1996 XP 787 low speed circuit

[TurnerBFC]

Good day all!

So i just rebuilt this 787 and dropped it in the ski full carb rebuilds with oem mikuni parts.
settings and pop off are as follows:

pop off: 31PSI on each side
LS - 1 turn out
HS- 0 turns out
Pilot - 70
Main - 142.5
Needle 1.5

I know these are rave valve engines and low end can feel sluggish but i think its still pretty rich down low. not necessarily bogging, but has a gurgle in the low end.
If i punch it out of the hole its great but if you want to do a half throttle cruise its very gurgly (best word i can use to describe it) or if you are up cruising on plane and let off the throttle very slowly, you can feel the circuit change over so drastically and the power falls off and just blub blub blub - gurgles.
also, fuel consumption is significantly worse than my other identical 96 XP. i played with low speed needle with little to no effect.

I was reading around the internet that 30 psi pop off is still pretty low. and that pop off and pilot jets have a direct relationship. so what are the opinions of putting 65 pilots in it?
any other suggestions? too lean? anyone have this issue?

TIA

 

[TurnerBFC]

Anyone? Really lost

 

[Mekanix]

Same issue for me on the gsx.
Tried smaller pilots, same issue. Tried higher pop off. Same but was too high and it ran lean/died above half throttle.


Next I'm trying:
lots of carb cleaner through the pilot circuit. Because my low speed screws do very little if anything.
Pop off from 20 up to 42 in small steps.
New oem rotary valve.
Proper squish clearance with the right base gasket.

Mine was always like this and I could never really get it right but with enough playing I could get it pretty smooth.untill below 1/4 throttle.
But I did notice one day that after a very long rescue cruise that the bottom end was even more gurgly. Turns out the wsm rotary valve I got was too thin and warped/ wore out.

I'm about to reinstall my engine and start troubleshooting. Ill let you know if i learn anything.


But to start off with I would rebuild again and this time use half a can of cleaner in the pilot jet with the ls scew closed.
Then more with it open 1 turn.

Here is one relationship between pop off and pilot jets:
High pop off means you need lower pilot jets or low pop off means you need higher pilot jets to get the same result. You can compensate with one or the other. But one combination might work better than the other though.

So with a 70 pilot I've been told they need 38 psi to work properly and that is with a black spring.

 

[Mekanix]

I have noticed that mine also has a pop off of 31 psi with the black spring. I used a shim under the spring to bring it up to 38 which is exactly 1mm thick. I'm assuming old carbs wear down and need a bit of help.

 

[Mekanix]

Finally got mine out today and its still the same crap... Feels rich on low end from idle to 4500 ...
Gurgly and almost like its missing or mixture is really wrong...
Have you found anything yet?

 

[JJinSC]

Are you sure the LS setting of 1 turn is correct? My shop manual for the 97 XP (787) stated the same setting for the LS but I found it ran like crap and started checking around. Other sources recommended 1.5 turns. So I set mine there and it runs great except for a hard restart issue I am chasing


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[TurnerBFC]

I managed to clear it up a little bit, fuel is much better but still have this weird "rev limiter" issue that comes up intermittently. Rectifier?

 

[Mekanix]

Are you sure the LS setting of 1 turn is correct? My shop manual for the 97 XP (787) stated the same setting for the LS but I found it ran like crap and started checking around. Other sources recommended 1.5 turns. So I set mine there and it runs great except for a hard restart issue I am chasing

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96 in the manual is 1 turn out
97 has an accel pump and smaller pilot jet so having the LS screw 1.5 sort of makes sense.

 

[Mekanix]

I managed to clear it up a little bit, fuel is much better but still have this weird "rev limiter" issue that comes up intermittently. Rectifier?

What do you mean by rev limiter? As in: bogs and kicks back in or as in miss as if you have a fouled plug or misfire?

 

[TurnerBFC]

What do you mean by rev limiter? As in: bogs and kicks back in or as in miss as if you have a fouled plug or misfire?

It will just start to limit the rpm's and then it kicks back in. I don't think fuel related, more like electrical.

 

[Mekanix]

Could you make a video of what its doing ?