Thread Tools
Jan 15, 2020, 06:41 PM
Registered User
Thread OP
Hi guys. Thanks for all the input. Today was a great day to fly here in Texas. I fly at home. I got in a couple of flights and since I had my stuff out, I put the engine back on the stand. It is running right now. I have it running very lean. I leaned out the top end then opened the needle one click. Next, I leaned out the low end as far as I could go and still run. If I pinch the fuel line it will just quit. I set it at about 4000 rpm and poked at the throttle barrel to see if I could get it to change any. All is good there. While the engine was running, I tightened the two screws holding the intake manifold to the head. I got them about ľ turn tighter. Anything under about 2700 rpm and the RPM starts to wander. I got it running at about 2500 RPM and used a C-clamp to lock the throttle linkage to the bench. It would still wander in RPM. I think I have a problem with the carb that could be a slight air leak someplace. I don’t think it has anything to do with the CDI. I will say with it running this lean it just sips fuel. When I slam the throttle rod, it now has a slight stumble going through mid-range. My thought is to take the carb apart and replace all the O-rings. Before it quits running right now, I want to squirt some fuel around the seal between the carb and the manifold to see if that causes an RPM change.

Thanks for all the help. I’m open to any ideas you might have.

Pete
Sign up now
to remove ads between posts
Jan 15, 2020, 07:22 PM
Registered User
Pete, | don't know the carburettor but so a question, can you adjust the depth of the position of the spraybar in the centre of the carburetor? Is the spraybar symmetrical at the end or has rotating the spraybar influence?

Cees
Jan 15, 2020, 07:50 PM
Registered User
Thread OP
Cees,
I do not think I can adjust the depth of the spray bar on this carb. I am just about to take it apart.

Pete
Jan 15, 2020, 07:52 PM
Still gassin' it.
Quote:
Originally Posted by modeltronics
Hi guys. Thanks for all the input. Today was a great day to fly here in Texas. I fly at home. I got in a couple of flights and since I had my stuff out, I put the engine back on the stand. It is running right now. I have it running very lean. I leaned out the top end then opened the needle one click. Next, I leaned out the low end as far as I could go and still run. If I pinch the fuel line it will just quit. I set it at about 4000 rpm and poked at the throttle barrel to see if I could get it to change any. All is good there. While the engine was running, I tightened the two screws holding the intake manifold to the head. I got them about ľ turn tighter. Anything under about 2700 rpm and the RPM starts to wander. I got it running at about 2500 RPM and used a C-clamp to lock the throttle linkage to the bench. It would still wander in RPM. I think I have a problem with the carb that could be a slight air leak someplace. I donít think it has anything to do with the CDI. I will say with it running this lean it just sips fuel. When I slam the throttle rod, it now has a slight stumble going through mid-range. My thought is to take the carb apart and replace all the O-rings. Before it quits running right now, I want to squirt some fuel around the seal between the carb and the manifold to see if that causes an RPM change.

Thanks for all the help. Iím open to any ideas you might have.

Pete
Sounds like very possibly some issue with the carb. Possibly the O-ring around the main needle, but just as likely the O-ring around the LS needle.
Also, a small but distinct possibility is some lint in the spraybar opening, causing the fuel to flow pattern in the carb to be disrupted, just to name a few possibilities. Could also be something completely different such as dirty valves, incorrect valve lash or valve timing off, but I would expect other issues as well if that were the case, and you don't mention those, so I guess valvetiming should be OK.

Another thing that is very likely to cause irregular running at idle, is the spring behind the throttle barrel. If that spring is missing, the barrel will move about a little bit, changing the fuel metering of the LS needle, making idle inconsistent.

Because that engine definitely should be able to idle much, much lower. 1800 RPM should be no issue at all on that propsize and using CDI.
Jan 15, 2020, 08:07 PM
Registered User
Thread OP
Maybe I found the problem. There is a groove in the carb barrel that a screw rides in. This cams the carb barrel in and out a little as the throttle is rotated. The grove in this barrel looks slightly defective at one end. This would be the end that the screw would be in at low throttle. It is slightly wider at this end. That would allow the barrel to move in and out more at low throttle. I looked at the barrel in an ASP FS70 and 90. The groove is a constant on those engines. From what I can tell the part number is 70813F. It looks like the carb is the same for the 70 and the 80. Just Engines and Mike Goes Flying does not have the part. Now I just have to try to find the part.

I tried to take some pictures of the defect. I had the camera very close to the part and the pictures came out bad but maybe you can see what I am looking at.

Pete
Jan 15, 2020, 08:21 PM
Registered User
Is there a spring inside that pushes the barrel in one axial direction?
Jan 15, 2020, 08:23 PM
Registered User
Thread OP
Yes. The spring pushes it outward.
Jan 15, 2020, 08:25 PM
Registered User
Seems complete but maybe the pressure isn't enough. Vacuum can suck the barrel inwards
Jan 15, 2020, 08:30 PM
Registered User
Thread OP
The problem is the groove in the barrel. One side of the groove hooks toward the throttle arm at the low speed setting. The other ones I've looked at don't do that.
Jan 15, 2020, 08:33 PM
Registered User
Yes but when spring pressure is enough, the barrel will follow the grove on the other side and that seems to be okay, isn't it?
Jan 15, 2020, 08:38 PM
Registered User
The vacuum in the carburettor at idle can suck the barrel inside direction in the carburettor housing, the spring has to prevent that.
Maybe enlarge the force of the spring a little bit?
When running the engine and wandering, pulling on the throttle lever maybe shows you the result.
Last edited by Taurus Flyer; Jan 15, 2020 at 08:49 PM.
Jan 15, 2020, 08:49 PM
Registered User
Thread OP
I was thinking the defect in the groove is what is causing me to vary in RPM at low throttle. If the spring is doing its job then I shouldn't have that problem even if the groove is not right.
Jan 15, 2020, 08:50 PM
Registered User
See my last post above.

The vacuum is varying.
Jan 15, 2020, 09:00 PM
Registered User
Thread OP
Got it. I can try that. Easy
Jan 15, 2020, 09:14 PM
Registered User
That's right Pete, I don't know the carburetor but when the rpm is wandering and the spring is in most expanded position it can be the cause.

03:20 in the Netherlands .... success
Last edited by Taurus Flyer; Jan 15, 2020 at 09:20 PM.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Percentage weight penalty convering from I/C to electric oneday Electric Plane Talk 13 Jun 02, 2017 04:28 PM
Discussion Lazy cell. OPtion to conver 4S to 3S jferfre Batteries and Chargers 3 Apr 18, 2016 05:55 AM
Discussion HP/Fuel Consumption Tests: FA180, FA180 CDI, FA180HC CDI BBC SrTelemaster Engines 18 May 07, 2013 03:35 PM
Convering for beginners khaoz Beginner Training Area (Aircraft-Electric) 8 Oct 03, 2005 04:25 PM