Topic: hard start 1971 honda cb 750

[pablo63]

I have re-built my 71 honda cb 750 , new rings , valves re-ground , new valve seals , carbs completely stripped and re-built .
here is my problem , the bike  hard starts ,my method of starting is 1/2 choke & twist the throttle 2-4 times ,most times  I run the battery down (new battery)  and am forced to put my charger in start mode , once out of five it will start before the battery dies ,once started and all the hiccupping and roughness disappears and the engine is warm  , usually a couple of minutes  , it runs good , starts with just one kick of the kick start or one press of the starter button , accelorates with no hesitation and will pull strongly in fifth gear .
   So far I have removed the tank and made sure the petcock is clear of any obstructions and that gas flows freely ,no problemo   , I have also increased the air screw to 1 1/4 turns from 1 turn out , removed two plugs and found them very slightly sooted ,the air cleaner is new ,  the bike has 200 miles since the  re-build , I also checked the points gap and found them to be 29,00 thous , way to big a gap as the book reccomends 12-16 thou , my problem is that when I line up the index line with the f mark for 1-4 and using a static timing light so the light just comes on at this mark ,then rotate the crank to the widest point opening I find that the points are opening to 29,000 ,if  I reset the gap to 14 thou then the static lightv does not come on  untill the rotor is past the timing mark for 1-4 cylinder , is this telling me that the part of the points which sits on the rotor is warn down and thus not allowing the proper gap setting ,when the bike is running there is no arcing at the points and the faces are fairly flat ,when I purchased the bike it had only 14,000 miles so maybe these are the original points .also the weather here has been cool  , probably 12-14 deg celsius ,would that cause hard starting ass once it does start and the engine warms up it is very easy to start .

thx ,Paul

[MD]

Hello from MN.

-MD

[Stev-o]

Welcome from Texas. 

Try this for cold starts:

1. Turn on ignition and fuel
2. Give it full choke
3.  Hit starter and no throttle til it fires
4. Hold throttle as necessary and reduce choke to half

My K4 will not start if I give it throttle on a cold start.   Good luck.

[b52bombardier1]

Yes, follow the cold start procedure in the owners manual which is largely described above.  My bike starts a lot easier if I follow those procedures.

Rick

[pablo63]

Well weather getting colder , 5 deg c today , snow on the ground ,have a new set of points on order , so will wait till the points arrive , try the reccomended cold start procedure as described , if no change then install new points and hope that solves the problem , will update once the have tried this .
Thx , Paul

BIKES :1982 HONDA CBX , 1976 HONDA GOLD WING ,2009 YAMAHA V-MAX , 1971 HONDA K-1

[Kevin D]

Welcome Paul.

 K1's are the best!

[Stev-o]

What brand of points did you buy? Hoping not ccc....

[scottly]

my problem is that when I line up the index line with the f mark for 1-4 and using a static timing light so the light just comes on at this mark ,then rotate the crank to the widest point opening I find that the points are opening to 29,000 ,if  I reset the gap to 14 thou then the static lightv does not come on  untill the rotor is past the timing mark for 1-4 cylinder

The point gap is set at the widest point, which is about 90 degrees past the "T" mark, first, then the points plate is rotated to set the timing to the "F" mark. Always set the gap first, before the timing.
Those carbs do not have accelerator pumps, so twisting the throttle 2-4 times does nothing. Like Steve said, in cold temperatures use full choke, and it may help to hold the throttle a bit open, as this will enrichen the mixture more than with the throttle closed with full choke. Every bike is a little different. The fact that the bike runs well and is easy to start when warm is a good sign.

[540nova]

Like Steve said, in cold temperatures use full choke, and it may help to hold the throttle a bit open, as this will enrichen the mixture more than with the throttle closed with full choke.

Quite the opposite: if during start, open throttle, eng not running, would be leaner.

It's amusing to watch people "prime" these bikes with a couple throttle twists before starting. As you said, no accel pump.


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[pablo63]

I have ordered a set of genuine Honda points & condensers from David Silver Spares , so twisting the throttle before attempting to start does not dump a little gas into the carb bore ,
just wandering what would cause a weak spark , it's not something I have checked as yet but plan on doing that once the new points arrive ,
Also when attempting to start it wants  to start but when moving the choke from 1/2 to full choke it just seems to drown out 

[540nova]

I have ordered a set of genuine Honda points & condensers from David Silver Spares , so twisting the throttle before attempting to start does not dump a little gas into the carb bore ,
just wandering what would cause a weak spark , it's not something I have checked as yet but plan on doing that once the new points arrive ,
Also when attempting to start it wants  to start but when moving the choke from 1/2 to full choke it just seems to drown out

Your first paragraph makes no sense. Is that a question?
Your last paragraph... if going from half choke to full choke, you're doing it backwards. Full choke means the butterfly valve is fully closed. Perhaps you mean going full open?



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[calj737]

I would abandon the start up sequence issues until you resolve your ignition problems. Improper timing and poor point gaps or weak spark will prevent your bike from firing whether you do a rain dance, a break dance, pray to Odin, or whatever else.

Once you know the bike has the correct, full strength spark, diagnose starting issues. If they persist, turn your attention to the carburetors. Did you rebuild them properly? Did you set the fuel level properly? Are they bench synched?

[Scott S]

 Welcome, and I'm moving this to the tech forum where you'll get more views and, hopefully, more help.

[Keith]

Why is point gap important? When the points are closed, the coil is energized, you could say "building up spark". When the points open, the magnetic field collapses, and the built up spark needs somewhere to go...it jumps the plug gap to ground. So, if your point gap is too wide, there is less time to build up spark. So, you have a weak spark, something you don't want with a cold engine.

[pablo63]

Problem solved
   I installed the new genuine honda points and condenser , set the points @14 th , set the ignition timing ,static method , turn on gas ,full ckoke & no twisting of throttle either before or while actually starting & bingo , she fires up , although very roughly at first on full choke , turned the choke to half and controlled engine speed @ 2000 rpm , allowed the bike to warm up as it was 5 deg cel , after couple minutes no choke and bike would idle smoothely @ 1000 rpm and responds with no lag to throttle twist .
   Just want to say thx for all the replies and the info ,so hoping that's all the headache for now ,but as with any 46 year old bike strange things can happen

[Stev-o]

Good news, thanks for coming back and giving us the update.
It may help someone else out in the future.

Now go ride!

[Kevin D]

Wow! That bike is mighty clean after riding around in the snow. A good looker!
Don't try to park it on the side stand.

[pablo63]

The picture was taken in mid january after the bike was re-built ,all the heavy snow has gone although woke up to a slight dusting this morning ,was able to get out yesterday and put on about 125 trouble free miles ,bike worked really good ,  starts easily and idles smoothely at 1000 rpm ,pulls strongly ,although I have never ridden a cb 750 before have owned a few 750 bonnevilles so have some idea how a 750 should feel , have borrowed a strobe light to double check the timing , and then eventually synch the carbs ,but for now happy with the way it's running , I used to own a 1974 triumph trident which was launched in 1969 and was  the bike the brits launched to do battle with the 69 honda cb 750 , now I understand why they lost .