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35GTR heads , cam , mikuni carb & exhaust . Only 34mph @ WOT
http://mudmotortalk.com/mmt_v2/viewtopic.php?f=3&t=72585		 Page 1 of 1
Author:  muddobbler [ Sun Jun 17, 2018 5:32 pm ]
Post subject:  35GTR heads , cam , mikuni carb & exhaust . Only 34mph @ WOT
I have an older ratio non-CDI , non-Revlimit ,35 Gator Tail with mods and only running 34MPH @ 4450RPMS WOT . It runs about 2MPH faster and 500RPMS more than when it was stock . The Vanguard engine only has about 4 hours on it . It will idle all day long on the trailer but kills as soon as its in the water . Idles erratic and sometimes backfires .1400RPMs in neutral and drops to 900 in FWD and kills .I've bent the float tab a good bit and Carb overflows at idle in gear . Ive tried 22.5-37 pilots with no different results . Runs great from 1/4-WOT . Makes no difference from 3/4-WOT in RPMS or speed . Carb is opening complety , sparkplug color ( Autolite APP3923 ) and valve lash look fine . engine has cam drive fuel pump .Just not seeing the speeds I thought . I ran it with a brand new Raptor 12x11 and a worn raptor and runs about the same . Any thoughts ?

Mods listed below:
180PSI compression +/- 5psi
Ported and shaved heads
Black Death intake w/ 42mm Mikuni 167.5 main & 37 pilot jet , idle air screw 1.5 out
BPS Q series muffler
305 Cam
Governor lock out
Author:  CypressCritter [ Mon Jun 18, 2018 7:58 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
What boat?
I run a mikuni with heads and exhaust, no cam, with a 175 main jet and it is just rich enough I was running way too lean with a 165.
Your speeds seem to be right.
Author:  muddobbler [ Mon Jun 18, 2018 8:09 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
18x46 1/8” thick . Lightly loaded . I only picked up about 2mph and 500rpms with mods .
Author:  Russ [ Mon Jun 18, 2018 8:27 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
4450 and 34 mph is about right as critter said. The work you did will now allow you to put more load in that boat and still run well. Change your lower gear to the 48t, then play with props if you want more speed. It's not a cheap game, but that's the one you have to play now if you wanna go faster.

As far as all the other issues (which should be far more important to you than speed until they're fixed), you should be running a 25 pilot, or one up or one down from that. Adjust the mixture from there. Set idle at 1250-1400, and in gear it'll be around 950-1100. Don't keep swapping pilots to the 30 and above. As far as the main, I'd be surprised if you needed anything more than a 165. Cypress critter is an anomaly, but is likely due to the cam. Most of these engines in the 180ish compression range are running 162.5 or 165 main jets, but yours is running where it should with the 167.5.

#CFFITR
#TheGhostBoat
#whoneedsphysicsihaveamudmotor
Author:  Russ [ Mon Jun 18, 2018 10:19 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
The backfire you mention: Is it happening at idle?

#CFFITR
#TheGhostBoat
#whoneedsphysicsihaveamudmotor
Author:  muddobbler [ Mon Jun 18, 2018 11:48 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
After it dies from idle and trying to start it back up it will back fire and throw a flame out of the carb with no air filter . I still have to order and replace the float valve to the 2.3 . Im sure thats causing my overflow problem at idle in gear . Just leaks a few drops out of the hose .
Author:  Russ [ Mon Jun 18, 2018 12:02 pm ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
Yes, that's the overflow problem. The stock float valve is too big.

Pm sent.

#CFFITR
#TheGhostBoat
#whoneedsphysicsihaveamudmotor
Author:  MaXXis85 [ Mon Jun 18, 2018 2:46 pm ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
If you have a 37 pilot jet you are WAY to fat on the idle circuit. My BIG motor with a EVT intake only runs a 32.5 pilot.
Also, who did you head work? Basic work will yield basic results.
Author:  muddobbler [ Fri Jun 22, 2018 6:39 pm ]
Post subject:  Update
Well after rejetting and putting a 2.3 float seat the final results are 165 main , 25 pilot and air screw 2 turns out . It was still hard to start and backfiring with the 27.5 pilot when hot . It still seems slightly rich at idle with the 25 pilot . Ran 30.8 WOT with a fishing load .
Author:  MaXXis85 [ Mon Jun 25, 2018 7:35 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
you know the pilot circuit is an Air system so the more the AFR screw is open the leaner the system is, and less it is open the richer it is.
Backfiring out of the carb at idle is a lean condition.
Author:  Wchauvin [ Mon Jun 25, 2018 10:47 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
Cypresscritter is right, those speeds sound about right for that set up
Author:  kirk [ Mon Jun 25, 2018 5:19 pm ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
Author:  muddobbler [ Mon Jun 25, 2018 8:01 pm ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
Author:  MaXXis85 [ Tue Jun 26, 2018 5:26 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
Author:  kirk [ Sun Jul 01, 2018 2:57 pm ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
I’m guilty for telling people it’s just a lawn mower engine with a prop haha


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Author:  trofazz [ Fri Jul 20, 2018 6:20 am ]
Post subject:  Re: 35GTR heads , cam , mikuni carb & exhaust . Only 34mph @
I'm new to this particular genre of engine performance, but was an ASE mechanic for 15 years and have tinkered a bit with old school hot rods and motorcycles. BTW, great info here from you guys regarding jetting, head work, etc... To the OP, I think you are focusing on your top speed and overlooking the real benefits the extra work has given. Meaning hole shot improvement and increased load capacity. Judging by the other guys' comments on here that know a heck of a lot more about this than me, it sounds like you are close to maxing out the top mph with your current gearing and prop set up. Once you get the drivability issues under control, I'd focus on props, pitch, and gearing to get you the speed you are looking for....
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