Vanguard twin missing at mid to wot

[TandEtractors]

model 386777-0111-e1

The engine is on a simplicity conquest thats been sitting 6 years. only 178 hours

Valves set, new fuelines and fuel pump, carb completely cleaned and new internal gaskets used, leakdown test was good on both clyinders , coil was swapped with another known good coil, kill wire unhooked and ran the same.
Head pulled and new head gasket installed, valves seems good , new intake gaskets.
Valve seats are good.
push rods are not bent.
I also have a 385777 vanguard engine on the shelf i know runs good so i put that carb on and still runs the same

When i ground Cylinder 1 the engine dies, when i ground cylinder 2 there is hardly a difference.

I'm stumped, the exhaust valve spring retainer has a purple tint to it, and the miss does seem to come from the muffler side, could the spring be weak?
i think i am pulling the head again today.

[KE4AVB]

Okay on this v-twin you must connect both coils kill wires to test to coils. The kill wire harness contains diodes that can short or open causing problems. I have a very similar problem last year and it appeared to be a bad coil and instead one of the diodes was shorted and the other one was leaky. In my case I replaced the shorted diode and was still having problems until I replaced the leaky diode in the other lead. In your harness there appears two diodes in series in lead that goes to each coil.

One other thing to check is for a spark plug that is failing under compression.

[bgsengine]

Agree with KE4 - sounds like your diode harness failed - have had 2 or 3 of these (All on Cub Cadet tractors) with the same exact problem - replaced the diode harness and all fixed - So definitely something to be checking on as unhooking kill wire at the engine housing won't eliminate a bad kill wire - you would have to unhook both wires from the coils to verify the problem - or run a spark tester on both cylinders to verify or not that there is ignition - and as mentioned, make sure there isnt a bad plug in the equation.

easiest way to confirm ignition is with 2 spark testers hooked between plug wire and plug and see if they are both sparking when engine runs... if they are, swap the plugs around (or replace them with new) and re-test - if you still have the miss and good spark on both sides, then more likely is NOT ignition, but I would bet on the diode harness between the coils... and the spark test would lead you that way - then you can consider pulling the engine cover and unhook ground wires from both coils, and re-test for spark (with plugs removed, or you gonna have a hard time shutting down the engine.. haha) - if you got spark then, your problem is in the diodes.

[TandEtractors]

I have unhooked both ground (kill) wires and ran the engine, same result a popping or missing sound in the muffler new plugs also.

I have the briggs inline spark tester and number 2 clyinder has good spark even when missing,

Has anyone ever had trouble with the compression release sticking on , since its on the exhaust valve it would hold the valve open at high rpms and cause a miss ....if the little ball was stuck?

[bgsengine]

when you had the head off was it checked for warp or cracks? a Leakdown test would pretty much answer the question I suspect. (as opposed to compression test) I think there is a thread here (or possibly it was at the old perr forum) to make a home made leakdown tester - if it is not spark, and not an intake leak (I assume you checked for air/vacuum leaks all along the manifold, verified the intake gasket did not slip out of place, etc.) then it has just got to be a compression leak - although I would think such a misfire due to compression would show up in a compression test, it isn't always the case.

[KE4AVB]

Haven't had my coffee yet but it sounds like the cylinder is dead for some reason other than ignition now that you have verified that it working. These v-twins can run fairly good even on one cylinder when not under a load. As BGS suggests a leak down test is order to find if it the rings/cylinder, valves, or a gasket that could be leaking. Also do cylinder balance compression test to verify that both cylinder are with 25% of each other. If the compression is off more than 25% you may have lower cylinder problems when the leak down is passed.

One thing addition that is not too invasive is pull the dead cylinder valve cover and check the valve lift (amount of rocker movement). It should be nearly the same for both in and ex. I have seen a couple cams here where the lobes were worn nearly round instead of the normal lobe shape. The strange thing both times it was the intake lobes. The engines wound run but had no power to engage things like the mower decks.

You ask about the decompressor system on the cam. I haven't seen this style of decompressor personally but anything is possible. It just Murphy's Law and my luck such a thing could and would happen to me.

[Arkie]

Did not read all of this real close, but if grounding #2 cylinder plug wire makes no difference at times #2 cylinder is the culprit. You say it has good spark, good compression, etc.
You have changed everything except the whole motor.
Valve clearance looks ok.

Next time you pull the head rotate the engine until each valve is fully open one at a time and take a o ring pick and magnifying glass and look for a valve seat that is getting ready to let go. (assuming you have installed new spark plugs!
Finding a bad spark plug after all that will really make you feel weak. If a valve seat is loose do a valve seat stake procedure with a sharp punch.
How do I know this!
Also you will find that during the finals on the twins that have the non-adjustable H jet, but a adjustable L jet that the setting of the L jet is real critical on some engines for good smooth mid range engine smoothness if the jet is adjusted for what appears to be real good low speed idle. Adjust L in 1/8 at time on a warmed up engines from what appears to be real good Low idle will sometimes smooth a slightly mid range roughness. Some carbs are just lean enough that slight adjustment of the L jet will smooth the non adjustable H side engine performance.

[bgsengine]

Next time you pull the head rotate the engine until each valve is fully open one at a time and take a o ring pick and magnifying glass and look for a valve seat that is getting ready to let go. (assuming you have installed new spark plugs!

Umm.. you do know it is an OHV we're talkin' about, right? rotating the engine won't do anything for the valve seats with the heads off.. hehehe but that's also a good point to check - but on OHV it is a whole lot easier - just pop the valve out of the head and check seat

[Arkie]

Next time you pull the head rotate the engine until each valve is fully open one at a time and take a o ring pick and magnifying glass and look for a valve seat that is getting ready to let go. (assuming you have installed new spark plugs!

Umm.. you do know it is an OHV we're talkin' about, right? rotating the engine won't do anything for the valve seats with the heads off.. hehehe but that's also a good point to check - but on OHV it is a whole lot easier - just pop the valve out of the head and check seat

No did not realize OHV. As you say even easier to check the valve seats.
I think he said it was a low hours motor, but I've seen neglected low hour motors pop or loosen a valve seat mainly due to owner not keep the cooling fins clean, but can also happen for no apparent reason. (aluminum and steel expand at different rates and out she comes or loosens or a valve stem will start seizing intermittently. I've also seen OHV valve stems start hanging (seizing) the valve springs are not very strong on a OHV as compared to a L head engine..
If a #2 head is loose on the other spare shelf motor that he mentions he might consider swapping in a head.

Appears he has changed about everything but his underwear. Clean underwear kinda makes you feel and smell better and also the engine likes you better sometimes.

[TandEtractors]

It's defiantly the exhaust valve and seat, it holds a seal but the face is slightly rounded not a nice looking valve face.
Would the valve being too tight have caused this?

I ended up putting the other vanguard I had on the machine, and am most likely ordering a new clyinder head on my next briggs order .