Topic: CB550 with CB750 carb, regrined cam - jetting for racing

[vintage_racer]

Hello,

I was racing my CB550 with Yosh pistons, really radical camshaft and CB750 (K2) carbs with velocity stalks and a 4-1.
I used 122.5 main jets and plugs were looking right (maybe just a tad fat, but on 120 mains they were a bit lean).
At this setup there was very little going on below 6500rpm but the motor was strong from 8000 and up.

Now I rebuilt the motor with stock Honda pistons and a mild camshaft regrined. I will keep same carbs and exhaust.
What main jets should I use for baseline? I was thinking 112.5 or 115 but really have no experience with close to stock setup.

[Bill/BentON Racing]

With your new set up ,those carbs are going to want those missing cc's! 1975 750K models came with 105's.You might want to try 100's or 105's,lean out your needle position one notch and try it.Could still be to much for your set up.Good Luck,Bill...pilots available to 38's,comes w/40's.

[bwaller]

I haven't been able to make a race yet, how's the twin go Dan?

[vintage_racer]

Hi Brent,
Only raced the CB450 in Shennonvile, On Sat it went very well, finished 3rd in heat race, one second per lap slower than on the CB550, considering it was my first time on track it went very well. This bike handles so much better than the 550, very stable in fast corners and easier to throw around. Obviously the drum brakes are a bit of an issue after 3 or 4 laps. the motor shakes as a paint mixer but the low end power and drive out of corners is much better than on the CB550, less top end power though.
Things didn't go as well on Sunday, I started running out of gas with 2 laps to go (my fault), finished the race one before last on a single cylinder (right hand side) and the tank was BONE dry.
Now here is why I'm worried: There was oil leaking of the exhaust port of the left cylinder, so it indicates that the exhaust copper seal is a goner and I new about it but how and why the oil will get into the exhaust port?
With fresh fuel the motor started in my driveway on both cylinders but the left one was running so much cooler and actually quite cold, both carbs are setup equally and both sides are very rich but why is there so much difference?
Any suggestions?
Can it be that in the process of running out of fuel, the left cylinder was so lean that it caused detonations and now there is some damage?

BTW: the CB550 motor needs another 2-3 hours before it's ready to go into the frame (now with much longer rear shocks).
The motor is rebuilt with fresh everything (pistons, gaskets, camchain,etc..) will start it over the weekend. It wouldn't be as strong as the Yosh motor was with it's radical camshaft (and very low compression due to humongous  valve pockets) but should have good bottom end.
At least will have two bikes for Mosport and will run 4 categories (can run even in more but figured out that 4 heat and 4 finals are enough).

Are you coming to Mosport?

Dan

[bwaller]

Yes I'll try to make it to Mosport. I've got to re-do Merciers fast school in order to secure my race license, it was called on account of rain a while back.

Why don't you do a leakdown test on the 450?

[Sam Green Racing]

Can it be that in the process of running out of fuel, the left cylinder was so lean that it caused detonations and now there is some damage?

Dan, it could be one of three things.
(1) I'm not sure if the 450 has seals on the valves/guides but that would let oil into the the exhaust port. The very weak condition caused by continuing to run when out of fuel would heat up the exhaust valve to such an extent, it could have damaged the seal.
(2) If you have holed a piston, this would also let oil into the exhaust port.
(3) It may not be oil at all. If you have burnt a valve due to the weak condition, the lack of compression would cause any fuel that did get in there not to burn properly, this would show as a leak at the exhaust gasket.
Like Brent said, do a compression test.

Sam.

[vintage_racer]

Sam, Brent,
Thanks for the information.

It was oil leaking from the port, not gasoline, for sure.
Compression test isn't possible since the bike has no kickstarter (or starter) at all.

I was trying to find a leak-down tester for a while and will try again.
As I mentioned, that cylinder is firing and running now, it's just running very cold ...

I more or less finished putting the CB550 motor together yesterday night and will put it in the frame tonight.

Brent - I had an occasional clutch slip before though it improved over the seasons. I decided to put in the Barnet clutch basket with Barnet plates and springs.
What I had noticed is that the first friction plate from Barnet wouldn't fit, it's too wide for the groove in the back of the clutch. I found other old friction plates that do fit as first plate. Am I doing anything wrong there? maybe the plate that I'm trying to use for the back of the clutch (the one with the 4 threads for on which the springs sit) is not for a Barnet clutch?
With all the plates in place I have about 1/8" of room in the basket for proper disengagement??
 

[bwaller]

Dan, sorry I'm not sure what a Barnett clutch basket is (pics man pics  ) I think 1/8" is probably enough free play. Here is a picture of three stacks, the one on the right are Barnett friction discs and stock steels, looks like about 1/8" play.






What year is the clutch, there were chamges to the 77/78 model years.

We've all seem disastrous results from lean conditions, but if you can get a tester & do a leakdown it may offer something. I'm not sure what else to tell you.

[vintage_racer]

Brent,
Thanks for the picks, looks like mine is a Barnet, I also only have stock steal plates and Barnet friction plates.

We are not only suffering from the "lean condition" what we are suffering from is stubbornness  .
It's hard enough to keep a 45 years old bike in running condition.
It's hard enough to keep even a modern racer running.
To try and race a 45 years old machine in a high state of tune is simply sick.

I had decided too many times to switch to modern (i.e P4 up to 1989) machines for the sake of being able to be around my son and so far I just keep going to even older race machinery.

One day I will have the brain to either quit road racing all together  or switch to some new (1989) FZR 600.
For now will keep spending my weekends in my basement trying to sort out those old Hondas.

[turboguzzi]

hei dan

remember you posts about ride height... so sounds like you took your sohc to the track, could be nice to see some pics.

regarding the fuel running low, cant believe that's the reason, how many times we run out of fuel and nothing happens to the motor....the engine might go through a short lean state but then its too lean to fire and thats it.

cant believe that's the reason for one cyl not working, sounds like ignition or clogged jet to me.

i have barnet frictions/stock steel in my 500, no problems to fit in the stock basket, like brent, never heard about a barnet basket!

if the engine is burning some oil, then black goo will leak through a bad ex. gasket, not sure thats a problem, did it smoke badly when it ran?

TG

 

[vintage_racer]

Hi TG,
I agree, if the cylinder is cutting due to insufficient fuel, it will pass lots of cooling air through the combustion chamber without ignition and wouldn't end up with overheating symptoms (i.e - detonation or burned exhaust valve).

The motor is burning some oil but not that much, probably mostly due to bad valve guide seals.
again, this cylinder is running now, it is just running much colder than the other one and I can't test ride the bike on the street (live in downtown Ottawa) to see if it lost any power after the incident of running out of fuel.

I didn't run the CB550 this season yet, just installed the rebuild motor in the frame yesterday night. Will see how the new long shocks will function on the track though at Mosport it doesn't mater much since this track is all about power and being brave, only one slower corner where you do need a flickable bike...

I don't post pictures of my CB550 since people claim it's the ugliest in our club (VRRA), the frame and tank are all rust spots and the tail cover all crashed and repaired with pieces of fiberglass. My car trailer and street bikes are all like that as well, I just don't care about appearance (accept woman's  ).