I messed with the acc. pump and the jetting of my 426

For those of you who have a 426. How many turns out on your acceleration pump timing adjustment screw?

Here is the story:

WR426 below 2000' and riding at temps between 40 and 85 most of the time.

FMF PC4

Yz timed

air lid removed

I run 92 octane fuel

Since I'm jetting with stock FCR jets, JD suggested 48 pj, DQR needle clip pos #4 or 5, 170 mj and all the rest is stock.

The bike pulls very hard everywhere but I have a bog as soon as I "gun" the throttle

As I read and ask on TT, many suggested that the problem was coming from my acc pump.

I checked it out. The acceleration pump timing adjustment screw was 5 1/2 turn out (the more you turn out, the less delay you have). I noticed that at this position, the slide was not clearing the hole from where the squirt occurs at the precise moment the pump actually squirts. The squirt probably partly land on the slide creating a lean bug.

I decided to put the screw at 2 turns out so that the squirt occurs when the slide is half way up.

The bike fired like a charm but I only created a bigger problem. I suspect that now the delay is too big (the squirt occurs only at half throttle which might not be soon enough) creating a lean bug again. I plan on adjusting the screw so that the pump starts to work right after the slide has cleared the little hole from where the squirt comes.

Does this all make sense to you?

I'd like to hear your setting suggestions. Hence my first question:

For those of you who have a 426. How many turns out on your acceleration pump timing adjustment screw?

Thank you very much for your help. :)

You have to bend the squirt needle so it points just below 1/4 throttle slide opening. You dont want it hitting the back of the slide unless it is at 1/8th throttle or less. :)

Bend the squirting needle? What do you call the squirting needle? I don't get it.

The squirt comes from the little "golden color" rod that shows up right in the middle of the carb right?

I tried to look through the carb venturi (engine head side) to see at which throttle position I could start to see the hole from where the squirt comes. I could see the little hole from under the slide appear at about 1/8 throttle position.

When set as stock, (5 1/2 turns out on the screw) the pump has already started to squirt at this throttle position (I can see the acceleration pump rod push on the diaphragm as soon as I touch the throttle - no delay). This means that from 0-1/8 throttle, the squirt hits the back of the slide.

When I set the screw at 2 turns out instead of 5 1/2, the pump starts to work only at half throttle. What I'm saying is, this might be too late. (No squirt until the throttle is half way twisted causing a lean condition).

What I'll do is: set the adjustement screw so that the squirt starts at the precise moment the slide gets away from being in front of the squirting hole (1/8) throttle.

What do you think?

When set as stock, (5 1/2 turns out on the screw) the pump has already started to squirt at this throttle position (I can see the acceleration pump rod push on the diaphragm as soon as I touch the throttle - no delay). This means that from 0-1/8 throttle, the squirt hits the back of the slide.

When I set the screw at 2 turns out instead of 5 1/2, the pump starts to work only at half throttle. What I'm saying is, this might be too late. (No squirt until the throttle is half way twisted causing a lean condition).

?

5.5 turns out....squirts too soon or 2 turns out is too late....hmmmmmmmmm .

How about somewhere between 5.5 turns out and 2 turns out. :)

5.5 turns out....squirts too soon or 2 turns out is too late....hmmmmmmmmm .

How about somewhere between 5.5 turns out and 2 turns out. :)

It makes sense does it? That's what I thought. I'll try that during the week-end.

It makes sense does it? That's what I thought. I'll try that during the week-end.

Can't hurt anything.

just checked mine and it's about 3.5 turns. WOuldnt say my bike is perfect in the pump department, I got to the 'Ahhh.. F%$# it , that'll do' stage. The only time mine plays up is if I am going to do a U turn with bars turned, bike leant over slightly virtually stopped, and I pull in the clutch and give the throttle a little snap ( to slide the rear around) and it normally stalls.

just checked mine and it's about 3.5 turns. WOuldnt say my bike is perfect in the pump department, I got to the 'Ahhh.. F%$# it , that'll do' stage. The only time mine plays up is if I am going to do a U turn with bars turned, bike leant over slightly virtually stopped, and I pull in the clutch and give the throttle a little snap ( to slide the rear around) and it normally stalls.

Thanks for the info Hamish. I'm pretty sure now that the best we can do is somewhere between 3-4 turns out.

I'll keep you informed :)

How do you time it like a YZ?

do you have to buy new cams?

How do you time it like a YZ?

do you have to buy new cams?

only need a new exhaust cam if it's 03 onwards.

400/426 just bump the ex 1 tooth. Do a search in YZ Timing.

only need a new exhaust cam if it's 03 onwards.

400/426 just bump the ex 1 tooth. Do a search in YZ Timing.

Yep, just like Hamish said.

Everything is in here, use the search engine. You'll even find pictures you need.

OK, now I've been setting the acc pump screw at 4 turns out.

Got rid of the backfiring that was occuring when hitting the gaz :) . I still have a bug everywhere each time I snap the throttle a little too fast from idle or from mid range. I was at the 4th clip pos on the DQR needle. I'm now trying the 5th to get richer (lower the clip pos=rising the needle=richer)

What is contradictory is that 1st, the bike gets hot quickly (which might indicate its too lean). On the other hand, it looks to me that the bug arise when the engine reaches its running temp... in other words, it runs better when cold which might indicate it's too rich.

Maybe the effect of running better when cold just comes from the fact that it's just been choked leading to more gaz to be available for a couple of seconds and thus making a lean jetting appear ok for those couple of seconds....

Later,

11h30pm

Ok just tried the 5th clip and nothing does. If something, it's even worse. As soon as the engine gets hot (1 minute), the bog kills it.

Will try clip #3 tomorrow to see if it improves.

I think that now that the acceleration pump throws at the right time, the bike might not need to be that rich...

I'm asking myself a question though. How much fuel runs through the pilot at full throttle? Is it the same all the time or does it vary as throttle opens.

Thanks

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