B12 dot head and cams but what carbs?

[Svsam]

Evening all,

Ive been quietly reading though this awesome forum for help with my current build (one I've always want to do)

so I bought a h reg (1990?) 750 with a 748 dot engine minus carbs, it had USDs some seemed a good place to start for the money. 

Instead of sourcing loads of bits I bought a 2002 bandit 1200 donor bike also at a very good price as it was cosmetically umm rough. 

So ive got the head off both, put the b12 cam sprockets on the dot cams. I'm going to take the head to be cleaned and maybe skimmed then start the rebuilt. 

My question is..

since I have only have bsr36ss carbs, are these okay to use on the build for now? Or will it restrict any potential gains I'm aiming for?

should I be looking for bst38/40s?

Trying to add a picture but this iPad is beating me!

 

Any help appreciated

Sam

[Oilyspanner]

Welcome to the site Sam.

36mm carbs are very good, flow well, easy to set-up and work well with dual pod filters - plus they're smaller than the 40mm carbs, so will fit in the under tank space more easily. Unless you're having the head flowed etc you'll not be stunting the top-end much at all, but you will have stronger power through the normal rev range - a couple of hp at the very top isn't much use on the road, more power and torque everywhere else is far more useful.

[MeanBean49]

36 carbs are plenty good enough,

Personally I wouldnt bother with the head swap at all, just use 750 cams in the bandit head, only makes a couple of hp difference doing the whole head over the cams and mid range is better with the b12 head. 

I wouldnt want to skim the 750 head if you are going to swap either, already makes the compression higher.

[wraith]

Welcome to OSS 

As above the 36mm carbs are a good carb, I have heard the 36mm carbs off the mk1 are better to set up.

I would not go for the 40mm, I run a set of 38mm carbs on my 1127 motor on the road and it always puts a smile on my face 

As to the DOT head, you don't need to get it skimmed as it will already increase combustion and I would put the DOT head on the B12  even get it ported.

Cams, as you have the gsxr cams use them much better than the standard b12 cams, but if you still want all over power instead of top end keep a eye open for a set of gsx1100f cams.

 

[MeanBean49]

1 hour ago, wraith said:

Welcome to OSS 

As above the 36mm carbs are a good carb, I have heard the 36mm carbs off the mk1 are better to set up.

I would not go for the 40mm, I run a set of 38mm carbs on my 1127 motor on the road and it always puts a smile on my face 

As to the DOT head, you don't need to get it skimmed as it will already increase combustion and I would put the DOT head on the B12  even get it ported.

Cams, as you have the gsxr cams use them much better than the standard b12 cams, but if you still want all over power instead of top end keep a eye open for a set of gsx1100f cams.

 

If your going to get a head ported you might as well just do the bandit one. At least then the combustion chamber is the right size for the bores.

[wraith]

1 hour ago, MeanBean49 said:

If your going to get a head ported you might as well just do the bandit one. At least then the combustion chamber is the right size for the bores.

This is the idea of the DOT head on the b12 to increase the combustion, as the b12 is a lower combination than say a 1127 gsxr, as the DOT head has the same size valves as the 1127 etc it will give you more oomph.

[MeanBean49]

10 minutes ago, wraith said:

This is the idea of the DOT head on the b12 to increase the combustion, as the b12 is a lower combination than say a 1127 gsxr, as the DOT head has the same size valves as the 1127 etc it will give you more oomph.

Arent 1127 and 1157 valves the same size anyway?

What i meant was a ported dot head doesnt work as well as a ported bandit head because the combustion chamber is sized for the 750 bore size.

Ported skimmed head thats made for the engine works better.

[wraith]

2 minutes ago, MeanBean49 said:

Arent 1127 and 1157 valves the same size anyway?

What i meant was a ported dot head doesnt work as well as a ported bandit head because the combustion chamber is sized for the 750 bore size.

Ported skimmed head thats made for the engine works better.

Now that make sense  if you skim the b12 head to give you the same combustion as the DOT head and port it, then yes 

[clivegto]

I have a 1052cc motor with a ported dot head & cams ran 36mm carbs on it, goes like a screaming banshee lots of fun lol.

[Sandman]

16 hours ago, MeanBean49 said:

If your going to get a head ported you might as well just do the bandit one. At least then the combustion chamber is the right size for the bores.

What is the right size for the bores?

 

[MeanBean49]

2 minutes ago, Sandman said:

What is the right size for the bores?

 

Ideally the same size as the bore.

[fatblokeonbandit]

Ive done it twice to b12,s

instant cheap bolt on power/fun.

first time in 2003, on the bike below, had it dynod with the pods and the exhaust in the picture, standard motor,  made 109 rwhp after a lot of faffing about, then fitted the DOT 750 cams and it went up to 127 rwhp, later on I fitted the head as well it went to 133rwhp after setting up on the same dyno..

 

dyno bloke did say that it would have been better if some chimp hadn't chopped the can in half..

 

was it worth it to fit the head???  it didn't really feel any different, the extra 5 donkeys weren't noticeable but it picked up much quicker and was an absolute hoot to ride,   pain in the bum sorting the fuel tap though...

 

 

 

[Sandman]

4 hours ago, MeanBean49 said:

Ideally the same size as the bore.

So the combustion chamber should be the same size as the bores?

 

[MeanBean49]

3 minutes ago, Sandman said:

So the combustion chamber should be the same size as the bores?

 

Where the head meets the top of the bores yes. 

[Sandman]

Why?

 

[MeanBean49]

8 minutes ago, Sandman said:

Why?

 

Efficiency, better fuel mixing, better burn, better heat distribution over the piston, etc

[Lachie04]

 

Ok no firm opinion here but I asked this on another thread once

 

"Question I've been meaning to ask would you not have to machine out the squish band to match the new cylinder diameter?

I would think the sharp edge would create a hot spot in the chamber"

 

And I got the reply

 

The squish isn't any different to an 1100 head, the chamber is shallower though which raises the compression ratio. 

The edge of the squish band is the only place in a combustion chamber you want a sharp edge, the turbulence it creates mixes the air fuel better and reduces the size of the fuel droplets, this creates a faster burn reducing the likely hood of detonation.

 

 

[MeanBean49]

1 hour ago, Lachie04 said:

 

Ok no firm opinion here but I asked this on another thread once

 

"Question I've been meaning to ask would you not have to machine out the squish band to match the new cylinder diameter?

I would think the sharp edge would create a hot spot in the chamber"

 

And I got the reply

 

The squish isn't any different to an 1100 head, the chamber is shallower though which raises the compression ratio. 

The edge of the squish band is the only place in a combustion chamber you want a sharp edge, the turbulence it creates mixes the air fuel better and reduces the size of the fuel droplets, this creates a faster burn reducing the likely hood of detonation.

 

 

If that was the case why wouldnt the OEM make the head like that in the 1st place?

[Gixer1460]

Can someone explain the last 5 or 6 posts to me cos it doesn't make any sense whatsoever!

Unless you have a Hemi, pretty much every engine combustion chamber will extend to near the bore diameter in some direction whilst being inset in others - this is usually matched by the piston crown. Where the inset is placed, is where the most 'squish' effect occurs and is designed to induce a turbulent mixture and squeeze it toward the plug to be ignited. A 750 matches an 1100 chamber shape but shallower which increases the CR. Throwing an 1216 kit at an 1100 increases squish (as the head chamber profile hasn't changed so the piston has more 'real estate' to work against) Adding a DOT head may increase that effect a bit more.

[MeanBean49]

Granted this i a bit exagerated and it probably doesnt have a huge effect in reality, but its still quite a difference using a head made for a 73mm bore on an engine thats 79mm bore or 81mm, i would still much rather have a ported and skimmed stock head over a Dot head.

Edited January 30, 2019 by MeanBean49

[Sandman]

Nice drawings, You know that the DOT head and GSXR 1100 head got the same valve size?

[clivegto]

1 minute ago, Sandman said:

Nice drawings, You know that the DOT head and GSXR 1100 head got the same valve size?

What is going on with those inlets on the top one ?

[Sandman]

I assume the inlets are reshaped.

 

[MeanBean49]

36 minutes ago, Sandman said:

Nice drawings, You know that the DOT head and GSXR 1100 head got the same valve size?

Yeah, i said they had same valve size earlier. 750 and 1100 dont have the same bore size though.

[wraith]

4 hours ago, Gixer1460 said:

Can someone explain the last 5 or 6 posts to me cos it doesn't make any sense whatsoever!

Unless you have a Hemi, pretty much every engine combustion chamber will extend to near the bore diameter in some direction whilst being inset in others - this is usually matched by the piston crown. Where the inset is placed, is where the most 'squish' effect occurs and is designed to induce a turbulent mixture and squeeze it toward the plug to be ignited. A 750 matches an 1100 chamber shape but shallower which increases the CR. Throwing an 1216 kit at an 1100 increases squish (as the head chamber profile hasn't changed so the piston has more 'real estate' to work against) Adding a DOT head may increase that effect a bit more.

No  

getting a bit SOC, only OME things to go on that bike shit  to me