FireSKip17 Posted December 18, 2023 Share Posted December 18, 2023 Helping a friend with an antique fire engine. I can not tell what size this Magnadyne engine is, attached are photos of the block stamping. We can not get it to idle with the choke off. Will only run with half choke or with a 1/4 or more throttle. Had the carb rebuilt. Going to do the ignition system next. Slowly been burning out the old gas to try fresh fuel. I disconnected the vacuum line first thinking a leak but that did nothing. Sprayed down the intake manifold with carb cleaner to listen for an engine change but nothing. Only adjustments I can see on the carb are the 2 idle mixture screws and that's made no difference either. As soon as you put the choke in and off the throttle it dies. Any thoughts on what else I maybe missing.? What is the device between the carb and the manifold, I assume some type of governor? Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/ Share on other sites More sharing options...
mrsmackpaul Posted December 18, 2023 Share Posted December 18, 2023 Sounds like you have a vacuum leak Out with some WD 40 and spray it about the manifold mating surfaces and joins etc and if the engine revs change your getting close to solving the problem Make sure the carby base is flat and square Mums glass chopping board from the kitchen and spray some glue on and stick a sheet of fine-ish sandpaper to it and rub the base of the carby in a figure 8 motion and this will soon show up if it is warpped If it runs fine with the choke part out it won't be spark as spark is working okay Paul 1 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543686 Share on other sites More sharing options...
FireSKip17 Posted December 18, 2023 Author Share Posted December 18, 2023 I sprayed carb cleaner all over it and couldn't get any rpm changes. Was frustrating. I soaked the back of the carb and the top half of the manifold where it bolts to the block Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543688 Share on other sites More sharing options...
mrsmackpaul Posted December 18, 2023 Share Posted December 18, 2023 If there is no vacuum leak then your idle jet is blocked There really is not much else on a really basic motor to stop it idling Paul 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543708 Share on other sites More sharing options...
FireSKip17 Posted December 19, 2023 Author Share Posted December 19, 2023 That was my thought too. But we stripped and had the carb rebuilt. Adjusting the idle mixture screws doesn't seem to do anything. Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543712 Share on other sites More sharing options...
mrsmackpaul Posted December 19, 2023 Share Posted December 19, 2023 Is the butterfly fully closing on idle If the idle is wound up to high the idle mixture will have no effect The butterfly should be pretty much fully closed at idle to make the idle jet have any effect at all If carb is spotless And the butterfly fully shut As a general rule of thumb, wind the mixture screw fully in and back out 1 1/2 turns as the starting point On just about every motor I have ever worked on, this gets them running good enough to start tuning them I still would be thinking you have a vacuum leak Does it have a power booster for the brakes if it has hydraulic brakes ? The diaphrams on these perish and cause vacuum leaks Pretty hard to be of much help when I'm in Australia and not standing beside the truck But I'll try and help if I can Paul Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543713 Share on other sites More sharing options...
FireSKip17 Posted December 19, 2023 Author Share Posted December 19, 2023 (edited) I'll check the carb butterfly's. The idle screws are set to 1.5 turns. I unhooked the vacuum line and plugged it to eliminate the possibilities of leaks in the vacuum system. Could the governor play any part in this issue? Edited December 19, 2023 by FireSKip17 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543718 Share on other sites More sharing options...
mrsmackpaul Posted December 19, 2023 Share Posted December 19, 2023 Also assuming you have vacuum advance on the dizzy If so the diaphragm here can leak Paul Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543731 Share on other sites More sharing options...
FireSKip17 Posted December 19, 2023 Author Share Posted December 19, 2023 No vacuum advance Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543732 Share on other sites More sharing options...
HarryS Posted December 19, 2023 Share Posted December 19, 2023 Years ago I had a Diamond T with a Hercules engine that had a large Zenith carb with a governor between it and the manifold. It had your same symptoms. I ended up hollowing out the governor and blocking all the vacuum holes and ports. It was then just a spacer. It ran fine after that. One other thing to check is the fuel pressure. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543751 Share on other sites More sharing options...
FireSKip17 Posted December 19, 2023 Author Share Posted December 19, 2023 (edited) Thanks. I'll drop an inline gauge on the fuel line but I feel it's good. I'll pull the governor and see what I can find. I absolutely soaked the intake manifold and got nothing. Edited December 20, 2023 by FireSKip17 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543767 Share on other sites More sharing options...
JoeH Posted December 20, 2023 Share Posted December 20, 2023 Far outside my expertise, but we have an old Oliver white tractor/forklift that we replaced the carb on a couple years ago and just hooked the governor back up a couple months ago. Man, it idles so much smoother now that it self regulates it's idle speed. I would look into your governor mechanism if all else checks out. Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543805 Share on other sites More sharing options...
JoeH Posted December 20, 2023 Share Posted December 20, 2023 Also, you don't have your air cleaner attached in those pictures, so you don't have as much vacuum pressure at the jets without air filter restrictions. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543806 Share on other sites More sharing options...
FireSKip17 Posted December 21, 2023 Author Share Posted December 21, 2023 On 12/19/2023 at 8:17 PM, JoeH said: Also, you don't have your air cleaner attached in those pictures, so you don't have as much vacuum pressure at the jets without air filter restrictions. I get no difference with the air cleaner on 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73664-1941-mack-a40-magnadyne-idle-issue/#findComment-543887 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.