make sure it is not plugged with dirt and airflow is proper. Also make sure the exhaust isn't full of mouse nests or similar. I've seen them full of stuff like that. That will cause your problem also.

Dave

[ajs350]

Having spoken to Dave last weekend it seemed pretty clear that my needles being made of brass were after-market and the likely culprit of my poor gas milage.

There was also a possiblity that some of my jets had been drilled so I purchased a cleaning kit and by comparing my jet sizes with known jets I was able to confirm all jets were correct and no drilling had occurred.

So with that, in went the correct needles and after a 60 mile ride today I got just under 30mpg which is much improved over the 21mpg I was getting. I suspect that over a full tank of fuel I will do a bit better.

My thanks again go to Dave who was able to zero in on the problem which would have taken me way too long to figure out.

Ross

[ajs350]

Well, with a full tank I did not do much better and maintained a 28.5mpg average so while the new jet needles helped tremendously there was something else a miss. I checked the air cut off and all was well there. Checked the timing and made sure the float levels were correct and consistent across all 6 carbs.

Being at a loss I do what many of you do and called Dave. He suggested I check the wheel bearings as if worn they would cause some drag.

Front wheel was OK so I took the rear wheel out and it was fine also. However, once back in I noticed that the wheel did not spin much so I decided to clean up the chain and re-lube. Once this was done the rear wheel spun much more freely.

Spirited riding now delivers 31mpg while a nice easy ride with the odd blast takes it up to 33mpg.

Here's a picture of the aftermarket Kawasaki needle (Brass colour) alongside the Honda needle. Notice how thin the needle is at the end. It must have been letting a huge amount of fuel through compared to the Honda needle.

[pistolgrip1]

glag to see this post. i just picked up my 2nd x this one being a 82. bike sat for 6 years without being ridden, just started every once and awhile, 12,500 miles on odomtr. there is a flat spot between 3 and 4k rpm, i'm sure carbs need to have a cleaning. previous owner had TIMS put in a jet kit and carb rebuild.first trip on the bike i averaged just over 50 mpg. next trip i topped off the tank drove 134 miles and topped off again, 2.3 gals. this is with bike on side stand and filling to the bottom of tank. average speed 65 mph. anyone else getting mpg like this?