Walbro Pulse Port Orientation WT-98

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NoPaint

NoPaint

Feeling the Heat
Hearth Supporter
Jan 2, 2009
269
USA
I posted a while back about rebuilding the WT-9B (EDIT FROM WT-98) on my Dolmar 100 and it running all wacky afterward. Didn't run at all before.

Anyway, no amount of tuning screw turning showed me any sort of progress that seemed to be in the right direction.

So tonight I went out to the shop, pulled the carb to rebuild again, and then thought "maybe the carb spacer with the pulse port was put in wrong!"

I had the pulse port from the engine side coming directly into the carb - straight shot. What is the correct orientation? Also, what are the H and L initial settings to get to when I go to start after reinstalling the carb?
 

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The pulse port coming from the engine should travel thru the groove you see and then to the pulse port on the carb.
So according to your picture it looks ok.
I would also double check the gaskets and diaphragms on the carb. The side of the carb where the lever and needle are the gasket goes on first then the diaphragm. Then the other side is the opposite. Also make sure that you have the correct diaphragm on the carb where the screen is. That diaphragm has those small reed type valves. If you have the old diaphragm compare it again to make sure it is correct.
 
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I've never worked on a Dolmar 100. Is the orientation not dictated by the design of the crankcase? Unless the spacer is symmetrical top-to-bottom I would've guessed there would only be one way for it to be assembled. In any case, if the gaskets on either side of the spacer seal properly and there's a path for the impulse to get from engine to carb then the orientation shouldn't matter. The carb doesn't care whether the pressure changes are traveling through a few turns or making a relatively straight shot.

Is it still running the same way it was back in January, i.e stalling when you let off the throttle or tip the saw nose-down at idle? If so, I'd still be looking for reasons that the idle mixture might be too rich. I just noticed, while reviewing the old thread, that you changed the welch plugs. It's not typically necessary to mess with the welch plugs so I don't have a lot of experience with them, but I've read that a leaking welch plug can cause the saw to run rich.
 
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You guys are right there's only one way the port can go. I think I need to take that carburetor apart again and make sure that the needle is moving correctly. While I am in there I will put in a new kit.
 
Also I should get some stronger glasses while I am at it because the carb is a WT-9B and not a WT-98!
 
I agree Jon1270 but I believe the other side of that intake has only one hole that lines up with the cylinder and in order for the carb to mount correctly the impulse will travel thru that groove. This is pretty common with a lot of saws and not just Dolmar.
 
Mag Craft said:
I agree Jon1270 but I believe the other side of that intake has only one hole that lines up with the cylinder and in order for the carb to mount correctly the impulse will travel thru that groove. This is pretty common with a lot of saws and not just Dolmar.
Click to expand...

Yep, I know. OP already indicated there's only one way it can go together, so that question is settled.
 
I did not know the OP had taken out the welch plugs on that carb. That is a big red flag unless you are experienced with doing that. You might even consider replacing the carb with a known good one if you do not figure it out.
 
Mag Craft said:
I did not know the OP had taken out the welch plugs on that carb. That is a big red flag unless you are experienced with doing that. You might even consider replacing the carb with a known good one if you do not figure it out.
Click to expand...

I'm an idiot. I was watching the Walbro video on youtube. They have a video on how to rebuild their small WT series. They indicate taking out the little welch freeze plug then tapping in the new on from the kit...looks like that step could also be my problem. Now they say in another video that they used to seal them with fingernail polish but with ethanol you cannot do that anymore. Is there something I can use to seal in the welch plug? Drop of red locktite?
 
Well I just took it apart. Started with the welch plug side and they do look fine in there. Couldn't see any signs of issue like a leaking trail. I checked to see if the needle arm is just barely level with the surrounding body and it was not - it was much higher. So adjusted arm down.

Flipped carb over and took off the fuel pump side and found that there was a compost pile covering the fuel screen. I replaced that screen not 20 minutes of running ago! So right after purchasing and before I did anything I took it to a mechanic who put on a new fuel line and said it was good. It didn't run when I got it home but had the new fuel line. Thats when I took the carb apart and cleaned it earlier in the year...I think its time to do the following:

1. Flush out tank
2. Leave welch plugs alone for now or pull again and clean behind...since there is a compost pile in the carb...
3. Put a new carb kit on it
4. New fuel line. I am guessing a new fuel line can be installed by attaching it to the old line and pulling through the fuel filler? The line the mechanic installed seems fine but gets harder when it enters the saw body toward the tank...maybe just leave the fuel line and clean the tank?
 

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Have you figured out how the dirt got into the carb? Maybe you forgot to check the inlet tube last time you had it apart? It can't have gotten through an intact fuel line or filter.

After making sure the line and filter are in good shape, I'd put it back together and see how it runs. The high metering lever might have been the problem, after all.
 
I bought the saw for $60 not running. Took it to the mechanic who said he put a new fuel line on it. I get it back - no run. So I take the carb off, notice the new fuel line, never looked at the tank or tank filter...

Carb was dirty originally so I rebuilt.

Carb is super dirty now! And only maybe 20 minutes of running at most.

So I just drained the tank, the filter in the tank looks just like the compost in the carb.

Theres a little sludge in the tank but nothing major. I think the mechanic reused the original in tank filter (probably 25 years old lol). He used to be awesome and recently not so much. Can't blame him. I am going to flush out the tank, just bought a new line and filter, and will probably just put a new fuel screen in the carb, and spray out the ports. I'll reuse the gaskets as they look good and I won't take off the welch plugs - no idea though if that gunk is behind them or not.
 
Sounds like a plan.
 
[Hearth.com] Walbro Pulse Port Orientation WT-98
 
I wish I was eating the popcorn instead of fiddling with this thing!

Got it running for about 10 seconds. Only stays on at full throttle and running ultra rich. Spraying fuel out of the intake. Take the carb back off...
 
I'm convinced some carbs have mojo installed at the factory and it leaks out - they never tune in after that. If you can't get this carb tuned in, there are cheap alternatives. Like MS 180 carb is $8-10 on fleabay and you can drill a new throttle link hole in the link plate. Or can take throttle rod out of bad carb and fit it in new carb. Same deal if choke rod is different. Make sure a new carb impulse hole is correct place.
 
I'm gonna take the carb top off and see if switching from that clear white fiber color diaphragm to the black one makes any difference. Otherwise I'm stumped on this carb. I've rebuilt 100 and never had this much issue! Any ideas on other carbs that could be modified to fit would be awesome because a replacement one that matches is $45 plus shipping. At that point the saw would become firewood.
 
Ultrasonic cleaning may help. I don't have one and that's why I may give up early. Compare MS180 carb (pics on ebay) to WT-9. I've used MS180 carb on Poulan 1800 and it worked after adapting. Keys are throttle and choke links, impulse hole. MS 180 carb will deliver enough fuel for a saw under 40cc. I have adapted an MS250 carb for some saw (can't remember which). But more work - the impulse hole is at 1:30 o'c. But there is an impulse hole at 12 o'c plugged with a brass plug. Drill 12 o'c plug out and JB Weld in 1:30 o'c hole.
So, look over MS180 and MS250 carbs.
 
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Yes the ultra sonic cleaner is an excellent idea. I have had some carbs that would not work after spraying every orifice with carb cleaner. Put them thru my ultra sonic cleaner and bingo they worked great after wards. The idea of substituting a different carb is another good idea.
 
Ok swapped gasket - nothing. Now no run at all. Really wet at intake. Spark is bright and blue.

So its time to get a replacment carb. The MS180 seems really similar. Just bought one and will report back. With the crud that was in the fuel inlet I am guessing behind those welsh plugs are probably full of gunk too. Time to move on from that stock carb.
 
Under one of those welch plugs I believe there is the main nozzle check valve. If that has been messed up then the carb will never work correctly.
 
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