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740 UB based off 670

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Thunderguy BOOSTED

Well-known member
Oct 5, 2008
1,324
68
48
Bc Canada
Hey~!

doing an engine swap into a 05 Rev chassis with my 740 Uning bay engine!!! My question is, anyone ever run this engine on a singe pipe? What worked? Custom???

please get back

:help:
 
L

letrbuck

New member
Jun 6, 2013
62
4
8
Montana
I have heard of guys running the psi 670 single pipe on them I know there were others building a fatty single style back then like to hear how it turns out just put a new top end in my 740 last week still makes lots of power
 
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nuttyn01

Well-known member
Dec 3, 2007
811
220
43
Sioux Falls, SD
I've run a few different pipes on that engine. Crank Shop twins are fun at 8600rpm if the engine is built around those pipes. Stock works great too. The best all around one IMHO is the PSI center dump single. Stock rpm and it was an aftermarket single that actually made more power. Oh and i happen to have one left if there's any interest. PM me for details.
 
hey guys I just got off the phone with Kurt at FTX I'm trying to gather the info I need to do my own 740. I just finished assembling my m-10 and my bulkhead and tunnel are out for blasting. Kurt said I can run a few diff pistons, one is Pro-X which I've never run and any input is accepted, the other an OEM 720 sea doo piston which will be more expensive but price is really no option. the hard part is the jugs from what I hear, what sleeves are you guys running to accept the 82mm bore, or where can I be pointed to have the jugs bored and sleeved in 1 shot vs. bouncing around, the company union bay used to use no longer exists. Also I've spoken with the Crank Shop about porting as well as Dyno port, I've used Dyno port for all my other motors but the crank shop has more experience with the 740 so I'm leaning towards them. this is going to be a multi purpose sled so it needs to be somewhat reliable (keyword somewhat) but also climb like its life depends on it, and lastly carbs the Crank shop did my last ones on my 617, what mm bore are you running as well as jets. I know its a lot but I've been on the forum since I was 14 and I know this is the right place to get what I need. Thanks, Tony.
 
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nuttyn01

Well-known member
Dec 3, 2007
811
220
43
Sioux Falls, SD
The Pro-X pistons are a very good choice for that application. I've used SPI pistons as well. You can buy the sleeves from LA Sleeve. I have the part numbers for them ill try to get for you tomorrow. I've used stock 44mm round slides and stock 40mm carbs. Any jetting I could give you would be based off the Summit X with DPM working. The cranks were keyed different on the X/HO motors compared to the standard motors. I know Union Bay didn't like using the X/HO cranks for the 720/740s. The last one I built I had over 7000 miles on it. The RV timing needs to be tweaked some too obviously on any RV motor. Mine aas set up using the PSI center dump single but with some Crank Shop twins and 8600-8800 RPM I've seen the 740 scream like mad. Shoot me a pm with your number and I can give you any information I have.
 
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letrbuck

New member
Jun 6, 2013
62
4
8
Montana
just put a new top end on mine this year took pro x pistons out put new 50 over pro x pistons back in they are the 720 xp pistons and I believe the sleeves are 720xp ports opened up to match cylinders 670 crank and rods jaws twins on mine
 
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nuttyn01

Well-known member
Dec 3, 2007
811
220
43
Sioux Falls, SD
The part number on the sleeves is 6031FB for a 670OB-748cc. The pistons are just off the shelf watercraft pistons for the Sea-Doo XP. I've used the WSM pistons too with good luck. The sleeves have a lot of meat for over bores past the standard watercraft bore. The top of the case will need machined for the larger OD on the sleeves and obviously recutting the combustion chambers. Or do a sleeper head with removable domes if you can still get any. Crank Shop should have these. Open up the case with some tunnel porting too on the intakes.150-160hp is very achievable with these in trail form. I've seen them spun way up in RPM with twins for more power and were still very reliable. They just don't last as long at 8600-8800 RPM.
 
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HANDSOME

Well-known member
Nov 28, 2007
1,400
177
63
51
Pouce Coupe
Any idea how much power with stock pipe ? I'm getting a 93 Mach 1 and would like to get some power out of it.
 
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