what kind of gas to use
March 24, 2009 9:00 PM Subscribe
Completemechanicalidiotfilter: Does it matter what kind of gas I use in a 1979 CB400?
I've recently bought a 79 CB400 Hawk Type I and wonder if it matters what kind of gas I should use. I have the manual but it doesn't mention anything about it. I've been putting in regular unleaded and it's been running fine except the throttle is not all that responsive in the lower gears but someone told me this might be more to do with the carbs needing to be synched. On a side note, aside from studying the manual what are some good complete idiot motorcycle maintenance sites? Most of the places I've tried expect a fairly decent knowledge and I have pretty much none.
I've recently bought a 79 CB400 Hawk Type I and wonder if it matters what kind of gas I should use. I have the manual but it doesn't mention anything about it. I've been putting in regular unleaded and it's been running fine except the throttle is not all that responsive in the lower gears but someone told me this might be more to do with the carbs needing to be synched. On a side note, aside from studying the manual what are some good complete idiot motorcycle maintenance sites? Most of the places I've tried expect a fairly decent knowledge and I have pretty much none.
In the last few years they reformulated the gasoline in some (most?) areas to have more oxygenating agents, ethanol for one. This new gas runs leaner than old gas, and thus runs hotter and often pings if not rejetted. So, since motorcycles get such good mileage, and excessive heat or pinging are bad for engines, it might make sense to run premium. The higher octane of premium will prevent pinging and to some degree running hot, but more for the pinging. Also, just because you can't hear pinging doesn't mean it isn't happening, engine clatter can cover the sound.
posted by jester69 at 9:22 PM on March 24, 2009
posted by jester69 at 9:22 PM on March 24, 2009
Response by poster: That's awesome jessamyn, had no idea you rode. I occaisonally get pinging but not after the bike has been warmed up for a while. I've read that higher octane gas can do damage to engines and I've also heard that it will improve performance and NOT harm the engine. So far I've been sticking with good ol cheap gas but maybe I'll try a few tanks of premium and see if the pinging goes away.
posted by BrnP84 at 10:02 PM on March 24, 2009
posted by BrnP84 at 10:02 PM on March 24, 2009
It was a long time ago, and I was mistaken it was a CJ360T.
posted by jessamyn at 10:15 PM on March 24, 2009
posted by jessamyn at 10:15 PM on March 24, 2009
Best answer: This is something you should ask the mechanic who services your bike for advice on.
Firstly, because fuel varies very significantly in quality and composition area to area, your mechanic has far more knowledge than most about who's selling good fuel and who is selling bad. Secondly, because in thirty years of use, your CB400 has almost certainly been serviced and reserviced to the point where the jetting in its carburettors is unique and unlike other bikes of the same model. Thirdly, because the composition of fuel is constantly changing in response to changes in regulation and world demand---the addition of ethanol to fuel in particular is a controversial point amongst bike mechanics.
posted by Fiasco da Gama at 10:25 PM on March 24, 2009
Firstly, because fuel varies very significantly in quality and composition area to area, your mechanic has far more knowledge than most about who's selling good fuel and who is selling bad. Secondly, because in thirty years of use, your CB400 has almost certainly been serviced and reserviced to the point where the jetting in its carburettors is unique and unlike other bikes of the same model. Thirdly, because the composition of fuel is constantly changing in response to changes in regulation and world demand---the addition of ethanol to fuel in particular is a controversial point amongst bike mechanics.
posted by Fiasco da Gama at 10:25 PM on March 24, 2009
I had a CB400-Four SuperSport for several years, and I always ran basic, low-octane unleaded in it. I never had a problem.
Actually, the headlight burned out. And it needed new tires and front brakes. And one of the rear turn signals shorted out...
posted by jon1270 at 1:45 AM on March 25, 2009
Actually, the headlight burned out. And it needed new tires and front brakes. And one of the rear turn signals shorted out...
posted by jon1270 at 1:45 AM on March 25, 2009
My Honda CB350 runs on plain-Jane "gas". No additives, no cleaners, no oxygenated hydro-fluoro-carbonizators... just take whatever the base octane is at the pump (87/89).
posted by Civil_Disobedient at 2:52 AM on March 25, 2009
posted by Civil_Disobedient at 2:52 AM on March 25, 2009
Best answer: I owned a CB500 for a little while and ran it on basic, cheap unleaded with no problems. (Well, the bike had lots of problems, but none of them were gas-related.)
With a bike that old, there's a decent chance that the rubber boots connecting to the carbs are cracking, which will mess up your fuel/air mix something terrible. Check those before putting lots of hours into the carbs.
Are you familiar with the SOHC Four website and owners group? I found it to be a good resource when I owned mine, but that was a few years ago. Oh, whoops, a little more looking on google tells me that your 79 is actually a twin, not a 4-cylinder, so that may be less perfect for your situation, though much of the technical info will carry over, as will questions about what gas to use.
posted by Forktine at 4:49 AM on March 25, 2009
With a bike that old, there's a decent chance that the rubber boots connecting to the carbs are cracking, which will mess up your fuel/air mix something terrible. Check those before putting lots of hours into the carbs.
Are you familiar with the SOHC Four website and owners group? I found it to be a good resource when I owned mine, but that was a few years ago. Oh, whoops, a little more looking on google tells me that your 79 is actually a twin, not a 4-cylinder, so that may be less perfect for your situation, though much of the technical info will carry over, as will questions about what gas to use.
posted by Forktine at 4:49 AM on March 25, 2009
Best answer: I know the UK version of the bike. Believe it or not, bikes like this are now becoming bona fide classics. I've seen mint superdreams selling for silly money.
Anyways, it's a tough old motor and should be able to handle anything you throw at it. It's not running particularly high compression, or high revs. Knocking/pinking/pinging will be causing abnormal loads in the engine, so you should sort this out before it causes problems.
Lead was added to petrol to prevent this, nowadays engine timing is adjusted to avoid the problem. There arent any obvious alterations you can make to the engine; theoretically reducing the compression would work (take the head off and put a second gasket in would work, but, technically, this is a 'bodge'). Otherwise, high performance petrol should have additives which avoid pinking, or you can use an additive.
One symptom of pre-ignition can be the engine running on; you close the throttle but the engine keeps going for a second or two. As Forktine says, this is more likely to be an air leak at the carb rubbers.
The 400T should be (relatively) responsive in the low gears; it's a twin so should be producing good power at low revs. It may be that your carbourettion is all screwed up. What air filter does it have?
posted by BadMiker at 7:20 AM on March 25, 2009
Anyways, it's a tough old motor and should be able to handle anything you throw at it. It's not running particularly high compression, or high revs. Knocking/pinking/pinging will be causing abnormal loads in the engine, so you should sort this out before it causes problems.
Lead was added to petrol to prevent this, nowadays engine timing is adjusted to avoid the problem. There arent any obvious alterations you can make to the engine; theoretically reducing the compression would work (take the head off and put a second gasket in would work, but, technically, this is a 'bodge'). Otherwise, high performance petrol should have additives which avoid pinking, or you can use an additive.
One symptom of pre-ignition can be the engine running on; you close the throttle but the engine keeps going for a second or two. As Forktine says, this is more likely to be an air leak at the carb rubbers.
The 400T should be (relatively) responsive in the low gears; it's a twin so should be producing good power at low revs. It may be that your carbourettion is all screwed up. What air filter does it have?
posted by BadMiker at 7:20 AM on March 25, 2009
Response by poster: I just checked the air filter and it looks like someone cut out the paper of a home air filter and fitted it in the bike's filter. It looked fairly clean and didn't have much dirt or grime but I'll probably get the real thing sometime soon. My roommate has a guy that works on his bike, he's been pretty busy for the past two weeks but hopefully he can take a look at it sometime next week, thanks for the advice so far everyone.
posted by BrnP84 at 11:48 AM on March 25, 2009
posted by BrnP84 at 11:48 AM on March 25, 2009
Best answer: Basically, what increased octane does is decrease pre-ignition/pinging/detonation. If you have pinging, premium is a good idea, if you don't have pinging it doesn't really do much of anything. Running premium in a motor that could get by on regular doesn't do any harm that I am aware of, it just may be a waste of money.
posted by jester69 at 2:29 PM on March 25, 2009
posted by jester69 at 2:29 PM on March 25, 2009
Get the proper air filter, change the spark plugs and change the oil. It sounds like the bike just needs a bit of TLC; the wrong air filter will be upsetting the fuelling and may be making it run rough when cold or on choke.
When you change the spark plugs check their colour; the ceramic bit at the engine end should be a light biscuit colour. If they are black or sooty then the fuelling is too rich (not enough air), if they are white or blasted looking then it's too lean (too much air). If they are different colours then the carbs need balanced (easy job on your bike).
From the sound of things I would guess they will be black. If changing the air filter doesnt sort it out the poor running then check them again; if they are still wrong then take the carbs off and check the jet sizes.
If you have a manual (I could only find the UK version) and are organised and methodical then none of this is hard to do by yourself. PM me if you want a walkthrough!
posted by BadMiker at 2:11 AM on March 26, 2009
When you change the spark plugs check their colour; the ceramic bit at the engine end should be a light biscuit colour. If they are black or sooty then the fuelling is too rich (not enough air), if they are white or blasted looking then it's too lean (too much air). If they are different colours then the carbs need balanced (easy job on your bike).
From the sound of things I would guess they will be black. If changing the air filter doesnt sort it out the poor running then check them again; if they are still wrong then take the carbs off and check the jet sizes.
If you have a manual (I could only find the UK version) and are organised and methodical then none of this is hard to do by yourself. PM me if you want a walkthrough!
posted by BadMiker at 2:11 AM on March 26, 2009
This thread is closed to new comments.
posted by jessamyn at 9:02 PM on March 24, 2009