Time To Play... Diagnose This! - Internal Carb Failure

[trains]

2 common sizes, but actual dimensions are beyond me at the moment (its dinner time and I should be at the table hehe), Tom should have them handy tho.

 

[Vintage Engine Repairs]

I had a carb I put to the side a while ago I suspected had an issue with the accelerator pump... decided to pull it apart yesterday. The primer would suck air & it was impossible to tune. First indication the O-ring wasn’t sealing was the fact the little piston would fall into/out of the hole with no resistance. To confirm it was leaking I removed the spring & placed the piston in the hole upside down (so the O-ring sat where it should to seal). I then spraywd soapy water into the hole & pumped the primer. The soapy water was sucked past the seal & into the primer bulb so it clearly wasn't working. On closer inspection the O-ring was cracking & hard as a rock.
I rummaged through a few other carbs & found another C1Q carb that appeared to be identical with the exception of the accelerator pump piston being smaller. Got me wondering if there are common sizes for these (& therefore common O-ring sizes to keep on hand). Original carb was 5mm piston, 5.5mm bore. Second carb was 4mm piston, 4.5mm bore.
Curious to know what size O-rings you've come across Tom

Sure, I’ll copy and paste my reply to Steve’s similar posts on OP3 forum:

“There is no documentation on these sizes, no.
I discovered it through comparing part numbers to dimensions. So far they have all been metric - expected as they’re a German company.

Example: 195503010 is a 3x1 metric o/ring

You take the last 4 digits

195503010 = 3.0 x 1.0
195503510 = 3.5 x 1.0
195702308 = 2.3 x 0.8

The above are the only 3 I have come across.

This is a common numerical ordering system making P/O very efficient to fill out.

Once I understood that was the likely means to decipher the size, I measured the bore, piston groove and checked o-ring clearance charts and confirmed it on a bunch of different carbs with accelerator pumps.

 

[J D]

Thanks for that... I take it those sizes are internal diameter?

 

[Vintage Engine Repairs]

Thanks for that... I take it those sizes are internal diameter?

That’s it mate, o-rings are measured internally and then cross section.

 

[SteveSr]

I just ordered both of the 1mm CS rings from here:
https://www.theoringstore.com/store/index.php?main_page=index&cPath=368_65

I also ordered in V75 (Viton compound). These are 75 durometer which is fairly close to the normal 70 for Buna-N.

I also got a chance to test run the saw with the new carb. Ran about 3 tanks through it. It didn't cut bad for a cheap saw.

When the o-rings come in I'll reassemble the original carb and try it to see if the o-ring was the root cause of the issues.

 

[SteveSr]

Well the verdict is in... Drum roll please...

Even though the existing Accelerator pump o-ring was compression set and likely leaking, replacing it did NOT fix the carb. It still had pretty much the same issues as before replacing the o-ring. The only remaining component is the main nozzle check valve which was not changed.

 

[Old2stroke]

Well the verdict is in... Drum roll please...

Even though the existing Accelerator pump o-ring was compression set and likely leaking, replacing it did NOT fix the carb. It still had pretty much the same issues as before replacing the o-ring. The only remaining component is the main nozzle check valve which was not changed.

There we go, didn't think that small of a leak would cause the problem. What's left now is maybe a nozzle check valve that is stuck slightly open enough to bleed air into the idle circuit but not open enough to flow max fuel for the high speed circuit. Easy to check with a suck-and-blow tube held against the main jet feed hole.

 

[SteveSr]

There we go, didn't think that small of a leak would cause the problem. What's left now is maybe a nozzle check valve that is stuck slightly open enough to bleed air into the idle circuit but not open enough to flow max fuel for the high speed circuit. Easy to check with a suck-and-blow tube held against the main jet feed hole.

Well this test is hard to perform as it is hard to get a tight seal to the "H" circuit to run the test.

 

[SteveSr]

Well, I decided to punch out the main nozzle for further examination. I cut a slot with a Dremel wheel in the back (metering chamber) end of the nozzle and then shoved the whole nozzle into the end of a piece of urethane fuel hose.

I then performed the blow/suck test on the other end of the hose. What I discovered is that the valve will seal if you suck hard enough but with only slight negative pressure it will leak. So yes, this valve was "sticky" open that could be masked by the traditional blow/suck test or the venturi pressurization test.

 

[trains]

Well the verdict is in... Drum roll please...

Even though the existing Accelerator pump o-ring was compression set and likely leaking, replacing it did NOT fix the carb. It still had pretty much the same issues as before replacing the o-ring. The only remaining component is the main nozzle check valve which was not changed.

I thought you had already tested that before and ruled that out after re post #31

 

[SteveSr]

I thought you had already tested that before and ruled that out after re post #31

I did. The valve clicked and it does open and close with enough pressurization. The issue is that the actual forces on the valve are much weaker during actual running conditions.

 

[trains]

I did. The valve clicked and it does open and close with enough pressurization. The issue is that the actual forces on the valve are much weaker during actual running conditions.

I find that putting some hose over the drilling for the main nozzle check valve, and blow/ suck with your mouth tells you exactly how its sealing or not sealing under both pressure and vac , and you can go from there, I prefer doing it that way than using a mitivac for example, the feedback you get by doing it yourself is useful.

 

[Vintage Engine Repairs]

Well, I decided to punch out the main nozzle for further examination. I cut a slot with a Dremel wheel in the back (metering chamber) end of the nozzle and then shoved the whole nozzle into the end of a piece of urethane fuel hose.

I then performed the blow/suck test on the other end of the hose. What I discovered is that the valve will seal if you suck hard enough but with only slight negative pressure it will leak. So yes, this valve was "sticky" open that could be masked by the traditional blow/suck test or the venturi pressurization test.

That notch will render it useless now - you’ll have unmetered fuel into the main nozzle. It’s also difficult to find many of these nozzles now days. Attaching a small hose to the end that enters the carby bore and a slightly larger one from the other end that will go over the first gives you clearance to test in the right direction.

Regarding leaking, ideally they should seal solid when wet with fuel however there is an allowance of a leak no faster than a loss of 7 psi in 4-5 seconds - I have come across new ones that don’t seal fully, but meet the above and work great. I’ve even made my own disks and they have worked well.

Other times they just need to be cleaned out and they work well again.