Surging suspect carb issue

whydahdvr · Post by **whydahdvr** » Thu Jun 02, 2011 9:47 am

Ah. I see what you mean now. An issue I'm dealing with, and am wondering if these two items are related, is surging in my engine. We'll get the idle adjusted, it sounds and runs great for a bit and then starts to rev up and then almost die out. I'm not sure what the issue is. We've played with the fuel mixture, the idle, it seems to run fine.... But.... I wonder if there is an issue with the fuel pump supplying a steady enough flow of fuel. Or is it something else? That's why I thought maybe there was a vacuum leak of some sort or this cross-over tube topic might be related.
If anyone has experienced something similar I'm interested to hear. I haven't pulled the fuel pump yet either and the float seemed right when it was set a couple years ago. But that might do it too, correct?

Hello, Moderator here, I split your new discussion from Reddog's thread to keep the topic specific.

Cacti_Ken · Post by **Cacti_Ken** » Thu Jun 02, 2011 11:09 am

Has your engine been running fine before? Or are just now trying to get it running after a restoration? <<Which was my first impression. But if it was running fine before then I doubt the carb vent has anything todo with the surging.

whydahdvr · Post by **whydahdvr** » Thu Jun 02, 2011 12:10 pm

It's part of on-going work. It ran decently in 2008, but I have only really had it on the road, aside from running in my workshop here and there, this year. It was something I was aware of before I deployed in 2009 but it has only been this year that I've started to identify and deal with the problem. It really came out during the St Patrick's Day parade this year.
Anyway, just not sure what's causing the issue.

Cacti_Ken · Post by **Cacti_Ken** » Thu Jun 02, 2011 4:55 pm

I am no expert on these or anything else. but if I were having any of the two problems noted I would do the simplist thing first. Take the caruretor apart and inspect it. Clean it up and install a new kit.

whydahdvr · Post by **whydahdvr** » Thu Jun 02, 2011 7:07 pm

Good suggestion. I had that done when I got the jeep in 2008. I had a mechanic familiar with carb's and older vehicles work on it while I was away. One suggestion I got was to run a carb cleaner in it and a fuel cleaner and see how that affects the surging.

Cacti_Ken · Post by **Cacti_Ken** » Thu Jun 02, 2011 7:23 pm

Did the mechanic install new diaphrams? It could be that the material they are made from is low quality. I've only read about this subject, and have not experienced any carb problems. Knock on wood!

Post by **wesk** » Thu Jun 02, 2011 8:03 pm

Before repairing, replacing or rebuilding anything you should check the basics first.

1-Remove gas cap and run the engine to see if the problem is still there. If the problem goes away you have a plugged tank vent system.

2-Check both fording valves if installed to insure they are in the NON-fording position.

3-Using fuel pressure gage check output of fuel pump.

4-Remove PCV valve and check it for correct operation.

5-Check all sections of the induction system for blockage or leaks.

6-Check exhaust system for blockage.

7-If you have the dash mounted hand primer pump check it for proper operation and air leaks.

oilleaker1 · Post by **oilleaker1** » Sat Jun 04, 2011 6:29 am

If it runs fine under throttle, and only surges at a idle, I had this problem with a new kit installed. By spraying carb clean as a fuel, into the flex tube leading to the carb, the idle would even out. Ended up a blocked hole on the idle circuit pickup tube (long brass one) that is pushed or pressed into the carb top cover. It has two very fine holes on the side of the tube. Got to be clean!

John

whydahdvr · Post by **whydahdvr** » Wed Aug 24, 2011 5:30 am

OK: I had the fuel pump rebuilt by Then and Now and rebuilt the carb with a kit from DeBella's. I got it all put back together last night, started, and it seems to run just fine now - no surging.
The exhaust seems more sooty than before which means it's probably running rich. I haven't gotten it out on the road yet though to see how it does. But big improvement. Clockwise on the fuel-air mixture screw means more air, less fuel? Or is it vice versa? Last night when I was looking through the Operator Manual I didn't see it elucidate that point but probably missed it.

oilleaker1 · Post by **oilleaker1** » Wed Aug 24, 2011 6:07 am

Turn it in and out until you get the best/fastest idle. I think it meters fuel that is sucked up into the long brass tube that is tied into the idle circuit. If it speeds up too high, turn the stop screw on the throttle shaft out or down a bit, and then go and re-adjust again for fastest idle. Make sure choke cable is adjusted to be all the way open for normal running. John

Post by **wesk** » Wed Aug 24, 2011 8:43 am

In the YS carb air and fuel are mixed together in the low speed circuit before the idle screw. So turning the idle screw in will reduce the amount of air/fuel mixture allowed to enter the venturi thus reducing the idle mixture while not actually changing it's air/fuel ratio but it does have a slight leaning affect on the overall transition and midrange fuel/air mixture.

The M38A1 metering rod inadvertantly installed in the M38 carb will result in a rich mixture as will a leaking metering rod diaphragm.

whydahdvr · Post by **whydahdvr** » Wed Aug 24, 2011 11:40 am

Because I'm slow, turning to the right effectively makes the mixture leaner? I am going to assume (I know....) that I have the correct rods and springs in there as I followed the diagram as best I could.
According the manual, the next issue might be the float level.
I'll see what I can do with the two screws - idle and throttle.
Thanks.

Post by **wesk** » Thu Aug 25, 2011 12:29 am

The reason I point out the metering rod mixing is because it is very common. The AJP/MWM kits come with both the M38 and the M38A1 parts so grabbing the wrong rod is easy to do and either rod fits in either carb.

For kits that came with parts for only one carb again it is easy to drop the wrong rod in the kit wherever the kit was filled.

The only way to know for sure which rod you put in is to read the part number on it.

Clockwise on the fuel-air mixture screw means more air, less fuel? Or is it vice versa?

In my post above I explained that turning the screw in and out does not change the ratio of air to fuel but only increases or decreases the amount of the already mixed air/fuel charge. Many carbs use the screw as an air screw and some use it as a fuel screw but the YS carbs use it to regulate the supply of fuel laden air that has already been correctly mixed. May be more info then you wanted but you did ask how the screw functions.