97 787 midrange performance issue

[IANONTOP]

Trying to see if i can get a little technical help.
From IDLE TO WOT. The ski gets up quick cruises at 20 and slowly climbs then for 4 secs up to 52mph. In short ski has a dead spot transitioning form low speed circuit to high speed circuit. Once im in the high speed circuit, the ski responds very well. If i feather the throttle it will take off from a dead stop. If i sit there a second or two it will do what i stated earlier.

The carbs are fairly new and rebuilt with mikuni kits and new mikuni N/S on both carbs. Completely cleaned all orfice holes are free and clear. The fuel line is direct from res to carb with inline filter. (No selector or water separator) also stock f/a’s. Have tried all amounts of pop off pressure from 20, 28, 34, 40, 50. Only difference is that higher pop offs lean out ski as expected and the lower it starts four stroking (too much gas) Also low speed adjuster makes no difference neither should it. No longer has affect on fuel delivery as of this point in transition. Im tired of messing with it and im at a wall. TIA

 

[bigJake]

Just a guess, have you cleaned your RAVE valves? Could be sticking

 

[mikidymac]

You need to install the correct pop off springs and stop trying to change pop off.

Get rid of the inline filter and go back to the stock separator with a new o-ring.

All adjuster screws should be at stock settings.

Given that your low speed screw doesn’t do anything there’s something still wrong with your carbs.

What model ski? If it’s an XP is your accelerator pump squirting correctly?

 

[IANONTOP]

Just a guess, have you cleaned your RAVE valves? Could be sticking

Yes they move freely ive been through the motor.

 

[IANONTOP]

You need to install the correct pop off springs and stop trying to change pop off.

Get rid of the inline filter and go back to the stock separator with a new o-ring.

All adjuster screws should be at stock settings.

Given that your low speed screw doesn’t do anything there’s something still wrong with your carbs.

What model ski? If it’s an XP is your accelerator pump squirting correctly?

Popoff is there for a reason and in different applications is useful. When you know what your doing. But popoff is at 40psi. 98 spx carbs.

Those stock seperators have more problems than just oring. The platic strips out lets air bubbles or they’ve been tightened so much they warped out and still unable to seat right.

Ive tried two sets of carbs pre 98 no accel pump and post 1998 with accel pump. Same thing.

the ski is a 1997 gtx. Stock no accel pump but since it had wot hesitation installed 98 spx carbs with accel pump and hesitation is gone typical. Im talking about a slower than usual climb to max rpm.

pump has also already been gone throug brand new oem wearing and good prop clearances are good.

 

[AKnarrowback]

Rave valve and housing updated? If not it is something to consider. Set the carbs to factory specs and leave them there, after that then start changing one thing at a time. Make sure the carbs are in good working condition.

 

[IANONTOP]

Rave valve and housing updated? If not it is something to consider. Set the carbs to factory specs and leave them there, after that then start changing one thing at a time. Make sure the carbs are in good working condition.

Yeah definitely! Next thing im shooting for is rave update. But everything is factory spec. Ive just tried other stuff. Nothings making a difference.

 

[IANONTOP]

Anyone else any ideas?

 

[AKnarrowback]

So you did or did not do Technical Service Bulletin 97-20?

Admitting that my experience on DOOs is mostly 89-95. I do own a 97 GTX and did fight with the typical 5000 rpm problem. A gradual increase in throttle would produce a situation where, around 5000 rpm, any additional throttle input would not make the rpm's rise and the motor to run poor. If I quick dropped to zero throttle then nailed the throttle the engine would reach full rpm's and run strong. Doing a drag race start would allow the motor to push through the 5000 rpm barrier and run strong.

The key thing here is that gradual increases in throttle allowed the motor to lock into 5000, abrupt changes would not allow the 5000 problem. Now the EXACT 5000 rpm was not dead on, the problem would happen 4600 up to 5200 on my machine. Doing TSB 97-20 almost eliminated the 5000 problem.

Another test that can indicate the raves as the problem is to go out on some choppy water, do the throttle to get the motor to lock into 5000 but then hold the throttle wide open and see if the motor jumps to full rpm's after hitting a wave allowing the motor to rev when the intake is in the air for a second.

You could be looking at a couple of problems. Based on my snowmobile experience (40 years) I can't count how many times people have gone overboard on trying to tune out a problem using the carbs when it was a clutching, gearing, etc problem.

The 97 787 has a rave problem, do your best to investigate this before giving up. Only change on thing a time when diagnosing. Put your carbs to stock, jets, pop off the see what happens.

 

[IANONTOP]

So you did or did not do Technical Service Bulletin 97-20?

Admitting that my experience on DOOs is mostly 89-95. I do own a 97 GTX and did fight with the typical 5000 rpm problem. A gradual increase in throttle would produce a situation where, around 5000 rpm, any additional throttle input would not make the rpm's rise and the motor to run poor. If I quick dropped to zero throttle then nailed the throttle the engine would reach full rpm's and run strong. Doing a drag race start would allow the motor to push through the 5000 rpm barrier and run strong.

The key thing here is that gradual increases in throttle allowed the motor to lock into 5000, abrupt changes would not allow the 5000 problem. Now the EXACT 5000 rpm was not dead on, the problem would happen 4600 up to 5200 on my machine. Doing TSB 97-20 almost eliminated the 5000 problem.

Another test that can indicate the raves as the problem is to go out on some choppy water, do the throttle to get the motor to lock into 5000 but then hold the throttle wide open and see if the motor jumps to full rpm's after hitting a wave allowing the motor to rev when the intake is in the air for a second.

You could be looking at a couple of problems. Based on my snowmobile experience (40 years) I can't count how many times people have gone overboard on trying to tune out a problem using the carbs when it was a clutching, gearing, etc problem.

The 97 787 has a rave problem, do your best to investigate this before giving up. Only change on thing a time when diagnosing. Put your carbs to stock, jets, pop off the see what happens.

Thanks alot for the reply and will do! And will get back with you in a few days gotta order the rave stuff in!

 

[AKnarrowback]

When I tried the rave TSB I "experimented" since I had almost zero budget at the time for toys. I used "JB qwik" to modify the old housings I had into something that I hoped would simulate the oem updated housings. My "mod" worked and made enough of an improvement to convince me ordering the factory parts was worth the money.

On rare occasions, when I am playing hard on the machine, I still have a rare hangup at the 5000. To be truthful though it has been a while since I pulled and cleaned the valves.

Other people on this site have far more technical knowledge than I do on these machines, I don't know squat about anything 98 and newer. But the approach to trouble shooting is the same regardless. I have had the pleasure of working, briefly, side by side with the Bombardier "test and tune" engineers when they came to my town to do early season testing on the Snowmobile lineup back in the 90's. They never changed multiple settings, if they were trouble shooting or tuning they only changed one component at a time. They were VERY thorough in dialing things in, this is why we on the site are suggesting you take and set your carbs back to stock. Get the machine in good working order as it came from the factory then start tracing the problem down.

Sometimes they do make mistakes, this is why there are TSBs. When I was trying to dial in my 94 XP I must have made 15 trips to the river to do a warm up and 30 second ride to see how a carb, impeller tweak would work.

Test and tune, test and tune.

 

[POPPS]

I agree with previous posts, get carbs to "superstock" and make sure ravs are working perfectly, An easy way to check ravs working is to remove the caps, start the motor, put 2 fingers on the plastic caps, hit the throttle, the ravs should respond and pop up of course., (you may want to run some water through it, but not needed if you are only running it for a short while). And have you ruled out compression?

 

[IANONTOP]

I agree with previous posts, get carbs to "superstock" and make sure ravs are working perfectly, An easy way to check ravs working is to remove the caps, start the motor, put 2 fingers on the plastic caps, hit the throttle, the ravs should respond and pop up of course., (you may want to run some water through it, but not needed if you are only running it for a short while). And have you ruled out compression?

Yes ruled out compression and and carbs are back to super stock! Including stock f/a’s. The raves work well . But is missing lower spring on bellow, maybe thats letting pressure slip by and its not fully pushing out?!

 

[mikidymac]

There’s no “lower spring “ on the bellows, only the top.

 

[AKnarrowback]

Lower clamp on the bellows maybe?

 

[IANONTOP]

There’s no “lower spring “ on the bellows, only the top.

Well then there should be. Plus the updated rebuild kits come with them now

 

[mikidymac]

I haven't seen the updated kits but I can tell you in all of mine and all our friends that I service, 11 787's in total, none have a lower clamp or spring and not a single one of them leaks because I am the lucky shmuck that services all of them. Maybe just lucky?

 

[Ckrawiec]

I haven't seen the updated kits but I can tell you in all of mine and all our friends that I service, 11 787's in total, none have a lower clamp or spring and not a single one of them leaks because I am the lucky shmuck that services all of them. Maybe just lucky?

"LuckySchmuck"....when/if you ever change your handle......hahahaha!

 

[Travlash]

I am also having a similar issue but i can hear the raves open on water. At around 5k it will jump to 6000 after the raves open, but it stays at 6k consistently. Runs great actually , but not getting full rpm. If i hit a wake, and the intake and nozzle gets into the air, the ski will rev to 6800. But back in the water nothing.

 

[Lothsahn]

I haven't seen the updated kits but I can tell you in all of mine and all our friends that I service, 11 787's in total, none have a lower clamp or spring and not a single one of them leaks because I am the lucky shmuck that services all of them. Maybe just lucky?

+1. My 96 GTX had original raves and housings. I replaced bellows with aftermarket and replaced both raves with slotted (one of the original raves had cracked at the stem), and I have an upper spring but no lower. I did not replace the housings.

Ski hits 6680rpm @54mph with a SD-CD 14/22 impeller. My high end rpm seems low but given the speed I'm assuming it's the impeller. So you should not need the lower springs.

 

[mikidymac]

Some of the cheap aftermarket bellows are too stiff and won't let the raves open all the way.

 

[Lothsahn]

Some of the cheap aftermarket bellows are too stiff and won't let the raves open all the way.

Think that's why I'm only hitting 6680, or do you think that's related to the impeller?