Topic: 1980 CB650C Hard Starting

[tosylate]

Hey all, I've been using the resources you fine folks have provided for some time now and they have helped immensely in the process of getting
a 1980 CB650C up and running. I have one final problem though, that I can't seem to figure out on my own.


Backstory:

I picked up this CB with low miles (16xxx) for a reasonable price, being told that it only needed some "carb work". I think you know where
this is going... near total overhaul. I was able to get the bike started on fluid, but only a few cylinders were firing. I replaced the spark plugs, this didn't help. Compression test: low.  Bingo! So I ended up rebuilding the top end with new rings, cleaned off carbon, lapped the valves, new head gasket, etc.

I replaced exhaust gaskets as they were leaking (note; this bike has a non-stock exhaust... a Jardine 4into1)

I then METICULOUSLY cleaned the carbs. All passages are clean and blow through with cleaner/ air / guitar string. The accelerator pump check
valves work.  The press in idle jets are new (stock, #35's), and the float needles are new, everything else is original and stock specs. The floats are set to the correct height (12.5mm). I replaced all rubber, including o rings, acceleration pump, air cutoff diaphragms.  I laser cut new felt washers to replace missing ones along the choke and throttle rods. Air screws set at 2.25 turns out.

I ensured that the cam timing was set properly (though it still has the original cam chain) and the pulse generators were properly gapped. I then static timed it. 

I checked that the coils had the proper resistance (2 ohms) and that the plugs were properly gapped and were getting spark. I also tested the charging system, good to go at ~14.x V while running.

I then set valve clearances and synched the carbs.


This is where I am now:

The bike won't start without a lot of jockying of the throttle and turning of the starter.  Sometimes I need to hold the throttle wide open as I hit the starter resulting in a huge RPM spike on a cold bike, probably not the best.  Additionally, I'm nervous to ride it and park it somewhere in case it decides to not start.  The bike runs alright when warm but still stumbles a bit under acceleration (especially from a stop). I'm also getting an intermittent pop or pffft sound coming back through the carbs followed by a stumble in the engine. Could this be blow-by past the slides? The engine re-starts right away while warm, however.

I'm confident that the carbs are clean and getting gas to the engine, but obviously something isn't right.  Could it be a jetting issue?  I'm at 4300 ft and have a non-stock exhaust but I'm still running stock jets.  I have the original airbox on there and I didn't think these factors were enough to keep it from starting. 

The engine also hasn't been run a lot since the top end rebuild (just a bit of tuning in the garage and a couple of trips around the block). Could it be that the rings haven't mated to the cylinders yet?



Any insight would be much appreciated.  I'm obviously rather new at this, but relish the opportunity to learn as I go. 

Cheers

[cb650]

Is the tank clean?   Try a new set of plugs.  What do the old ones look like?  You might try one or two size bigger jets if still having problems.
And post pics.  Especially of the jardine.  I also have one on my 650.

[tosylate]

The tank was cleaned with a phosphoric acid etch (but not coated) and I'm running an inline filter.  The in-tank filter was toast so I removed it. 

Worth a shot on new plugs.  The current ones are relatively new - can a poorly running engine foul them quickly?

I'll post pics of the current plugs and the bike when I get home from work today.

[harisuluv]

You're having problems starting, but only mentioned choke one time and that was referencing washers.

You are using choke to help start, yes?  If you are, it is not uncommon for people to have the choke setup incorrectly.  You need to visually verify the choke flaps are closing when you actuate the choke (in all 4, because you might get half working and the other half not working if you didn't connect it properly).  You can have pull the choke knob, and see the cable lift the arm connected to the choke assy, but it might not actually be enough to actuate it.  Add to that the assy is a little counterintuitive as to how it is put together, you should verify visually.  This type of problem is usually just error in choke usage/setup.  As you said, other than that the bike mostly runs fine.

You "synced" the carbs.  Bench sync or vacuum sync?

And yes a poorly running bike can foul the plugs in one ride.

1980 CB650 is just a CB650, the CB650SC was an 82 bike and had CV carbs.

If you want REAL help, start posting pictures.  The more the better.  You won't have to explain your exhaust, air intake, carb setup, carb model, 650sc or cb650, we will definitively KNOW.

[tosylate]

Thanks for the quick replies!

I am using the choke to help start, and I have verified that the butterflies are actuating properly.  It's become part of the ritual of taking the carbs on and off before attaching the airbox boots.  I have also split the rack a couple of times in this whole process and am confident that the two choke shafts are interfacing properly. 

I vacuum synced the carbs using a homemade manometer.

Plugs don't look awesome (or consistent for that matter).  I'll pick some up tomorrow and give that a shot since they're cheap.

I can take more pictures of specific things tomorrow if there is anything in particular that would be helpful







Plugs are #s 1 - 4, left to right

[harisuluv]

Those plugs are done.  #3 is the only one that's not fouled, maybe even lean. 

[cb650]

Your jardine looks way diff than mine.

[tosylate]

Installed brand new plugs today, no dice.  Still having the same issues... I got it running, but it took awhile (no difference in how hard it was to start) then died whenever I tried getting it rolling in first.

My Jardine is stamped "9500" if that means anything.

[harisuluv]

The plugs are not the cause of your problem, they are the symptom. 

You are fouling plugs at some point in the power band.  If your bike runs basically normal otherwise you have a problem with your choke or idle circuit.

Try your air screws at 1.5 out.  Where did you get the 2.25 number?

Next thing I would try is squirting a combustible (starter fluid, sparingly) into the cylinders and see if that helps it start.  If so, you have a fuel problem.  Pinpoint the problem with data.

What is your compression now after your rebuild?

[Vinhead1957]

Take a plug and ground it to the head while turning the engine over. You might want to use insulated pliers to do it . See what color spark you are getting from each wire. A good spark is purplish blue. Strong!  A yellow is weak. Start there!

[rtnoble]

choke working?  timing off?  that or mixture is way off.  compression + right mix + right timing = boom.

[Gene]

Points?  or are you running an electronic ignition?

[grasscutter]

What is your compression now after your rebuild?

+1. 

Do this! 
And if you can get it to start, then re-do the test with a warm engine. 
And then a 3rd time with a little oil in each spark plug hole. 

[strynboen]

in line filters give a bad flow..try not use it..and see if its get better..

..get those plugs glass blast..then they often can be reused

..wet and dirt plugs get shorted up..use a megger(heighvolt) to test them..any elektrician have a test app..and can test them for you....