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Discussion Starter · #1 ·
Honda 1970 cb 450 .. bike starts easily and idles ok , right cylinder ( kick start side ) has a miss . When I manually throttle up at carburettor near idle screw there is a Hugh miss like a dead spot.. other cylinder if fine . So far I have cleaned carbs. but have not put a kit through although I have a kit here .I read on forum original needles were better. new petcock , inline fuel filters, swapped fuel lines over between carbs, adjusted floats. I have not synchronised the carbys . The Air Filters are origional . I hosed them out and cleaned as best I could.On the electrical side of things I have new points but did not time with a timing light ( but used other method)... Both exhausts get hot... new spark plugs and caps (not new leads )original coils but did check with a multi meter and are within specs. I have new coils here . I put in a new condenser and thought it made a bit of a difference( but maybe wishful thinking) I have not adjusted valves or timing chain but motor seems to runs quiet, appreciate any advice, thanks I advance, Kanga
PS the bike was bought as a project and was not running when I got it .
 

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+1 ^^^^^^
Stop throwing parts at it.

Lots of threads on "cleaned" carbs that turned out not to be, dropping the bodies in carb cleaner won't always do it, the passages need to be blown through.
Follow the FREE TO DOWNLOAD FSM on the forum.

Sounds like a fuel/carb problem to me but you need to follow the FSM or you will be chasing your tail, getting frustrated and broke throwing parts at it
 

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... he has the right idea.. just not in the correct order..
Swapping the coils(and trigger leads) will help if its "electrical"
Also.. having a small "spray" bottle with fuel or starting fluid, bring the motor to the dead/ flat spot.. spray the carb intake.. if it smooths out.. you know it's a fuel issue.., I have also used a small propane bottle with a hose (minimal gas flow).. the same way in locating a vacuume leak on a car motor..
Since this was not running at one point, you have already made progress. You may want to do s compression and leakdown test to give yourself an assesment of the health of the compression chamber, rings, valve seats.. simple easy tests. They provide a lot of info..
Hitting it with a timing light wouldn't hurt, if it is advanced to far, it could hit a "flat spot"( but I believe the idol would be high)..check advanced plates and springs..

Just simple easy stuff.. I'm sure your close to figuring it out.. your checking off all the correct boxes.., just not in "order"
 

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+1 ^^^^^^
Stop throwing parts at it.

Lots of threads on "cleaned" carbs that turned out not to be, dropping the bodies in carb cleaner won't always do it, the passages need to be blown through.
Follow the FREE TO DOWNLOAD FSM on the forum.

Sounds like a fuel/carb problem to me but you need to follow the FSM or you will be chasing your tail, getting frustrated and broke throwing parts at it
If it IS a carb problem, it sounds likely that it is the transitional fuel port, just on the upstream side in this drawing, #13:
carb_schem.jpg
Note that you can get to it for cleaning by removing the large brass plug. You can also make sure the passage to it is cleared with compressed air or spray carb cleaner through the pilot jet.
 

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+2 on the stop throwing $ sentiment. However, have you considered the age of the gas inside your ride? Even with Sta-bil to keep the gas fresher I can personally attest that draining gas that has sat for a bit and adding a nice amount of fresh(in NY I use 93 premium and always with some drops of Sta-bil) can help cure starting & idling woes.

as for the blowing out of carb passages....absolutely! take great care when removing any debris from the main jets and slow jets as the brass material is very soft and you can mistakenly enlarge the holes. I use fishing line and gently pull it thru the holes and then blow clean with compressed air. Have you verified the float heights at 20mm? there is really no give with that adjustment; Must be 20mm. For kicks you can manually drain both carbs after running it and measure the gas in a measuring cup which shows the amount of gas flowing in the carbs. I periodically do this. What size main and slow jets do you have?
Synchronizing carbs is still a chore for me but once correctly completed the adjustments stays consistently for me during the season. Verify you correctly timed the bike. Again, this is a chore but if done carefully with attention to detail the adjustment will stay. I find myself timing my bike twice a year; once in the beginning of the season to double-check what I did prior to winter storage and once later on during the riding season to confirm the adjustments held true.

what plugs you running? Can be NGK B8ES OR B7ES. Personally, I'm running better on the B7S.
 

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I gota agree with doode, start with the big three and then go from there.
 

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Discussion Starter · #8 ·
thanks very much for the replies everyone, I was running out of ideas (but not any more) . not sure what size jets from what I can tell the carbs are original.I have a new the carby kit here but decided to use clean up existing , but I might install new one to see what happens. NGK B8ES plugs .I will try all suggestions and get back. really appreciate the help
 

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Discussion Starter · #9 ·
hi just put two new plugs NGK B7ES as suggested by jseconds77 when I removed old plugs they were really black and fouled
the bike run beautifully with new plugs
so went for a short ride maybe 3 or 4 miles. the flat spot returned removed new plugs and they were already black carboned up.
does this narrow down cause of problem or is it likely to be many things ? appreciate your thoughts,Kanga
PS I have new fuel and oil. I hope this is just a timing issue
 

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... do like Santa.. get your list.. check it twice..
 

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You have checked that your choke lever doesn't move when you are riding? Because that sounds like its pretty rich.
 

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Discussion Starter · #12 ·
thanks Nige .no way the choke lever is moving. I am going to do the carbs again . I bought kits from Common motor and will see what happens ,. I am going to do valves and timing chain as well which I haven't done ...thanks Doode for the advice
 

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I don't think you need do all of the carb kit, since jets don't wear, unless you damage them while cleaning. The float valves, however, do wear, and can cause over-filled bowls. The needle jet can also get pounded out of round by the needle vibrating around, but you have to remove the main jet to replace it, assuming your kit includes the jet (many do not). This kind of damage to the needle jet is less common in the 450 carbs, as the jet needle is better held in the CV piston; something to look at.
If you haven't got one, you can make a float height gauge from thin stainless, or plastic that doesn't melt in fuel:
floattool.jpg
The tool just spans the surface that mats the bowl, and the first step should be exactly 20mm; the second step is to clear the bit that holds the main jet. I use mine with the carbs mounted. I use the tool to hold the float up, and open the petcock and, if fuel flows, reset the little bit that lifts the valve pin. If fuel doesn't flow, adjust it so it just seeps past the valve.
 

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Discussion Starter · #14 ·
thanks Rick, I didn't adjust the floats heights when I cleaned carbs (i realise now that was a stupid thing to do) i also only used carb cleaner as I have no access to compressed air . Now I read using fishing line through jets to make certain they are clear. This bike sat for 25years and the fuel tank had a layer of varnish which was a nightmare to clean . Maybe there is varnish in the jets , but the petcock was turned off .One other thing which may be significant is the right carb was flooding so I removed bottom and jiggled float and it stopped flooding. thanks for the float adjustment tool diagram .I.will be working on the bike today or tomorrow. I started with an old bike that sat for 25 years,2 stuck motors . very little expertise . . I had a lot of advice via email from 450roo hear in Oz (different states) I didn't do a compression test or adjust valves or timing chain , just put it all together the best I could, it started first try and I nearly fell over because I wasn't expecting it to. I ride the bike nearly every day(short trips) it doesn't blow smoke . I realise how lucky I was to get it going.
 

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These 450s come back to life pretty well. Mine sat 27 years in my basement garage, so temps didn't vary a lot, and I did prep it a bit, not knowing how long it would be there. Put a quart of fuel in it, and it popped on the third kick, started on the fifth. That's when I decided to get it back on the road.
 

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Discussion Starter · #16 ·
hi Rick have just remove the bottom of carbs while still on the bike and even to the naked eye I can see they are different have just come up to the house to get your diagram ... how to set floats while still on bike. here's hoping!
 

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Discussion Starter · #18 ·
hi Rick yep floats were out.. have now set correctly .. no needle valves in kit .. bike runs a lot better .. Thankyou I am now going to do what I should have done in the first place and do tappets and timing chain and set the points again. I now have the Honda Shop Manual .I will get a compression test also . I appreciate all the help. love the bike and learning heaps
 

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I now have the Honda Shop Manual.
Best tool you can have for any machine, IMO.

The tappets can be frustrating, and a third hand to tighten the lock nuts makes it tons easier. Just pay attention to the little marks on the adjusters, to make sure they stay in the range shown if Fig. 3.25-b. There are two spots in the total rotation of the adjusters that will reach the spec gap, but only the one in that range works properly.
 

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Discussion Starter · #20 ·
hi Rick I did the tappets before I read your post , probably have another go when I can get someone to help me with lock nut. did the cam chain tensioner and
checked timing . right cylinder was slightly advanced so now have static timed but have ordered a timing light from ebay. the bike is running well no flat spot at all.The Air Filters are original so I will have to look into that .Thankyou to everyone for helping me solve the problem. Kanga
 
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