WR400 carb problems and Q's

G'day all,

I've just recently re-entered the motorcycle world after a six year lay off from bikes and a fifteen year layoff from dirt bikes, with the purchase of a 1998 WR400 (cheap). Since I purchased it, I've stripped it down and replaced parts such as the swing arm/shock linkage bearings etc, checked valve clearances (outside of tolerance on the low side and in need of re-shimming, but still clearing ok) and cleaned/straightened a bunch of parts. The engine top end has been rebuilt about 2000kms (1250miles) ago (according to the previous owner) about half dirt and half in motard trim. He purchased it from a dealership where his friend was working as a salesman.

I rode it probably 50km before stripping it down as mentioned above. I got it all reassembled yesterday (motard setup) and rode it to work today. About a 35km trip each way, mostly highway. From my initial local test rides, I had determined that the carb needed some tuning as it displayed all the stock carb traits (based on my very limited knowledge of carbs and research from TT forums) of hanging idle when warm, off idle bog etc, and despite the fact it it would only lift the front wheel in first (throttle only) and not second and third like so many others here have expressed, it still accelerated like a bullet (compared to the 125/200/250's I grew up on in the 90's) and I've been drifting sideways out of nearly every corner, so I was expecting to have some carb tuning work to do in the near future. Little did I know how near that future would be!

I stopped on the way home for fuel (first time I gassed the tank since purchasing) and all was good. Got to within a short distance from home (still on the highway) and the bike started to bog and surge repeatedly

and when I pulled the clutch in it idled at about 5000rpm in a very sporadic and rough manner, although once past about 1/4 throttle it accelerated in a fairly normal manner. I managed to nurse it home, let it cool down while I had a beer and cooled down too. Then I tried to start it again. Was really strange to kick over. I normally can't kick it past the compression stroke without the de-comp lever, exhaust valves must seal well I guess. Anyway, when kicking it over post high idle issues, I would bring the piston to compression and past with de-comp lever, give it a usual full kick, but it wouldn't kick smoothly all the way through the stroke. At the bottom of the stroke it would kick back a bit as if I was hitting the compression stroke again. Maybe that is normal/happens at times (this is my first bike with a de-comp), I'm not sure, but after a few kicks it started and immediately went back to the 5000rpm idle. I wound off the idle screw all the way and it dropped to 4000rpm. So I shut her down, started searching TT again and reading the service manual. I figured it had to be a pilot jet issue as once the throttle was opened past about 1/4, the bike accelerated like there was no issue.

I pulled the carb off (it was filthy and obviously hadn't been removed/touched for the full 2000km, maybe more) and now's where I get nervous! I inspect the carb once out and immediately notice that the engine side slide has the bottom right hand corner (looking back into the carb) broken off and missing! I worriedly inspect the intake valves through the inlet port with a mirror and light, with no evident damage. (Whhoooo) May have been the reason for the last rebuild?

I should also mention that the left hand corner is cracked in the same manner as the right, but is still holding on, so at this stage I'm thinking, a good thorough clean and a new OM slide and I'm back in business. Not the end of the world right!

So I continue to dismantle the carb and soon notice that the accelerator pump shaft that actuates the diaphragm, is frozen. Disassembly shows it to be horribly frozen with rust!

At this point I'm beginning to think a replacement (more modern than gen1) carb might be the best option until a search on eBay Australia, shows them to be going for big$, but about half what it costs for a new replacement!

So, all that said, my main Q's are.....

What do all of you carb gurus think caused the issue with my bike?

What WR426/450 carbs can I rig to run on my 400 without major surgery and added expense?

Please share any thoughts on repairability of the carb pictured? I don't intend to have this bike forever and will be upgrading when budget constraints allow (2/3years).

Does anyone out there in TT world have a good/working, carb they can sell me for a reasonable price?

Thank you all heaps for reading my (long) post and thanks in advance for the wisdom you are about to share!





In your case, if it revved up to 5000rpm and will not idle it sounds like-

Fuel metering screw O-ring perished.

See here for more info-

http://www.thumperta...hl__ high idle

IMHO- I would not bother trying to fix that carb and would be looking to replace it with a Gen 2, it is a POS!

If you want to fix it you 're looking at roughly $114.00 AU to replace the slide plate, $8.00 for a seal, $30.00 for a new float needle valve set (you can't buy the O-ring separate) unless yours is okay (not likely), $16.00 for a new diaphragm, $18.00 for the fuel metering screw set (you cant buy the O-ring separate, that I know of) and any other parts that you happen to need. This alone without seals or O-rings would cost roughly $186.00. This is from the cheapest source online (that I know of) without shipping/postage. With postage it would be more like $210.00. Is it worth it to still have the POS gen 1 attached? IMO - HELL NO!

The other option if your float needle valve is okay is find the correct size O-rings needed from a hardware store and buy only the parts needed (still not much of a saving).

What carb fit's what is already answered in the post above. All you have to do is block the manifold to carb pipe off, EASY!


Edited by Barra8

I would not put one dime of time or effort into that carb. it is abused.

Any FCR CR flat 39 or 41 past 2002 will work on that bike.

Thanks for reading my post gents. I copy/pasted it into the intake/jetting forum as soon as I'd posted it in WR and for whatever unknown reason, only the replies that I received in that forum showed up on my iPad, so I've only just got your advice today!

The bad news is that I didn't follow my gut feeling and what you have all advised :-( but the advice that I got from mates that seemed to know more about carbys than I did, combined with the uninspired reply's that I got in the jetting section (although still appreciated), I spent over $200 on an OM vacuum release plate and some orings/seals and stripped and cleaned the carb inside and out until spotless. Ran the jets through an ultra sonic bath. Reassembled with all original jets (pilot jet-45, main jet-170RD, starter jet-85, pilot air jet-75, main air jet-200, needle-LBDTM-clip pos. 4, float needle valve seat-3.8).

Mixture screw set to 1.5 turns out (forgot to mention that I only pulled the carb the first time after measuring +3 turns in on the mixture screw when trying to establish a baseline, knew something wasn't right, pulled the carb and discovered the vacuum release plate damaged/partly missing)

Installed new vac release plate with original seal (looked in good condition). Seal facing same way as removed from damaged plate.

Installed carb and connected up. Kicked it over with usual cold routine (choke on). Started 2nd/3rd kick and settled into perfect 2g idle. I was stoked until I realised that the choke was on and the revs weren't climbing. Closed the choke and the engine died. Started again, wound idle screw in to max position revs increased as would be expected with choke on but engine will not idle without choke. Revs can be held at normal idle position with delicate throttle control but after 10/20 seconds engine will snuff out with a mild backfire. Engine runs quite smoothly with choke on but decel pops badly when when blipping throttle with choke off.

Have tried adjusting mixture screw between 0 and 3 turns out with no discernible difference. Have removed carb and cleaned and recleaned jets/passages, checked and rechecked float height etc, reversed the seal on the vac release plate.

Should also mention that I was able to free the accelerator pump actuating rod and clean it up. Is now squirting strongly. Have tried disconnecting it to rule out negative idle involvement.

Have tried disconnecting TPS with no noticeable improvement to idle/pilot circuit.

Forgot to mention, checked valve clearances, exhaust clearances within spec. Intake clearances were half to one thou under. So shouldn't be contributing to the problem.

So where to from here?

Mixture screw oring is very slightly deformed from having been previously installed between washer and spring. I wouldn't have thought this was the end of the world considering how well the bike was running prior to the high idle incident but reading around seems to suggest otherwise so it is next on the list for replacement along with the vac release place seal as I have read that a new one can make a difference (not confident in any improvement here as I inspect orings and seals every day in my job as a Heli engineer, but I have been wrong before).

I have a mate with a keihin jet kit that we have organised to play with tomorrow afternoon.

I'm not feeling terribly confident thought and I can't understand why! It's a mechanical thing and has to be able to be troubleshot! Right!

If things don't go really well tomorrow, I'll be seriously considering chalking the wasted money up as a lesson learned and coughing up the $450/500 bucks for a second hand fcr-mx.

Would love to hear your thoughts and will post results of tomorrow's troubleshooting.



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