Problem with #2 at idle

[CG901]

Freshly rebuilt Lotus Twin cam with original 40DCOE18 that were rebuilt as well (5 years ago). The engine has 97% on each cylinder on leakdown. 195-210 psi compression test in each cylinder.
Head was professionally rebuilt with new valves, guildes and seats, etc.
Always has run rough at low rpm. #2 cylinder is isolated as not responsive to any adjustment in idle mixture screw. Exhaust gas temperature runs 260F vs. 450-550 on other tubular exhaust manifold.
#2 cylinder pulls 1.5 kg/h on the Syncrometer vs. 4.5 kg/h on all other throats (800 rpm idle). Inspection of the butterflies shows they are in same position at idle.
We replaced the front carburetor with another identical carbutetor, and expereicned the same problem.
Good spark on all four plugs.
Above 1500 rpm engine pulls strong, although still feels abnormal compared with our other twincams of same vintage.
We can pull the spark plug lead on #2 at idle and hear no change in engine sound or rpm. Pulling any other plug will produce the predicable stumbling.
We believe #2 has no or little fuel in the idle circult.
What do you think?

[SENC]

If you swapped carbs and had the same results, I'd think that eliminates the issue being internal to a carb (unless both carbs happen to have the same issue). Does sound like a vacuum leak somewhere, though. I chased similar symptoms on my Seven before finding a weak valve spring, but would expect that to show on leakdown testing. Is the mounting o-ring good and appropriately seated? Have you tried starter fluid or propane around the spindle seals and mounting o-rings to search for a possible leak around the carb?

Do you have a way to test vacuum at each intake? If the 18s don't have a port, Keith Frank has designed a hollow version of the idle mix screw that can be used for measuring vacuum.

[Craven]

Check tappet clearance.

[CG901]

#1 Intake 0.004" Ex 0.009"
#2 Intake 0.007" Ex 0.012"
#3 Intake 0.005" Ex 0.010"
#4 Intake 0.005" Ex 0.009"

[Craven]

#1 Intake 0.004" Ex 0.009"
#2 Intake 0.007" Ex 0.012"
#3 Intake 0.005" Ex 0.010"
#4 Intake 0.005" Ex 0.009"

Close enough, unlikely to be your problem.

[sprintsoft]

Hi,

this is the root of the issue... "1.5 kg/h on the Syncrometer vs. 4.5 kg/h."

...except the cause is not obvious, normally I'd go for a butterfly not installed straight, or the progression holes not feeding enough, but then you said you had tried another carb.

When you swapped the carbs was there anything common between them that you re-used, like jets or auxiliary venturi, chokes?

Iain

[CG901]

When carbs were swapped, only the linkage and trumpets were reused.

[sprintsoft]

There's not much left to look at unless that exhaust port or pipe is blocked?

[HCA]

And the plugs? You have not pulled them out. Their condition will tell you a lot. Take them out after a long idle.

At what RPM does the engine pick up and run smoothly?

There is no difference at idle when you disconnect the plug lead - do it again and try for a spark against the head. Note the colour and intensity of spark, if any. (To do this you may have to use capless connections and use an insulated pliers) Do this with each cylinder and compare. Repeat at different RPM.

[CG901]

Plugs have been pulled. All appear to be normal color. The engine does not hesitate to rev freely in neutral or under load. Plenty of smooth power.
We did get strong spark from #2 plug wire against the head.
We will plan to pull each cap and compare color and intensity of spark.

[bitsobrits]

Try swapping idle jets around to see if the problem moves with the jet.

[2cams70]

Easy to create a spark in free air much harder to create a spark in high gas pressure conditions.
Did you try replacing the no. 2 spark plug? Did you try replacing the no. 2 plug lead?

[2cams70]

Easy to create a spark in free air much harder to create a spark in high gas pressure conditions.
Did you try replacing the no. 2 spark plug? Did you try replacing the no. 2 plug lead? Double check the carb to manifold "O" rings too. Low vacuum strongly suggests an air leak

[HCA]

I was leading up to this, but since he says the engine revs freely under load, discounts anything ignition related I think. i do remember a while back a mini that had a hairline crack on the cap that mysriously manifested issues at low revs. So maybe worth a look in case it was knocked.

I missed the low vac symptom. As the carburetter has been disturbed and if the plugs are a uniform colour and the problem is only apparent at idle, then I go along with the sealing ring. Or idle jet…

[HCA]

It is times like these where a Colortune is a worthwhile investment.

I had an MGB with a crossflow head fed by twin 45s. It was a useless piece of junk always going out of tune. I bought four Colortunes that made it all so easy just for the time taken to revove the plugs!