Topic: Help getting this bike running please.

[Logan80]

I bought a $400 1972 CB750 a couple months ago and am just now getting around to trying to get it ready to ride. The bike had been sitting for 15 years before I got it.

I already replaced the exhaust, although the eBay auction lied and it's definitely not for a CB750K, although the headers fit, the muffler is certainly not correct, as the bracket on it is nowhere near attaching to anything on my bike. Maybe it's for an F model? For now it will provide backpressure and allow me to get the bike running/semi-tuned.

I took the float bowls off and there was nothing in them. I removed and cleaned the jets, although they were mostly fine. The PO must have ran the carbs dry before storing the bike, they really were pretty clean inside. The gas tank was also very nearly empty and clean. A little swish of some new gas and draining and it's clean and good to go.

I've also cleaned and adjusted the points, checked the valves, changed the oil/filter, checked compression (100/100/100/95 or so) with a automotive tester, and replaced the battery.

I've put about 2 gallons of gas in the bike and managed to get it to fire up. It was missing on a couple cylinders and running pretty poorly, but it would idle. This was with open headers before I bought my "new" eBay set. I've since clipped the ends off of the spark plug wires and reinstalled the caps, cleaned and gapped the plugs, and installed this new-to-me exhaust. It still runs very poorly, and still is running on only two cylinders, except now it's two different ones than before!

I've decided the best thing to do is probably to clean all the wire connections and make sure the coils are getting the electricity they need. What would you guys suggest I do first? I don't want to put money into rebuilding seized brakes and replacing all of the cables until I know it will run correctly. That's got to be my priority right now I think.

Anyone in or near Tacoma, WA that wants some good beer can feel free to come help me!  

[bender01]

It will run. What 2 cylinders are firing now? As your sitting on the bike left to right 1>4. Carbs may not be as clean as you think! Did you buy a new Mac exhaust? 4>1? They send no instructions with them. Any black flat pieces leftover? Make sure your battery is strong. Check for spark and buy new plugs! theyre only 8-10 $. Did you pull the rack of carbs off the bike?

[ron.cieri.313]

Hmmm, I would start back in the carbs...but that's just me...eventhough it looks clean to the naked old gas could have plugged your slow jets easily, especially for a bike thats been sitting that long....and I'd want to be sure to elimante that as a problem first thing....so, I guess I'm saying, I'd disassemble the carbs and really give everything a good cleaning...the jets, the body of the carbs, etc.

[Stev-o]

Charge the battery and give it a full tune up. If it's not getting proper spark, tuning carbs won't help much.

Good Luck.

[Logan80]

I bought a "used mac 4 into 1" so it came with no info at all. Just a header, muffler, and clamp.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=330449402103&ssPageName=STRK:MEWNX:IT here is an ebay link to the actual auction I won. I can't complain, the price was right...

Cylinders 3 and 4 are firing. It WAS firing on 1, 2, and usually 3. With 4 only firing if I wiggled the plug cap/wire around just right before starting it...

I didn't actually remove the carbs from the bike. I just took off all the bowls and pulled the jets out of the center of the carb bodies. Also made sure the floats weren't sticky or anything. I was honestly a bit scared to actually pull the carb setup off the bike. Seems like a disaster waiting to happen hehe. Although I suppose I really should do that...

[tlbranth]

About the exhaust: If the headers fit it's the right one. I welded on a mount piece when I got mine. I live in Carnation, if you want to come see it.

[bender01]

You know what needs to be done! Pull em and clean em once twice or more. Get a piece of flat metal and make a bracket for the exhaust.Drill 2 holes. Use your rear foot peg as the mount. The tune up includes the carbs. Once you do this you will be talking all your friends into buying old CBs and amazing them by getting them running. Of course you can shop off your friends bikes a little too.
Win..Win!

[Old75_ratafe]

Hey logan good score your bike is my exact model and the exact same story as mine although mine sat since 89 lol.  My carbs "looked" ok too.... Trust me clean them the proper way and rebuild them.  I found two internal passages clogged solid on my "looks good" carbs (twisted some thing gauge strands of electrical wire together tightly to chaise them).  My PO was a good friends dad so I was lucky enough to know the bikes history he tried to replace the points after it stopped running... He had them so off that one side would not open at all and the other side would not close all the way    I fixed that only to determine no or intermitant spark....  Soo I replaced ended up replacing the coils which I would bet money was the original problem he had not the points he misgapped.  I got a good deal upgraded to magnetic trigger.... she fired right up 3rd or fourth crank.   After I got it running I figured out the carbs weren't as good as I thought.....  it would only run with 3/4 choke on and #4 would foul out after about half an hour to an hour of running and would be hemorging gas so bad it would come out the FRONT of the carb lol.  

That said I have a few questions for you.  

1. Do we know WHY it was parked and sat so long.... my money is the same reason your fighting now.  

2. Have your replaced the 15 year old plugs and 15+ (probably 35) year old points yet?

3. Pics man pics! we want to see this score lol!  

OH I almost forgot pop the headlight and the tank off and go through every little bullet connector and clean and dielectric grease them.... trust me...  Its a PITA but well worth it in the long run...

[Logan80]

Well, the alternator cover had a hole in it, and the guy had a good used one in a box. I'm assuming that it was dropped (I can tell the bike's been down at some point) and the cover was cracked open. The man who owned it was an older guy and he died while the bike was "parked" and it was left to his niece, it then sat in her and her husbands shop for ten years and I bought it from them. They had no interest in it at all and hadn't made any attempt to repair it. Took me about 45 minutes to clean up and replace the alternator cover and it's as good as new now.

I haven't replaced the plugs, I just cleaned them up on my bench grinder's wire wheel and put them back. I know I need to replace them.

No new points yet either. I was debating on going with Hondaman's setup with new points, or Dynatek ignition. Leaning toward Hondaman's parts.





Sadly that "Hooker" branded exhaust was horribly rotted out. Holes as big as my hand throughout the bottom of it.

I'm also aware of the fact that it's got 1973 paint, and a 1973 engine code if I remember right. But the frame is titled as a 1972, and marked as one as well. Might be quite a story to this old bike...

[Old75_ratafe]

Well, the alternator cover had a hole in it, and the guy had a good used one in a box. I'm assuming that it was dropped (I can tell the bike's been down at some point) and the cover was cracked open. The man who owned it was an older guy and he died while the bike was "parked" and it was left to his niece, it then sat in her and her husbands shop for ten years and I bought it from them. They had no interest in it at all and hadn't made any attempt to repair it. Took me about 45 minutes to clean up and replace the alternator cover and it's as good as new now.

I haven't replaced the plugs, I just cleaned them up on my bench grinder's wire wheel and put them back. I know I need to replace them.

No new points yet either. I was debating on going with Hondaman's setup with new points, or Dynatek ignition. Leaning toward Hondaman's parts.





Sadly that "Hooker" branded exhaust was horribly rotted out. Holes as big as my hand throughout the bottom of it.

I'm also aware of the fact that it's got 1973 paint, and a 1973 engine code if I remember right. But the frame is titled as a 1972, and marked as one as well. Might be quite a story to this old bike...

Yea before you do anything else tuning wise do plugs and at the least check to see if the points are pitted and gaped properly.  I would have done the hondaman ignition except when I bought the dyna I didn't know it exsisted lol.  But then again I got the Dyna 3ohm coils and S ignition for under $100... so a steal of a deal.  And I thought you said this bike was a 75 at the start of the post?  Too bad about the hooker exhaust btw.

[Logan80]

I honestly forgot the year, so I checked. For some reason I had 75 stuck in my head, but it's a 72. May of 1972 according to the frame.

[Old75_ratafe]

I honestly forgot the year, so I checked. For some reason I had 75 stuck in my head, but it's a 72. May of 1972 according to the frame.

Thats ok I called mine a 74 for about 3 years till I went to transfeer the title (forgot I never did it and it was under a tarp in the back yard anyway) and realized I had it wrong all that time lol.  Either way nearly the same bike, looks about the same shape too mine spent about 5 or 6 of those years (think it was more) outside unprotected by anything.  I tried to clean  and polish up my chrome but in the end painted most of it flat or gloss black (gauges, rims, guards, handle bars) the only thing still chrome on it I think are the turn signals, headlight ring and aftermarket mirrors.  The exception was the original 4 into 4 exhaust was good yet.

[Alan F.]

I'd also make sure your coils are good, get a test meter (even a cheapo will do) measure the primary leads (small wires coming out of the coil bodies) and measure the secondary windings (each coil has two plug wires exiting, they're connected to a coil inside)
Measure the resistance of your plug caps too, they should read 5K ohms, they go out of spec when they get old.

This'll help: http://forums.sohc4.net/index.php?topic=60263.0;all

Cheers and good luck, its gonna be a nice bike.
Alan

[Logan80]

Ok, just tested the coils.

The small leads on the "200" scale measure:

Coil 1 - 5.3
Coil 2 - 5.4

The caps on the "20k" scale measure:

1 - 4.5
2 - 4.5
3 - 4.95
4 - 4.85

The plug wires on the "20k" scale measure:

Coil 1 - 14.53
Coil 2 - 14.55

[TwoTired]

Tests show that the coils should work.

[Old75_ratafe]

yep time to check connections and replace those plugs, also make sure the plugs are getting a firm connection those connectors on the end sometimes widen and don't grip so well on them.

[Logan80]

And so it begins...

[Old75_ratafe]

Oh geesh logan your gonna make me do mine again now arn't you... I just pulled them off the bike tonight myself... Isn't simple green great stuff.

[Logan80]

Absolutely! I never thought it would work THAT good. The first two carbs and all their parts came out today, had to use a toothbrush to take off a little residual grime, but other than that they are spotless and look new!

I have the other two in the bucket for tonight and during work tomorrow. Once I get them all back together I still want to do my best to clean my engine since it's more accessible right now. Also need to replace the plugs and get some new exhaust gaskets, I didn't change them when I put on the new header and it seemed to leak pretty badly when I did get it to run for a few seconds at a time...

[Old75_ratafe]

ha ha ha ha I just did the same thing left them in buckets overnight to soak.... I have a bucket of that chemical carb cleaner and am gonna do a follow up dip tomarrow.

[Logan80]

It worked. Here's a bunch of clean parts, thanks to Simple Green and an old toothbrush...

[Logan80]

Ok, before I re-rack these carbs, does anyone know where I can get the little rubber boots that cover the slider adjuster nuts on the tops of the carbs? Mine are all torn and ratty...

[Old75_ratafe]

Ok, before I re-rack these carbs, does anyone know where I can get the little rubber boots that cover the slider adjuster nuts on the tops of the carbs? Mine are all torn and ratty...

Yea mine too I just left them off.  Honda may still make them?

Hey Logan http://forums.sohc4.net/index.php?topic=76073.0
Have fun with them I know I did and if you have a question feel free I'll keep my eyes on your post.  Mmmmm simple green.

[Duanob]

hey Logan nice score! And you didn't even have to pay the West Coast Tax. I paid twice what you did for 550 in about the same shape. After shopping around the Seattle/Portland area I decided that was what I had to pay even for a fixer. I wish I had free shipping from the mid-west that's where the great deals seem to be.

Did you ever get the carbs mounted? Have any luck getting it started? I'm always enthralled by successful carb stories. I'm hoping mine works out to the same happy ending.

[Hush]

Just a thought but the coils on most of these SOHC4 bikes run different plugs, #1 and #4 run off the same coil as do #2 and #3, I was just thinking as you said originally that the bike fired up on 2 cylnders but after you snipped the wires it fired up on the other 2, did ya mix the wires?

"gapped the plugs, and installed this new-to-me exhaust. It still runs very poorly, and still is running on only two cylinders, except now it's two different ones than before!"