1275 slight miss, what's the cause???

[vagt6]

My 1275 Midget has a slight stutter or miss at various RPM and I've been trying to diagnose it to no avail.

Today, I synched the carbs and it made a significant difference in power but the slight miss is still there. I didn't mess with the fuel mixture because the plugs look really good.

But still, that slight miss or so it seems. I'm not sure but it seems to miss or stutter just a little at idle, and also at higher RPM. The engine is a tweaked 1275 bored 30 over with fast road cam, roller rockers and Pertronix module in the dizzy. The carbs seem okay, pistons both fall at the same time, carbs are clean.

Again, the spark plugs look <span style="text-decoration: underline">very</span> good: no oil deposits, not a lot of carbon and the porcelain is a nice, white-to-brownish color.

What gives? What could cause the minor miss? OR, am I imagining it? The car runs well, gets good mileage. Do these engines run rough at idle?


Ideas?

 

[jlaird]

Did you check points gap? If they OK check wires tonight in the dark, with engine running of course.

 

[Trevor Jessie]

Be sure your distributor is snug and the pertonix/plate is not loose.
Fresh fuel?

 

[Billm]

Any car with a hotter cam will idle poorly- the hotter the cam the worse the idle (in general).
Your's is probably fine but check what the other guys have said too.
BillM

 

[Gerard]

Check the easy stuff first. Check for dashpot oil and if it's indeed full, try a slightly heavier weight. Also, try richening the mixture a flat or two at a time. Sounds like you are too lean. Also, check for vacuum leaks at the throttle shafts and any other potential location. If you are using a Smith's PCV, check that the diaphragm is still pliable and not cracked, or that a hose in that circuit isn't leaking.

 

[nomad]

I'm a pessimist, check compression. Never know it may be the start of something big. Valve leak, head gasket leak, cracked head, all kinds of nasty things!

Kurt.

 

[vagt6]

Did you check points gap? If they OK check wires tonight in the dark, with engine running of course.

Thanks, Jack. It's a Pertronix module so no points. I will check the wires, though. Good idea.

 

[vagt6]

Be sure your distributor is snug and the pertonix/plate is not loose.
Fresh fuel?

Trevor, good idea. Dizzy is snug and plate is secure.

Fuel is good, I drive the car at least every week or two and keep fresh fuel in it.

 

[vagt6]

Check the easy stuff first. Check for dashpot oil and if it's indeed full, try a slightly heavier weight. Also, try richening the mixture a flat or two at a time. Sounds like you are too lean. Also, check for vacuum leaks at the throttle shafts and any other potential location. If you are using a Smith's PCV, check that the diaphragm is still pliable and not cracked, or that a hose in that circuit isn't leaking.

Gerard, thanks a million for the idea.

Dashpot oil is good. I'll try richening the mixture a flat.

Not sure about the PCV, however, the carbon cannister is probably shot. I do know that if I remove the PCV hoses the car will not run right. I'll post a photo of the PCV setup, maybe someone can tell me what I've got.

Gerard, what's a good way to check for vacuum leaks??

 

[vagt6]

While were at it . . . my "vapor adsorption cannister" (p. 23 in the latest Moss catalog) is a mystery to me. What the heck does it do? My Midget is a a '74, looks like it may be the original adsorption canniser?

The adsorption cannister has three outlets on top: 1) a hose goes from both fuel float bowls over to the cannister; 2) hose #2 goes from the cannister to the front of the car, it's connected to nothing and just points downward; #3 hose (attached to a hard line) goes to the aft of the car, probably to the fuel pump. Also, there's a bottom outlet on the cannister with no hose or anything attached to it. What's up with that? Is the bottom outlet supposed to have a hose attached to something?

Of course, the PCV hoses run from each carb to the carbon cannister on front of the engine. I've never opened or inspected the cannister. Can this cause problems?

Should I service these contraptions? Clean, replace, repair?

Ideas?

Thanks, guys.

 

[vagt6]

<span style="font-weight: bold">Here's the engine bay:</span>

<span style="font-weight: bold">Here's the emission control setup from the carbs:</span>

<span style="font-weight: bold">Here's the Vapor Adsorption cannister:</span>

<span style="font-weight: bold">Here's my dizzy, no vacuum hose: should I plug that vacuum port???</span>

 

[Gerard]

Something like WD40 or stater fluid around the throttle shafts (both sides of each carb) and all manifold joints. If you detect a change in idle speed, that's an indication. You could also use a piece of rubber hose, one end you your ear and the other near potential problem areas. With the engine running you will pick up "rushing" sounds from different things, even the fan whirring, , so listen for a change in pitch, especially if high frequency. It will get more distinct as you get closer to the source.

I sometimes use MM0 in the dashpots, but some engines don't run well with lighter oil and have sometimes cured the miss or stumble by changing to 30 wt or 20-50w.

They canister is just a vapor recovery system to capture gas fumes from the carbs and fuel tank. Connection from a <span style="font-style: italic">restricted</span> valve cover vent would provide some vacuum and should go to the center connection of the canister. Fuel bowl overflows are connected via one pipe to the right (rear of car) connection when viewed from the carburetors. There is an air intake at the bottom. The 3rd top connection goes to the fuel tank, I believe. Assuming much of the rest of the system has been removed, check that what is left has been dealt with correctly, but I don't think it should have any effect on performance unless something is connected in a weird way.

 

[vagt6]

Thanks for the great info, Gerard.

I've been using the Moss carb dashpot oil in the carbs. Not sure what kind of oil it is. What's your opinion of the Moss oil?

 

[Gerard]

Thanks for the great info, Gerard.

I've been using the Moss carb dashpot oil in the carbs. Not sure what kind of oil it is. What's your opinion of the Moss oil?

You are welcome. Glad you find it helpful.

I've never used the Moss oil, so I have no idea. I would assume it's a correct/suitable product, but I have no idea of the weight. A little searching around indicates it's pretty light weight... I'm guessing 20 wt or maybe 15.

 

[regularman]

Does it just miss at idle or all the way through the range? Can you feel the miss when when taking off easy?

 

[vagt6]

Kim, it seems to miss at idle, and when I rev it up to about 3,000 while sitting still. It's a very slight miss or stutter.

However, the car seems to run fine under load, on the rolling road. If it's missing on the road while driving/accelerating, I really can't percieve it. I ran her through the gears up to about 4,500 RPM to about 80 MPH yesterday, seemed okay.

Again, plugs look great (maybe a little lean), car burns zero oil, etc. There's no miss under acceleration that I can notice.

Strange, maybe I'm imagining it?

How would I check (electronically) to determine if there's a miss?

 

[JeffS]

I'd bet my own Midget that you're running lean, and it can either just be the carb setting, or more likely a vacuum leak. I use propane (from an unlit torch) to check for leaks. Test it at the air cleaner first to see if the idle goes up or down, then check all hoses and the intake manifold/carbs.

If you find no leaks, go 3 flats rich at the jets. If you have a problem, 1 flat will NOT be enough to repair it. You're always safer a bit rich than a bit lean!!! Typically cars with this issue need to go 3-6 flats rich. Look into the carbs with a flashlight with the air cleaners removed as well, and verify that both jets are approximately the same distance down from the bridge. You can always turn them both all the way in, then back them out 12 flats as a starting point. Your plug color means very little to me - its deceiving.

I also see that your vacuum advance is not hooked up to anything - probably because the diaphragm is torn out (yet another vacuum leak). You should consider getting that repaired, since it will improve throttle reponse to have it hooked up.

 

[vagt6]

Thank, Jeff.

With the propane cylinder, how does it work? Just point the unlit torch at various spots to see if the propane gets sucked in?

I'll check the diaphragm.

Great info, guys.

 

[JeffS]

Yep, propane is fuel, so if you're lean idle will increase when it gets sucked in. If you're rich, idle will drop when it gets sucked in. You can do the same thign with carb/brake cleaner more effectively if you're not too worried about losing paint or grime.

 

[vagt6]

Thanks everyone for the great tips.

Best wishes, all. :thumbsup: