Honda Twins banner
1 - 20 of 21 Posts

·
Registered
Joined
·
10 Posts
Discussion Starter · #1 ·
Hey guys - so my 74 CB450 runs excellently, when the plugs are clean. The problem is I have to change them about every 2 months. After two months of riding, the engine will start to sputter when at high rpm's (around 7,000 and up). When this starts to happen, I know the plugs are getting bad. When I remove them, the left plug looks OK - blackened but not too bad, but the right plug is absolutely coated in carbon - totally black. Any idea how to fix this? The strange thing is that the bike idles normally and runs perfectly but something is obviously wrong on the right side or the plug wouldn't get so bad over there!

Thanks in advance,
Andy
 

·
Premium Member
Joined
·
5,610 Posts
Could either be a jetting issue or float level problem. Something is causing it to run over-rich. You should probably check the jet sizes to see if they are in keeping with your setup and also check/adjust the float levels to stock height. At least that will help rule out those possibilities.

Could also mean that the spark is not the best and the air/fuel mixture is not burning like it should.
 

·
Registered
Joined
·
10 Posts
Discussion Starter · #3 ·
When you say check the jet sizes, you mean that they may not be the correct size? I believe everything inside the carbs is stock but I can check. Novice question: are the jets actually numbered so you can tell what size they are?

Also, is there any potential harm being done to my engine (other than quickly dirtying the spark plugs) by riding it with one side a bit rich?

Thanks again.
 

·
Registered
Joined
·
148 Posts
the jets should be numbered--usually a number will be etched into the top or side. it's a good idea (i've found and been told) to take written note of the jet sizes you find once you get in there. i think 'stock' jets for that year/model are 38 for the slow jets and either 145 or 150 for the main jets. if you've got something else in there, chances are someone made modifications at some point.
good luck!
 

·
Premium Member
Joined
·
667 Posts
I agree that your problem is either incorrect jet size or float level. One thing I ran into though on jets once was that the number was correct on the main jet but someone had reamed it out trying to clean it with a drill bit or something and made a new size jet that was too big.
As far as harm to your bike, I think that too rich would be better than too lean but you are building up quite a bit of carbon in the combustion chamber probably and that in time will make the bike run bad.
Don
 

·
Registered
Joined
·
3,926 Posts
It depends on if the carbon flakes off easily by hand. If it does then it's likely a rich mixture but if it's more of a burnt on black carbon then it's burning oil. I agree with Deacon about checking for the weak spark and also the float levels..


GB :mrgreen:
 

·
Registered
Joined
·
10 Posts
Discussion Starter · #9 ·
Thanks for the info. How can you tell if the spark is too weak? I pulled each individually and let the engine idle and the spark looked the same on both sides. Also - the bike seems to idle fine w/either plug is pulled. The air filter is clean.

I did take a pic of the right plug though, keep in mind, I just changed this 2 days ago and have probably only put 15-20 miles on it since changing it. Looks pretty bad already.


Thanks again!
 

Attachments

·
Registered
Joined
·
10 Posts
Discussion Starter · #10 ·
Starting to think this may be beyond my skill level. Perhaps it's time for a crash course in fixing my own carbs!
 

·
Registered
Joined
·
148 Posts
given your knowledge and experience, i'll take your word over the fiche! and lord knows i've been wrong far too many times :D
 

·
Registered
Joined
·
10 Posts
Discussion Starter · #15 ·
Location: Cincinnati, edited profile. I'll get into the carbs and see what the jets are. Assuming they are the same on both sides but the left side is running appropriately, would you leave the jets the way they are and assume floats?
 

·
Registered
Joined
·
6,495 Posts
With stock exhaust and air cleaners, a K7 would indeed run 140-150 main jets.
Mixture screws should be the "stepped" profile instead of the earlier plain taper.
Most 450's should never require rejetting unless really radical changes have been made - for the most part they compensate for altitude changes nicely by themselves, just the nature of the carbs.

However, things change in 40 years - I'd save messing with the jets as a last resort.
Try the floats first, I do them "wet" - here's a PDF - http://home.comcast.net/~tbpmusic6/carbsetup.pdf

Ever check your compression??
 

·
Registered
Joined
·
3,926 Posts
Ok. Sorry to everyone for the wrong information. Apparently my memory is getting REALLY BAD as I get older. I would've put money on the jets being 130-135's as I thought that was correct. I just went and pulled 4 carbs out and looked at them. Low and behold - 150 main jets.

Again, sorry for my mistake.


P.S. I don't have that much experience (with 450's only, especially) and I'll be the first to admit that fact.



GB :mrgreen:
 

·
Premium Member
Joined
·
667 Posts
My 1970 CL450 has 130 mains. I havent looked at the number on my 1972 CB450 but I had bought some parts carbs from ebay one time that were listed as fitting that bike and they have 145 main jets. I am in the Cincinnati area and would be glad to let you try the jets that I have. I know I read somewhere that the reason for the difference in the main jet sizes from a CL and a CB is because of the pipes.
Don
 

·
Registered
Joined
·
30 Posts
I've also had the same problem. But mine fouled the left side. It used to do it after 20-30 miles of riding. Here's what I have done trying to fix the issue ---
* - Clean carbs about ten times
* - Adjusted the timing, static and dynamic
* - Swapped carb parts from one side to other, jets etc
* - New points
* - New coils and leads
* - New needles and needle jets
* - Compression test - around 150psi
* - Rebuilt top end, rebore and valves done. Had a broken second ring - was sure I had now found the problem
NO - Still there

At this stage I took it to a bike shop with an old guy, and a dyno. I spent the afternoon with him, tuning the bike on the dyno with a gas analyzer. We checked the ignition, was fine, the left side ran richer than the right side under load and cruising, but was still ok. He checked the carb sync, which was fine, the only thing that was out was the idle mixture was too lean. The tech stated that it as carbureting very nicely.

The bike was better after this, although we really did not change much. I then cut the paper from the air filters and glued in Uni Filter, and oiled it. The bike lightly fouled a plug about 100mls, I made sure the oil was well displaced on the foam, I did not replace the black filter covers, just the side covers and changed the plugs to BR8IEX. I have now done around 500 miles on the same plugs.
I firmly believe there was no one single issue that caused this problem and the left side plug is still darker than the right.

Issues like this are very frustrating - don't go crazy trying to fix it :shock:
 
1 - 20 of 21 Posts
Top