Author Topic: Questions re Yoshi mods  (Read 225 times)

Offline Seamus

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Questions re Yoshi mods
« on: April 17, 2019, 08:59:44 AM »
My 500 has had a 605 Yoshi big bore kit fitted and also a cam with a Yoshi grind, it is running standard 4 pipes and airbox. Seems to run well enough, but not had a standard 500 to compare.
Out with some friends and they complain of a smell of petrol when following me, odd because there are no leaks and it runs smoothly and idles well.
Anyone know if the valve clearances change for the Yoshi cam?
Not sure whats in the carbs as I did not do the work, but assume the jets would have to go up a bit.
Ignition is by dyna and is correctly set, but will recheck as I have a new strobe that needs a test.
I know I need to do a plug chop to see how its running, so hopefully this weekend as the weather is supposed to be good.
Anyone with any general knowledge of this conversion as there is not much on the internet.
Thanks in advance

Offline Johnwebley

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Re: Questions re Yoshi mods
« Reply #1 on: April 17, 2019, 09:23:40 AM »
I also don't know any information on different jet sizes.
Suggest you check plugs gap etc and  colours. The do runs at 5K.And 7,5K and full throttle.

See what colours you get.
I would think running 600cc on standard carbs would give lots more mid range grunt.But the carbs could strangle it at top end. They would need to pass 25% more air to fill the cylinders.

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Offline K2-K6

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Re: Questions re Yoshi mods
« Reply #2 on: April 17, 2019, 09:50:50 AM »
Undoubtedly specific knowledge would help here with straight numbers for jets if anyone has relevant experience.

In the past people have just assumed that things like this "just need jetting up " but the same carbs used for the smaller engine will have a greater volume of air going through them......... Which also takes more fuel from standard jets,  so it's never as big a change as presented or believed.

You can usually smell an old bike or car ahead in a line of traffic currently as most engines don't kick out unburnt fuel,  so it's very noticeable when you follow one that does.

A basic start view could be to get a reading from each pipe on MOT test equipment,  HC (unburnt hydrocarbons) level will tell you straight away if it's really bad.

I know you'd not want to run it lean,  but heading to bore wash levels is just as bad with people not seeing it as much of a risk.

Offline Bryanj

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Re: Questions re Yoshi mods
« Reply #3 on: April 17, 2019, 11:16:13 AM »
Depends on the type of cam but i remember the "street" cam needed a lot more advance than a standard cam plus we had to fit a bigger back sprocket as the extra power from the grind only came in at higher revs and low down power vanished

Offline ka-ja

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Re: Questions re Yoshi mods
« Reply #4 on: April 17, 2019, 07:32:09 PM »
I don't know about the 500 yohi kit
The 400 to 460 kit would run quite well on standard carb settings, they can then be assessed for final tuning once running, the bigger bore increases power at a lower rev range, the "road and track" cam puts the power band back up the rev. range, it also allowed a one tooth increase in the final drive sprocket to use the extra power.
nice bike,nothing in the bank

Offline Seamus

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Re: Questions re Yoshi mods
« Reply #5 on: April 18, 2019, 08:17:18 AM »
Thank you all for your comments.
Will get to play with it soon, I can take it to work next week and put the mot emissions probe in each exhaust and see what I get, (good idea K2-K6).
Will also do some digging and see if I can find out which Yoshi grind was used. I have a receipt somewhere.
Will post any useful info that I find

Offline Seamus

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Re: Questions re Yoshi mods
« Reply #6 on: April 18, 2019, 08:59:31 AM »
Had a dig round and found the original advert for the bike. My friend bought it and spent a lot of money to get it looking right. Unortunately, he could not ride it due to a serious illness and has given it to me.
Specification :
1972 CB500/4 K1
605 Yoshimura big bore using cb 550 barrels, decked to compression ratio of 10:1
1/2 race camshaft, street special. YH500-121-00
gas flowed and polished head
Dyna ignition
Dyna coils
twin disc conversion
Oil cooler.
It came with the original forks and hub (500/4), originla 500 barrels and pistons and a spare set of big bore liners.
The chap who did this work restored classic racing cars for a living, so assemed he knew what he was doing. I may be able to get his contact details from my friend.


Offline K2-K6

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Re: Questions re Yoshi mods
« Reply #7 on: April 18, 2019, 05:52:42 PM »
It'll be interesting to see roughly where it is to give you a guide.

I know it's not run under load for those about to point that out,  but it does give something to understand and work on.

Offline Seamus

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Re: Questions re Yoshi mods
« Reply #8 on: April 18, 2019, 07:04:39 PM »
Will report back next week. To be honest, I dont know if it goes well for what it is as I have nothing to compare it with. Last bike was a 600 bandit and a fireblade before that. Kind of throws the seat of the pants thing

Offline Seamus

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Re: Questions re Yoshi mods
« Reply #9 on: April 20, 2019, 12:30:42 PM »
Well, I popped into work today for something else and checked the bike while I was there. At idle, I got the following.

                      Cyl 1       Cyl2       Cyl3        Cyl4
CO                    2.0        1.8          2.0         3.3
CO2                  2.6         2.3         2.3         3.2
Lambda            2.7         3.3         3.0          2.1
HC                  840          293       600         335
O2                  15.2         15.5      15.5        13.1

At approx 4000 rpm

CO                    3             3            2.2           4
CO2                  2             2            2.2           3
Lambda           2.5           2.7         3              2
HC                  900          310        110        250
O2                  14             15.6       15          13

Going by the HC values, it is running rich, a lot, especially No 1. Will try float levels first and a general tune up, valve clearances (assume to be standard). Running dyna coils and dyna ignition and non ressistant caps and plugs. Perhaps a new set of resister plugs, but will do a search to see if these are required with this set up

Offline K2-K6

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Re: Questions re Yoshi mods
« Reply #10 on: April 20, 2019, 01:49:57 PM »
That gives an interesting look at the current status. No wonder it's smelling of petrol.

As you've said,  something well out of range on no1,  seems a bit of a gusher on that one.  Although if it had any mis-fire on that cylinder it wouldn't help the unburnt fuel total.

Also appears to have idle jets 1 or 2 steps higher than needed,  be interesting to see what numbers are in there.

looks like carb balnce is ok for 1,2,3 with 4 sitting slightly lower judging by the O2 readings.

Unless there's anything specific about valve clearance to give confirmed non std,  then I can't see why normal tolerancing wouldn't be in place.

Offline Seamus

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Re: Questions re Yoshi mods
« Reply #11 on: April 20, 2019, 06:18:18 PM »
Yes indeed. Some sporty cams have increased valve clearances. Put a Piper in my MGB and that increased clearances by a couple of thou on the exhaust and 1 on the inlet. May be worth opening them up to about 0.04mm inlet and 0.05 exhaust.
Oddly, it idles really well and sounds perfect. Runs smoothly and pulls well through the gears. Holds a steady 80 up hill and still will accelerate.

As you say, carb balance is about there.

Guess, I will pop the bowls off and have a look. If necessary, I can remove the carbs and see if there are any oddities