[Casey_m]

Hello all! I recently became the proud owner of an '88 TRX350D and '88 TRX300 2wd (my first quads, but not my first rodeo in Honda motorcycle rebuilds over the years). They were a 2-fer deal from an old rancher here in Idaho. Both bikes had been sitting outside for about 8-10 years, but the 350D was more complete, so I started there.

I got her running great (won't go into everything I did on the bike, but will focus on engine-related work) after a carb kit, new fuel filter, new fuel pump, new fuel lines, new plug, air filter, oil & filter change change. I scrubbed a mountain of gunk of the cylinder head, to find that one of the valve adjustment covers was leaking (installed new o-rings) and one of the cylinder head cover bolt holes had stripped threads (Helicoil worked perfectly).

It was running pretty well, but wouldn't hold idle unless the idle screw is cranked way in. I pulled the carb and rechecked my rebuild work - broke it all down, sprayed compressed air through all the ports and passages. Set idle screw to factory, minus 1/4 for elevation (I'm at 4000ft), so it is 1.25 turns out from seated. Still no improvement, but ran well otherwise, lots of power.

One note: when reassembling the first time, I noticed the o-ring/seal on the body of the starter valve was cracked, so I replaced with an o-ring I had. Not sure if this is causing it to stick, and precipitating the other issues - but I wanted to note it here.

I did a SeaFoam Top-End treatment, let it hot-soak, then ran it more. Shortly after that I started getting smoke (didn't look like sea-foam smoke, nor did it look like burning oil) and A LOT of carbon blowing out of the exhaust at mid- and high- throttle. Then yesterday, after giving the kids a ride around the block twice, it crapped out completely - died and wouldn't start, unless I was manipulating the throttle. And it will run as long as I keep thumb on throttle, but it runs REALLY rough until it hits a few thousand RPM.

This morning I tore it down, checked valve lash (minor adjustment on one intake valve, but not enough to cause this level of hell).

Its got a beautiful fat spark when I tested the plug, so thats out.

Unfortunately I didn't check compression before, because it seemed really strong (very healthy sounding thump, and very solid in the kickstarter. But I did check it with my gauge this morning, and compression is consistently ~90psi, using the electric starter. I added about a tablespoon of oil in the spark plug hole and readings shot up to ~120psi, which would usually indicate worn rings/cylinder wall.

So I'm wondering: maybe the SeaFoam knocked some crap loose, it wedged in between a valve and seat, and caused a burned valve. Or, the choke was stuck on, causing idle issue and engine damage to the rings/piston/cylinder wall (hence the higher readings when I added oil).

So I'm wondering where I turn next? I'm afraid the next step is to pull the head (was really hoping I wouldn't have to tear down the top-end) and see whats going on in there. Unless you guys have another suggestion or hypothesis? Thanks!!

 

[shadetree]

welllllllllll, first thing, if it won't run on idle ?, this tells me the idle circuit is not clean/right ?. second, chances are, the rings/cylinder/piston will need to be fixed. 90 psi is wayyyyyyyyyyyyyyy to low !. these call for a ton of compression to run right. check the cam chain to see if its stretched ?, if its really bad ?, this can lower your compression. where and what fuel pump you replace with ?, oem from Honda ?, or so cheap china junk from ebay ?. is the fuel pump even working right ?, as in, could/can be a faulty fuel pump relay.

 

[Casey_m]

welllllllllll, first thing, if it won't run on idle ?, this tells me the idle circuit is not clean/right ?. second, chances are, the rings/cylinder/piston will need to be fixed. 90 psi is wayyyyyyyyyyyyyyy to low !. these call for a ton of compression to run right. check the cam chain to see if its stretched ?, if its really bad ?, this can lower your compression. where and what fuel pump you replace with ?, oem from Honda ?, or so cheap china junk from ebay ?. is the fuel pump even working right ?, as in, could/can be a faulty fuel pump relay.

Thank you for chiming in Shadetree - I've enjoyed reading through your builds and advice.

I replaced fuel pump with cheap junk from Amazon, not ebay (couldn't afford $160 for a Honda fuel pump).

Its pumping fuel (pulled line to carb and it pumps well). Slow click at idle, clicks speed up when you add throttle. Haven't done a pressure test see what its pumping.

Anything else to check for the idle issue? Carb is clean as a whistle, not clogs or gunk. Could it be that starter valve I jimmy-rigged with an o-ring instead of replacing? Also - I should probably pull the tank and flush it as well. Looks like a PO added some sort of liner at some point, cuz I think I see some bubbling up in a couple spots - could be a chunk of it broke free and is obstructing the fuel inlet...

The low compression is new - within the last 48 hours (since SeaFoam, which Im regretting now). Anything else you would suggest checking prior to teardown, or do I have to just break it all down and see whats going on?

 

[retro]

The seafoam cylinder washdown was a very bad idea. It won't idle until the motor has enough compression to run so no sense in messing with the carb anymore at this point. I think I saw those OEM fuel pumps listed at about $110 or so just recently. You'll need one.... along with a topend refresh at minimum.

Welcome to the forums! Got any pics of your old tank you care to share?

 

[Casey_m]

Hi Retro, thanks for your input. Yes I’m regretting the SeaFoam, though I’ve never had issues with it before. Going to tear it down tomorrow and see what’s going on. I’ll share more once I get in there.

 

[shadetree]

my money says it needs a refresh on the top end. chances are, rings/cylinder are worn too much, which lowers compression. i would work on getting the engine fixed before i messed with the fuel issue. as for the china junk fuel pump ?, bad..just bad in my book !..lol. not worth pizzing on !!. just because it '' clicks '' ?, does not mean its putting out the correct psi !?!. they call for anywhere from 4 to 6 psi if i remember right ?.

 

[Camchate]

What should the compression be on the 350D?

 

[Duck0fDeath]

178-206 Psi

 

[Casey_m]

I got the top end disassembled this morning. I don’t have a bore gauge but my micrometer puts it at 81mm. No scoring on piston or bore, no lip on the bore. Cam and rockers look clean. No obvious burnout on the valve seats, but they are pretty crusty. See pics for the rest.

 

[Casey_m]

A couple more

 

[shadetree]

I got the top end disassembled this morning. I don’t have a bore gauge but my micrometer puts it at 81mm. No scoring on piston or bore, no lip on the bore. Cam and rockers look clean. No obvious burnout on the valve seats, but they are pretty crusty. See pics for the rest.

top, side of piston has grooves in it.

 

[shadetree]

run your rings down in the cylinder, see what the end gap is on all rings ?.

 

[Casey_m]

run your rings down in the cylinder, see what the end gap is on all rings ?.

1.25mm on top ring, 1.06mm gap on second ring

 

[retro]

Valve seals are shot, piston rings are worn razor sharp and are shot, piston looks better than I expected, but your pic only shows one side and we don't know whether its the thrust side of the piston or not. Can't see any detail in the bore at all, but if your ring gaps measure that far out, its probably gonna need a fresh bore & piston kit. The head should be completely reconditioned and cam chain/possibly cam sprocket replaced. Looking at about $500-$600 to get 'er done right.

EDIT: Look the cam journals in the head over good, can't have any deep scoring or rolled out aluminum. Journal diameters must be in spec or you'll have to replace the head and rocker cover. Also check the rocker arms over good where each one has been contacting the cam lobes. Its common to find rockers shot because many folks don't perform valve adjustments... not ever.

 

[Casey_m]

Valve seals are shot, piston rings are worn razor sharp and are shot, piston looks better than I expected, but your pic only shows one side and we don't know whether its the thrust side of the piston or not. Can't see any detail in the bore at all, but if your ring gaps measure that far out, its probably gonna need a fresh bore & piston kit. The head should be completely reconditioned and cam chain/possibly cam sprocket replaced. Looking at about $500-$600 to get 'er done right.

EDIT: Look the cam journals in the head over good, can't have any deep scoring or rolled out aluminum. Journal diameters must be in spec or you'll have to replace the head and rocker cover. Also check the rocker arms over good where each one has been contacting the cam lobes. Its common to find rockers shot because many folks don't perform valve adjustments... not ever.

Thanks Retro. I took everything to the machine shop on my way to work.

In addition to the bore and hone, they said that cam is shot, as are the followers.

They also suggested a valve job and new valves (they said they always suggest new valves because "those Honda valves don't have enough material to grind" - is this true?).

They also suggested decking the cylinder, as there was evidence of compression leak on the left side.

Their total for machine work is $185.

As for parts, I can find the piston/rings/top-end gasket set pretty easily (I found Wiseco sets for ~$130) and I found valves for about $25/ea, and I found cam for $175 ( Web Cam Inc. - Performance and Racing Camshafts / Honda TRX 350 (ATV) (87-89) SOHC 4v).

I can't seem to find rockers - any suggestions where I can source those?

Thanks all!

 

[Jeepwm69]

Casey I have a used 350 Cam I just pulled out of an 86. That webcam might give you a little oomph. If you want to go cheaper let me know.

 

[Casey_m]

Casey I have a used 350 Cam I just pulled out of an 86. That webcam might give you a little oomph. If you want to go cheaper let me know.

Thanks for the offer Jeepwm69. I checked and the '86 cam is different part number than the '88, so not sure if it will work - thoughts?

webcam can also weld and surface the rockers for $65/ea. Man, this "cheap old ATV project" is turning into a not-so-cheap old ATV project!

 

[Jeepwm69]

No idea whether they're different or not. Maybe one of the 350 Foreman/ 350D gurus will know.

 

[retro]

The 86 cam is a drop-in. The early motors had ATC350X camshafts in them which was ground on a larger base circle. Later motors have a cam with a different part number because they were ground using a different base circle, but nothing else differs. You can interchange them at will...

The Wiseco 10.25:1 piston kit is a good choice. Give the new piston to your machinist BEFORE they bore the cylinder, so final honing clearance can be determined. If they say "We don't need the piston" run and get all of your stuff and find a shop that knows better.

You can still buy the OEM camshaft new, but its pricey... but the rockers are not available new, so you'll need to either find a used set of rockers in great shape or have yours hard-welded and refaced by Webcams. If you decide to buy used rockers then you may as well buy a used cam to match? If pockets are deep, the Webcams camshaft and hardened rockers refurb is fine too.

As for valves, I'd buy a set of four from Shindy if I were doing the job. Yours are probably worn out... don't waste your time on them. Check the fLeebay for a set of Shindy valves or search the 'net for the best price.

EDIT:
Its not uncommon to find these old motors with sacked valve springs. You may have to buy some... have them tested for sure. Specs are listed in the service manual for everything you/your machinist needs to know.

D.I.D sells a good cam chain at a fair price or you can pay more for OEM which is the same chain. Don't buy any other aftermarket chain, cause they are all china garbage.

 

[Jeepwm69]

I should have a valve cover with rockers in it too.

Let me see what they look like.

How can I make sure this cam is good? I'm guessing I can use a micrometer to measure the lobes (guess I'm off to read the FSM for specs)