96 787 Very rough midrange.

[Mekanix]

Working on a 787 new rebuild with original carbs just cleaned and reinstalled.
Top end is awesome from 5500 on to 7100.
1400-3500 is smooth
From 3800 to 5500 its rough and sounds like misfiring or 4 stroking.
Choke makes no difference. Just kills it even with small applications.
No adjustment makes a difference.
Haven tried without the airbox yet to look for leaking through the ventury.

Its doing this just before the main starts to open at 4000 and continues to get worse until its completely on the main jet.
I cleaned the carbs, Nothing seemed wrong, even pop off was good and needles held for a long while. Just weird that nothing seems out of the ordinary...

Any ideas?

This one might be a candidate for brand new carbs. Never bought a new set before

 

[mikidymac]

From my experience this is very common on the 787 and not to worry about. This is the rpm that the carb circuit is transitioning and the rave’s opening so you have a lot of things transitioning at once.

 

[Mekanix]

Its just so weird though. Some I've built were perfectly smooth and some were like this including my own GSX.
I've never really solved it or figured out how to even effect it and its a pain in the butt

I do build them to spec and usually end up at close to 170 psi though. Possibly an issue ? Maybe its just too lean at that transition point.

 

[etemplet]

I've been working on both my GTXs with performance enhancement issues since May. I bought them last year a 96 and a 97. Built the engine on the 97 from parts off ebay.... replaced everything. 96 had no fire issues and MPEM problems.

Rebuild carbs on both skis then did it again and apart for checks, more cleaning.... reset pop off to different settings, etc. I was almost ready to rejet the 96. Here is what I'd do. If you are sure you have mikuni needle and seats and diaphragm and all the good parts in the carbs..... I know you know about the 3 holes in the barrel.

Install slotted rave valves (I machined my own slots in my raves) and buy "new" upgraded valve bodies. A little information on the problems I was experiencing.....After all the work I did to both skis, the 97 still had acceleration issues, a flat "smoke a cigarette and get back to you" attitude from 2500 to 5 or 6K, then came on like a raped ape. I raced my wife often on the 96. I'd jump out to lengths and stay there till she hooked up and drove by me. Her's is faster top end but I just built the engine. The 96 was the smoothest with regard to transition RPM. Had I not had the 97 I would have figured the 96 was pretty darn good. On the speed difference 2-3mph on the GPS I refuse the resort to cheating to make my ski faster.

That said... after the rave valve work she told me her ski didn't come on as hard at 6K. We lined up for a race.... she pulled way from my ski from the start and walked off into the distance. I couldn't get a sniff. I have not rode her ski but judging by what happened, her flat spot ain't flat no mo. LOL Someone on the forum posted about a technical service bulletin to do what I did for customers complaining about a flat spot or hesitation on the 97 models. Give that some due diligence. Good Luck.

 

[mwat65]

Sounds like a possible semi sticky rave valve. Also check the water regulator.

 

[kmarts]

...Someone on the forum posted about a technical service bulletin to do what I did for customers complaining about a flat spot or hesitation on the 97 models...

That was me

TSB is attached.

 

[etemplet]

That was me

TSB is attached.

Thanks man !! That shiznit WORKED !! I am about to slot my rave valve stems on the 96. I already replaced the body section. Where did you find this info ??

 

[kmarts]

Thanks man !! That shiznit WORKED !! I am about to slot my rave valve stems on the 96. I already replaced the body section. Where did you find this info ??

Because of the issues i was having i was doing a lot of research into potential problems. That subject came up in many forum threads i found. To actually find the TSB wasn't so easy but i eventually did.

I'm only getting to take my ski out this weekend since replacing all the seals and my fingers are crossed that i don't have this problem too... I just want to ride the damn thing lol.

 

[jjsinaz]

I've been chasing this problem too...on a new rebuild. The RAVEs I have are not the slotted ones though. They were in the engine before it blew though and did not have the problem, at least not as severe as it is now.

 

[jjsinaz]

Here is a video. You will notice in the video that when slowly backing off the throttle, at about 4500 RPM, the RPM will suddenly drop to 4000 and the engine gets very rough. Then continuing to back off it begins to smooth out again at about 3500 and gets smooth again at 3000. Then slowly advancing the throttle it again becomes rough at about 3500 and really gets rough at 4000 then continuing to slowly advance the throttle it will abruptly jump up to 5000.

787 Midrange issue

 

[esahlin]

jjsinaz- I know you have been on the other recent threads about the water control regulators being problems So I know you know about that. I just wanted to point out that I watched your video and that is how my 96 GTX acted and it was a blown apart (loose top clamp /rusted lower clamp/hole in bellows) water regulator valve. I know it might not be the solution for every case. Mine seemed to be more throttle position dependent than strictly rpms but I don't know for sure. Any throttle position between 1/4 to 3/4 would give me the rough running that your video shows. Above 3/4 throttle it would blast like you said to 5K and be golden at anything above. Also, a slow advance of the throttle would be rough running as the rpms climbed then when I hit about 3/4 throttle it would clear up. I think this is what you describe too. I don't get why but fixing the water regulator vastly improved the condition.

 

[jjsinaz]

esahlin, you are spot on and it is definitely more throttle position related (just used the RPM as a reference) but between 1/4 and 3/4 throttle is where it occurs. I had the WR apart the other day and found what appeared to be a very small hole in the bellows from the rusted clamp that was previously there before I replaced it with the zip tie, and now with an o-ring. I sealed the hole on both sides with black RTV, I haven't been back out yet but I will let everybody know what happens when I do

 

[esahlin]

jjsinaz….Im hoping that your work on your water regulator fixes it for you. I know what a pain it is to fight these problems.

 

[etemplet]

jjsinaz….Im hoping that your work on your water regulator fixes it for you. I know what a pain it is to fight these problems.

I like "the chase." When you get it solved... you've learned so much in the process.

 

[jjsinaz]

Thanks guys, but like kmarts said earlier, "I just want to ride the damn thing!"

 

[Mekanix]

Been chasing this one for a while.
That drop from 4500 down is when the main jet turns off. Its normal but you can help with that by installing a thinner or softer antiblowback valve. It's the little peice of plastic on the half moon part. The jump from 4500 up to 5k is where it opens and around 4400 it closes again. So you see the big drop.
But with a genuine mikuni peice of plastic in there it is much smoother.
I bend mine up a little bit just so that they close completely but also so that they are not too stiff

 

[Mekanix]

The problem your all describing is entirely the low speed circuit. And at this point popoff is less of a contributor and low speed adjustment has almost no effect.

It's only the pilot jet itself that will have an effect on this.

Try a 75 or 80 pilot and see what happens.

I always thought I was too rich when this was happening but turns out it was too lean.


I haven't seen a water regulator problem that has caused this yet.

 

[etemplet]

You got a problem somewhere else. Due Diligence. Sometimes a number of little problems all add up. Good Luck !!

 

[jjsinaz]

Thanks Stephen. Whenever I rebuilt the carbs just for reference sake, I compared the thickness of the mikuni blow back valve to the aftermarket one the PO had in there. The aftermarket one measured .005 whereas the mikuni one measures .0025. So I think I have the less stiff one in there, and I have checked to make sure they weren't stuck down, etc.

What you are saying about the pilot jets makes perfect sense due to the throttle point where this occurs and also your observation that its a lean condition rather than rich is very helpful. I think I will order a couple of of each of those sizes and give them a try.

 

[suke]

I had a mid RPM stumble and could never get rid of it. Bought new carbs.......gone. No stumble anywhere at any RPM.

 

[Mekanix]

Before I learned about the anti blowback valve mine would jump from 4 to 4500 and when coming down just below 5000 it would drop to 4200 but after I rebuilt them again and reuse the original blowback valves and bent them up slightly that transition was gone and it was very smooth. But I am soon going to be purchasing brand new carbs just for the hell of it because I never have before and figured why not try. And maybe I'll learn something from them that is different than my current carbs.

The reason why I'm thinking it's a lean misfire instead of being overly rich and four stroking is because of the sound of the engine at that point feels like a bad misfire that is fixed by adding more fuel and it doesn't feel like it's loaded up. It really wants more fuel.

 

[suke]

Yeah I had all kinds of niggling little issues and got tired of not being able to tune them out. These things are now 20+ years old. There's going to be a point where we can't get new carbs. So might as well bite the bullet now and just replace them. I'm likely never going to rebuild carbs again as long as I can buy new. You're already $100+ in by the time you get the rebuild kits, factor in your time and the fact you can likely get discount codes from OSD and/or if you're on the FB group OSD will give you 10% off, and new carbs start sounding much more worth it to me.

 

[Mekanix]

I'm traveling through the states right now and currently in Baltimore. Is there anywhere around here that I could get some carbs.
Otherwise I was just going to go through winner Circle Canada again.

 

[suke]

I'm traveling through the states right now and currently in Baltimore. Is there anywhere around here that I could get some carbs.
Otherwise I was just going to go through winter Circle Canada again.

I think osd is in Ohio. Might could contact him and see.

 

[Trailrider]

I have a 2000 787 DI with the identical symptoms as you have described. I will be watching to see what helps others solve it.