GX340 Questions - Bob's 4 Cycle Karting

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You are using an out of date browser. It may not display this or other websites correctly.You should upgrade or use an alternative browser. GX340 Questions

• Thread starter Thread starter kurly154
• Start date Start date Apr 9, 2014

K

kurly154

New member

Picked up a gx340 pretty cheap. Looking to build it for good power and torque. I run a 1/5 and a 1/4 mile dirt track. Is it possible to bore the 340 out to 390 specs? Also are stock size valves pretty good on these or do they need to be stepped up when running a .360+ lift cam? Any and all help is appricated. T

truck

New member

Stock valves are not long enough with that cam, if you bore it just perfect you can barely get a stock 390 piston in it, but will break out of sleeve 3 out of 5 times........with that much lift I think your gonna need different rockers..... K

Kart43

Member

It really depends on how old your GX340 is as the GX340 and 390 have had the same bore of 88mm for several years now. K

kurly154

New member

I'm not sure how old it is. Picked it up for $30. I know it says gx340 and 337cm right under it. I knew the newer ones were the same size but haven't torn into this one just yet. I was hoping I could fit the 390 piston in it. J

jsstump#70

Member

If it says 337cc on it, its more than likely an "old" one. Every true 340 I have had bored (only 4 or 5 of them) I have had no trouble what so ever going to 88mm. I have found the exterior dimensions of the sleeve to be the same as the 390 so boring to 390 specs was no problem. In fact Ive had to go .020 and .030 over 390 spec in 2 of them. Thats just the ones I have seem mind you. Possible difference in the block/sleeve in the 340 over time I suppose.The shop boring them should have no trouble telling if the sleeve is thick enough or not. Good luck. M

metalmagic

New member

call parsons racing he will hook u up....i say stick with the 340 piston and stroke it...... his number is 812-639-1802 K

kurly154

New member

metalmagic said: call parsons racing he will hook u up....i say stick with the 340 piston and stroke it...... his number is 812-639-1802 Click to expand...

Why stay 340 bore but stroke it? I've looked at parsons website and like the rod piston kit. K

kurly154

New member

Anybody else? M

metalmagic

New member

Stroke gets u out boys and u can gear these things for speed with the torque.... Its a totally different ballgame lol L

level600a

Member

Bore it to 390 spec.

CarlsonMotorsports

Site Supporter

Bore and stroke if it's within your means. "There's no replacement for displacement." -- Thanks and God bless, Brian Carlson Carlson Racing Engines Vector Cuts www.CarlsonMotorsports.com Celebrating 25 years of service to the karting industry 765-339-4407 [email protected] 6

65ShelbyClone

Member

metalmagic said: i say stick with the 340 piston and stroke it...... Click to expand...

metalmagic said: Stroke gets u out boys and u can gear these things for speed with the torque.... Its a totally different ballgame lol Click to expand...

Generally speaking, two otherwise identical engines with the same displacement will make about the same power and power delivery regardless of the bore and stroke. If we're talking about a GX390 as the baseline, then IMO, stroking a 340 is the most expensive way to get there. 6

65ShelbyClone

Member

Gold star for you. J

jsstump#70

Member

The cheapest stroke increase IMO is to install a 420 crank in a 390 (or 340 I guess). Mill the dish off the stock piston (or most of it) and you have about a 402cc. Any other stroke increase would involve offset grinding the crank, using a special rod and hoping for a proper comp. height piston to be available. Is there a crank readily available to get the added stroke? This could get expensive as apposed to simply boring to 390 spec. Im all for added displacement but the case is already pretty full with a 66mm (420)stroke crank. Boring is relatively cheap in comparison to any measurable gains by stroking these things. C

cubcadet70

New member

Vegas Carts has 68mm stroke cranks right now in stock. First come first serve! They are cast. R

rkcarguy

New member

Vegas's 440 is $400, why bother stroking/boring, etc etc? Add billet rod, cam, flywheel, carb, go collect trophies M

metalmagic

New member

cubcadet70 said: Vegas Carts has 68mm stroke cranks right now in stock. First come first serve! They are cast. Click to expand...

If it's not a special grind a stock 440 crank is 66mm K

kurly154

New member

I got this whole 340 for $30. Figured it'd be a good platform for a big block build. I was planning on just boring it to 390 spec and put a parsons rod and piston in it with either a .360-.370 lift cam and 1.3 rockers. Not sure if I should go big valve or stay with stock size valves. Also would a slide carb or the super bn carb be better? Methanol or race gas? You must log in or register to reply here. Share: Facebook X Bluesky LinkedIn Reddit Pinterest Tumblr Email Share Link

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