View Full Version : Exhaust Leak Causing Misfire?

40 Nuck
03-17-2008, 02:26 PM
I have a 1940 EL, and noticed that the engine would miss (as best I could tell, only one cylinder), and have gone through the points and plugs and tested the coil. All seem to be fine. In checking things out, I noticed that the exhaust pipe from the front head felt a little loose at the exhaust port, and wondered if that could be causing a miss. Can anyone tell me if that could be the culprit? If so, any suggestions on tighter fit? How about using some sort of gasket sealer around the exhaust pipe? I'm a little leary about doing that with it being so close to the chamber. The exhaust pipe seems to fit up into the exhaust port about 3/8", but has a little wiggle.


40 Nuck
03-17-2008, 02:28 PM
One more thing causing me to think it might be the exhaust ... It only misses in low-to mid-RPM, and usually when I initially apply throttle. Once it revs up, or when idling, there is no issue. Thanks again.

03-17-2008, 05:51 PM
Typically an exhaust leak like that would cause her to bakfire quite badly when you rolled off the throttle hard. Have you checked for an intake leak? I would also test the coil hot and make sure all wires and mounting bolts were tight, low to mid RPM is usually were the good vibrations are.

40 Nuck
03-25-2008, 11:58 AM
Thanks, Brian ... I'll check the coil again ... and the intake. Appreciate your input.

T. Cotten
03-29-2008, 08:33 AM
40 Nuck!

The best way to test for a vacuum leak is with air pressure and soapy water:

Good luck!


40 Nuck
04-02-2008, 01:37 PM

Thanks for the link ... great info and detail.

04-02-2008, 04:19 PM
i have a 41 u (for sale!) that was doing the same thing you describe , and some other neat stuff to.i built a intake leak testing device ,as describe by cotten in the virtual indian piece and started testing . i had leaks in places i never would have thought to check!after many hours of fixing all the isms the bike runs great now , i have since checked all my other bikes and repaired those as well .making your own tester is well worth the effort ,especially if your bike isnt a trailer queen and gets ridden good luck in chasing down this gremlin , tom

05-15-2008, 05:18 PM
All of the above info is good. Make sure you have the correct main nozzle, and I would definitely recommend one of Cotten's floats. If you are running a brass float, or one that is fuel-logged, the air hole in the main nozzle won't kick in at the proper time causing a very rich mixture at low speeds. You can pull the bowl, cork the bottom hole, and connect it to the fuel line to get a wet fuel level. It should be 5/8" below the top of the bowl. Having this right makes a HUGE difference.

Also, make sure you have the correct tophat washer under the low-speed needle spring. The ID size meters the idle and low speed circuits. If it's missing or way off in size, you can adjust for the idle, but the low speed mixture will still be off. There are at least 3 different ones. I think I'm running a .106" ID.

05-15-2008, 08:57 PM
I was unaware of the different tophats. Where would one find a list of wich carbs used wich one?

George Greer
06-29-2008, 10:26 PM
The info about the tophat "washer's" id is interesting...

I had one on my WLA (M-88)....that I thought was just worn out...I mean the ID was huge..

Got a NOS one and now it runs good..

Wish I could remember where I put the old one....I seem to remember that it was not oval from wear....I'll try and find it and measure the ID..

A list of the possible ID's and applications would be nice.

T. Cotten
06-30-2008, 09:57 AM
I attempted to keep track of different spring collars for some time.

It is my impression that the IDs were probably the same for all, even for Scheblers.
If the collar does not slide easily when the choke is operated, the holes wear larger.
Occasionally the needles will "hourglass" or have a flat side as well.

The thicknesses of the collars vary however.
In general, flathead models including Indians used a thinner collar, whereas postwar HD OHV's nearly always are found with thick collars.

Too many exceptions to these rules cross my benches to declare any gospel.

Since there is little height difference of the orifice hole for the two, I cannot imagine that there is a difference in air metering.

Next quandry: Why are there differences in the lowspeed needle wells, if the collar does the metering? (The hole in the casting, as shown in the attachments.)
And why do the last M74Bs with no serifs on the B have a needle well that is choked towards the bottom?
Without L&L literature, we may never know.


06-30-2008, 06:08 PM
I come up with two sizes on the washer. 27345-27 is about .103 - .104" ID and 27345-41 is about .106 - .108" ID. A couple of thousandths make a big difference when coming off of idle.

07-01-2008, 09:15 PM
From the Harley manual-

"CAUTION: The lift spring seat hole is calibrated and limits the amount of air bleed to idle circuit and must be in place; otherwise carburetor cannot be adjusted for satisfactory engine idling."

.003" difference in the ID of the washer adds about 30% more air to the low speed circuit.

07-04-2008, 04:06 PM

I may have found a partial answer to your quandry about the differences in the M74-B castings. I was trying to identify this carburetor, which someone destroyed by buffing it to death, and I noticed there is no hole for an idle enrichment lever. The idle mixture needle threads directly into the casting, and there is a separate idle bleed orifice threaded into the boss on the side. i don't have any literature for this carb, but I'm guessing it's an M53 off a K model. This would explain the additional material cast into the later bodies. Can anyone help?