Engine Mechanical TopicsDiscussion for motor builds, cams, head work, stripped bolts and other engine related issues. The good and the bad. If it goes round and around or up and down, post it here.
Hey guys I'm looking for some insight on some issues I'm having with my twin cam build. Rebuilt top end, 103 jugs, pistons, rings, cams, lifters, push rods, oil pump, cam plate, etc. Also rebuilt primary side stator, rotor, screamin eagle compensator Sprocket, stater clutch, etc. The issues I'm having are misfire, backfire, rough idle and stalls out. This is on initial first start up after build. I guess another thing to mention i had the bike painted so the tank sat dry for couple months. I have opened cam cover and rechecked my timing marks all line up. However, I believe my tdc on front cylinder is off somehow. Maybe I'm over thinking this but the timing marks on sprockets are not 180 degrees apart and in line. They are over on the right a bit. I'm checking for tdc with a paint brush. The rear cylinder tdc is lined up with marks closest together. Could be over thinking and maybe it's just a fuel issue but I wanted to start here since I made big changes and bike ran ok prior to rebuild. Thank you all in advance Tdc Tdc
The crankpin is shared between rear piston and front piston, so if the crank to cam timing is OK on the rear ( and it looks to be) the front has to be- can't jump timing. If you verified the rear cam to front cam timing is correct (easily done using a compression tester- remember to disconnect any ACR before testing), then it's time to check fuel pressure.
I don't believe the OP has checked cam timing; he seems to be in denial that the sprockets could be one tooth off. A compression test will tell the tale as has been suggested; known good gauge, ACRs disconnected, fully charged battery, both plugs out, WOT, etc. Coil is OK, have cranked motors many times with ungrounded plug wires with no damage to the coil.
I don't believe the OP has checked cam timing; he seems to be in denial that the sprockets could be one tooth off. A compression test will tell the tale as has been suggested; known good gauge, ACRs disconnected, fully charged battery, both plugs out, WOT, etc. Coil is OK, have cranked motors many times with ungrounded plug wires with no damage to the coil.
The problem is that when you do this you are testing the insulation inside the coil. It takes about 3kV to jump a 0.040 gap in air. It can be considerably more under cylinder pressure but it's still not as high as the output voltage with an open connection. If the insulation has any weak point, it can ark and burn through.. At that point the coil is toast. It wasn't an issue when points were around but the output voltage on these things is way higher than back then. Yeah you can get away with it but the potential is still there.. (Pun intended)
The problem is that when you do this you are testing the insulation inside the coil. It takes about 3kV to jump a 0.040 gap in air. It can be considerably more under cylinder pressure but it's still not as high as the output voltage with an open connection. If the insulation has any weak point, it can ark and burn through.. At that point the coil is toast. It wasn't an issue when points were around but the output voltage on these things is way higher than back then. Yeah you can get away with it but the potential is still there.. (Pun intended)
I guess I have just been lucky. I should have indicated that when this happened, it was unintentional but it has happened several times with no coil damage so the OPs coil is probably not the problem and he should stay focused on cam timing.
Could crossed injector wires present the symptoms the OP is experiencing or would that present differently?.
I guess I have just been lucky. I should have indicated that when this happened, it was unintentional but it has happened several times with no coil damage so the OPs coil is probably not the problem and he should stay focused on cam timing.
Could crossed injector wires present the symptoms the OP is experiencing or would that present differently?.
I've made the same error and also been lucky with the coils. The issue is likely something a lesser chance of failure but still a possibility and therefore worthy of proper precautions.
As far as injector wire swap goes, the fuel would be going to the wrong port at the wrong time. It's going to cause running issues, Particularly at low speeds. Remember the firing order is not symmetrical so with the wires swapped one cylinder could get more fuel then the other..
It's also possible that something got into one of the injectors and is either not supplying fuel to supplying too much.
I am going to pick up new injectors today and run it again. Coil, wires tested good, and plugs are new. The front cylinder plug looks like it is not combusting the plug is completely white where the rear plug has some black from combustion. I believe spark is good so yes injectors are next. You guys think rings aren't set yet thats why compression on cylinders is off? Maybe run it a bit and test again? The cams are 100% timed correctly i triple checked tore it all back apart. Thanks guys
That is pretty high CCP and you do have roughly a 20 lb difference as stated. You never mentioned what Pistons, what compression, what kind of build you really did... You just said you did a top end and now that you're showing that kind of CCP it looks like your front cam is advanced one tooth.. Hence the higher compression allowing for an earlier IVC which will bump up the CCP.. curious as to what your build was to static compression and what your IVC is to determine CCP..
Sometimes it makes a big difference in determining what is causing an issue... Once we have all the relevant information It will be easier to diagnose.....
Harleys with their common crankpin are not 180 degrees apart. If you notice as you turn it, watching the pistons, it will make sense. 405° from rear to front, then 315° from front to rear.
Last edited by Jackie Paper; 01-02-2021 at 10:30 AM.
IIRC the individual cam timing marks line up when the gear marks line up.. You can rotate the motor so the cam mark (green star) is opposite the bolt and check with a straight edge. In that case the red start should line up with the 2 cam centers. BTW you can get some difference between the cylinders due to slight differences in the how each cam is ground, distance between cams and length of the chain.
01-01-2021, 01:50 PM
