Return to the home page...

Go Back   Moyer Marine Atomic 4 Community - Home of the Afourians > Discussion Topics > Troubleshooting

Reply
 
Thread Tools Display Modes
  #1   IP: 69.72.50.34
Old 11-19-2005, 07:09 AM
Unregistered
Guest
 
Posts: n/a
Question Erratic Engine Speed

Having just put in new coil, plugs, points, HT leads and fixed the source of 3 years of heartache (an intermittent break in the 12v wire from the coil to the distributor) I find that the engine will race for a couple of minutes then slow down, race again and so on. Doesn't seem to matter wether the engine is warm or cool. I have not touched the carb, so am discounting that. Any suggestions ?
Reply With Quote
  #2   IP: 69.72.50.34
Old 11-19-2005, 07:11 AM
Don Moyer's Avatar
Don Moyer Don Moyer is offline
 
Join Date: Oct 2004
Posts: 2,811
Thanks: 0
Thanked 183 Times in 124 Posts
Even though you haven’t touched the carburetor, I don’t know of any history of erratic RPM which was not caused by the carburetor.

Assuming that the condition you’re describing is while the engine is at idle, here is a technical note we prepared on the subject:

The following problems within the idle system (listed somewhat in decreasing order of likelihood) can cause erratic or unreliable idling:

IMPROPER IDLE SET SCREW ADJUSTMENT: Your idle RPM may be set – not by the idle set screw – but by the throttle cable itself. In other words, the idle set screw is backed off so far that the lower limit of the throttle cable is reached before the idle set screw. The problem with this scenario is that throttle cables do not have an accurate end point, and they don’t always stay where you put them. If you pull the throttle cable to its minimum position and walk away, it’s likely to spring back up or down a bit, moving the throttle control arm with it.

To check the idle stop, place the throttle lever in the cockpit at idle, then go down to the carburetor (with a flashlight and mirror if necessary) and note whether or not the end of the idle set screw is actually touching the idle stop coming out from the housing of the carburetor. If it is not, reposition your throttle cable so that the idle set screw does touch the stop.

This adjustment will probably result in too slow an idle, and the next time you start the engine it will probably shut down as you slowly move the throttle to idle. So the next (and final step) is to readjust the idle set screw to the idle RPM that you want. In this new setting, the idle RPM will actually be set by the idle stop and not by the random positioning of the end of the cable.

CARBURETOR ISSUES AFFECTING IDLE:

1) Dirt in the idle jet.

2) A small speck of dirt in or near the upper of two tiny outlet ports of the idle system. This is the port (about the size of a fly speck) that you see just off the edge of the throttle valve when looking down from the mounting flange of the carburetor.

3) A faulty seal around the idle passage way between the upper and lower halves of the carburetor housing. This passage way is in the very center of the carburetor housing, and is about the size of a drinking straw. If the gasket between the upper and lower housing is not completely sealed around this passage way, air can be drawn in and break the suction necessary to draw fuel up to the upper idle port.

4) A leaky float valve which provides such a rich mixture that it floods out the engine at idle RPM.

5) There is an air bleed hole in a small brass plug that is pressed into the lower face of the upper half of the carburetor, that sometimes becomes blocked. This plug is located between the idle fuel jet and the venturi tube in the main throat of the carburetor, and can be easily seen when the upper half of the carburetor is separated from the lower half. Sometimes a small speck of dirt lays on top of this small plug and interferes with the air being drawn in through the bleed hole to mix with the idle fuel.

MANIFOLD LEAK: You could have a vacuum leak in the manifold gasket or carburetor flange gasket. Some folks have been successful in diagnosing a vacuum leak in these areas by spraying some starting fluid around the manifold gasket and carburetor flange gasket while the engine is at idle. If the engine RPM changes at all while spraying the starting fluid, the gasket is almost certainly leaking.

Don
Reply With Quote
The Following User Says Thank You to Don Moyer For This Useful Post:
TimBSmith (10-01-2020)
  #3   IP: 50.132.111.15
Old 01-24-2012, 08:10 PM
anian anian is offline
Senior Member
 
Join Date: Dec 2007
Posts: 10
Thanks: 0
Thanked 0 Times in 0 Posts
Idle/rpm control problem

At a fast idle (1500rpm +/-) after reaching operating temperature, engine rpm will momentarily decrease 100-200 rpms at 15 second to 30 second intervals for about 5 seconds or so and then return to the previous rpm. This characteristic is not noticeable at a slow idle of 700 +/-. The throttle control is extremely sensitive in neutral as well; moving the throttle control lever roughly a sixteenth (or less) of an inch causes rpms to change about 200-600 +/-. The only way to increase/decrease rpms in small increments is to tap the throttle lever VERY gently. Under load/in gear these phenomena for the most part disappear and engine control seems normal. Any ideas?
Reply With Quote
  #4   IP: 67.78.241.34
Old 01-24-2012, 10:12 PM
hanleyclifford's Avatar
hanleyclifford hanleyclifford is offline
Afourian MVP
 
Join Date: Mar 2010
Posts: 6,978
Thanks: 172
Thanked 280 Times in 226 Posts
You may have a blockage in the carb idle system. There is a link in the diagrams section for the original A4 Manual which has an interesting detail and discussion of the idle system.
Reply With Quote
Reply

Tags
None

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Moderation Tools:
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Engine Squeal at speed eriesailor Troubleshooting 20 09-11-2010 02:15 PM
Removing the engine from your boat Don Moyer General Interest 13 06-27-2008 11:06 AM
Overheating problems jdaly986 Cooling System 1 01-16-2008 10:32 AM
Engine dies under load. Unregistered General Interest 2 08-20-2005 03:35 PM
Engine dies after it warms toscaindub Troubleshooting 2 08-13-2005 03:39 PM


All times are GMT -4. The time now is 05:18 AM.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2024, vBulletin Solutions, Inc.


Universal® is a registered trademark of Westerbeke Corporation

Copyright © 2004-2024 Moyer Marine Inc.

All Rights Reserved