jayroc

I've got a Sterling carb on a stock 12a with headers. One afternoon I got in to drive it after driving it earlier in the day and it was down on power and seemed to have a misfire.

It doesn't want to idle below 2k, and the header attached to Rotor 1 glows red within a few minutes.

I've checked ignition, timing, the carb seems to be putting out even amounts of fuel to each rotor. Comp test shows 105psi on both rotors with a steady bounce during the turnover test. Fuel pressure shows 2 to 2.3psi. (It fluctuates rapidly off idle)

I do have some rust build up in the tank so I thought something might be clogging the idle circuit, but I have two inline fuel filters and neither showed signs of letting rust through, (I replaced them anyway, though.) and the carb sprays fuel normally.

The previous carbs I tried before the sterling had the same no idle below 2k problem, but when I put this Sterling on all my carb problems went away and it ran great. Now I'm back to square one and it's frustrating.

I don't know what else to check..Vacuum leak, or bent throttle rod on the carb??

Any ideas for me to try?

DivinDriver

iTrader: (15)

Vacuum leak would be the obvious choice, someplace close to the front rotor. Intake mani a likely candidate, since the problem survived across multiple carbs.

Partially-clogged jet would be second choice, or a low bowl level that's reducing fuel feed pressure on one side of the carb.

Kentetsu

1. With the engine running, look at the float bowls and see if both are at the halfway point.
2. Verify that you do not have the plug wires reversed between leading/trailing on that rotor.
3. Inspect or replace the cap/rotor. If one contact was bad, you could be running on trailing only on that rotor.

A bent throttle rod would affect both rotors, as would a clogged filter. Carb is probably not to blame, especially since it was running right, then stopped. A vacuum leak is also likely, but since they can be such a bitch to track down I'd start with the list above first.

Good luck, and please post back with results. I'll help you out if I can, but I'm sure Sterling will be along soon with even better advice.



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Sterling

Well, no better advice than what's been given so far.
Two ways to bend a shaft are to drop the damned thing, or to have the cable too tight and the pedal stop not adjusted (wayyy out of whack). The primary shaft can turn another good 30* beyond WOT until it will stop on the linkage and twist.
A twisted shaft will cause one rotor to run lean at idle, but it will be the rear rotor.
The fact that this is so sudden would lead me to believe you have a cracked hose.
Check the vacuum advance hose & the brake booster hose for cracks.
Post back. The more info you give, the more we can all help narrow it down.

Kentetsu

See that, what'd I tell you?

jayroc

Ignition is all good. I checked all plug wires with a timing light, they fire properly and timing is good.

I'll check the float bowls and poke around for a vac. leak. Wish there was an easier way to find vac. leaks...

What about the air pump block off? Does it feed both rotors or just one?

Thanks guys

bumpstart

kero on a brush works fine for me when tracing leaks
( motor will surge when you find the leak )
- some use carb cleaner spray, but it hurts hoses , is very flammable and rather indiscriminate

when reading from top i also suspected the trailing only was firing that chamber
the fact that its the front rotor rules out a leak on the brake booster line
or even a leak around the shaft for the decel throttle blades
( found in the rear of some later manifolds under the carb on rotor 2 )

so i will go with air leak around throttle shaft, or phenolic spacer on that front side

- if the kero test rings OK
and once you have checked those throttle blades for twisting -

then you can try testing the tank vent circuit
blow it back from the emission rats nest pipe
expect rust and all sorts of crap is blocking it
( in aussie, wasps make a habit of setting up in the rats nest pipe )

this can affect some later carbs bowl venting, and cause all sorts of issues
( though unequal running isn't typically one of them, its still definitely worth a try )

DivinDriver

iTrader: (15)

The air pump air injection channels go to both rotors, but on the exhaust side: I don't think they can be contributing to a lean condition.

jayroc

Okay, I just finished another round with it:

Float bowls are fine, levels are good in both. I tried the kerosine spray to find a vac. leak, but it didn't show anything. I'm not sure the kero was sufficient.
I did also check all the vacuum plugs and they seem fine.

I'm beginning to think this may be a coolant seal failure. I took the rad cap off and started/ran it, but didn't notice any bubbling.

Tomorrow I'm going to pull the intake manifold off and see if coolant is leaking into the chamber. BTW, I have the coolant passages on the motor plugged with a freeze plug, so a lot of coolant can't be getting into chamber that way..

Kentetsu

Coolant seal eh? Is she blowing white smoke out the tail pipe?

jayroc

No.
I was thinking maybe a large enough leak would be dousing the plug and causing it not to spark, or extinguish it..

I'm kinda grasping at straws here.

jayroc

Got it fixed!

Apparently there was a vacuum port tucked away next to the power booster's vac. line hookup. That plug had popped off and landed in the small spot on the motor mount crossmember somehow. Weird.

Anyway, I popped it back on and it ran fine. Didn't see the plug until after I took the carb off, but it's working now. Thanks to everyone that replied.

I did notice, though, that both pipes on the header started to glow when I had it idling at 2k while it warmed up. Easily noticed now that it's night time.
Is this normal behavior? Should I up my fuel pressure to 2.5psi, or is this more of a jetting issue? The carb is a Sterling Nikki. In general I thought it ran rich on average based on my gas mileage.

bumpstart

surprised you didnt find that leak with kero on a brush !

running hot on the extractors=
timing is overly retarded , and hasn't the time to allow combustion to all happen within the motor

- either cut back on the fuel for same timing
( yes, over rich causing the after-burn to happen which was a factory intention with the air pump )
or increase the timing to have it all happen within the cranks torque phase


- or alternately, particularly if one side, a huge lean out caused by vac leak

jayroc

The bugger is really tucked away, pretty hard to see even during the day. Kind of fortunate I noticed before I took the manifold off.

timing is set slightly advanced, but vac. advance is disabled.

I set the fuel pressure at 2.5psi, the car seems to like it a bit better. Got a trip planned to Tues, so I'll log my mileage.

Kentetsu

Put a hose clamp on that plug so it doesn't fly off again.

For the glowing header, I agree with timing issue. Make sure the plug wires are routed correctly, and that the trailing ignitor is actually firing the trailing coil/plugs (weird things do happen).

The other possibility would be running lean, so playing with the fuel pressure might help. 2.5 psi is a bit light for a Sterling carb. I believe I'm running about 3.2 psi. You'll know if you go too high, because you'll start losing high rpm power...



.

jayroc

Good thinkin..

Timing and wires are correct. I believe it's 5 degrees ATC and advances to 15-20. Whatever mechanical advance on the stock dizzy allows for anyway.
I am in the process of installing a o2 sensor/gauge, actually if anyone has a meter for sale let me know.

I bought the Sterling 2nd hand and the guy had planned on boosting it so he wasn't sure the psi to set. I asked rx7carl and he said to start at 2.5psi. I accidentally set it at 2psi, but it ran fine so I didn't change it. It's at 2.5 now.

How can I know the proper fuel pressure? It doesn't starve out at all and it's returnless so I was cautious about setting it too high thinking it may overwhelm the float bowls or something.
Can I just idle it at 2-3k and check the float bowl level? Watch for loss in high rpm power?

Viperx7

iTrader: (4)

Good to here you got it squared away.

Kentetsu

You should probably send Sterling a PM and get a dialog going with him. It won't matter to him that you bought the carb second hand, he still gives outstanding support even if he hasn't received a singe red cent from you. I know this because I also bought mine used. Good luck...

P.S. What I've found is that if you go too high on the fuel pressure, you'll see a reduction of high rpm power due to being overly rich. Another symptom of too high a pressure will be bogging on hard left turns. So don't be afraid to play around with it a bit.

RotaryMelon

iTrader: (7)

Having the same problem... also running a Sterling carb..
Also tried everything you did with no result.. grr!
Figure it out yet?

jayroc

Yeah, it's been fixed for a while now.

The problem was a vacuum line on the back of the manifold. I had it capped, but it came off somehow. I put the cap back on, the car runs great now.

The line in question is difficult to see and below/behind the brake boost feed. I don't remember what the line was for on a stock emissions system... PCV maybe?

Good luck.