Ross’ 1975 CB750

[Galactica]

Just because now I’m into CB750s, this is interesting.  It appears to be a completely untouched, low mile, ‘76 CB750.  I love the Antares Candy Red on the ‘76s. 

https://bringatrailer.com/listing/1976-honda-cb750-19/

[hardrockminer]

That one looks nice!  Price is approaching KZ900.

There are a few on Facebook Marketplace.  Right now I'm cash poor, otherwise I might get serious.

[Galactica]

17 quick bids, then stuck at $9K.  By the time it’s done and fees are paid, approaching $13,500 CDN.  Plus shipping etc, etc, etc. 

[Galactica]

Sold for $9,100 USD.  Plus 5% commission = $9,555 = $13.377 (+-) CDN on your VISA card.

[hardrockminer]

A very nice bike!  There is a 73 for sale on FB for around the same price.

[Galactica]

I’m getting fed up with this thing.  I’ve had it firing on all four cylinders a time or two.  But #3 won’t fire now.  Has fuel.  Timing light shows it has spark. 

Today I took the rack of carbs off.  Cleaned the pilot jet, switched it to a different carb.  Blew out the #3 carb with my compressor.  Reinstall.  No joy.  WTF?  Next I’ll try switching the #2&3 spark plug wires.  When I feel like it, but not now.

[kilowop]

Sounds much like the problems of old BRIT bikes. I was told by 'older' riders to push it over a cliff, then report it stolen.

[Steve G.]

  Internal combustion engines require compression. Fuel, and ignition. Regarding my own love/hate/love of my Norton Commando with original Amal Concentric carbs,  it was , after seemingly years of ‘bother’ that the upgrade to an easily sourced electronic ignition for the bike, whereby it’s become one of my most reliable engines in my busy little garage.  I’m not saying the specific issue with Ross’s Honda is carburetors, or electronics,  but I will say that old electronics have been the only component group that has left me at roadside with my old clunkers.   Carburetors generally have issues in their idle circuit, you can B.C. still get home on a main circuit,,,, I’m currently having issues with my  semi modern Honda XR650, after replacing all electric components, OEM and very cheap,,,,and Installing both Dynojet carb kits, and Dave’s Fix kits, it’s plainly a bad marriage of a CV carb and this engine.

[hardrockminer]

I would think if fuel is getting to a couple of cylinders its probably getting to all of them.  But you could check plugs to see if they are wet.

My focus would probably be on the electrics.  Plug wires, coils and ignition. 

[Twisted_Twin]

Run it and spray some carburetor cleaner into the throat of #3 while it's running.

If that cylinder lights off, it's a carburetor problem.

If not it's somewhere else.

Look for dead spark plugs. I've had them fire just fine out of the cylinder but when installed, under pressure they don't fire.

Are the valve clearances in spec?

It's just a big lawnmower engine, you can handle it.

[Galactica]

It’s just the one cylinder, #3.  1,2,&4 are all hot.  Fuel in the bowl.  Timing light say’s there is spark.  That says the problem is in the carb.  But to be definite, this afternoon I’ll switch the 2&3 spark plug leads.  If it follows the wire, then it’s in the coil/wire.  If not, then it’s definitely in the carb.  

[Galactica]

Compression is good.  Valve clearance is set.

[Galactica]

I’ll take off the airbox and try spraying the carb cleaner before I tear apart the rack.

[Galactica]

This bloody thing.  Aaaaggghhh. 

I came home after coffee and switched the 2&3 plug wires, no joy.  Switched plugs, no joy.  Re&re plug cap, no joy.  Checked resistance through the coils, both at 24 with 5ohm caps.  Drained bowls, turned on fuel & filled bowl.  #3 fired but not really hot but hotter than before when it was just plan cold. 

I dunno.  Could be an intermittent or inconsistent coil/electrical issue, or it could still be in the carb.

[Galactica]

After all that.  I took the carbs off….again.  Pulled the float, fuel needle, and jets.  Sprayed carb cleaner in all the holes and blew it out with my compressor.  One more time.  Reinstall.  Start up, same same, warm but not as hot as the others. The whole time I’ve been using a laser thermometer so I can tell quite accurately individual exhaust temperatures. 

I’ve had the air screws set at stock, one turn out.  I’ve played with them before without any appreciable effect.  But I thought, what the hell, I’ll try again.  Turned the #3 screw in 3/4 turn.  Immediately audibly better.  Temperature came up right away.  Played around with it a bit and got it better.  They’re all still different temperatures, but at least they’re all HOT.