My engine develops a shake when in gear once it reaches operating temperature. The engine is 1971 340, with a Holley classic 600 CFM vacuum secondaries carb. I have been running the engine for the past 3 years and just recently developed this issue. When the car is in park or netural it idles fine. Any ideas what I should check out to troubleshoot the issue would be great.
Engine Idle Shake
- Thread starter ant72
- Start date
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Is the rubber in the motor mounts soft? They can get really soft if exposed to a lot of oil. What rpm does it idle at? Might just need to bump it up a hair. Could also be timing related.
I'll check the motor mounts, in gear the idle is 700 RPM out of gear 1000. I have the timing set at 20 degrees initial. The trans has a 2200 to 2400 RPM stall but it may be to tight for the cam in the car, which I don't know what it is. The motor makes between 12 - 13 Hg out of gear and drops down to 7 - 8 Hg in gear.
Frank Mopar
Well-Known Member
Could be as simple as a spark plug change.
That's a large drop in rpm.
I agree with check the spark plugs first. See what they may tell.
My guess, and its just that, a guess...
One possibility is the plugs are carbon fouled, possibly from too rich for some condition, and now its built up.
What cam?
And at what rpm was the 20* initial measured at?
I don't know what cam, engine was already together I never pulled it apart. I did change out the spark plugs, no change. The RPM is at 1000 when I measure initial timing.
pishta
I know I'm right....
That drop with that convertor sounds drastic. I had a 284/484 cam in a 340 with a 2800 stall convertor and I got the idle at 700 in and 800 out of gear. you may have a vacuum leak causing a lean issue. Did the idle rise without reason>
The idle remains consistent in and out of gear.
What did they look like? That's always useful to check.
What else can you observe? Smell the exhaust, listen to the engine - these are all clues - even if they mean little right now.
Some specific suggestions.
When it 'warms up' is this when the choke fully opens (it is opening, yes?) or when the coolant termperature opens the thermostate (around 180 F)?
If its the initial, it could be the choke opening too quick for the situation.
If its in the coolant and oil up to operating termperatures, then maybe lifters or something. Using a vacuum gage onto the manifold vacuum port of the Holley will show how strong its pulling and if there are variations (pulsing) due to misfires.
As far as the drop in rpm placing it into gear - its probably too lean and too far advanced.
It could be a deteriorated vacuum cap or PCV hose or something like that. It could also be dirt of varnish in the carb.
Before blathering on, its worth mentioning that most distributors are set up to start advancing around 600 to 700 rpm. So keep in mind that while you measured 20* BTC at 1000 rpm, at 700 rpm its probably less than 20*.
See if you can get the engine to idle at lower rpm in neutral by richening the idle with the mixture screws, and then slightly reducing the idle speed screw.
Then try it in gear.
If the mix screws have no effect, then either the throttle at idle is too open, or the passages are blocked with dirt/varnish, or the fuel level is off - probaly too high and fuel is dribbling out the boosters.
Should I adjust the timing with the car in gear?
I would not. Too dangerous.
Even checking the timing in gear is best with two people or a bulletproof method of ensuring you don't get pinned to a wall or something like that.
My suggestion was meant to say make the adjustments, and then check for the drop in rpm and vacuum in gear.
See if you can get the engine to idle at lower rpm in neutral by richening the idle with the mixture screws, and then slightly reducing the idle speed screw.
Then try it in gear.
If the mix screws have no effect, then either the throttle at idle is too open, or the passages are blocked with dirt/varnish, or the fuel level is off - probably too high and fuel is dribbling out the boosters.
Even checking the timing in gear is best with two people or a bulletproof method of ensuring you don't get pinned to a wall or something like that.
My suggestion was meant to say make the adjustments, and then check for the drop in rpm and vacuum in gear.
See if you can get the engine to idle at lower rpm in neutral by richening the idle with the mixture screws, and then slightly reducing the idle speed screw.
Then try it in gear.
If the mix screws have no effect, then either the throttle at idle is too open, or the passages are blocked with dirt/varnish, or the fuel level is off - probably too high and fuel is dribbling out the boosters.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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I'm liking this.
The tune is definitely off. But it might have nothing to do with the rough idle.
unless it only started right after an oilchange
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