D ports and jetting

TBW · 2026-09-13T15:18:47-0600

YING said:
Schwany,
I thought I should throw in my thoughts on your post. I have a stock 850 with Premiers and the d-port inserts on the exhaust. I don’t know if they are a plus or not at the high end above 6K but my bike could not run better as a daily rider in my opinion.
Mike

Mike, Thanks for your info Just running in after rebuild so 6K a way off.

TBW · 2026-09-13T15:20:01-0600

baldy said:
I tried the d exhaust ports on my 850. No jetting change but had to reduce idle speed. They do something but hard to say if they increase power.
George

George, what caused you to reduce idle speed ?

TBW · 2026-09-13T15:23:56-0600

jseng1 said:
The main change with the ex port floors is that they have an anti reversion effect. because of the step at the bottom of the port where it meets the ex pipe. Outbound exhaust gas have enough momentum to blast past the step smoothly, but low-energy return pulses at the bottom of the port hit the wall of the step and break up instead of re-entering the port and contaminating the combustion chamber. I don't know for sure if this has enough effect to require rejetting or not. In a highly tuned engine I think it would.

Hybridracer - the earlier twin carb XR750 had round ports. The cobra head ports didn't come in untill the mid/late 1980s and gave a signifitant HP increase. Now you see that shape in nascar motors.

Jim, thanks for clarity. I’d thought they would speed up exhaust gas extraction (but in a different way) with associated increased charge volume and additional fuel requirements.

At least the Alan Cathcart question got resolved

TBW · 2026-09-13T15:26:28-0600

Torontonian said:
All I can say on this subject is :
Very happy with the reduction intake tract sleeves . Perfect fit and 30 mm. performance attained .
The D-Port exhaust kit , not so much . Mostly the fit . A bit sloppy fit and then carbon built up between the floor insert sleeve area , so I will figure out a better way to make it work . Back to stock on that one and now happy . Blasted about today . All good .

Hey Torontonian, I had same reaction to inlet tract sleeve…noticed better / faster pull - fitted before recent rebuild.
My D ports fitted just fine FWIW.

Schwany · 2026-09-14T09:21:01-0600

TBW

With all due respect and some excessive curiously, if these sleeves are working great, why ask about carburetor jetting?

To anyone else that might know, did maaseyracer ever put a 750 head fitted with these parts on a flow bench or a complete bike on a dyno before and after installing the sleeves to verify theoretical speculation?

TBW · 2026-09-14T10:43:21-0600

Schwany said:
TBW

With all due respect and some excessive curiously, if these sleeves are working great, why ask about carburetor jetting?

To anyone else that might know, did maaseyracer ever put a 750 head fitted with these parts on a flow bench or a complete bike on a dyno before and after installing the sleeves to verify theoretical speculation?

Hi Schwany, no explanation needed.

As mentioned, inlet tracts were fitted before D ports ( 250 miles prior “rebuild” -prompted by sudden oil pressure loss resulting in bore hone, new rings and shells thus preventing proper test. Oil pump seal…thank goodness for o8l pressure gauge!)

So was still breaking in / assessing Inlet tracts but noticed smoother pull during accel .

D ports fitted during this rebuild - I posed question on assumption of inlet and exhaust ports’ cumulative effect on carburettor and knowledge of the forum.

First shakedown /local test ride today and my “tuned” ears detect exhaust tone change as revs gently past 3000.

D ports and jetting

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Schwany

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