Strange Warm-Up Symptoms

[MisterCinders]

First, I did SEARCH the forums (like a pro ) for this issue, but only found info about checking valves. My valves are in spec as of this summer when I put the top end together. Here are my symptoms.

Warming up the bike is bizarre.

She starts right up with the choke circuit opened up, as you'd expect. Instead of idling and then climbing in RPM, however, the engine idles almost immediately at 2,500 - 3,000 RPM, as if it was warmed up already. If you kill the choke, however, she dies.

Of course, if I give any throttle, she dies (because the choke is open).

Even after running for a fair amount of time (several minutes), the idle climbs as expected, and I can keep it below 2,000 RPM by easing down the choke, but closing the choke below around 25% -- she dies. Again, blipping the throttle kills the engine, like it should with the choke open.

At this point, I would guess lean condition, etc. I continue to fiddle with the pilot jets, so that is not completely crazy.

Here is the funky bit.

Once the bike has idled a bit, I can get it to run using a mix of two methods.

First, if I dial the idle screw in a bit, she will catch and run without choke after a normal warm-up period of about 30 seconds to a minute, even though blipping the throttle kills it. Once running, though, I soon find my idle running too high at a stop, so I have to reach under and back the idle screw out. This happens a few times once I am riding around, but after 2-3 small adjustments, the idle settles into the 1,050 - 1,200 range.

Second, the other thing that works is a quick combo of kill choke and blip throttle (sometimes after turning in the idle screw a bit first). Basically, if I can catch the motor running before it dies without choke and blip the throttle, it "catches" and idles normally, at about 1,500 RPM. Later, I find the idle has climbed up a bit, and need to reset it a bit to level out.

Adding to the oddity is that once I get over this weird warm-up hurdle, the bike runs nicely. I still have a slight decel popping, but I was getting that before with a lot of rich plug readings, so I think that is a small exhaust leak. Otherwise, the engine responds well to throttle, the idle does not hang, and the bike is generally very responsive -- running better than it has since I got it. Also, once warmed up, the bike starts with no problems. So this only happens at first start up.

Having to tinker with the idle screw a bit is not that big of a deal, but the symptoms are so contradictory that it concerns me. What might I look into to root out this problem. Is it just a lean idle circuit? Forgot to do a plug chop this weekend, but I will do that anyway. Last idle chop that I did, however, came out pretty good, finally losing that richness that plagued me.

Before I get the "usual suspects," I adjusted my valves, fully cleaned and rebuilt the carbs, have jetted the bike for pods and exhaust (though I continue to fine-tune the idle circuit), pet cock works fine, electrics are tight (with coil relay and Dyna-S). After my many "Noob" threads, this is not my first rodeo . Not to say that there won't be first rodeos for me in the future, of course.

 

[Steve]

What are the settings on your fuel and air screws?

With your pipes and pod, your fuel screws (the ones on the bottom) should be out 1 full turn, the air screws should start about double that.

If you still have problems, try turning the fuel screws out another 1/8 turn.

.

 

[MisterCinders]

Fuel screws are at 3/4 turn out, as I was having richness problems before. I started the air screws at about 2 turns and then adjusted to highest idle, as best I could.

 

[BassCliff]

Hi,

Did you also replace the intake boots and/or intake O-rings? Yes, that's a pretty weird scenario you have there.


Thank you for your indulgence,

BassCliff

 

[DanTheMan]

I don't understand why you think opening the throttle with the choke open causes the engine to die. That's not normal either. The choke circuit (it is a circuit) is similar to your low speed circuit, extra fuel flows through the carbs because you've opened another fuel path to "enrichen" the mixture, not to deplete the air flow, as in a butterfly style choke. Opening the throttle just admits more fuel through different jets, and the enrichening of the mixture by the choke has no effect at that point, either positive or negative. When you close the throttle and the choke is still open, it should be running 2500-3000 rpm or so.
Your carbs are leaning out and not getting enough fuel flow from the sounds of things.

 

[MisterCinders]

Hi,

Did you also replace the intake boots and/or intake O-rings? Yes, that's a pretty weird scenario you have there.


Thank you for your indulgence,

BassCliff

Intake boots are nice and soft, but I did change the intake o-rings.

 

[MisterCinders]

I don't understand why you think opening the throttle with the choke open causes the engine to die. That's not normal either. The choke circuit (it is a circuit) is similar to your low speed circuit, extra fuel flows through the carbs because you've opened another fuel path to "enrichen" the mixture, not to deplete the air flow, as in a butterfly style choke. Opening the throttle just admits more fuel through different jets, and the enrichening of the mixture by the choke has no effect at that point, either positive or negative. When you close the throttle and the choke is still open, it should be running 2500-3000 rpm or so.
Your carbs are leaning out and not getting enough fuel flow from the sounds of things.

When the choke is on, opening the throttle floods the engine with an over-rich mixture (i.e., normal mixture + choke fuel), so I am used to it struggling/bogging if I open the throttle with the choke open.

Ordinarily, with the choke open, I strictly leave the motor at idle and wait for it to warm up before closing the choke and working the throttle.

 

[BigD_83]

I know you've gone through this in the past, but I'm not going back through some of the older posts to find the answer...

In line fuel filter...Yes/no? it COULD be rich at warmup and lean out on throttle if the increased vacuum (and air draw) is not being matched due to an upstream fuel constriction.

 

[Steve]

Fuel screws are at 3/4 turn out, as I was having richness problems before.

I still think your symptoms sound like it's running lean, so try opening them up another 1/4 turn.
Of course, you will also have to adjust the air screws to match.

I don't understand why you think opening the throttle with the choke open causes the engine to die. That's not normal either. The choke circuit (it is a circuit) is similar to your low speed circuit, extra fuel flows through the carbs because you've opened another fuel path to "enrichen" the mixture, not to deplete the air flow, as in a butterfly style choke. Opening the throttle just admits more fuel through different jets, and the enrichening of the mixture by the choke has no effect at that point, either positive or negative.

Except for one small detail, Dan. :-k

Yes, the "choke" circuit admits its own fuel and air, but the fuel has to go much higher to reach the plunger that controls the mixture. To get that extra height, it is MANDATORY to leave the throttle closed, for high vacuum. If you open the throttle, you eliminate the vacuum, and you will be running (or trying to run) on just the normal idle mixture.

.

 

[MisterCinders]

I know you've gone through this in the past, but I'm not going back through some of the older posts to find the answer...

In line fuel filter...Yes/no? it COULD be rich at warmup and lean out on throttle if the increased vacuum (and air draw) is not being matched due to an upstream fuel constriction.

No problem. I do not have an inline filter.

 

[MisterCinders]

I still think your symptoms sound like it's running lean, so try opening them up another 1/4 turn.
Of course, you will also have to adjust the air screws to match.



Except for one small detail, Dan. :-k

Yes, the "choke" circuit admits its own fuel and air, but the fuel has to go much higher to reach the plunger that controls the mixture. To get that extra height, it is MANDATORY to leave the throttle closed, for high vacuum. If you open the throttle, you eliminate the vacuum, and you will be running (or trying to run) on just the normal idle mixture.

.

Hmm. Then I had my reasoning for no throttle with choke backwards. I figured that the choke overkilled the mix, not that throttle canceled it out.

Of course, that makes my symptoms even stranger. I suppose, since it will idle on the normal mixture, once I get over this strange warm-up hump.

I will back out the fuel screws and see if that helps. If that richness comes back, ugh.

 

[MisterCinders]

Didn't get to do chops, but I pulled my plugs to see what they looked like after my ride on Sunday. I ride mostly in the city, so the bulk of my throttle is low-end. Figured they'd give me some indication of the idle circuit.

This gets weirder.

The No. 1 plug was pretty sooty, still running rich there.

The other three seemed middle of the road, darkish tan with no soot.

Here's the weird part -- the plugs on 2-4 all had a red colored residue on them. Not rust AFAICT, but a brick colored dust. It wiped off with only a little effort.

Popped them back in and started the bike, with a little choke. She warmed up after a bit (had a fan on the engine), and idled without too much struggle after a slight idle screw adjustment. I let her idle a few minutes, and tried to fine tune the air screw.

Shut her down, pulled the No. 1 plug. It hadn't gotten sooty yet (but the engine was not under load), but it too picked up this weird reddish residue.

Any ideas on what might color the plugs this way?

I have a fresh tank of gas. No inline filter. No additives to the fuel.

The only suspect I can think of is Bondo. I have been working on another tank in the garage, sanding filler, etc. But all my sanding has been by hand, so there aren't big clouds of dust everywhere from power sanding.

But, the red color residue is similar to that Bondo dust. Could dust from hand sanding in the garage have gotten past the filters to color the plugs? Seems unlikely to me. But what else would do that?

 

[Steve]

Bondo dust is a remote possibility, and you say that YOU have not added anything to the gas, but what about the gas company? :-k

.

 

[MisterCinders]

Bondo dust is a remote possibility, and you say that YOU have not added anything to the gas, but what about the gas company? :-k

.

Dunno. The gas was just regular unleaded from mainstream service station. Either Chevron or BP, I don't recall.

 

[DanTheMan]

I still think your symptoms sound like it's running lean, so try opening them up another 1/4 turn.
Of course, you will also have to adjust the air screws to match.



Except for one small detail, Dan. :-k

Yes, the "choke" circuit admits its own fuel and air, but the fuel has to go much higher to reach the plunger that controls the mixture. To get that extra height, it is MANDATORY to leave the throttle closed, for high vacuum. If you open the throttle, you eliminate the vacuum, and you will be running (or trying to run) on just the normal idle mixture.

.

Yeah, I knew I was missing something in my line of thinking there. Thanks for clearing that up for me (and the OP)

 

[jimfj]

I would look pull the fuel hose and put the petcock on prime. Drain the fuel into a glass jar and see what color the very fine particles are. You stated no fuel filter. I am betting there is some rust in your tank some somwhere.

I just finished cleaning a rack of VM carbs off my 77 550 that were covered in the nasty brick red stuff. Not big chunks, just super fine dust. I believe that when the contaminated fuel burns away it leaves this crud. i would also pull a float bowl and look at the little recess where the drain is located and I would also bet some has collected there.