I’m starting this thread because I want (step by step) try to gain more power from a CB72 engine. I’ll give myself a year to get as much (but within reasonable limits) power out of a CB72, but not without making it unreliable.
Any ideas are welcome, tips, tricks, and storys.
A modern 250 cc twin engine delivers more than 30 HP, very reliable., and within noise limitations forced by low. I think I can get to 30 HP or more without increasing the volume.
I have a standard CB72 engine (late) as a basic start.
Since I don’t want to change it’s volume, I’m stuck to the standard bore. In this case power is gained mainly by the cilinder head, pistons, carbs and exhaust.
A few months back I bought a few cylinder heads for a CB72. I decided to take one (one with some fins broken off) and start to make the entrance aswell as the exit a little more up to it’s task.
Some people cut off (grain) the valve guide to clear the entrance and exit area, I decided not to because the live cycle time of the valve / valve guide will be shortened and that’s conflicting with the first sentence.
I also decided to use steel valves, Using titanium valves will shorten the live of the guides, because titanium does interact strongly with the material of the guide, in a negative way. The only way to avoid that is making guides from other material, and that’s going a little above my head and budget.
Valves out of stainless steel do have a shorter live time, are not lighter, so there's no real advantage’s.
I’m still struggling with the carbs, I can make a choice out of these :
Yamaha RD350 carbs (good standard choose, almost no atmospheric influences)
Kawasaki KR1-s karbs (these are flats-sides) (good for top speed, but atmospheric influences)
CB77 carbs (26 mm) (simple and easy to bolt on)
TZR 250 carbs (good carbs, I have gained a lot of experience with these)
CR carbs (exclusive, nice but a lot of atmospheric influences on the fuel / air ratio)
I decided to let the inlet 22 diameter, for as long I didn’t decided what carbs to use.
A few weeks ago I started with the head, pictures of what it became are displayed.
I doesn’t seem to be a lot of work, but in this head are more then 4 ours of graining and measuring.
Please compare with a standard head, and see the differences.
30 HP out of an CB72 ?
30 HP out of an CB72 ?
Last edited by jensen on Tue Apr 13, 2010 1:33 am, edited 1 time in total.
assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen, I have heard that two stroke carbs don't work well on four strokes. I believe the idle circut is too rich, and can't be leaned out enough with jetting. CB450 carbs are supposed to give more power, I haven't tried it yet but would like to on my CL77 with 3mm over pistons. Megacycle, about 20 minutes away from me offers regrinding of cams, and hard facing of rockers. Not cheap at all. here's a link to thier website.
davomoto http://www.megacyclecams.com/
Hi dave,
Thanks for the input about the carbs, but do you know someone who put these carbs on a CB72 ? I did work with the flat sides (KR1s) on a cb72, and it run great, except in the middle regions. The needle wasn't good, and Kawasaki didn't had a needle other then the standard needle. The top was perfect, so where the low regions (after an extensive job on the dyno). A RD350 carb was a little too much on a standard CB72, but this head will have a much bigger flow, so I don't know what will happen. Now I have a very good Emco compact 5 turning machine, and I may be able to design my own needles in all kind of shapes. Now about the cams. I 'll use the early CB72 cam's, I have a new set somewhere. If this isn't enough to get to 30 HP I'll remember the megacycle cams. I know them well, I have a megacycle cam in my CB400F, it runs fine after re-jetting the carbs. I don't want constant vacuum carbs on my CB72, I like the response of the direct carbs, it gives the CB72 it's character. Jensen
Last edited by jensen on Tue Apr 13, 2010 1:33 am, edited 1 time in total.
assembly of Japanese motorcycles requires great peace of mind (Pirsig)
Jensen, I had a megacycle cam in my CB400F back in 1979! Also head was ported and polished, great running little bike. Sold it to buy a Harley, long story, sure wish I hadn't ! I would love to get my hands on two Honda CRF150R carbs, Kehien flat slides, and endless jets and needles available. They usuall sell for $150-$200 a piece on Ebay, and manifolds would have to be made. i believe a huge performce gain could be realized with these carbs. Again, maybe too much carb for the 72s.
davomoto
Which compression piston are you planing on using?. ......lm
Jensen
On one of the motors I'm playing with with I'm going to make phosphor-bronze valve guides and slim down the ends rather than cut them down. I read somewhere - may have been CB160 site - that you need to add a little metal to the floor of the exhaust port to improve the flow. I always like to do something to improve the flow from the inlet valve at low lifts. On the 500 triumph twins they used to put a small chamfer at the top of the cylinder liner to remove masking at low lifts. Do you have a flow rig? I have always fancied making one. All you need are some manometers, a vacuum cleaner and a big box to attach the head / barrel. You could have great fun. I went for an interview with Ricardo Consulting once and turned down their job offer - stupid boy! What are you using to get the valve seats the right width? Phil Irvine's "Tuning for Speed" is a very old but excellent book on engine tuning. Have fun! G '60 C77 '60 C72 '62 C72 Dream '63 CL72
'61 CB72 '64 CB77 '65 CB160 '66 Matchless 350 '67 CL77 '67 S90 '77 CB400F
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