MGB & GT Forum

The subject line says it all: "RPM drops below idle when stopping then (sometimes) recovers."

My newly, beautifully rebuilt SU HS4 carburetors are balanced with idle set at 1,000 RPM. After a full warmup, the engine idles nicely (with maybe a little "chuffing" from time to time. Acceleration is good then with no flat spots. The problem shows its ugly self when I come to a stop. Letting up on the gas fully often causes the RPM to drop below idle -- like down to 500 RPM -- then recover (well, sometimes -- a stall is not uncommon. Plus, "hunting" for the normal idle speed occurs, too.

Also, I suspect that my fast idle setting is incorrect. Please review the proper adjustment.

Many thanks!

Craig
Washington, DC or thereabouts (not my fault - I was sent here)

Drop when coming to a stop, doesn't sound like a fast-idle or carb issue if the car is fully warmed up and choke is off. Leave the carbs along for now, futzing with them will just complicate diagnosis.

Sounds more like a vacuum leak somewhere or something going one with the vacuum advance unit. Could also be a mechanical issue with the mechanical advance inside the distributor not returning fully at low rpms. Could also be a broken or worn mechanical advance spring (there are two under the plate). Also, the plate could be hanging due to a mechanical issue or the points/trigger hold down screws are binding the advance plate. If you turn the distributor shaft, it should twist about 5 degrees and spring back.

Quick check, draw a vacuum on the vacuum advance, if it holds the vacuum the diaphragm is okay, if not the diaphragm is bad. Try making a test run with the vacuum advance line plugged, that could tell you if its hanging.

Start by checking closely for vacuum leaks on the intake manifold, vacuum operated devices and any plumbing associated with the manifold. Plug off all vacuum connections to the manifold, with warm engine idling, make small sprays of a non-flammable solvent to all joints and fittings between carbs and head, if the engine stumbles you have a leak at that point.

Are you SURE your gas tank is vented, if not test drive with the gas cap off to see if the problem persists.

Are you SURE the float bow vents on the carbs are open to atmosphere? If not, pull the vent lines off the carbs and test drive.

I think that HS4 carbs can go lean under deceleration. Try enrichening by one or two flats. I think you may be on the edge of being too lean.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

I agree with Terry. IF "CHUFFING" IS OUGHING. .

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Edited 1 time(s). Last edit at 2024-02-18 10:30 PM by oleanderjoe.

Thanks, fellas!

I certainly don't suspect my Harry Singleton-rebuilt carburetors. His excellent work and advice cured several issues.

Here's an additional bit of possibly pertinent information: the tendency to stall lessens -- but does not disappear altogether -- as the engine warms up fully. Also, the problem pre-dates the carb rebuild.

I'll try Terry's mixture adjustment tip first 'cuz -- well -- it's easy. If that doesn't work, I'll take Clay's thorough, comprehensive troubleshooting steps.

Thanks again!

Craig

Why 1000rpm idle?

There are variations across years which I honestly can't be bothered to look up but normally it's is in the range of 600-750.

If you need 1000 to maintain an idle then something is very wrong.

Does your car have a brake servo (power braking)? If so, you could have a vacuum leak there. Which, in turn, affects the carbs on deceleration.

Harry rebuilds them, but you still need to adjust mixture, balance, and idle speed. He just makes them similar. Your job is to tune them after the installation.

Idle quality is a combination of fuel and spark tuning. What is your timing set at? Does the timing light show a rock steady timing mark at idle or does it waver? Stretched out timing springs or a loose timing chain tensioner can cause a drop in rpms at idle.

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jeff@advanceddistributors.com

That depends. When my engine was rebuilt in 2011, a Moss "Street Cam" was installed and ever since, it doesn't like to idle below 1000 RPM. I've put almost 75k miles on it with no issues. Runs great.

Classic symptoms of water in the fuel. On decel, water, which is heavier than fuel and therefore collects on the bottom of the float bowl, can slosh to the pickup point, causing the engine to stumble and, perhaps, die. At road speeds with higher engine speeds there's enough fuel being picked up to sustain decent performance. Suggest you look carefully for water in the fuel...

Dick

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Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Should also mention that these tachs were often not well calibrated. My car's tach always read 1100 RPM when the actual RPM was 900 (the specified idle on 18GK). So a 1000 RPM idle might not be unusual and might not actually be 1000 RPM.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

High lift or longer duration cams require a higher idle. 1000 is normal enough even for a stock cam.

The stall and cough at idle is running too lean. When rebuilding a carb one may set the jet level to an accepted starting point, but it is just a starting point. Easy enough to find out.

GMc

Terry makes a good point. The 1000 RPM to which I referred is from the car’s tach. According to my external meter - a vintage Hesthkit Engine Analyzer - the actual speed is 850 RPM (or is it?).

Craig

Just under 1000 RPM is a happy motor. 850, is WAY too low.

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IN ALL SUBJECTS.: For those who believe, no proof is needed. For those who don’t believe, no proof is possible.

"Strive for perfection in everything. Take the best that exists and make it better. If it doesn't exist, create it. Accept nothing nearly right or good enough"
Sir Henry Royce
co-founder of Rolls-Royce .

"I take no credit for my own, of another man's fame".
Joe Baba 2021

Hmmm. Interesting thought. Thanks, Dick.

Craig