Tune-up Question

[Bucolic]

OK, I got my 151A1 running after working on it for about 18 months and it didn't run in that time. It started easily enough but there are a couple of issues with it. At a fast idle it backfires. What would cause that? I pulled the carb and cleaned it up. No varnish in the float bowl. Diaphram was replaced. Gas is 94 octane and fairly fresh. Before I started it I pulled the plugs and cleaned them up. I checked the distributor cap and points, all looks good there. I just can't figure out why it pops and backfires. Any ideas?

[rickf]

Is the backfire from the intake or exhaust? Is it doing it while idling or when you hit the gas? Does it have an audible exhaust leak? Is the backfire intermittent or every rotation of the engine? Need a bit more information before I can begin to diagnose.

[Bucolic]

OK, I haven't driven it yet but I've run it parked in the driveway. There doesn't seem to be any exhaust leaks, I sprayed brake cleaner on the carb, hitting all the seams, anywhere there's a gasket and I sprayed the intake manifold as well as the exhaust manifold and the joint where the exhaust pipe attaches to the manifold. No changes in the engine speed. The backfire is coming out of the tailpipe and it's most pronounced when I run the engine speed up and hold it, I guess it was around 2500 to 3000 rpm. (I used brake cleaner because that's all I had that was flammable in a spray can.) As far as the backfire being intermittent or not, it's a steady popping but not very loud with an occasional loud one when I back off.
I hope this helps.
Would changing to a different ignition be a good idea. There's a thread here wherein they talk about a Swiss Controls Module, should I maybe look into getting one?

[rickf]

From what you are describing it sounds like a lean misfire which in turn causes unburned gas into the manifold which is lit off by either oxygen from a leak or possibly the lean condition. At this point I would just wait until you are ready to drive it and see if it feels like it is running lean. As in falling flat at cruise speed or really bogging at light acceleration. If that s the case I would say you have an intake manifold leak. I have not seen many brake cleaners that are very flammable, not saying they are not out there. For that test it rally does not have to be flammable but it helps, it is a more noticeable change in the engine with something flammable. Remember, not all carb cleaners are flammable either. try setting the hand throttle so the idle speed is above idle but not real fast, maybe 1200 rpm and then try the spray test. If it is running to fast you will not pick it up because the change will happen to quickly and be gone. If you have points then be sure the condensor is good, that will give a misfiring and or backfiring condition. I will never tell anyone it is a bad idea to change to electronic ignition, just know that it may not solve the problem. It will most definitely be a step up going forward though.

[SturmTyger380]

You might like to check the valve adjustment for the correct lash / gap. An exhaust valve that is not fully closing will allow gas on the intake cycle to flow through to the exhaust side. Then it might ignite in the exhaust system. Also this would cause loss of power under load. On my M38A1 rebuild I set the valves and it ran fine for 3 months and started loosing power. I readjusted the valves and more power and no popping.

[Bucolic]

I've been reading the TMs for the 151s regarding tune-ups and there are numerous references to a distributor adapter. It seems it's used in conjunction with a dwell meter, a tach and/or a timing light. Can someone tell me what this adapter is? Is it something that's easily fabricated? Do I really need one?

[Fil Bonica]

It is an adaptor that screws in to the distributor cap after you remove the large hex nut.
As you are aware it will allow you to connect to the ignition coil terminal.
If you are careful you can do the test with a simple probe.
If you have electronic ignition the test is not necessary.
If you really need the piece and promise to return it you can borrow mine.

Fil Bonica

[Bucolic]

It looks as how I do not have a points type ignition. There's no condensor and no visible points so I probably don't need to mess with the distributor adapter.

Thanks

[Fil Bonica]

Life just became a bit simpler!

Fil Bonica

[Bucolic]

OK, I took my 151 for a short drive yesterday and it seems to run ok. (It's been about 18 months since it was driven.) I adjusted the mixture screw and it idles fine. But, in order to accelerate I have to feather the gas peddle. I did do a "rebuild" on the carb but maybe I didn't get the accelerator pump assembled right? I replaced the red rubber diaphram, cleaned everything up and put it all back together. Oh, the carb is, I believe, an A-2 emissons Zenith. (The previous owner removed the seal over the mixture screw.) Any ideas?

[rickf]

Did you put the spring on the outside of the diaphragm? It goes between the cover and the diaphragm. Most carbs are the other way around. This could cause the pump not to work which is what it sounds like is happening. And the diaphragm has a hole on one edge that has to line up with the hole in the body.

[Bucolic]

Did you put the spring on the outside of the diaphragm? It goes between the cover and the diaphragm. Most carbs are the other way around. This could cause the pump not to work which is what it sounds like is happening. And the diaphragm has a hole on one edge that has to line up with the hole in the body.

This is what was posted to my inquiry over on the G503 site.
W. Winget » Mon Feb 22, 2016 8:57 am

You did go: end cover, diaphragm, spring into the carb housing in that order?

Who's right? Or does it not matter? Got me confused now.

[muttguru]

This is the correct order:-
Diaphragm cover - spring - diaphragm - carb.

Note that the diaphragm will fit either way round BUT....only one way is correct.The metal core of the diaphragm is larger and cup-shaped on one side. This side faces AWAY from the carb and TOWARDS the diaphragm cover.

Ken

[Bucolic]

Thanks for clearing that up Ken, I appreciate it.
Mike Schnitzius

OK, I went out and removed the carb, pulled the diaphragm cover and I had installed the diaphragm backwards. I reinstalled it correctly, as per Ken's instructions and fired it up. Hesitation nearly gone now. I still need to adjust things as it still backfires but not all the time.
Thanks to everyone that responded.

[america2002]

My best suggestion is to send your carb in to Billy Brooks. I forget what he charged but I want to say it was somewhere in the $150-$200 area but it is well worth it. If you google his name you can watch the YouTube video of his carb. When you send him your carb make sure you let him know about what sea level you will be running it at as he will set it for you. When I got my carb back (I think it took 2 weeks) it looked brand new and even comes with a new manifold gasket. Bolt it on and go. No setting mixture screws or anything. He builds them better then a factory NOS carb as he somehow gets rid of that dead spot when you first step on the throttle. He builds them for our modern crappy unleaded gasoline rather then the older leaded gasoline. And no i do not receive any money from Billy but his carbs really are bad ass.