EZGO With Honda GX670 Engine Lack of Performance

nightmare

New Member
I have a modified gx670 engine in 1999 custom EZGO golf cart. My problem is lack of performance. It will not pull over 4200 rpm out on the road. It has a pipe and cold air kit, the heads have been milled 50 and ported, and have 1.3-1 rockers and hd valve springs. The engine will free rev to 5500 RPM plus but will only pull 4200 on the road at 39 mph. I put 102 jets in carb and didnt help. I dont think the carb is big enough after the mods and thinking about a 40 Mikuni. Anybody have any suggestions or experience on rpm issue.
 

nightmare

New Member
plugs are normal,slightly dark. pulls good to about 3800 and then not as hard to 4200 probably because of the road speed . it doesnt miss at allpulling up to there. you can hold your foot on the floor and it will not accelerate or miss.pulled cold air pipe totally off engine and no difference. pulledchoke on slightly and engine started to die a bit. i gapped spark plugs to 25 and it pulled to 4300.
 

Nubs

Cartaholic - V.I.P.
Sounds like its running out of air, the cam will fall flat after 3600 so the torque is gone and your only left with rpm's. should be able to dummy down a 40, 36 with accl pump may be better. i'd switch to 8:1 gears and let it work. :twocents:
 

nightmare

New Member
i am running 8:1 gears and 27 inch tall tires. i just bought a 38 mikuni sbn carb thinking that would be larger than the keihen but they are the same square inches of venturies and thinking bigger. do you think the cam is not big enough or the carb does not flow enough on top end???????
 

Nubs

Cartaholic - V.I.P.
yep, would need to fab up a adapter to raise N mount it. not much lower clearance in the valley on those. intake opening is small also.
 

nightmare

New Member
i dont mean to be ignorant but the keihan on the engine now has the same venturie asthe 38mm sbn mikuni and the tm 36 has a 36mm bore size ,so would a smaller carb be defeating the purpose. maybe a tm 40?????should flow more air. is it only available in spigot mount?
 

nightmare

New Member
have you tried this mod. or am i the guinny pig????does the accelerator pump make the flat slide work way better with a 4 stroke?? know where to start or order for jetting??
 

Nubs

Cartaholic - V.I.P.
I've done a few, can't say I'm looking to send anybody on a wild goose chase. as I'm sure you know is that anything can be done given some effort. I myself would not spend the time or money on the Honda engine. there just not build to take rpm's and there is almost 0 performance parts out there. :twocents:
 

nightmare

New Member
well yesterday i proved the theory. pulled the carb off and removed the choke shaft and butterfly. put it back on and it cranked up to 4400 rpm on the road, so it pulled 200 more revs than before. pulled carb off again and drilled out main jets to .046 [ probably just a hair too much ] and put it back on. This time it hauled ass right up to 4400 rpm and then struggled to get to 4500 rpm so it could use a bigger carb :thumbsup:
 

nightmare

New Member
nubs are you out there?? you said you had done some mikunis on the gx670.did you build your own adapter for a tm 40 to the gx manifold and what would one cost ? nice work on the tractor...
 
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