Honda GX390 Stroker Build

[KartFab]

Camshaft Selection

Hi all,

So i have been doing a bit more research and am wanting this engine to really pump out more torque, hp, and rpms for drag racing in 300 ft, 600 ft, and 1/8 mile.

The engine currently has the dynocams 275 cam (keep in mind ive got 1.2 ratio rockers), and im considering the following:

Dynocams 315 cam
nr racing 306 or 307 series cam
dynocams 308

sending the head to bullfrog racing for a real port job, 3 angle valve job, shaving head, etc

12.5-13.9 compression ratio

Imi not sure about torque converter as im in uncharted territory. It does GREAT on launch, but it seems almost like the bike revs super high, then it locks in over drive, drops rpms, and then accelerates like crazy after that (belt slipping?) .... but i dont see anyone drag racing these things, but who knows. Im considering getting the bully turbo clutch.

So basically $800 worth of mods im considering

 

[KartFab]

Took out the mini bike to a T.O.M.B. Event over the weekend. Drive belt slips on the driven unit when in overdrive. I’ll need to tinker with the spring tension to see if I can fix it.

 

[itsid]

Don't mind me.. ..just putting a foot in the door.

'sid

 

[KartFab]

https://www.youtube.com/watch?v=mBMhE9j1m4Q

Been sitting on this footage, about 160 gigs of it for a while and finally got around to editing it. Engine is not 'complete' even though it runs and drives.

I got a race cylinder head that will put the compression ratio at 13.5:1 and a methanol 33mm pumper carb and some stainless steel I will fabricate a header out of.

torque converter slips on driven unit as the hp/torque overcomes the belt friction. Im thinking about running a bully turbo 1" clutch on it and the using some slicks on it for drag racing.

 

[bob58o]

So what is the other easy way to remove the countershaft bearing? LMAO

Heat up the block around the bearing, then turn the block over and let it fall out?
I think I've seen RBG weld a slide hammer to the inner race?

 

[KartFab]

There is a tool for it. A bearing puller.

 

[65ShelbyClone]

I wanted to give props on just how thorough you were in doing the engine build. Great attention to detail not often seen.

 

[KartFab]

Getting serious here. Compression ratio is now going to be 13.5:1. The top babbit bearing may not hold up. Head should seal with an annealed copper head gasket with hondabond high temp sealant.

33 mm pumper carb on M5 methanol. But first some dyno runs before i put all these goodies on it.

 

[65ShelbyClone]

The rod bearing is going to be fine IMO unless it somehow experiences pinging. Even at 13.5:1 it probably won't.

The TC won't be happy though.

 

[KartFab]

The rod bearing is going to be fine IMO unless it somehow experiences pinging. Even at 13.5:1 it probably won't.

The TC won't be happy though.

The TC definitely wont be happy. It was sad the day i took it off of the go kart that made 18 hp.

I will likely have to switch over to a bully turbo and use the existing jackshaft, but that will be down the road when i have saved up a few more coins.

Decided to go with the Dynocams 310 cam and use the 1.2 ratio rockers for this build. I know the valve and carb size should be bigger for this type of cam, e.g. 38-40mm intake, but I'm thinking with the increased vacuum for the bore/stroke, carb, 36mm intake valve and the "max hp" port job on this head that i should be able to pull some serious velocity without too much of a restriction. It is not ideal for these parts to have such "small valves and carb" that could potentially reduce maximum high rpm horsepower, but I think it will still be pretty good for what I've got.

I can always swap out the head for one of those GAGE Motorsports big block heads that may or may not be coming to fruition in the future. The small block heads are already out flowing the welded heads with much less work. Cant wait to see what they can do with a big block head that isnt a belmit or some crazy expensive head.

Max rpm is unknown with this cam, but i expect it to be 1000+ rpm lower than a comparable Honda clone with a 310 cam. My guess is above 7200, and up to 8000 tops?

This frees up the .308 and the .275 cam with 1.3 ratio rockers that i have for future big block projects.

 

[KartFab]

Jetting Woes

Tearing down the engine, but now that i have the wideband lets see here:

Turns out (not surprised here) i was running super rich!

About 9.8 AFR with the stock tm32 flat slide. Owch.

Had a few main (230, 210) and pilot (35, 27.5 ) trying to dial it in and the results are improved, but they were so far off that I have to buy more jets. No big deal, that's an easy one, just going to buy smaller pilots and main jets....

Main jet is the closest to being spot on
pilot jet shows improvement.
needle and needle jet are another story... Makes me want to cry!

So what i know here is that the TM-32 takes the 389 series of Needle Jets and mine is a 389 series Q-2 jet, and then there are the Jet needles on top of that, like hundreds of combinations here, and they aren't cheap.

Looking at this logically, there are the O, P, and Q jets, listed from lean to rich.... okay... There are a total of EIGHT different needle jets to choose from here that are leaner than the needle jet i currently have...
http://www.wahlracing.com/389-Series-Needle-Jets.html

But then I'm looking at this chart, which seems to indicate Q-0 through R-8 are the available needle jets for the TM-32... so does that mean i can only get a Q-0 needle jet as it is the only one that is leaner for the TM-32?
http://www.pjmotorsports.com/mikuni-needle-jets.html

Now i know the carb takes the 5 series needles, but then some sites have it listed as 6 series needles.... that's going to be a problem.....

So the default needle jet (Q-2) and the default needle (5FP17) are just too stinkin rich. Top clip only gets me to 10.4 AFR at half throttle under full load, you can see the rest in the attachment... not looking good.

I have a feeling that the the needle and needle jet are going to need to be replaced no matter what. Im hoping i could just drop down to the Q-0 needle jet, and keep the existing needle, but then i might just end up in the 11's at part throttle instead of where i think i should be, which is about 12.5-12.6 AFR under full load.

Anyway, i hope i can maybe just buy 1 more needle jet and maybe what 3-4 needles to figure this out?

Im inclined to purchase a "small" needle jet or a "large" needle, and chuck up the needle to my drill and just run it on some sandpaper in my fingers a tiny bit off at a time and test bit by bit, throttle position by throttle position. Sounds exhausting.

Any help would be appreciated.

 

[65ShelbyClone]

Jets R Us also lists series 389 needle jets from O-0 to R-8 here:
https://jetsrus.com/a_jets_by_carburetor_type/needlejet_jets_mikuni.htm

And that section specifically mentions the TM32.

They also indicate that the TM32 uses a #5 series needle on this page:
https://jetsrus.com/a_jets_by_carburetor_type/needle_mikuni.html#5

 

[KartFab]

Jets R Us list series 389 needle jets from O-0 all the way to R-8 here:
https://jetsrus.com/a_jets_by_carburetor_type/needlejet_jets_mikuni.htm

They also indicate that the TM32 uses a #5 series needle on this page:
https://jetsrus.com/a_jets_by_carburetor_type/needle_mikuni.html#5

Thanks! That is confirming what I thought. I have a tendency to get stuck on things that are trivial.

Ok, so looking at the D-1 through D-5 measurements, and what needles are not discontinued, the field is quickly narrowed down to the following needles:

5F3
5F21
5FL7

I'm liking the 5FL7 so far as it is the most similar in length, and i might be able to sand it down with a smaller needle jet.

Crap shoot on the needle jets so i just said screw it and ordered P2 through Q0. Its not worth my time to order a few and then find out im one or two off and have my whole day shot because of it.