In line external mukuni DF62 fuel pump verses internal carburetor fuel pump for a GP 1200.

[Radical Riche]

I changed from internal to external fuel pump on my GP1200 build. I've been told that the external fuel pump gives the wring "signal" and I should not use. Also I've been told they work great on a 1100. Anyone change over to external fuel pump on a GP 1200?? Thanks

 

[DylanS]

I changed from internal to external fuel pump on my GP1200 build. I've been told that the external fuel pump gives the wring "signal" and I should not use. Also I've been told they work great on a 1100. Anyone change over to external fuel pump on a GP 1200?? Thanks

Guys have been running externals for a long long time, including me on several gp1200 builds, you will have zero issue with your setup this way.
Onboard and external pumps are both driven by pulse signal from the engine and your delivered fuel pressure will be regulated in carb on unmodified carbs or via the return line on carbs with the restricter drilled out.

 

[Radical Riche]

Do you mount your external fuel pump lower than fuel tank? I heard that pulse can't be no longer than 11 inches. I never put a restrictor jet in my fuel line..do I need to? I have the mikuni DF62 which I drilled and tapped for a 3rd fuel out to carb line and I drilled and tapped for a additional fuel line coming into fuel pump. My friend did the same adding a 3rd output for carb but he never added a 2nd fuel line coming into pump. The pump is marked with arrow on where to add a external fuel line to carb and it's marked to add 2nd fuel line coming into fuel pump. My worry is, is the pump strong enough to pull duel fuel lines into pump? I had fuel rail made up of dual fuel lines when it was a 1100 but I modified the case and added the 1200 which I heard the 1200 likes it's fuel...thoughts? Thanks

 

[DylanS]

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Do you mount your external fuel pump lower than fuel tank? I heard that pulse can't be no longer than 11 inches. I never put a restrictor jet in my fuel line..do I need to? I have the mikuni DF62 which I drilled and tapped for a 3rd fuel out to carb line and I drilled and tapped for a additional fuel line coming into fuel pump. My friend did the same adding a 3rd output for carb but he never added a 2nd fuel line coming into pump. The pump is marked with arrow on where to add a external fuel line to carb and it's marked to add 2nd fuel line coming into fuel pump. My worry is, is the pump strong enough to pull duel fuel lines into pump? I had fuel rail made up of dual fuel lines when it was a 1100 but I modified the case and added the 1200 which I heard the 1200 likes it's fuel...thoughts? Thanks

I typically mount my outboard pumps on the front face of my kart tanks so about half way down.
I’ve also run one bolted to the old ebox mount threads on a b2 so well above the tank.
The pump is plenty strong enough to pull fuel from wherever you decide to mount it even with dual feeds.

I typically use the single inlet pumps with the 5/16ths (I believe) inlet and dual 1/4 outlets and I’ve personally never heard of anyone tapping a outboard pump for a second inlet but I can’t say I’ve researched it.

All factory carbs that haven’t been modified have return restrictors built into them so you will be alright.
Keep in mind stock gp1200’s feed the entire rack of carbs off one 1/4 fuel line!

 

[kevbo]

The DF62 flows a lot. You want to mount the pump high so the pulse line can completely drain any liquids back into the engine. The pulse line should be as short as possible in order to get best performance. If each carb has a restrictor just make sure there is no pressure drop on the pump side at full power.

 

[Radical Riche]

The DF62 flows a lot. You want to mount the pump high so the pulse line can completely drain any liquids back into the engine. The pulse line should be as short as possible in order to get best performance. If each carb has a restrictor just make sure there is no pressure drop on the pump side at full power.

I never ran restrictors, I have 44s that are tapper bored. I ran ski for 15 mins last summer and it was lean with screws turned completely out. I have 110 pilot 135 main, 2.0 N/S, 95 gram spring with like 50lbs pop off. My friend put otter rear seal in an it blew out but the inner seal was in place as well as the front seal. I either had a case leak or not jetted high enough. I bought 125 low and 145 pilot to try but thinking that a lot of fuel. My pistons are scuffed as well as cylinders so I 'll getting bore. To change jets the motor gotta come out so I'm not sure if I should up jets or keep what I had when I had lean condition..not sure if I had case leak or not. What's your thoughts?

 

[WFO Speedracer]

What flame arrestors are you running aftermarket or stock, without that info all bets are off., At any rate take care of sealing everything else up properly before you go changing jets.

 

[Radical Riche]

What flame arrestors are you running aftermarket or stock, without that info all bets are off., At any rate take care of sealing everything else up properly before you go changing jets.

Hot products with decked head and cylinder, 155 psi compression with stock exhaust. Thanks

 

[WFO Speedracer]

Then yes you need to rejet and change the popoff pressure and possibly the needle and seats, to be very basic the small needle and seats work with high restriction and high popoff pressures, once you change to low restriction flame arrestors you need bigger needle and seats , bigger jets and less popoff pressure.

 

[Radical Riche]

Then yes you need to rejet and change the popoff pressure and possibly the needle and seats, to be very basic the small needle and seats work with high restriction and high popoff pressures, once you change to low restriction flame arrestors you need bigger needle and seats , bigger jets and less popoff pressure.

I called Pro Tec and Havasu PWC and they both told me the jets, N/S, spring and pop off. Like I was saying the lean could have been caused by a case leak. I wish someone on here was running the same engine and carbs to see what jetting they're using. When I called Pro Tec after engine failure they said just to up the jets and n/s, spring and pop off stays the same. I'm not carb guy. Thanks!

 

[kevbo]

I was referring to the internal restrictor inside the carb, many aftermarket carbs have them drilled so they can use an external one. As you have found, large two strokes are very easy to burn up! Always set the main jet/ top screw overly rich whenever any changes are made. Its very difficult to damage an engine that way.

 

[Radical Riche]

First, the Compression, 155 psi, is too high for Pump Gas operation. Needs 105 Octane at 40:1, especially for long WOT runs.

Second, with a 2.0 N/s and 90 gr Spring, should have a Pop-Off Pressure of about about 25 psi. This is about right for Flame Arrestors on the GP1200 Engine.

I've run a Group K Ported Pump Gas GP1200 Engine with Stock Carbs w/ FA's with each having it's own dedicated Pulse Line from the Crankcase. And with only ONE Fuel Supply Line from the Tank to the Rear Carb and the Return Line out of the Front Carb. OEM Jumper Hoses 'tween Carburetors.

Can't remember the Jetting. Would have to crack open the 44's to see what Jets are in it. And the Center Carb High Speed Screw gets ONE complete turn out more than the Front and Rear Carbs.

Note that Group K added a Cooling Water Bypass to the Center Cylinder Head.

View attachment 433290

Yeah I added a 3/8 cooling line to my manifold and it exits out of the center of my cylinder head, also, the one extra turn makes sense too. If I'd go 25lbs pop off what exactly does that change? Id have to run a much lower spring than 95 gram. Im running a high flow external fuel pump so I only run one pulse line. I ran 3 pulse lines when I had engine as a 1100 now I modified the case and running 1200 jug. Brandon from Havasu PIC said you can't run external pump on a 1200 that it sends the wrong signal. I also was told you can run 93 pump fuel with 155 compression..160 psi is max on pump fuel. Thoughts?? Thanks for your help and input keep it coming this build is tricky. I'm lost on pop off reasoning

 

[kevbo]

The maximum compression will depend on the ignition timing, air/fuel mixture, fuel quality, and how long you run it wide open. A Zeeltronics ignition will make setting the carbs safer since you can pull timing back more than stock. You wont hurt anything if a richer than peak RPM fuel setting is maintained on all three cylinders. Thats hard to do on a jetski without an additional pump and air seperator. Most standups wont be at full power long enough for air in the line to matter but be aware. Each cylinder must have the top screw tuned individually. This is more difficult on a triple. I use a tachometer and thermocouples on each plug and it still takes some time to get all three running just right. You can sometimes do it by reading plugs but that is dangerous if one carb is running a little leaner. You may want to start with a stock compression head until the carbs and pump load is perfected. The main effect of lowering popoff will be a richer idle (bottom srew) and pilot jet mixture. I like to get all my SBN44s up to 25psi.

 

[Radical Riche]

The maximum compression will depend on the ignition timing, air/fuel mixture, fuel quality, and how long you run it wide open. A Zeeltronics ignition will make setting the carbs safer since you can pull timing back more than stock. You wont hurt anything if a richer than peak RPM fuel setting is maintained on all three cylinders. Thats hard to do on a jetski without an additional pump and air seperator. Most standups wont be at full power long enough for air in the line to matter but be aware. Each cylinder must have the top screw tuned individually. This is more difficult on a triple. I use a tachometer and thermocouples on each plug and it still takes some time to get all three running just right. You can sometimes do it by reading plugs but that is dangerous if one carb is running a little leaner. You may want to start with a stock compression head until the carbs and pump load is perfected. The main effect of lowering popoff will be a richer idle (bottom srew) and pilot jet mixture. I like to get all my SBN44s up to 25psi.

I was wrong in my previous post. I have like 25-28 pop off using a 95 gram spring. What spring are you using...95? I mix my fuel 32:1, I used to mix it 40:1 in race days and wanted a engine that was going to last. I took engine out to install a 155 @torrent, Carter B tunnel and the real deal GP trim. After 8 years of hammering I still had cross hatch in my cylinders, I'd rather be a tad rich then lean and it didn't make any difference in performance maybe a little sluggish if you hit it from a stand still. Do you run a external fuel pump? I have the mikuni DF62 which is high flow. I was lean when I rode it for 15 mins last summer, had screws clear out, it improved but I think I had a case leak. I was running 110 pilot and 135 main with 2.0 n/S. I had fuel wash on top of all 3 cylinders so that makes me think I had air leak. My outside rear seal blew out cause friend got the one with 4 tans mixed up and had the 4 tabs on the inside which didn't allow the outter seal to seat. Now the inner rear seal was seated and so was the front so that's way I'm leaning toward case leak. I bought 125 pilot and 145 main jets Pro tec said 1200 likes fuel but don't know if I should use them. You gotta pull engine out of B1 to change jets..it sucks. What jets are you running? Thank you very well your input!

 

[Radical Riche]

Download a free Mikuni SBN Watercraft Manual and refer to the Pop-Off Chart, Page 15.

Read the Mikuni Manual (several times) to understand how Pop-Off Settings affect Fuel Flow in the Carbs.

If it's the same Crankcase, why can't you use the 3 Pulse Nipples to actuate each Carburetor Pump? Do you have 38's or 44's Carb Racks?

The Stock 38 mm Carb Rack came with one External Fuel Pump off one Pulse Nipple to feed all 3 Cylinders. I've never tried to feed a 1200 Cylinder with 38's. But the 1200 Intake Manifold will bolt right on to the 1100 Crankcase and then a Stock OEM set of 44's will bolt right on too, if you have a set. Then they each run off an independent Pulse Nipple.

I was told to upgrade to external fuel pump because if pump fails the engine won't run verses if you have independent internal carb pumps and one fails then your engine is toast. I had 3 pulse lines, sodered and capped 2 closed. I'm not liking the external pump but that's what everyone's using. I have the mikuni DF62 which I drilled and tapped for a 3rd fuel out to carb and drilled tapped for for a additional fuel line in to fuel pump. I have engine tore down, going for slight bore.. cylinders just scuffed a little. Thanks for your help

 

[Radical Riche]

I was told to upgrade to external fuel pump because if pump fails the engine won't run verses if you have independent internal carb pumps and one fails then your engine is toast. I had 3 pulse lines, sodered and capped 2 closed. I'm not liking the external pump but that's what everyone's using. I have the mikuni DF62 which I drilled and tapped for a 3rd fuel out to carb and drilled tapped for for a additional fuel line in to fuel pump. I have engine tore down, going for slight bore.. cylinders just scuffed a little. Thanks for your help

Forgot, I have tapper bored 44s.

 

[Radical Riche]

"if you have independent internal carb pumps and one fails then your engine is toast"

How often does this happen? What if the External Fuel Pump fails?

It's the Carb Regulator Diaphragms that wear out and need replacement on a regular basis.

Use Genuine Mikuni Rebuild Kits and that Engine will run happy for many years.

If the external fuel pump fails the engine won't run verses if one internal carburetor fuel pump fails the engine is toast because you won't realize it till the damage is done

 

[WFO Speedracer]

On an 1100 it happens when the rear crank seal goes out , I have seen that a few times, you ask how often it happens, on stock machines it happens quite often on 1200's , usually the plastic check valves in the fuel pumps get creased of crinkled and it causes an issue with one cylinder running lean.

I have also seen it numerous times on both 66E and 66V engines, I chased a fuel related problem for a long time on a customer's 66E engine it was crinkled check valves on both fuel pumps that caused the issue.

Hard to diagnose but thankfully an easy fix, I don't like the stock setup at all and I won't run it on any of my skis , at the very least I am doing the O side Bill carb mod, this is where you drill out all the returns and install a jet in the return line, what this mod does is equalize the fuel pressures and it saves a lot of wear and tear on the fuel pump check valves, of course reducing the ridiculous OEM popoff pressure helps on that aspect as well.

The way the stock setup is it acts like three separate fuel systems all with slightly different fuel pressures, add to that the ridiculous popoff pressures of the stock system and you have a recipe for disaster.

 

[Radical Riche]

On an 1100 it happens when the rear crank seal goes out , I have seen that a few times, you ask how often it happens, on stock machines it happens quite often on 1200's , usually the plastic check valves in the fuel pumps get creased of crinkled and it causes an issue with one cylinder running lean.

I have also seen it numerous times on both 66E and 66V engines, I chased a fuel related problem for a long time on a customer's 66E engine it was crinkled check valves on both fuel pumps that caused the issue.

Hard to diagnose but thankfully an easy fix, I don't like the stock setup at all and I won't run it on any of my skis , at the very least I am doing the O side Bill carb mod, this is where you drill out all the returns and install a jet in the return line, what this mod does is equalize the fuel pressures and it saves a lot of wear and tear on the fuel pump check valves, of course reducing the ridiculous OEM popoff pressure helps on that aspect as well.

The way the stock setup is it acts like three separate fuel systems all with slightly different fuel pressures, add to that the ridiculous popoff pressures of the stock system and you have a recipe for disaster.

So you're saying that you run the external fuel pump?

 

[Radical Riche]

So you're saying that you run the external fuel pump?

Why would carb issue cause the rear seal on a 1200 to go out? Why is it common for rear seals to go out on the 1200? Thanks