Just gone 701 to 753 with porting..

[Superjetjim]

I've just upgrade my 701 to 753cc and I'm after the input from you tuning guru's!..

Here's what I have now:

FS Ported 753cc Cylinders (from stock 701)
Twin 38MM Carbs with Choke Butterfly removed
Tau Ceti Flame Arrestors
Factory B Mod Pipe
185 PSI ADA Girdled Head & BR8ES Plugs
MSD Ignition
Carbon Tech Reeds & Riva Stuffers
13/15 Solas Concord Setback

Everything was running great - good bottom and top end when it was as above but with stock 701 cylinders - since the porting/753cc has been done (no other changes) it has no bottom end at all (less than stock) - its like there's a second or so of labouring then it clears after mid RPM it pulls cleanly and alot stronger than before - below half RPM it feels like its hardening off but richening it up at the bottom jet just makes it smoke and doesn't help... keep it above half RPM's and its great..

So is my jetting out? (not sure what the jets are - but I know there not stock - might be 135)

Am I getting to much water through the pipe?

Could the pop off/springs be wrong?

I've reduced the water going to the 'B' Pipe stinger and opened the screw on top of the header pipe 3/4 turn and its a little better but still not right.

Factory pipe screws are 3/4 turn open on top of the pipe - middle closed - underneath pipe is as it came from factory.

Carbs are 1 & 3/4 turns out high needle and 1 & 3/4 on low needle.

Any ideas whats up with my settings or jetting or what you think it should be?

Thanks for any help you can give:shrug:

Jay

 

[Matt_E]

:bigeyes: :bigeyes: :bigeyes:

Whoa there buddy....if you went from stock porting and stock bore to a ported 753, you absolutely have to change the jetting. You are lean, most likely. Don't run that motor until you change the jetting.

Are these dual 38mm carbs? Start with 80 pilots, 142.5 mains, 1.5N&S and 95g springs.
Work from there.

 

[Big Kahuna]

not enough info

 

[Superjetjim]

Matt E - yes they are twin/dual stock 38mm Carbs - thanks for your help so far.. I take it richening up the screws with same jets as when it was 701 (and had all the other items already fitted) would not allow enough fuel to flow to cover the extra 53cc?.. I'm over simplifying this by the sounds of things... take the porting allows for loads more fuel too?

I'm a newbee at all this carb setup!!

 

[sprocketq]

jimwhenyou builtthis have youjust boredit out??have you changedthe exhaust port height? have you strippped the carbs and checkedthe filters? if its labouring ie ( judders) thenitsway 2 rich ifit dropstonothingthen goes like a scolded cat thenits lean. 135 mains willwork but you may have to take the top out a bit ( its not all on the bottom jet mate)

 

[Matt_E]

The adjustment screws will NOT do the job!

I assume this a Sport Port port job? The added displacement doesn't change the jetting as much as the porting.
Porting can and will throw the jetting off by quite a bit.
Post some info on the port job, i.e., who did it, and what was done.

 

[Superjetjim]

Matt,

Similar to Sport port job.. but not identical - I could get details tomorrow...

Sprocket,

Yeah carbs had a full strip down and clean by me - all filters are clean, installed new gaskets all round, new crank seals etc.. I've got a side exhaust so I can see the colour of the exhaust gas (it rarely looks clear/lean) and has a healthy smoke to it as before.. it just has NO power low down.
I know what lean and rich feels like (used to have 2 Stroke RC cars/plane/boats ect) and it just doesn't feel like either.. hence why I thought it could be the B pipe water settings.

Wouldn't pop off/wrong springs cause this feeling of flatness?

Thanks for your help guys..

 

[Big Kahuna]

Your jetting is wayy off.

 

[Matt_E]

Matt,

Similar to Sport port job.. but not identical - I could get details tomorrow...

Sprocket,

Yeah carbs had a full strip down and clean by me - all filters are clean, installed new gaskets all round, new crank seals etc.. I've got a side exhaust so I can see the colour of the exhaust gas (it rarely looks clear/lean) and has a healthy smoke to it as before.. it just has NO power low down.
I know what lean and rich feels like (used to have 2 Stroke RC cars/plane/boats ect) and it just doesn't feel like either.. hence why I thought it could be the B pipe water settings.

Wouldn't pop off/wrong springs cause this feeling of flatness?

Thanks for your help guys..

No...ALL of your jetting is off. You cannot go from a bonestock 701 to a ported 760 and not change the jetting. You are lucky you didn't seize the engine with too lean a mixture.
It may be worth your while to have someone knowledgable with carbs help you out in person.
Your popoff is likely okay at 1.5NS & 95 grams springs.
But your Hi & Lo jets are off.

 

[Superjetjim]

Big,

Do you concur with Matt's recommendations of:

Start with 80 pilots, 142.5 mains, 1.5N&S and 95g springs?

I need to order the correct items tomorrow so I can get them fitted for the weekend..

Thanks,

 

[Superjetjim]

Matt,

Thanks - just to clarify it wasn't bone stock before - it had all the mods listed just without porting...

 

[meatball]

porting and B pipe are both major items that both require a change in jetting. Extra CC's doesnt effect as much, porting definitely does.

 

[Waste Land]

Matt,

Thanks - just to clarify it wasn't bone stock before - it had all the mods listed just without porting...

RC cars and planes are NOTHING!!!!!! like a jet ski carb. the only the thing close is the direction the screws turn.

 

[Big Kahuna]

Be safe, start out at 110 lows 150 high, then work your way down from there, none of us know what will be perfect, I only know of one other cylinder out there that I could tell you what jetting would work, Superjett use to have the matching TLR Ported Cylinder to what I am running, other than that, it is a crap shoot.............

Good luck.......... start rich, go leaner from there......... your N.S. should be ok, you could try a 2.0 N.S. also, but no larger.

 

[Matt_E]

Big,

Do you concur with Matt's recommendations of:

Start with 80 pilots, 142.5 mains, 1.5N&S and 95g springs?

I need to order the correct items tomorrow so I can get them fitted for the weekend..

Thanks,

Oh no....you need to order a whole sleuth of jets. Order everything from 72.5Pilots to 100 pilots, and 130 mains to 150 mains. And I predict you will spend your weekend tuning more than riding.

 

[robertg]

I almost had the same set up last year but mine was 83mm not 84mm. I am assuming that you bored your stock 61X cylinder to 84mm. Mine ran the best with 1.5 needle and 115 gr spring. The pilots were 75 and mains were 135.

It sounds to me like you are too rich down low. Download the Mikuni manual and read it several times.

 

[Matt_E]

Robert,

Did you have it ported? The porting likely was not identical to what the OP has.

 

[ski4]

i am in the middle of the same problem, i started with a 1.5 n/s 95 gram spring, started jetting at 137.5 and 75
ran like crap
fattened her up made it up to 85 on the bottom only to get worse ( wouldn't go past idle) brought her back to 80's
and then took the main to 145 eventually, also made things worse never better

i have leaned her out, now i think i am at 80 on the pilot and 132.5 on the main ( which scared the hell out of me as factory pipe calls for a 135 i think on a stock motor) but it seems to be working the best and i am still on the rich side, and need to tweak some more but i have some top end at least now my set up is similar to your
ported, b pipe, twin 38's stock air box and 82.5 mm bore

i have learned that porting can actually lower the fuel demand ( thanks matt e) and that was hard to swallow for me just seems counter intuitive

 

[robertg]

Robert,

Did you have it ported? The porting likely was not identical to what the OP has.

Im sure my porting will be different. My exhaust was raised 1.5mm and widened, and the transfers were cleaned up.

If you compare several 61X cylinders the porting will be slightly different in all of them. It looks like Yamaha cut the transfer ports with a torch.

 

[Matt_E]

LOL...true. Even slight differences in porting can make a huge jetting difference.