Budget GX390 stroker build.

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KartFab

Member
Hi all, this is KartFab from the youtube interwebs.

I am working on my next engine build and need some good input on my budget honda gx390 stroker build (budget+stroker=oxymoron right?)

So far i have a used honda gx390 block that I am taking to a shop to have bored to 90mm. I also purchased the -0.050" piston that arc racing sells. According to my research, if i put in the +.200" clone crank, that will put the top of the piston .010" in the hole at TDC with a stock length rod.

The purposes I am using this engine for will be to put on a mini bike -baja heat to be exact (dumb yes I know) and have to ride around on the trails as well as zip around doing a little drag race.

Now, I dont want to go ALL OUT to be the top dog with the biggest baddest, highest revving engine out there. The goal is to keep this within a reasonable budget, and have a target max rpm of around 5K, and keep the stock flywheel.

I am hoping I wont need to get chromemoly rods because the lift isnt very high, also hoping I can use stock valves and lifters as well. Just do a lap job and set lash after it is pieced together.

With that said, here is what I plan on purchasing (mostly from NR racing):

1)4.410" billet rod (stock length)
2) high torque cam - requires single valve springs, says it wont beat up valve train (http://www.nrracing.com/product-p/390clonecamshaft460.htm?CartID=1)
3) chinese 69mm crank (+.2 stroke) http://www.nrracing.com/product-p/69crank.htm?CartID=2
4) UT1 vs UT2: gotta check into this, but if mine is UT2, ill have to purchase this coil? http://www.nrracing.com/product-p/30500-z5k-801.htm?CartID=4
5) cheapo chinese tachometer to monitor my rpms.
6) will get a high flow intake adapter and filter, but will get htat after i determine if i can use the stock gx390 carb with good results.

From what I have gathered, ill check the valve clearance with modeling clay (i dont think there will be an issue as the cam is pretty mild). I will also need to check cam lobe clearance, and i will likely need to clearance the counterbalance from the crank as well as the rod from the bore. I feel pretty confident that I can do this. I know there are balanced cranks you can buy, but im trying to avoid spending 4-$550 on a balanced crank, and hoping that the counterbalance in conjunction with the ~5000 rpm target limit will keep things from flying apart.

So I am now trying to figure out:
A) when valve float occurs on the stock gx390 springs. Again, i only want to go to about 5000 rpms as my max target rpm.
B) what single valve springs are suitable for this application (e.g. not too heavy)
C) carb setup. Can i get away with the stock carb, and rejet? If so, what is a good starting point? Emulsion tube replacement, or is it good?
D)I plan on welding a new exhaust (can mig/tig) and am wondering what i need to stage it to? E.g. x length of x ID+ xlength of xID
E) what would be a good muffler for this thing? Are those open RLV mufflers even worth it? I dont want it to be ungodly loud, but i still want the flow to be good.
f) i have seen some modified engines run a fuel pump to the carb. Is this necessary for my application? What sup with the crank case cover being tapped and drilled in some?
g) can i use the electric start on this setup? (e.g. another reason I want to keep the stock flywheel yes i can always get the billet electric start one, but im trying to avoid this).
h)timing?
i) valve lash?


Anyway, Just thought I would ask these few questions in hopes that somebody with some foresight will reply with good information.
 
Imo when You are going to get it bored out anyways, i think You should buy the New billet rod and piston combo from vegascarts. I think it's 92 mm and Will give you extra cc's that way. The rod is longer thank a stock one, giving better rod ratio, and the two items Are only 100 dollars. Then spend the money on a bigger carb, cam and headwork. That will give You more power and more relyability because of the much better rod ratio.

I honestly think You should not go down the stroking road if You just want a good engine. It can be done both better and cheaper in other ways.
I Also think You May want to to increase your target rpm if You want power. If You want to stay at 5k, slap the valle springs for some stronger ones and keep the stock cam and carb. That combo Will have fine low end power and stop pulling at around 5500 rpm.

If the engine is not pretty New it's ut1.
 
Already bought the 90mm piston should come in today. I forgot to mention that the stock crank on my engine had turned over a key and completely jacked up the keyway to the point where a clutch wont spin true on it (was like that before I purchased so who knows how it happened).... so I have to buy a rod no matter what. Seems like the perfect excuse no matter what.

I always wanted to build a stoker. I understand I could get great performance and increased compression without going with the increased stroke, but I'm set on building the stroker now.That is a really good point though with the longer rod to get the same displacement, higher compression, better torque, better ratio, less mechanical wear......

Yes the increased rod angle can cause accelerated/excess wear to the cylinder wall and rings, but one could argue that any performance modification can cause accelerated wear. Not trying to quantify it, just saying I think its worth it (to me) to get a nice torquey powerful stroker fired up just for fun and enjoy it for what it is. It was either this or put the v-twin honda gx620 on the mini bike, but that would mean I would have to raise the horizontal support bar. The gx390 will fit height wise, with less modifications for mounting.
 
seems like you have your mind set then - let me just remind you that i don't think it's the right way but whatever.
Stroking and boring this engine will increase the capacity to 440 cc from 390 that's a 12 % increase. That means that you only get around 12 % more torque and thus 12% more power for all the money you spend stroking this thing. It's the worst value performance mod you can do!

To answer your questions:

a)
I don't know but probably before you target max rpm with a hot(er) cam.

b)
I think around 35 lbs of preassure would be suitable. I run that myself on the stock cam and see no float.

c)
I would just get a 32 mm PWK carb from ebay to not be missing out on any power. with that said, the stock carb will probably have more low end torque and almost have as good high end power as a bigger carb, when you don't turn more rpm

d)
no idea, don't think it's a big deal.

e)
some use small car mufflers, but those take up a lot of space. maybe a bike muffler for a similar displacement bike would work well.

f)
the fuel pump is neccesary for gas tank relocation, but not needed if the tank is placed higher than the carb.

g)
if you have a cam with the compression release intact, which it looks like the one you want to buy has, it should work just fine.

h)
just get an advanced timing key, think they're around 32 degrees.

i)
The lash gets looser as these engines heat up (at least the 200 cc counterparts), so it's not a big deal. just run a little bit.
 
Vbruun said:
seems like you have your mind set then - let me just remind you that i don't think it's the right way but whatever.
Stroking and boring this engine will increase the capacity to 440 cc from 390 that's a 12 % increase. That means that you only get around 12 % more torque and thus 12% more power for all the money you spend stroking this thing. It's the worst value performance mod you can do!

To answer your questions:

a)
I don't know but probably before you target max rpm with a hot(er) cam.

b)
I think around 35 lbs of preassure would be suitable. I run that myself on the stock cam and see no float.

c)
I would just get a 32 mm PWK carb from ebay to not be missing out on any power. with that said, the stock carb will probably have more low end torque and almost have as good high end power as a bigger carb, when you don't turn more rpm

d)
no idea, don't think it's a big deal.

e)
some use small car mufflers, but those take up a lot of space. maybe a bike muffler for a similar displacement bike would work well.

f)
the fuel pump is neccesary for gas tank relocation, but not needed if the tank is placed higher than the carb.

g)
if you have a cam with the compression release intact, which it looks like the one you want to buy has, it should work just fine.

h)
just get an advanced timing key, think they're around 32 degrees.

i)
The lash gets looser as these engines heat up (at least the 200 cc counterparts), so it's not a big deal. just run a little bit.
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Thanks for the reply. Although stroking may be the worst value mod I could do, its not the worst mod I have seen suggested (like adding stickers, or indexing the spark plug, or cutting out fins from the flywheel, or bypassing the governor spring).

Ill still have to look into this side cover thing, but i swear i thought i saw a modified engine have lines from the top of the crankcase cover to the valve cover. I dont know why, maybe to equalize pressure between valve cover and crankcase? Who knows.

I think ill get either the PWK or the mikuni flat slide 32mm, and make my own little adapter. Ill keep you all posted on my progress
 
Are you still using the governor? Removing every other fin on the fan is actually a cheap mod that does help. The lines from the crankcase to the valve cover will not do anything for you, they actually don't do much for anyone.
 
Just before You do get the New carb, i suggest checking the size of your intake valve and going with a 2 mm smaller carb than that. Also doing a light porting of the head should be good.

If he plans to replace the carb, i guess the governor is gutted.
 
saving intake/exhaust for the last step. Whole engine is gutted to bare block, its in the shop now getting bored to 90mm. Ive seen some conflicting info about ring end gaps. Ive done the math and it looks like my end gap shoudl be .014" for top, .017 for second compression, and .015 for oil (wiseco). (even though my piston is a 'arc racing' forged 90mm piston, seems like the consensus is make top ring piston diameter X.004 and second piston diameter X.005, keep oil at .015"

ARC racing is showing less on their blog article "why does it matter" . I don't want to have my ring ends butt together. I was told that the rings come to about .006-.008 and dont need to be gapped. Also read that after rings seat that the gap grows by .004-.005".

also read that in vehicles that ring end gap set to .010" and the rings butted together and lost hp because rings scraping on cylinder wall.... siiiigh. Thanks internet for thoroughly confusing me.
 
KartFab said:
saving intake/exhaust for the last step. Whole engine is gutted to bare block, its in the shop now getting bored to 90mm. Ive seen some conflicting info about ring end gaps. Ive done the math and it looks like my end gap shoudl be .014" for top, .017 for second compression, and .015 for oil (wiseco). (even though my piston is a 'arc racing' forged 90mm piston, seems like the consensus is make top ring piston diameter X.004 and second piston diameter X.005, keep oil at .015"

ARC racing is showing less on their blog article "why does it matter" . I don't want to have my ring ends butt together. I was told that the rings come to about .006-.008 and dont need to be gapped. Also read that after rings seat that the gap grows by .004-.005".

also read that in vehicles that ring end gap set to .010" and the rings butted together and lost hp because rings scraping on cylinder wall.... siiiigh. Thanks internet for thoroughly confusing me.
Click to expand...

I think you should just have the gaps set the way you calculated. Most of the engines that run really tight ring end gaps are run on methanol which burns cooler.
 
Industry standard is typically .004 for each inch of bore. Problem arises when folks use rings for a .020 or .030 over bore so they can get the gaps down to what they "think" they need. The added ring drag from doing it this way simply kills power from the huge ring drag this causes. To properly get super small end gaps, you must use rings for the size bore intended BUT in a longer length so you can "trim" them. Or detention overbore rings to reduce drag. Much too much emphasis is placed on getting small end gaps IMO. Reduced drag will yield measurable gains on the dyno. Super small end gaps will not. Be sure your bore is round. Be sure you have a good finish. Be sure there are no flaws in the rings. Be sure they are installed properly. Be sure the bore is clean clean clean, then clean it again before assembly. Be sure the engine fires up on initial start pretty quick so your not flooding the crap out of it. Spend less time on getting a super small ring end gap. .014 and .017 you listed above should be fine. The tightest I would go on a 90mm bore would be .010 IMO but I wouldn't spend time or money getting it If I had .014 already. Good luck.
 
jsstump#70 said:
Industry standard is typically .004 for each inch of bore. Problem arises when folks use rings for a .020 or .030 over bore so they can get the gaps down to what they "think" they need. The added ring drag from doing it this way simply kills power from the huge ring drag this causes. To properly get super small end gaps, you must use rings for the size bore intended BUT in a longer length so you can "trim" them. Or detention overbore rings to reduce drag. Much too much emphasis is placed on getting small end gaps IMO. Reduced drag will yield measurable gains on the dyno. Super small end gaps will not. Be sure your bore is round. Be sure you have a good finish. Be sure there are no flaws in the rings. Be sure they are installed properly. Be sure the bore is clean clean clean, then clean it again before assembly. Be sure the engine fires up on initial start pretty quick so your not flooding the crap out of it. Spend less time on getting a super small ring end gap. .014 and .017 you listed above should be fine. The tightest I would go on a 90mm bore would be .010 IMO but I wouldn't spend time or money getting it If I had .014 already. Good luck.
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Alright. So here is what I got from ARC. Set them to .006-.008", and if you are worried about it, set to .008-.010". This should be done on a 'perfectly' straight bore (e.g. not one you have been running for a while, that is egg shape, out of round tapered blah blah blah. You run into issues if you dont get engine heat up before you put it under load (e.g. think hot rings, cool block, you get the idea). So with that said, since its getting bored today/tomorrow, I'm having them set the top compression to .008", second to .010" and leaving the oil scraper at .015. ARC estimates that about .001-.003" gap increase will occur until rings seat during break in so....

final gaps should end up around .009-.011 top, .011-13 second, and who really cares about scraper anyway as long as its .015" to start out with.

Also, bullfrogperformance said that .006-.014" is a good range so both pieces of advice jive.

Next item: head gasket. I will be checking cylinder head volume and having my buddy cut the piston (if needed) to be at .010" below the deck, and will likely have the head mill about .050" off (pending 1st cc check).

What are my head gasket options at a compression ratio of 10.2:1 -10.5:1 at 90mm bore on the GX390? Im hoping i can keep (e.g. use a new) the stock gasket, but will get whichever one will hold up/fit the bore.
 
I would get the thin one and then mill correspondingly less off of the head to maximise its structural integrity.

The thin one is 10 dollars at vegascarts anyways.
 
The 90mm arc piston isn't a stroker piston it will come out of the block alot I've payed shipping 3 times for their mistake to get a actual stroked piston it's a 88mm they don't make a 90mm stroker piston like their web site says
 
7orange said:
The 90mm arc piston isn't a stroker piston it will come out of the block alot I've payed shipping 3 times for their mistake to get a actual stroked piston it's a 88mm they don't make a 90mm stroker piston like their web site says
Click to expand...

Okay whelp..... you are definitely right. I will be measuring the piston stick out, with my machinist friend one day this week. The pop out seems to be quite a bit. It even says on their site that .050" needs to be cut from the top to get proper compression height, BUT it appears to be popping out much more than .050" complete BS as I had called in before and they said that it should sit .010" below the deck. Now this is with a genuine honda block and buying parts that say, piston may need to be machined.050 and you get something different in the mail that what you were expecting. Ill report back when I get the measurement, but from what I can see just eyeballing it, I may need to cut a lot more than .050" to get it to sit flush with the deck.

Also, I did have my block bored to the 90mm piston, so I am starting to think I may be screwed now :-/ kind of ****ed off right now, but am wondering what to do at the same time to make it work. My compression calculations show that I need a .020" head gasket (which i already bought and verified that it works with the gx390) with the piston .010" in the hole at TDC. This really throws a wrench into it unless I can somehow use a much thicker headgasket with the piston popping up from the deck to get the right compression ratio (10.5:1)

Also have noticed after the block was bored out that the sleeve may not go 100% through the block? Is that right? It looks like the very top of the deck is all aluminum like the block, then a little bit down the steel sleeve starts. If the top compression ring sits on aluminum, then transitions to steel, i just cant see how that would be beneficial at all.... Will need to measure though.
 
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I ended up using a dome piston in my 90mm block with a thick head gasket it seem to run good I'm not sure about the sleeve I've never had one bored
 
Ok, took the block to my machinist friend and here is the following info:

1) piston pops out of deck by .093" at TDC, so if i use a stock head gasket of .045" (which i honestly dont think i can since i bored to 90mm???!?!) and milled .050" off, we are still looking at the piston coming .002" from contacting the head at TDC in a perfect world. Also, compression ratio would be about 11.6", which is too high for what I am trying to achieve.... I think. Cam calls for 10.5:1 and ill be running 93 octane in it. The bigger problem I am facing now is that I dont want .002" clearance because I know for sure that we dont live in a perfect world, and i will be wanting to mill just a tad from the head(when i say head, i mean combustion chamber only, NOT the mating surface) to get that compression ratio and clearance that I need. Afterwards, ill CC the head and see what we can do to get it down to 10.5:1 as well as get the proper clearance from piston head to head.

Speaking of that, what is the 'proper' clearance from the piston to the head anyway?

Speaking of head gaskets, what is the thickest head gasket i can get for the gx 390 head that will accomodate a 90mm bore? (stock is 88 mm) Does the predator 420 head gasket fit the gx390 head?
 
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Ive used .035 as my minimum many times. The more RPMs you intend to run, the more you may want. Stock type gaskets come in thicknesses ranging from .009 to.050. some "390" gaskets have a 88mm hole, some have a 90mm hole. Vegas Carts has a 92mm gasket that is .045 thick that seals well. Some of the MLS (multi-layer-steel) gaskets can be opened up to 90mm if they are the 88mm type. I have found that the gaskets with a "fire ring" seal very well. I coat all my gaskets with Copper-Coat just to be safe. Be sure when checking for pop-up, you are using the stock length rod. Stock length is 4.41. Some after market rods also come in a 4.451 length. One company's rod is 4.423. They list this as "stock" length. Also some rods have been made .200 to .400 longer to mate with short compression height pistons. Point is, if you are getting .093 pop-up, that's a lot, and I just wanted to be sure you are using the shortest rod possible so as to make it a possible simple fix.
 
I used the arc racing stock rod that is 4.41" The problem is the piston was advertized as a "stroker piston" which it is clearly not. It is basically a stock compression height piston with a short skirt... I did the math with the 0.093" pop up and if i switched to a standard crank, it would be .005" in the hole at TDC. Frustrating to spend $150 on a 'stroker' piston that may need to be milled .050" but in reality its an invitation for my engine to turn into an expensive paperweight if I milled it or ran it as is.

I called arc racing and they said that their stroker piston should sit flush or pretty close to flush at TDC with their stroker crank. After about 5 minutes on the phone, the answer is currently 'we dont know why it sticks out of the block so far' and 'we will call you later when we find out more'.

I get a call back later (arc gets these from NR racing). Russel at NR racing said that he cuts .050" from the top to get piston to fit flush in stroker applications...... which does not acount for why mine sticks out .093" at TDC. So now the question is did i get a wrong rod? Is the gx390 block shorter for some reason? What makes the GX390 shorter if its a genuine gx390? I thought all gx390 blocks have the same dimensions.

Time to check more into this. Based on what 7orange has said, they have sent me the wrong piston? Well, i had my block bored, so what are my options? The Inevitable option that I am thinking i will have to take is return the stroker crank, and buy a gx390 stock crank, sent it in to NRRacing and have him balance the crank.
 
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