Author Topic: 1978 CB750k, not starting-Has spark and fuel to the carbs (Read 2320 times)

jpresler.adm · « **on:** February 26, 2012, 09:46:12 PM »

About a month ago I went on a long trip bout 300 miles, I was sitting at a stop sign and idling when the engine gradually turned off, like if you hit the kill switch. It would not start after that with both the kick and electric start. I have since then replaced the condensers, points, coils, sparkplugs and wires and connectors, and ignition switch. I currently have spark on all plugs and have fuel to the bowls in all four carburetors. I don’t really know what could keep fuel from reaching the cylinders if it is making it to the carbs, I did clean them two days ago, everything looked great for it sitting with fuel in it but went ahead and thoroughly cleaned each carburetor. I have tried using both the electric and kick start after replacing each item with no luck on starting the bike. I am really confused and would appreciate any advice and or insight. If you need any additional information about the bike respond to the forum and I will reply asap.

Thank you all in advance

scottly · « **Reply #1 on:** February 26, 2012, 09:59:00 PM »

Do you have compression? Have you checked tappet clearances? Are the spark-plugs wet or dry after attempting to start it?

jpresler.adm · « **Reply #2 on:** February 26, 2012, 10:01:27 PM »

plugs have been dry, i am checking compression tomorrow

scottly · « **Reply #3 on:** February 26, 2012, 10:07:27 PM »

Try squirting some starting fluid into the carb throats with the throttle opened, then try starting it; if it fires and runs for a few seconds, it indicates you have a fuel/carb problem.
PS Welcome to the forums!

SOHC Digger · « **Reply #4 on:** February 26, 2012, 11:24:51 PM »

Quote from: scottly on February 26, 2012, 10:07:27 PM

Try squirting some starting fluid into the carb throats with the throttle opened, then try starting it; if it fires and runs for a few seconds, it indicates you have a fuel/carb problem.
PS Welcome to the forums!

Agreed. You may have fixed your original problem already, but if the bike has been sitting, the fuel may have turned on you. This happened to me just a few months ago. If the bike fires even once on starting fluid, drain your tank and carbs and fill with some fresh gas.

bollingball · « **Reply #5 on:** February 27, 2012, 05:21:09 AM »

jp Getting fuel to the carbs is good but you have to get it out.
+1 on the start fluid. That will be the fastest way to tell. If you have good spark and your air filter is good it will fire with the start fluid even with very low compression. Did you remove the slow or pilot jets? Tell us exactly how you cleaned your carbs. Did you do a 3k tuneup after you put all those parts in?
Why did you change all those parts?? Did they all test bad?

Ken

steam-powered man · « **Reply #6 on:** February 27, 2012, 06:42:04 AM »

check fuel flow, you may have a partially plugged in-tank mesh filter, or in-line fuel filter.

check or replace the fuel lines, they may have crud in them that re-fouls your idle circuits after you've cleaned the carbs. then re-clean the idle circuits with a guitar string

turn up the idle screw

remove any droop in the fuel lines

i've had all of the above at some point, that stopped me from starting.

jpresler.adm · « **Reply #7 on:** February 27, 2012, 06:33:28 PM »

got called into work today so missed getting a compression tool today, I started with the replacing of the electronic componets by starting with no spark and working form cheapest to expensive. The coils fail on the resistance test and for fun and ideas I took the pair to 4 cycle shops, two Honda dealerships and 4 small motor shops to see if they could test them, no one could so replaced those. as for condensers and points they were only 25 for the sets so easy and cheap replace. The spark plug wires 1 and 4 had some corrosion so decided to replace all four caps and usually replace my plugs every two years, just happened to be this year. I did try the starting fluid, first by spraying through the air filter and then without them on,( i have the K&N filters 4 separate) still no firing, I also tried spraying some into the ports that i would use if i were carb syncing, all with no firing of the engine. I pulled the spark plugs out and all four smelt like gas and starting fluid.

Steam-powered man in response to your questions-I removed the in line filter and have a direct downhill feed from the tank to the carbs, hose is brand new and tank was cleaned a week ago by draining, using a small amount of solvent and and after draining again i added some small metal balls and shook around then again with the solvent and draining. I left it in my garage to air dry for the rest of the week.I just refilled it with fresh gasoline three days ago. Is it possible for the lines in between the carbs to also be in need or replacement. Also I am sure as soon as you reply I will figure out what the idle circuits are but as of now I don't know what they are.

bollingball-
As for cleaning the carbs all i did was to remove the bowl covers and clean the bowls, also removed the single jet and cleaned the jet and port. I replaced the gaskets not that they were leaking but they looked thin. not sure what the slow jets are. and i did not do a 3k tune up but if that is the next step I will start it tomorrow.

scottly · « **Reply #8 on:** February 27, 2012, 07:45:37 PM »

Check your ignition timing. You have fuel, you have spark, you need to make sure the spark happens at the right time. (It's doubtful that your compression suddenly went away on all four cylinders) Search "static timing".

jpresler.adm · « **Reply #9 on:** February 27, 2012, 10:43:10 PM »

hey scottly, checked the timing against the static point. that as i rotate the points should release just after 'F' lines up with the static point and each contact releases 2-3 and 1-4 respectively. Was this what you ment by checking it?

lucky · « **Reply #10 on:** February 28, 2012, 07:12:19 AM »

What makes you think this is an electrical problem???

Just take a clamp on inductive timing light and clip it onto each spark plug wire while it is running. That will tell you if each cylinder id firing.

Timing lights are not just for dynamic timing ,they are good diagnostic tools.

BTW since this is a 1978 you better find out if your accelerator pump and spray nozzles are working. If they do not work that bike will be running very lean part of the time.

One more thing disconnect the fuel hose from the carbs(#2) and but the end of the hose in a container. TURN on the fuel valve. Is there fuel coming out of it?

Does it have a inline fuel filter???
If it does not have a inline filter the screen in the gas tank fuel valve could be clogged with rust.

Check basic things first, then move onto timing ,dwell, points gap, and dynamic timing.

jpresler.adm · « **Reply #11 on:** February 28, 2012, 12:58:21 PM »

so checking some wiring today and noticed the black wire from the turn signal buzzer is not pluged into anything. Tracing on the block diagrams I dont think this would have an effect on the ignition system. Any thoughts or am I just hoping for the "magical fix"?

Spanner 1 · « **Reply #12 on:** February 28, 2012, 06:06:54 PM »

When your bike died at a stop light and you took it home, did you charge the battery before doing the plug firing test ?? When it died, did the starter still spin the motor fast ? Was the headlight good and bright ?
Seems to me your battery just slowly depleted enough for the ign. to fail and your problem may be a no charging/ low charging deal..........

bollingball · « **Reply #13 on:** February 29, 2012, 09:02:39 AM »

quote from jp) I don’t really know what could keep fuel from reaching the cylinders if it is making it to the carbs, I did clean them two days ago, everything looked great for it sitting with fuel in it but went ahead and thoroughly cleaned each carburetor.

From jp) Also I am sure as soon as you reply I will figure out what the idle circuits are but as of now I don't know what they are.

This does not add up you can not thoroughly clean them with out removing the jets.
Sounds like spanner could be on to something. Check your battery and charging.

Ken

my78k · « **Reply #14 on:** February 29, 2012, 09:15:10 AM »

3 words...plugged idle jets

Keep in mind on the 78's the idle jets are pressed in and do not unscrew. They will need to be pulled....use vise grips with a rag to protect the brass and gently pull/twist.

I would bet money they are plugged up.

Dennis

Gonzowerke · « **Reply #15 on:** February 29, 2012, 10:22:28 AM »

My '78 sat for years before it was given to me. Inside of the tank looked good, being my first bike, I never looked at the carbs, just threw a new yuasa at it and some tires. I got 10 miles or so away from my house before she died. Gunk that was in the carbs was loosened by fresh gas and clogged my slow jets and a passage or two. A good cleaning with gum cutter or the like will do wonders. Also, if you are not comfortable pulling the slow jets, use a guitar string or a welder's torch tip cleaner to clean it out. Remember! Brass is soft, and you do not want to enlarge the hole! Seeing as you rode 300 miles though, I'm going to go with either rust from your tank, or a bad batch of gas from your last fill-up. That also happened to me at Daytona once, I filled up, and got a tank of mostly water!

All this assumes it is a fuel problem, and not electrical. Are you running the stock points in the ignition? If your condensers die, so does the bike. Again, this happened to me! It made me go out and get a Dyna S the next day.

bollingball · « **Reply #16 on:** February 29, 2012, 02:11:12 PM »

From Gonzo if you are not comfortable pulling the slow jets, use a guitar string or a welder's torch tip cleaner to clean it out.

I would like to see how you do this with out taking them out. See those tiny little holes on the side. You have to remove the jet to clean those.
jp this is your slow jet. #35 stock Should have eight holes. I still think you need to make sure you have a good hot battery.
What does your in tank filter look like?

Ken

Gonzowerke · « **Reply #17 on:** February 29, 2012, 08:10:08 PM »

Quote from: bollingball on February 29, 2012, 02:11:12 PM

From Gonzo if you are not comfortable pulling the slow jets, use a guitar string or a welder's torch tip cleaner to clean it out.

Having never seen one out, I did not know about the side holes. I used a tip cleaner on the main hole with my jets in and everything was okay, maybe because I also used compressed air to blow it out, I don't know, it just worked for me.

jpresler.adm · « **Reply #18 on:** April 03, 2012, 01:07:48 PM »

First I want to thank everyone who took the time to read the post and respond. It helped so much everytime i got stuck and my brain couldn't think. Sorry for the long time in response but i was required to leave the country for a bit but back now and after taking the valve cover off, discovered that the two bolts holding the camchain sprocket to the camshaft had come out. I found one inside a groove near the chain and the other i found several parts of after it had been ground up by the gears. I ended up taking the whole case apart to clean and check for additional damage. Everything looked great and found most of the other bolt stuck to a magnet inside the crank case. I just finished putting everything back together and am waiting for the pipes to return from the chrome shop. I hope to have everything back and ready for the sun whenever it shows up. if not I'll get my rain gear out and enjoy it either way. Again thank you so much for the support, advice, and insight.

jp

LesterPiglet · « **Reply #19 on:** April 03, 2012, 01:51:58 PM »

Wow, what a turnup.

Let's hope no other damage has occurred.

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Author Topic: 1978 CB750k, not starting-Has spark and fuel to the carbs (Read 2320 times)

bollingball

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