GP 1100 with 1200 cylinders port matched

[crazymanme2]

I have a Gp1100 that's been machined to accept 1200 cylinders & has been port matched & forged crank. I was told this was a big thing back in the 90's. Motor completely rebuild WMS pistons & all new seals. Case pressure checked. Rebuilt carbs with genuine Mikuni kits. Bill O-side return mod. Stock reeds, intake & heads. Running Rec fuel @ 32:1 premix. Checked fuel pressure (2-3 idle 5-6 WOT. Pop off at 55 psi with 1.5 needle & seat. Running 135 mains with 95 pilots. High speed set at 1 1/2 turns out & lows at 1 1/4 out. starts right up & idles (1500). Warmed up had a slight hesitation from idle to WOT, turned low speed adj. in 1/4 turn & hesitation cleared. 1/8 throttle to WOT great. Motor still idled & started right up. Now 1/4 throttle to WOT falls on its face for about 4 secs sometimes completely dies. if I let off throttle before it dies goes back to idle. Tried turning low speed in 1/4 turn but was too lean on idle motor would not start just hitting starter, turned back out started right up. So I feel that I need to lower my pilot to (92.7). When running WOT & back off to like 1/2 throttle it falls on its face unless I play with throttle. Any input greatly appreciated. I have learned tons from this site. Where I learned to use low speed adjuster to dial in pop-off & pilot jet. 73 year old man who is still a kid. Thanks for any input.

 

[WFO Speedracer]

It sounds to me like your popoff is too high, popoff controls the midrange circuit which is where you are having issues , you left out some important details here , are you running 38 carbs or the 1200 44 carbs setup , are you running the stock air box or individual air filters , are the choke plates still in the carbs or are you using a primer setup , if it's the latter you will need to lower the popoff to compensate for the loss in carb signal.

Bogging is almost always a lean condition , it needs more fuel , if you can grab the choke slightly or hit the primer and it pulls on through the midrange correctly then for sure it's lean in the midrange and you need to install different springs to lower the popoff pressure.

 

[crazymanme2]

It sounds to me like your popoff is too high, popoff controls the midrange circuit which is where you are having issues , you left out some important details here , are you running 38 carbs or the 1200 44 carbs setup , are you running the stock air box or individual air filters , are the choke plates still in the carbs or are you using a primer setup , if it's the latter you will need to lower the popoff to compensate for the loss in carb signal.

Bogging is almost always a lean condition , it needs more fuel , if you can grab the choke slightly or hit the primer and it pulls on through the midrange correctly then for sure it's lean in the midrange and you need to install different springs to lower the popoff pressure.

Sorry for lack of info.
Running the 1200 44 carbs with stock air cleaner with choke plates but running primer. Tried just running bottom half of air box didn't seem to have any effect. You have a ball park suggestion on pop-off__35-40. Thanks alot for your feedback.

 

[crazymanme2]

Sorry for lack of info.
Running the 1200 44 carbs with stock air cleaner with choke plates but running primer. Tried just running bottom half of air box didn't seem to have any effect. You have a ball park suggestion on pop-off__35-40. Thanks alot for your feedback.

Looks like if I go to a 95 gram spring will give me pop-off of 43

 

[WFO Speedracer]

The problem is pretty much everyone that mods these ditches the stock airbox so there are very few jetting recommendations out there that retain the stock airbox and anything using individual filters is going to be way off. I usually just use the Oside Bill jetting specs with the Pro K filters and it's good to go .

I don't have a lot of expertise in this specific case , I did set one up similar to yours in a Waveventure 1100 hull but I used the stock 38 carbs , which ran just fine once I swapped to bigger main jets and readjusted the low speed screws . all of that being said :

I am going to refer you to this thread : https://www.greenhulk.net/forum/per...wc-performance/yamaha-old-school-skis/280800-

 

[crazymanme2]

The problem is pretty much everyone that mods these ditches the stock airbox so there are very few jetting recommendations out there that retain the stock airbox and anything using individual filters is going to be way off. I usually just use the Oside Bill jetting specs with the Pro K filters and it's good to go .

I don't have a lot of expertise in this specific case , I did set one up similar to yours in a Waveventure 1100 hull but I used the stock 38 carbs , which ran just fine once I swapped to bigger main jets and readjusted the low speed screws . all of that being said :

I am going to refer you to this thread : https://www.greenhulk.net/forum/per...wc-performance/yamaha-old-school-skis/280800-

Thanks for the input

 

[WFO Speedracer]

Ok I had a minute to think here and I am not sure how far out the low speed adjusters already are right now , before you change anything out try this first , back out the low speeds screws about 3/4 to 1 turn from where they are right now , make sure they are not out over two full turns, idle it back up using the idle adjustment , water test it and see where that gets you , this works on most stock or fairly stock 760's and 1200's , if it doesn't work no big loss and not a lot of time or ,money spent.

I will say that the stock low speed adjustments on these are way too lean and I have done many exactly this way mostly to solve serious starting issues , if it is adjusted correctly it should now start with little to no choke , most of then I have done you just press the green button and it starts before you can get your finger off the button.

You may have to play with that adjustment some , this is usually done with stock jetting so yours may be a little different .

 

[kevbo]

The bottom screws are the low idle adjustment only. 1/4 throttle is the pilot jets. Lowering the pop off will do the same as using the next larger pilot jet. Once 1/4 throttle is slightly fourcycling, open the top screws to get good response. This will make full throttle a little rich of peak and prevent melted pistons.

 

[WFO Speedracer]

The bottom screws are the low idle adjustment only. 1/4 throttle is the pilot jets. Lowering the pop off will do the same as using the next larger pilot jet. Once 1/4 throttle is slightly fourcycling, open the top screws to get good response. This will make full throttle a little rich of peak and prevent melted pistons.

It all works together but the carb pulls fuel through the low speed circuit at all times and throttle settings , what I described is basically an over-rich situation on the low speed screws which quite coincidently is how most carbs on freestyle skis are set up . It gives makes for much easier starting and usually more bottom end power in my experience , reverse jetting works on a similar principle.

 

[kevbo]

An overly rich part throttle setting will usually help response. Couches spend a lot of time at full throttle so setting the top screws are first priority. After that a mukuni sbn44 needs to be only slightly rich at 1/4 throttle for great response. The “reverse” jetting fad works great when the rider constantly fans the throttle which is a standup thing.

 

[crazymanme2]

An overly rich part throttle setting will usually help response. Couches spend a lot of time at full throttle so setting the top screws are first priority. After that a mukuni sbn44 needs to be only slightly rich at 1/4 throttle for great response. The “reverse” jetting fad works great when the rider constantly fans the throttle which is a standup thing.

So what if I put 100 PJ in instead of 95 & leave pop-off at 55?

 

[crazymanme2]

So what if I put 100 PJ in instead of 95 & leave pop-off at 55?

I had 100 PJ in at first but as I turned the LS in, the take off from idle was better so I thought I was running rich is why I put in 95 PJ, but it sounds like my mid range was lacking. Thanks for the input.

 

[kevbo]

The bottom screw feeds the lowest fuel inlet hole. It’s to adjust idle mixture only. Pilot jet feeds the holes just above lowest one, it directly affects 1/4 throttle. You can set the bottom screw out of the water. Just slowly close them until idle rpm peaks then open them until it slows back down. The pop off pressure directly affects how much the screws need to be open.