Author Topic: Fuel efficiency on a CB750K1 (Read 6334 times)

Raul CB750K1 · « **on:** May 03, 2005, 02:01:33 PM »

Maybe some of you remember I posted the same topic in the old forum. I'm getting 6,1 l/100 km, or 38.72 mpg. I have to switch to reserve at 125 miles on a full tank. For me it seems way too high consumption. The plugs are sooty black, that means clearly a rich condition.

I was putting the blame on the air filter, that was for the later models. But this weekend I rode with the airbox open (to let the air bypass the filter) and there was no difference. Carburetors were rebuilt about 1000 miles ago, and for the life of me I would say the float height was correct. Measured a the point where the float tab starts to press the needle (before compressing the needle spring). The rest, jets and needle are completely stock.

Timing is also well adjusted, static and dynamic. Where can the problem be? Or is that fuel efficiency just normal?

I poured some Vegemite on the tank but to no avail. The only problem is that now my bike is surrounded by birds at all times...;-)

Jonesy · « **Reply #1 on:** May 03, 2005, 02:38:10 PM »

On my K3 I have to switch over to reserve at 180 Miles. Looking at a few specs, this sort of makes sense; If I remember right, the K1 used 120 mains, had bigger openings in the airbox and the HMC300 pipes, which had less restriction. My K3 has 110 Mains, later airbox (=smaller intakes for noise control) and HMC341 pipes (more baffling=restriction). My plugs look good, but I run D7EA's, one notch hotter than normal.

This being said, it seems to me you'd go through the fuel a bit faster, but I don't know if a spread of 55 miles would be acceptable. When you rebuilt the carbs, did you install all brand-new jets, needles, etc? Or were they cleaned and re-used? One thought could be if the parts were re-used, the needle jets could be worn slightly, richening your mixture. I'm not sure what else it could be at the moment. If I think of anything else I'll post it.

Dennis · « **Reply #2 on:** May 03, 2005, 02:53:46 PM »

I'm getting about the same as Raul.
'75 CB750F, with aftermarket muffler and K&N in the airbox it was lean so it has 115 MJ

Raul CB750K1 · « **Reply #3 on:** May 03, 2005, 02:55:05 PM »

Supersport, I don't live in US but in Spain, and here on liter is about one euro, or .80 GBP, so it's not that cheap.

jonesdp, I rebuilt carburetors with the original parts, thoroughly cleaned. The bike had only 5000 miles, but after being sitting for 10+ years it needed a good cleaning, but everything was fine. Used spray carb cleaner and compressed air to clean jets, not wire or similar so they were not enlarged.

Anybody out there with a CB750K1 who can give some info on how much his bike drinks?

Raul

Gordon · « **Reply #4 on:** May 03, 2005, 05:46:32 PM »

Raul,

I'm getting about the same mileage (kilometerage?) as you. Sometimes a little less, but I'm pretty heavy on the throttle in city traffic.

Stock everything except no baffles in exhaust. I'm in Denver, CO, so I'm sure the thin air plays a part in reducing the efficiency.

MetalHead550 · « **Reply #5 on:** May 03, 2005, 06:12:23 PM »

I ride a 75 550 but can I sneak in? I get 37mpg....normal?

Jonesy · « **Reply #6 on:** May 03, 2005, 07:23:03 PM »

Quote from: Raul CB750K1 on May 03, 2005, 02:55:05 PM

jonesdp, I rebuilt carburetors with the original parts, thoroughly cleaned. The bike had only 5000 miles, but after being sitting for 10+ years it needed a good cleaning, but everything was fine. Used spray carb cleaner and compressed air to clean jets, not wire or similar so they were not enlarged.

OK, I didn't know the bike had such low mileage. That rules out worn carb parts and it sounds like you cleaned things very carefully. Hmmmm.... I don't know what else to say. For all I know, it could be perfectly normal.

Kevin D · « **Reply #7 on:** May 03, 2005, 08:07:44 PM »

I get about 45mpg - 18/48 stock sprockets, 341 pipes with baffles pulled.

kghost · « **Reply #8 on:** May 03, 2005, 09:10:38 PM »

I'd say the gearing and driving conditions will play a factor.

More RPMS = more fuel

A steady state cruise = less fuel as opposed to WOT

Raul CB750K1 · « **Reply #9 on:** May 04, 2005, 12:24:20 AM »

I ride it on open roads, no traffic, cruising between 120-140 km/h (70-90 mph), so about 4k-5k rpm. I think plugs are not stock. I will replace them for the NGK d8S. If the ones that are fitted now are colder that could be a reason for them to get sooty black, but that wouldn't explain the high consumption.

C'mon, the rest of you, what is your average mpg? Say bike, riding style and if something is not stock (air filters etc).

Raul

P.S. I guess that if you are in the U.S. you don't know because you've never give a damn about it!!!!

Raul CB750K1 · « **Reply #10 on:** May 04, 2005, 12:26:56 AM »

I forgot to mention: chains and sprockets are the original (don't know how many teeth etc).

SteveD CB500F · « **Reply #11 on:** May 04, 2005, 01:50:57 AM »

Not a 750 but 45-49 mpg (UK gallon) everything stock.

andy750 · « **Reply #12 on:** May 04, 2005, 06:43:01 AM »

Stock CB750K4 (18/48 sprockets) -bafled OEM 4-4 pipes, stock air filter, cruising at 70 - 80mph on long road trips gets 150miles to the tank before reserve. i..e ~42mpg.
Cheers
Andy in Boston

eldar · « **Reply #13 on:** May 04, 2005, 07:32:56 AM »

Well i have a 78 750k and last I checked, I got about 40 - 45 mpg. If your plugs are sooty, you are running rich and should lean it out a bit. No matter the year of 750, plugs should not be sooty unless you are using the wrong ones.

I would say to put in new plugs that are in the "normal" range. If you still soot up, then lean the carbs out a bit. This a an older 750 but you should still get close to 40 mpg like about 38 or so.

Raul CB750K1 · « **Reply #14 on:** May 04, 2005, 07:59:21 AM »

I thought about it already (replacing the plugs). Correct me if I'm wrong, but shouldn't I fit a hotter type to lean the mixture?

In case I have to lean the carbs out, how can I do that? If I replace the jets, bike would not be stock anymore. Should I have to reduce the float height?

Raul

Dennis · « **Reply #15 on:** May 04, 2005, 08:09:59 AM »

Since you have not done plug chops, and since it appears that your plugs are getting sooty at cruise speeds, have you considered lowering your jet needles?

Raul CB750K1 · « **Reply #16 on:** May 04, 2005, 01:49:01 PM »

Dennis, yes I did. But shouldn't they remain in the stock position? (center notch)

Raul

Dennis · « **Reply #17 on:** May 04, 2005, 02:39:38 PM »

You would think so, but who knows how they ran 30 years ago. I worked on a few back then but I don't recall. If they were set to be borderline rich to protect the valves a little wear to the needle or jet could require an adjustment.
BTW I have the needles lowered in mine. Remember no internal emgine mods, just a minor exhaust and airbox change and larger main jets to accomodate that, which should not affect part throttle operation.
I don't know what else to tell you except that the only person I can think to ask is Bryan Jones. I will try to remember to ask him the next time we speak. You can send me an email to ask until I finally get an answer if you like. Or email him direct.

eldar · « **Reply #18 on:** May 04, 2005, 02:49:00 PM »

well adjusting your main needle will change the mixture, same with your pilot screw. I would use these first since changing to a hotter plug will also cause the engine to run hotter I think.

cb650 · « **Reply #19 on:** May 04, 2005, 03:47:49 PM »

With only a couple exceptions I have always got around 60 mpg with my 650. High of 67 and a low of 55. Dad always did to. 55 is some blasts of gas and 67 is puttin with my girlfriend on her hardly.
My 360 is getting close to 50. But that is city and my lard ass twistin the gas alot.

Terry

BobbyR · « **Reply #20 on:** May 04, 2005, 07:07:15 PM »

Here in the Peoples Republic of NY I get about 35 MPG on the "winter mix". The "summer mix" has a bit less alchohol so I can get 38 MPG. Gas here is $2.45 of 87 Octane which the bike runs OK on. I did get my hands on some real gasoline used in the Craftsman Truck Series. It made a real differnce in performance I am not sure about mileage since I spent every weekend spanking Harleys.

kghost · « **Reply #21 on:** May 04, 2005, 07:21:52 PM »

Whatever gas costs where your at, and regardless of the milage your getting......

Its still cheaper than Therapy!

Einyodeler · « **Reply #22 on:** May 04, 2005, 08:06:46 PM »

Quote from: kghost on May 04, 2005, 07:21:52 PM

Whatever gas costs where your at, and regardless of the milage your getting......

Its still cheaper than Therapy!

Amen to that,brother!
I`m originally from Germany and we were paying the equivalent of $4.00 a gallon 20 years ago,now that gas here in the States has gone over $2.00 all you here is complaining at the pumps while they are drinking a 12oz. bottle of water that cost $1.19.Maybe if it goes up a little more we can get some of these house sized SUV`s,with the drivers talking on cell phones and putting on makeup or shaving while driving, off of the roads more often.
My 500 gets between 35-40 mpg but I don`t putt around,I like to keep those revs up in the sweet spot

kghost · « **Reply #23 on:** May 04, 2005, 09:18:50 PM »

Das gut Mein Herr. Nothing better for blowing out the "cobwebs".

I do find it interesting that Motorcyclists don't tend to suffer from "road rage".

I will confess to occasionally suffering from "Road Terror"

followed by rage

, but really only when they are trying to kill me

!

Raul CB750K1 · « **Reply #24 on:** May 05, 2005, 01:29:39 AM »

Guys, thanks for the anwers. I'll wait for Bryan's advice, but I think I will lower the jet needles a notch and see what's the difference.

But I have a question about the spark plugs. Fitting a hotter type can lead to avoid the sooty black, but it shouldn't affect the gas mileage as the mixture is done before the ignition, isn't it?

And second, the heat grade of the spark plug shouldn't affect the running temperature of the engine, or should it? If I'm not wrong, the heat type means the the plug have more porcelain to dissipate heat, so depending on how hard the engine runs you need a type that can cope with the environment it works in. Isn't it?

Raul

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Author Topic: Fuel efficiency on a CB750K1 (Read 6334 times)

MetalHead550

eldar

eldar