Poor idle when hot

My first post. I recently received my BN1 from a ground up restoration. It is intended to be a driver. After a few teething problems I still have 2 outstanding. The cockpit heat (I can live with) and bad idleling when engine is hot in traffic. Many things have checked out ok, my mechanic thinks another session with the carburettor guy might be the answer. I was wondering if there is something less obvious. The car has done 1000 Ks since restoration including the engine. Overheating is resolved. An electric fan comes in at 70 and runs at 80-90. RPM at idle 950-1000, when hot rpm drops to less than 500 then cuts out. In traffic I need to keep throttle open to avoid stalling. I a week I will be attending the national rally (300K) and I would like to have this resolved.
Thanks, Peter

Hi Peter, I am not an expert on tuning but I have done all my own tuning for 40 years with satisfactory results. I believe on really hot days that your healey should usually still run well without aftermarket cooling accessories. Here in Pa in really hot summers (day time temps in the high 90s my Bj7 temp gauge would reach 210*, 220* sometimes230*) and still it idled well. The lowest around 600rpm. My standard idle spd was 800 - 850 rpm. I am running HD8 carbs which are bigger than standard. So if your carbs are acting up my first question is are you blowing black smoke. (not blue smoke but black smoke). If your car is puffing black smoke at idle and maybe when you shift gears, then the carbs are too rich. My carbs are just alittle too rich and show a very faint hint of black smoke when it idles slow on hot days but not enough to affect any driving quality. If the engine is new and maybe alittle tight, I believe after 1000 miles it is ready for its last tune and adjustment. Give it one more go around with the turner, if it isn't better, find another mechanic. Also I would have them retorque the head and then they will also need to readjust the valves. Good Luck, Dave. Ps: Conversely, extremely lean carb adjustment can give you the same symtoms and will contribute to alot of engine heat as well as damage the engine. SU carbs are not that difficult, they should be able to get it close enough.

This is (mostly) a SWAG, but it sounds like fuel in the float bowls is getting too hot, possibly even boiling. What is the ambient temperature (last I heard, it was pretty warm Down Under)? If your car accelerates well and runs OK at speed, and isn't blowing clouds of black smoke the mixture setting is probably in the ballpark.

The BN1/2s only have a token heat shield--the 6-cyl cars have a much more substantial shield over the exhaust manifold--and it sounds like your carbs are getting too hot sitting with no airflow through the engine compartment (my BJ8's idle will drop some when the coolant temp rises sitting still on hot days). Your best long-term remedy may be to get a ceramic coating on the exhaust manifold and downpipe. You could try wrapping the downpipe with header tape or fashioning some sort of heat shield under the carbs--that may help a bit (you could at least try it to see if that is indeed the problem).

Could this be caused by a carburetor piston sticking? Pull off your air cleaners and check to make sure both are falling smoothly all the way down with an audible click.

We have several 100-4 owners in SoCal who have added bilge blowers in front of the carbs to blow air at them. This fixes the heat/vapor lock problems.
Some folks are also blocking up the rear of the bonnet to increase the airflow.

Thank you for the reponses. I have discussed these with my mechanic. The engine has been 'tight' and it was not until the electric fan installed was I could get some kilometers up. Yesterday (hot) I was able to do 120k on the local motorways (not cheap). There was no problem until the end in the traffic near home. The mechanic and I have concluded it is most likely vapour lock. Since stop/start traffic is unlikey on the way to the ralley I will risk it. On return we will investigate more but ceramic coating will be short listed. I had been thinking of a bilge fan for air flow in the cabin, I imagine 2 would be double the cost.
Thanks again
Peter

PeterBL said:

Thank you for the reponses. I have discussed these with my mechanic. The engine has been 'tight' and it was not until the electric fan installed was I could get some kilometers up. Yesterday (hot) I was able to do 120k on the local motorways (not cheap). There was no problem until the end in the traffic near home. The mechanic and I have concluded it is most likely vapour lock. Since stop/start traffic is unlikey on the way to the ralley I will risk it. On return we will investigate more but ceramic coating will be short listed. I had been thinking of a bilge fan for air flow in the cabin, I imagine 2 would be double the cost.
Thanks again
Peter

Click to expand...

The bilge fan I installed on the front of my fresh air duct is very noisy - like a hair dryer. I'd do one for the carbs then see what you think. Just my 2 cents.

My BN1 has a slowing idle when hot, just like yours. It has done that for years. I found the best solution was to configure the choke so it brings up the fast idle cam a bit before it pulls down the jets to enrich the mixture. This way the choke cable serves as a trim on idle speed -- pulling it out slightly brings that slow idle back into the 700-1000 RPM range.

This is a work around, not sure what causes the drift in idle. But I have driven other four cylinder Healeys that do it too. It's very easy to get used to making this little adjustment. You need a choke cable that can be twisted and locked in any position.

Good luck

Bill