2003 Viper Miss at Idle after rebuild

M

mg_sxviper

New member
Joined
Jan 28, 2021
Messages
7
Location
Chelan, WA
Problem started back in Feb, on a ride, came to a stop and it was backfiring and barely running. Towed it back home, checked compression and #3 dead. Pulled the head and looked like part of a ring broke off and grenaded the piston, cylinder & head. Decided to do a full rebuild, tore all the way down, pulled the crank and didn't find any fragments in the bottom end, installed new crank seals. Replaced cyl #3 & head, new top end rebuild from mcbperformance for all 3 cylinders, torqued cylinders and head to spec. Since the snow is now gone, I can't take it on a ride. TORS is bypassed. It starts right up and I varied RPM from idle to 3500 until heat exchangers warmed up. Let cool down and repeated a few times.
It runs smooth off idle but when comes back down to idle it is visually a little rough, occasional miss (every 10 sec or so) and backfire (once in 5 minutes). Can't tell which cylinder. I can pull each plug wire and it idles down to 1400 (from 1800), the same on each so they seem balanced. Checked temp of each cylinder and all match. Can rev back up to 3000 and it's smooth. Then new plug caps and plugs, no change. What worries me is that I've never heard it backfire before until the breakdown in Feb. I have not gone through carbs again as I did carb kit on all 3 last summer and only run 92 non-ethanol.
Am I down to carburation or could it be something mechanical I missed?
Have not checked power valve gap yet, could this cause it? I did totally clean them during the rebuild.
Thank you for any help.
 
First thoughts for me may be rusty flywheel or weak stator?
Also, double check wiring harness ground to make sure its not loose.
 
The flywheel was clean when I did the rebuild. Pulled the stator and cleaned with electronic cleaner, have read that they could be wonky so I will measure it next. Is this the wiring harness ground behind the panel on the left side?Inked0411231415.jpg
 
It has the Moto Tassinari VForce 3 carbon fiber reeds from the prev owner. I did inspect them and blew them with air hose, especially the one for cyl 3 but might need to remove the petals and inspect closer. I checked the stator and the pickup measured 205 ohms, stator 0.4 ohms which is within spec. I have read other posts though where it measured ok but problem fixed if replaced. I also checked the connector which had some corrosion on the terminals so I cleaned and applied dielectric grease. Also measured each coil and cleaned it's ground lead. Removed the frame ground wire, found rust underneath so I cleaned the frame and wired, applied dielectric grease hoping that was it but no improvement.
 
I think you nailed it when you said " havent gone thru the carbs again".

Check float height. Adjust as needed.
Pull needle & seats. Clean the hat screens above the seat.
Inspect and clean the pilots.
Replace the o-rings on the seat. probly 20 years old.
 
I to have a set of V force 3 reeds as well. Inspected them closely and could not "see" a problem. Unknown mileage on them as they came with the motor I purchased. Chased my tail trying to figure out why one cylinder wasn't functioning properly. Moved reeds around and problem moved with one set of reeds. Installed a factory set of reeds on all 3 and all was well. V Force reeds are now in a box as I still can't "see" what the problem is. Maybe fatigue from age??? Just trying to help!!!
 
Mac said:
I think you nailed it when you said " havent gone thru the carbs again".

Check float height. Adjust as needed.
Pull needle & seats. Clean the hat screens above the seat.
Inspect and clean the pilots.
Replace the o-rings on the seat. probly 20 years old.
Click to expand...
Thanks!
 
northern srx said:
I to have a set of V force 3 reeds as well. Inspected them closely and could not "see" a problem. Unknown mileage on them as they came with the motor I purchased. Chased my tail trying to figure out why one cylinder wasn't functioning properly. Moved reeds around and problem moved with one set of reeds. Installed a factory set of reeds on all 3 and all was well. V Force reeds are now in a box as I still can't "see" what the problem is. Maybe fatigue from age??? Just trying to help!!!
Click to expand...
Ah, could be, thanks, I found a set of oem on ebay. How did you determine which cylinder was acting up?
 
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I "THOROUGHLY" cleaned the carbs (hidden o-rings replaced ETC) so they were ruled out. RE-checked coils / wires / caps / plugs... all good. Moved on to the reeds and inspected them again (kept them in order!). Decided to switch 2 of them and lo and behold - problem moved the other cylinder. Middle cylinder not firing consistently, colder than the other 2, verified by checking the exhaust manifold with a heat gun. Further verified by switching the 2 reed blocks back to their original position and the problem followed.
 
Update... I got in the OEM reeds and installed. Went through the carbs, checked the o-rings which I had replaced last year. Still doing it. Here's some additional clues:
Starts on first pull and doesn't seem to miss/slight backfire until after 4 minutes warming up, it's still occasional every 5-10 sec.
If I rev up to 3000-3500 it's smooth but off throttle it hangs at 2500 for about 20 seconds then comes back down to 1800.
So my questions:
1) Even though power valves were totally cleaned and checked for smooth cable operation, can these cause this? Can the YPVS be disconnected as a test?
2) Is a backfire related to a lean or rich condition?
3) Could this simply be because it's not broken in yet because I can't go ride?
 
I'm not thinking power valves have anything to do with your problem. Yes you could disconnect. They only pull up when the engine RPM is over 7500 RPM? don't quote me on the exact number.
When the RPM hangs like what you describe. Its lean. Back out the fuel screw and you will see it wont hang for 20 seconds. What sucks is the location of the fuel screw.
 
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