6401 Big Bore won't fire Over

[mstang1988]

All, looking for help. I Big bored my 6401. It was running before dis-assembly. I added the piston/cylinder using the factory gasket and can't get it to fire up. Any thoughts on what could be wrong? I haven't checked the compression but that could be suspect. I would think it would still fire over even with lower then average compression. It did seem to fail the compression test as it slowly would drop holding it only from the pull starter. I tried to adjust the carb after it not starting for a while and have it 1 turn out right now.

 

[w8ye]

You need to go through the basics . . .

Do you have spark?

Is the spark plug wet?

 

[nstueve]

need more info like the actual compression and if the saw has spark. knowing those two things narrows it down quite a bit. You didn't pull the flywheel did you? if so did you shear the flywheel pin and miss align the flywheel?

what was the squish on your BBK? mine was quite big and so I deleted the cylinder gasket and used liquid sealer.

Honestly not sure how many turns out my carb is right now...

If you have spark and compression I usually start at 1 turn out on the "L" and then turn out 90* every couple of pulls to find the firing point. Once you can get it to fire you can tune in the "L" and the idle screw... then move to reving the saw and finding the sweet spot on the "H".

 

[mstang1988]

You need to go through the basics . . .

Do you have spark?

Is the spark plug wet?

Yes, I have spark and the plug is not wet (I even got the torch out to clean it up a bit).

 

[mstang1988]

need more info like the actual compression and if the saw has spark. knowing those two things narrows it down quite a bit. You didn't pull the flywheel did you? if so did you shear the flywheel pin and miss align the flywheel?

what was the squish on your BBK? mine was quite big and so I deleted the cylinder gasket and used liquid sealer.

Honestly not sure how many turns out my carb is right now...

If you have spark and compression I usually start at 1 turn out on the "L" and then turn out 90* every couple of pulls to find the firing point. Once you can get it to fire you can tune in the "L" and the idle screw... then move to reving the saw and finding the sweet spot on the "H".

I need to get some solder so I can measure the squish. I looked for some but seem to be out. I didn't pull the flywheel.

 

[Jimmy in NC]

If the plug isn't wet and you have spark, there is your problem. Did you go through the carb while you had it down? If so could you have possibly put a diaphragm in backwards or something? I've done it and have been through 100's of these carbs. Sure you have the impulse line hooked up? It's got to be simple.

 

[w8ye]

One guy hooked is impulse and fuel lines up backwards to the carb when he put it back together.

 

[mstang1988]

One guy hooked is impulse and fuel lines up backwards to the carb when he put it back together.

Definitely didn't do that. It's getting fuel to the motor too as I pulled the plug and pulled the cord and it sends foggy gas smell out.

 

[procarbine2k1]

Definitely didn't do that. It's getting fuel to the motor too as I pulled the plug and pulled the cord and it sends foggy gas smell out.

Which gasket in the kit did you use? Should have been one for the 7900 and the 681 (thick gasket). Sorry if you already answered that.

 

[mstang1988]

Which gasket in the kit did you use? Should have been one for the 7900 and the 681 (thick gasket). Sorry if you already answered that.

The factory 6401 gasket, the others seemed way too thick.

 

[blsnelling]

Definitely didn't do that. It's getting fuel to the motor too as I pulled the plug and pulled the cord and it sends foggy gas smell out.

It's flooded. You need to check squish and compression.

 

[rsbhunter]

won't fire

I just got done putting on a 79cc oem kit on a used 6401 Sat. used the metal gskt that came with kit...when i asked about carb settings on this forum, i was told 1-1/2 to 2 turns from lightly closed on the "H" . I got lucky, as mine fired right up, idled ok with the "L" setting the way it was, have since adjusted the idle and low speed to get the smoothest idle point...maybe try the 1-1/2 to 2 setting? I'm a noobie, so just trying to give back what little i can, because of all the help the guy's here gave me....rsbhunter

 

[mstang1988]

If the plug isn't wet and you have spark, there is your problem. Did you go through the carb while you had it down? If so could you have possibly put a diaphragm in backwards or something? I've done it and have been through 100's of these carbs. Sure you have the impulse line hooked up? It's got to be simple.

Plug is wet but I'm beginning to think it's not getting a good or maybe not a consistent spark. When I went to tested it originally I shocked myself and assumed I had a spark in that case. Last night I ripped it apart and used Hondabond for the gasket and tightened the compression release valve down. While at it I decided to check the spark again. I couldn't see a spark. I changed the plug with the one from my Dolmar 9000 and again, still couldn't see the spark. I assume the ignition system works by grounding the external of the plug through the cylinder and the plug wire is the other end of course. I tested it by putting the outside of the plug against bare metal in many forms/fashion but never saw a spark when pulling it over. Any thoughts how it could go from functioning to non functioning?

I didn't rebuild the carb, just wanted a straight slug and jug swap.

Additional info:
I decided to see if I could speed the process up and clean the plug off with a micro torch and burn the excess fuel out of the cylinder as well. No problem igniting the mixture in there each and ever time. This further makes me suspect the ignition/spark.

 

[mstang1988]

Plug is wet but I'm beginning to think it's not getting a good or maybe not a consistent spark. When I went to tested it originally I shocked myself and assumed I had a spark in that case. Last night I ripped it apart and used Hondabond for the gasket and tightened the compression release valve down. While at it I decided to check the spark again. I couldn't see a spark. I changed the plug with the one from my Dolmar 9000 and again, still couldn't see the spark. I assume the ignition system works by grounding the external of the plug through the cylinder and the plug wire is the other end of course. I tested it by putting the outside of the plug against bare metal in many forms/fashion but never saw a spark when pulling it over. Any thoughts how it could go from functioning to non functioning?

I didn't rebuild the carb, just wanted a straight slug and jug swap.

Additional info:
I decided to see if I could speed the process up and clean the plug off with a micro torch and burn the excess fuel out of the cylinder as well. No problem igniting the mixture in there each and ever time. This further makes me suspect the ignition/spark.

Alright, I think the plan is to pick this up and test the spark. If that says I have spark then I'll be taking the saw to the shop to see if they can run a compression test for me (don't have the tools).

90º Inline Ignition Spark Checker

 

[Jimmy in NC]

Have you tried another spark plug just for giggles? I've had plugs spark out of the cylinder that won't fire under compression more than once.

 

[mstang1988]

Have you tried another spark plug just for giggles? I've had plugs spark out of the cylinder that won't fire under compression more than once.

Yup, sure did. Pulled the plug from my 9000 and put it in. Still didn't fire. I guess it could be mixed wrong.

 

[mstang1988]

Yup, sure did. Pulled the plug from my 9000 and put it in. Still didn't fire. I guess it could be mixed wrong.

Ok, got the plug tester and I'm getting spark. Just to make sure, I checked the brightness vs my Dolmar 9000 and it's about the same. I also got a crappy harbor freight compression tester... it showed 60PSI. I got excited as I thought I might have narrowed it down but I think it's a crappy tester as I tested my 9000 to double check. 60 PSI also which is BS as it runs like a champ.

So back to square one, I turned the carb needles all the way in and was going to start from there and work my way out. When I release the compression release valve I notice it's blowing loads of fuel out of the valve, so much so that you can see a fog even with the needles all the way in! WTH, how could that much fuel be there. Am I wrong in thinking that the fuel supply should be almost complete chocked?

 

[rsbhunter]

no fire

I take it by your name that you may have a good amount of experience working on cars? This thread brings me back to the days of points, condensers and generators...i was always taught, there are three things to get an engine to fire....fuel, spark, and timing....is there ANY way the flywheel key was broken, or timing could have changed? I know squat about 2 cycles, but R & R ing the C&P is a pretty straightforward swap, heck, even i did it..(luck i promise) But, if you were running before, and all you did was the c&p, it should run , even with all the settings unchanged.Is there any chance that the p&c aren't sealing? Air is getting into the carb/manifold junction, Is there any possibility that the spark is grounding to the cyl BEFORE reaching the spark plug ( it takes alot less voltage/amperage to fire a test light)? Was the piston ring gap at the locator pin in the groove(although i think you wouldn't be able to install it in the cyl. if it wasn't).Arrow on the piston pointing toward exhaust? Most of these questions are needless, but once in a million, asking, and checking, can come up with the answer..one last question, the choke is operating correctly? Not staying closed?....You have me stumped , but i can't wait to find out the soloution...rsbhunter

 

[gw165]

Any chance that the metering lever is incorrectly adjusted (too high)? I have had that happen, it flooded the saw pretty heavy. Good luck to you!

 

[mstang1988]

I take it by your name that you may have a good amount of experience working on cars? This thread brings me back to the days of points, condensers and generators...i was always taught, there are three things to get an engine to fire....fuel, spark, and timing....is there ANY way the flywheel key was broken, or timing could have changed? I know squat about 2 cycles, but R & R ing the C&P is a pretty straightforward swap, heck, even i did it..(luck i promise) But, if you were running before, and all you did was the c&p, it should run , even with all the settings unchanged.Is there any chance that the p&c aren't sealing? Air is getting into the carb/manifold junction, Is there any possibility that the spark is grounding to the cyl BEFORE reaching the spark plug ( it takes alot less voltage/amperage to fire a test light)? Was the piston ring gap at the locator pin in the groove(although i think you wouldn't be able to install it in the cyl. if it wasn't).Arrow on the piston pointing toward exhaust? Most of these questions are needless, but once in a million, asking, and checking, can come up with the answer..one last question, the choke is operating correctly? Not staying closed?....You have me stumped , but i can't wait to find out the solution...rsbhunter

Yup, some experience with cars and other two cycles.

To answer your questions:
-Yup, I'm positive the timing has not changed
-The piston arrow is facing to the exhaust
-the "up" side on the ring is faced up and placed proper to the locator pin
-choke operating correctly
-I guess it's possible air could be getting to the carb manifold junction but looking around I doubt it is
-Actually because you aren't pull the cord very fast and the lack of continuous voltage means it takes a lot to heat the element

For the metering level, I'm not sure. How could I check this?

A bit more info. When I was making the swap I had scratched the piston trying to put the circlip in so I waited several months for Baileys to send the replacement I ordered. I guess the carb could be gummed up or something dried up and broke.

I'll take a video of it later today just to make sure I'm not a complete idiot and missed something obvious.