What causes RPM increase at cold starts? By twowheeler (Read 1,913 times)

[twowheeler]

If i understand the choke it allows more fuel after sitting for a few minutes or more but what causes the high RPMS during that period? Thers no extra air coming in like we had back in the day on carburetor cars where the butterflies and choke worked together.

[Bashan]

Jan 2, 2014 10:36:33 GMT -6 twowheeler said:

There's no extra air coming in

Actually, there is extra air. This is the air inlet for the enricher:

It leads to the port with the blue arrow below. So there's a port for gas that is pulled up from the float bowl. There is also the air port:

When the plunger is retracted (cold engine) both ports are open supplying the engine with more gas AND air which is delivered through the large opening to the right in the pic below. The extra gas and air bump your RPMs up. FYI, the small holes in the circle are the delivery ports for the low speed circuit; the one that is adjusted by the A/F screw:

When the heater element gets the thermogel warm enough in the auto-choke, it expands and pushes the plunger down into the choke well shutting off the extra gas and air. That's when the RPMs drop down.

[twowheeler]

Ahhh i see.

[tomcat]

Good info!
So to "override" the enricher could heat be applied to it (say, a blow drier) prior to firing up the engine, and then just use the throttle to avoid stalling?
My problem is that the cold start idle is 3400 rpm (2500 would be ideal) and the clutch engages at 3000 rpm, so I have to lock the trike brakes while the enricher does it's thing. I'm sure this isn't doing the clutch plates any good during my weekly winter-warmup session. Only other option I can think of is to put the axle on jack stands for the winter and just let the rear wheels spin away to their heart's content during warmup.
Or am I overthinking this and no harm is being done during the 2-3 minutes of brake torquing during warmup?

[Bashan]

No, it's not good on them for the drive to be engaged and holding the brake. I've had to do it just like all of us has had to do on occasion and I grimaced the whole time. It's not only the clutch pads but the variator takes a hit too.

Using external heat wouldn't push the plunger into the well, the thermogel wouldn't get hot enough because it's shielded. This diagram not only shows the themogel (wax) but shows how the device works:

I see four options for you:
1. Install a manual choke. Sage Alley is the expert on this but I think you could not only leave it off but just use maybe half choke.
2. You could remove the enricher. However, I see you're from Taylor, Mi and having lived the first 27 years of my life in Toledo I know that sometimes you may need the choke. Hey, by the way, did you know it's warm down here in the south?
3. I've toyed with this idea but never got around to trying it. How about run 12v DC directly from the battery and switch the circuit? That way you could hit the switch, the gel warms up, plunger comes out, boom, the enricher is off. A couple drawbacks would be can you remember to turn the switch off when you park it (I couldn't ), and you'd have to wait 5 minutes before cranking while you wait for the plunger to come out (I couldn't ).
4. Install stronger tensioner springs in the clutch. That way the clutch would engage at a higher RPM. However, with it engaging at 3K already you're looking at a possible hole shot everytime. Yes, Alley is also the expert on this ....and, sigh...Trikes. Hey, I know how to remove the enricher! Just don't ask me about the air cut valve.... .

[Alleyoop]

I do not understand why when Tom starts his Trike it Idles over 3000K and he cannot lower it. Few things that may not be adjusted right on his carb:
1. Throttle cable
2. Idle Speed
3. Fuel Ratio Mixture
4. Has an Air Leak and after it warms up it seals
Alleyoop

[Bashan]

Well I wondered about that too but forgot to ask him. I thought it might have something to do with the HP intake. I'm not sure exactly what that entails and figured that might be what was keeping his RPMs up on choke while he was happy with his RPMs otherwise. The sealing air leak is very intuitive, grasshopper is impressed.

[Alleyoop]

HAHAHA, they call me SCOOTER WHISPERER, some call me CARB WIZARD then the names they call me after that goes down quite a bit
Alleyoop

[tomcat]

Thanks Bashan & Alley for your helpful responses.
Here's a bit more detail:
The original carb that came on the trike idled at around 2400 rpm during warmup. This carb was defective from the get-go (could only do 30 mph before massive sputtering) so Jim from Rapid Repair ordered me a new one under warranty.
Before the new one arrived, I ordered the HP carb from Scrappy on the recommendation of my local scooter mechanic (he said the construction quality is far superior to OEM). Kept the replacement carb as a spare.
Cold idle was immediately about 1000 rpm higher with the HP carb. This wasn't a big issue during riding weather, as I would just immediately take off & let the high idle propel the trike to the end of my street. By the time I reached the main drag the idle had settled down to a normal 1800 rpm.
I've adjusted the idle speed & it had no effect on the enricher's steroid-enhanced action.
When the weather warms up (if ever) I'll adjust the fuel ratio mixture to see if that helps. Also I'll instal the HP intake, which I ordered after the carb & never got around to installing. I should change my sig to reflect that, my bad.
Thing is, the trike runs like a bear, pulls strong through the whole power band.
The only problem I'm experiencing is the ridiculously high cold-idle.

[Bashan]

OK, here's another hair brained idea (my specialty). Say you liked everything except the auto-choke idle. You tried adjusting the A/F to no avail and wanted to keep your set up the way it is. Do something like this:

But leave the air and gas ports half open. If it doesn't work take the gasket material out and no harm done. All of that is blocked off when the plunger comes down anyway.

[Alleyoop]

Which HP carb from scrappy did you get TOM? When you say HP do you mean High Performance Carb or the Make?
Alleyoop

[tomcat]

Got lotsa things to try now, thanks!
It's the Performance CVK Carb ($99.95) on this page Alley.
www.scrappydogscooters.com/125_150cc_Performance.html
Have to scroll down a bit to see it.

[Alleyoop]

That is what I thought a 30mm carb on a 157QMJ stock motor is to much fuel. You could try putting in a smaller PILOT JET or slap in the 24mm carb and tune the carb.
Alleyoop

[twowheeler]

Im told that the possible reason the overall rpm range is higher while riding cold is the contra may be stiffer and relates to higher rpms as well.

[Alleyoop]

Contra Spring has nothing to do with high IDLE. The problem is the 30mm carb on there, I knew I would find the cause of his high cold start idle that he cannot adjust. What I didn't know was that he had a 30mm carb on there that is a heck of a lot of fuel for a stock 157QMJ motor. Sometimes if you keep asking questions sooner or later things get said that you did not know about.
Alleyoop