Engine Doctor Posted October 29 Share Posted October 29 11 hours ago, i12fly said: Sorry bit off topic, but I have an SC 70FS where the valve inserts leak in the head. Only happens when it is hot but enough to cause the engine to stop. Is there any cure other than a new head? No cure im afraid that I know of . A new head, now available again might cure it providing it doesnt have the same manufactuting fault. Good luck hope you get it sorted Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted October 30 Author Share Posted October 30 Bearings replaced and end float reduced to acceptable amount Engine still wont idle below 3500, fiddled with low speed idle but thats not the issue, got fed up, whipped it out and changed it for an os 91 fs i had, immediately did 9600 on a 14x6 and would idle 2500 nicely. Was able to go to field and practise landings with Mustang, much improved Need to pull sc 91 fs carb apart and perhaps try new o rings etc? Hmmm Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 31 Share Posted October 31 I can’t recall if it is a direct copy and will fit straight on, but you might answer a few questions by trying the OS carb on the ASP… Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted October 31 Share Posted October 31 17 hours ago, Simon Lumsdon 1 said: Bearings replaced and end float reduced to acceptable amount Engine still wont idle below 3500, fiddled with low speed idle but thats not the issue, got fed up, whipped it out and changed it for an os 91 fs i had, immediately did 9600 on a 14x6 and would idle 2500 nicely. Was able to go to field and practise landings with Mustang, much improved Need to pull sc 91 fs carb apart and perhaps try new o rings etc? Hmmm Does your SC have any real compression when warm ? tested with throttle fully open . Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted October 31 Author Share Posted October 31 Its on the test bench (upright) ready for the weekend and some more running in and testing. Compression seems fine, my moneys on the carb … Quote Link to comment Share on other sites More sharing options...
John Wagg Posted October 31 Share Posted October 31 1 hour ago, Simon Lumsdon 1 said: Its on the test bench (upright) ready for the weekend and some more running in and testing. Compression seems fine, my moneys on the carb … A different or same plug ? Quote Link to comment Share on other sites More sharing options...
RottenRow Posted October 31 Share Posted October 31 On this series of engines, if someone overtightens the two bolts that hold the inlet tube onto the cylinder head, it can cause the threads in the head to pull outwards, meaning the mating face is no longer flat. The inlet flange can also get distorted. Air can then get drawn in, even with the gasket fitted, messing up the mixture. Check that the faces of the head and the inlet tube are both dead flat. Will the engine idle better with the glow plug still powered? If it does it’s a good indication that the mixture setting is rich. Brian. Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted November 2 Author Share Posted November 2 Well ran it on the bench this morning and end float issue resolved, only normal float now spent some time dissling with the low speed idle and could just about get it down to 2800 revs, wot gives 9100. compression seems fine to answer a prev question. Will give it another chance in a model but wondering if inverted position is a factor. Loathe to use an onboard flow Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted November 2 Author Share Posted November 2 (edited) Edit.mp4 Edited November 3 by Martin Harris - Moderator File format changed plus a little housekeeping Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 2 Share Posted November 2 Just to be a little boring, but I would always do the measurement from behind the prop - the leading edge of the prop even just touching a knuckle would be bad enough, with the consequences of a full blown prop strike at full rpm being horrendous. Bruised knuckle from behind...hospital visit from in front...unless of course it's a pusher prop. I have the scars from 1976 to remind me how dangerous they can be! 1 Quote Link to comment Share on other sites More sharing options...
Jon H Posted November 2 Share Posted November 2 I think the forum video player is broken as it keep saying it cant find the file when clicked on. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 2 Share Posted November 2 I think it's a file format issue - its fine on my iPhone but not on my PC. It's posted as a .mov which is an Apple format. Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted November 3 Author Share Posted November 3 Sorry, tried to save it in a compatible format but failed, not easy is it … Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 3 Share Posted November 3 Changed to MP4 format... 1 Quote Link to comment Share on other sites More sharing options...
Jon H Posted November 3 Share Posted November 3 Slow run needle is too lean. That smooth fast idle is symptomatic of a lean slow run. i assume it wont go any slower if you throttle back? Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted November 3 Author Share Posted November 3 Correct, That was the absolute slowest i could get it to run, screwing the low speed idle clockwise, inwards until it cut. (12x6 prop, optifuel 12% fuel) in comparison the os 91 runs at 9600 max revs and will idle slightly slower but definitely a more reliable idle that the sc. thanks to all for all the help, i find this incredibly useful, so much to learn Quote Link to comment Share on other sites More sharing options...
Jon H Posted November 3 Share Posted November 3 27 minutes ago, Simon Lumsdon 1 said: screwing the low speed idle clockwise, inwards until it cut Is this how you are setting the slow run? If so its not the right procedure. You need to bring the engine to idle, and then firewall the throttle. Normally engines are too rich but they make it to full power. In this case you lean the slow run and go again, and again...until you get to a case where the engine no longer accelerates from idle to full power. This will be a pronounced bog or total cut, and indicates a lean condition. I suspect your engine will bog/cut given how lean it is. Note that the slower you accelerate the engine the leaner you can run the slow run needle and a servo is generally slower than whipping it open with your hand on the bench. I did have a video of me setting up a laser 150 carb but i cant access the laser youtube channel any more so i will have to shoot another one some day. Quote Link to comment Share on other sites More sharing options...
RottenRow Posted November 3 Share Posted November 3 On my SC / ASP 52s and 120s (similar design) I find it’s not possible to accurately set the low idle needle with the engine running at idle speed. Just pushing a screwdriver into the slot in the needle pushes it inwards sufficiently to make the engine too lean, and cut. The entire barrel moves inwards, a very small amount, but that is all that is required to stop the engine. Either stop the engine, adjust the idle slightly (1/16th of a turn) and try again, or increase the speed to well above idle whilst making the tiny adjustment. Then go back to idle speed. Brian. Quote Link to comment Share on other sites More sharing options...
Jon H Posted November 3 Share Posted November 3 55 minutes ago, RottenRow said: The entire barrel moves inwards, a very small amount, but that is all that is required to stop the engine. Sounds like the barrel or more likely the barrel screw is worn. I have come across this many times and old irvine carbs on lasers were notorious for it. A new barrel screw, or even a custom one made to fit your specific carb will work wonders. I have an RCV 120 that didnt idle well at all and a new barrel screw cured the problem almost entirely. Fortunately i have a lathe to do the work, but even if you dont there must be someone locally who can help. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted November 3 Share Posted November 3 The behaviour noted seems to be the norm for the clone carbs and caused by the manufacturer’s tolerances. Unless you have a very light touch, there will always be a significant reduction in rpm as the barrel moves against the internal spring’s tension. I usually set engines at a slightly higher tickover speed while making initial adjustments before fine tuning by the method detailed by Jon. In use, the spring stops this play causing problems while ensuring free rotation. I suppose that if you follow the advice in the instructions to make adjustments with the engine stopped, it isn’t a problem! Quote Link to comment Share on other sites More sharing options...
Simon Lumsdon 1 Posted November 3 Author Share Posted November 3 Hi Jon, sorry, to be clear i am screwing idle in a bit and checking it throttles up ok as you describe. Next step is to replace o rings and manifold paper gasket to eliminate possible air leaks Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.