Topic: Need help with a no start

[scorched101]

Hello everyone, I'm Jeremy. I searched the web for different help and have had no real luck at finding an answer to my problem with my 1975 CB750. I've looked through some of the post and it has offered some help, but still does not correct my problem. Here is my situation, I am working on a garage build. I have replaced most parts with new and rewired the bike. I have fuel, spark, and new jets. However, when the bike kind of starts, it sputters, backfires from the carb and muffler. I have cleaned the carbs, set the air screws to 1 1/2 turns. It will not stay running, with out the throttle fully on, then it only lasts for about 10 seconds before dying.  Used the timing lite and it seems way off, though I never did anything with the timing or engine except for removing the valve cover and painting it. Any suggestions.

[scorched101]

Just to add one bit of info I forgot to write, the 1-4 point is sparking a lot and the 2-3 point is barely sparking to no spark. Is this normal, could this be a reason for the non starting problem, or timing problem.

[mrbreeze]

It's a 750....not a 550. Removing valve cover won't change valve clearences.

[scorched101]

If removing the cover doesnt change it. Is there anything that you can think of that would affect the bike this way. It has not ran for at least a year. It will get a valve check, just trying to get every bit of info or potential solutions, so i can start tackling them one by one.

[mrbreeze]

How well did you clean the carbs? If it has sat for over a year, it is bound to have a bunch of crap built up in the carbs unless it was put away properly (most times not). There are lots of things that it could be. Some people say they have spark but it might not be strong enough to fire. Do you have a manual? If not, it would be a good thing to get. Keep at it and if you want it to run bad enough, you can make it happen.

[scorched101]

In regards to the carbs, disassembled the upper portion(removed needles), pulled float bowl, pulled main and pilot jets, used carb cleaner and sprayed into the pilot jet hole, everything looked clear with a good stream of spray. Reassembled.

In regards to the spark plugs, replaced with new, tested each against block for visual. Both a tester to test the volts, I basically used a tester similar to the Thaxton model #458, but in the Advance Auto line. Each test at 20,000  and a little higher. So according to that I have spark. I believe it is something to do with the timing. Do not know if this is the most accurate way, or if it is even right, used the timing light while cranking the bike, marks are way off. It may be the wrong way to test, but can not get it to run for more that 10 sec or so. 

[setdog]

I'm a little confused.  The thread title says "no start", but it sounds like the bike does start, just doesn't
Run well. 

I just went through a non-start problem, no spark at all, i fully believed i had bad coils, turned out it was a electrical short.  When I got spark it didn't appear strong, but its plenty to ignite gas. 

Now I just have to tune the valves, timing, points, carbs, etc...

PS. There is a proper order to tune valves, timing, ignition, and carbs, but I forget the proper sequence.

If the bike is starting and not running well, then he problem is more likely a tuning issue.

[Davez134]

Do a complete tune-up per honda manual. (Valve clearance adjustment, clean air filter, oil change if not done yet, cam chain tension, set points gap and timing.) Then look at carbs last. If you took them apart, including needles, sounds like you may need to vacuum synch them.

[scorched101]

Just to clarify, the bike will not idle or start on its own without holding the throttle on full twist. When it partially starts, it backfires through the carbs and exhaust, even with the throttle fully on, it will die out in about 10 sec. I am thinking it has to do with the timing somehow. It ran well before I took it apart. I will be in the garage today so I will start on the tune up procedures. Can the valves be adjusted without the valve cover on? It is 90% still oem style frame, and really tight to get into the 2-3 valves.

[seanbarney41]

timing can be set statically, without the engine running,  all you need is 12v test lamp and a manual

[scorched101]

I'm sorry, I keep adding tidbits of info, but here is another thing I did to the bike, I did remove the stator to replace the gasket on that side, could the stator be placed back in wrong, and could this effect the starting of the bike? Unfortunately, I am thinking that I am may just tear it down and redo everything, just in case I missed something when I rebuilt the bike, (never rebuilt engine). Any thoughts?

[lucky]

Idle jets are probably clogged as usual.

[Vinhead1957]

Backfire.  Only run at full throttle . Are the plugs soaked. Get a new set and try starting it again