Topic: My old 1972 cb750 won't start in cols weather

[FondaHonda]

I checked the archives and I may have overlooked it. This has probably been discussed before, so I apologize if this is a repeat. I bought this bike this summer and it ran ok (rough idle). I have replaced plugs, cleaned carbs, adjusted valve clearance, replaced the battery. Once the weather began to cool it was a #$%* to start. If I put it out in the sun or put space heaters beside it for 20-30 minutes, it would finally start. The timing is questionable. If I turn it as far counterclockwise as it will go it will run but idle very roughly. Any adjustment the other way causes it to stall immediately. On the first crank I hear exhaust like it wants to start, after that it doesn't sound like it is even trying to start. I may need to recheck the valve clearance to make sure I did that right. Compression was hard to test, getting the gauge in the right spot, the best I remember it was in the high 90's if I was getting a good test. The only carb sync I have done is a manual, getting it close adjustment after I cleaned the carbs. Since winter is coming I may have to do some serious work, but I hope not. Is this common for older cb750's?

[MetalHead550]

Well my Hondas hate to start in the cold, especially if theyve sat for a couple days.  Both run very well, so I imagine a bike that isnt running top notch will have an even harder time.  Id go ahead and confirm fuel and spark though to be sure.  Mabey getting flooded?

[TwoTired]

First off, all the standard tuneup items should be at book value, including the timing.  If you're having trouble with adjustment ranges, check out the timing plate lateral movement with the springs of the points loose.  Either peen near the bottom the of the plate for a tight fit in the mounts, or make a feeler gauge shim so the plate can't slop around.
Make sure your plugs aren't resistor type, are correct heat range, clean and gapped properly.  Check you plug caps for inline resistance and that they screw tightly into the ignition wires making good contact.

And, check the voltage being presented to the coil primaries.  All the bike's wiring connectors (and switches) can build an oxide over time.  Ten inline connections that loose .1 volts at each connection, means 1 volt less deliver to the coils, and a corresponding weaker spark available for starting.  A cold battery puts out less voltage,too.  So, you get the double whammy and a hard to start bike.  Cleaning all the interconnects inline between battery and coil should restore maximum voltage to the coils and spark plugs.

[ProTeal55]

I agree with MetalHead , all of these bikes are cold blodded.
So , if they aren't in "perfect tune" , and it gets cold , they are even harder
to start.

[KB02]

First of all, I agree with everything that TwoTired said about making sure you have all the tune up specs spot on.

Secondly, I also agree with Metalhead and Proteal -
It's a:
HONDA (haun-duh); Noun. Japanese for cold blooded. 
I have owned five old hondas (the newest being an 82) and I ain't had a one of 'em yet that wasn't a cold-blooded beast when it came to starting it when it was cold. If your tune is not right, good luck.

[amattel]

I was having trouble starting my cb750k2 when cold... until I found "the trick" 

For my bike the key is not to touch the throttle until after it sounds like it is beginning to start...

I used to give a little blip of the throttle in the beginning and would be cranking it for a good 5 minutes before I got it going, but now, at 35 degrees, it starts right up.

I have been adjusting points and timing every few hundred miles since I learned how from scondon.

It is running strong these days!

Adam

[ofreen]

I don't mean to be contrary, but my 750 has always started well even when very cold.  It was 22 degrees F in the garage the morning before last and it fired up immediately.  It has a Dyna S ignition installed, so that may help.  Even before I put the Dyna S in, it started well when I lived in Alaska.  It starts well even with temps into the teens and the engine cranking noticeably slower.  The motor sure rattles at startup at those temps, tho.  Or maybe it is my teeth chattering.

Anyway, my point is that I don't think these bikes are inherently hard to start at low temperatures.  If your bike doesn't want to start, there is probably something not quite right.  I do believe the Dyna improved starting at low temps.

Greg
'75 CB750F

[mrblasty]

Same as ofreen, my starts right up when its cold outside.  A tune-up, carb cleaning and replacing any worn ignition parts should help with the problem.

[dusterdude]

mine starts tho it is a bit cold blooded until it warms up

[Gordon]

7 deg. (F) on Tuesday morning and started on the second kick.
 I have a Dyna-S in it, but the biggest difference came from thoroughly cleaning out the combustion chamber and tops of the pistons about 18 months ago when replacing the gaskets. Carbon buid-up can make it much harder to start and maintain a smooth idle. 

[cadler]

Make sure you are not using high octane gas. I had the problem with my cb450(I was using 94oct). put in 87 oct. and now it starts right up....
Higher octane gas is harder to ignight under compression than lower octane gas,componded by cold weather,...give it a shot.