Idle RPM drops after high speed driving

[Irish911]

I find that when I get off the highway my rpm's will drop down to around 400-500 at idle, almost causing the engine to stall. I need to tap on the gas periodically to keep the rpm's up. When starting and doing routine city driving the idle sits around 900. It's only right after I have been doing high speed driving I find the idle drops very low. Its almost like the engines not getting sufficient gas.

I have Weber 40 IDTP carbs. I have not taken them apart so I don't know all the numbers except the idle jets are 55. I removed all of them to make sure they were clear and clean. Would it be reasonable to replace the 55 idle jets with 60's? Is there anything else I can do or should be looking at so the idle rpm doesn't drop off so much? Thanks for any help.

[friggens]

Hi Irish -

I am not an expert, but my experience in low idle change with recent careful consideration of my IDS carbs is related to the throttle stop. I changed my springs slightly to overcome, adding moment arm. If I pushed slightly on each throttle level on each carb - idle went lower and stayed there. Slightly more spring tension overcame that so now the throttles are properly 'closing' to that very special place that correctly taps into the progression circuit, or moreover does not tap into it at idle. So you may be tuned to 900 rpm with the throttles not quite closed and when you are nice and warm, throttles are closing better and you get lower idle. Now if it continues to drop off to stall, gotta be different reason. Mine will drop low idle rpm, say 500-700, but it sits there even, not dropping off. Warm also means very different atomization, etc.

my 2 cents - wouldn't be jets if you have not had this problem in the past in the same place (altitude).

I definitely like the topic... Webers... very nice instruments!!!

[RSTarga]

Apart from throttle issues, it could be a bad advance mechanism in your distributor.

[Irish911]

Thanks Mike and David for your responses. I hadn't thought of the springs and it may be that long periods with the throttle valve being open while highway driving they are not seating in the same position as when driving in town and the throttle is constantly being worked. I will check it on my next drive. Yes, I too enjoy working with the carbs, they are incredible pieces of engineering.

The advance mechanism I'll look into if that doesn't help. If anyone has any other ideas please let me know.

[1QuickS]

Carburetors experience cooling during your driving and the faster you go the cooler they run. This is due to the air flow through the engine compartment due to higher fan speeds and due to the cooling effect of the air passing through the venturis. In-town driving would have your carbs running at warmer temperatures than open road running. Insulators between the heads and the intake manifolds would help isolate the conduction of engine heat into the carburetors. Other cars incorporate methods to supply heat to the intake manifolds by routing engine coolant through the intake manifolds to normalize the effects of cooling our air cooled engines experience.

So, the efficiency of atomization of the fuel when the carbs are cool would be less than when they are warm which would tend to lower engine speed. A mixture that is rich will suppress idle speed since there isn't enough air to provide efficient combustion.

Your idle jet sizes are probably OK at 55.

I assume you line in Ontario, Canada which provides more opportunities for chilly highway runs than more Southern climates.

[Irish911]

Thanks Paul. I was going to e-mail you this question but thought I would post it here and let you get some work done after taking up so much of your time earlier with my other carbs. I'll keep the 55's until I pull these off in the fall.

Your description would explain why the rpm's return to normal once getting off the highway and doing some city/town driving. Is installing insulators between the heads and manifolds typical and what sort of insulators are you talking about? As you say, our spring and fall can be quite cool (like this year, it just snowed today) so driving in cooler temps is probably 30-40% of my driving.