Jonsereds 49sp does not start

[Bart01]

Hello,
I have a jonsereds 49sp chainsaw for restoration.
It has good spark, good compression and a rebuilt carb. I also replaced the fuel lines. But for some reason the carb does not draw in fuel...
I am pretty sure the carb works fine. it's completely rebuild and holds its pressure good. But for some reason, if i pull the starter rope, the fuel does not travel up the fuel line. You can see it in the picture.
I removed the fuel filter in the tank, to make sure that that is not the problem. I tought that perhaps the tank vent line was the problem, so i removed that as well. But stil the fuel won't go up...
I also checked the hole between the intake manifold and the carb and it seems free as well...
Any ideas what could be the problem ? I have restored chainsaws before and always got them running, but this one is a real challenge... Not sure where to look next...
thanks !!

 

[shug360]

Do you know if it’s building vacuum/pressure in the crank case? And if the gasket for your carb is not flipped and covering the pulse circuit by chance. I have gotten in a hurry and done it before.

 

[Bart01]

Hi,
Gasket in carb is fine. checked that a few times...
How can you test if crank case is building up pressure ? I used a compression tester in the spark plug hole, but i guess that only checks the "upper" part of the piston pressure ?

 

[shug360]

Someone may know of an easier way to check but you have to block the intake and the exhaust to leak check the saw. And a way to pressurize it. I couldn’t imagine the crank seals being that bad without the crank bearings being junk but never know.

 

[2StrokeFondler]

Plug your pressure/vacuum tester into the impulse line/hole/whatever and pull. You should see the needle moving. On either setting, vacuum or pressure. If it doesn't, there's your problem.
You don't need to plug anything, just to check the impulse.

 

[Bart01]

So if there is a crank case leak, it is possible that the impuls hole (jonsereds 49sp works with a hole from the intake manifold to the carb) does not create pressure to pump the carb ? Even if the compression test of the engine is good ?
Hope this makes sense...

 

[Bart01]

this is how it looks like. I'll do a pressure test soon...

 

[shug360]

Yes crank case pressure/vacuum operates the diaphragm in the carb. The piston plays a role in creating that. But not Cylinder compression so much.

 

[sundance]

I don't know details and haven't had the carb off mine. I do recall comments about the carb gaskets in rebuild kits blocking holes. Make sure yours matches the original. If needed i can try to dig in saved notes.

 

[jonsereds 621]

I have a few 49sp's, with a little care and attention you will end up with a useful 50cc saw. From my experience, before you go any further change seals on crankshaft as they are likely 50yrs old. while you are at it check for play up and down on crank especially on sprocket side. Check to that bolts holding cylinder to case are present and tight. The carb to intake gasket supplied with the carb kit almost certainly won't suit the 49 sp, if you check closely you will likely see that while it looked right, it does not produce the intended air tight route for pulse to carb....make another with larger piece of gasket paper. Look at carb exploded diagram of carb and understand order of asembly of diaphragms , one side goes under gasket, the other side on top of gasket. I would also replace fuel pickup hose and filter even if it looks ok. use branded Tygon fuel hose or quality similar hose. (not fishtank or home-brew pipe). with carb off, simple check for pulse is a small splurge of grease over pulse hole from crank case. pull cord and grease will be blown from hole. if this is not the case, pulse may be blocked by gasket compound at crank case / cylinder interface ( by previous oaf , of course ! ). This all assumes you have some compression to start with. Lots of info on this site for this grand saw. Let us know how you get on (and how many of these you have in six months time ). I'm sure others will come to your aid as well if you proceed with it . If not, move your questions to Jonsereds saw section.

 

[Cantdog]

Welcome to the site.
All good advice but it boils down to basically two problem areas if you're impulse is not pumping fuel. The first and most common has been alluded to but not explained completely enough. In your post of the IPL see gasket 504 13 04 01. If you used the supplied diamond shaped gasket supplied with the carb kit (ANY carb kit) this will not work as, though the impulse holes line up to the carb body the outer area of this gasket will not cover the impulse corridor in the isolation block (504 80 28 03). Look at the pic of the isolation block/manifold.....at 9:00 the impulse hole is evident, this corridor then extends up and around intake bore to about 10:30 to where it lines up with the impulse hole in the gasket and carb. Put your diamond shaped gasket against it and line up the impulse hole and bolt holes and you will see this gasket does not cover the entire wiggly impulse corridor. You will need to make a new gasket with enough surface area to cover this or if you have a Husky dealer close by get replacement gasket that fits a 61,266, 272 etc. This is part # 501 25 09 01.

The other place where a common problem occurs is a blown cyl base gasket from the bolts at some time getting loose. Don't assume on a used saw that just because the bolts are tight now that they haven't loosened up on a previous owner and the gasket has blown out. These are very thin paper gaskets (around 0.007" when compressed) and fail from time to time opening up the crankcase to outside air and not allowing base pressure or vac to build enough to pump fuel.

Both of these can be located easily with a conventional pressure/vac test. 99% of the time the problem can be traced to the diamond shaped carb to manifold gasket

I would not argue against replacing the crank seals due their age but this is very unlikely to cause any problems with impulse. This would manifest more with running issues like lean conditions, not being able to achieve or hold a tune etc.

 

[Bart01]

Thanks guys for all the advice !
The gasket between the carb and intake manifold is the correct one, it's not the diamond shaped one that came with the carb kit. See first picture.
I followed Jonsereds 621 his advice and put some grease on the impuls hole (second picture). Pull the cord a few times and the grease did not move... (see third picture). So i still have to do a pressure test, but i think i can assume that the problem now lies with no pressure buildup in the lower part of the piston.
Next : looking for the cylinder base gasket.... To be continued...

 

[Old2stroke]

You may be chasing the wrong thing. The impulse and operation of the fuel pump has very little to do with starting an engine. The fuel pump has to operate properly once the engine is started and you may still have a problem with that (failed grease test?), but for starting it's all about the choke, not the fuel pump. The choke must be COMPLETELY closed and the engine must be healthy enough to develop a reasonable intake vacuum. When you pull the engine over, the vacuum has to pull down the metering diaphragm which will open the inlet valve and the vacuum will pull fuel up the fuel line, through the check valves in the pump diaphragm and fill the fuel chamber. With the throttle partly open, the engine should start and run for a minute or so with this fuel even if the pump is not working. Things that prevent this are stiff diaphragms, inlet valve stuck shut, incorrect assembly of the pump diaphragm, leaking carb gaskets, nozzle check valve stuck open, etc.
With a separate impulse line, checking it with a gauge is better than the grease test as you should get both negative and positive pulses. Also, if you pressure test the carb impulse fitting, it should not leak.

 

[pioneerguy600]

One of the best reasons a vac and pressure test is so important in diagnosing these 2 stroke problems. I had a Huskvarna 359 come my way once, it had multiple problems but one of the worst was the flywheel side crank seal had not one shred of the viton/rubber seal left on it, was wide open to the bearing, only the metal housing left there. The saw owner had been priming the saw dumping fuel in through the carb for months, could get it started that way and run it WOT but would stall immediately when throttle was let off. I was totally surprised the P&C were not scored. The saw also had the leaking plastic sleeve between he carb and cylinder and a fuel tank vent blockage, they ran it that way for months.

 

[dave ensign]

I would have bet carb gasket, my second choice is crank seals. Could be that the muffler screen is plugged solid.
I just built another 49 this week from my parts box. Had everything except a top handle and a choke rod.

Dave

 

[dave ensign]

You are missing a the tank vent.

 

[jonsereds 621]

0ne other problem that may be why your fuel is not getting to carb. Under one of the carb diaphragms there is a very small sunken gauze filter, The first one I ever took apart, (1980 ish ) I completely missed the fact that this was a filter as it was completely plugged with fragments of felt from the original felt type tank filter, so plugged that it appeared to be a brown felt pad that was there intentionally so knowing nothing I left it alone. It was some years before someone with more experience showed me this and another Jonsereds 621 burst back to life. (same carb). I apologise if if your level of experience well exceeds mine at the time, but if so this may be the revelation to someone else that it was to me. At that time getting a carb kit was as I remember it a question of ordering individual parts from a store man who would MUCH rather you brought the saw in for a service at vast expense. Now it is possible to get advice and get the bits without leaving your seat. It is also possible to take a close up photo and transmit it to whoever might be interested for their perusal. Be assured that your difficulty WILL be resolved using these pages !

 

[Cantdog]

0ne other problem that may be why your fuel is not getting to carb. Under one of the carb diaphragms there is a very small sunken gauze filter, The first one I ever took apart, (1980 ish ) I completely missed the fact that this was a filter as it was completely plugged with fragments of felt from the original felt type tank filter, so plugged that it appeared to be a brown felt pad that was there intentionally so knowing nothing I left it alone. It was some years before someone with more experience showed me this and another Jonsereds 621 burst back to life. (same carb). I apologise if if your level of experience well exceeds mine at the time, but if so this may be the revelation to someone else that it was to me. At that time getting a carb kit was as I remember it a question of ordering individual parts from a store man who would MUCH rather you brought the saw in for a service at vast expense. Now it is possible to get advice and get the bits without leaving your seat. It is also possible to take a close up photo and transmit it to whoever might be interested for their perusal. Be assured that your difficulty WILL be resolved using these pages !

This is very true and another real easy thing to check by simply removing the top cover on the carb. Right in line with where the fuel enters the carb is a 1/4" dia hole with the final fuel filter at the bottom. It is included in the carb kit. I have seen these, as 621 said, packed totally full of filter fuzz before. Another choke point to getting fuel for sure.

 

[Cantdog]

One of the best reasons a vac and pressure test is so important in diagnosing these 2 stroke problems. I had a Huskvarna 359 come my way once, it had multiple problems but one of the worst was the flywheel side crank seal had not one shred of the viton/rubber seal left on it, was wide open to the bearing, only the metal housing left there. The saw owner had been priming the saw dumping fuel in through the carb for months, could get it started that way and run it WOT but would stall immediately when throttle was let off. I was totally surprised the P&C were not scored. The saw also had the leaking plastic sleeve between he carb and cylinder and a fuel tank vent blockage, they ran it that way for months.

That is certainlly a possibility as well. I have replaced dozens and dozens of crank seals on this model saw and have never seen one that bad...thankfully!! Actually I have never had a seal fail on a 49sp, just replaced during a rebuild due to age and/or them seeming harder than what they should be. But that should not be taken that I think that they are immune to failure either as they are just a simple rubber seal in a metal carrier like any other.

A good pressure/vac test certainlly eliminates a lot of the guess work!!!

 

[Cantdog]

You may be chasing the wrong thing. The impulse and operation of the fuel pump has very little to do with starting an engine. The fuel pump has to operate properly once the engine is started and you may still have a problem with that (failed grease test?), but for starting it's all about the choke, not the fuel pump. The choke must be COMPLETELY closed and the engine must be healthy enough to develop a reasonable intake vacuum. When you pull the engine over, the vacuum has to pull down the metering diaphragm which will open the inlet valve and the vacuum will pull fuel up the fuel line, through the check valves in the pump diaphragm and fill the fuel chamber. With the throttle partly open, the engine should start and run for a minute or so with this fuel even if the pump is not working. Things that prevent this are stiff diaphragms, inlet valve stuck shut, incorrect assembly of the pump diaphragm, leaking carb gaskets, nozzle check valve stuck open, etc.
With a separate impulse line, checking it with a gauge is better than the grease test as you should get both negative and positive pulses. Also, if you pressure test the carb impulse fitting, it should not leak.

This is true......if all is well, even with a totally plugged impulse the choke will pull enough fuel to fire the saw but not run. However, if it has a blown cyl base gasket or a seal as bad as Jerry spoke of, the saw can't develope enough base vaccume to pump at all.