Topic: '77 CB750 starting issues SOLVED!

[Radski]

Fellow Forum Fellows,

So I've finally got all my parts back together and for some reason the engine is not firing up. Upon putting oil and gas is last night, one of the fuel rails on the carb decided to leak like a sieve. SO today I went and got some o-rings and low and behold its sealed up and holding everything.

I have spark on all spark plugs. I have proper timing and spark in the points. The battery just came off a fresh trickle. It turns over and is trying so hard to start...even has a little thump like its catching but it just wont fire up. I did have it fire up last night for about 6-8 seconds but I turned it off as this was before I fixed the carbs and had gas leaking everywhere.

I had this running 2 months ago before I took it all apart and cleaned it up. The ONLY thing different now is that I have a Cycle X 4-into-1 exhaust now where as before I was running just straight headers (no muffler) but had to basically run with choke on at all times. The carb is setup to run pods and jetted properly.

What am I missing here?? Any other suggestions of troubleshooting?

[BenelliSEI]

If you had to use the choke full time, in the past, the idle and main jets must be clogged. When you pull the choke on you create an intake vacuum and pull fuel in. Sounds like time to pull those carbs and really clean them out. The K7/8 carbs have notoriously small idle jets, I bet they are plugged up totally....... with the screw on float bowls, it’s really hard to do anything in place. They need to come off.

[Erny]

Did you check plugs? Are they dry or wet? Dry = no fuel

[Radski]

If you had to use the choke full time, in the past, the idle and main jets must be clogged. When you pull the choke on you create an intake vacuum and pull fuel in. Sounds like time to pull those carbs and really clean them out. The K7/8 carbs have notoriously small idle jets, I bet they are plugged up totally....... with the screw on float bowls, it’s really hard to do anything in place. They need to come off.

My plan is to do just that tonight. Clean all jets. And check float heights

[Radski]

Did you check plugs? Are they dry or wet? Dry = no fuel

In fact someone told me exactly the same thing. The plugs were generally dry so yes = no fuel or little. Gonna clean out the jets tonight.

[Erny]

Are you absolutely sure you have petrol flowing from tank?

Asking because it happen to me that by stupid mistake I turn petcock to main while I had just little fuel so only reserve would work... Took me 20 mins to find the problem

[Radski]

Are you absolutely sure you have petrol flowing from tank?

Asking because it happen to me that by stupid mistake I turn petcock to main while I had just little fuel so only reserve would work... Took me 20 mins to find the problem

Yes I verified the gas is above the petcock. Plus I see gas running through the petcock down the fuel line (clear line) into the carbs.

[Airborne 82nd]

Did you remove the slow jets and see light through every hole? Did you set up the choke to work with the fast idle cam and is your accelerator working on all four carbs?

[Radski]

Did you remove the slow jets and see light through every hole? Did you set up the choke to work with the fast idle cam and is your accelerator working on all four carbs?

woah woah woah there Tex! lol I'm a newb at best. When I had the bike running last time I had not cleaned the slow jets. THIS time however I took them out and even though I could see through them, I cleaned them with a VERY small wire, carb cleaner and compressed air.

I did find out however my floats were WAY out of spec. Turns out theyre supposed to be 12.5mm and well lets say the first one was at 20.  So I got all them very close to spec and after a few mins of humping and hollering....the bike started!

Without choke even! And now I barely have to touch the start button ...it bursts to life. First kick even....well tonight so far.

So thanks for all the suggestions...def cleaning the carbs and jets and making sure the floats were in spec was what it needed. I couldnt be happier.

I am and have been too lazy to be doin a "project" thread but I do have all the pics. Took this thing from a rust bucket project bikes (actually two) and made something I'm loving so far. I get the seat back tmrw or friday so after that its just small minor things like a duckbill rear fender and lowering (by cutting) the front forks. Maybe a paint job down the line but a buddy stopped by the other day and said that the patina is "sick". SO I gotta figure out what I'm going to do with the brand new black ones I have. Was considering a paint job but maybe someone can match the patina and paint the side covers?

Tomorrow bleed the front brakes!

[BenelliSEI]

Hey...... good fo you and keep moving forward. Good Luck with the rest. How do you “lower the front forks by cutting”? I’m not sure that is a sound idea?

[PeWe]

That much wrong floats means that a non technical person had messed with them!

All my carbs have always been close to a little bit too high fuel levels as stock. Just matter of time to overflow, adjust float height just a little and solved.

[madmtnmotors]

lowering (by cutting) the front forks

How do you “lower the front forks by cutting”? I’m not sure that is a sound idea?

You can lower the forks without cutting them:

http://forums.sohc4.net/index.php/topic,178969.msg2076593.html#msg2076593

This method preloads the spring as well if you leave the spring length as is.

[Radski]

lowering (by cutting) the front forks

How do you “lower the front forks by cutting”? I’m not sure that is a sound idea?

You can lower the forks without cutting them:

http://forums.sohc4.net/index.php/topic,178969.msg2076593.html#msg2076593

This method preloads the spring as well if you leave the spring length as is.

Thanks for posting that! I was trying to find it to respond with. Thumbs up!

[Radski]

Turns out I have a wobble in the sprocket carrier and therefore the sprocket. After inspection I think the bearing isnt seated all the way. Appears to be out 1mm on a side. Gonna have it re-pressed tonight.

Also found that the choke wasnt working. After inspection and fiddling, I found that one of the butterfly shields was keeping it from working properly. A little loosening and re-tightening of that shield and its working all good now although I still dont hear an audible difference. Weird. But I verified that the cable is pulling and turning all butterflies when activated. So it SHOULD be doing what its supposed to.

Updates later! ...and hopefully some pics!

[Erny]

Maybe it's because I'm not natural English speaker, but I don't understand..

[Radski]

Maybe it's because I'm not natural English speaker, but I don't understand..

Anything specifically?

[Erny]

I don't understand that lowering of forks with spacer

[madmtnmotors]

I don't understand that lowering of forks with spacer

By placing the spacer under the fork piston it prevents the lower part of the fork from fully extending by the length of the spacer.

i.e. a 2" spacer placed below the piston will prevent the fork lower from extending the last 2", effectively shortening the fork by 2". No cutting required! 

[Radski]

I don't understand that lowering of forks with spacer

By placing the spacer under the fork piston it prevents the lower part of the fork from fully extending by the length of the spacer.

i.e. a 2" spacer placed below the piston will prevent the fork lower from extending the last 2", effectively shortening the fork by 2". No cutting required! 

so even though the link you posted shows to cut the spring (i think)  you say its not needed?

[madmtnmotors]

I don't understand that lowering of forks with spacer

By placing the spacer under the fork piston it prevents the lower part of the fork from fully extending by the length of the spacer.

i.e. a 2" spacer placed below the piston will prevent the fork lower from extending the last 2", effectively shortening the fork by 2". No cutting required! 

so even though the link you posted shows to cut the spring (i think)  you say its not needed?

Depends on how well (or confident) you are at overcoming the additional preload on the spring when reinstalling the fork cap. Cutting the spring will reduce (or eliminate) the preload, but I felt that there would be an increased chance of bottoming out the fork as a result. The preload is not an issue since I'm using this on my dragbike. I have considered marking or labeling the fork cap warning me of the spring preload lest I forget upon future disassembly and send the fork cap flying, possibly putting an eye out! 

Another advantage is if I want to put it back to stock length all I have to do is remove the spacer and voila'!