RPM is dropping from 1,100 to 600-500

[jalexquijano]

Took my 240z for a spin yesterday and got into heavy traffic. RPM mantained at 1,100 in light traffic but once i started going bumper to bumper, the rpm started to lower to 800 rpm and the raised back to 1100. The car carries Datsun L24 motor with crank and rods from a L26, special oversized 84mm flat top black coated pistons from D.L. Potter Engineering with E88 Head and Far 311 New Race Cam and 2 original SU Hitachi round top carburetors rebuilt reason why it has a rough idle.

Anyway,i would like to mention that i did cancel the EGR system and that the fast idle Actuator was removed as the diaphgram is broken:

Could there be some air escape somewhere on the engine which is causing this RPM lowering. FInally when i reached to my destination point, i parked the car for some minutes at idle. The Rpm lowered to 500 and the car engine shut down. I would also like to mention that i can feel some failure of the engine when speeding up at 2000 RPM.

[madkaw]

68 threads started in less than one year

[jalexquijano]

68 threads started in less than one year

Madkaw,

Im trying to get a solution without changing the Far 311 Race Cam back to the stock cam. Its gonna get really expensive if i need to revert to the original cam. Unfortunately in Panama there is a lot of traffic Jam and heat, you always get stucked in 20 to 30 minutes traffic and with this kind of cam i wont be able to install an AC System to the car.

[Stanley]

Check carefully for vacuum leaks and plug anything threaded that's removed from balance tube with correct BSPT thread plugs?

I would change to 65 series tires rather than the usual 70 series, only my opinion. That will put your rpm at any speed a little higher which might be better around town with the cam. It will also make your speedometer optimistic, but there's a calculator on this site.

I expect it would also drop your top speed by about 10 or 15 mph, and reduce gas mileage at cruising speed (unless you reduce your speed).

Also the smaller tire might not look right unless the car is slightly lowered.

[Zed Head]

heavy traffic. RPM mantained at 1,100 in light traffic but once i started going bumper to bumper, the rpm started to lower to 800 rpm and the raised back to 1100.

2 original SU Hitachi round top carburetors rebuilt reason why it has a rough idle.

when i reached to my destination point, i parked the car for some minutes at idle. The Rpm lowered to 500 and the car engine shut down. I would also like to mention that i can feel some failure of the engine when speeding up at 2000 RPM.

Doesn't this seem more like the classic vapor lock from the fuel supply lines? Hot weather, original carbs, probably mechanical fuel pump.

[jalexquijano]

My car has a recently installed Electric Fuel Pump! So this should not be the issue! I believe it must have a vacumm leak somewhere which is causing this rough idling.

Any clues? How do i check for vacuum leaks? I did indeed removed the EGR system as the Servo Diaphgram was broken and thus avoid any leak. I attaching a picture of the system as it is.

Edited September 29, 2014 by jalexquijano

[HuD 91gt]

Sounds like a vacuum leak. But I am no expert. I've heard of people using propane to detect leaks, if you spray it near a leak, the isle should technically smooth out. Once the propane is removed, your idle issue will return. Any connection in which you recently disconnected, make sure the connection is tight, and the hose is in good condition.

I know nothing about deleting the EGR, but some of that vacuum tube routing looks sketchy. Including the bolt at the end of one of those tubes.

Edited October 1, 2014 by HuD 91gt

[siteunseen]

I don't see a fuel filter. There's 3 on my 240. Clogged carburetors cause a rough idle.

[siteunseen]

What PSI is the "recently installed Electric Fuel Pump! So this should not be the issue!" set at? You know, the fuel pressure regulators that come with " I want perfect and powerfull flow for my car."

[madkaw]

Really no reason for your car to idle that fast in the first place, the cam is not that big. You probably have a vacuum leak as suggested. Search you tube for troubleshooting vacuum leaks

[jalexquijano]

Really no reason for your car to idle that fast in the first place, the cam is not that big. You probably have a vacuum leak as suggested. Search you tube for troubleshooting vacuum leaks

Madkaw,

 

the only way to check for vacuum leak is spraying carb cleaner? Maybe there could be a leak under the car from the manifold or the exhaust? Please confirm