Non BMW machine

[simonpeter]

I dont know if this is allowed but i will ask anyway. I have just aquired a 1981 CB900f. Cant get it to fire up. There is a good spark, plenty of petrol getting through and the coils are wired correctly. Not a peep or even a backfire! Any ideas?

[notacop]

Are the plugs getting wet with petrol? You may have fuel to the carbs, some of these things had vacuum dealies that was a fuel cut off, are the hoses good? Sure about the timing?

Are the rubber connectors on the carbs dried out?

Does it cough with starter fluid? Use WD40 if it's available. That will seal some vacuum leaks too.

[T__]

I dont know if this is allowed but i will ask anyway. I have just aquired a 1981 CB900f. Cant get it to fire up. There is a good spark, plenty of petrol getting through and the coils are wired correctly. Not a peep or even a backfire! Any ideas?

 

 

Simon, well IF you have compression,, ENOUGH spark at the correct time,, ignitable fuel/air reaching the cylinders it WILL run.. What are you missing?

 

 

_Do you have decent compression?

 

_You say spark? Do you get a bluish colored spark across a 3/16” gap? Sometimes you can get spark out in the open but not enough to spark under compression..

 

_You say it is getting fuel? Are the spark plugs wet with fuel after cranking? If not maybe not as much fuel as you think..

 

_Is the spark appearing at or near TDC compression? If not find out why..

 

_Is the battery FULLY charged & in good shape.. If it cranks slowly the system voltage could be below what it takes to allow good spark under compression.. Check the coil voltage during cranking..

 

_Is the gasoline any good? Is it gasoline & not some strange storage fluid?

 

Twisty

[simonpeter]

The plugs are getting wet with petrol, as for the ignition timing I am not sure. It has an electronic system, can they go out of setting?

[T__]

The plugs are getting wet with petrol, as for the ignition timing I am not sure. It has an electronic system, can they go out of setting?

 

Simon, anything is possible.. Maybe try a timing light to see where the spark is occurring..

 

Take an old (known good ) spark plug (any type),, open the plug gap to 3/16” (use a 3/16” drill bit as a gap gauge),, place that in a plug wire & lay on the cylinder,, DO NOT remove the original spark plugs,, then crank the engine & look for a bluish colored spark across that 3/16” gap.. If red, orange, pale or erratic suspect low battery voltage or a spark problem..

 

If a good spark per above suspect poor ignitable fuel or bad spark plugs.. Does it have compression?

 

Twisty

[sardineone]

Simon, just one suggestion if the bike has old fuel in the gas tank. Get some fresh fuel and a new set of spark plugs as old fuel can foul spark plugs. I had a bike in perfect running order in the past and let it sit for 3 months waiting to sell for a new bike on order. When trying to start it got more and more difficult come time to sell. Fresh plugs and fuel made all the difference. If the bike has been sitting for awhile with old fuel your low speed jets in the carbs may be plugged as well.

 

PS: I used to have a bike related to your 900, an 83 1100F.

[Rich_O]

Sometimes, old fuel just plain does not work.

 

Just went through that experience dusting off my roto-tiller that was sitting for a year or two.

Rich

[simonpeter]

Thanks for all your advise gents. Certainly some food for thought! The petrol is not an issue. I syphoned a gallon from my 1100rt. Fuel only 4 days old. I will check the spark under compression next. I think i need to do a compression test. If there is any progress i will let you know.

[T__]

Thanks for all your advise gents. Certainly some food for thought! The petrol is not an issue. I syphoned a gallon from my 1100rt. Fuel only 4 days old. I will check the spark under compression next. I think i need to do a compression test. If there is any progress i will let you know.

 

 

Simon, if the compression is low (might be if all the oil has drained/washed off the cylinder walls) just use an oil can & squirt some light motor oil into each cylinder through the spark plug holes.. In a lot of cases that will restore the compression enough to start the bike..

 

Twisty

[simonpeter]

Thank you for that little nugget of wisdom. Sounds like a plan. I have just checked the spark under compression, held clear of any earth there was no spark. However with the plug resting on the rocker cover there was lots of blue sparkyness!

I will continue to plod on. We will get there in the end. something daft i am reasonably sure. Cant see the wood for the trees.

[harleyjohn45]

How long has the bike been sitting. Hondas have 4 carbs with very small jets. they get clogged up. you may be mistaken about getting fuel. if you have fire, then it probably will be clogged carbs. will cost more to rebuild the carbs than the bike is worth.

[simonpeter]

Success at last! I was talking to a mate at work who used to have a 750 version. We discussed what had been done so far. It came up that the air box has no cover. Tonight i made one from an old paint tray, pressed the button and bingo! Lots of TLC now required to get her finished. Thanks for all your advice. When its don I will post some pics.