Technical Correct jetting for 594cc engine

[the hobbler]

Some time ago Ima

Good point.....at least @Goldnrust has some decent suggestions.

Some time ago, I managed to get hold of a "Gunson Gastester". A relatively simple device, but surprisingly accurate. With the Gunson, you have a 'sensor' pipe which is put up the exhaust, and that enables you to obtain a mixture reading at practically any revs, without having to take the (very hot!) plugs out every time to inspect them. They do occasionally come up on e-bay (which is where I got mine from), but I will admit, they are now getting scarce.

 

[Toshi 975]

Like Tom the Hobbler I picked up a 2nd hand gas analyser at what I considered a reasonable price, a very handy piece of kit.

 

[smart51]

I've done back to back tests with the jets I have. I'm not sure what to make of them.

115 and 190 (652cc standard jetting, so should be a bit lean on my 594). Starts, but not as cleanly as with the other jetting combinations. Sounds better at 30MPH in top, pulls up hill slightly better. Snap open the throttle from low opening to full and it pauses for about a quarter of a second before accelerating. Will rev through 5000 RPM in 3rd. I suspect it is down on power at 62 MPH in top (4300 RPM).

125 and 215 (early 594cc standard jetting). Starts straight up. Sounds more annoyed with an open throttle at 30 MPH in top and doesn't climb hills quite as well. Smooth response under all kinds of throttle opening and closing tests. Will not get to 5000 RPM in 3rd. Noticeably down on power at higher speeds.

125 and 190. Same as 125 and 215, except worse up hill and at high speed.

The bogging down when snapping the throttle open suggests leanness, but that could be the idle circuit or the main jet.
While it starts better with the bigger main jet (!?) It clearly doesn't run as well. The other jetting combination for the 594 is 125 and 225 which would lean up the top end a bit. It wouldn't affect running at low RPM and full throttle though.

Should I split the difference and try a 120 main jet and a selection of air jets? What would you do?

 

[Goldnrust]

I like the methodical approach

It sounds to me that there is some benefit on smaller throttle openings with the 125 main jet.

Logically if 125/215 is better than 125/190, then a larger air jet would seem to be the right direction?

Knowing that 125/225 was a factory option, that would be my next move.

 

[smart51]

I've been thinking about why the "correct" settings made things worse, aided by a few google searches. My new hypothesis is that a 125 main jet is probably right for a 594 with a stock exhaust. Stock exhausts restrict gas flow a bit, leaving some gas in the cylinder. A non-empty cylinder doesn't 'pull' quite as hard as a result, so needing a large main jet to flow enough fuel. Freer flowing exhausts empty the cylinder better (you'd hope!). An empty cylinder pulls more air through the carb. Carbs are not linear, so more pull = way too much fuel. I might need something smaller than a 125.

My carb came with a 115 main, which is two sizes smaller than the 125 stated in the book. It is too small and so a bit lean, at least in places. While the 125 is a bit too big. It fixes a couple of lean spots compared to the 115, but causes more problems than it solves. A bigger air jet might only fix the top end.

If my set up draws more air than a stock 594 but less than a stock 652, perhaps a 120 is the right size main for my setup. I'll order one, and a selection of air jets, and try them out.

If the 120 keeps the better starting but avoids the hill climbing problems at 30 and 40 MPH, and the bogging down, it is probably the right one. I can then try a few different correctors and keep whichever gives the best top speed. Otherwise, the choice is 115 with a smaller air corrector, or 125 with a bigger corrector.

The trouble is, I can't find a good source that says what the difference is between "matched pairs" of bigger main and corrector. Adding together little snippets I've found, a bigger main with a bigger corrector makes it richer in the middle / leaner at the top. Smaller mains and correctors make it less rich in the middle and less lean at the top. Bigger correctors make move the transition to the main circuit to lower air speeds. Smaller does the opposite.

 

[smart51]

I've found a copy of the weber tuning manual. It has a good description of how the carb works, but little advice on tuning it. www.lainefamily.com/images/WeberTuningManual.pdf

taking account of our engines only having 2 cylinders, the graphs suggest that the 26IMB is the right size for a 500cc engine and the 28IMB is the right size for 600s and 650s. Good to know.

What it does say is that for every size change in main jet, you need to go 3 sizes in air corrector jet. By implication, changing the air corrector outside that affects the top end more than everywhere else. Changing the main jet affects the whole range.

 

[smart51]

More progress today and some good news. First, meet my new friend the tiny handled wide blade screwdriver. It can take the top of the carb off in situ, where a stubby screwdriver can't quite fit.

The new jets arrived today. I can highly recommend Eurocarbs. Order on eBay and you get next working day delivery. They even combine postage and refund the difference.

120 main and 205 air corrector is today's trial combination. Like the 125 main, it starts much better with a 120 than a 115. It is richer at idle too. For the first time, I've been able to adjust the idle mixture until it got worse (at 3 turns out) then back it off. It sounds counter intuitive, but as all the fuel goes through the main jet, even for the slow circuit, it does make sense.

The car runs well in the mid range like with the 115 main jet. It will climb steep hills at low speed acceptably will. With the 125 main, it wouldn't. The throttle response is good too.

It will rev to 5k too, which is wouldn't with a 125 main jet. Tops speed is down to 59 MPH. It's not a surprise though, 205 is the leanest air jet to go with the 120 main. The test was to see if the top end is too lean. I have a 200, a 190 and a 185 air jet to try. My thinking is that the highest top speed will be found with the engine running between 5% and 10% rich, so one size bigger than that is about right.

 

[smart51]

Another day, another outcome that I didn't expect. Today I tried 200 and 190 air correctors with the 120 main jet. As expected, the 200 was richer than the 205, but only giving 1 MPH better top speed. The 190, of course, is richer still, but the top speed was the same, but only just. 190 is too rich for the 120 main. I'd guess that the 200 is probably the best match, maybe a 195. It's still worse than the 115 main jet though.

That has led me down another path. Perhaps the 120 and 125 mains giving better idle is a red herring. Perhaps the 45 idle jet is too small after all, and that a 50 would be better. Fiat pair the 50 with the 115 main after all. So is a 115 +190 combination is down on top end power, a 115+185 should be the next combination to test.

I also seem to have lost the throttle adjusting screw and spring. I'm not sure where it fell off, but it isn't in my garage. I can't find them for sale on line either. Do any of you have a spare? part numbers 47600.084 64625.003

 

[Goldnrust]

I think if you really want to dial it in, you're going to need some more accurate and consistent ways of measuring than just 'feel' and the top speed. The human body is great at comparative measurement, but not great at objective measurement. So for example it's easy to say that the mid range pull is better than before, but then hard to say if the top end is worse or stayed the same as your judgement is coloured by the comparison to the midrange. Add to that the atmospheric condition changes, 5mph wind speed difference could easily knock 1-2mph off your top speed, not to mention barometric pressure and humidity changes affecting the mixture, and you can really run yourself in circles when hunting for small improvements.

With your 120 main jet, you tried various air correctors and found the best match in the 200 (205 was slower and 190 no faster), that suggests you've found the best mixture for full throttle high revs.

If we take that to be true then logically going back to a 115 main will mean you have to retune the air corrector to once again find that sweet spot, and for that top speed run you'll end up at the same mixture and the same speed. It might make for a better low end or mid range feeling on the way to that point, or it might not?

The route you're heading I'd seriously consider an AFR gauge, it will take all the guesswork out of what you're doing and then you can really tune it in. But also be prepared that carbs are a compromise (unlike EFI that can in theory be mapped to be perfect everywhere) and so you may just end up with a lesser of two evils decision to make, do you want the better mid range or the better top speed.

It's fun challenging yourself to make it faster though... reading this thread is really making me want to fit my AFR gauge to my 500 and start seeing where I can make it faster! haha.

 

[the hobbler]

I think if you really want to dial it in, you're going to need some more accurate and consistent ways of measuring than just 'feel' and the top speed. The human body is great at comparative measurement, but not great at objective measurement. So for example it's easy to say that the mid range pull is better than before, but then hard to say if the top end is worse or stayed the same as your judgement is coloured by the comparison to the midrange. Add to that the atmospheric condition changes, 5mph wind speed difference could easily knock 1-2mph off your top speed, not to mention barometric pressure and humidity changes affecting the mixture, and you can really run yourself in circles when hunting for small improvements.

With your 120 main jet, you tried various air correctors and found the best match in the 200 (205 was slower and 190 no faster), that suggests you've found the best mixture for full throttle high revs.

If we take that to be true then logically going back to a 115 main will mean you have to retune the air corrector to once again find that sweet spot, and for that top speed run you'll end up at the same mixture and the same speed. It might make for a better low end or mid range feeling on the way to that point, or it might not?

The route you're heading I'd seriously consider an AFR gauge, it will take all the guesswork out of what you're doing and then you can really tune it in. But also be prepared that carbs are a compromise (unlike EFI that can in theory be mapped to be perfect everywhere) and so you may just end up with a lesser of two evils decision to make, do you want the better mid range or the better top speed.

It's fun challenging yourself to make it faster though... reading this thread is really making me want to fit my AFR gauge to my 500 and start seeing where I can make it faster! haha.

If you look on the "WEBCON" site, you will find the 'idle-speed screw' (their part number:---6462500300)---£2.62p + VAT, and what they call "The Idle Mixture Screw Spring" (their part number:---4760008400)---£3.86p + VAT. Now, the funny thing is that according to the 'numbered parts list' for the 28IMB, this spring is listed as the spring for the IDLE-SPEED screw! I think that for what it costs, it would be sensible to get the spring and see if it will do the job.

 

[smart51]

Thanks Hobbler. I bet I could have found something that fits, but I'd rather have the genuine part.

 

[smart51]

Well isn't this something. Today's combinations were a 115 main jet with a 185 and a 200 air corrector, plus a 50 idle jet. The new throttle screw and spring were fitted too. Starting is better than the 115 + 45 jet combination, so that is good. The 185 gave me a top speed of 59, which is disappointing. The 200 gave me 61 MPH. Previously, a 190 air jet gave 62.

So it seems that after all this fiddling about, the main and air jets were right to start with, even if they were wrong according to the book. Changing the idle jet to the "wrong" one (that matches the main and corrector) has improved it a bit. I'm still left with an engine that seems down on top end
power, but at least I've ruled out carburettor jetting.