836cc kit choices - do I need a new cam?

[theshootist]

Hi,

I would be interested on member's thoughts on my SOHC rebuild. I'm having a set of Wiseco 836cc pistons fitted along with a copper gasket. The head is also being flowed, although the valves will remain standard size and I am sticking with the original carbs. Since I'm having the engine apart to this extent would it be a good idea to go to a different cam, along with stringer valve springs etc, or could I just have the existing cam re-profiled or left alone? Some of what I'm reading suggests a cam is necessary to get much benefit from the slight displacement increase, but there are plenty of builds where the standard item is retained.

Many thanks 

[MrDavo]

I think it depends what engine characteristics you want, and how you are going to ride it. More cc and nothing else will usually give you more torque and 'rideability', from my experience with performance cams, including a CB750 with a Yoshimura cam, they tend to only work well on full bananas WOT where they give you better acceleration, the rest of the time you lose something in the flexibility and smooth running stakes. They weren't designed to pootle through slow traffic.

I used to have a race bike with full race cams, and big bore carbs, that ran so badly on the warm up lap that I considered going back to the pits, ran perfectly in the race, pulling like a train, then coughed and spluttered around for the slow in lap.

[theshootist]

Thanks, point taken.

I didn't say what I was after. I'm not after performance at the top end, a bit more go at the bottom end would be welcome, but I'm fitting the pistons because I'm currently burning oil and have pretty poor performance, so thought I might as well go the whole hog. I'm going to have a word with Kent cams to see whether they can suggest a re-work of the existing cam.

[Greebo]

Hi,

I would be interested on member's thoughts on my SOHC rebuild. I'm having a set of Wiseco 836cc pistons fitted along with a copper gasket. The head is also being flowed, although the valves will remain standard size and I am sticking with the original carbs. Since I'm having the engine apart to this extent would it be a good idea to go to a different cam, along with stringer valve springs etc, or could I just have the existing cam re-profiled or left alone? Some of what I'm reading suggests a cam is necessary to get much benefit from the slight displacement increase, but there are plenty of builds where the standard item is retained.

Many thanks

These engines don't like tuning at all, even just fitting K&N's will upsets the running until you getting the jetting & needle settings right, as Mr Davo has mentioned if you are going to race the bike OK go for it, but as he also says it will run like a dog at low speeds..

Take a look at this video
https://www.youtube.com/watch?v=N8tZ8Vmb7HQ

[philward]

I've been running a cruz image 836 kit (there standard compression) for about 800 miles on standard cams/airbox/exhaust - plenty of bottom end - 'Hondaman' on .net reckons about 8 bhp on top end. Just fitting same with otherwise standard engine to the CR750 replica I'm building. And at £120ish, can't be beaten in my opinion - trigger rates the cruz image piston which are made by NPR I think (Graham will correct me if I'm wrong - they're good brand anyway)
Phil

[JamesH]

Phil - did you need to machine the upper cases to allow the Cruzinimage pistons to clear m? Trigger advised me not to use the 836 kit as this required additional work to clear the cases?

[philward]

No - just got them rebored at a local firm in St Helens - he's a Z1 owner (£50) - my previous research on .net never mentioned re-machining cases

Sent from my MotoG3 using Tapatalk

[theshootist]

Well the more I've looked into it the more it seems costs would get out of control for little gain.

I'm having the heads inspected and since I've already had new valve seals if the valve guides are okay I will probably just fit the existing head with the 836cc pistons. Once I added up gas flowing the head, potentially fitting new valve guides, a new cam, stronger valve springs, it starts to add up to silly numbers!
 

[Yoshi823]

Because I modified the standard 750F2 frame I could work on the top end easier. So for riding abroad I used a standard cam to go with the fitted Yoshi 823 pistons I bought from Dixon Racing. At the time 812 pistons wern't available hence the reason I got the 823 kit. As far as I remember the 836 kit is 0.5mm up on the kit I had. This was the furthest you could safely go with the standard liners. For a 900 kit new liners needed to be pressed into place.

I polished the ports but didn't remove much metal. I fitted ally/bronze valve guides, S&W valve springs, had the seats recut, had the crank & conrods polished, balanced & lightened, fitted a 750K0 solid cam sprocket & degreed in the reprofiled cam I used for fast road use. The base circle of a standard F2 cam was ground to give slightly more duration & lift. This worked perfectly with the F2 carbs & Andrews high output coils, gold plugs & Boyer-Bransden ignition. The standard exhaust was welded up & a CG exhaust bolted up. Worked a treat 




My bike was in a CB750 feature in Superbike magazine back in 1982

[Greebo]

Because I modified the standard 750F2 frame I could work on the top end easier. So for riding abroad I used a standard cam to go with the fitted Yoshi 823 pistons I bought from Dixon Racing. At the time 812 pistons wern't available hence the reason I got the 823 kit. As far as I remember the 836 kit is 0.5mm up on the kit I had. This was the furthest you could safely go with the standard liners. For a 900 kit new liners needed to be pressed into place.

I polished the ports but didn't remove much metal. I fitted ally/bronze valve guides, S&W valve springs, had the seats recut, had the crank & conrods polished, balanced & lightened, fitted a 750K0 solid cam sprocket & degreed in the reprofiled cam I used for fast road use. The base circle of a standard F2 cam was ground to give slightly more duration & lift. This worked perfectly with the F2 carbs & Andrews high output coils, gold plugs & Boyer-Bransden ignition. The standard exhaust was welded up & a CG exhaust bolted up. Worked a treat 




My bike was in a CB750 feature in Superbike magazine back in 1982

Yoshi What do you gain in performance / genuine BHP after all this work ?

[Yoshi823]

I didn't have the bike Dyno tested as they were not very numerous back in the day. I'm not sure which one modification made the biggest difference either, as I did many of them in one go. What I do know from my backside dyno is that the engine accelerated quicker with much more torque in the bottom & mid-range & on several occasions beat my brother on his standard GS1000S in straight line acceleration & top speed runs. The bike also handled better than his bike, much to his annoyance 

[Rozabikes Tim]

RE BHP I used a big bore kit on my my DOHC 750, taking  out to 823cc + cams + flowed head and carb mods. Gave 95 BHP at rear wheel after proper dyno set up at TTS. Big percentage gain over 78 BHP ish (at crank) for stock I think?

Keeping in mind you are talking SOHC / 8v, therefore poorer breathing and less output to start with, something less than the 95? Not very scientific I know....

It did transform my bike both in terms torque and top end but yes to get all the bits working together it is a big expense to consider....

[theshootist]

RE BHP I used a big bore kit on my my DOHC 750, taking  out to 823cc + cams + flowed head and carb mods. Gave 95 BHP at rear wheel after proper dyno set up at TTS. Big percentage gain over 78 BHP ish (at crank) for stock I think?

Keeping in mind you are talking SOHC / 8v, therefore poorer breathing and less output to start with, something less than the 95? Not very scientific I know....

It did transform my bike both in terms torque and top end but yes to get all the bits working together it is a big expense to consider....

Thanks. My engine was not performing very well to start with, if an 836cc bore with standard head and cams can get me anything above the stated standard output then it will feel a huge improvement.

[Greebo]

I have to say after the complete rebuild [to stock] I am very happy with the stock performance of my 750K6...IMPO if you want faster get another bike..
My Thruxton is faster than the Honda & My Ducati is faster than the other 2... But I get a Real Buzz riding them all ....